Final Rapid Science Synthesis Report: 

Findings from the 

Second Texas Air Quality Study (TexAQS II) 

A Report to the 

Texas Commission on Environmental Quality 

by the 

TexAQS II Rapid Science Synthesis Team 

Prepared by the 

Southern Oxidants Study Office of the Director at 

North Carolina State University 

by 

Ellis B. Cowling, Director of SOS 

Cari Furiness, Research Associate 

Basil Dimitriades, Adjunct Professor 

and 

David Parrish 

Earth System Research Laboratory, 

National Oceanic and Atmospheric Administration 

In cooperation with Mark Estes of TCEQ and 

47 other members of the Rapid Science Synthesis Team 

TCEQ Contract Number 582-4-65614 

31 August 2007 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 2 31 August 2007 

Acknowledgments 

The Texas Commission on Environmental Quality (TCEQ) and the Rapid Science Synthesis 

Team (RSST) for the Second Texas Air Quality Study (TexAQS II) gratefully acknowledge 

significant contributions to the success of the TexAQS II field research campaign by the many 

different organizations listed below. These contributions include significant financial and inkind organizational support as well as the time and creative energy of individual scientists, 

engineers, post-doctoral fellows, and graduate students who voluntarily joined in this 

collaborative enterprise. 

These contributions include: 

• Defining the objectives of TexAQS II, 

• Instrumenting the various air-quality measurement platforms, 

• Implementing and quality assuring the myriad field measurements during the course of 

the study, 

• Completing the analysis, interpretation, and intercomparison of research results, and 

• Formulating carefully crafted statements of research findings to be used for policy 

purposes by TCEQ and other stakeholders. 

Cooperating State Government Organizations 

 California Air Resources Board 

 Texas Commission on Environmental Quality 

 Chief Engineer’s Office (CEO) 

 Air Quality Division (AQD) 

 Air Modeling and Data Analysis (AMDA) 

 Industrial Emissions Assessment (IEA) 

 Toxicology 

 Office of Compliance and Enforcement 

 Field Operations Division 

 Region 4 – Dallas/Fort Worth 

 Region 12 – Houston 

 Monitoring Operations (MonOps) 

 Ambient Monitoring 

 Air Laboratory and Mobile Monitoring 

 Data Management and Quality Assurance 

Cooperating Texas Local Government Organizations 

 Alamo Area Council of Governments (AACOG) 

 Capital Area Council of Governments (CAPCOG) 

 City of Corpus Christi 

 City of Victoria 

Cooperating Federal Government Agencies 

 Department of Defense (DOD) 

 Navy Postgraduate School (NPS) 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 3 31 August 2007 

 Department of Energy (DOE) 

 Pacific Northwest Laboratory (PNL) 

 Argonne National Laboratory (ANL) 

 Los Alamos National Laboratory (LNL) 

 Department of Interior (DOI) 

 Minerals Management Service (MMS) 

 Environment Canada (EC) 

 Meteorological Service of Canada (MSC) 

 National Aeronautics and Space Administration 

 Headquarters Science Mission Directorate and science teams for the 

 Atmospheric Infrared Sounder (AIRS), 

 Cloud-Aerosol Lidar Infrared Pathfinder Satellite Observations (CALIPSO), 

 Moderate Resolution Imaging Spectroradiometer (MODIS), 

 Multi-angle Imaging SpectroRadiometer (MISR) 

 Ozone Monitoring Instrument (OMI), and 

 Tropospheric Emission Spectrometer (TES) 

 Jet Propulsion Laboratory (JPL) 

 Langley Research Center (LRC) 

 National Oceanic and Atmospheric Administration (NOAA) 

 Office of Oceans and Atmospheric Research (OAR) 

 Earth System Research Laboratory (ESRL) 

 Pacific Marine Environmental Laboratory (PMEL) 

 Atlantic Oceanographic and Meteorological Laboratory (AOML) 

 Air Resources Laboratory (ARL) 

 National Weather Service (NWS) 

 National Environmental Satellite Data and Information Service (NESDIS) 

 Office of Marine and Aviation Operations (OMAO) 

 National Science Foundation (NSF) 

 National Center for Atmospheric Research (NCAR) 

 Tennessee Valley Authority (TVA) 

 United States Environmental Protection Agency (USEPA) 

 Office of Air Quality Planning and Standards 

 Air Quality Analysis Division 

 Air Quality Modeling Group 

Cooperating Research Universities 

 Baylor University 

 California Institute of Technology 

 Chalmers University of Technology, Göteborg, Sweden 

 Fort Hayes State University 

 Georgia Institute of Technology 

 Howard University 

 Lamar University 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 4 31 August 2007 

 Texas Air Research Center (TARC) 

 North Carolina State University 

 Pennsylvania State University 

 Portland State University 

 Rice University 

 Scripps Institution of Oceanography 

 Texas A&M University 

 Texas Tech University 

 University of Alabama in Huntsville 

 University of California at Los Angeles 

 University of California at Santa Cruz 

 University of Colorado 

 Cooperative Institute for Research in Environmental Science (CIRES) 

 University of Houston 

 University of Manchester, Manchester, UK 

 University of Maryland Baltimore County 

 University of Massachusetts 

 University of Miami 

 University of New Hampshire 

 University of Rhode Island 

 University of Texas in Austin 

 University of Washington 

 Joint Institute for Study of Atmosphere Oceans (JISAO) 

 Valparaiso University 

 Washington State University 

Cooperating Private Sector Organizations 

 Aerodyne Research Inc. 

 Baron Advanced Meteorological Systems 

 Exxon Mobil Corporation 

 Houston Regional Monitoring Network 

 Science Systems and Applications, Inc. 

 SensorSense, The Netherlands 

 Shell Oil Corporation 

 Sonoma Technology, Inc. 

 URS Corporation 

Other Cooperating Organizations 

 Houston Advanced Research Center (HARC) 

 Texas Environmental Research Consortium (TERC) 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 5 31 August 2007 

Executive Summary 

The Rapid Science Synthesis Team (RSST) for the Second Texas Air Quality Study (TexAQS II) 

has been charged to address a series of 12 High Priority SIP-Relevant Science Questions 

identified by the Texas Commission on Environmental Quality (TCEQ). Answers to these 

important questions were needed by TCEQ and other stakeholders in Texas to help fulfill the 

Commission’s responsibility to develop scientifically sound State Implementation Plans (SIPs) 

by which to attain the recently implemented 8-hour National Ambient Air Quality Standard 

(NAAQS) for Ozone. SIPs for both the Houston-Galveston-Brazoria and the Dallas-Fort Worth 

Ozone Non-Attainment Areas were submitted to EPA in June 2007. 

This Final Report of the Rapid Science Synthesis Team for TexAQS II is designed to provide 

statements of Findings in response to each of TCEQ’s 12 High Priority SIP-Relevant Science 

Questions. This report addresses: 

 1) Significant sources of ozone and aerosol pollution in eastern Texas, 

 2) Photochemical and meteorological processes involved in the production, transport, and 

accumulation of these pollutants in various parts of Texas, 

 3) An assessment of the adequacy of emissions inventories for both biogenic and 

anthropogenic sources of ozone and aerosol precursor chemicals, and 

 4) An assessment of the skill of current air quality modeling and forecasting systems and 

recommendations for improvement of these systems. 

This Executive Summary provides a short introduction to the scientific Findings from TexAQS 

II research for use by TCEQ managers and other air-quality decision makers and stakeholders in 

Texas. It contains a complete list of the 12 High Priority SIP-Relevant Science Questions that 

TCEQ asked our Rapid Science Synthesis Team to address a series of carefully crafted 

statements of Findings that have been developed in response to each of these questions. 

We emphasize that the statements of scientific Findings developed as part of the Rapid Science 

Synthesis effort are based on analysis and interpretation of results from TexAQS II research 

within the limited time that was available between completion of the last TexAQS II field 

measurements on 15 October 2006 and our Rapid Science Synthesis contract ending date – 31 

August 2007. More thorough and comprehensive analyses are already underway and will yield a 

great deal of additional important information in the future. 

The institutional affiliations of the scientists responsible for the analyses leading to these 

Findings are given in the Acknowledgments section, immediately preceding this Executive 

Summary. Each section of this report is structured to contain one of the 12 SIP-Relevant Science 

Questions, then a numbered sequence of Findings in response to that question. Following each 

Finding, acknowledgment of the individuals whose analyses and data contributed to that Finding 

are listed. It also provides a brief discussion of the evidence that supports each Finding. The 

concerned reader should carefully consider the caveats contained in these discussions. For the 

convenience of all readers, a glossary of acronyms is provided in Appendix 1. 

Please note that questions printed in blue were designated by TCEQ for special emphasis. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 6 31 August 2007 

Findings 

Question A 

Which local emissions are responsible for the production of high ozone in Houston, Dallas, 

and eastern Texas? 

Are different kinds of emissions responsible for transient high ozone and 8-hour-average 

high ozone (i.e., ≥84 ppbv)? 

Finding A1: The highest (i.e. > 125 ppbv) ozone concentrations in the HGB area result 

from rapid and efficient ozone formation in relatively narrow, concentrated plumes, which 

originate from HRVOC and NOx co-emitted from petrochemical facilities. The Houston 

Ship Channel (HSC) is the origin of the plumes with the highest ozone concentrations. 

Finding A2a: Winds carry the emission plumes from the Ship Channel throughout the 

Houston area. The wind direction determines the location of the ozone maximum relative 

to the Ship Channel; shifts in wind direction lead to the transient high ozone events 

observed at monitoring sites within the Houston area. 

Finding A2b: The general characteristics of the highest (i.e. > 125 ppbv) ozone formation 

in Houston did not change between 2000 and 2006, although the maximum observed ozone 

concentrations were lower in 2006 than in 2000. 

Finding A3: Emissions from the Houston Ship Channel play a major role in the formation 

of the highest 8-hour average ozone concentrations (and ozone design values) observed in 

the Houston area. 

Finding A4: Observations during TexAQS 2006 showed that nitryl chloride (ClNO2) is 

formed within the nocturnal boundary layer when NOx emissions and marine influences 

are both present. Following sunrise, ClNO2 photolyzes to yield chlorine atoms, which may 

lead to earlier and more rapid O3 production in the Houston region. 

Question B 

How do the structure and dynamics of the planetary boundary layer and lower 

troposphere affect the ozone and aerosol concentrations in Houston, Dallas, and eastern 

Texas? 

Finding B1: Boundary layer structure and mixing height near and over Galveston Bay and 

the eastern Houston ship channel area are spatially complex and variable from day to day. 

Vertical mixing profiles often do not fit simple models or conceptual profiles. High 

concentrations of pollutants are sometimes found above the boundary layer. 

Finding B2: Complex coastal winds, resulting from weak larger-scale winds over the area, 

occurred during many, but not all, ozone exceedance days in the Houston-GalvestonBrazoria nonattainment area. Almost no high-ozone days during TexAQS 2000 resembled 

any high ozone days from TexAQS 2006, but collectively the 2000 and 2006 field intensives 

sampled the full range of meteorological conditions associated with high ozone events. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 7 31 August 2007 

Finding B3: After sea breeze days, the Houston plume was broadly dispersed at night 

through the formation of low-level jets. 

Finding B4: The Dallas ozone plume can extend well beyond the monitoring network. 

Question C 

Are highly reactive VOC and NOx emissions and resulting ambient concentrations still at 

the same levels in Houston as they were in 2000? 

How have they changed spatially and temporally? Are there specific locations where 

particularly large quantities of HRVOC are still being emitted? 

Are those emissions continuous or episodic? 

How well do the reported emissions inventories explain the observed concentrations of 

VOC and NOx? 

Finding C1: There are indications that ethene (the lightest HRVOC) emissions from 

industrial sources in the Houston area decreased by 40 (±20)%, i.e., by a factor of between 

1.25 and 2.5, between 2000 and 2006. 

Finding C2: Measurements of ethene emission fluxes from petrochemical facilities during 

TexAQS 2006 indicate that the 2004 TCEQ point source database underestimates these 

2006 emissions by one to two orders of magnitude. Repeated sampling of the same 

petrochemical facility showed that the ethene emission flux remained constant to within a 

factor of two. 

Finding C3: Close to petrochemical HRVOC sources, the OH reactivity of propene is 

generally greater than that of ethene. 

Finding C4: The latest available emission inventories underestimate ethene emissions by 

approximately an order of magnitude. 

Finding C5: Inventories for NOx point sources at petrochemical facilities equipped with 

CEMS appear to be relatively accurate. Substantial decreases in NOx emissions in the 

Houston Ship Channel are suggested by the inventories, and measurements from aircraft 

are qualitatively consistent with the NOx decreases. 

Question D 

What distribution of anthropogenic and biogenic emissions of ozone and aerosol precursors 

can be inferred from observations? 

Finding D1a: Several rural electric generation units (EGU) in the Houston area and in 

eastern Texas have substantially decreased their NOx emissions per unit power generated 

since the TexAQS 2000 study. With one exception, SO2 emissions have not changed 

appreciably since 2000 for the plants sampled in 2006. 

Finding D1b: Comparisons of emissions derived from ambient observations with those 

measured by continuous emission monitoring systems (CEMS) indicate that the emissions 

from point sources equipped with CEMS are very accurately known. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 8 31 August 2007 

Finding D1c: Underreporting of CO emissions at several EGU noted in 2000 (Nicks et al., 

2003) has been reconciled by large increases (by factors of 5 to 50) in the inventory values 

between 2000 and 2006, as a result of newly implemented CEMS monitoring of CO at these 

plants. 

Finding D2: On-road mobile emission inventories developed from MOBILE6 have 

significant shortcomings. MOBILE6 consistently overestimates CO emissions by about a 

factor of 2. It accurately estimated NOx emissions in the years near 2000, but it indicates 

decreases in NOx emissions since then, while ambient data suggest NOx emissions have 

actually increased. Consequently in 2006, NOx to VOC emission ratios in urban areas are 

likely underestimated by current inventories. 

Finding D3: Emissions from ships constitute a significant NOx source in the HGB region. 

Literature results provide accurate emission factors for inventory development. 

Finding D4: Mixing ratios of isoprene over Texas measured from the WP-3D were used 

for evaluation of the BEIS-3 emission inventory. On average, the isoprene emissions from 

the inventory and emissions derived from the measurements agree within a factor of ~2. 

There may be areas south of Dallas-Fort Worth and southwest of Houston, where isoprene 

emissions are lower than indicated by the BEIS3 inventory based upon the biogenics 

emission land cover data (BELD-3.0). 

Finding D5: The speciation of VOC from mobile sources in the Houston and Dallas-Forth 

Worth areas agrees with detailed measurements in the northeastern U.S. However, initial 

results suggest that the agreement with the NEI-99 emission inventory is poor. 

Question E 

Are there sources of ozone and aerosol precursors that are not represented in the reported 

emissions inventories? 

Finding E1: The observed mixing ratios and regional distribution of ambient 

formaldehyde are broadly consistent with daytime photochemical production from reactive 

VOC. An upper limit for primary formaldehyde emissions from mobile sources is obtained 

from nighttime measurements, and is small in comparison with the secondary, daytime 

formation. 

Finding E2: Concentrated plumes of ammonia were observed occasionally in the Houston 

Ship Channel area. These plumes often led to the formation of ammonium nitrate aerosol. 

Finding E3: Concentrated plumes of gaseous elemental mercury from at least one point 

source were observed repeatedly in the Houston Ship Channel area and once in the 

Beaumont-Port Arthur area. The sources of the plumes could not be identified with 

current inventory sources of mercury. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 9 31 August 2007 

Question F 

How do the mesoscale chemical environments (NOx-sensitive ozone formation vs radicalsensitive ozone formation) vary spatially and temporally in Houston, Dallas, and eastern 

Texas? 

Which mesoscale chemical environments are most closely associated with high ozone and 

aerosol? 

Finding F1: Both Eulerian and Lagrangian plume modeling approaches indicate that in 

2000 high ozone concentrations in the HGB area were sensitive to both VOC and NOx

emission reductions. 

Finding F2: An observation-based approach to determine the sensitivities of high ozone in 

the HGB non-attainment area to the precursor VOC and NOx emissions has been 

investigated; it has yielded ambiguous results. 

Finding F3: At the highest ozone concentrations, the observed relationship between ozone 

and the products of NOx oxidation indicates less efficient ozone production in the Dallas 

area than in the Houston area. In the observation-based indicator species approach, this 

behavior corresponds to less NOx-sensitive and more VOC- or radical-sensitive ozone 

formation in Dallas compared to Houston. 

Finding F4: Tests of the ability of models to reproduce observed relationships between 

ozone and other photochemical products have the potential to provide very fruitful 

approaches to improving models. 

Question G 

How do emissions from local and distant sources interact to determine the air quality in 

Texas? 

What meteorological and chemical conditions exist when elevated background ozone and 

aerosol from distant regions affect Texas? 

How high are background concentrations of ozone and aerosol, and how do they vary 

spatially and temporally? 

Finding G1: The maximum background ozone concentrations encountered in 2006 

exceeded the 8-hour NAAQS. On average, air of continental origin had higher background 

concentrations than marine air. The average background ozone concentrations measured 

in 2006 in eastern Texas complement a previously developed climatology. 

Finding G2: The net ozone flux transported out of Houston averages about a factor of two 

to three larger than the corresponding flux from Dallas. The fluxes from these urban areas 

are significant contributors to the background ozone in the eastern Texas region. 

Finding G3: Elevated background ozone concentrations for urban areas can include 

contributions from the recirculation of locally produced ozone or local precursor emissions. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 10 31 August 2007 

Finding G4: Plumes from Texas urban areas make substantial contributions to the ozone, 

aerosol, and precursor concentrations in the rural regions of eastern Texas. 

Finding G5: Dust of African origin and sulfate aerosol advected into the Houston area, 

under southerly flow conditions from the Gulf of Mexico, can make significant 

contributions to the background aerosol in the eastern Texas region. 

Finding G6: Nighttime chemistry influences the availability of oxides of nitrogen (NOx), 

highly reactive VOC (HRVOC), and O3. 

Finding G7: Low rural nighttime ozone concentrations have been observed at some, but 

not all, rural locations in northeast Texas; these low nighttime ozone concentrations are not 

replicated in the regulatory modeling. 

Question H 

Which areas within Texas adversely affect the air quality of non-attainment areas in 

Texas? 

Which areas outside of Texas adversely affect the air quality of non-attainment areas in 

Texas? 

Finding H1: Ozone can be transported into the Dallas area from the Houston area. 

Finding H2: High ozone concentrations in eastern Texas result from both in-state sources 

and transport of continental air from the east and northeast. 

Finding H3: A synthesis of satellite and in situ measurements with photochemical 

modeling and Lagrangian trajectory analyses provides a quantification of regional 

influences and distant sources on Houston and Dallas air quality during TexAQS 2006. 

Finding H4: In the Dallas area, local emissions and transport each contributed about 

equally to the average 8-hr ozone exceedance in 2002. Transported ozone alone can bring 

the Dallas area close to exceeding the 8-hour ozone standard. 

Finding H5: In the Houston area, local emissions and transport each contributed about 

equally to the average 8-hr ozone exceedances investigated by aircraft flights in 2000 and 

2006. As in Dallas, transported ozone alone can bring the Houston area close to exceeding 

the 8-hour ozone standard. 

Question I 

Why does the SAPRC chemical mechanism give different results than the carbon bond 

(CB-IV) mechanism? 

Which replicates the actual chemistry better? 

Finding I1: Air quality modeling for both 2000 and 2006 shows substantial differences in 

ozone concentrations predicted by the SAPRC99 and CB-IV chemical mechanisms. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 11 31 August 2007 

Finding I2: In regions with very high VOC reactivity and high NOx emission density, 

differences in ozone formation and accumulation predicted by regional photochemical 

models using the SAPRC99 and CB-IV mechanisms are due to differences in: (1) the 

chemistry of aromatics, especially mono-substituted aromatics (e.g., toluene), (2) nitric acid 

formation rates, and (3) the rates of free radical source terms in the SAPRC and CB-IV 

mechanisms. 

Finding I3: The differences in the predictions of the SAPRC99 and CB-IV mechanisms 

can be probed using simulations of model compounds and comparisons of the simulations 

to environmental chamber data. For simulations involving CO and NOx (inorganic 

chemistry), the predictions of the CB-IV and SAPRC99 mechanisms that are used in 

regional photochemical models (with no chamber wall corrections) converge if the rate 

constants for the OH + NO2 reaction are made consistent between the two mechanisms. 

The predictions of the Carbon Bond mechanism, version 5 (CB05) also converge to the 

same predicted values if the OH + NO2 rate constant is made consistent with the other 

mechanisms. 

Finding I4: The performance of the mechanisms in simulating olefins chemistry can be 

improved through more explicit representation of internal olefin chemistry, which has been 

added in CB05. In addition, performance of the SAPRC99 mechanism in simulating 

chamber data was improved for some experiments when propene was modeled explicitly, 

as opposed to being represented by a lumped chemical species. 

Finding I5: For high reactivity chamber experiments involving olefins, sensitivity analyses 

indicated that mechanism adjustments that would lead to increased radical concentrations 

(increasing the radical yield in olefin-ozone reactions and changing the OH+NO2

termination rate constant) had little impact on predicted ozone concentrations. 

Finding I6: The SAPRC99 mechanism performed better than the CB mechanisms in 

simulating some chamber experiments with toluene; the mechanism performances were 

more comparable for xylenes and other multiply substituted aromatics. 

Finding I7: The differences between the SAPRC and CB-IV mechanism predictions for 

toluene chemistry decrease substantially if the yield for the lumped species CRES, and rate 

constant for the OH + NO2, are made consistent between the two mechanisms. The 

predictions of the CB05 mechanism also converge to the same predicted values if the CRES 

yield and the OH + NO2 rate constant are made consistent with the other mechanisms. 

Finding I8: For simulations of ambient surrogate mixture experiments in the UCR EPA 

chamber, all mechanisms underpredict O3 at low ROG/NOx ratios, with the bias decreasing 

as the ROG/NOx ratio increases. In simulations of mixture experiments in chambers 

without aromatics, CB05 performs the best, and with no dependence of bias on the 

ROG/NOx ratio. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 12 31 August 2007 

Finding I9: SAPRC99, CB-IV, and CB05 all successfully predicted concentrations of ozone 

and other species during simulation of a stagnation event, if the chemical mechanisms were 

initialized after initial high concentrations of olefins had reacted. None of the mechanisms 

successfully predicted a rapid rise in radical concentrations and ozone concentration 

concurrent with initially high C2, C3, and CMBO concentrations during the event. 

Question J 

How well do air quality forecast models predict the observed ozone and aerosol formation? 

What are the implications for improvement of ozone forecasts? 

Finding J1a: Most forecast models exhibit skill in predicting maximum 8-hr-average O3

but none of the models is better than persistence in predicting 24-hr-average PM2.5 levels. 

Finding J1b: Seven air quality forecast models and their ensemble were generally unable 

to forecast 85 ppbv 8-hr-average O3 exceedances with any reliability. 

Finding J2: Sophisticated data assimilation of meteorological and even chemical 

observations is essential for improving photochemical model forecasts. 

Finding J3: Model performance evaluations and intercomparisons require a 

comprehensive, best-guess emissions inventory for the TexAQS 2006 Field Intensive. 

Question K 

How can observation and modeling approaches be used for determining (i) the sensitivities 

of high ozone in the HGB non-attainment area to the precursor VOC and NOx emissions, 

and (ii) the spatial/temporal variation of these sensitivities? 

Finding K1: A simple, heuristic model based upon the Empirical Kinetic Modeling 

Approach (EKMA) method suggests that the HGB region may ultimately require drastic 

NOx emissions controls to reach compliance with the NAAQS for ozone. 

Finding K2: The same model suggests that in a projected future scenario with very strict 

VOC emission controls, but without drastic NOx emission controls, biogenic VOC 

emissions plus background concentrations of CO and methane may be sufficient to cause 

ozone exceedances. 

Question L

What existing observational databases are suitable for evaluating and further developing 

meteorological models for application in the HGB area? 

The Final Report for Question L can be found in Appendix 2 of this Report, as submitted 31 

October 2006. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 13 31 August 2007 

Introduction 

The Rapid Science Synthesis Team (RSST) for the Second Texas Air Quality Study (TexAQS II) 

was charged to address the series of 12 High Priority SIP-Relevant Science Questions listed in 

the text box below. The Texas Commission on Environmental Quality (TCEQ) posed these 

questions to elicit responses needed to help TCEQ fulfill its responsibility to develop State 

Implementation Plans (SIPs) by which to attain the recently implemented 8-hour National 

Ambient Air Quality Standards (NAAQS) for ozone in the 8-county Houston-GalvestonBrazoria and 8-county Dallas-Fort Worth ozone non-attainment areas. TCEQ’s interests and 

questions also deal, to a lesser extent, with fine particulate matter. This report presents the final 

Responses of the RSST to these 12 TCEQ Science Questions. 

As part of the Rapid Science Synthesis effort, TCEQ and the Office of the Director for the 

Southern Oxidants Study (SOS-OD) established Working Groups to address each Science 

Question. Each Working Group consisted of 8-15 experts drawn from various university-, state-, 

federal-, and private-sector organizations. The members of the Working Groups are listed in the 

text box below; contact information is given in Appendix 3. 

The Working Groups developed research approaches for responding to each of the 12 TCEQ 

Science Questions; the approaches are described in a Progress Report from the RSST dated 31 

July 2006. On 12 and 13 October 2006, shortly before completion of the TexAQS II Field 

Study, the Rapid Science Synthesis Team participated in a day-and-a half-long meeting in 

Austin, Texas. Presentations from this initial Rapid Science Synthesis Workshop were posted on 

the TCEQ website, http://www.tceq.state.tx.us/implementation/air/airmod/texaqsfiles/TexAQSII.html. These presentations formed the basis for an RSST report, Preliminary 

Findings from the Second Texas Quality Study (TexAQS II), dated 31 October 2006 [8 November 

revision]. 

The RSST participated in the Principal Findings and Data Analysis Workshop for TexAQS II/ 

GoMACCS 2006, held 29 May – 1 June 2007, in Austin, Texas. Presentations from this 

Workshop were also posted on the TCEQ website, 

http://www.tceq.state.tx.us/implementation/air/airmod/committee/scc.html#workshops. These 

presentations and subsequent analyses of research results by various TexAQS II researchers 

provided the foundation for this final report of the Rapid Science Synthesis Team, Final Rapid 

Science Synthesis Report: Findings from TexAQS II. The sections of this Final Report present 

carefully crafted statements of scientific findings in response to each of TCEQ’s High Priority 

SIP-Relevant Science Questions. In this connection, we found very helpful the “Guidelines for 

Formulation of Scientific Findings to be Used for Policy Purposes” shown in Appendix 4. 

Electronic copies of the three RSS reports to TCEQ are available on websites maintained by 

TCEQ (listed above) and by the National Oceanic and Atmospheric Administration (NOAA) 

(http://esrl.noaa.gov/csd/2006/). 

The following introductory information provides some background and perspective for the 

Science Questions and the Responses developed by the RSST. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 14 31 August 2007 

TCEQ’s High Priority SIP-Relevant Science Questions and Leaders (L), Participants (P) 

and Observers (O) in Working Groups within the Rapid Science Synthesis Team 

A Which local emissions are responsible for the production of high ozone in 

Houston, Dallas, and eastern Texas? Are different kinds of emissions responsible 

for transient high ozone and 8-hour-average high ozone (i.e., ≥84 ppbv)? 

L – David Parrish, P – Tom Ryerson, Joost deGouw, Basil Dimitriades, David 

Allen, Mark Estes, Bernhard Rappenglück, O – Noor Gillani 

B How do the structure and dynamics of the planetary boundary layer and lower 

troposphere affect ozone and aerosol concentrations in Houston, Dallas, and 

eastern Texas? 

Co-L – Robert Banta & John Nielsen-Gammon, P – Allen White, Christoph Senff, 

Wayne Angevine, Bryan Lambeth, Lisa Darby, Bright Dornblaser, Daewon Byun, 

Bernhard Rappenglück, O – Carl Berkowitz, Noor Gillani 

C Are highly-reactive VOC and NOx emissions and resulting ambient concentrations 

still at the same levels in Houston as they were in 2000? How have they changed 

spatially and temporally? Are there specific locations where particularly large 

quantities of HRVOC are still being emitted? Are those emissions continuous or 

episodic? How well do the reported emissions inventories explain the observed 

concentrations of VOC and NOx? 

L – David Parrish, P – David Allen, Joost deGouw, Tom Ryerson, Mark Estes, 

David Sullivan, John Jolly, Eric Williams, Barry Lefer, O – Yulong Xie, Carl 

Berkowitz, Noor Gillani. Note: To answer the last part of question C, TCEQ must 

define the inventory to which the observations must be compared. 

D What distribution of anthropogenic and biogenic emissions of ozone and aerosol 

precursors can be inferred from observations? 

Co-L – David Allen & David Parrish, P – Tom Ryerson, Charles Brock, Joost 

deGouw, David Sullivan, Mark Estes, John Jolly, Eric Williams, Barry Lefer, 

Bernhard Rappenglück, O – Yulong Xie, Carl Berkowitz, Noor Gillani 

E Are there sources of ozone and aerosol precursors that are not represented in the 

reported emissions inventories? 

L – David Parrish, P – Tom Ryerson, Charles Brock, Joost deGouw, David 

Sullivan, John Jolly, David Allen, Eric Williams, Barry Lefer, Bernhard 

Rappenglück 

F How do the mesoscale chemical environments (NOx-sensitive ozone formation vs 

radical-sensitive ozone formation) vary spatially and temporally in Houston, 

Dallas and eastern Texas? Which mesoscale chemical environments are most 

closely associated with high ozone and aerosol? 

Co-L – Basil Dimitriades & David Parrish, P – David Allen, Harvey Jeffries, 

William Vizuete, Daewon Byun, Mark Estes, Kenneth Schere, Barry Lefer, 

Bernhard Rappenglück, O – Yulong Xie, Carl Berkowitz 

Note: Letter designations are for convenience only and do not denote priority. Questions in blue have been 

designated by TCEQ to receive special emphasis.
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 15 31 August 2007 

TCEQ’s High Priority SIP-Relevant Science Questions and Leaders (L), Participants (P) 

and Observers (O) in Working Groups within the Rapid Science Synthesis Team 

(continued) 

G How do emissions from local and distant sources interact to determine the air 

quality in Texas? What meteorological and chemical conditions exist when 

elevated background ozone and aerosol from distant regions affect Texas? How 

high are background concentrations of ozone and aerosol, and how do they vary 

spatially and temporally? 

Co-L – David Allen & David Parrish, P –Bryan Lambeth, David Sullivan, Basil 

Dimitriades, Charles Brock, Michael Hardesty, Steve Brown, Joost deGouw, 

Bernhard Rappenglück, Brad Pierce, Wallace McMillan, Kevin Bowman, David 

Winker, Tim Bates 

H Which areas within Texas adversely affect the air quality of non-attainment areas 

within Texas? Which areas outside of Texas adversely affect the air quality of 

non-attainment areas within Texas? 

Co-L – David Allen & David Parrish, P – Mark Estes, Greg Yarwood, Basil 

Dimitriades, David Sullivan, Charles Brock, Michael Hardesty, John Jolly, Bryan 

Lambeth, Brad Pierce, Wallace McMillan, Kevin Bowman, David Winker 

I Why does the SAPRC chemical mechanism give different results than CB-IV? 

Which replicates the actual chemistry better? 

Co-L – David Allen & Greg Yarwood, P – Harvey Jeffries, William Vizuete, Bill 

Carter, David Parrish, Stuart McKeen, Daewon Byun, Joost deGouw, Barry Lefer, 

Bernhard Rappenglück, O – Mark Estes, Noor Gillani 

J How well do forecast air quality models predict the observed ozone and aerosol 

formation? What are the implications for improvement of ozone forecasts? 

L – Stuart McKeen, P – Gregory Carmichael, Bryan Lambeth, Kenneth Schere, 

James Wilczak, Greg Yarwood, Daewon Byun, John Nielsen-Gammon, Michael 

Hardesty 

K How can observation and modeling approaches be used for determining (i) the 

sensitivities of high ozone in the HGB non-attainment area to the precursor VOC 

and NOx emissions, and (ii) the spatial/temporal variation of these sensitivities? 

Co-L – Basil Dimitriades & David Parrish, P – Ted Russell, Harvey Jeffries, 

William Vizuete, Mark Estes, David Sullivan, Tom Ryerson, Greg Yarwood, 

Barry Lefer, Bernhard Rappenglück, O – Noor Gillani 

L What existing observational databases are suitable for evaluating and further 

developing meteorological models for application in the HGB area? 

L – Lisa Darby, P – Robert Banta, John Nielsen-Gammon, Daewon Byun, Wayne 

Angevine, Mark Estes, Bryan Lambeth, Stuart McKeen 

Note: Letter designations are for convenience only and do not denote priority. Questions in blue have been 

designated by TCEQ to receive special emphasis.
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 16 31 August 2007 

Non-attainment Areas Requiring SIP Development 

TCEQ initiated TexAQS II to obtain additional SIP-relevant understanding of the processes that 

lead to formation and accumulation of ozone and particulate matter in two very different ozone 

non-attainment areas within Texas: 

1) The Houston-Galveston-Brazoria (HGB) ozone non-attainment area is a coastal urban 

region of about 5 million people. It consists of eight counties in southeastern Texas and 

is subject to distinctive coastal (sea-breeze) meteorological conditions and large 

petrochemical sources of industrial emissions, especially the Houston Ship Channel 

(HSC) and other nearby industrial sites. In 2005 HGB was classified as a non-attainment 

area of “moderate” status, with an attainment deadline of June 2010. The most recent 

SIP revision (June 2007) does not demonstrate attainment by that date, and the state of 

Texas has requested a reclassification of the HGB non-attainment area to “severe,” which 

requires an attainment date of June 2019 and a SIP revision deadline of March 2010. 

HGB is in attainment of the PM2.5 NAAQS. 

2) The Dallas-Fort Worth (DFW) ozone non-attainment area is an inland urban region, also 

of about 5 million people. The DFW non-attainment area includes 8 counties in northcentral Texas, with relatively typical inland metropolitan meteorological conditions and 

only limited industrial sources within the non-attainment counties, but with several power 

plants in nearby locations within northeastern Texas. DFW also was classified as in 

“moderate” non-attainment of the ozone standard, with a June 2010 attainment date. The 

June 2007 SIP revision submitted to EPA demonstrates attainment of the 8-hour standard 

by that deadline. DFW is in attainment of the PM2.5 NAAQS. 

Recent Air Quality Studies in Texas 

During recent years, the State of Texas provided substantial funding for scientific studies to 

improve understanding of ozone formation and accumulation in eastern Texas. These scientific 

studies have been focused around two major air-quality field research programs. 

 First Texas Air-Quality Study (TexAQS 2000) 

TexAQS 2000 was a relatively short-term (six-week-long) intensive field measurement program 

conducted during the summer of 2000 that included aircraft-based, tall tower-based, and groundbased field measurements; the study period was imbedded in a 16-month long study on 

particulate matter in the Houston area. The research results demonstrated that the 

extraordinarily stringent decrease in NOx emissions proposed in the 2000 SIP for the HGB nonattainment area of Texas was not an optimal approach; a more realistic plan for attainment of the 

NAAQS for ozone should involve decreases in emissions of both VOC and NOx – with a 

particular focus on the highly reactive VOC (HRVOC) emissions in the industrial areas 

surrounding the Houston Ship Channel and Galveston Bay. 

 Second Texas Air Quality Study (TexAQS II and TexAQS 2006) 

TexAQS II was a much longer-term (18-month-long) program focused on the photochemical and 

meteorological processes leading to the formation and accumulation of ozone and particulate 

matter air pollution in eastern Texas. The study began in June 2005 and extended through 15 

October 2006. This period includes not only the 2005 and 2006 summer ozone seasons, but also 

the intervening fall, winter, and spring months when occasional exceedances of the 8-hour ozone 

standard did occur, both together with and separately from, occasional episodes of high 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 17 31 August 2007 

concentrations of airborne particulate matter. The field measurements of TexAQS II constitute 

one of the most comprehensive air quality field research studies ever undertaken in the United 

States; the analysis and interpretation phases of TexAQS II will extend for years after completion 

of the field measurements. 

TexAQS II culminated during the months of August, September, and October 2006 with an 

intensive series of coordinated chemical and meteorological measurements and modeling studies. 

In order to distinguish this relatively short-term but very intensive study from the earlier parts of 

TexAQS II, the 2006 summer intensive study has been dubbed TexAQS 2006. 

The air-quality measurement platforms during TexAQS 2006 included: 

1) Multiple instrumented aircraft operating in both the HGB and DFW areas of Texas, 

2) A series of ground-based sites with continuing measurements of air chemistry, a network 

of ground-based wind profiler and rawinsonde locations, a mobile solar occultation flux 

research van, and both aircraft-based and ship-based ozone and aerosol lidar 

measurements throughout eastern Texas, 

3) NOAA’s Ronald H. Brown Research Vessel operating within the Houston Ship Channel 

and Galveston Bay, 

4) The 200-foot-tall Moody Tower at the University of Houston, and 

5) The National Aeronautics and Space Administration (NASA) Aura, Aqua, Terra, and 

GOES satellites. 

The multimillion-dollar 18-month-long TexAQS II field research study and its embedded shortterm intensive study, TexAQS 2006, were conducted jointly by staff of TCEQ and by scientists 

and engineers working under contracts issued by TCEQ, the Texas Environmental Research 

Consortium (TERC), and through formal and/or informal agreements for cooperation with the 

many other research organizations listed in the Acknowledgments of this Report. 

All the research studies and plans for analysis and interpretation of results obtained during 

TexAQS II and TexAQS 2006 were undertaken with specific scientific research objectives in 

mind. But many of these investigations also were designed, undertaken, and funded by various 

federal, state, and private-sector organizations with specific policy purposes in mind – and most 

particularly in order to be used by TCEQ in developing State Implementation Plans that were 

required by the US EPA in 2007 for the Houston-Galveston-Brazoria and Dallas-Fort Worth 

ozone non-attainment areas. The very limited time available between completion of many of the 

TexAQS II field measurements and the deadline for preparation and final submission of the SIPs 

required for Houston and Dallas-Forth Worth was extraordinarily short – thus the need for a 

rapid synthesis of SIP-relevant science. 

History of Ozone Exceedances in Houston and Dallas 

Figure 1 shows that maximum ozone concentrations throughout HGB and DFW decreased from 

1978 to 2005. It is notable that: 

1) Most of the decreases in HGB occurred before 1990; 

2) The decreases were greater in HGB (from over 150 ppbv to near 100 ppbv) than in DFW 

(from about 120 ppbv to near 95 ppbv); and 

3) The ozone design values in HGB are now approximately the same as in DFW. The large 

variability in ozone maxima since 1990 – both among monitoring sites and between years – 

make it difficult to determine whether these apparent decreases in ozone maxima since 

1999 are due to changes in emissions or to meteorological variations. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 18 31 August 2007 

Although the ozone design values for the HGB and DFW areas are now very similar, acute 

ozone episodes are much more frequent in Houston than Dallas. During 2000-2005, the HGB 

area had 219 8-hour ozone exceedance days while the DFW area had 203. In contrast, however, 

during this same period, the HGB area had 147 one-hour exceedance days (ozone concentrations 

Figure 1. Temporal trends of 8-hour O3 design values at a) HGB and b) DFW monitoring sites. The 

8-hour O3 design value for each monitor is the 3-year average of the fourth-highest daily maximum 

8-hour-average ozone concentration measured at that monitor; these values are representative of the 

maximum ozone observed and the air concentrations of specific concern in developing control 

strategies. (Figure from Bryan Lambeth, TCEQ) 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 19 31 August 2007 

> 120 ppbv) but the DFW area had only 24 such days. Over 60 percent of the HGB 8-hour 

exceedances were accompanied by 1-hour exceedances. In the HGB area the local design value 

maxima have not changed locations during that period, and have persisted in the vicinity of Deer 

Park, Bayland Park, and Aldine. There is a local minimum in ozone design value at sites located 

in the urban core of Houston, presumably because of the abundant fresh NO emissions there, 

which can suppress ozone formation, or titrate ozone transported into the area. 

Regional Character of Ozone Exceedances in Eastern Texas 

Maximum daily ozone concentrations generally occur in multi-day episodes, and episodes often 

are region-wide, with similar patterns throughout eastern Texas (Figure 2). These region-wide 

episodes of high ozone concentrations are seen in all years. Two hypotheses can be suggested to 

explain this behavior: 

1) The episodes occur during meteorological conditions under which elevated ozone 

concentrations, produced in upwind regions, are transported into the whole region of 

eastern Texas. 

2) The high ozone episodes represent periods when the regional meteorology is particularly 

conducive (sunny, hot, stagnant) to local or regional formation of ozone from locally or 

regionally emitted precursors, and the intervening periods represent meteorological 

conditions that are particularly poor (cloudy, cool, windy) for ozone formation within the 

region. 

Figure 2. Daily maximum 8-hour ozone averages for August-September 2004 for all monitoring sites in 

eastern Texas. Each column represents one day, and each row represents one monitor within each 

community (TLM-Tyler, Longview, Marshall; BPA-Beaumont, Port Arthur; Aus-Austin; SAo-San 

Antonio, CCV-Corpus Christi, Victoria). The increasing color intensity indicates increasing ozone 

concentrations (yellow ≥65 ppbv, orange ≥85 ppbv, red ≥105 ppbv, purple ≥125 ppbv. (Data from TCEQ; 

Figure adapted from David Sullivan-U. Texas) 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 20 31 August 2007 

A decreasing temporal trend in regional ozone concentrations and exceedances has occurred over 

the past 13 years. A trend analysis covering 1994 to 2004 shows that the average of the 

maximum 8-hour-average ozone concentrations observed at upwind surface monitoring stations 

in the Houston area have declined since about 2000 (Figure 3). Values were approximately 80 

ppbv in the 1990s and have dropped to between 60 and 70 ppbv in 2002 – 2004. This suggests 

that background ozone concentrations during high ozone episodes in the Houston area have been 

declining in recent years. There has been a concomitant decrease in the total number of 

exceedance days recorded each year in August and September at twelve representative eastern 

Texas monitoring sites (Figure 4). These sites were selected for completeness of data coverage 

(>75% each year) and regional representation (one site per county, except for two in Harris 

County). It is important to note that the year of the TexAQS 2006 field study had the fewest 

number of regional exceedances during this 13-year period. 

 

 

Figure 4. Total number of monitordays with maximum 8-hour-average 

ozone above 85 ppbv in August and 

September during the years between 

1994 and 2006. Twelve regional 

monitoring sites in eastern Texas were 

considered in this analysis. (Data from 

TCEQ; Figure from David Sullivan-U. 

Texas) 

Figure 3. 1994 to 2004 trend of 

highest 3 - 6 values of 8-hour 

background ozone from upwind 

surface stations in Houston area 

(from Nielsen-Gammon et al., 

2005).
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 21 31 August 2007 

An Important Distinction: Ozone Production versus Accumulation 

From the standpoints of understanding air quality science and the practical business of air quality 

management, it is important to recognize the distinction between ozone formation and/or ozone 

production (which are essentially synonymous), on the one hand, and ozone accumulation on the 

other hand. The ambient concentration of ozone in a particular, near-surface volume of air at a 

fixed location is the net result of six processes that occur in the lower atmosphere: 

• In situ ozone formation (or production) from chemical precursors in an air parcel, 

• In situ ozone destruction by chemical reactions in that air parcel, 

• Deposition of ozone from the air to vegetation or other surfaces, 

• Vertical transport of ozone from an ozone reservoir aloft, 

• Horizontal transport of ozone from up-wind locations, and 

• Dispersion and dilution as a result of mixing with cleaner air during advection or vertical 

mixing when the height of the planetary boundary layer increases. 

In effect, the rate of change in the concentration of ozone in a particular, near-surface volume of 

air is an “algebraic sum” of the rates of all six of these processes. The time-integral of this 

“algebraic sum” then represents the accumulation of ozone in that volume of air at that specific 

location (if the integral is positive, or a decrease in ambient ozone concentration if the integral is 

negative.) 

Atmospheric scientists and engineers are well aware of these important processes and 

distinctions. They devote much effort to experimentally determining the rates of each of these 

processes, and to developing models that accurately calculate the resulting accumulation of 

ozone to compare with ambient observations of ozone concentrations. 

Air quality managers are concerned with management of near-surface ozone concentrations; 

thus, they are not concerned with the production of ozone as such, so long as the ambient 

concentration of ozone does not exceed the NAAQS. Air quality managers are required to 

develop State Implementation Plans that are aimed at keeping ozone from accumulating in the 

atmosphere in concentrations that exceed these NAAQS. 

In the mixed dialog that occurs between atmospheric scientists and air quality managers, this 

important distinction can be lost, and a degree of confusion may result. Some of this confusion 

arises because, among the six processes listed above, only the rate of ozone formation can be 

effectively addressed through local emission control efforts. Thus, ozone formation receives 

particular attention from both atmospheric scientists and air quality managers. 

In this report, the term “accumulation of ozone” is used when the distinction is important. 

However, in many situations, the distinction between “accumulation of ozone” and “ozone 

production” is not so important. These situations arise when the rate of production dominates 

the “algebraic sum” above. In such situations, the discussion will focus on “ozone production” 

and the contribution of the other five processes will not be discussed. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 22 31 August 2007 

Reactivity of VOC 

Ozone Precursors 

Photochemical production 

of ozone requires two 

classes of precursors: 

oxides of nitrogen (NOx, 

which include NO and 

NO2) and volatile organic 

compounds (VOC). 

During ozone formation 

the VOC are 

photochemically oxidized 

while NOx catalyzes the 

formation of ozone during 

that oxidation. Oxidation 

of carbon monoxide (CO) 

and methane also can 

contribute significantly to 

the formation of ozone. 

Figure 5 shows the rate 

constants for the reactions 

of many different VOC with 

the hydroxyl radical (OH). 

This radical is the active agent 

in the atmosphere that initiates the photochemical oxidation of VOC. The OH concentration in 

ambient air varies widely, but a representative, 24-hour average value is 1 x 106

 OH radicals per 

cm3 of air. The right hand axis in Figure 5 gives the lifetimes of various VOC at that 

representative OH concentration. The OH rate constants span such a wide range that the VOC 

lifetimes vary from many days to less than one hour. The effectiveness of any given VOC in 

ozone formation is approximately proportional to its OH reactivity, which is the product of the 

VOC concentration times its OH rate constant. 

The TexAQS 2000 study highlighted the important role played by highly reactive VOC 

(HRVOC) in the HGB area. These species are the alkenes, five of which are included in Figure 

5 (above the “Alkenes” label in the figure). Their importance is due to their high OH 

reactivities, which result from both their large OH rate constants and their high ambient 

concentrations. The high ambient concentrations result from large quantities of HRVOC emitted 

from petrochemical facilities in the HGB area. 

The biogenic VOC, especially isoprene, are particularly important in forested regions of eastern 

Texas (Figure 6). Recently, new land cover and vegetation data have improved the spatial 

resolution and provided more quantifiable estimates of uncertainties for these emissions. 

Biogenic emissions derived from these new data are about 40% lower than previous calculations. 

It is important to recognize that some biogenic VOC, especially isoprene, are also emitted from 

anthropogenic sources in the industrial facilities in HGB. 

Figure 5. Rate constants and representative atmospheric 

lifetimes for some VOC. CO is included for comparison. Rate 

constant data are from the literature. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 23 31 August 2007 

Figure 6. Biogenic VOC 

emissions in southeast Texas 

on a representative summer 

day. The emissions are 

calculated at 4 km x 4 km 

spatial resolution based on 

the latest University of 

Texas-Center for Space 

Research land cover data. 

(Figure adapted from Mark 

Estes, TCEQ) 

A Reflective Note 

Although this is the Final Report of the Rapid Science Synthesis Team, all of us on the RSST 

recognize that we have not yet derived maximum benefit from all the research data and 

information that was accumulated during TexAQS II. The accelerated pace of our analyses and 

interpretations did not allow full utilization of the results. Fortunately, however, the future will 

provide many additional opportunities for more detailed analyses, for more intercomparisons of 

results, and for more integrative interpretations. The findings developed in this Final Report will 

be subject to more rigorous examination when TexAQS II results are presented at scientific 

meetings and when papers deriving from TexAQS II research are submitted for publication in 

refereed scientific journals. 

We are proud of what has been accomplished so far – the new scientific insights obtained, and 

the new policy insights provided. We hope this Final Report will be useful to TCEQ and the 

people of Texas, as they continue the practical business of air quality management in Houston 

and Dallas-Fort Worth. 

We wish to express our thanks to the many staff in TCEQ who organized our several workshop 

meetings and helped us gain access to additional data and information from existing databases – 

especially those that go back in time to observations made long before both TexAQS 2000 and 

TexAQS II were conceived and implemented. We have also enjoyed the excellent cooperation 

among so many of the diverse groups of scientists, engineers, graduate students, and post-docs 

that joined in various parts of this very large study. It was fascinating to have the chance to 

understand more fully the very challenging chemical, biological, physical, meteorological, and 

mathematical factors that are involved, as well as some of the social, cultural, industrial, 

political, professional, and economic forces involved in managing air quality – especially in the 

Houston-Galveston, and Brazoria ozone non-attainment area. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 24 31 August 2007 

Response to Question A 

QUESTION A 

Which local emissions are responsible for the production of high ozone in Houston, Dallas, 

and eastern Texas? 

Are different kinds of emissions responsible for transient high ozone and 8-hour-average 

high ozone (i.e., ≥84 ppbv)? 

BACKGROUND 

Question A is related to Questions F and K, which focus on the sensitivities of high 

concentrations of ozone to VOC and NOx emissions. Here the response to Question A gives an 

overview of the photochemical processes that produce the highest observed ozone 

concentrations, and the emissions that allow those processes to proceed rapidly. These highest 

ozone concentrations are limited to the HGB region, so the focus is on that area. Understanding 

8-hour-average high ozone requires very different considerations and approaches than the 

understanding of transient high ozone; an initial investigation of the longer-term-average ozone 

in HGB is included. 

FINDINGS 

Finding A1: The highest (i.e. > 125 ppbv) ozone concentrations in the HGB area result 

from rapid and efficient ozone formation in relatively narrow, concentrated plumes, which 

originate from HRVOC and NOx co-emitted from petrochemical facilities. The Houston 

Ship Channel (HSC) is the origin of the plumes with the highest ozone concentrations. 

Analysis: Trainer-NOAA; Data: Ryerson, de Gouw et al.-NOAA, Fried et al.-NCAR, Atlas et al.-

U. Miami. 

The aircraft flights of TexAQS 2000 and 2006 reveal a persistent feature over Houston – on all 

days conducive to photochemical ozone production, narrow and concentrated plumes of ozone 

formed and were transported downwind from the HSC. Figure A1 shows measurements from 

one flight; on this day the wind was from the east-northeast. The top two graphs show the 

measurements of two primary ozone precursors. NOy represents the NOx emissions plus their 

oxidation products, primarily PAN and nitric acid. Ethene, a HRVOC, is an important ozone 

precursor. The concentrations of these two primary emissions were highest over the HSC, and 

defined a narrow downwind plume with decreasing concentrations. Figure A1 indicates that the 

NOx and ethene emission sources are not precisely collocated, but the primary sources of both 

are in the HSC region. The downwind decrease in the concentration of NOy was primarily due to 

dilution. Clearly, ethene concentrations decreased more rapidly than NOy, due to its much faster 

removal through photochemical oxidation. The bottom two panels of Figure A1 show the 

concentrations of two secondary photochemical products, ozone and formaldehyde, formed 

within the plume of primary emissions. Formaldehyde, produced by the photochemical 

oxidation of ethene and other HRVOC, formed during the downwind transport as the HRVOC 

were oxidized; formaldehyde itself is an important ozone precursor. Ozone accumulated as a 

concentrated (up to 147 ppbv), narrow plume within the much less concentrated (≤ 100 ppbv), 

but wider ozone plume from the larger Houston urban area. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 25 31 August 2007 

-96.5 -96.0 -95.5 -95.0 -94.5

4 8 12 16

 

CH2O (ppbv)

0 4 8 12

C2H4 (ppbv)

30.5

30.0

29.5

29.0

28.5

0 10 20 30 40

NOy (ppbv)

30.5

30.0

29.5

29.0

28.5

-96.5 -96.0 -95.5 -95.0 -94.5

60 90 120 150

 

O3 (ppbv)

Figure A1. Measurements of NOy, ethene (C2H2), ozone, and formaldehyde (CH2O), plotted along 

the flight track of the WP-3D on 6 October 2006. The symbols are sized and color-coded according to 

the indicated concentrations. Only measurements below 1.5 km altitude are shown. (A close approach 

to the Parish power plant has been removed from the NOy plot.) 

The high temporal resolution measurements of ethene made by the laser photo-acoustic 

spectrometer (LPAS) and the wide species coverage of VOC measurements conducted on the 

canisters from the whole air sampler (WAS) provided detailed characterization of the HRVOC 

sources in HSC. The map in Figure A2 shows the concentration of ethene measured by these 

two systems downwind from six petrochemical complexes; the plumes of ethene emissions from 

the complexes are clearly delineated. The four pie charts in Figure A2 indicate the total OH 

reactivity of the VOC in four of the WAS samples, and show how that reactivity was divided 

among four of the lightest HRVOC species, and the total of all of the other alkanes, aromatics, 

and biogenic VOC measured in the canister samples. These four canister samples illustrate the 

general finding that the HRVOC dominate the VOC reactivity immediately downwind of the 

petrochemical facilities, and thus are responsible for a large fraction of the high ozone 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 26 31 August 2007 

production in the HGB area. The total reactivity and the fraction of the reactivity contributed by 

the HRVOC decrease downwind in the plumes, as expected from their very short lifetimes. 

The pie charts in Figure A2 do not include aldehydes. These oxygenated VOC, which also 

account for a significant fraction of the OH reactivity, are primarily secondary products of the 

oxidation of the HRVOC. Thus, the contribution of the aldehydes to ozone production increases 

the fraction of the total VOC reactivity in the HSC area that is attributable to the emissions of the 

HRVOC. 

Figure A2. Measurements of ethene along the section of the WP-3D flight track included in the 

rectangle in the upper right panel of Figure A1. The symbols are sized and colored according to the 

indicated concentrations. The triangles indicate the 20-second-average measurements by LPAS, and 

the circles indicate the measurements from WAS. The pie charts are sized according to the total OH 

reactivity for four of the WAS samples, and the colors indicate the OH reactivity contributed by the 

four indicated HRVOC. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 27 31 August 2007 

Finding A2a: Winds carry the emission plumes from the Ship Channel throughout the 

Houston area. The wind direction determines the location of the ozone maximum relative 

to the Ship Channel; shifts in wind direction lead to the transient high ozone events 

observed at monitoring sites within the Houston area. 

Finding A2b: The general characteristics of the highest (i.e. > 125 ppbv) ozone formation 

in Houston did not change between 2000 and 2006, although the maximum observed ozone 

concentrations were lower in 2006 than in 2000. 

Analysis: Ryerson et al.-NOAA; Data: Ryerson, Williams et al.-NOAA. 

Ryerson et al. (2006) examined the four Electra flights during TexAQS 2000 that encountered 

ozone concentrations above 150 ppbv. Figure A3 shows the flight track segments where the 

highest ozone concentrations were observed. In each case, back-trajectory analysis attributed 

these high ozone concentrations to plumes from industrial emission sources in the HSC area. 

Measured chemical characteristics of the plumes (simultaneous high NOx, SO2, CO2, and 

oxidation products of HVROC) confirmed this source attribution. The relationship between the 

transport times derived from the trajectory analysis and the observed enhancements in ozone 

concentration provided a measure of the net average ozone production rates in the plumes. 

Figure A3 also shows the observed relationship between ozone and the products of NOx

oxidation for those four flights; the slopes of these relationships provide an estimate of the net 

ozone production efficiencies in these plumes. 

Figure A3. Highest ozone (red points) observed by the Electra aircraft during four flights in TexAQS 

2000. In the left panel flight track segments are color-coded by observed ozone, and in the right 

panel the dependence of ozone on the products of NOx oxidation are shown with approximate ozone 

production efficiencies estimated from fitted slopes (Ryerson et al., 2006). 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 28 31 August 2007 

Figure A4 presents a similar analysis for the three WP-3D daytime flights and the RHB ship 

cruise segment during TexAQS 2006 that encountered ozone concentrations near or above 120 

ppbv. A simple wind direction analysis, coupled with the chemical plume signatures, indicate 

that in each case these plumes also originated from industrial emission sources in the Houston 

Ship Channel area. Figure A4 also shows the relationship between ozone and the products of 

NOx oxidation in these plumes. 

Comparison of Figures A3 and A4 show that the ozone production environment in the Houston 

area was similar in 2000 and 2006. Similar ozone vs. (NOy-NOx) slopes are seen in each year; 

the maximum concentrations of ozone observed in both years were associated with slopes near 7. 

Lower maximum ozone concentrations were observed in 2006, but this difference may be 

partially due to meteorological factors; the background ozone, as indicated by the y-intercepts in 

the two right panels in Figures A3 and A4, was lower in 2006 and lower background 

concentrations led to lower maximum concentrations. The lower 2006 background may indicate 

smaller effects of stagnation and recirculation that year. Finally, the maximum ozone 

concentrations were observed at greater distances from the HSC in 2006 than in 2000 (note 

difference in area covered in the two maps). This difference may be attributable to slower ozone 

production or to greater transport speeds in 2006 than in 2000. 

Figure A4. Highest ozone observed by the WP-3D aircraft and RHB research vessel during 

TexAQS 2006. The figure is in the same format as Figure A3. The dotted rectangle in the left panel 

indicates area covered in Figure A3. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 29 31 August 2007 

Finding A3: Emissions from the Houston Ship Channel play a major role in the formation 

of the highest 8-hour average ozone concentrations (and ozone design values) observed in 

the Houston area. 

Analysis: Sullivan et al.-U. Texas; Data: TCEQ. 

The general flow patterns that give rise to the highest 8-hour-average ozone concentrations in the 

HGB area have been investigated through back-trajectory analysis. Monitoring sites to the south 

and west of downtown Houston have the highest ozone design values in the HGB area (defined 

as the 3-year average of the fourth-highest daily maximum 8-hour-average ozone concentration). 

In Figure A5 the blue dots highlight the four highest design values: 104 ppbv at Tom Bass 

CAMS 558, 103 ppbv at Bayland Park CAMS 53, 102 ppbv at West Houston CAMS 554, and 98 

ppbv at Monroe CAMS 406. The green dot indicates the Aldine site north of the city; as recently 

as 2002 Aldine had the maximum design value in the area, but it declined from 108 and 107 

ppbv in 2001 and 2002, respectively, to 92 and 88 ppbv in 2005 and 2006, respectively. 

Relationship between Ozone Production Efficiency and the Slope of the O3 versus 

NOy-NOx Correlation – A Cautionary Note 

Many investigations of atmospheric photochemistry have interpreted the slope of the 

correlation of the measured O3 concentration versus the measured concentration of the 

oxidation products of NOx (i.e. the difference between measured concentrations of 

NOy and NOx) as a direct measure of the ozone production efficiency (i.e. the number 

of O3 molecules formed for each NOx oxidized.) The first paper that examined this 

correlation noted that nitric acid deposition will affect the observed slope between O3

and NOy-NOx correlations. If nitric acid is removed from the atmosphere, then a 

measurement of NOy-NOx will underestimate the concentration of NOx oxidation 

products, and consequently, the slope of O3 versus NOy-NOx will overestimate the 

ozone production efficiency. During TexAQS 2006, ozone production was studied 

under a variety of meteorological conditions. High wind speeds were found to 

enhance nitric acid loss, which caused the O3 versus NOx oxidation products 

correlation slope to increase, often dramatically. The observed slopes varied from 2 to 

16 in the coalesced plume from Houston, which was followed up to 170 km 

downwind. In many cases, the particularly high O3 versus NOy-NOx slopes were 

primarily caused by particularly rapid loss of nitric acid from the transported plume, 

rather than from particularly high ozone production efficiency within the plume. 

Thus, when interpreting the correlation slope of O3 versus NOy-NOx as a measure of 

the ozone production efficiency, it is critical to consider the effect of nitric acid loss. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 30 31 August 2007 

Figure A5. Harris County monitoring sites with the 4 highest 2006 design values marked in blue, and 

Aldine marked in green. In 2006, Bayland Park monitoring site (C53) had the area-wide highest 

regulatory design value of 104 ppbv. 

A set of near-surface back-trajectory ensembles was generated for 8-hour ozone exceedance days 

at Bayland Park and Monroe, the two regulatory sites with the highest 2006 design values, and at 

Aldine to provide geographical contrast. The analysis covers the period from 2000 through 

2006. On each day that a given monitor recorded an ozone exceedance, 7-hour back trajectories 

were calculated for all hours in which the measured ozone was above 85 ppbv. Hourly wind 

speed and direction data from 14 locations covering Harris County provided the data for the 

back-trajectory calculations. Wind speeds were adjusted to account for varying anemometer 

exposure among sites using factors provided by Bryan Lambeth of TCEQ, and further increased 

by 20 percent to account for generally higher speeds above each monitor’s 10-meter anemometer 

tower. The data from all sites were pooled in an un-weighted average to provide one urban-scale 

wind speed and direction vector for each hour. Air parcel back trajectories were calculated in 

10-minute time steps beginning from the start time of each appropriate hour. The resulting 

trajectory ensembles include 79 days at Aldine, 104 at Bayland Park, and 56 at Monroe. Since 

exceedance days generally had multiple hours with ozone concentrations over 85 ppbv, the 

number of trajectories totaled 252 at Aldine, 281 at Bayland Park, and 110 at Monroe. One 

composite trajectory was produced from each ensemble by averaging the x-y coordinates of all 

trajectories in the ensemble as a function of transport time. The resultant composites are 

expected to be biased short, but based on maps of the ensembles and their centerlines, this effect 

is judged to be minimal. 

Figure A6 shows that each of these three composite back-trajectories reaches back to the HSC 

area earlier in the day, with approximately 7-hour transport times to the respective monitoring 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 31 31 August 2007 

sites. For Aldine the trajectories cluster to the south-southwest, for Bayland Park they wrap 

around the south side of central Houston, and for Monroe they cluster to the northeast. The 

consistency of the trajectories coming from the HSC provides strong support for the conclusion 

that emissions from the petrochemical industrial facilities in the HSC play a major role in the 8-

hour ozone exceedances in the HGB area. 

Figure A6. Composite 7-hour near-surface back trajectories for three Houston monitoring sites. All 

hours with O3 ≥ 85 ppbv on 8-hour exceedance days for the 2000-2006 period were included in the 

composite analysis. 

Finding A4: Observations during TexAQS 2006 showed that nitryl chloride (ClNO2) is 

formed within the nocturnal boundary layer when NOx emissions and marine influences 

are both present. Following sunrise, ClNO2 photolyzes to yield chlorine atoms, which may 

lead to earlier and more rapid O3 production in the Houston region. 

Analysis and Data: Roberts, Osthoff et al.-NOAA. 

The first reported atmospheric observations of nitryl chloride, ClNO2, were made aboard the 

Ronald H Brown during TexAQS 2006. Simultaneous measurements of ClNO2, N2O5, and 

aerosol size and composition, show that ClNO2 is a general product formed when N2O5 reacts on 

chloride-containing aerosol. N2O5 is produced from nighttime reactions of NO2 with O3. ClNO2
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 32 31 August 2007 

is relatively stable at night, but photolyzes upon sunrise to yield Cl atoms and NO2. These 

reactions link the nitrogen oxides to halogen activation. 

An episode of high concentrations of ClNO2 was observed on 2 September 2006 when the RHB

was anchored in the Barbour’s Cut inlet located off Galveston Bay near HSC. Figure A7 shows 

the measured N2O5 and ClNO2 concentrations along with the Cl atom production rate calculated 

from the measured ClNO2 and solar radiation flux. This Cl atom production reached almost 106

sec-1, which is highly significant since Cl atoms react with VOC up to 100 times more rapidly 

than OH radicals. Figure A7 compares O3 measured on 2 September with the average (±1 

standard deviation) of the O3 levels observed during the other days that the RHB was in the 

Houston-Galveston area during TexAQS 2006. On 2 September the O3 concentration increased 

more rapidly and reached a higher peak concentration compared to the average increase. This 

observation, while not definitive, is indicative of a potentially significant role for Cl atoms in O3

production in polluted marine air. 

A preliminary box model study was conducted to investigate the ClNO2 formation chemistry and 

to examine the effect of this Cl source on VOC-NOx photochemistry. The model consisted of 

the Master Chemical Mechanism (Jenkin et al., 2003; Saunders et al., 2003), to which the N2O5

aerosol chemistry and ClNO2 photochemistry were added. The model results show that a 

reaction efficiency of 25% for N2O5 on chloride-containing aerosol can account for the 

measurements in Figure A7. The Cl atoms produced upon sunrise led to a 2- to 3-fold increase 

in total peroxy radicals during the morning hours, which resulted in about a 15% higher O3

concentration at the end of the afternoon production period. These results indicate the need to 

add this Cl atom source to current regional photochemical models. 

Figure A7. N2O5 and ClNO2

measured on 2 September 2006, 

and calculated Cl atom production 

in the atmosphere (a); and the 

measured O3 on 2 September, and 

the average from the other 

measurements in the HoustonGalveston area (b). 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 33 31 August 2007 

KEY CITATIONS AND INFORMATION AND DATA SOURCES 

Jenkin, M.E., S.M. Saunders, V. Wagner, and M.J. Pilling. 2003. Protocol for the development 

of the master chemical mechanism MCMv3 (Part B): Tropospheric degradation of aromatic 

volatile organic compounds. Atmospheric Chemistry and Physics, 3:181-193 (also 

Atmospheric Chemistry and Physics Discussions, 2:1905-1938 (2002)). 

Ryerson, T.B., K.K. Perkins, M. Trainer, D.K. Nicks Jr., J.S. Holloway, J.A. Neuman, F. Flocke, 

A. Weinheimer, S.G. Donnelly, S. Schauffler, V. Stroud, E.L. Atlas, D.D. Parrish, R.W. 

Dissly, G.J. Frost, G. Hübler, R.O. Jakoubek, P.D. Goldan, W.C. Kuster, D.T. Sueper, A. 

Fried, B.P. Wert, R.J. Alvarez, R.M. Banta, L.S. Darby, C.J. Senff, and F.C. Fehsenfeld. 

2006. Chemical and meteorological influences on extreme (>150 ppbv) ozone exceedances in 

the Houston metropolitan area. Draft Report to TCEQ, Contract No. 582-4-65613. 

Saunders, S.M., M.E. Jenkin, R.G. Derwent, and M.J. Pilling. 2003. Protocol for the 

development of the master chemical mechanism MCMv3 (Part A): Tropospheric degradation 

of non-aromatic volatile organic compounds. Atmospheric Chemistry and Physics, 3:161-180 

(also Atmospheric Chemistry and Physics Discussions, 2:1847-1903 (2002)). 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 34 31 August 2007 

Response to Question B 

QUESTION B 

How do the structure and dynamics of the planetary boundary layer and lower 

troposphere affect the ozone and aerosol concentrations in Houston, Dallas, and East 

Texas? 

BACKGROUND 

Meteorology in the boundary layer affects ozone levels through regulation of near-source 

concentrations (wind speed, mixing height), background levels (transport winds), photochemistry 

(solar radiation, temperature), and air parcel history (local winds). 

FINDINGS 

Finding B1: Boundary layer structure and mixing height near and over Galveston Bay and 

the eastern Houston ship channel area are spatially complex and variable from day to day. 

Vertical mixing profiles often do not fit simple models or conceptual profiles. High 

concentrations of pollutants are sometimes found above the boundary layer. 

Analysis: Nielsen-Gammon-Texas A&M; Senff, Darby, Banta, Angevine, Tucker, White-NOAA; 

Breitenbach, Dornblaser, Lambeth-TCEQ; Morris-Valparaiso U.; Rappenglück, Perna-U. 

Houston. Data: Nielsen-Gammon-Texas A&M; Senff, Tucker, White, Darby, Angevine, BantaNOAA; Morris-Valparaiso U.; Rappenglück, Perna-U. Houston. 

Mixing depth (synonymous and used interchangeably here with the term “mixing height”) exerts 

an important control on the concentrations of pollutants including ozone and its precursors. For 

example, when pollutants are released into a shallow mixed layer, high concentrations of 

emissions can accumulate; this was observed using airborne ozone lidar during TexAQS 2000 

(Banta et al., 2005). On the other hand, deep mixing can significantly dilute pollutants. 

Well away from the coastal region, the mixed layer was relatively uniform over broad spatial 

areas. Well inland, peak afternoon mixing heights were generally between 11⁄2 and 21⁄2 km, but 

could reach as high as 4 km. The mixed-layer heights generally grew into late afternoon, often 

leveling off above 2 km. Over the Gulf of Mexico, shipboard lidar and platform radar windprofiler measurements indicated a relatively constant maritime mixed-layer depth of 600 m, with 

weak positive heat fluxes at the surface, both day and night. 

The Houston urban area and major industrial sources are located in the coastal region, where the 

mixing depth is highly variable in space and time, as determined by shipboard lidar and landbased sensors. Reasons for this variability include land-sea contrast and the sea-breeze cycle, 

land-use differences, and along-shore coastal irregularities, the major one in this area being 

Galveston Bay. The meteorology of the previous day and night is another influence on mixedlayer variability, but that effect is much more difficult to generalize. The coastal zone of 

southeast Texas is a transition region between the maritime boundary layer, with the relatively 

constant 600-m mixed-layer depths over the Gulf of Mexico, and the deeper daytime mixed 

layers inland. The coastal influence on Houston mixed-layer heights can be seen in their diurnal 

behavior. In contrast to the inland sites, where the mixed-layer height generally increases until 

late afternoon, in the coastal zone the mixing height was observed to reach its maximum earlier 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 35 31 August 2007 

in the day (as early as late morning for sites near the shore) and then to decrease, as the seabreeze front brought in cooler marine air through the afternoon. 

Land-cover and land-use differences include the cross-region gradients in soil moisture and 

urban heat island processes. Climatologies and vegetation indicate moister conditions to the east 

of Houston and nearer to the coast, and drier conditions to the west and inland. Airborne lidar 

flights revealed a significantly deeper mixed layer over the arid regions to the west, consistent 

with mixing-depth data provided by the profiler array, which indicated increases in peak 

afternoon mixing depths with distance from the coast. 

Urban heat island. The issue of the urban heat island (UHI) is also complicated in the coastal 

zone by the sea breeze. The effects of the sea breeze can be appreciated by contrasting Houston 

with an inland urban area, Dallas. Both Dallas and Houston have comprehensive networks of 

surface meteorology and chemistry sensors. The similarities of the networks and lack of terrain 

in Dallas and Houston allow for the comparison of their UHIs. The strength of the UHI is 

measured by taking the temperature difference from representative urban and rural sites. The 

Dallas UHI, unperturbed by thermal flows driven by land/sea temperature differences, was a 

well-defined phenomenon during the summers of 2000-2006 (Fig. B1a). Including all weather 

conditions, the average nighttime Turban – Trural temperature difference was between 1.5o and 2.0o 

C and the average daytime difference was ~ 1.0o C. Analysis of Houston temperature data, 

however, revealed a different picture due to the Bay and Gulf breezes (Fig. B1b). Although the 

Houston UHI was a distinct phenomenon, even when including all weather conditions, the Bay 

or Gulf breezes modified the Houston UHI by cooling the city. Average nighttime Turban – Trural

temperature differences in Houston were between 1.75o and 2.75o C. However, during the day, 

the rural areas to the north and west of the city were often warmer than the downtown area 

during afternoon hours as a result of the sea breeze. Averaging the Houston Turban – Trural

temperature differences over the summers of 2000-2006 indicated a very small urban-rural 

temperature difference between 1400 and 1600 local standard time (LST), in contrast to Dallas, 

which had a Turban – Trural temperature difference of ~1° C. In some individual years, such as 

2000, 2003, 2005 and 2006, the Houston urban areas were actually cooler than the rural areas, on 

average, in the mid-afternoon. These years had more Bay-breeze/Gulf-breeze activity to cool the 

urban area. 

Figure B1. Turban – Trural for each of the 7 summers analyzed, and the average for all summers. 

“Summer” includes all days from 1 June– 30 September. a) Dallas. b) Houston. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 36 31 August 2007 

The effects of along-shore irregularity combined with the other coastal effects can be seen in the 

distributions of daytime mixed-layer heights. Midday mixed-layer heights measured at an inland 

site at Moody, Texas (near Waco) were relatively consistent from day to day, thus exhibiting a 

rather narrow distribution with a mean of ~1.5 km. Near the shore of Galveston Bay at LaPorte, 

a much broader distribution of mixed-layer heights, including high frequencies below 1 km, 

indicates the much more complex influences on mixing depth in the coastal zone. Spatial 

variability of mixing depth was evident in airborne lidar flights, with shallow mixing layers over 

Galveston Bay and deeper mixing layer depths over the Houston urban area as a result of heatisland effects. Variations in mixing depth at nearby locations often reflected advection of high or 

low mixing layer depths downwind of the urban area or Galveston Bay, for example. 

Sounding observations from the University of Houston (UH) campus (e.g., Fig. B2) and from the 

Ronald H. Brown (when stationed in Galveston Bay) often showed a very complex vertical 

ozone structure, with no well-marked transition between the local boundary layer and the air 

above; this was consistent with shipboard Differential Absorption Lidar (DIAL) and Doppler 

lidar profiles. This complexity is likely to be underrepresented in numerical models. 

Figure B2. Ozonesonde profiles of ozone (left, 0-100 ppb) and temperature (right, 270-300 K) from 

the surface to 6,000 m above ground level (AGL), for a 1200 UTC ascent on 31 August 2006, 

released from the University of Houston campus. The minimum in the ozone profile near 1,000 m 

AGL was associated with a layer of high relative humidity and clouds. 

Spatial variability of mixing height during TexAQS 2000 and TexAQS 2006. Multi-sensor spatial 

analyses of mixing heights, using data from the TexAQS field campaigns, illustrate the effects of 

these diverse influences, often showing mixing height varying by a factor of two or more across 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 37 31 August 2007 

Houston. An example of large spatial variability in mixing heights is shown in Figure B3. A 

combination of fixed-location ground-based sensors and mobile airborne sensors is essential for 

determining the spatial and temporal mixing height variability across the Houston metropolitan 

area. 

Figure B3. Mixing height measurements in early afternoon in Houston, 1 September 2000. Colors 

indicate observed or estimated mixing heights. Shown are mixing heights from airborne instruments 

(colored lines) and mixing heights from profilers, soundings, and aircraft ascents and descents 

(circles). Note the large spatial variability north and east of Houston (Nielsen-Gammon et al., 2007). 

Mixing height and O3 concentrations. Attempts to determine general relationships between 

daytime mixing heights and ozone concentrations have been mostly fruitless. The expected 

relationship is that higher concentrations would be associated with lower mixed-layer heights. 

Northerly or northeasterly large-scale flow, however, is often associated with higher 

concentrations from distant continental sources, and continental air masses often have higher 

mixing heights. Conversely, and also opposing the expected mixing-height/ozone relationship, 

out over Galveston Bay, large-scale southerly flow off the Gulf of Mexico was observed to be 

associated with more clean and moist air but shallower mixed layers. On the other hand, when 

pollutants are emitted into the shallow mixed layer of Galveston Bay and carried over the Bay, 

pollutant concentrations have been shown to remain very high – in agreement with expectation. 

In another scenario, aircraft sampling of urban or other plumes some distance downwind of the 

sources finds well mixed regions outside and inside of the plumes where the ozone is low and 

high, respectively, on the same day, but the mixing heights are essentially the same, yielding no 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 38 31 August 2007 

systematic relationship. All of these effects confound the search for general relationships, 

producing seemingly random or otherwise meaningless scatter plots of O3 and mixing heights 

Nighttime mixing. Nighttime boundary layer structure in the Houston-Galveston area is less easy 

to characterize. UH tethersonde and ozonesonde data indicate that nighttime depletion can occur 

at elevations as high as 200 m, implying that the top of the urban boundary layer tends to be 

~200 m or less at night. Thus, nighttime mixing and transport effects in the Houston-Galveston 

area remain an area of longer-term research needs. 

Layering of pollutants. O3 and aerosol pollutants in the Houston-Galveston area were often 

confined to the mixed layer (Fig. B4a,b). But occasionally significant concentrations of 

pollutants were found above the mixed-layer height (Fig. B4c,d). Pollutants, especially aerosol, 

sometimes exhibited a complex layered structure, in which the origin of pollution in the 

individual layers was difficult to determine (Fig. B2). In the analysis performed to date, high 

concentrations of pollutants found in deep layers (more than a few hundred meters thick) above 

the mixed layer (Fig. B4d) were found to originate in distant regions outside of Texas to the 

northeast or east. 

Figure B4. Airborne lidar time-height cross sections of aerosol backscatter (top, in units of 10-8 m-1

sr-1) and ozone concentrations (bottom, in ppb) for a), b) 30 August 2006, when high ozone and 

aerosol concentrations were confined to the mixed layer, and c), d) 4 September 2006, when the 

mixed layer was indicated by higher aerosol concentrations, but the high ozone was above the 

boundary layer. 

Finding B2: Complex coastal winds, resulting from weak larger-scale winds over the area, 

occurred during many, but not all, ozone exceedance days in the Houston-GalvestonBrazoria nonattainment area. Almost no high-ozone days during TexAQS 2000 resembled 

any high ozone days from TexAQS 2006, but collectively the 2000 and 2006 field intensives 

sampled the full range of meteorological conditions associated with high ozone events. 

Analysis: Banta-NOAA; Nielsen-Gammon-Texas A&M; Bryan Lambeth-TCEQ; Gary MorrisValparaiso U.; Perna, Rappenglück-U. Houston; Senff, Darby, Angevine-NOAA; Breitenbach, 

Dornblaser-TCEQ. Data: Nielsen-Gammon-Texas A&M; Senff, Tucker, White, Darby, 

Angevine, Banta-NOAA; Morris- Valparaiso U.; Rappenglück, Perna-U. Houston. 

High ozone concentrations were most often associated with light winds and daytime sea-breeze 

reversals (often leading to stagnation occurrences; (Darby, 2005]), but some days with stronger 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 39 31 August 2007 

winds also had O3 exceedances during both 2000 and 2006. The interplay between local wind 

patterns and large-scale transport produced a wide variety of meteorological scenarios for ozone 

events in the Houston area. Broadly speaking, when mean winds (here measured using 24-hrmean resultant winds from nearshore buoy 42035) were light, recirculation was likely, whereas 

when mean winds were strong, the wind direction tended to be steady throughout the day. The 

direction of the mean wind determines when stagnation will occur if winds are light enough, and 

also determines the direction of transport of the Houston ozone plume. 

By plotting the west-east and south-north components of the mean winds on a scatter diagram, 

the range of meteorological conditions during an ozone episode or an extended period may be 

determined. Such a diagram is shown in Fig. B5 for the late summer ozone season in Houston. 

During TexAQS 2000, most high-ozone events occurred when the mean wind was from the 

southeast (regime 1) or southwest (regime 2), but no light northeast wind events occurred at all. 

In contrast, during TexAQS 2006, wind conditions were much more representative of 

climatology, and several high-ozone events occurred under light northeast winds. Those days in 

which the wind was light from the southeast during 2006 tended to feature widespread clouds 

and precipitation. Thus, almost no high-ozone days during TexAQS 2000 resembled any high 

ozone days from TexAQS 2006, but collectively the 2000 and 2006 field intensives sampled the 

full range of meteorological conditions associated with high ozone events. 

Figure B5. Scatter diagram of daily mean winds at buoy 42035, near Houston, during the period 1 

August – 15 October, 1998-2006. Grid lines are every 5 m/s; a dot in the upper left quadrant implies 

winds from the southeast. The wind regimes most conducive to high 1-hr ozone in the Houston area 

are circled. Days during TexAQS 2000 are indicated with pink squares, and days during TexAQS 

2006 are indicated with yellow triangles; blue dots represent data from all other years. 

Peak ozone and wind speed. The overall relationship between peak O3 concentrations and wind 

speed is shown in Fig. B6, which indicates the expected strong negative correlation between 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 40 31 August 2007 

maximum in-network ozone enhancements (or “add-on”) above background and wind speed in 

the Houston area (see red data and line on Fig. B6). The 10-hr trajectory displacement is a 

surrogate for mean wind speed. Stronger winds (larger displacements) produce greater dilution 

of the pollution emissions, and thus lower concentrations of pollutants. Analysis of groundbased measurements over several years indicates a threshold effect in Houston (NielsenGammon et al., 2005). Large local add-ons are typically present when large-scale winds are 

weaker than about 3 m s-1, light enough to allow recirculation, and the add-on decreases rapidly 

for wind speeds larger than 3 m s-1. This effect is evident especially in the airborne data in Fig. 

B6. A steady 3 m s-1 wind would produce a 10-hr displacement of 108 km, and below this value 

is where peak O3 add-on concentrations become very large. 

R2 = 0.5033

R2 = 0.3902

-20

0

20

40

60

80

100

120

140

160

180

200

0 100 200 300 400

10-hr displacement (km)

O3 add on

Airb add on

Sfc add on

Linear (Airb add on)

Linear (Sfc add on)

Figure B6. Houston ozone enhancement (peak ozone values in urban plume minus background 

values) plotted vs. displacement of 10-hr trajectories, representing the vector-mean wind for the 

period, starting at Houston at 8:00 a.m. CST. Red symbols indicate data from surface measurement 

network (1-hr average), and blue symbols indicate data from airborne ozone lidar and WP-3D. “Add 

on” represents peak ozone concentrations observed minus background concentrations. Lines are 

linear best-fit lines. 

The blue points in Fig. B6 show the airborne-lidar-determined peak ozone concentrations vs. 10-

hr trajectory displacements. The concentrations again decrease with increasing speed, but are 

everywhere larger than the surface measurements. Reasons for this discrepancy include that the 

airborne data represent conditions aloft not subject to surface processes such as removal, that the 

airborne lidar data represent averaging over shorter intervals, and that on days with stronger 

winds, plumes are narrower and harder to sample by fixed networks and also high concentrations 

may be blown out of the surface network before being sampled. 

The last issue was expected to be a greater source of discrepancy between airborne and groundnetwork sampling, because the aircraft samples the pollution plume wherever it is, even though 

the maximum values may be occurring beyond the surface measurement network before the 

photochemical reactions are complete. Indeed, individual days have been identified with large 

discrepancies. However, Fig. B6 shows that, in general, light winds (small 10-hr displacements) 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 41 31 August 2007 

were associated with large O3 enhancements by the Houston area, and stronger wind speeds, with 

smaller enhancements. Several cases of stronger-wind days have been identified when the total 

O3 concentrations (background plus add-on) exceeded the 1-hr standard. Those days, most often 

associated with high background values, are being studied further to determine all processes 

responsible for the high concentrations. 

On such stronger-wind days with high background levels, high pollutant concentrations were 

generally being advected into the Houston area under easterly or northerly continental flow. 

Airborne ozone lidar flights sampling air masses entering Texas from the east on three different 

days found concentrations of 50, 80, and 90 ppb in the inflow air. Another typical scenario is for 

post-cold-frontal northerly flow to be associated with high pollutant concentrations. When the 

offshore winds are weak enough, sea-breeze stagnation occurs, and when they are stronger, they 

are associated with high background. During 2006 all ozone episodes in the month of September 

occurred in a post-frontal environment. For example, a four-day sequence of ozonesonde 

profiles (Fig. B7) showed an increase in background ozone within the free troposphere up to 

5000 m AGL, corresponding to transport from the continental United States after the passage of 

the front. 

Figure B7. Ozonesonde profiles at ~1200 CST on 28, 29, 30, and 31 August 2006. Passage of cold 

front occurred on 29 August 2006. Before the passage of the front, marine air masses with O3

concentrations of 30 ppb had concentrations even lower than the EPA boundary-condition benchmark 

values (40 ppb, red dotted vertical line). In subsequent days, background ozone increased throughout 

the troposphere up to 5000 m AGL due to the shift in wind direction after the frontal passage. 

Many of these episodes were associated with strong elevated inversions. Higher afternoon ozone 

peaks occurred where there was an extremely dry air mass aloft, although it was not always 

associated with the strongest inversion observed. The air masses aloft with the strongest winds 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 42 31 August 2007 

(greater than 8 m s-1) and driest air did not have extremely high ozone values associated with 

them, mainly because these episodes occurred right after a frontal passage where the northerly 

flow was strong enough to keep ozone peaks in check. 

Thus, the Houston-Galveston-Brazoria area produces a tremendous amount of ozone. Even 

under relatively strong wind conditions, the result can be high pollutant concentrations, although 

the very highest concentrations are produced on days with weak-wind or sea-breeze-reversal 

conditions. Even with the 8-hr standard, most, but not all, 8-hr exceedances during the TexAQS 

II field intensive (and other 2005-6 episodes) occurred when winds were light enough to allow 

stagnation/recirculation. 

Finding B3: After sea breeze days, the Houston plume was broadly dispersed at night 

through the formation of low-level jets. 

Analysis: Nielsen-Gammon-Texas A&M; Banta, Senff, Ryerson, Darby, Tucker, White-NOAA; 

Lambeth-TCEQ. Data: Nielsen-Gammon-Texas A&M; Senff, Tucker, White, Darby, AngevineNOAA. 

Trajectory analysis of nighttime transport of the Houston plume, based on the wind profiler 

network, indicates that the Houston plume sometimes remains a coherent entity, subject to little 

wind shear (see Fig. B8a,b), but at other times is dispersed over a broad portion of Texas by 

strongly sheared winds (Fig. B8c,d). Broad dispersal is favored after sea breeze days when 

nighttime decoupling allows a strong low-level jet to form from the remains of the sea breeze 

circulation. The wind speed is strongest at 300-500 m, decreasing above and below. 

Figure B8. Overnight trajectories for four elevations above ground level, based on hourly radar wind 

profiler measurements, for the nights of a) 16-17 August, b) 14-15 August, c) 31 August-1 

September, and d) 1-2 September. Red trajectories were averaged over the vertical interval from 200 

to 500 m;, black, from 500-800 m; green, from 800-1200 m;, and blue, from 1200-1800 m. Lower 

trajectories or those ending up to the north of their origin are not likely to be contaminated by signal 

from migrating birds. 

An inland surge of winds found in the 200-600 m layer at night has been called the sea-breeze 

low-level jet. The essential characteristics of this jet are shown in a sequence of images from an 

MM5 model forecast from 8 June 2006 (Fig. B9). In the first panel, at 1500 local standard time 

(LST), the winds are light and onshore. This wind pattern is particularly conducive to the 

formation of a sea-breeze low-level jet. Along the coast, a sea breeze has developed, and 

onshore wind speeds are locally 4-7 m s-1. Around sunset, in the second panel, the sea breeze 

wind maximum has moved inland and has increased in speed to over 9 m s-1 in places. The LLJ 

occurs in a broad area along the coast, rather than a narrow along-wind band. In the lower left 

panel, at 2100 LST, the sea-breeze low-level jet is about 200 km inland and has continued to 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 43 31 August 2007 

increase in intensity, with peak wind speeds over 12 m s-1. The leading edge of the jet is very 

sharp, implying a sudden increase in wind speed, whereas along the trailing edge the decline in 

wind speed is gradual. Note also that the wind direction, initially southeasterly, has veered to 

southerly under the influence of the Coriolis force. This veering continues through the night, and 

the wind direction is southwesterly by 0000 LST (final panel). By this time, with the winds no 

longer blowing toward lower pressure, the wind speed within the jet has begun to decrease. A 

similar wind evolution is simulated whenever winds are light and onshore during the day and 

precipitation is sparse or absent. Sea-breeze low-level jet passages are also evident in wind 

profiler observations from 2005-6. 

Figure B9. Forecasted 300 m winds (indicated with barbs and isotach shading) and pressure (black 

isobars), 1500 LST 8 June 2006 (upper left), 1800 LST (upper right), 2100 LST (lower left) and 0000 

LST, 9 June (lower right). The sea-breeze low-level jet is the band of strong wind speeds originating 

along the coast at 1500 CST and accelerating inland through the evening. 

A dramatic example of nocturnal transport has been documented for 8 September 2006, when 

high daytime concentrations of ozone that had accumulated over Houston during the previous 

day traveled to Dallas overnight, contributing to enhanced concentrations in Dallas the next day. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 44 31 August 2007 

This episode is discussed in more detail in the Response to Question H. Other examples of 

overnight transport of the Houston plume were identified in data from the instrumented tower at 

Moody, Texas (near Waco) and in the Tyler-Longview areas of northeast Texas. In a further 

example from 1 September, the Houston plume moved to the south of the city overnight, and 

then was brought back in the sea breeze the next day to produce enhanced O3 concentrations in 

the western suburbs. Calculations of the O3 flux from Houston indicate that if these 

concentrations were spread over an area 100 by 100 miles by 2 km deep, it would raise the 

concentrations by 10 ppb over the entire volume. These calculations are further discussed in the 

Response to Question G. 

Finding B4: The Dallas ozone plume can extend well beyond the monitoring network. 

Analysis: Senff, Darby, Banta-NOAA; Breitenbach, Lambeth-TCEQ. Data: Senff, Darby, BantaNOAA; Lambeth-TCEQ. 

An airborne O3 lidar flight, sampling the DFW plume on 13 September in northerly flow of ~ 5 

m s

-1, found peak O3 values of 90-95 ppb against a background of ~ 65 ppb (Fig. B10), 

representing an enhancement of 25-30 ppb, in agreement with previous estimates. These high O3

values were observed in a distinct urban plume extending past the southernmost cross-wind 

flight leg at a distance of 85 km downwind of DFW, indicating that the plume extended much 

farther downwind than this. It seems likely that the highest ozone occurs beyond the margin of 

the monitoring network on such occasions, as was previously noted for the Houston network. 

Under southerly large-scale flow, recently installed ozone sensors in southern Oklahoma also 

indicated elevated ozone concentrations resulting from the DFW plume, at a distance of at least 

240 km (near Lawton, Oklahoma). 

Figure B10. a) Flight track of the Twin Otter airborne ozone lidar, showing color-coded ozone 

concentrations averaged between 500 and 1000 m MSL (ozone scale from 50 to 100 ppb), for 13 

September, a day with stiff north-northeasterly flow. b) Vertical time-height cross section of ozone 

[ppb, same scale as in a), vertical scale from 0 to 1800 m MSL, horizontal scale from 2145 to 2230 

UTC] for southernmost cross-wind leg, showing plume of higher ozone from Dallas-Ft. Worth. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 45 31 August 2007 

KEY CITATIONS AND INFORMATION AND DATA SOURCES 

Banta, R.M., C.J. Senff, T.B. Ryerson, J. Nielsen-Gammon, L.S. Darby, R.J. Alvarez, S.P. 

Sandberg, E.J. Williams, and M. Trainer. 2005. A bad air day in Houston. Bull. Amer. 

Meteor. Soc., 86:657-669. 

Darby, L.S. 2005. Cluster analysis of surface winds in Houston, Texas and the impact of wind 

patterns on ozone. J. Appl. Meteor. 44:1788-1806. 

Nielsen-Gammon, J.W., J. Tobin, and A. McNeel. 2005. A Conceptual Model for Eight-Hour 

Ozone Exceedances in Houston, Texas. Part II: Eight-Hour Ozone Exceedances in the 

Houston-Galveston Metropolitan Area. HARC/TERC/TCEQ Report, 79 pp. 

http://www.harc.edu/Projects/AirQuality/Projects/Projects/H012.2004.8HRA

Nielsen-Gammon, J.W., C.L. Powell, M.J. Mahoney, W.M. Angevine, C. Senff, A. White, C. 

Berkowitz, C. Doran, and K. Knupp. 2007. Multisensor estimation of mixing heights over a 

coastal city. J. Appl. Meteor. Clim., in press. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 46 31 August 2007 

Response to Question C 

QUESTION C 

Are highly reactive VOC and NOx emissions and resulting ambient concentrations still at 

the same levels in Houston as they were in 2000? 

How have they changed spatially and temporally? Are there specific locations where 

particularly large quantities of HRVOC are still being emitted? 

Are those emissions continuous or episodic? 

How well do the reported emissions inventories explain the observed concentrations of 

VOC and NOx? 

BACKGROUND 

Questions C, D, and E all deal with emissions. Here, Question C specifically addresses highly 

reactive VOC and co-located NOx emissions in the Houston area. Question D addresses all other 

ozone and aerosol precursor emissions, biogenic as well as anthropogenic, that are included in 

emission inventories. Question E addresses evidence for additional, unrecognized sources of 

precursor emissions. 

FINDINGS 

Finding C1: There are indications that ethene (the lightest HRVOC) emissions from 

industrial sources in the Houston area decreased by 40 (±20)%, i.e., by a factor of between 

1.25 and 2.5, between 2000 and 2006. 

Comparison of ethene/NOx emission ratios measured during TexAQS 2000, TexAQS 2006, 

and one 2002 aircraft flight 

Analysis: de Gouw et al.-NOAA. Data: de Gouw, Ryerson et al.-NOAA; Atlas et al.–U. Miami 

The emissions of ethene from numerous point sources were individually characterized using the 

Electra and WP-3D aircraft measurements made in TexAQS 2000, TexAQS 2006, and on one 

flight in the southeast Texas region in April 2002. Determination of ethene/NOx emission ratios 

was most straightforward for the isolated petrochemical plants to the south of Houston. These 

facilities were extensively investigated in 2000 and 2006, and an additional research flight was 

made in 2002 when the WP-3D transited to a study in California. Figure C1 shows the flight 

track in 2002 (Ryerson et al., 2003) and an example of a flight made in 2006, along with results 

of the NOx and ethene measurements. Apart from Texas City, there appear to be changes in the 

ethene/NOx emission ratios. The results are summarized in Table C1, and show decreases in the 

ratio by a factor of 3-7 for Sweeny, Freeport, and Chocolate Bayou. Further research is 

necessary to determine if the lower emission ratios were systematic, or only happened to be 

lower on the 29 September flight; preliminary analyses have shown that emission ratios are 

variable within factors of 2-3 between different flights. In addition, the high-time-resolution 

ethene data from the WP-3D have indicated that ethene and NOx enhancements are not always 

well correlated. Evidently the sources of these species are sometimes not co-located, making it 

difficult to define emission ratios in those cases. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 47 31 August 2007 

Figure C1. Measurements of NOx and ethene downwind from 4 isolated petrochemical plants to the 

south of Houston. The top panels show the flight tracks of the WP-3D on 22 April 2002, (Ryerson et 

al., 2003) and on 29 September 2006. The lower panels show the measurement results plotted as a 

function of longitude. 

Table C1. Ethene/NOx emission ratios for petrochemical complexes determined from measurements 

in 2000, 2002, and 2006, compared with emission inventories. 

Ethene/NOx Emission Ratios 

Inventories Measured 

 1999 a 2004 b 2000 c 2002 c 2006 

Sweeny 0.05 0.019 3.6 1.7 0.5 

Freeport 0.05 0.030 1.5 0.62 0.32 

Choc. Bayou 0.08 0.048 2.0 1.2 0.62 a

 TNRCC emission inventory. 

b

 TCEQ point source emission inventory with 1999 VOC speciation. 

c

 Ryerson et al. (2003.) 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 48 31 August 2007 

Comparison of ethene and formaldehyde concentration distributions during TexAQS 2000 

and 2006 

Analysis: de Gouw et al.-NOAA. Data: de Gouw, Ryerson, Holloway, et al.-NOAA; Atlas et al.–

U. Miami; Fried et al.-NCAR. 

Figure C2 compares the 2000 and 2006 airborne ethene measurements made below 1000 m 

altitude inside a box around Houston (Figure C2, panel A). The median ethene from the whole 

air samples (WAS) was 40% lower in 2006 than in 2000 (Figure C2, panel B). The median 

ethene from the laser photo-acoustic spectroscopy (LPAS) measurements was even lower; one 

can show that the targeted filling of WAS inside pollution plumes leads to a bias in the WAS 

median. A similar comparison of the 2006 data from the Ronald H. Brown while in Barbour’s 

Cut with the 2000 data from La Porte airport (these two locations are in close proximity) 

indicates a similar 40% decrease (not shown here). 

Figure C2. Cumulative probability diagrams for ethene, wind speed, and temperature measured 

from the NOAA WP-3D inside the box around Houston shown in panel A and below 1000 m 

altitude. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 49 31 August 2007 

The question arises whether the differences between 2000 and 2006 are due to a decrease in 

HRVOC emissions or to a difference in meteorology. Average wind speeds in 2006 were higher 

than in 2000 (Figure C2, panel C), which would lead to a greater dilution of ethene emissions 

and lower mixing ratios in 2006. However, average temperatures in 2006 were lower than in 

2000 (Figure C2, panel D). Higher temperatures can be associated with deeper boundary layers 

and thus a larger volume within which the emissions are mixed and diluted, which would imply 

higher mixing ratios in 2006. 

All emissions are subject to the same meteorology. Comparison of other emitted species 

between 2000 and 2006 can provide additional insight into possible meteorological differences. 

If the decrease in ethene between 2000 and 2006 were due to meteorology, then other trace gases 

should exhibit similar trends. If, on the other hand, the decrease were due to a decrease in 

emissions, then other trace gases would show ambient concentration trends characteristic of their 

own emission trends. Figure C3 shows the difference in median mixing ratio between 2000 and 

2006 for six species. On-road mobile emissions dominate the emissions of ethyne and CO; their 

mixing ratios showed only small changes between 2000 and 2006. (The CO mixing ratios were 

corrected for the observed background mixing ratios.) A decrease in CO is expected because of 

the modernization of the vehicle fleet between 2000 and 2006; the observed decrease of ~3% per 

year is slightly smaller than the reported historical trend of 4.6% per year (Parrish, 2006). NOy

showed a significant decrease between 2000 and 2006; this is attributed to the installation of 

emissions reduction technologies in power plants and other industrial NOx point sources (see 

Finding C3 below). SO2 increased slightly between 2000 and 2006; reductions were not 

expected as emissions controls have focused on NOx. The largest decreases were observed in the 

mixing ratios of ethene and formaldehyde (HCHO), its main photoproduct. From this 

comparison we conclude that the meteorological differences had compensating effects on the two 

years, and that the decrease in ethene between 2000 and 2006 was due to a reduction in its 

industrial emissions. 

Figure C3. Difference in median 

mixing ratio for several trace gases 

measured from the WP-3D inside a 

box around Houston and below 1000 m 

altitude.
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 50 31 August 2007 

Long-term data sets 

Analysis: Estes et al.-TCEQ; Data: TCEQ. 

In the Houston area there have been extensive VOC measurements made by as many as eight 

auto-GC systems and by canister-based methods. Measurements were begun at some sites as 

early as 1997, giving temporal coverage over one decade by the end of 2006. Figure C4 presents 

results from two sites near the Ship Channel. The median ambient ethene levels at both sites 

decreased by about a factor of two between 1997 and 2005. Similar decreases were also seen for 

propene (not shown). Analysis by Sather and Cavender (2007) also shows significant decreases 

of ambient concentrations of ethene and propene, in qualitative agreement with Figure C4. 

Figure C4. Results of ethene measurements by auto-GCs at two sites near the Houston Ship Channel: 

9 years of data from Clinton (on the western end) and 8 years of data from Deer Park (on the eastern 

end). 

Summary of evidence 

Emission ratios of ethene relative to NOx from several isolated petrochemical plants in 2006 

were lower in comparison with results from 2000 and 2002. This decrease in ratios must be due 

to a decrease in ethene emissions, since the NOx emissions have also decreased in many cases 

(see Finding G3). Further, lower mixing ratios of ethene and formaldehyde were observed in 

2006 in comparison with 2000, and this change cannot be explained by differences in dilution 

rates between the two years. 

Thus, analyses based on four different measured parameters (ethene/NOx emission ratios in 

plumes of petrochemical facilities, the ambient distribution of ethene concentrations, the ambient 

distribution of formaldehyde, and long-term auto-GC ethene measurements) have all found 

evidence for a significant decrease in ethene emissions. The unanimity of these four analyses 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 51 31 August 2007 

increases our confidence that a significant decrease in HRVOC emissions from Houston area 

petrochemical facilities has actually occurred in the period between 2000 and 2006. Considering 

all of these analyses, the best estimate is that ethene emissions have decreased by about 40% 

(i.e., a factor of 1.7) between 2000 and 2006. The much more limited evidence available for 

propene suggests that emissions of this HRVOC may have decreased similarly. 

It is important but difficult to assign confidence limits to the magnitude of the decrease. Our best 

estimate is that the decrease is 40 ± 20%, which corresponds to a decrease by a factor of 1.25 to 

2.5. We consider it unlikely, but cannot categorically exclude the possibility that there has in 

fact been no significant decrease. 

Finding C2: Measurements of ethene emission fluxes from petrochemical facilities during 

TexAQS 2006 indicate that the 2004 TCEQ point source database underestimates these 

2006 emissions by one to two orders of magnitude. Repeated sampling of the same 

petrochemical facility showed that the ethene emission flux remained constant to within a 

factor of two. 

Analysis and data: de Gouw et al.-NOAA; Mellqvist et al.–Chalmers U. 

Emissions of ethene were determined during TexAQS 2006 by two completely independent 

methods. First, column measurements of ethene by absorption of solar IR radiation were 

obtained from the solar occultation flux (SOF) mobile laboratory operated by Chalmers 

University (Gothenburg, Sweden). In this method, the ethene emission flux was estimated by 

combining the column data with measured vertical wind profiles and then integrating across the 

width of the plume in close proximity to the emission source. Second, ethene was measured 

onboard the WP-3D aircraft using a new laser photo-acoustic spectroscopy (LPAS) instrument. 

An example of the resulting data is shown from 25 September when the WP-3D and the SOF van 

both sampled the emissions from the Mont Belvieu complex (Figure C5). 

Figure C5. Transect of the WP-3D just downwind from the Mont Belvieu complex to the northeast of 

HSC, color-coded by ethene measured by LPAS. Ethene sources from the 2004 TCEQ point source 

database are indicated by the blue triangles, with the size proportional to the source strength. The 

measured time series of ethene for this transect is shown on the right. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 52 31 August 2007 

The ethene flux was estimated from the aircraft measurements by integrating across the width of 

the plume and by assuming that the plume was homogeneously distributed over the height of the 

boundary layer. A flux of 280 kg h-1 is the result for this example. The result from the SOF van 

from the same complex on the same day was 500 kg h-1, i.e., agreement within a factor of 2. 

Flux estimates from the SOF and WP-3D measurements were compared for different sources 

(Sweeny, Freeport, Texas City) and generally agreed within a factor of 2, with no systematic 

difference as to which technique was higher or lower. 

The WP-3D determined the flux from Mont Belvieu on a total of 10 downwind transects. Figure 

C6 shows a histogram of the WP-3D results (grey bars); the average calculated flux is 470 kg h-1

with a standard deviation of 160 kg h-1. Two considerations can serve to put these fluxes in 

perspective. First, the 2004 TCEQ point source database estimated the ethene emissions from 

Mont Belvieu to be 29 kg h-1, a factor of 10-40 lower than the values measured during TexAQS 

2006. Second, Murphy and Allen (2005) investigated the role of large, accidental releases of 

HRVOC in ozone formation in the HGB area. They identified 763 HRVOC emission events in a 

one-year period (31 January 2003 to 30 January 2004). More than half of these events released 

less than 1000 lbs (454 kg) total HRVOC. Thus, the Mont Belvieu complex, as an example of 

petrochemical facilities in the HGB area, routinely emits more ethene each hour than the total 

released in most of the individual accidental release events considered by Murphy and Allen. 

These facilities represent a significantly larger source of HRVOC and more substantial 

contribution to ozone formation than indicated by current emission inventories. 

Figure C6. Histogram of ethene fluxes determined from aircraft measurements of 10 transects. Also 

indicated are the SOF result from September 25, the average SOF result from 3 days of 

measurements, and the 2004 TCEQ point source database. 

Finding C3: Close to petrochemical HRVOC sources, the OH reactivity of propene is 

generally greater than that of ethene. 

Analysis: de Gouw et al.-NOAA; Data: Atlas et al.-U. Miami. 

Close to emission sources, the reactivity of propene generally outweighs that of ethene. To 

illustrate, Figure C7 shows the OH reactivity of propene (the product of the propene 

concentration and the rate coefficient for its reaction with OH) versus that of ethene from all the 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 53 31 August 2007 

whole air sample measurements onboard the WP-3D. At high reactivity (>0.5 s-1) the data points 

are well above the blue line that indicates equal reactivity of ethene and propene. At lower 

reactivity (<0.1 s-1), the reactivity of ethene is higher than that of propene; the lifetime of ethene 

is longer by about a factor of 3, and therefore it persists further downwind from sources. Thus, 

VOC measurements made at routine monitoring sites, generally located relatively distant from 

the facilities, will not accurately reflect the propene OH reactivity contributions unless the 

relative lifetimes of the HRVOC are carefully considered. 

Figure C7. OH reactivity of propene versus that of ethene from all the data from the WP-3D. The 

blue line indicates equal reactivity of ethene and propene; the gray area shows the range where the 

reactivities of ethene and propene differ by no more than a factor of 3, higher or lower. 

Finding C4: The latest available emission inventories underestimate ethene emissions by 

approximately an order of magnitude. 

Analysis: Jolly et al.-TCEQ; Data: TCEQ. 

The TexAQS 2000 study established that inventories underestimated emission fluxes of HRVOC 

from petrochemical facilities by one to two orders of magnitude (Ryerson et al., 2003.) A 

remarkable feature of Table C1 and Figure C6 is that appreciation of this finding has not been 

reflected in the inventory evolution. For example, total HRVOC emissions included in the 

Harris County Point Source EI for 2000-2004 were fairly steady across those years, with the 

lowest year (2002, at 3300 tons) being about 83 percent of the highest year (2004, 4000 tons). 

Total VOCs in the county in the 2000-2005 period differed approximately 13 percent between 

the lowest year (2003, ~29,000 tons) and the highest year (2000, ~33,500 tons). Consequently, 

the latest available emission inventories still underestimate HRVOC emissions (as judged by the 

ethene comparisons summarized in Table C1 and Figure C6) by at least an order of magnitude. 

In an effort to improve this situation, TCEQ collected the special 2006 Hourly Emission 

Inventory during the TexAQS 2006 intensive from 15 August through 15 September. During 

this period, 141 sites in eastern Texas reported their hourly emissions of VOC, NOx, CO, and 

SO2 from predetermined industrial sources, which were selected based upon the following 

criteria: sources subject to HRVOC rules, NOx and sulfur dioxide sources equipped with CEMS, 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 54 31 August 2007 

and emissions sources located near ambient air monitoring sites. For HRVOC the reported 

emissions were based upon process flow monitoring (flares, cooling towers) that was required 

beginning in January 2006. This inventory was the first in HGB conducted since compliance 

deadlines passed for key HRVOC regulations promulgated by the TCEQ. These hourly 

estimates were made only for a subset of process units and emission points within the reporting 

plants, and only a subset of plants that normally report in the HGB EI did so in the Hourly EI. 

Both of these factors add uncertainty to how well this inventory represents the total HGB VOC 

inventory. In HGB, about 10-11% of the process units and emission points that emit ethene and 

propene in the annual (periodic) emission inventory were included in the Hourly EI. However, 

these units accounted for 23-24% of all ethene/propene emissions reported in the annual EI. 

0

50,000

100,000

150,000

200,000

250,000

300,000

350,000

400,000

450,000

500,000

Hourly EI OSD EI Hourly EI OSD EI Hourly EI OSD EI Hourly EI OSD EI

Cooling Towers Flares Equipment Leak Fugitives Unknown source type

Lb per 32 days

Propylene

Ethylene

Butenes

_1_3__Butadiene

The results from the hourly EI suggests that reported emissions of total VOC and HRVOC 

species may increase in the coming years. Figure C8 shows emissions changes in inventoried 

emissions from cooling towers and flares, as well as for equipment leak fugitives (which were 

poorly represented in the hourly EI). The total HRVOC emissions from inventoried flares, 

increased about 2.4 fold (95 tons to 231 tons), although the much smaller emissions from cooling 

towers decreased by 28% (9.9 to 7.7 tons in the 32 day period). Overall, the total reported ethene 

and propene emissions in HGB approximately doubled. 

In summary, the discrepancy between the HRVOC emission measured in the field and those 

included in inventories has improved to some degree between the 2000 and 2006 TexAQS 

studies. The measured emissions have decreased by about 40%, while the inventoried emissions 

(at least as indicated by the limited Special 2006 Hourly Emission Inventory) have increased by 

Figure C8. HRVOC emissions, by process unit type and compound/group. The special 2006 Hourly 

Emission Inventory is compared to the Ozone Season Daily Emission Inventory (OSD EI) summed 

over the 32 days (15 August – 15 September) included in the special inventory. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 55 31 August 2007 

about a factor of 2. However, in spite of this progress, the latest available emission inventories 

still underestimate ethene emissions by approximately an order of magnitude. 

Finding C5: Inventories for NOx point sources at petrochemical facilities equipped with 

CEMS appear to be relatively accurate. Substantial decreases in NOx emissions in the 

Houston Ship Channel are suggested by the inventories, and measurements from aircraft 

are qualitatively consistent with the NOx decreases. 

Analysis: Trainer et al.-NOAA; Data: Ryerson, et al.-NOAA. 

Data from the WP-3D flights in 2006 compared to similar data from the Electra aircraft in 2000 

indicate that substantial reductions in NOx emissions from certain petrochemical facilities have 

occurred in the intervening period. Figure C9 presents an example of a transect downwind of 

HSC that was flown under similar conditions during both field studies. Plumes with welldefined correlations between NOy and CO2 concentrations were seen in both years from the east 

end of the HSC and from the Cedar Bayou complex (at the northern end of Galveston Bay). The 

slopes of these correlations give the NOx/CO2 emission ratio from the respective sources. These 

observed slopes compared well to the ratios calculated from the CEMS systems for the times of 

emissions of these plumes. It is clear that the NOx/CO2 emission ratio decreased dramatically for 

the Cedar Bayou complex (by a factor of 6) as a result of NOx emission controls implemented at 

this plant. NOx emissions also decreased detectably (by about 30%) from the facilities at the 

eastern end of the HSC. 

Figure C9. Relationships between NOy and CO2 from flights during TexAQS 2000 and 2006. Linear 

least squares fits are given for correlations with r2

 > 0.80. Slopes of the fits give the NOx to CO2

emission ratios. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 56 31 August 2007 

KEY CITATIONS AND INFORMATION AND DATA SOURCES 

Murphy, C.F. and D.T. Allen. 2005. Hydrocarbon emissions from industrial release events in the 

Houston-Galveston area and their impact on ozone formation. Atmos. Environ. 39:3785–

3798. 

Parrish, D.D. 2006. Critical evaluation of US on-road vehicle emission inventories. Atmos. 

Environ. 40:2288–2300. 

Ryerson, T.B., M. Trainer, W.M. Angevine, C.A. Brock, R.W. Dissly, F.C. Fehsenfeld, G.J. 

Frost, P.D. Goldan, J.S. Holloway, G. Hübler, R.O. Jakoubek, W.C. Kuster, J.A. Neuman, 

D.K. Nicks, Jr., D.D. Parrish, J.M. Roberts, D.T. Sueper, E.L. Atlas, S.G. Donnelly, F. 

Flocke, A. Fried, W.T. Potter, S. Schauffler, V. Stroud, A.J. Weinheimer, B.P. Wert, C. 

Wiedinmyer, R.J. Alvarez, R.M. Banta, L.S. Darby, and C.J. Senff. 2003. Effect of 

petrochemical industrial emissions of reactive alkenes and NOx on tropospheric ozone 

formation in Houston, Texas. J. Geophys. Res. 108(D8), 4249, doi:10.1029/2002JD003070. 

Sather, M.E. and K. Cavender. 2007. Trends analysis of ambient 8 hour ozone and precursor 

monitoring data in the south central U.S. J. Environ. Monit. 9:143–150. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 57 31 August 2007 

Response to Question D 

QUESTION D 

What distribution of anthropogenic and biogenic emissions of ozone and aerosol precursors 

can be inferred from observations? 

BACKGROUND 

Questions C, D, and E all deal with emissions. Question C specifically addresses highly reactive 

VOC and co-located NOx emissions in the Houston area. Here, Question D addresses all other 

ozone and aerosol precursor emissions, biogenic as well as anthropogenic, that are included in 

emission inventories. Question E addresses evidence for additional, unrecognized sources of 

precursor emissions. 

FINDINGS 

Finding D1a: Several rural electric generation units (EGU) in the Houston area and in 

eastern Texas have substantially decreased their NOx emissions per unit power generated 

since the TexAQS 2000 study. With one exception, SO2 emissions have not changed 

appreciably since 2000 for the plants sampled in 2006. 

Finding D1b: Comparisons of emissions derived from ambient observations with those 

measured by continuous emission monitoring systems (CEMS) indicate that the emissions 

from point sources equipped with CEMS are very accurately known. 

Finding D1c: Underreporting of CO emissions at several EGU noted in 2000 (Nicks et al., 

2003) has been reconciled by large increases (by factors of 5 to 50) in the inventory values 

between 2000 and 2006, as a result of newly implemented CEMS monitoring of CO at these 

plants. 

Analysis and Data: Ryerson et al.-NOAA. 

NOx, SO2, CO, and CO2 are emitted directly, in varying ratios, from electric generation units 

(EGU). Enhancement of the first three species, relative to CO2 enhancements when sampled in 

plumes immediately downwind of EGU point sources, provide a measure of pollutant emissions 

per unit energy generated by the plant (e.g., Ryerson et al., 2003). Comparisons of emissions 

ratios between the TexAQS 2000 and TexAQS 2006 studies permit an assessment of EGU 

emissions control strategies, intended primarily to reduce NOx emissions, that have been 

implemented since 2000. 

Near-field plumes from numerous rural EGU in Texas were characterized using aircraft 

measurements in 2000 and 2006. Data were generated as shown in Figure D1a, which depicts 

the NOAA WP-3D ground track for a flight designed to assess several large EGU point sources 

in the eastern Texas area. The data are taken from the closest transects, within 10 km downwind 

of the plants, and plotted in Figure D1b as enhancement ratios versus CO2. The slopes of linear 

fits to these data provide a direct measure of the plant emissions ratios. 

Analyses of rural EGU emissions ratios to CO2 have been performed for Monticello, Welsh, 

Martin Lake, Big Brown, and W.A. Parish power plants. Table D1 compares derived emission 

ratios from the 2006 WP-3D data with the ratios measured from the NCAR Electra aircraft in 

2000. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 58 31 August 2007 

The TexAQS 2000 study demonstrated quantitative agreement between emissions estimates from 

the Electra aircraft data and the tabulated emissions from Continuous Emissions Monitoring 

Systems (CEMS) data, for NOx and SO2 at each plant. Some variability is expected on time 

scales of hours to years, and is reflected in the data in Table D1. Nonetheless, conclusions from 

the 2006 study can be drawn that are well outside of the uncertainties due to normal emissions 

variability over time. 

34.0

33.5

33.0

32.5

32.0

Latitude

-95.5 -95.0 -94.5 -94.0

Longitude

Monticello

Welsh

Martin Lake

10

8

6

4

2

0

NOx emissions, 1024molec/sec

Figure D1a. Sept. 16, 2006 WP-3D ground track (green) with 

observed SO2 enhancements (blue) plotted along the track.

120

100

80

60

40

20

0

NOx and SO2, ppbv

7:48 PM

9/16/06

7:50 PM 7:52 PM

Time, GMT

420

410

400

390

380

CO2, ppmv

200

180

160

140

120

100

80

60

CO, ppbv

SO2

CO

CO2

NOx

Welsh

Monticello

Figure D1b. Time series of CO2, SO2, NOx, and CO

measured immediately downwind of Welsh and Monticello.

Table D1. Measured emissions relative to CO2 for EGU in East Texas. 

NCAR Electra 

aircraft data 

2000 

NOAA WP-3D 

aircraft data 

2006 

EGU name SO2 CO NOx SO2 CO NOx 

NOx emissions 

decreased 

by factor of:

Monticello 3.5 6.4 1.0 2.8 5.4 0.80 1.25 

Welsh 1.5 1.7 0.80 1.7 1.7 1.20 1.5 (increase)

Martin Lake 1.4 4.0 1.3 3.0 6.1 0.80 1.6 

Big Brown 4.8 2.9 1.5 7.8 6.8 0.66 2.3 

W.A. Parish 2.1 (variable) 0.88 2.1 (variable) 0.25 3.5 

Emissions values presented as molecules per 1000 molecules of CO2 emitted. 

Analysis of all available transects of the Parish power plant plume shows that inventory values 

derived from CEMS data and estimates from the WP-3D are in quantitative agreement. Figure 

D2 shows that both the magnitude and the variability in time of emissions are captured by the 

data in Texas 2006 for both SO2 and NOx. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 59 31 August 2007 

3.0

2.5

2.0

1.5

1.0

SO2/CO2 (ppb/ppm)

9/21/06 10/1/06 10/11/06

Date

SO2/CO2 CEMS

P-3 data

0.6

0.5

0.4

0.3

0.2

NOx/CO2 (ppb/ppm)

9/21/06 10/1/06 10/11/06

Date

NOx/CO2 CEMS

P-3 data

Conclusions from this body of evidence are: 

• NOx emissions have decreased substantially in several EGU, by factors of 2–4, 

qualitatively consistent with NOx controls implemented since the 2000 study. NOx

emissions from other EGU are essentially unchanged. 

• SO2 emissions are generally unchanged between 2000 and 2006 in the plants studied to 

date. The small variability observed is within that expected from CEMS data due to 

normally changing plant loads. An exception is the Martin Lake plant, where SO2

appears to have increased by a factor of 2 relative to CO2 compared to the 2000 study. 

• Large CO emissions discrepancies, noted in the 2000 study, have been reconciled by 

substantial increases in the inventory CO values for several EGU in east Texas. 

• Quantitative agreement between inventory values and aircraft estimates of NOx/CO2 and 

SO2/CO2 emissions ratios from rural EGU suggests that emissions from point sources 

equipped with CEMS are very well known. 

Finding D2: On-road mobile emission inventories developed from MOBILE6 have 

significant shortcomings. MOBILE6 consistently overestimates CO emissions by about a 

factor of 2. It accurately estimated NOx emissions in the years near 2000, but it indicates 

decreases in NOx emissions since then, while ambient data suggest NOx emissions have 

actually increased. Consequently in 2006, NOx to VOC emission ratios in urban areas are 

likely underestimated by current inventories. 

Analysis: Parrish et al.-NOAA; Data: TCEQ, Lefer et al.-U. Houston. 

Figure D3 compares CO to NOx ratios from ambient measurements with those from emission 

inventories. The Dallas and Houston routine ambient data are in excellent agreement with the 

nationwide AIRS data. The TexAQS 2006 data from Moody tower agree reasonably well with 

the routine monitoring data. Significant differences are seen in El Paso and San Antonio, which 

have older vehicle fleets. 

Figure D2. Ratios of SO2 and NOx to CO2 emitted from the Parish power plant plume during six 

weeks of the TexAQS 2006 study. The ratios derived from CEMS data are compared to the 

determinations from multiple near-field transects of the emission plume by the WP-3D aircraft. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 60 31 August 2007 

The HGB inventory overestimates the CO to NOx emission ratio, and that overestimate becomes 

worse with time as the inventory does not show a significant temporal decrease. The HGB 

points are from the “SIP Quality” emission inventories 

(www.tceq.state.tx.us/assets/public/implementation/air/am/committees/pmt_set/20070424/20070

424-rthomas-0506EI_Modeling_Update.pdf; accessed 17 August 2007). It should be noted that 

the on-road emissions inventory value for 2000 was calculated with actual data for 2000, 

whereas the on-road mobile inventories for later years are projections, based upon projected 

population growth, economic data, and estimates of expected emission decreases. The 

projections, therefore, are more uncertain than the base case 2000 emissions. Parrish (2006) 

showed that the rapid decrease (6.6%/yr) in the ratio is partially due to a slower decrease in CO 

emissions (4.6%/yr), which implies a significant increase in NOx emissions (approximately 

2%/yr). The large inventory overestimates in the CO to NOx ratio at the present time are 

attributed to a factor of 2 overestimate in CO emissions, and an underestimate in present NOx

emissions. This will cause NOx to CO emission ratios in urban areas, which are often dominated 

by on-road mobile emissions, to be underestimated by current emission inventories. 

Urban emission ratios sampled by the WP-3D aircraft in 2006 and the NCAR Electra aircraft in 

2000 are consistent with measurements carried out at a Houston highway tunnel in 2000 

(McGaughey et al., 2004). These measurements demonstrate the weekday increase in CO/CO2

and CO/NOx emission ratios from midday to the afternoon rush hour correlated with increases in 

the proportion of gasoline vehicles during rush hour. While aircraft and tunnel measurements 

were not made during the morning rush hour, their afternoon rush hour emission ratios are 

consistent with the routine monitoring data for the morning rush hour. Similar to the routine 

monitors, the aircraft and tunnel observations indicate that inventories overestimate mobile 

source CO by at least a factor of 2. 

The VOC to NOx ratio in on-road emissions is of more critical importance to photochemical 

modeling in the HGB area than is the CO to NOx ratio. Parrish (2006) showed that VOC 

Figure D3. Determination of CO to 

NOx ratio in Texas on-road mobile 

emissions from monitoring data (solid 

symbols) compared to the HGB 

emission inventory (open symbols), 

color-coded according to urban area. 

The black symbols are for all stations 

in the EPA AIRS network. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 61 31 August 2007 

emissions from on-road vehicles have decreased at a similar rate to CO emissions. This 

correspondence is expected because catalytic converters are the principal control measure for 

both species. Although MOBILE6 accurately calculated the VOC to NOx ratio in 2000, the 

decrease in the ratio is not accurately captured. Consequently, in 2006, NOx to VOC emission 

ratios in urban areas are likely underestimated by current inventories. 

Finding D3: Emissions from ships constitute a significant NOx source in the HGB region. 

Literature results provide accurate emission factors for inventory development. 

Analysis and Data: Williams et al.-NOAA. 

During TexAQS 2006, emissions in exhaust plumes from over 200 marine vessels in Galveston 

Bay and the Houston Ship Channel were measured on the Ronald H. Brown. Table D2 presents 

the average derived emission factors for slow speed diesel (SSD) engines, which are those with 

maximum power greater than ~10 MW, and medium speed diesel (MSD) engines, which are of 

lower power. The NOx values are within 20% of the average values reported in the Lloyd's 

(1995) study for both MSD and SSD engines although the measured variability is large in both 

cases. These data are sufficient to provide emission factors classified by ship type (e.g., 

freighters, container ships, tankers, tugs, etc.). It is concluded that Lloyd's (1995) provides an 

accurate characterization of NOx emissions from underway vessels in the HGB region. 

Table D2. Summary of average marine vessel emission factors. 

Engine type NOx CO SO2 H2CO LAC 

Medium speed diesel 

(MSD) 

60 8.7 9.1 0.17 0.41 

Slow speed diesel (SSD) 74 6.6 28 0.20 1.16 

All units are grams of species per kilogram fuel consumed. 

Emission factors for CO are median data. 

The emission factors for CO are within 20% of the value reported by Lloyd's (1995). There is no 

trend of increasing CO at lower vessel speeds (used as a surrogate for engine load), which was 

seen in the Lloyd's data. Measurements of formaldehyde (H2CO) emissions from ships show 

little distinction between MSD and SSD engines; emission of H2CO is less than 5% of the 

emission of CO. Emission factors for SO2 vary with fuel sulfur content. In the HGB region the 

mean fuel S derived from the measurements is 0.46% for MSD engines and 1.4% for SSD 

engines. Measurements of light absorbing carbon (LAC; also known as black carbon) were also 

derived. 

Given detailed activity data (i.e., marine fuel consumption) for ships in the HGB region, an 

emissions inventory for this source could be constructed. The 2007 report from Eastern 

Research Group (ERG) to TCEQ (Eastern Research Group, 2007) has such data, but there 

appears to be a significant underestimate (factor of 2-8) of fuel consumption, when compared to 

an estimate from a more comprehensive model (Wang et al., 2007). The NO2 emission factors 

used in the ERG report agree with our data to within 10%. Thus, for current ship emissions 

inventory modeling, there is less uncertainty contributed by emission factors than by activity 

data. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 62 31 August 2007 

Table D3 gives emissions of NOx, CO, and SO2, relative to the emission of CO2, from ships, 

compared to similar emission ratios from electric power generating units (EGU) from the 2004 

point source emission inventory, updated to 2006 with CEMS data. This comparison shows that 

ships emit 10 to 100 times more NOx (and somewhat more CO, and SO2) per unit fuel burned 

(i.e. CO2 emitted) than large stationary sources. Though the emissions from an individual vessel 

might be 10-100 times lower than from an EGU, the volume of ship traffic in the HGB region is 

sufficient that emissions from commercial shipping, in aggregate, cannot be neglected. Accurate 

fuel consumption or other ship activity data are needed to accurately quantify these emissions. 

Importantly, while emissions from stationary sources are a focus of ongoing control measures, 

emission controls on commercial shipping are not likely to be implemented because of technical 

constraints and complications arising from international law. 

Table D3. Comparison of marine vessel and power plant emissions. 

Emission source NOx/CO2 CO/CO2 SO2/CO2

Commercial vessel, SSD 22 6.1 6.1 

Commercial vessel, MSD 18 8.1 2.0 

Electric power generating unit 

W. A. Parish 0.23 0.51 1.9 

Welsh 0.88 1.4 1.8 

All units are molecules of species per 1000 molecules of CO2. 

Finding D4: Mixing ratios of isoprene over Texas measured from the WP-3D were used 

for evaluation of the BEIS-3 emission inventory. On average, the isoprene emissions from 

the inventory and emissions derived from the measurements agree within a factor of ~2. 

There may be areas south of Dallas-Fort Worth and southwest of Houston, where isoprene 

emissions are lower than indicated by the BEIS3 inventory based upon the biogenics 

emission land cover data (BELD-3.0). 

Analysis and Data: de Gouw, Warneke et al.-NOAA. 

Isoprene was measured from the WP-3D aircraft both in-situ by PTR-MS and off-line by 

laboratory analysis of whole air samples (WAS) collected in flight. Both measurements agreed 

within the respective measurement uncertainties. The PTR-MS has the highest time resolution, 

and thus spatial resolution, and the data illustrate the large variability of the isoprene 

concentration due to the in-homogeneity of its sources and its extremely short photochemical 

lifetime. 

The PTR-MS data allowed the isoprene emissions to be estimated along the flight track using the 

measured height of the boundary layer (BL) and the concentration of OH radicals calculated 

from the parameterization of Ehhalt et al. (1998). The emissions were then converted to standard 

temperature and photo-active radiation using the formalism from Guenther et al. (1995) and 

compared to the BEIS3 emission inventory extracted along the flight track. The results for a 

flight on September 16th over NE Texas are shown in Figure D4. On this flight, the isoprene 

emissions from BEIS3 and those estimated from the aircraft data agree within a factor of ~2. A 

better agreement cannot be expected given the uncertainties in some of the relevant factors for 

this comparison (BL height, OH concentrations, and vertical gradients in the BL). 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 63 31 August 2007 

We also used the BEIS3 isoprene emission inventory as input for the Lagrangian particle 

dispersion transport model FLEXPART (Stohl et al., 2003). A comparison between isoprene 

measured by PTR-MS and modeled using FLEXPART after an average transport time of 1 hr is 

shown in Figure D5. As before, the model and measurements agree within a factor of ~2. Work 

is in progress to compare measured isoprene mixing ratios to 3-D chemistry-transport models; 

however, for a good comparison the models need to get boundary layer heights and OH levels 

correct, which is not true in all cases. 

Figure D4. Isoprene emissions for a flight of the NOAA WP-3D on 16 September over NE Texas. 

Emissions extracted from the BEIS3 inventory based upon the biogenics emission land cover data 

(BELD-3.0) along the flight track are shown in red. Emissions derived from the PTR-MS data are 

shown in blue. 

Figure D5. Comparison between isoprene mixing ratios measured by PTR-MS from the WP-3D 

on 16 September, and those modeled by FLEXPART. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 64 31 August 2007 

Comparisons between isoprene measurements and FLEXPART model results have been made 

for all flights. Figure D6 shows a map of the flight tracks in the boundary layer color-coded by 

the differences between measurements and model. The flight tracks in red indicate areas where 

measurement and model agree well. Flight tracks in yellow indicate areas where modeled 

isoprene was higher than the measurements; in particular to the southwest of Houston there is an 

area where this is the case. A possible explanation includes a difference between the actual land 

use and the land use assumed in the BEIS3 inventory; work is in progress to investigate this 

possibility. There is also an area in between Dallas-Fort Worth and Houston where the model 

indicates higher isoprene than the measurements. This area, however, was not as frequently 

sampled. Flight tracks in black indicate areas where the modeled isoprene was lower than the 

measurements. This is the case for NE Texas, but because isoprene was very high over this 

region, the difference of -400 pptv only represents a small relative difference between 

measurements and model. 

 

Figure D6. Flight tracks of the NOAA WP-3D aircraft in the boundary layer color-coded by the 

difference between modeled and measured isoprene mixing ratios. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 65 31 August 2007 

Finding D5: The speciation of VOC from mobile sources in the Houston and Dallas-Forth 

Worth areas agrees with detailed measurements in the northeastern U.S. However, initial 

results suggest that the agreement with the NEI-99 emission inventory is poor. 

Analysis and Data: de Gouw, Warneke et al.-NOAA. 

For specific wind directions, the emissions from urban, mostly mobile, sources in Houston are 

spatially separated from industrial emissions from the Ship Channel area. Specific WP-3D 

flights were selected based on wind direction to determine the speciation of VOC from mobile 

sources, using airborne data from the PTR-MS and WAS instruments. In addition, data from the 

flights downwind from Dallas-Fort Worth were studied. Emissions ratios of VOC versus CO 

were calculated from the airborne data and compared with the results of a similar study 

downwind from New York City in 2004 for different classes of compounds (Figure D7). There 

was a good quantitative agreement between urban emissions in Houston, Dallas, and New York 

City, which is not surprising. Warneke et al. (2007) showed previously that the VOC 

composition of urban emissions in New York City and Boston are not represented well in the 

NEI 99 emissions inventory. Initial results suggest that the same is true for Houston and DallasFort Worth.

Figure D7. Emissions ratios (ER) of various 

VOC to CO from urban, mostly mobile, sources 

in Houston and Dallas-Fort Worth in 2006 

versus the same ratios from New York City in 

2004. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 66 31 August 2007 

KEY CITATIONS AND INFORMATION AND DATA SOURCES 

Eastern Research Group. 2007. Houston/Galveston Routine Vessel Identification and Traffic 

Study, ERG Rep. No. 0196.00.023, TCEQ Contract No. 582-04-65564-23, January 26, 2007. 

Ehhalt, D.H., F. Rohrer, A. Wahner, M.J. Prather, and D.R. Blake. 1998. On the use of 

hydrocarbons for the determination of tropospheric OH concentrations, J. Geophys. Res. 

103:18981-18997. 

Guenther, A., C. Hewitt, D. Erickson, R. Fall, C. Geron, T. Graedel, P. Harley, L. Klinger, M. 

Lerdau, W. McKay, T. Pierce, B. Scholes, R. Steinbrecher, R. Tallamraju, J. Taylor, and P. 

Zimmerman. 1995. A global model of natural volatile organic compound emissions. J. 

Geophys. Res. 100(D5):8873-8892. 

Lloyd's Register Engineering Services. 1995. Marine Exhaust Emission Research Programme. 

Lloyd's Register House, London. 

McGaughey, G.R., N.R. Desai, D.T. Allen, R.L. Seila, W.A. Lonneman, M.P. Fraser, R.A. 

Harley, J.M. Ivy, and J.H. Price. 2004. Analysis of motor vehicle emissions in a Houston 

tunnel during the Texas Air Quality Study 2000. Atmos. Environ. 38(20):3363-3372. 

Nicks Jr., D.K., J.S. Holloway, T.B. Ryerson, R.W. Dissly, D.D. Parrish, G.J. Frost, M. Trainer, 

S.G. Donnelly, S. Schauffler, E.L. Atlas, G. Hübler, D.T. Sueper, and F.C. Fehsenfeld. 2003. 

Fossil-fueled power plants as a source of atmospheric carbon monoxide. J. Environ. Monit. 

5:35-39. 

Parrish, D.D. 2006. Critical evaluation of US on-road vehicle emission inventories. Atmos. 

Environ. 40(13):2288-2300. 

Ryerson, T.B., M. Trainer, W.M. Angevine, C.A. Brock, R.W. Dissly, F.C. Fehsenfeld, G.J. 

Frost, P.D. Goldan, J.S. Holloway, G. Hübler, R.O. Jakoubek, W.C. Kuster, J.A. Neuman, 

D.K. Nicks, Jr., D.D. Parrish, J.M. Roberts, D.T. Sueper, E.L. Atlas, S.G. Donnelly, F. 

Flocke, A. Fried, W.T. Potter, S. Schauffler, V. Stroud, A.J. Weinheimer, B.P. Wert, C. 

Wiedinmyer, R.J. Alvarez, R.M. Banta, L.S. Darby, and C.J. Senff. 2003. Effect of 

petrochemical industrial emissions of reactive alkenes and NOx on tropospheric ozone 

formation in Houston, Texas. J. Geophys. Res. 108(D8), 4249, doi:10.1029/2002JD003070. 

Stohl, A., C. Forster, S. Eckhardt, N. Spichtinger, H. Huntrieser, J. Heland, H. Schlager, S. 

Wilhelm, F. Arnold, and O. Cooper. 2003. A backward modeling study of intercontinental 

pollution transport using aircraft measurements. J. Geophys. Res. 108, 4370, 

doi:10.1029/2002JD002862. 

Wang, C., J.J. Corbett, and J. Firestone. 2007. Modeling energy use and emissions from North 

American shipping: Application of the ship traffic, energy, and environment model. Environ. 

Sci. Technol., doi:10.1021/es060752e. 

Warneke, C., S.A. McKeen, J.A. de Gouw, P.D. Goldan, W.C. Kuster, J.S. Holloway, E.J. 

Williams, B.M. Lerner, D.D. Parrish, M. Trainer, F.C. Fehsenfeld, S. Kato, E.L. Atlas, A. 

Baker, and D.R. Blake. 2007. Determination of urban volatile organic compound emission 

ratios and comparison with an emissions database. J. Geophys. Res. 112, D10S47, 

doi:10.1029/2006JD007930. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 67 31 August 2007 

Response to Question E 

QUESTION E 

Are there sources of ozone and aerosol precursors that are not represented in the reported 

emissions inventories? 

BACKGROUND 

Questions C, D, and E all deal with the evaluation of emission inventories. Question C 

specifically addresses highly reactive VOC and co-located NOx emissions in the Houston area. 

Question D addresses all other ozone and aerosol precursor emissions, biogenic as well as 

anthropogenic, that are included in emission inventories. Here, Question E addresses evidence 

for ozone and aerosol precursor emissions not presently represented in inventories. 

FINDINGS 

Finding E1: The observed mixing ratios and regional distribution of ambient 

formaldehyde are broadly consistent with daytime photochemical production from reactive 

VOC. An upper limit for primary formaldehyde emissions from mobile sources is obtained 

from nighttime measurements, and is small in comparison with the secondary, daytime 

formation. 

Analysis: de Gouw et al.-NOAA, Herndon et al.-Aerodyne, Rappenglück et al.-U. Houston; 

Data: Herndon et al.-Aerodyne, Fried et al.-NCAR, de Gouw et al.-NOAA, Atlas et al.-U. Miami, 

Rappenglück et al.-U. Houston. 

Diurnal variations in the concentrations of ethene, propene, and formaldehyde were measured at 

several locations in the HSC. Figure E1 shows the data from Barbour’s Cut, at the eastern end of 

the HSC. The mixing ratios of ethene and propene (VOC from direct emission) were highest at 

night, when photochemistry was absent and emissions of these species accumulated in a shallow 

boundary, and lowest during the day, when the boundary layer was deeper and alkenes were 

removed by reaction with OH. In contrast, formaldehyde is highest during the day – indicating 

that daytime photochemistry was the dominant source of this ozone precursor in the HSC area. 

Similar diurnal patterns also were observed at the Moody Tower site at the University of 

Houston. 

Figure E1. Average diurnal variation of 

mixing ratios of ethene, propene, and 

formaldehyde measured from the Ronald 

H. Brown over approximately ten days 

of measurements in Barbour’s Cut. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 68 31 August 2007 

Nighttime measurements onboard the Ronald H. Brown (RHB) and the WP-3D and at the Moody 

Tower site provide a useful means to estimate the direct emissions of formaldehyde from mobile 

sources. The left panel of Figure E2 shows the relationship of formaldehyde to CO measured in 

the turning basin at the western end of the HSC with winds coming from the central urban 

Houston area. The data are color-coded by the measured photolysis rate of formaldehyde; the 

nighttime points, in black, show a clear correlation between formaldehyde and CO with a slope 

of about 3 pptv ppbv-1. After sunrise, the formaldehyde to CO ratio increased strongly due to 

photochemical formation of formaldehyde (daytime points in yellow). Similar observations were 

made onboard the WP-3D during a missed approach at Montgomery airport to the north of 

Houston during the night (Figure E2, right panel). In this graph the data are color-coded by 

altitude. Below 200 m the aircraft penetrated a shallow layer that was strongly impacted by local, 

primary emissions; the formaldehyde to CO ratio in this layer was 1.8 pptv ppbv-1. Above the 

layer, the aircraft sampled the remnants of the daytime boundary layer that is isolated from 

primary emissions, and the formaldehyde to CO ratio was much higher. The corresponding 

slope from the Moody Tower measurements is somewhat higher (5 to 7 pptv ppbv-1); the cause 

and implications of this larger slope is under investigation. From these observations we estimate 

the primary emissions of formaldehyde from mobile sources to be 0.18-0.3 percent of the CO 

emissions; this estimate is an upper limit, since it is not possible to exclude the possibility that 

the sampled air had been photochemically processed to at least some extent during the preceding 

daytime period. These findings are consistent with the conclusions of Anderson et al. (1996) 

who found formaldehyde to CO emission ratios of 0.10 to 0.14 percent in measurements made in 

Denver, Colorado. 

Figure E2. Relationships of HCHO to CO concentrations obtained onboard the Ronald H. Brown in 

the turning basin at the western end of the HSC (left panel), and from the WP-3D during a nighttime, 

missed approach at Montgomery County airport to the north of Houston. 

Finding E2: Concentrated plumes of ammonia were observed occasionally in the Houston 

Ship Channel area. These plumes often led to the formation of ammonium nitrate aerosol. 

Analysis and Data: Nowak et al.-NOAA. 

Ammonium nitrate aerosol is formed from the reaction of gas-phase ammonia (NH3) and nitric 

acid (HNO3). Anthropogenic emissions of NH3 and NOx, which in sunlight is oxidized to form 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 69 31 August 2007 

HNO3, can result in elevated concentrations of ammonium nitrate. Sources of NH3 in the 

Houston area are thought to include motor vehicles, industrial facilities, outlying agricultural 

activity, and possibly power plants. High-time-resolution (≈1 s average) NH3 measurements 

were made from the WP-3D aircraft by a Chemical Ionization Mass Spectrometry technique and 

from the Ronald H. Brown by quantum cascade laser absorption during TexAQS 2006 with the 

goals of characterizing sources and examining the effect of NH3 on atmospheric aerosol 

formation. 

Figure E3 shows the altitude profile of all 1-second average NH3 mixing ratios measured aboard 

the WP-3D aircraft during the TexAQS 2006 study. Typically, NH3 mixing ratios over the urban 

area ranged from 0.2 to 3 ppbv, and generally decreased with increasing altitude. Though 

infrequent, plumes with NH3 mixing ratios from 5 to greater than 50 ppbv (highest values not 

indicated in Figure E3) were observed in the boundary layer below 1 km altitude. These plumes 

were encountered over the Houston metropolitan area in the vicinity of the HSC, around 

Beaumont, and in St. James Parish, Louisiana. NH3 mixing ratios as high as several hundred 

ppbv were measured in the HSC from the RHB. 

It is difficult to trace the sources of plumes with high NH3 concentrations to particular industrial 

facilities. Figure E4 illustrates observations made in one of these plumes intercepted 

approximately 13 km downwind from the Cedar Bayou Station power plant. This plant is the 

location of the only significant upwind NH3 point source listed in the National Emission 

Inventory of 1999 (NEI99v3). However, the lack of a strong correlation of the concentration of 

NH3 with the enhancements of the CO2 and NOx concentrations (upper panel of Figure E4) 

suggests the NH3 is not released in the Cedar Bayou stack emissions. Future analysis will focus 

Figure E3. TexAQS 2006 NH3

observations from the WP-3D plotted 

as function of altitude. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 70 31 August 2007 

on using the 2006 Toxic Release Inventory along with measurements of VOC made aboard the 

WP-3D aircraft to unambiguously identify this NH3 plume source. 

The observed NH3 enhancements were typically found to be accompanied by corresponding 

increases in the particulate nitrate (NO3

-

) and decreases in HNO3 mixing ratios as illustrated in 

the bottom panel of Figure E4. These correlated variations indicate the formation of ammonium 

nitrate. The magnitude of the observed HNO3 lost and NO3

-

 formed is consistent (within a factor 

of 2) with ammonium nitrate formation. The air quality implications of such ammonia plumes 

should be considered, particularly during cooler wintertime months when the ammonium nitrate 

will make a longer-lived contribution to the PM2.5 concentrations. 

Power plants that have installed Selective Catalytic Reduction (SCR) units constitute a possible 

NH3 source. This process adds aqueous NH3 to the exhaust gases as a reagent to decrease NOx

emissions. NH3 “slippage,” i.e., unwanted emissions of NH3 into the atmosphere, occurs when 

exhaust gas temperatures are too low for the SCR reaction to proceed to completion, or when too 

much NH3 is added. The W.A. Parish electric generating facility is equipped with these units. 

To characterize emissions from this plant, the WP-3D aircraft sampled the Parish plume on 

numerous flights during TexAQS 2006; Figure E5 shows a time series of measurements from 

one example transect of the plume. Clear enhancements of CO2, NOy, and SO2 coincide with a 

depletion in the ozone concentration due to its reaction with the freshly emitted NOx, while no 

difference in NH3 (black line) mixing ratios can be discerned in or out of the plume. The lack of 

NH3 enhancement in the power plant plumes sampled by the WP-3D indicates that NH3 slippage 

was not significant during any of the TexAQS 2006 plume transects. 

Figure E4. Time series of 

concentration measurements of 

primary emissions (NH3, NOx, CO2) 

and secondary products (HNO3

nitrate aerosol) during a transect of 

the Cedar Bayou power plant plume. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 71 31 August 2007 

Finding E3: Concentrated plumes of gaseous elemental mercury from at least one point 

source were observed repeatedly in the Houston Ship Channel area and once in the 

Beaumont-Port Arthur area. The sources of the plumes could not be identified with 

current inventory sources of mercury. 

Analysis and Data: Ryerson et al.-NOAA. 

High-time-resolution (≈1 s) measurements of gaseous elemental mercury (Hg) were made from 

the Ronald H. Brown during TexAQS 2006. Figure E6 shows examples of the plume encounters 

in the HSC and Beaumont industrialized area. The magnitude of the detected plumes varied 

widely; the HSC plume in Figure E6 was detected during each of four transects of the HSC 

under southerly to easterly winds, but the magnitude of the plume varied by a factor of 

approximately 25. During the plume transects, the measured Hg concentration did not correlate 

with the measured concentrations of any other measured species. This lack of correlation rules 

out most known sources of Hg emissions, which can be expected to co-emit one or more of the 

wide range of chemical or aerosol species that were measured concurrently on the RHB. Further, 

the measured Hg concentrations are not consistent with the latest TCEQ or EPA AIRS, eGRID, 

and TRI inventory source locations for known Hg sources in the HSC region. A potential source 

not included in these inventories would be re-release of Hg from historically contaminated soils. 

More research into this possibility is needed to better understand the unexpectedly high 

concentrations of Hg observed in coastal industrial areas in eastern Texas. 

Figure E5. Time series of NH3, NOy, SO2, CO2, and O3 observations during transect of the W.A. Parish 

power plant plume. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 72 31 August 2007 

80

60

40

20

0

8:30 PM

8/9/06

8:40 PM

Time, UTC

BeaumontPort Arthur

1-second Hg(0) data

250

200

150

100

50

0

Hg, ng/m3

7:00 PM

8/2/06

7:04 PM 7:08 PM

Time, UTC

Houston

Ship Channel

1-second Hg(0) data

KEY CITATIONS AND INFORMATION AND DATA SOURCES 

Anderson, L.G., J.A. Lanning, R. Barrell, J. Miyagishima, R.H. Jones, and P. Wolfe. 1996. 

Sources and sinks of formaldehyde and acetaldehyde: An analysis of Denver’s ambient 

concentration data. Atmos. Environ. 30:2113-2123. 

Ryerson, T.B., M. Trainer, W.M. Angevine, C.A. Brock, R.W. Dissly, F.C. Fehsenfeld, G.J. 

Frost, P.D. Goldan, J.S. Holloway, G. Hübler, R.O. Jakoubek, W.C. Kuster, J.A. Neuman, 

D.K. Nicks, Jr., D.D. Parrish, J.M. Roberts, D.T. Sueper, E.L. Atlas, S.G. Donnelly, F. 

Flocke, A. Fried, W.T. Potter, S. Schauffler, V. Stroud, A.J. Weinheimer, B.P. Wert, C. 

Wiedinmyer, R.J. Alvarez, R.M. Banta, L.S. Darby, and C.J. Senff. 2003. Effect of 

petrochemical industrial emissions of reactive alkenes and NOx on tropospheric ozone 

formation in Houston, Texas. J. Geophys. Res. 108(D8), 4249, doi:10.1029/2002JD003070. 

Wert, B.P., M. Trainer, A. Fried, T.B. Ryerson, B. Henry, W. Potter, W.M. Angevine, E. Atlas, 

S.G. Donnelly, F.C. Fehsenfeld, G.J. Frost, P.D. Goldan, A. Hansel, J.S. Holloway, G. 

Hübler, W.C. Kuster, D.K. Nicks Jr., J.A. Neuman, D.D. Parrish, S. Schauffler, J. Stutz, D.T. 

Sueper, C. Wiedinmyer, and A. Wisthaler. 2003. Signatures of terminal alkene oxidation in 

airborne formaldehyde measurements during TexAQS 2000. J. Geophys. Res. 108(D3), 

4104, doi:10.1029/2002JD002502. 

Figure E6. Measurements of gas-phase elemental mercury (Hg). Note the change in scales 

between the two graphs. Periods of missing data are due to automatic instrument calibration or 

zero procedures. 

Figure E6. Measurements of gas-phase elemental mercury (Hg). Note the change in scales 

between the two graphs. Periods of missing data are due to automatic instrument calibration or 

zero procedures. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 73 31 August 2007 

Response to Question F 

QUESTION F 

How do the mesoscale chemical environments (NOx-sensitive ozone formation vs radicalsensitive ozone formation) vary spatially and temporally in Houston, Dallas, and eastern 

Texas? 

Which mesoscale chemical environments are most closely associated with high ozone and 

aerosol? 

BACKGROUND 

Determination of NOX versus VOC (or Radical-Formation) Sensitive Ozone Formation 

The accurate prediction of the relative response of ozone concentrations to future reductions in 

NOx and VOC emissions is a much sought, but very elusive goal. The prediction is central to the 

very important SIP-relevant question of “direction of control” – that is, should ozone control 

efforts in an ozone non-attainment area be focused on: a) decreasing emissions of NOx alone, b) 

decreasing emissions of VOC alone, or c) decreasing emissions of both NOx and VOC? The 

following paragraphs discuss some important considerations concerning this question. 

In the period between the two TexAQS field studies, a change in the NAAQS for ozone was 

implemented, with at least two important implications for evaluating the “direction of control” 

question. The older standard, based upon a relatively high (120 ppbv) ozone concentration 

averaged over a short period (one hour), has been supplanted by one based on a lower (80 ppbv) 

concentration averaged over a longer period (eight hours). One implication is that the older, 

shorter-period average was much more amenable to analysis based upon in situ observations; the 

“snap shot” of the relationships between simultaneously measured concentrations of ozone, its 

precursors, and other photochemical products provided direct clues to the limiting precursor in 

an observed ozone exceedance. The longer-period average requires a more comprehensive 

analysis of the integrated accumulation of ozone over the full 8-hour exceedance period. A 

second implication is that the background ozone transported into an urban area will constitute a 

much larger fraction of the ambient ozone concentration constituting an 8-hour exceedance than 

was the case for a one-hour exceedance. Thus, much more attention must be paid to the effect of 

control strategies in upwind regions. 

Some approaches to determining the “direction of control” question rely, in effect, upon 

comparing the response of ambient ozone to differential reductions in VOC versus NOx

precursor emissions. Such approaches give useful guidance for effectively achieving 

incremental improvements in ozone air quality; however such approaches cannot determine if 

compliance with the NAAQS can be reached through such incremental emission reductions, and 

do not identify the most effective emission control approach for reaching compliance. 

A rigorous approach to answering all aspects of the “direction of control” question would require 

a thoroughly tested Eulerian air quality model that treats the region of interest with sufficient 

accuracy. Multiple simulations would then be run with that “perfected” model to test possible 

NOx and VOC emission control strategies. These simulations would then provide the required 

ozone response information. Unfortunately however, it is not yet generally possible to develop 

such models because of deficiencies and uncertainties in many critical areas, including 

emissions, meteorological modeling, and photochemical mechanisms. The thorough testing of 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 74 31 August 2007 

the model must verify that the model adequately reproduces the observed ozone for the correct 

reasons. This testing would include at least two model-measurement comparisons: first, the 

model’s ability to reproduce observed relationships between the concentrations of ozone and its 

precursors and, importantly, other secondary photochemical products (organic nitrates, nitric 

acid, formaldehyde and other oxygenated VOC, peroxides), and second, the model’s ability to 

reproduce the observed ambient concentrations of the radicals that drive the photochemical 

reactions. The first comparison should pay particular attention not only to the slopes and 

correlation coefficients of the relationships, but also to the range of the observed and modeled 

concentrations of ozone and the related species. From this perspective, the air quality 

community is still in the early stages of the testing process; a process that is limited both by 

resources for model development, and by the availability of observational data with which to 

compare the models. 

The eastern Texas region provides particular advantages, as well as particular challenges, for 

achieving the required model development and application. The TexAQS 2000 and TexAQS II 

field studies have collected data sets that provide unprecedented opportunities for model testing. 

However, experience has shown that development of needed emission inventories is a challenge 

that has not been met in all respects (see discussion in Responses to Questions C, D, and E.) A 

further challenge is presented by the concentrated plumes that determine the highest ozone 

values in the Houston area (see discussion in Response to Question A.) Such resolution may be 

a severe challenge for Eulerian models, especially if they attempt to include the whole eastern 

Texas region. At this point it is not possible to provide a rigorous answer to the “direction of 

control” question. 

In the absence of a perfected model, it has been necessary to take a variety of heuristic 

approaches to provide guidance to air quality managers. Heuristic implies a model that is 

simplified, but that is designed and used to learn more about some specific but important aspect 

of the more complex air-quality system that is to be managed. In this report the closely related 

Questions F and K both deal with aspects of this issue. Here in Question F the application of 

both observation and modeling based approaches for approximately determining the sensitivity 

of ozone production will be discussed; the photochemical environment of the entire eastern 

Texas region will be considered. Question K presents an EKMA model-based approach to 

address the same issue, but from a different perspective; its application is restricted to the HGB 

area. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 75 31 August 2007 

Figure F1. Relationship between 

ozone and formaldehyde 

concentrations observed from the 

Electra aircraft on 1 September 2000 

and modeled by a Lagrangian plume 

model (Wert et al., 2003). 

FINDINGS 

Finding F1: Both Eulerian and Lagrangian plume modeling approaches indicate that in 

2000 high ozone concentrations in the HGB area were sensitive to both VOC and NOx

emission reductions. 

Lagrangian Plume Model 

Analysis: M. Trainer-NOAA; Data: Parrish et al.-NOAA, Fried et al.-NCAR, Atlas et al.-U. 

Miami. 

Wert et al. (2003) presented a Lagrangian plume model designed to closely reproduce the 

emissions, ozone formation, other secondary photochemical product formation, and plume 

dispersion observed during the TexAQS 2000 study. Figure F1 shows that the model accurately 

reproduced the rapid production of ozone and formaldehyde that was measured in a concentrated 

plume originating from the HSC region. (Figure A3 shows the location of this plume intercept). 

The model required only two HRVOC – ethene and propene. To successfully reproduce the 

highest observed ozone concentrations, it was critical that the model reproduced two factors: a 

high ozone production efficiency as shown by observations (e.g. Figures A3 and A4 of this 

report), and a rapid rate of ozone production. High ozone production efficiency assures that high 

ozone can be produced from the emitted precursors, and the rapid rate of ozone production 

assures that high ozone is produced before the plumes of emissions dilute and disperse. 

200

150

100

50

0

Ozone (ppbv)

0 5 10 15 20 25 30

CH2O (ppbv)

Measured

Model

Figure F2 shows the modeled sensitivity of the ozone accumulation to both NOx and VOC 

emission reductions for the plume illustrated in Figure F1. The upper panel indicates that 

decreases in NOx emissions lead to a more rapid ozone production, but a lower peak ozone 

concentration, while HRVOC emission reductions lead to both slower ozone production and a 

lower peak ozone concentration. The lower panel indicates that HRVOC and NOx emission 

reductions are almost equally effective in reducing the total ozone flux in this plume. It is 

noteworthy that the ozone response is highly non-linear; reduction of either precursor by onehalf reduces the ozone flux by only about one-quarter. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 76 31 August 2007 

The ozone flux produced and transported in 

this plume from the HSC accounts for a 

substantial fraction of the total ozone produced 

in the entire Houston area. In Response to 

Question G of this report, the total flux from 

the Houston urban area is calculated from 

airborne ozone lidar measurements. The base 

case calculated plume flux in Figure F2 

represents 45 to 80% of the total Houston flux 

determined on six days during TexAQS 2000 

and 2006. These calculations led Wert et al. 

(2003) to conclude that targeted reductions in 

either or both emission categories would 

effectively reduce the highest observed ozone 

levels. 

Eulerian Modeling 

TCEQ (2004, 2006) has conducted Eulerian 

modeling of the entire HGB nonattainment 

area in order to determine which controls are 

necessary to reach attainment. This modeling 

indicates that both VOC and NOx controls 

were effective in reducing ozone in 2000. 

In particular, two tests have indicated 

sensitivity to both VOC and NOx in Houston: 

reductions of biogenic VOC emissions by 30%, and 

modeling of weekend/weekday differences in 

mobile source emissions. However, in both cases, 

the effects on ozone concentrations varied by 

location within the HGB area and meteorological 

conditions, suggesting that VOC and NOx

sensitivity in HGB varies spatially and temporally. 

TCEQ modeling for the HGB area indicates that in 2009 further NOx emission reductions will be 

by far more effective than VOC emission reductions in decreasing O3. In the 2009 future 

baseline scenario, the emissions of anthropogenic NOx and VOC are projected to be reduced by 

56% and 24%, respectively, relative to the 2000 emission inventory. Despite these large 

projected decreases in emissions, the maximum ozone design value in HGB is predicted to be 97 

ppbv, well above the 84 ppbv level of the NAAQS. Thus without additional emission 

reductions, the HGB area is predicted to remain out of compliance with the NAAQS in 2009. 

Figure F3 shows the predicted effects of additional anthropogenic emission reductions beyond 

the 2009 future baseline. Additional reductions in VOC emissions result in only modest 

improvement in the ozone design value, and compliance with the NAAQS cannot be achieved 

with VOC decreases alone. The modeling does suggest that reductions in VOC emissions would 

allow compliance with the NAAQS to be reached with somewhat less drastic NOx emission 

reductions (53% versus 58%) than is the case for NOx emission reductions alone. 

Figure F2. Ozone sensitivity calculated by 

the Lagrangian plume model. The red lines 

give the base case calculation, and the green 

and red lines indicate the effect of a factor of 

two decrease in emissions NOx and VOC, 

respectively. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 77 31 August 2007 

“Radical Starvation” in Selected Modeling Scenarios 

A photochemical grid modeling scenario has shown that under certain conditions with high 

concentrations of HRVOC, ozone formation can be inhibited by "radical starvation." The radical 

starvation can be alleviated by adding large quantities of primary formaldehyde emissions, CO 

emissions, or aromatic emissions, or by changing to the SAPRC99 chemical mechanism. It is 

unclear at this point which, if any, of these solutions are appropriate for Houston modeling. 

Figure F3. Predicted future ozone design values (O3 DVf; the 3-year average of the fourth-highest 

daily maximum 8-hour-average ozone concentration) for HGB as a function of reductions in emissions 

of NOx, VOC, and both. The reference calculation (i.e. 0 percent reduction in the table in the figure) is 

based upon the projected emissions for 2009, and the reductions from that reference case are assumed 

constant across all emission categories. (Analysis by TCEQ; Figure adapted from D. Karp) 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 78 31 August 2007 

Finding F2: An observation-based approach to determine the sensitivities of high ozone in 

the HGB non-attainment area to the precursor VOC and NOx emissions has been 

investigated; it has yielded ambiguous results. 

Analysis: Parrish-NOAA; Data: Ryerson, Neuman, Parrish et al.-NOAA. 

The observation-based approach selected is based upon the relationships between indicator 

species developed by S. Sillman (http://www-personal.engin.umich.edu/~sillman/obm.htm). 

This approach was selected because: 1) it addresses integrated total O3 produced (not 

instantaneous rate of O3 production), and is thus designed to answer the question of how 

maximum O3 responds to changes in VOC versus changes in NOx emissions; 2) it is arguably the 

most fully developed 

observation-based method and 

is widely used; 3) it is related 

to the ozone production 

efficiency relationships 

exemplified in Figures A3 and 

A4 of this report; and 4) it 

utilizes measurements made 

with high precision and 

accuracy on the Electra aircraft 

during TexAQS 2000 and the 

WP-3D and Ronald Brown

during TexAQS 2006. 

Figure F4 compares the 

measurements made from the 

Electra during TexAQS 2000 

with the modeled indicator 

species relationships of 

Sillman. These two 

relationships are those he 

discusses most thoroughly. 

The data shown include all 5-

second average measurements 

made in the greater Houston 

area (94.3-96.3 deg. E Long., 

28.7-30.7 deg. N. Lat.), which 

includes the entire urban area, 

all petrochemical facilities 

including the isolated Gulf 

Coast facilities, and the Parish 

power plant. 

Figure F4. Indicator species relationships from models [Sillman] and from the TexAQS 2000 Electra 

measurements (red symbols). The relationships expected for NOx-, mixed-, and VOC-sensitive ozone 

production are given by green, violet and black symbols, respectively. The blue highlighted symbols 

indicate air masses dominated by NOx titration. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 79 31 August 2007 

The indicator species relationships in Figure F4 give ambiguous results. In both figures most of 

the data are localized in the predominately NOx-sensitive region, but some points are in the 

mixed and predominately VOC-sensitive regions. Further, the model results themselves are 

interspersed between regions. Definitive conclusions cannot be drawn from these figures. This 

ambiguity is particularly clear in the examination of ozone data above 200 ppbv. These data 

were collected on three days in 2000: 25 and 30 August and 1 September (see Figure A3). The 

relationship of ozone with HNO3 suggests that all three days represent NOx-sensitive conditions, 

while the relationship of ozone with NOy - NOx suggests that the three days span the full range 

from NOx-sensitive to VOC-sensitive. It is clear that no definitive conclusions can be drawn. It 

may be that the indicator species relationships may be most useful as a basis of comparison of 

models with measurements. 

Finding F3: At the highest ozone concentrations, the observed relationship between ozone 

and the products of NOx oxidation indicates less efficient ozone production in the Dallas 

area than in the Houston area. In the observation-based indicator species approach, this 

behavior corresponds to less NOx-sensitive and more VOC- or radical-sensitive ozone 

formation in Dallas compared to Houston. 

Analysis: Parrish-NOAA; Data: Ryerson, Neuman, Parrish et al.-NOAA. 

Figure F5 shows the relationships between ozone and the oxidation products of NOx. These 

oxidation products include only nitric acid on the left plot and nitric acid plus organic nitrates on 

the right plot. The generally shallower slopes in the Dallas area indicate less efficient ozone 

production in that area. From an observation-based indicator approach, this behavior 

corresponds to less NOx-sensitive and more VOC- or radical-sensitive ozone formation in Dallas 

compared to Houston. 

Figure F5. Indicator species relationships from the TexAQS 2000 Electra measurements. The gray and blue 

symbols indicate data collected in the Houston and Dallas areas, respectively. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 80 31 August 2007 

Finding F4: Tests of the ability of models to reproduce observed relationships between 

ozone and other photochemical products have the potential to provide very fruitful 

approaches to improving models. 

Analysis: Parrish-NOAA; Modeling: TCEQ; Kim et al.-NOAA. 

Relationships between ozone and other photochemical products, such as those illustrated in 

Figure F5, have been found to be remarkably robust in observations. Examination of the ability 

of models to reproduce these observed relationships is expected to provide powerful tests of the 

models’ performance and to suggest possible model improvement. Figures F6 and F7 give some 

example model output for the relationships of O3 with HNO3 and with NOy-NOx. The figures 

are plotted in the same format as the observations in Figure F5 for ease of comparison. 

The results of the WRF-Chem model (Figure F6), which covers most of the continental U.S. at 

relatively coarse (27 km x 27 km) resolution (Kim et al, 2006), shows some similarities and 

differences when compared to the observations (Figure F5). The model produces relatively well 

defined relationships, but the slopes of the relationships are significantly smaller than those 

observed. There is no marked difference in the modeled relationships in the different regions of 

eastern Texas. The very high O3 concentrations observed in the Houston area are not reproduced 

by the model. Significantly higher resolution, particularly over the urban areas, is an avenue of 

model development that would be expected to improve the model performance. Further, the 

HRVOC emissions for the petrochemical facilities are undoubtedly underestimated in these 

calculations, since they are based upon the NEI 1999 emission inventory. 

The TCEQ modeling results (Figure F7) are focused on the HGB region with much higher 

resolution, and generally give a better reproduction of the observed relationships. The slopes of 

the relationships through the majority of the data are accurately reproduced. However, there is 

an indication that the slopes decrease at the highest O3 concentrations, so that the highest 

modeled O3 concentrations are significantly lower than those observed. One explanation that has 

been suggested for this behavior is that the HRVOC emissions are underestimated in the model 

emissions and/or that they are not properly collocated with the NOx emissions from the 

petrochemical facilities. The model also predicts relatively low concentrations of O3 at relatively 

high concentrations of HNO3 and NOy-NOx. Such behavior is not seen in the observations. The 

cause of the disagreement in these few points should be investigated. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 81 31 August 2007 

Figure F6. Indicator species relationships calculated by the WRF-Chem model for 2000 UTC each day 

between 1 April and 31 October 2004. The color coding indicates results for different areas of eastern 

Texas. 

Figure F7. Indicator species relationships derived from 2006 modeling by TCEQ with the Base 1b 

configuration. The data points include all hours, 12:00-18:00 local time, on the days between 16 August 

and 6 September 2000. The color coding indicates two different monitoring sites in the HGB area. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 82 31 August 2007 

KEY CITATIONS AND INFORMATION AND DATA SOURCES 

Kim, S.-W., A. Heckel, S.A. McKeen, G.J. Frost, E.-Y. Hsie, M.K. Trainer, A. Richter, J.P. 

Burrows, S.E. Peckham, and G.A. Grell. 2006. Satellite-observed U.S. power plant NOx

emission reductions and their impact on air quality. Geophys. Res. Lett. 33, L22812, 

doi:10.1029/2006GL027749. 

Sillman, S. Observation-based methods (OBMs) for analyzing urban/regional ozone production 

and Ozone- NOx-VOC sensitivity (http://www-personal.engin.umich.edu/~sillman/obm.htm) 

TCEQ. 2004. Houston-Galveston-Brazoria Mid-Course Review SIP Appendixes (2004-042-SIPNR). http://www.tceq.state.tx.us/implementation/air/sip/dec2004hgb_mcr.html

TCEQ. 2006. Weekday-Weekend Effect Analysis, Part Two. Presented by Jim Smith, June 8, 

2006. 

http://www.tceq.state.tx.us/assets/public/implementation/air/am/committees/pmt_set/200606

21/20060621-smith-weekday_weekend_effects_part2.pdf

Wert, B.P., M. Trainer, A. Fried, T.B. Ryerson, B. Henry, W. Potter, W.M. Angevine, E. Atlas, 

S.G. Donnelly, F.C. Fehsenfeld, G.J. Frost, P.D. Goldan, A. Hansel, J.S. Holloway, G. 

Hübler, W.C. Kuster, D.K. Nicks Jr., J.A. Neuman, D.D. Parrish, S. Schauffler, J. Stutz, D.T. 

Sueper, C. Wiedinmyer, and A. Wisthaler. 2003. Signatures of terminal alkene oxidation in 

airborne formaldehyde measurements during TexAQS 2000. J. Geophys. Res., 108(D3), 

4104, doi:10.1029/2002JD002502. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 83 31 August 2007 

Response to Question G 

QUESTION G 

How do emissions from local and distant sources interact to determine the air quality in 

Texas? 

What meteorological and chemical conditions exist when elevated background ozone and 

aerosol from distant regions affect Texas? 

How high are background concentrations of ozone and aerosol, and how do they vary 

spatially and temporally? 

BACKGROUND 

Question G is closely related to Question H. Here Question G focuses on characterizing the 

background ozone and aerosol distributions, and the chemical and physical processes that affect 

the background concentrations of ozone and aerosol in Texas. Question H focuses on the 

transport processes and source-receptor relationships of those background concentrations. 

FINDINGS 

Finding G1: The maximum background ozone concentrations encountered in 2006 

exceeded the 8-hour NAAQS. On average, air of continental origin had higher background 

concentrations than marine air. The average background ozone concentrations measured 

in 2006 in eastern Texas complement a previously developed climatology. 

Analysis: Senff et al.-NOAA, Sullivan et al.-U. Texas; Data: Senff et al.-NOAA, TCEQ. 

Daily 8-hour-average ozone maxima from upwind suburban or rural sites are taken as indicators 

of the local background in a particular region. In Figure G1, the red line segments indicate the 

average background for four areas in Texas on days in April – July 2006 when the area 

maximum 8-hour average reached or exceeded 80 ppbv. These segments are higher than the 

corresponding average curves, which were compiled for all days, not just the higher ozone days. 

The marine influence in the Houston area accounts for the lower background (49 ppbv) 

compared to the Dallas (63 ppbv), Beaumont (61 ppbv), and Northeast Texas (60 ppbv) areas. 

The NOAA lidar provided a different approach for obtaining mesoscale estimates of background 

ozone for four regions in eastern Texas: 1) offshore over the Gulf of Mexico, 2) Gulf of Mexico 

coastal areas (less than 50 km from shore), 3) southeast Texas including Houston (all of 

southeast Texas more than 50 km from shore), and 4) northeast Texas including Dallas. 

Background ozone was determined by averaging lidar ozone profiles between the surface and the 

top of the boundary layer over all data points that were identified to be outside of pollution 

plumes. The blue line segments in Figure G1 indicate the background ozone values for the four 

regions identified above using measurements from all suitable lidar flights. The average 

background values for the southeast Texas and Gulf coast areas compare well with the Houston 

area background on high-ozone days (red line segment). The northeast Texas lidar background 

ozone value is close to the Dallas and northeast Texas TCEQ network-based background values 

for high-ozone days. This good agreement is not surprising because the lidar was typically flown 

on days when an ozone exceedance was forecast. The average ozone background measured over 

the Gulf of Mexico was the lowest of all four areas (39 ppbv) and is close to the average curves 

for Houston-Galveston-Brazoria (HGA) and Beaumont-Port Arthur (BPA) for the 1 August to 15 

September period of the measurements. The highest observed ozone background value was 86 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 84 31 August 2007 

ppbv measured on 8 September in east central Texas near the Louisiana border, after several 

days of continuous easterly flow conditions. 

Figure G1. Continuous curves give six-year (1998-2003) average background ozone in various 

regions in eastern Texas, smoothed with a 31-point running mean filter (Nielsen-Gammon et al., 

2005). The blue (lidar data) and red line segments (TCEQ surface network data) indicate 2006 

background ozone determinations with greater emphasis on exceedance days. The widths of the line 

segments approximately indicate the periods considered in 2006. 

A characterization of synoptic flow based on classes of regional ozone concentrations showed 

higher background ozone concentrations under continental air compared to maritime air. Twelve 

sites were selected for completeness of data coverage (>75% each year in August and September 

from 2001 to 2006) and regional representation (one site per county, except for two in Harris 

County). Each of the days included in this data set was classified as “high regional ozone,” if 

and only if four or more stations reported maximum 8-hour-average O3 of 75 ppbv or higher, 

(this included 40 days) or “low regional ozone,” if and only if all stations reported maximum 8-

hour-average O3 of at least 40 but less than 50 ppbv (this included 43 days). The probability 

distributions for the 1-hour time step points of the HYSPLIT back trajectories (72-hour, 300 

meter above ground level/mid-day starting point) for the twelve sites are given in Figure G2. On 

“low regional ozone” days air parcel trajectories tended to cluster to the south and southeast with 

relatively longer fetches from the Gulf of Mexico, while on “high regional ozone” days the 

trajectories cluster to the northeast with relatively short fetches, indicating transport from within 

eastern Texas or from areas to the northeast of Texas. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 85 31 August 2007 

Finding G2: The net ozone flux transported out of Houston averages about a factor of two 

to three larger than the corresponding flux from Dallas. The fluxes from these urban areas 

are significant contributors to the background ozone in the eastern Texas region. 

Analysis: Senff et al.-NOAA, Sullivan et al.-U. Texas; Data: Senff et al.-NOAA, TCEQ. 

The horizontal flux of O3 downwind of Houston and Dallas during TexAQS 2000 and TexAQS 

2006 was calculated from airborne lidar measurements of O3. The O3 flux was computed by 

vertically integrating excess O3 concentration in the plume (plume O3 minus background O3) 

between the surface and the top of the boundary layer and horizontally between the plume edges 

(see Figure G3 for example of a plume cross section). The integrated plume excess O3

concentration was then multiplied by the horizontal wind speed (estimated from nearby wind 

profilers) to yield O3 flux, which is expressed in molecules per second. Using data from the 

TexAQS 2000 and 2006 studies, fluxes were computed for the Houston area on five days (8/28 

and 9/06/2000; 8/12, 8/14, and 8/30/2006) and the Dallas/Fort Worth area on one day 

(9/13/2006). The ozone flux from Houston ranged from 3.2 * 1026 to 6.0 * 1026 molec s-1, and - 

within the uncertainties - was similar for the 2000 and 2006 cases. The average ozone flux from 

Houston of 4.6 * 1026 molec s-1, emitted over the 8-hour photochemically active part of one day, 

is sufficient to produce a 10-ppbv increase in ozone over an approximately 10,000 square mile 

area, assuming a 2-km deep boundary layer. The flux for the one Dallas case was estimated as 

1.7 * 1026 molec s-1, about a factor of two to three smaller than that observed for Houston. 

Although this comparison is based on only one flux determination for Dallas, we expect this 

comparison to generally hold, since Dallas lacks the large ozone production from the 

petrochemical industry, which accounts for a large fraction of the ozone production in Houston. 

Figure G2. Contour plots showing the probability distribution of origins of 72-hour HYSPLIT 

back trajectories ending at 12 representative TCEQ surface ozone monitoring stations in eastern 

Texas. Red color denotes high probability, yellow indicates low probability. Left panel: high 

regional ozone days; right panel: low regional ozone days. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 86 31 August 2007 

As a complement to airborne lidar data, which are only available for rather short time periods, a 

similar assessment was performed using data from TCEQ’s surface ozone monitoring network to 

estimate horizontal ozone fluxes. The horizontal plume integral was calculated from ozone 

measurements at downwind surface sites. The ozone was assumed constant through the mixed 

layer and the vertical plume extent was estimated from mixed layer height measurements from 

nearby wind profilers. For the days of the lidar analysis the differences between lidar- and 

surface-network-based flux estimates were generally on the order of 20 to 25%, except for one 

case where the difference was nearly 60%. This one large discrepancy may be due to 

uncertainties in the estimation of mixing height and horizontal plume dimensions when using 

surface network data. The agreement between the airborne lidar and surface monitoring results 

suggests that routine flux assessments using surface data may be useful. 

Figure G3. Time-height cross section of ozone plume about 50 km downwind of the Houston 

and Ship Channel area measured with NOAA’s airborne ozone lidar on 8/14/2006 at about 

17:00 CST. Note the strong variation of mixing height (shown as black line) across the plume. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 87 31 August 2007 

Finding G3: Elevated background ozone concentrations for urban areas can include 

contributions from the recirculation of locally produced ozone or local precursor emissions. 

Analysis: Ryerson et al.-NOAA, Sullivan et al.-U. Texas; Data: Ryerson et al.-NOAA, TCEQ. 

Background ozone concentrations entering an urban area can be elevated by recirculation of the 

ozone produced in that same urban area on a previous day, or by ozone production from 

precursors emitted within that urban area and recirculated. A past example provided by TCEQ is 

a May 31, 2003 episode in DFW, for which O3 concentration contours at mid-afternoon are 

shown in Figure G4. In this event, the one-hour-average peak O3 was 161 ppbv, with the eighthour-average peak at 130 ppbv. The event was influenced by a stalled frontal passage – note the 

contrasting direction at the wind vanes associated with monitoring sites on the north side versus 

the south side of DFW. The color code for ozone concentrations in the figure is: Gray 85-99 

ppbv, Orange 100-124 ppbv, Red 125-149 ppbv, Purple > 149 ppbv. 

Figure G4. (Left) Surface weather map for 6 CST 31 May 2003 shows stalled front in north Texas 

(http://weather.unisys.com/archive/index.html). (Right) Mid-day contour of O3 concentrations shows 

“pancake” of elevated O3 between Dallas and Fort Worth. Wind barbs show northerly winds north of 

the city and southerly winds south of the city, trapping local pollution (TCEQ, 2003). 

A recirculation example was observed in the Houston area during TexAQS 2006 (Figure G5). 

On 26 September the Houston plume with ozone concentrations approaching 120 ppbv was 

carried to the southeast by the prevailing winds. On the following day, winds from the south 

brought air from over the Gulf of Mexico into the Houston area. However, this background air 

contained ozone concentrations much higher (64 to 84 ppbv, average 74 ppbv) than the more 

characteristic concentrations of about 40 ppbv (see Figure G1). These elevated ozone 

concentrations were accompanied by elevated concentrations of other photochemical products 

(e.g. formaldehyde and nitric acid) and CO, a tracer of primary emissions. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 88 31 August 2007 

Results from FLEXPART particle dispersion modeling (Figure G6) and back-trajectories 

calculated from wind profiler data (not shown) both indicate that the elevation in background 

ozone concentrations on 27 September were due to recirculation of the 26 September Houston 

area plume back into the HGB region. 

Figure G5. WP-3D flight tracks within the boundary layer in the HGB area on two successive 

flights color-coded and sized according to the measured O3 concentration. 

Figure G6. Footprint emission 

sensitivity for air sampled by WP-3D 

on 27 September at the position 

marked by the asterisk. The integral 

of the product of this quantity with 

the surface emission density gives 

the expected concentration that 

would be measured at the aircraft 

position. The numerals 1 and 2 

indicate days of backward transport 

time. The near coincidence of the 

asterisk and the numeral 1 indicates 

that emissions from 26 September 

have been returned to near the 

original emission location. The shape 

of the red to violet colors indicates 

the primary transport pathways. The 

color scale is logarithmic in arbitrary 

units. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 89 31 August 2007 

Finding G4: Plumes from Texas urban areas make substantial contributions to the ozone, 

aerosol, and precursor concentrations in the rural regions of eastern Texas. 

Analysis: Brock et al.-NOAA, Data: Middlebrook, de Gouw et al.-NOAA. 

Figure G7 shows one example of model output and measurements of transported plumes of 

emissions and their photochemical products from the Houston area to northeastern Texas. The 

model results are from the FLEXPART Lagrangian particle dispersion model, and the 

measurements are from the WP-3D aircraft on 16 September 2006. There is excellent agreement 

between the model and the measurements, with both showing that the SO2 plume, primarily from 

the Parish power plant, was transported parallel and to the west of the NOx and benzene plumes, 

primarily from the Houston urban and Ship Channel areas. Table G1 shows that these plumes 

significantly increased the total aerosol concentrations in rural northeast Texas (for reference, the 

NAAQS for PM2.5 aerosol are 15.0 μg m-3 annual mean, and 35 μg m-3 24-hour mean). 

Table G1. Measured concentrations of aerosol and gas phase species in the Parish power plant and 

Houston area plumes in northeast Texas. 

Background Parish Petrochemical 

Industry 

Total mass (μg m-3) 4.3 7.9 5.7 

 Sulfate (μg m-3) 1.9 4.7 2.0 

 Organic (μg m-3) 1.2 1.9 2.3 

 Black Carbon (μg m-3) 0.08 0.12 0.17 

Benzene (ppbv) 0.094 0.103 0.26 

SO2 (ppbv) 0.29 0.97 0.53 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 90 31 August 2007 

Figure G7. Modeled and measured concentrations of aerosol and gaseous species in transported 

plumes from the Houston area to northeast Texas. The upper panel shows the FLEXPART model 

calculations (Cooper et al.–NOAA) for SO2 (primarily Parish power plant) and NOx (primarily HSC) 

plumes, and the lower panel shows the measurements from the WP-3D of sulfate aerosol and benzene 

concentrations. A plume of sulfate aerosol associated with the Parish power plant SO2 emission 

plume is evident in the measurements, as is a plume of benzene associated with HSC emissions. 

(Lower panel covers only a fraction of the area in the upper panel.) 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 91 31 August 2007 

Finding G5: Dust of African origin and sulfate aerosol advected into the Houston area, 

under southerly flow conditions from the Gulf of Mexico, can make significant 

contributions to the background aerosol in the eastern Texas region. 

Analysis and Data: Bates and Quinn–NOAA. 

Figure G8 summarizes 2006 aerosol chemical composition measurements made on the Ronald 

H. Brown in the Houston area. The onshore southerly flow of background aerosol (low radon 

concentrations indicating no contact with land for several days) was substantially impacted by 

Saharan dust and what appear to be ship emissions (acidic sulfate and nitrate). Mean (median) 

mass concentrations of the total submicrometer and supermicrometer aerosol were 6.5 (4.6) μg 

m-3 and 17.2 (8.7) μg m-3, respectively. These mass loadings of “background” aerosol are much 

higher than typically observed in the marine atmosphere, and are large enough to substantially 

impact the PM loading in the Houston-Galveston area. The integrated PM2.5 mass at ambient 

relative humidity (Figure G9) includes the accumulation mode (primarily acidic sulfate and dust 

under southerly flow conditions, seen in Figure G8) and part of the coarse mode (primarily sea 

salt, dust, and the acidic nitrate and sulfate absorbed by these basic components seen in Figure 

G8). The average PM2.5 mass advecting into the Houston-Galveston area during TexAQS 2006 

was 19 ± 11 μg m-3 (Figure G9). Aerosol composition and mass size distributions during 

northerly flow conditions are shown in Figures G8 and G9 for comparison with the 

“background” southerly flow. Air quality forecast models need to include ship emissions and 

dust transport to correctly characterize aerosol loadings in SE Texas. Compliance with PM2.5

regulations in the Houston-Galveston area may require stricter controls on upwind aerosol 

sources (e.g. ship emissions). 

POM

(NH4)HSO4

Nitrate

EC

Seasalt

Dust

Northerly Flow

20.8 μg m-3

36%

51%

8%

POM

(NH4)HSO4

Nitrate

Seasalt

Dust

Northerly Flow

7.4 μg m-3

9% 30%

11%

37%

13%

POM

(NH4)HSO4

NitrateECSeasalt

Dust

Southerly Flow

Gulf of Mexico

6.5 μg m-3

61%

30%

6%

POM

(NH4)HSO4

Nitrate

Seasalt

Dust

Southerly Flow

Gulf of Mexico

17.2 μg m-3

38%

33%

15%

7%

6%

Figure G8. Average 

submicrometer (top) and 

supermicrometer (bottom) 

aerosol composition in the 

marine boundary layer, 

measured (at 60% RH) from 

the Ronald H. Brown in 

TexAQS 2006 during 

periods of southerly and 

northerly flow. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 92 31 August 2007 

70

60

50

40

30

20

10

0

dM / dlogDp (ambient RH)

3 4 5 6 7 8 9

102 2 3 4 5 6 7 8 9103 2 3 4 5 6 7 8 9104

Dp (nm)

 Southerly flow

 Northerly flow

Integrated PM 2.5 Mass

 Southerly flow 19 ± 11 μg/m3

 Northerly flow 34 ± 20 μg/m3

Figure G9. Average aerosol mass size distributions at ambient relative humidity measured from the 

Ronald H. Brown during TexAQS 2006. 

Finding G6: Nighttime chemistry influences the availability of oxides of nitrogen (NOx), 

highly reactive VOC (HRVOC), and O3. 

Analysis and Data: Brown et al.-NOAA. 

Nocturnal measurements of key nitrogen oxide species, nitrate radical (NO3) and N2O5, were 

made on the Ronald H. Brown and WP-3D. These compounds are important to regional air 

quality because their formation and subsequent reactions affect the nocturnal loss rates for NOx, 

O3, and HRVOC. Analysis of the measured NO3 and N2O5 led to the following conclusions. 

• Hydrolysis of N2O5, typically the most important reaction in the nocturnal conversion of 

NOx to HNO3, was generally inefficient in air masses sampled aloft around Houston and 

elsewhere in Texas. As a result, NOx emissions occurring late in the day or at night could 

be transported overnight in the form of N2O5 to regions distant from the NOx source 

regions. Transport of O3 was also efficient since N2O5 is a reservoir for odd oxygen (Ox) 

as well. This transport may affect O3 concentrations well downwind of major urban and 

industrial areas on the following day (Figure G10). 

• The reduced rate of N2O5 hydrolysis aloft in Houston, and the relatively warm 

temperatures in this area, enhanced the availability of NO3 as an oxidant. This was 

important in plumes containing both NOx and HRVOC from industrial sources in 

Houston. Observed nocturnal loss rates for HRVOC due to reaction with NO3 from 

plumes originating in the Houston Ship channel were 0.5 – 4 ppbv hr–1 (Figure G11). 

• Surface measurements of N2O5 from the RHB and vertical profiling to low altitude from 

the WP-3D indicated more rapid loss rates for N2O5 near the surface than aloft, possibly 

arising from concentration of sinks for NO3 and N2O5 from emissions within the shallow 

nocturnal boundary layer. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 93 31 August 2007 

Figure G11. Total VOC loss rates due to reaction with NO3 measured from the WP-3D on 11-12 

October 2006. The flight track in the map is color-coded by NO3 mixing ratio; these values are among 

the largest ever observed. The pie chart insets show the relative contributions to NO3 loss on this 

flight and the relative contribution of NO3 loss to anthropogenic VOC, which is dominated by the 

reaction with alkenes. Apparent "Biogenic" NO3 losses on this chart are dominated by isoprene from 

anthropogenic sources. 

Figure G10. Fraction of NOx

[F(NOx) = (NO3 + 2N2O5)/(NO2 + 

NO3 + 2N2O5)] present in the form of 

N2O5 and predicted to be transported 

overnight from the Parish power 

plant near Houston to northeast 

Texas, based on the observed N2O5

loss rates within the plume sampled 

in the Houston area and a forward 

overnight meteorological trajectory. 

The WP-3D flight track is color 

coded by F(NOx) and the forward 

trajectory is color and size coded by 

the predicted F(NOx) during 

transport. Sunrise for the trajectories 

occurs over east central Texas where 

F(NOx) quickly drops to zero, 

indicating re-conversion of N2O5 to 

NOx and O3 in a rural area. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 94 31 August 2007 

Finding G7: Low rural nighttime ozone concentrations have been observed at some, but 

not all, rural locations in northeast Texas; these low nighttime ozone concentrations are not 

replicated in the regulatory modeling. 

Analysis: Sullivan et al.-U. Texas; Data: TCEQ. 

Photochemical models fail to reproduce the low nighttime ozone concentrations observed at 

some rural sites in northeast Texas (Figure G12). Possible causes of this discrepancy include the 

presence of: 1) shallower nighttime boundary layers than predicted by the model at the affected 

sites, and 2) larger local NO emissions than included in emission inventories. If the latter is the 

cause, then corrections to the emissions inventory are needed to accurately assess NOx

concentrations and atmospheric chemistry upwind of northeast Texas cities and the DFW area. 

Compressors at well-heads and on pipelines emit NOx, and are potential sources of local NO 

emissions that may be underestimated in inventories. The map in Figure G12 shows that 

northeast Texas is a region of intense natural gas exploitation. Four other monitors in that area 

also consistently show lower than expected nighttime O3 concentrations. 

KEY CITATIONS AND INFORMATION AND DATA SOURCES 

Nielsen-Gammon, J.W., J. Tobin, A. McNeel, and G. Li. 2005. A Conceptual Model for EightHour Ozone Exceedances in Houston, Texas - Part I: Background Ozone Levels in Eastern 

Texas. HARC Report No. H012.2004.8HRA, January 29, 2005. 

http://www.harc.edu/Projects/AirQuality/Projects/Projects/H012.2004.8HRA. 

Figure G12. Time series of ozone concentrations for 19-22 August 1999, an episode included in 

the DFW SIP modeling (left). Ambient data at the Cypress River site in northeast Texas are 

compared with modeling results. The map shows natural gas well regions color-coded for 

production (right), and the arrow locates the Cypress River site. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 95 31 August 2007 

Response to Question H 

QUESTION H 

Which areas within Texas adversely affect the air quality of non-attainment areas in 

Texas? 

Which areas outside of Texas adversely affect the air quality of non-attainment areas in 

Texas? 

BACKGROUND 

Question H is closely related to Question G. Here Question H focuses on the source-receptor 

relationships that determine the background concentrations of ozone and aerosol in Texas and 

the meteorologically driven transport processes. Question G focuses on characterizing the 

background concentrations and the chemical and physical processes that affect those background 

concentrations. 

FINDINGS 

Finding H1: Ozone can be transported into the Dallas area from the Houston area. 

Analysis and Data: Senff et al.-NOAA. 

An example of direct transport of ozone from Houston to Dallas was observed during TexAQS 

2006 and is depicted in Figures H1 and H2. During 4–8 September 2006, there was a regional 

buildup of background ozone in eastern Texas, indicated by Dallas-Fort Worth (DFW) surface 

station ozone monitors (not shown) and airborne ozone lidar measurements (Figure H1). From 

4–7 September, large-scale winds tended to be northerly to easterly. While the ozone was 

building up in Dallas, Houston also experienced a daily increase in ozone, resulting in 8-hr ozone 

averages up to 110 ppbv in Houston on 7 September (not shown). Between 7 and 8 September, a 

shift in the position of a synoptic-scale high caused the transport winds to Dallas to change from 

a weak northeasterly component to a stronger southerly component. This major shift in transport 

had a strong impact on Dallas, as 24-hour forward trajectories from Houston, beginning at 3 pm 

local time, indicated transport from Houston to Dallas (Figure H2). Also, elevated overnight O3

concentrations were measured at two rural O3 stations sited for TexAQS II (Palestine and Italy). 

This transport brought additional ozone to a region that was already approaching 8-hr 

exceedance levels, resulting in 3 stations exceeding the 80 ppbv 8-hr-average O3 in the DFW 

network. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 96 31 August 2007 

Figure H1. Airborne ozone lidar measurements on 4 and 8 September 2006, showing the large 

increase in background ozone in eastern Texas between the two days. 

Figure H2. Forward trajectories 

starting at 3 pm local time in 

Houston, 7 September, and ending at 

3 pm local time on 8 September 

2006. The trajectories show direct 

transport from Houston to Dallas. 7 

September was an exceedance day in 

Houston; 8 September was an 

exceedance day in Dallas. (The 

trajectory map was created using the 

NOAA Physical Science Division 

(PSD) upper air back-trajectory tool 

[http://www.etl.noaa.gov/programs/2

006/texaqs/traj/]). 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 97 31 August 2007 

Finding H2: High ozone concentrations in eastern Texas result from both in-state sources 

and transport of continental air from the east and northeast. 

Analysis: Sullivan et al.-U. Texas. 

Ensembles of historic HYSPLIT (Draxler and Rolph, 2003; Rolph, 2003) 5-day back trajectories 

have been run from major Texas cities on high (≥ 75 ppbv 8-hr max) and low O3 days to 

characterize the upwind “dirty” and “clean” typical fetches. A residence time analysis of the 

“dirty” fetch trajectories yields high O3 air residence maps for northeast Texas, as exemplified in 

Figure H3. These maps (particularly the right one) show that transport of air from the east and 

northeast support many high O3 episodes in August through October, but this analysis also shows 

that on high O3 days in eastern Texas, the upwind air is resident within the state for several days, 

potentially building up background ozone from in-state source emissions. 

Using a new statistical analysis tool in HYSPLIT, an ensemble of back-trajectories can be 

categorized into clusters of similar behavior. Back-trajectories from East Texas from August and 

September 1997–2006 cluster into 5 categories: “short fetch” within Texas, Arkansas, and 

Louisiana; “E-SE” running through the southeastern US and the northern Gulf of Mexico; “SSE” running through the Gulf to the Yucatan; “NE” running to the Midwest; and “north” running 

through the north-central US. An assessment of back-trajectory clusters by year compared with 

regional O3 levels for August and September 1997–2006 shows that the “NE” cluster is best 

associated with high regional ozone, with the “short fetch” having the second highest correlation. 

There is some evidence that the year-to-year variation in the distribution of back-trajectory 

clusters helps explain the relative severity of O3 seasons. 

Figure H3. Contouring of HYSPLIT 500 meter 5-day back-trajectories on moderate and worse O3

days in San Antonio (left) and Tyler/Longview/Marshall (right), color coded by density of end 

point frequency. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 98 31 August 2007 

Finding H3: A synthesis of satellite and in situ measurements with photochemical 

modeling and Lagrangian trajectory analyses provides a quantification of regional 

influences and distant sources on Houston and Dallas air quality during TexAQS 2006. 

Analysis: Pierce et al.–NASA; Data: MODIS, AIRS, CALIPSO, and TES teams–NASA, 

NOAA/NESDIS, UMBC. 

Realtime Air Quality Modeling System (RAQMS) (http://rossby.larc.nasa.gov/RAQMS/

accessed October 2006) chemical analyses provide a means to quantify regional influences and 

distant sources on local air quality in Texas. Total column ozone from the Ozone Monitoring 

Instrument (OMI) and ozone and carbon monoxide profiles from the Tropospheric Emission 

Spectrometer (TES) onboard the NASA Aura satellite constrain these analyses, and fire counts 

from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard the 

NASA Terra and Aqua satellites are utilized to generate biomass burning emissions. The fidelity 

of the RAQMS ozone analysis for the TexAQS 2006 period was assessed with independent 

satellite observations and aircraft data. The resulting bias-corrected RAQMS chemical analyses 

were used to provide estimates of background composition along ensemble back trajectories 

initialized daily at 18Z from surface EPA AIRNow ozone monitoring stations within the 

Houston and Dallas metropolitan statistical areas (MSA). Lagrangian averaged ozone 

production minus loss (O3 P-L) rates along the back trajectories were used as a metric to classify 

back trajectories. The Lagrangian averages were computed during time periods when the back 

trajectories were outside the respective MSA, defined as more than 2o

 in longitude or latitude 

away from central Houston or Dallas. Three trajectory classes were considered: Class 1 (O3 P-L 

> 10ppbv/day); Class 2 (0 < O3 P-L < 10ppbv/day); and Class 3 (O3 P-L < 0ppbv/day). 

A time series depiction of the RAQMS back-trajectory analysis of regional influences on 

Houston and Dallas O3 is shown in the upper panels of Figure H4. The red line shows the 

observed mean and variability of surface O3 measurements in the urban area at 18 UTC extracted 

from EPA’s AIRNow data system (http://www.airnow.gov/ accessed October 2006). The blue 

lines show the RAQMS chemical analysis with the solid blue for the AIRNow mean (biascorrected), and the dashed blue for the background mean (bias-corrected) immediately prior to 

entering the urban area. The color bar along the upper part of each time series indicates the 

regional influence classification code for the modeled extent of O3 production and loss upwind 

(see key). The Lagrangian analysis shows that periods of enhanced (P-L >10ppbv) regional O3

production preceded 6 out of 9 Houston periods and 7 out of 15 Dallas periods with elevated O3

(greater than 60 ppbv) during the 15 July – 15 October 2006 TexAQS II period. Maps of source 

regions for days with enhanced regional O3 production are shown in the lower panels of Figure 

H4. Houston enhanced regional O3 production events have a Midwest/Ohio River Valley source 

with significant O3 P-L (40ppbv/ day) due to NOx sources along the southern Great Lakes. 

Dallas enhanced regional O3 production events have a broad Great Plains/Midwest/Ohio River 

Valley source with significant O3 P-L (30ppbv/ day) due to Chicago NOx sources along the 

southern Great Lakes. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 99 31 August 2007 

Case studies were conducted to investigate the processes responsible for enhanced regional O3

production rates during the TexAQS II period. One such case is early September 2006. Twelveday boundary layer back trajectories were initialized at locations where the High Spectral 

Resolution Lidar (HSRL) instrument (used to look down from the NASA King Air airplane to 

=Net O3 Loss 

=0< O3 P-L <10 ppb/day 

= O3 P-L >10 ppb/day

Trajectory Classification 

Figure H4. Lagrangian analysis of 

regional influences on 

Houston/Dallas O3. Left, time series 

of Houston and Dallas observed, 

analyzed, and background ozone. 

Color bar along upper part of each 

time series indicates regional 

influence trajectory classification. 

Below, maps of Houston and Dallas 

Class 1 (see text) NOy source 

contributions (ppbv/day). Red=AIRNow MSA mean+Std 

Blue=RAQMS MSA (solid) Regional (dashed) bias corrected mean 

Houston Dallas
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 100 31 August 2007 

assess the aerosol loading of the vertical column of air below) observed enhanced aerosol optical 

depths (AOD) indicating the presence of elevated particulate matter concentrations in the vicinity 

of Houston on 4 September 2006. Based on calculations of regional 24-hour rolling averages of 

TCEQ’s hourly PM2.5 concentrations, 4 September was one of only three days in the TexAQS 

2006 period with widespread rolling 24-hour concentrations above 20 μg m-3. In addition, a 

speciation monitoring site in Deer Park showed elevated sulfate and carbon material for that 

date. The back trajectories link the local 4 September HSRL measurements to satellite column 

and profile measurements from various sensors on low-earth orbit satellites (see Table H1 and 

Figure H5) on 23 August. 

The 12-day back trajectories indicated two primary source regions: southern Canada and the 

eastern US. The MODIS sensor and AIRS sensor show column AOD and carbon monoxide 

(CO) enhancements associated with Pacific Northwest wild fire emissions within the Canadian 

branch, and CO enhancements associated within the eastern US branch of the back trajectories. 

CALIPSO attenuated aerosol backscatter cross-sections through the central US show an elevated 

aerosol layer associated with wild fire emissions and boundary layer aerosol enhancements over 

the eastern US. TES CO vertical cross-sections, which follow the same orbit as CALIPSO, show 

both lower and upper tropospheric CO enhancements. This case study illustrates the influence of 

remote emissions from the southeastern US and Pacific Northwest on Houston air quality. This 

analysis underscores the importance of integrating satellite, aircraft, and surface measurements 

of aerosol and trace gases in conjunction with advanced modeling techniques, for characterizing 

the impact of emissions from remote sources on local Texas air quality. Additional case studies 

are presented on the TCEQ website (http://www.tceq.state.tx.us), and a detailed case study is 

available from the 29-31 May Principal Findings Data Analysis Workshop for a 20 July 2004 O3

and PM2.5 episode in the Dallas area. 

Table H1. Satellite sensors and resulting data products utilized for 4 September 2006 case study. 

NASA Sensor Product 

MODIS - Moderate Resolution Imaging Spectro-radiometer Aerosol optical depth 

CALIPSO - Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation Aerosol optical depth profile 

TES - Tropospheric Emission Spectrometer Carbon monoxide profile estimate 

AIRS - Atmospheric Infrared Sounder Carbon monoxide column 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 101 31 August 2007 

Yellow here at 30 deg. latitude is suspected sulfate from eastern U.S. 

Here, aloft at 45 deg. latitude, is suspected Canadian fire aerosol. 

Here also at 45-50 deg. latitude is elevated CO, believed to be from fire. 

Figure H5. 

MODIS and 

AIRS satellite 

sensor 

observations, 

with CALIPSO 

and TES orbits 

given by white 

lines. Center 

CALIPSO/TES 

ground track in 

two top panels 

represents 

8/23/06 orbit, 

with colorcoding for AOD 

(left) and CO 

(right). 

Bottom two 

panels show 

corresponding 

vertical profiles 

along the center 

ground tracks, 

with x-axis 

roughly the 

north-south 

direction.
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 102 31 August 2007 

Finding H4: In the Dallas area, local emissions and transport each contributed about 

equally to the average 8-hr ozone exceedance in 2002. Transported ozone alone can bring 

the Dallas area close to exceeding the 8-hour ozone standard. 

Analysis: Kemball-Cook et al. –ENVIRON; Parrish et al.-NOAA. Data: Ryerson et al.–NOAA. 

Transport contributions to Dallas area O3 were quantified for June–September 2002 using the 

CAMx photochemical model with emissions from EPA’s 2002 National Emissions Inventory 

(NEI) (updated by the Central Regional Air Planning (CENRAP) consortium) and meteorology 

from the MM5 model (Kemball-Cook et al., 2006). The Dallas area had 35 days in 2002 with 

monitored 8-hour ozone levels of 85 ppbv or higher (Figure H6). Averaged over these days, 

Dallas area emissions contributed about 48 ppbv and other sources 54 ppbv to the total modeled 

ozone 8-hour maximum of 102 ppbv on the average exceedance day. The modeled average 

transport contribution from other parts of Texas was 6 ppbv, and there were days when northeast 

Texas, Houston/Beaumont, south Texas, and central Texas individually contributed 9 ppbv or 

more. The average modeled transport contribution from other US states was 28 ppbv, and the 

largest contributing states were Louisiana, Arkansas, Mississippi, Alabama, and Tennessee. 

These findings are consistent with back trajectory and residence time analyses. The example 

shown in Figure H7 compares CAMx O3 transport contributions to back trajectories for 7 August 

2002. The 5-day back trajectories cross NE Texas, Arkansas, and Tennessee, extending to the 

mid-Atlantic region; the CAMx modeling finds these same regions contributing to the Dallas 

area O3 exceedance at the Ft. Worth NW monitor on 7 August 2002. 

Estimates of the O3 imported into the DFW area from aircraft data are in general agreement with 

the trend from the CAMx model, although the aircraft data suggest that the transported 

background ozone contributed an even larger fraction of the ozone in the DFW region than 

suggested by the model calculations. The Electra aircraft in TexAQS 2000 and the WP-3D 

aircraft in TexAQS 2006 measured O3 upwind, across, and downwind of Dallas on two days 

during each field study. The green symbols in Figure H6 show the results from the 23 August 

and 7 September 2000 flights, and the orange symbols the results from the 13 and 25 September 

2006 flights. The peak 8-hour-average O3 for these four flights are from the monitoring data, 

and the DFW contribution is estimated from the difference between these peak values and the 

background O3 measured upwind of DFW. 23 August 2000 is the one day investigated by the 

aircraft flights that had a significant exceedance of 98 ppbv. The background ozone transported 

into Dallas was about 72 ppbv, or 73% of the total, on that day. On some days transported ozone 

can bring the Dallas area close to an 8-hour exceedance without any added contribution from 

local emissions. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 103 31 August 2007 

August 7, 2002--106 ppb at Fort Worth NW (CAMS 13), 

 95 ppb in CAMx

0

5

10

15

20

25

30

35

40

DFW NNA

North

Texas

Arkansas

Alabama

Tennessee

Mid Atlantic

States

Georgia

Mississippi

Biogenics

Boundary

Conditions

Ozone Contribution (ppb)

Figure H7. Modeled contributions to one monitor’s ozone 

total concentration. Local is largest single contributor, but 

transport from upwind source areas significantly increase 

concentrations (analysis by Kemball-Cook et al., 2006). 

Trajectories from 3 different 

altitudes over Ft. Worth to 

characterize upwind areas for 

the column of air 2 pm CST on 

7 August 2002. 

Figure H6. Measured peak 8-houraverage O3 in the DFW area as a 

function of the DFW contribution to 

that peak. The DFW contribution is 

derived from the difference between 

the measured peak from the regional 

monitoring network, and the 

background ozone transported into 

the DFW region. The small symbols 

are from the model results from the 

CAMx model for each day 1 June -

30 September 2002. The larger 

symbols are from aircraft transects 

upwind and across the DFW region 

on four days in 2000 and 2006. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 104 31 August 2007 

Finding H5: In the Houston area, local emissions and transport each contributed about 

equally to the average 8-hr ozone exceedances investigated by aircraft flights in 2000 and 

2006. As in Dallas, transported ozone alone can bring the Houston area close to exceeding 

the 8-hour ozone standard. 

Analysis Parrish et al.-NOAA; Data: Ryerson et al.–NOAA. 

Houston area emissions contributed about 48 ppbv and transported ozone about 59 ppbv to the 

average total ozone 8-hour maximum of 107 ppb on the seven exceedance days investigated by 

aircraft measurements during 2000 and 2006. Figure H8 shows the apportionment of the 

measured maximum 8-hour-average ozone for all days characterized by aircraft measurements. 

The linear least-square fits give slopes near unity for the separate years, and for the total data 

sets, which indicates that the local and transported background contributions are largely 

independent of each other. The background contribution was similar between the two years, but 

the local contribution was considerably larger in 2000 than 2006. The transported background 

ozone reached a maximum of 70 ppbv, which can lead to an exceedance with a relatively small 

additional contribution from local sources. 

140

120

100

80

60

40

20

0

HGB Maximum 8-hr Average O 3 (ppbv)

0 20 40 60 80

Transported Ozone Background (ppbv)

y = (0.98 ± 0.19)x + 28 ± 10 r 2 = 0.80 

y = (0.77 ± 0.66)x + 57 ± 35 r 2= 0.19

 y = (1.00 ± 0.30)x + 35 ± 16 

R

2

 = 0.42

0 20 40 60 80

HGB Ozone Contribution (ppbv)

 y = (1.00 ± 0.22)x + 50 ± 9 

R

2

 = 0.58

 y = 0.91x +51 r 2 = 0.17 

y = 0.92x +56 r 2

 = 0.69

HGB

TexAQS 2006

TexAQS 2000

Figure H6. Measured peak 8-hour-average O3 in the HGB area as a function of the background 

ozone transported into the region (left) and the local HGB contribution to that peak (right). The 

background ozone was determined from aircraft transects upwind and across the HGB region on 

seventeen days in 2000 and 2006. The HGB contribution is derived from the difference between 

the measured peak and the background. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 105 31 August 2007 

KEY CITATIONS AND INFORMATION AND DATA SOURCES 

Draxler, R.R. and Rolph, G.D. 2003. HYSPLIT (HYbrid Single-Particle Lagrangian Integrated 

Trajectory) Model access via NOAA ARL READY Website 

(http://www.arl.noaa.gov/ready/hysplit4.html). NOAA Air Resources Laboratory, Silver 

Spring, MD. 

Kemball-Cook, S., M. Jimenez, and G. Yarwood. 2006. Seasonal Model of Northeast Texas 

Ozone for Summer 2002. http://www.netac.org/0405closeout/Task_6.1.pdf (accessed July 6, 

2007). 

Rolph, G.D. 2003. Real-time Environmental Applications and Display sYstem (READY) 

Website (http://www.arl.noaa.gov/ready/hysplit4.html). NOAA Air Resources Laboratory, 

Silver Spring, MD. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 106 31 August 2007 

Response to Question I 

QUESTION I 

Why does the SAPRC chemical mechanism give different results than the carbon bond 

(CB-IV) mechanism? 

Which replicates the actual chemistry better? 

BACKGROUND 

Gridded, regional photochemical models, used in developing State Implementation Plans (SIPs), 

use simplified photochemical reaction mechanisms. The two mechanisms that are most 

commonly used are the [California] Statewide Air Pollution Research Center (SAPRC) 

mechanism and the Carbon Bond (CB) mechanism. Both mechanisms are approved for use by 

the US EPA and are updated periodically to incorporate new experimental findings. For most 

urban areas, the CB mechanism, version IV (CB-IV) and the 1999 SAPRC (SAPRC99) 

mechanism yield similar results, but for conditions found in Houston, the SAPRC99 mechanism 

leads to concentrations of ozone that are up to 30-50 ppbv greater than in CB-IV and is more 

sensitive to reductions in NOx emissions, especially on days with high predicted ozone 

concentrations (Figure I1). These differences in the sensitivity of chemical mechanisms to 

emission reductions could have significant consequences for determining the magnitude of 

decreases in emissions of ozone precursors that will be required to demonstrate attainment of the 

current 8-hour, 80 ppbv NAAQS for ozone. 

Figure I1. Predictions of domain-wide maximum O3 concentrations in Comprehensive Air Quality 

Model, with extensions (CAMx) on 30 August 2000. (Faraji et al., 2007a) 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 107 31 August 2007 

FINDINGS 

Finding I1: Air quality modeling for both 2000 and 2006 shows substantial differences in 

ozone concentrations predicted by the SAPRC99 and CB-IV chemical mechanisms. 

Analysis: Faraji et al. (2007a,b); Byun-U. Houston. 

Simulations performed using multiple regional photochemical modeling packages 

(Comprehensive Air Quality Model, with extensions [CAMx] and Community Multi-scale Air 

Quality model [CMAQ]), and multiple emissions preprocessing systems (Sparse Matrix Operator 

Kernel Emissions [SMOKE] and Emissions Preprocessing System version 2 [EPS2]) for 2000 

and for 2006 predicted higher ozone concentrations when the SAPRC99 mechanism was used 

than when the CB-IV mechanism was used. The differences in predicted ozone concentrations 

were greatest when predicted ozone concentrations were high. 

Finding I2: In regions with very high VOC reactivity and high NOx emission density, 

differences in ozone formation and accumulation predicted by regional photochemical 

models using the SAPRC99 and CB-IV mechanisms are due to differences in: (1) the 

chemistry of aromatics, especially mono-substituted aromatics (e.g., toluene), (2) nitric acid 

formation rates, and (3) the rates of free radical source terms in the SAPRC and CB-IV 

mechanisms. 

Analysis: Faraji et al., 2007a. 

In reactions of mono-substituted aromatics, the CB-IV mechanism predicts a higher proportion 

of ring-retaining products, such as cresols, than SAPRC99. These ring-retaining products are 

less reactive than the ring opening products. The CB-IV mechanism also predicts: (1) more 

extensive nitric acid formation, (2) less extensive formation of peroxyacetyl nitrate (PAN), and 

(3) decreased formation of free radicals than the SAPRC99 mechanism. The SAPRC mechanism 

also has several source reactions for higher aldehydes that are not present in CB-IV. The 

enhanced free radical production in SAPRC99, as compared to CB-IV, leads to accumulation of 

higher concentrations of radicals. 

Finding I3: The differences in the predictions of the SAPRC99 and CB-IV mechanisms 

can be probed using simulations of model compounds and comparisons of the simulations 

to environmental chamber data. For simulations involving CO and NOx (inorganic 

chemistry), the predictions of the CB-IV and SAPRC99 mechanisms that are used in 

regional photochemical models (with no chamber wall corrections) converge if the rate 

constants for the OH + NO2 reaction are made consistent between the two mechanisms. 

The predictions of the Carbon Bond mechanism, version 5 (CB05) also converge to the 

same predicted values if the OH + NO2 rate constant is made consistent with the other 

mechanisms. 

Analysis: Faraji et al., 2007b. 

Equating the rate constant for the OH + NO2 reaction in CB-IV, CB05, and SAPRC99 

mechanisms, in the form in which they are used in regional photochemical modeling, led to 

convergence in the concentrations of O3, OH, HO2, NO, and NO2. This suggests that in 

photochemical modeling simulations, the major differences in the inorganic chemistry between 

the mechanisms are due to differences in the rate parameters of the OH + NO2 reaction. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 108 31 August 2007 

However, CO - NOx experiments may not be sensitive to all aspects of inorganic mechanisms 

that affect simulations under ambient conditions. 

Finding I4: The performance of the mechanisms in simulating olefins chemistry can be 

improved through more explicit representation of internal olefin chemistry, which has been 

added in CB05. In addition, performance of the SAPRC99 mechanism in simulating 

chamber data was improved for some experiments when propene was modeled explicitly, 

as opposed to being represented by a lumped chemical species. 

Finding I5: For high reactivity chamber experiments involving olefins, sensitivity analyses 

indicated that mechanism adjustments that would lead to increased radical concentrations 

(increasing the radical yield in olefin-ozone reactions and changing the OH+NO2

termination rate constant) had little impact on predicted ozone concentrations. 

Finding I6: The SAPRC99 mechanism performed better than the CB mechanisms in 

simulating some chamber experiments with toluene; the mechanism performances were 

more comparable for xylenes and other multiply substituted aromatics. 

Finding I7: The differences between the SAPRC and CB-IV mechanism predictions for 

toluene chemistry decrease substantially if the yield for the lumped species CRES, and rate 

constant for the OH + NO2, are made consistent between the two mechanisms. The 

predictions of the CB05 mechanism also converge to the same predicted values if the CRES 

yield and the OH + NO2 rate constant are made consistent with the other mechanisms. 

Analysis and Data: Faraji et al., 2007b. 

The lumped group of chemical species, CRES, as used in the photochemical reaction 

mechanisms, is often referred to as cresol, and comparisons between chamber experiments 

involving aromatics and observed cresol concentrations are generally better for the SAPRC 

mechanism than for the CB mechanisms. However, in the CB mechanism, the CRES species is 

used to represent both cresol yields and other products (such as unsaturated dicarbonyls) that 

react rapidly with the nitrate radical. Therefore direct comparisons of CRES yields with cresol 

concentrations for the CB mechanisms are not appropriate. It is clear that there are substantial 

differences in predictions of aromatics chemistry between the CB and SAPRC mechanisms, that 

the toluene mechanisms all need to be revised to improve their consistency with chamber data, 

and that much still remains unresolved about aromatics chemistry. 

Finding I8: For simulations of ambient surrogate mixture experiments in the UCR EPA 

chamber, all mechanisms underpredict O3 at low ROG/NOx ratios, with the bias decreasing 

as the ROG/NOx ratio increases. In simulations of mixture experiments in chambers 

without aromatics, CB05 performs the best, and with no dependence of bias on the 

ROG/NOx ratio. 

Analysis and Data: Faraji et al., 2007b. 

Figure I2 shows the results of comparisons of modeled simulations of environmental chamber 

experiments performed at University of California, Riverside (UCR). The performance of 

SAPRC99 in simulating the UCR EPA mixture experiments (both with and without aromatics) 

improved considerably if compounds are modeled explicitly, rather than in lumped groups. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 109 31 August 2007 

Full Surrogate - Arc Lights Full Surrogate - Blacklights

ROG/NOx Ratio ROG/NOx Ratio

Non-Aromatic Surrogate - Blacklights

ROG/NOx Ratio

Δ([O3]-[NO]) Model Error

Note: Full surrogate experiments include experiments 

with formaldehyde removed.

-90%

-75%

-60%

-45%

-30%

-15%

0%

15%

30%

1 10 100 1000

-90%

-75%

-60%

-45%

-30%

-15%

0%

15%

30%

1 10 100

-90%

-75%

-60%

-45%

-30%

-15%

0%

15%

30%

1 10 100 1000

Detailed SAPRC-99

CAMx SAPRC-99

CB05

CB4

Figure I2. Comparison of CB-IV, CB05, and SAPRC99 model errors for UCR EPA surrogate runs. 

(Faraji et al., 2007b) 

Findings I3-I8 were arrived at by analyzing model compound simulations and environmental 

chamber data. These six findings are consistent with Findings I1 and I2, which are based on 

sensitivity analyses performed using the regional photochemical models. Additional analyses 

were performed using ambient data from Houston collected during a stagnation event with high 

olefin concentrations. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 110 31 August 2007 

Finding I9: SAPRC99, CB-IV, and CB05 all successfully predicted concentrations of ozone 

and other species during simulation of a stagnation event, if the chemical mechanisms were 

initialized after initial high concentrations of olefins had reacted. None of the mechanisms 

successfully predicted a rapid rise in radical concentrations and ozone concentration 

concurrent with initially high C2, C3, and CMBO concentrations during the event. 

Analysis and Data: Faraji et al., 2007b. 

Analyses have been performed on ambient data collected during TexAQS 2000 at the heavily 

instrumented LaPorte site (Faraji et al., 2007b). Specifically, a period of stagnation at the 

LaPorte airport was simulated. This stagnation period had high concentrations of C2 and C3 

alkanes and alkenes, as well as high concentrations of a marker of chlorine chemistry (CMBO – 

1-chloro-3-methyl-3butene-2-one) that lasted for approximately one hour. The data suggest that 

a source of radicals, not accounted for in the current mechanisms was present, but no definitive 

source of the radicals was identified. 

RECOMMENDATIONS 

Model compound analyses and chamber data, as well as sensitivity analyses in CAMx, indicate 

that major causes for the differences in SAPRC and CB mechanisms under conditions 

encountered in eastern Texas are the chemistry of aromatics, and to a lesser extent, radical 

termination rates. In choosing a mechanism that will be most effective for eastern Texas, several 

guiding principles emerge. 

1. Over the past several decades, comprehensive updates to the CB and SAPRC chemical 

mechanisms have been released roughly every 5-10 years. These updates incorporate 

changes to the mechanisms that reflect evolving knowledge about rate constants and key 

chemical pathways. For example, the rate parameters for the critical OH + NO2 rate 

constant have been adjusted in CB05, as compared to CB-IV, and in SAPRC07, as 

compared to SAPRC99. In addition, the updates often provide explicit representations of 

chemical species that have recently been identified as being especially important. For 

example, explicit isoprene chemistry was incorporated into both SAPRC and CB 

mechanisms in the 1990s and the newly released CB05 incorporates internal olefins as an 

explicit compound class. These updates generally reflect more current knowledge, and, 

as a guiding principle, should be used when possible. 

2. Recognizing that it may be difficult to unambiguously define the most appropriate 

mechanism for use in Houston, the effectiveness of proposed control strategies should be 

evaluated using multiple chemical mechanisms. 

3. As new comprehensive updates to the CB and SAPRC mechanisms are developed, more 

explicit representation of both key chemical pathways (e.g., the low NOx routes for 

aromatics) and key chemical species (propene, toluene, and possibly other aromatics) 

would likely make the mechanisms more useful in analyzing the complex industrial 

emission sources in the Houston area. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 111 31 August 2007 

KEY CITATIONS AND INFORMATION AND DATA SOURCES 

Carter, W.P.L. 2004. Evaluation of a Gas Phase Atmospheric Reaction Mechanism for Low NOx

Conditions. Final Report to the California Air Resources Board, Contract No. 01-305. 

http://pah.cert.ucr.edu/~carter/bycarter.htm. 

Faraji, M., Y. Kimura, E. McDonald-Buller, and D. Allen. 2007a. Comparison of the Carbon 

Bond and SAPRC photochemical mechanisms under conditions relevant to southeast Texas. 

Atmospheric Environment, in press. 

Faraji, M., G. Heo, Y. Kimura, E. McDonald-Buller, D. Allen, G. Yarwood, G. Whitten, and W. 

Carter. 2007b. Comparison of the Carbon Bond and SAPRC photochemical mechanisms. 

Report to the Texas Commission on Environmental Quality, August, 2007. 

Yarwood, G., S. Rao, M. Yocke, and G.Z. Whitten. 2005. Updates to the Carbon Bond 

Mechanism: CB05. Report to the U.S. Environmental Protection Agency (December). 

www.camx.com/publ/pdfs/CB05_Final_Report_120805.pdf. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

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Response to Question J 

QUESTION J 

How well do air quality forecast models predict the observed ozone and aerosol formation? 

What are the implications for improvement of ozone forecasts? 

BACKGROUND 

An assessment of seven air quality forecast models 

(AQFMs) operating in real time during TexAQS 2006 

focused on skill at predicting maximum 8-hour-average 

O3 at 119 sites and 24-hr-average PM2.5 levels at 38 sites 

in eastern Texas, western Louisiana, Arkansas and 

Oklahoma. These models included: the NCEP CMAQWRF model (12-km horizontal resolution), two versions 

of the NOAA/ESRL WRF-Chem model (12- and 36-km 

res.), the Canadian CMC AURAMS (28-km) and 

CHRONOS (21-km res.) models, one version of the 

Baron-AMS MAQSIP-RT model (15-km res.), and the 

University of Iowa STEM model (12-km res.). Details 

on each of these modeling systems can be found in 

McKeen et al. (2005), and internet web links listed 

below. Three standard statistical parameters, Mean 

Bias, Root Mean Square Error (RMSE), and correlation 

coefficients were evaluated in this preliminary work. Skill for these three statistics is measured 

relative to a persistence forecast, or the forecast that tomorrow’s AQ levels are the same as 

today’s observed levels. In addition, bias-corrected model forecast values are calculated based 

on the mean O3 or PM2.5 bias at each site at each hour of the day, averaged over the previous 7 

days. The statistical parameters are also calculated from the ensemble of the models, and the 

ensemble of bias-corrected models. The summary statistics for the models and their ensemble 

are shown in Fig. J1 for O3 and Figure J2 for PM2.5. Also shown are the RMSE for persistence 

and climatology forecasts, and correlation coefficients for persistence. Categorical statistics 

(measures of how well a model predicts an event) related to the predictions of daily maximum 8-

hour-average O3 levels exceeding the 85 ppbv regulatory threshold are also summarized for the 

seven models, their mean ensemble, and with two bias-correction options (Figures J3 and J4). 

The four categorical statistics evaluated in this work were: Probability of Detection, False Alarm 

Rate, Critical Success Index, and Bias Ratio. 

The forecasts of roughly 25 chemical, aerosol, meteorological, and radiation variables from the 

seven models have been compared with data collected on board the WP-3D aircraft for 

individual transects and vertical profiles on a flight-by-flight basis for the first 12 flight legs of 

the aircraft deployment. These comparisons are available for viewing at the publicly available 

NOAA web page, http://www.esrl.noaa.gov/csd/2006/modeleval/. Concentrations and fluxes of 

key atmospheric constituents upwind and downwind of Houston and Dallas have also been 

compared, allowing assessments of model reliability in terms of emissions inventories, and 

efficiencies of O3 and PM2.5 formation from these large urban sources. 

The University of Houston also made nine AQ forecasts using three model resolutions and three 

emission scenarios with the MM5/CMAQ model. Detailed statistical summaries for O3 and 

Air Quality Forecast Models 

AURAMS – A Unified Regional AirQuality Modeling System 

MAQSIP-RT – Multiscale Air Quality 

Simulation Platform-Real Time 

CHRONOS – Canadian Hemispheric 

and Regional Ozone and NOx

System 

CMAQ5x/WRF – Community Multiscale Air Quality model/Weather 

Research Forecast model 

University of Iowa STEM – Sulfur 

Transport and Emissions Model 

WRF/Chem – Weather Research 

Forecast model/Chemistry version 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 113 31 August 2007 

PM2.5, for each forecast, and for each of the CAMS surface monitors can be found at the 

University of Houston web page, http://www.imaqs.uh.edu/ftp/AQF_usa/ (password protected). 

Figure J1. Summary statistics for 8-hour maximum ozone for nine air quality forecast models and 

their ensemble mean and bias-corrected ensemble mean. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 114 31 August 2007 

Figure J2. Summary statistics for 24-hour-average PM2.5 for seven air quality forecast models and 

their ensemble mean and bias-corrected ensemble mean. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 115 31 August 2007 

FINDINGS 

Finding J1a: Most forecast models exhibit skill in predicting maximum 8-hr-average O3

but none of the models is better than persistence in predicting 24-hr-average PM2.5 levels. 

Analysis: Wilczak et al.-NOAA. Model Data: McQueen et al.-NOAA/NWS, Grell et al. 

NOAA/GSD, Carmichael et al.-U. Iowa, McHenry et al.-Baron AMS, Gong et al.-Met. Srv. 

Canada, Bouchet et al.-Met. Srv. Canada. 

When all O3 data are considered: 

– Nearly all models beat persistence RMSE and r-coefficients without bias correction. 

– The ensemble performed better than all individual models, and the bias-corrected 

ensemble showed the best statistical performance. 

– Bias correction improved RMSE scores appreciably and r-coefficients to some degree. 

When all PM2.5 data are considered: 

– In contrast to O3, no model or ensemble, or bias correction, beat persistence. 

– All but one model was biased low, and bias correction improved r-coefficients 

significantly. 

– Since they are not included in the models, the occurrence of Saharan dust events during 

August and September influenced the bulk PM2.5 statistics. Removal of these events 

from the statistical evaluations is another required step in model analysis. 

Figure J3. Categorical statistics related to the 85 ppbv exceedance for 8-hour maximum ozone for 

seven air quality forecast models and their ensemble. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 116 31 August 2007 

Finding J1b: Seven air quality forecast models and their ensemble were generally unable 

to forecast 85 ppbv 8-hr-average O3 exceedances with any reliability. 

Analysis: McKeen et al.-NOAA. Model Data: McQueen et al.-NOAA/NWS, Grell et al. 

NOAA/GSD, Carmichael et al.-U. Iowa, McHenry et al.-Baron AMS, Gong et al.-Met. Srv. 

Canada, Bouchet et al.-Met. Srv. Canada. 

– Although the 7-model ensemble had the best overall bulk statistics, and was biased high 

by 5 ppbv, its ability to predict an exceedance event was significantly worse than 

persistence. 

– The ensemble, like most of the models, predicts far fewer events, and missed most of the 

exceedance events in the Houston area (see Figure J4). This collective model underprediction could be due to the lack of an important source component within all the 

models, such as highly reactive VOC emissions. 

– Only one model beat persistence for both probability of detection and false alarm rate. 

– Since most models and the ensemble are biased high, bias correction tended to reduce a 

model’s ability to predict an exceedance event even further. This was in sharp contrast to 

a similar categorical analysis for New England during the summer of 2004 (McKeen et 

al., 2005), where bias corrections nearly always improved the critical success index. 

Figure J4. Number of times the 85 ppbv exceedance for 8-hour maximum ozone occurred within the 

reported AIRNow observations, and the 7-model ensemble forecast. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 117 31 August 2007 

Finding J2: Sophisticated data assimilation of meteorological and even chemical 

observations is essential for improving photochemical model forecasts. 

Analysis: Nielsen-Gammon et al.-Texas A&M U., Byun et al.-U. Houston, McKeen et al.-NOAA. 

Model Data: Nielsen-Gammon et al.-Texas A&M U., Byun et al.-U. Houston, McQueen et al.-

NOAA/NWS, Grell et al. NOAA/GSD, Carmichael et al.-U. Iowa, McHenry et al.-Baron AMS, 

Gong et al.-Met. Srv. Canada, Bouchet et al.-Met. Srv. Canada. 

Spatial and temporal accuracy of AQ forecasts are to a large degree limited by the accuracy of 

the underlying meteorological forecasts within the AQ models. Most models rely on the 

available NCEP/NAM model product for initialization and boundary conditions, which may 

contribute to model biases in AQ model wind fields and pressure patterns documented 

throughout the field study. Retrospective forecasts and sensitivity studies are needed to assess 

the impact of the NAM forecasts available in the summer of 2006 to AQFMs, particularly in 

light of recent upgrades to the WRF-NMM model used in the NAM forecasts. Assimilation of 

wind profiler data (Nielsen-Gammon et al., 2006) has been shown to improve forecast 

meteorology in the Houston area. Similar research related to the assimilation of photochemical 

and aerosol data within AQFMs should be encouraged, utilizing the comprehensive data sets 

from the TexAQS 2006 field study. 

Daytime planetary boundary layer (PBL) heights from seven of the models were compared to 

those derived from wind profiler measurements at several sites in east Texas. Some models 

showed persistent bias in daytime PBL, and no model appeared to stand out in terms of 

significantly better comparisons. Deficiencies in AQ forecasts from several of the models appear 

to be related to the formulation of PBL height and vertical transport within the PBL. This is 

shown in Figure J5, which shows a wide spread in the models’ ability to simulate water vapor 

and NOy profiles compared to WP-3D aircraft data for afternoon conditions on a particular clear 

day. It is quite common for models to exhibit unrealistic high biases in O3 and PM2.5 precursors 

downwind of source regions when PBL heights are under-predicted. The analogous 

misrepresentation of the stable PBL often affects model-predicted offshore pollutant transport 

and pollution precursor buildup from emissions along the coastlines. Persistent errors in the 

forecast of the low-level nocturnal jet are also characteristic of many models. Preliminary 

sensitivity studies with the MM5/CMAQ model also have demonstrated a case of over-predicted 

O3 associated with a missed forecast of widespread precipitation (24, 25 August). The collection 

of available satellite, radar, and surface network data sets for comparing cloudiness, 

precipitation, and radiation with model output are needed to perform further evaluations of these 

parameters, and their relationships to O3 and PM2.5 forecasts. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 118 31 August 2007 

Figure J5. Profiles of water vapor, relative humidity, and NOy on 25 September 06, south and 

downwind of Dallas. Black dots indicate observations. The horizontal transect shows high biases 

of NOy for those models with the lowest PBL heights and reduced vertical transport. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 119 31 August 2007 

Finding J3: Model performance evaluations and intercomparisons require a 

comprehensive, best-guess emissions inventory for the TexAQS 2006 Field Intensive. 

Analysis: Byun et al.-U. Houston; Data: TCEQ. 

Ozone and PM2.5 forecasts are highly dependent on the emissions inventories of precursor 

emissions, and PM2.5 forecasts are also dependent on primary emissions at many of the urban and 

suburban CAMS locations. A high priority in the model evaluation effort should be placed on 

using TexAQS 2006 field data to determine the accuracy of the inventories that drive AQ 

forecasts. Ozone forecasts are particularly sensitive to emissions estimates of HRVOC, such as 

ethylene and propylene, from large petrochemical facilities, especially in the Houston ship 

channel region. Quality-assured VOC measurements from the various platforms and 

comparisons with AQ model results are currently in progress in order to evaluate the model 

emissions inventories. Two preliminary sensitivity results from the University of Houston 

MM5/CMAQ model relate directly to emissions inventory validation. That model shows 

generally improved NO2 and O3 comparisons with CAMS site data using an emission inventory 

based on projections to 2005 as opposed to an inventory based on 2000 estimates. A narrow 

plume of extremely high O3 observed downtown and west of Houston (7 September) that was 

significantly under-predicted by the original forecast was found to be replicated accurately in 

retrospective runs that included a VOC source in the ship channel region much larger than 

specified in the base emissions inventory. 

KEY CITATIONS AND INFORMATION AND DATA SOURCES 

McKeen, S., J. Wilczak, G. Grell, I. Djalalova, S. Peckham, E.-Y. Hsie, W. Gong, V. Bouchet, S. 

Menard, R. Moffet, J. McHenry, J. McQueen, Y. Tang, G.R. Carmichael, M. Pagowski, A. 

Chan, T. Dye, G. Frost, P. Lee, and R. Mathur. 2005. Assessment of an ensemble of seven 

real-time ozone forecasts over eastern North America during the summer of 2004. J. 

Geophys. Res., vol. 110, D21307, doi:10.1029/2005JD005858. 

Nielsen-Gammon, J. W., R.T. McNider, W.M. Angevine, A.B. White, and K. Knupp. 2007. 

Mesoscale model performance with assimilation of wind profiler data: Sensitivity to 

assimilation parameters and network configuration. J. Geophys. Res., vol. 112, D09119, 

doi:10.1029/2006JD007633. 

Model Acronyms and Web Links 

AURAMS (A Unified Regional Air-Quality Modeling System) 

http://www.msc-smc.ec.gc.ca/research/icartt/aurams_e.html

BAMS (Baron Advanced Meteorological Systems, Inc.) 

http://www.baronams.com/projects/SECMEP/index.html

CHRONOS (Canadian Hemispheric and Regional Ozone and NOx System) 

http://www.msc-smc.ec.gc.ca/aq_smog/chronos_e.cfm

CMAQ5x/WRF (Community Multi-scale Air Quality model/Weather Research Forecast model) 

http://wwwt.emc.ncep.noaa.gov/mmb/aq/

University of Iowa STEM (Sulfur Transport and Emissions Model) 

http://nas.cgrer.uiowa.edu/MIRAGE/mirage-2k6.html

WRF/Chem (Weather Research Forecast model/Chemistry version) 

http://www.wrf-model.org/WG11
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 120 31 August 2007 

Response to Question K 

QUESTION K 

How can observation and modeling approaches be used for determining (i) the sensitivities 

of high ozone in the HGB non-attainment area to the precursor VOC and NOx emissions, 

and (ii) the spatial/temporal variation of these sensitivities? 

BACKGROUND 

The accurate prediction of the relative response of ozone concentrations to future reductions in 

NOx and VOC emissions is a much sought, but very elusive goal. This prediction is central to 

the very important SIP-relevant question of “direction of control” – that is, should ozone control 

efforts in an ozone non-attainment area be focused on: a) decreasing emissions of NOx alone, b) 

decreasing emissions of VOC alone, or c) decreasing emissions of both NOx and VOC? In this 

report the closely related Questions F and K both deal with aspects of this issue. The 

Background Section of the Response to Question F discusses some issues relevant to this 

Question and Response; the interested reader should peruse that discussion as a prelude to this 

Response to Question K and also review the recently revised Guidance on the Use of Models and 

Other Analyses for Demonstrating Attainment of Air Quality Goals for Ozone, PM2.5, and 

Regional Haze (USEPA, 2007). One point made in the background section of the Response to 

Question F is that a definitive answer to these questions requires a thoroughly tested Eulerian 

emissions-based air quality model that treats the region of interest with sufficient accuracy. 

Another point is that this model testing should involve comparisons between model results and 

direct observations of the relationships between ozone, its precursors, and the radicals that drive 

these reactions. Unfortunately however, it has not yet been possible to develop such generally 

applicable models because of deficiencies and uncertainties in many critical areas, including 

emissions, meteorological modeling, and photochemical mechanisms. 

In the absence of a “perfected” Eulerian air quality model for the HGB non-attainment area, it 

has been necessary to take a variety of heuristic approaches to provide guidance to air quality 

managers. Heuristic implies a model that is simplified, but that is designed and used to learn 

more about some specific but important aspect of the more complex air-quality system that is to 

be managed. The Response to Question F discusses the application of both observation-based 

and emissions-based modeling approaches for approximately determining the sensitivity of 

ozone production and concludes that both VOC and NOx controls are presently effective in 

reducing the maximum observed ozone concentrations. Continued incremental reductions in 

emissions of either VOC or NOx emissions can be expected to yield incremental improvements 

in ozone design values in the HGB region. 

Here the response to Question K addresses two very important questions that are related, but 

importantly different, in that they address how compliance with the current NAAQS for ozone 

can be achieved in an attainment demonstration: 

• What emission controls will ultimately be necessary for the HGB area to achieve 

compliance with the 8-hour-average, 80 ppbv ozone standard? 

• What is the most effective strategy by which to reach this compliance? 

To address these questions, a very simple, emissions-based model was applied to the HGB 

region. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 121 31 August 2007 

FINDINGS

Finding K1: A simple, heuristic model based upon the Empirical Kinetic Modeling 

Approach (EKMA) method suggests that the HGB region may ultimately require drastic 

NOx emissions controls to reach compliance with the NAAQS for ozone. 

Finding K2: The same model suggests that in a projected future scenario with very strict 

VOC emission controls, but without drastic NOx emission controls, biogenic VOC 

emissions plus background concentrations of CO and methane may be sufficient to cause 

ozone exceedances. 

Analysis: Dimitriades-NCSU; Luecken-USEPA. 

A simplified model of an inherently complex system can sometimes provide valuable insight into 

the behavior of the complex system (Dimitriades, 2006; Dimitriades and Luecken, 2007). As a 

case in point, here we investigate a very simple, Empirical Kinetic Modeling Approach (EKMA) 

that includes an Ozone Isopleth Plotting Research Package (OZIPR) [see e.g. Finlayson-Pitts and 

Pitts, 2000]. This approach comprises a photochemical box model, and was applied to the HGB 

area in order to illuminate one aspect of the complex chemical-transport system that defines the 

near-surface atmosphere in the HGB region. The model was formulated to provide an answer to 

the very critical question: To reach compliance with the NAAQS ozone standard in HGB, what 

emission controls are necessary and most effective? 

The model utilized an advanced version of the EKMA method for translating early morning (8 

AM) ambient concentrations of VOC and NOx into the maximum 8-hour-average ozone 

concentration that accumulates by the end of the day. The model incorporates a SAPRC99

chemical mechanism and requires model and observational inputs on VOC composition, sunlight 

intensity, diurnally varying VOC (including biogenic VOC) and NOx emissions, diurnally 

varying boundary layer height, and background ozone concentration. The results presented here 

were obtained for: 

(i) A “typical” Harris County VOC mix, based on measurements at the LaPorte site during 

TexAQS 2000. 

(ii) A constant 30-ppbv concentration of background ozone. 

(iii) Time-varying post-8 AM VOC and NOx emissions based on average daily 

anthropogenic emissions reported for Harris county in 2002, apportioned as hourly 

emissions based on modeled profiles for total NOx and anthropogenic VOC emissions from 

EPA’s 2001 National Emissions Inventory processed using the SMOKE emissions 

processor (http://www.smoke-model.org). 

(iv) Time-varying post-8 AM isoprene emissions estimated from the area average isoprene 

emissions for Harris County from the HGB 2002 emissions inventory 

(http://www.tceq.state.tx.us/assets/public/implementation/air/sip/hgb/hgb_sip_2007/append

ices/06027SIP_Appendix_F_PEI.pdf). These emissions totaled 29 kg km-2day-1. 

(v) a typical time-varying mixing height based on measurements made at the Moody Tower 

site. 

The results of the model are presented in the form of a traditional EKMA diagram in Figure K1. 

For illustration purposes, the method was applied to 8 AM NOx and VOC concentrations 

measured at five sites (Clinton, Channelview, Wallisville, Lynchburg Ferry, and HRM-3) within 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 122 31 August 2007 

Harris County during April-October of 2005 and 2006. A single ozone concentration isopleth 

diagram was constructed from the calculated maximum 8-hour-average ozone concentrations 

that accumulated in the model initiated with those 8 AM measured concentrations. 

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

8 AM anthropogenic VOC (ppmC)

g pp p p

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

8 AM NOx (ppm)

A

B C

C

Figure K1. EKMA diagram constructed for the HGB region. The contour lines give the maximum 8-

hour-average ozone concentration (in ppbv) that accumulated starting with the morning VOC and 

NOx concentrations given on the respective axes. The critical, dashed line isopleth represents the 

maximum ozone concentration allowed in an area in compliance with the NAAQS. 

Figure K1 suggests that the ozone sensitivity conditions within Harris County vary from VOCsensitive to NOx-sensitive from site to site and from day to day. For example, consider the 

points labeled A, B, and C in the figure. The two points labeled C represent non-exceedance 

conditions – they lie below the critical (84 ppbv) dashed line isopleth. Point A represents NOxlimited exceedance conditions. The predicted maximum 8-hour average ozone concentration is 

above 84 ppbv, but by decreasing NOx emissions sufficiently, the 8 AM NOx concentrations will 

decrease so that Point A will cross the critical dashed line isopleth. Similarly, Point B represents 

VOC-limited exceedance conditions; by decreasing VOC emissions sufficiently, the 8 AM VOC 

concentrations will decrease so that Point B will cross the critical dashed line isopleth. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 123 31 August 2007 

There is a critical difference in the behavior that the EKMA model predicts for the NOx-limited 

Point A and the VOC-limited Point B. Point B suggests that attainment can be most easily 

reached by reducing VOC emissions, but attainment can also be reached by reducing NOx

emissions if the reductions are drastic enough. Initially at point B, NOx emission reductions 

alone would first increase the predicted maximum ozone concentration, but after a peak is 

reached, further NOx emission reductions would lead to a decrease in the predicted maximum 

ozone concentration until Point B crosses the critical isopleth. In contrast, VOC reductions alone 

applied to Point A can never bring that point below the critical isopleth, because the maximum 

ozone concentration is predicted to be above 84 ppbv even when the 8 AM anthropogenic VOC 

concentrations are zero. This model behavior occurs because biogenic VOC emissions alone 

(with background CO and methane concentrations), in the presence of sufficient NOx, can 

produce ozone concentrations in exceedance of the ozone standard. This behavior of Point A is 

similar to the predictions from the Eulerian modeling presented in Figure F3, which show that 

even extremely stringent VOC controls cannot decrease the HGB ozone design values enough to 

achieve compliance. Importantly, the EKMA model suggests that NOx-sensitive conditions as 

exemplified by Point A are relatively common in HGB. Of the days investigated, 54% for 

Clinton, 17% for Channelview, 35% for Wallisville, 39% for Lynchburg Ferry, and 43% for 

HRM-3 exhibited Point A-like, NOx-limited behavior. 

Clearly the EKMA model is highly simplified, but it is notable that the NOx-sensitive behavior 

exemplified by Point A may be reasonably well represented by the model. One of the most 

critical simplifications in the model is the assumption of a “typical” Harris County VOC mix. It 

is well known that the VOC composition in HGB changes markedly, from the typical urban mix 

of downtown Houston to the widely varying VOC emissions from the petrochemical facilities. 

However, the critical Point A behavior is focused at zero anthropogenic VOC concentration, so 

the assumed VOC composition will have no effect. 

Given the EKMA model prediction that drastic NOx emission controls will be required to 

eventually reach ozone compliance in HGB, the extremely difficult but central question arises: 

How should air quality managers in the HGB area divide ozone management resources 

between NOx and VOC emission controls? 

Several considerations favor control of VOC emissions, including: 

• First, the Response to Question F indicates that incremental VOC emission reductions 

presently will result in incremental decreases in ambient ozone concentrations. Any 

progress in decreasing the maximum ambient ozone concentrations will have immediate 

health and welfare benefits. 

• Second, Figure K1 does indicate that there is a modest advantage to VOC control when 

used in conjunction with NOx control. For example, if the 8 AM VOC concentration is 

reduced from 0.4 to 0.1 ppm C, then NOx controls need only reduce the 8 am NOx

concentration to about 25 rather than about 20 ppbv. Figure F3 also indicates a similar 

benefit of VOC control. 

• Third, VOC emissions reductions can decrease ambient concentrations of VOC 

hazardous air pollutants. For example, the ambient concentrations of formaldehyde, 

which in the HGB area is primarily a secondary product of HRVOC oxidation, will 

respond to HRVOC emission controls. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 124 31 August 2007 

Similarly, several considerations favor control of NOx emissions, including: 

• First, NOx emissions will reduce not only locally produced ozone, but also regional 

background ozone. Outside of central urban areas and point source plumes, the regional 

ozone production environment is generally strongly NOx-limited. 

• Second, all of the modeling approaches agree that sufficiently drastic NOx control is 

certain to lead to ozone attainment. Further, to the extent that the EKMA results reflect 

reality, such drastic control may be required to ultimately reach compliance. However, 

there is uncertainty whether the high degree of NOx control required for ozone attainment 

is socio-economically acceptable or even technologically feasible. 

• Third, NOx controls will reduce local and regional levels of both nitrate aerosol, a 

significant contributor to PM2.5 concentrations, and nitric acid, a significant component of 

acid deposition. Both of the nitrate contribution to PM2.5 and the nitric acid contribution 

to acid deposition are particularly important issues in the western U.S. 

The most efficient emission control strategies will then depend upon practical and economic 

considerations associated with the alternatives. Given that the Eulerian and EKMA models upon 

which all the preceding discussions are based are still imperfect, it is prudent to implement what 

VOC controls and/or NOx controls are presently beneficial and practical, while modeling 

improvements are effected. 

KEY CITATIONS AND INFORMATION AND DATA SOURCES 

Dimitriades, D. 2006. A Proposed Study for Determining the Optimum Direction of Control in 

the Houston-Galveston-Brazoria Ozone Non-Attainment Area. NC State University Report to 

the Texas Commission on Environmental Quality. Submitted October 10, 2006. College of 

Natural Resources, NC State University, Raleigh, NC.

Dimitriades, B. and D. Luecken. 2007. An observational investigation of ozone sensitivity to 

VOC and NOx and it variation in the Houston-Galveston-Brazoria (HGB) ozone nonattainment area. Presentation at TexAQS II Principal Findings Data Analysis Workshop, 29 

May-1 June 2007. Austin, TX. 

http://www.tceq.state.tx.us/assets/public/implementation/air/texaqs/workshop/20070529/200

7.06.01/6-Spatial+Temporal_Patterns-Dimitriades.pdf

Finlayson-Pitts, B.J. and J.N. Pitts. 2000. Chemistry of the Upper and Lower Atmosphere. pp. 

892-893. Academic Press, New York, NY. 

US Environmental Protection Agency. 2007. Guidance on the Use of Models and Other 

Analyses for Demonstrating Attainment of Air Quality Goals for Ozone, PM2.5, and Regional 

Haze. EPA Report 454/B-07-002. April 2007. EPA Office of Air Quality Planning and 

Standards, Air Quality Analysis Division, Air Quality Modeling Group, Research Triangle 

Park, NC. 262 pp. http://www.epa.gov/ttn/scram/guidance/guide/final-03-pm-rhguidance.pdf
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 125 31 August 2007 

APPENDIX 1. GLOSSARY OF ACRONYMS 

(The meaning of some of the acronyms more commonly used in the report are listed here.) 

AIRS Atmospheric Infrared Sounder 

AIRS Aerometric Information Retrieval System 

AOD Aerosol Optical Depth 

AQ Air Quality 

AQFM Air Quality Forecast Model 

AURAMS A Unified Regional Air-Quality System 

BEIS Biogenic Emissions Inventory System 

BL Boundary Layer 

CALIPSO Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation 

CAMS Community Air Modeling System 

CAMx Comprehensive Air Quality Model, with extensions 

CB Carbon Bond 

CB05 Carbon Bond Photochemical Mechanism, version 5 

CB-IV Carbon Bond Photochemical Mechanism, version IV 

CEMS Continuous Emissions Monitoring System 

CHRONOS Canadian Hemispheric and Regional Ozone and NOx System 

CMAQ Community Multi-scale Air Quality Model 

CMBO 1-chloro-3-methyl-3butene-2-one 

CRES Lumped group of chemical species in photochemical reactions 

mechanisms that includes cresol 

CST Central Standard Time 

DFW Dallas-Fort Worth 

DIAL DIfferential Absorption Lidar 

EGU Electric power Generating Unit(s) 

eGRID Emissions and Generation Resource Integrated Database 

EPA US Environmental Protection Agency 

EPS2 Emissions Processing System version 2 

EKMA Empirical Kinetic Modeling Approach 

HGB Houston-Galveston-Brazoria 

HRVOC Highly Reactive Volatile Organic Compound(s) 

HSC Houston Ship Channel 

HSRL High Spectral Resolution Lidar 

HYSPLIT HYbrid Single-Particle Lagrangian Integrated Trajectory model 

LPAS Laser Photo-Acoustic Spectroscopy 

LST Local Standard Time 

MAE Mean Absolute Error 

MAQSIP-RT Multiscale Air Quality SImulation Platform-Real Time 

MM5 Meteorological Model version 5 

MOBILE6 EPA Mobile Source Emission Factor Model version 6 

MODIS Moderate Resolution Imaging Spectroradiometer 

MSA Metropolitan Statistical Area 

MSD Medium Speed Diesel 

NAAQS National Ambient Air Quality Standards 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 126 31 August 2007 

NASA National Aeronautics and Space Administration 

NCAR National Center for Atmospheric Research 

NCEP National Centers for Environmental Prediction 

NEI National Emissions Inventory 

NOAA National Oceanic and Atmospheric Administration 

NOAA/ESRL NOAA Earth System Research Laboratory 

NOAA/NESDIS NOAA National Environmental Satellite Data and Information Service 

OMI Ozone Monitoring Instrument 

OZIPR Ozone Isopleth Plotting Research Package 

PAN PeroxyAcetyl Nitrate 

PBL Planetary Boundary Layer 

PM2.5 Particulate Matter less than 2.5 micrometers in diameter 

PTR-MS Proton Transfer Reaction-Mass Spectrometer 

RAQMS Realtime Air Quality Modeling System 

ROG Reactive Organic Gases 

RHB research vessel Ronald H. Brown 

RMSE Root Mean Square Error 

RSST Rapid Science Synthesis Team 

SAPRC [California] Statewide Air Pollution Research Center 

SAPRC99 [California] Statewide Air Pollution Research Center (photochemical 

mechanism 1999 version) 

SAPRC07 [California] Statewide Air Pollution Research Center (photochemical 

mechanism 2007 version) 

SIP State Implementation Plan 

SMOKE Sparse Matrix Operator Kernel Emissions 

SOF Solar Occultation Flux 

SOS-OD Southern Oxidants Study-Office of the Director 

SSD Slow Speed Diesel 

STEM Sulfur Transport and Emissions Model 

TCEQ Texas Commission on Environmental Quality 

TES Tropospheric Emission Spectrometer 

TexAQS 2000 First Texas Air Quality Study, completed during the summer of 2000 

TexAQS 2006 Intensive field measurement study conducted during 11 weeks of TexAQS 

II 

TexAQS II Second Texas Air Quality Study, completed during 18 months in 2005 and 

2006 

TRI Toxics Release Inventory 

UHI Urban Heat Island 

UTC Coordinated Universal Time 

VOC Volatile Organic Compound(s) 

WAS Whole Air Samples 

WP-3D Instrumented aircraft operated by NOAA 

WRF/Chem Weather Research Forecast model/Chemistry version 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

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APPENDIX 2. FINAL REPORT FROM QUESTION L WORKING GROUP. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 128 31 August 2007 

Question L Final Report 

QUESTION L

What existing observational databases are suitable for evaluating and further developing 

meteorological models for application in the HGB area? 

QUESTION L WORKING GROUP 

Leader: Lisa Darby; Participants: Robert Banta, John Nielsen-Gammon, Daewon Byun, Wayne 

Angevine, Mark Estes, Bryan Lambeth, Stuart McKeen. 

BACKGROUND 

In order to address this question, databases that are potentially useful to individuals performing 

air quality modeling for Texas, including both permanent measurements and enhanced 

measurements from TexAQS II deployments, were compiled. The databases were evaluated 

based on several criteria, including quality control, accessibility, regional coverage, and time 

resolution. Web links to the databases are given, followed by a brief description and evaluation. 

FINDINGS 

Surface Meteorology and Chemistry Data 

COOP observations 

http://www7.ncdc.noaa.gov/IPS/CDPubs?action=getstate

Consists of monthly printed pages (available as PDFs online) containing station observations. 

No description of QA is provided on the web site. 

Because these data are not in machine-readable format, they are unlikely to be useful for any 

systematic study. They are redundant, in the sense that the same observations should be in the 

normal NWS data streams. 

Crop Weather Program, Texas A&M University 

http://cwp.tamu.edu/cgi-bin/htmlos.cgi/6742.2.1749378041063346623

The Crop Weather Program for South Texas (CWP) was developed to help farmers and 

consultants make management decisions conducive to profitable crop production. It replaces an 

earlier cotton monitoring system known as the Weather Station Network Program. The CWP is 

the gateway for access to weather data measured by a network of 21 automated weather stations 

spread across 10 South Texas counties and provides hourly measurements of air temperature, 

relative humidity, solar radiation, wind direction and speed, precipitation, and soil temperature at 

1", 3", and 8" depths. The wind direction is reported based on a 16-point compass and the wind 

speed appears to be arithmetic (no vector average direction or speed). The wind also appears to 

be measured about 10 feet above ground level based on an example site photo provided (this 

could exacerbate exposure problems where buildings and/or trees are nearby). 

Harris County Office of Homeland Security and Emergency Management 

http://www.hcoem.org/

The Harris county rainfall map site allows you to enter an amount of time (in days, hours, or 

minutes) before the current time, and it produces a map of accumulated rainfall amounts from 

each site, over the time requested. The data come from 163 automatic remote sensors (part of 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 129 31 August 2007 

the flood alert system) across the metropolitan area, and they are “unofficial” (probably means 

not quality-controlled). The density of the network allows for detailed information regarding the 

horizontal distribution of the rainfall. Their locations can be found on a map link and a text link, 

which includes latitudes and longitudes. There is a link to an archive site where you can indicate 

a given amount of time before your date of interest to obtain a map of accumulated rainfall, but I 

could not get this part to work. If this does eventually work, this could be a useful site for 

modelers, although it looks like the only output would be a map (i.e., no text dump). I suggest a 

following up on this site to determine if there is a way to order the archived data. 

Also on the main page for Harris County Office of Homeland Security and Emergency 

Management is a link to a real time Houston speed map. Along the outlines of the major 

highways the current speed of traffic is shown in color (indicating speeds <20, 20-29, 30-39, 40-

49, and 50+ MPH, or no data). On this site is a link to the Houston speed map archives 

(http://traffic.houstontranstar.org/map_archive/map_archive.aspx). From this site you can select 

a date and time (down to 15-minute intervals) and you get a traffic speed map for that time. This 

could be useful to determine if gridlock was worse on some days compared to others. 

Texas A&M data 

http://dallas.tamu.edu/Weather/index.html

This web site has data from two sites near Dallas. The sites are run by the Texas A&M Dallas 

Agricultural Research and Extension Center (phone: 972.231.5362), and details are sketchy. 

The locations are not specified, although one is on a research farm (Prosper) and the other is just 

called “Dallas.” The “Dallas” site has, by date, max/min soil temperature, max/min air 

temperature, max/min RH, a single column labeled “wind” (no units indicated on any of the 

columns), max/min soil moisture, and total rain. Some years have a column labeled ET_o 

(evapotranspiration?). At the end of each month is a row for monthly medians for each column 

and another row with the max, min, or total for each column (depending on the variable). The 

Prosper site has the same variables, plus “RAD” (radiation?), wind speed, wind direction and 

battery voltage. The Dallas site has data archived from 2000 and the Prosper site has data 

archived from 1997. Given how important soil moisture measurements are for modelers, it may 

be useful to investigate this database further to determine the location of the sites and the 

robustness of the soil moisture data. 

Lower Colorado River Authority network 

http://hydromet.lcra.org/index2.shtml

Lower Colorado River Authority network. This web page has a wealth of information regarding 

measurements throughout the Colorado River watershed (which extends from NW to SE of 

Austin, becoming quite narrow at Matagorda Bay). The network is most dense around Austin. 

They have: rainfall (24-hr accumulation, accumulation since midnight, and the most recent 

measurement); stage, flow, lake level, air temperature, relative humidity, and conductivity data, 

shown on maps. You can download historic data for a single site (precipitation, air temperature, 

relative humidity, wind speed and direction), but this is not very practical for obtaining data from 

many sites. It is stated that real-time data are provisional, but there is no indication about the 

quality of the archived data. It may be worth investigating if it is possible to obtain archived data 

directly from the agency. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 130 31 August 2007 

Texas A&M agricultural weather site 

http://texaset.tamu.edu/weatherstns.php

This one would be useful for Texas meteorological comparisons of precipitation (not many 

CAMS sites have precipitation), temp, RH, wind speed, wind direction, and solar radiation. 

Pretty good coverage in East Texas. Hourly data should be simple to download and compare 

with model results. 

Soil Climate Analysis Network, US Agriculture Department 

http://www.wcc.nrcs.usda.gov/scan/

This site only gives soil parameters - no standard met. There is only one site (Prairie View) in the 

region of Texas that may be useful to TexAQS 2006 participants. Nonetheless, it may useful for 

comparison of soil models and parameterizations in meteorological models, since soil data is so 

sparse in the east Texas region. 

Louisiana agricultural weather data network 

http://www.agctr.lsu.edu/subjects/weather/

This site is specific to the state of Louisiana. It gives meteorological and soil parameter data at 

about 20 sites evenly distributed throughout Louisiana. The data are not so convenient to 

download. But the soil parameters may be useful for meteorological model soil data 

comparisons. 

Louisiana Universities Marine Consortium weather network. 

http://weather.lumcon.edu/

This web site includes measurements from 5 sites in Louisiana run by LUMCOM (Louisiana 

Universities Marine Consortium). Four of the sites are on platforms over water. The 5th site is 

somewhat inland, but looks like it’s in a marshy/wetlands type of area. The Lake Pontchartrain 

station is in the northern part of the lake. Meteorological data include: atmospheric pressure, 

humidity, temperature, winds, solar radiation, and precipitation. Hydrographic instrumentation 

include: chlorophyll probe, conductivity probe, and a sonde, 6600. Three other sites have the 

same instruments: Tambour Bay, Southwest Pass/Miss River and LUMCOM. The Tambour 

Bay and Southwest sites are off the LA coast, on platforms. LUMCOM is the slightly inland site 

and also has a co-located 915-mHz wind profiler with RASS. The Audubon/Miss River site is 

on a floating structure near the coast (from the picture is looks like it’s in a harbor). It only has 

atmospheric pressure, humidity, and temperature for meteorological measurements. It has the 

same hydrographic measurements as Lake Pontchartrain, plus a wet chemical in-situ nitrate 

analyzer. 

The web site is comprehensive, with a map and much information for each station. There are 

records regarding calibrations and inspections, implying that these sites are well maintained. 

These appear to be good sites for modelers to obtain coastal meteorological data for Louisiana. 

The archived files are easy to access. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

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CAMS (TCEQ organized surface meteorological and chemical data) 

http://www.tceq.state.tx.us/nav/eq/mon_sites.html

This website includes information regarding the details of the TCEQ measurements sites. Many 

sites have both meteorology and chemistry measurements. Some have just one or the other. All 

chemistry sites appear to have ozone measurements, but some also include NO, NO2, and 

perhaps other important constituents. Those with meteorology tend to have temperature and 

winds, perhaps precipitation. This site has two links: 

1) TCEQ’s Air Monitoring Sites (Regional Map) provides details about the TCEQ's air 

monitoring sites and air pollution, weather and other parameters measured at each site. 

2) Air Monitoring Sites (Table) 

Provides a user interface to view sortable list of locations and descriptions of monitoring sites 

operated by the TCEQ and other entities around the state as well as link to photos of sites, lists of 

parameters monitored, and current measurements. 

This is useful for modelers who want to know the locations of monitoring stations, and what is 

monitored at each station. 

http://www.tceq.state.tx.us/compliance/monitoring/air/monops/historical_data.html

This page provides access to two sources of pollutant and weather data. The first source, the 

Texas Commission on Environmental Quality (TCEQ) and local monitoring networks, provides 

hourly pollutant and weather data from 1972 to 2004. The second source, the U.S. 

Environmental Protection Agency (EPA), provides data summaries and hourly data collected 

since 1982 on numerous pollutants and meteorological parameters in Texas and other states. 

A useful site for modelers to download hourly surface data for model evaluation. 

METARs (NWS surface data) 

http://www.nndc.noaa.gov/cgi-bin/nndc/buyOL-001.cgi

The Unedited Surface Weather Observations product consists of unedited hourly observations 

from over 700 U.S. locations. There is a charge to access this data online, but not if your domain 

is .gov, .mil, or .edu. (More details on the web site.) The time range of the available data is from 

July 1, 1996 to two days ago. A useful site for obtaining surface observations for model 

evaluation. 

Oklahoma air quality monitors 

http://www.deq.state.ok.us/AQDnew/monitoring/index.htm

This site has the details of the air quality monitoring stations in Oklahoma. There are several 

monitoring sites north of the Texas-Oklahoma border that would be useful for southerly flow 

events (e.g., looking at transport from Dallas to Oklahoma). For a graphical display, the site 

links to the EPA site http://www.airnow.gov/index.cfm?action=airnow.currentconditions, where 

the user can click on the state of Oklahoma to see the Oklahoma observations. Text data 

includes real-time data (for today and yesterday). The user can sort by pollutant or by station. 

Archived data includes 8-hour averages of ozone and CO, organized by year. Within each year 

is the date and amount of the 4 highest readings for each station. One-hour ozone exceedances 

are also available in this format. It does not appear that data other than the 4 highest readings per 

year are available via the web. This site is probably somewhat useful for modelers. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

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Upper Air Data 

ESRL (formerly ETL) Profiler Network, South Central Texas 

http://www.etl.noaa.gov/et7/data/

The ESRL (formerly ETL) network page allows access to real-time and archived plots of profiler 

winds and other profiler data. Real-time plots are provided through a clickable map interface. 

Archived plots and ASCII data can be downloaded for single profilers. A trajectory tool allows 

the calculation of forward and backward trajectories using profiler data. The site includes all 

regular wind profilers from the NOAA and TCEQ network as well as all those installed for the 

TexAQS-II field program. The data include profiler winds and signal-to-noise ratio, RASS 

virtual temperature and virtual potential temperature, and surface meteorological observations 

from profiler sites. Data should remain available for several months after the experiment, as well 

as the profiler trajectory tool. 

NOAA National Profiler Network graphical display 

http://www.profiler.noaa.gov/npn/

The NOAA site used to include all permanent profilers, but now it appears to contain only the 

profilers in the NOAA demonstration network, including Ledbetter, Palestine, and Jayton in 

Texas. Users can request real-time plots or generate plots using archived data. There is 

considerable flexibility in the online data plotting interface. Archived data are available from the 

web site hosts. 

Rapid Update Cycle (RUC) soundings 

http://rucsoundings.noaa.gov/

This sounding page allows the user to generate plots or ASCII data dumps of soundings from 

rawinsondes, profilers, and RUC/MAPS forecasts. The output is Java-based, allowing mouseover data information and animation/looping of soundings. The interface requires the user to 

know the name or site ID’s of the stations to plot. Most of the data are available only in realtime or near-real-time, except that an online rawinsonde archive was begun early in 2006. 

Perhaps the most useful aspect of the web site is the ability to plot forecast soundings from the 

RUC model. These forecasts are available for any arbitrary location and extend up to 12 hours 

into the future, so they provide detailed guidance for mixing heights, vertical wind shear, and 

convection. 

University of Wyoming sounding page 

http://weather.uwyo.edu/upperair/sounding.html 

This web site allows the user to select a station using a clickable map and generate graphical 

soundings or ASCII data output from real-time or archived rawinsonde observations. The output 

format includes all common sounding diagram types and ASCII data formats. Large amounts of 

data would be difficult to obtain, but this site is the best available on the web for individual 

archived soundings. 
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ACARS aircraft observations 

http://amdar.noaa.gov/

ACARS observations are in situ meteorological observations made by commercial aircraft. The 

data include temperature, wind, and often dew point. The wind precision is not very good, but 

the temperature and dew point data are useful for estimating mixing heights and their diurnal 

variation. Most ACARS observations in Texas come from the Dallas-Fort Worth area, usually 

about two dozen per day. Much less frequent observations are available from Houston and other 

major airports. The data are not freely available in real time on the web, but they are available 

for research purposes upon approval by NOAA. Texas A&M presently receives ACARS data 

but is not funded by TCEQ to process or use the data for analysis or forecasting during 2006. 

Coastal and Buoy Data 

Texas Coastal Ocean Observation Network, Texas A&M Corpus Christi, Conrad Blucher 

Institute 

http://lighthouse.tamucc.edu/TCOON/HomePage

Large network of coastal stations. Some of the reported stations are regular NOAA or other 

agency stations, and these are not identified as such. The additional stations seem to primarily 

provide water level, water temperature, and air temperature. Machine-readable historical data 

are available. Some QA is apparently done, but specifications are not easily found on the web 

site. 

Possibly useful for improving resolution of model validations for simple parameters. 

NDBC (National buoy data) 

http://www.ndbc.noaa.gov/Maps/WestGulf.shtml

Provides listings of hourly meteorological data (air and sea-surface temperature, winds, pressure, 

etc.) and wave data for each meteorological buoy in the Gulf of Mexico (and elsewhere around 

the U.S.). Meteorological data are archived back as far as 1990 for some sites. Also a section 

gives data on ocean currents as a function of depth. Buoy and other instrument locations are 

displayed on a map, and data are obtained by clicking on the site of interest. Recent ship 

observations are also listed at this site, and the tri-annual Mariners Weather Log. 

Houston/Galveston Port Meteorological Office 

http://www.srh.noaa.gov/hgx/marine/pro.htm

Houston/Galveston Port Meteorological Office

Site includes a description of needs for maritime meteorological data and the role of this office in 

facilitation of the Voluntary Observing Ship (VOS) Program. The office also “works within the 

framework of the Shipboard Environmental (Data) Acquisition System (SEAS), by which 

meteorological data are collected and transmitted to NCEP, for inclusion in the major data bases. 

Under Past Weather, this site has climatological data and daily information for several Texas 

land stations around the Gulf of Mexico, including daily high and low temperatures, wind, 

precipitation, and some other meteorological data. The monthly Texas Climatic Bulletins and 

other climatological products and information are available at this site. We were unable to locate 

any actual shipboard data from this site (however, some current data could be found on the 

NDBC site). 
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Satellite Data 

Space Science and Engineering Center, University of Wisconsin, Madison 

http://www.ssec.wisc.edu/

Comprehensive archive of data, products, and downloadable processing software for 

geosynchronous and polar-orbiting satellites, including GOES-11 and -12 and MODIS data from 

Terra and Aqua. A host of real-time satellite images and products are also available, some stored 

for 7 days. Routine meteorological data are also available for McIdas users. 

NASA Aura TES step and stare observations 

http://tes.jpl.nasa.gov

TES is an infrared, high resolution, Fourier Transform spectrometer covering the spectral range 

650 - 3050 cm-1 (3.3 - 15.4 μm) at a spectral resolution of 0.1 cm-1 (nadir viewing) or 0.025 cm1 (limb viewing). Launched into a polar sun-synchronous orbit (13:38 hrs local mean solar time 

ascending node) on July 15, 2004, the TES orbit repeats its ground track every 16 days (233 

orbits), allowing global mapping of the vertical distribution of tropospheric ozone and carbon 

monoxide along with atmospheric temperature, water vapor, surface properties (nadir), and 

effective cloud properties (nadir). TES has a fixed array of 16 detectors, which in the nadir 

mode, have an individual footprint of approximately 5.3 x .5 km. In order to increase the signalto-noise ratio, these detectors are averaged together to produce a combined footprint of 5.3x8.4 

km. TES has two basic observational modes: the global survey mode, where observations are 

taken 1.3 degrees apart in latitude, and the "step-and-stare" mode, where the separation between 

observations is approximately 35 km along the orbit. This step-and-stare mode was used 

extensively throughout the TexAQS 2006 campaign. 

Maps of these profiles can be found at 

http://tes.jpl.nasa.gov/TexAQS_2006/main_SS_TEXAQS_2006.html

Contact information: kevin.bowman@jpl.nasa.gov

NASA CALIPSO observations 

http://www-calipso.larc.nasa.gov/

The Cloud-aerosol lidar and Infrared Pathfinder satellite Observations (CALIPSO) satellite 

mission was launched on April 28, 2006 for a planned 3-year mission. CALIPSO is flying in 

formation with Aqua, Aura, CloudSat, and Parasol satellites as part of the Afternoon 

Constellation or A-train. The CALIPSO payload consists of three instruments: the CloudAerosol LIdar with Orthogonal Polarizaton (CALIOP); the Infrared Imaging Radiometer (IIR) 

and the Wide Field Camera (WFC). CALIPO is a nadir-pointing instrument which provides 

profile measurements of aerosol backscatter at 532 and 1064 nm and depolarization at 532 nm. 

The IIR provides calibrated radiances at 8.65 μm, 10.6 μm, and 12.05 μm over a 64 km swath 

centered on the lidar footprint and a pixel resolution of 1 km. The WFC consists of a single 

channel covering the 620 nm to 670 nm spectral region providing images of a 61 km swath with 

a spatial resolution of 125 m in a band 5 km about the nadir track and 1000 m elsewhere. For 

TexAQS 2006 campaign, CALIPSO quick turn-around browse images were produced to identify 

aerosol and cloud layers for flight planning activities. Observations were also synthesized into 

aerosol modeling systems to better understand the origin of surface and elevated aerosol features 

from regions outside the experiment domain. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 135 31 August 2007 

Examples of aerosol browse images used during TexAQS can be found at 

http://www-calipso.larc.nasa.gov/products/lidar/

Contact information: Charles.R.Trepte@nasa.gov

NOAA and DoD satellite images 

http://www.class.noaa.gov/nsaa/products/welcome;jsessionid=1C0E54F015C2813E5A9ACFC2

2C675F90

The National Oceanic and Atmospheric Administration (NOAA) Comprehensive Large Arraydata Stewardship System (CLASS) is NOAA's premier on-line facility for the distribution of 

NOAA and US Department of Defense (DoD) Polar-orbiting Operational Environmental 

Satellite (POES) data and derived data products. CLASS is operated by the Information 

Processing Division (IPD) of the Office of Satellite Data Processing and Distribution (OSDPD), 

a branch of the National Environmental Satellite, Data and Information Service (NESDIS). 

CLASS maintains an active partnership with NOAA's National Climatic Data Center (NCDC). 

NCDC, the permanent US Archive for POES data and derived data products, supports CLASS 

through a user-interactive Help Desk facility and through the provision of POES supporting 

documentation, including the NOAA Polar Orbiter Data (POD) User's Guide and the NOAA 

KLM User's Guide. Additionally, NCDC and CLASS share data distribution responsibilities for 

Defense Meteorological Satellite Program (DMSP) data under a Memorandum of Understanding 

with the National Aeronautics and Space Administration (NASA) for the Earth Observing 

System (EOS) Program. 

CLASS provides data free of charge. Anyone can search the CLASS catalog and view search 

results through CLASS's World Wide Web (WWW) site. Users who wish to order data are 

required to register with their names and email addresses. CLASS distributes data to those users 

via FTP services. 

CLASS (originally called Satellite Active Archive), was established as a demonstration 

prototype for electronic distribution of POES data in 1994, and became operational in July 1995. 

During that first month, 379 Advanced Very High Resolution Radiometer (AVHRR) Level 1b 

data sets were distributed to 27 customers via the emerging Internet. During the first five years of 

operation, the average monthly volume of data distribution increased to 65,000 data sets with a 

total size of 1.2 TB, and the SAA customer base grew to more than 10,000 registered customers. 

The active archive was expanded during that period to include TIROS Operational Vertical 

Sounder (TOVS) data, Defense Meteorological Satellite Program (DMSP) data, Radarsat 

Synthetic Aperture Radar (SAR) imagery, operational (near-term) satellite-derived products, and 

climatic (time-series) satellite-derived products. 

NASA Earth Observatory natural hazards 

http://earthobservatory.nasa.gov/NaturalHazards/

This is a NASA site that has awesome satellite images due to the following natural phenomena: 

crops & drought; dust & smoke, fires, floods, severe storms, and volcanoes. The images are 

organized by event, and are free to all. They just ask for proper acknowledgment. This site is 

probably of limited value to modelers, but for certain events, such as the Saharan dust events that 

occurred during TexAQS II, the images may add some visual interest for a case study 

presentation. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 136 31 August 2007 

NASA MODIS Rapid Response System images 

http://rapidfire.sci.gsfc.nasa.gov/realtime/2006297/

This site has images from MODIS (Terra and Aqua). Images are archived by day, and can be 

downloaded. This site may be somewhat useful for modelers. 

NASA Aqua AIRS retrieved CO profiles 

http://asl.umbc.edu/pub/mcmillan/www/index.html#calendar

AIRS output for the TexAQS II field campaign. There is a clickable calendar for a view of the 

data. Please work with Dr. Wallace McMillan if interested in using the data (contact information 

is on the web site). 

Solar Radiation Data 

Texas Solar Radiation data, from a solar energy research group at UT 

http://www.me.utexas.edu/~solarlab/tsrdb/

This site has solar radiation data for 15 sites throughout Texas. The data intervals and times of 

coverage vary by station, ranging from 15 minute data to monthly averages. Data stops in 2003 

or earlier for many of the stations. Data reported: Global horizontal, direct normal, and diffuse 

horizontal (W m-2). Monthly averages include temperature (degrees C). Data are easy to access. 

This site may be moderately useful for modelers. 

National Renewable Energy Lab (solar radiation data) 

http://rredc.nrel.gov/solar/new_data/confrrm/

Cooperative Networks for Renewable Resource Measurements (CONFRRM). This network was 

designed to capture long-term solar radiation and wind measurements. There are 5 sites in 

Texas, however the last month showing data for all sites is March 2000. Therefore, data on this 

site are not useful for modelers working on summers 2000 – 2006. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 137 31 August 2007 

Large, Multi-field Data Sets 

MADIS 

http://madis.noaa.gov/

The Meteorological Assimilation Data Ingest System (MADIS) is dedicated toward making 

value-added data available from the National Oceanic and Atmospheric Administration's 

(NOAA) Earth System Research Laboratory (ESRL) Global Systems Division (GSD) (formerly 

the Forecast Systems Laboratory (FSL)) for the purpose of improving weather forecasting, by 

providing support for data assimilation, numerical weather prediction, and other hydrometeorological applications. 

MADIS subscribers have access to an integrated, reliable and easy-to-use database containing 

the real-time and archived observational datasets described below. Also available are real-time 

gridded surface analyses that assimilate all of the MADIS surface datasets (including the very 

dense integrated mesonet data). The grids are produced by the Rapid Update Cycle (RUC) 

Surface Assimilation System (RSAS) that runs at ESRL/GSD, which incorporates a 15-km grid 

stretching from Alaska in the north to Central America in the south, and also covers significant 

oceanic areas. RSAS grids are valid at the top of each hour, and are updated every 15 minutes. 

• Observations 

o Meteorological Surface

 METAR 

 SAO 

 Maritime 

 Modernized NWS Cooperative Observer 

 Integrated Mesonet

 Observations from local, state, and federal agencies and private mesonets 

(including GPSMET water vapor) 

o Radiosonde

o NOAA Profiler Network

o Hydrological Surface

o Automated Aircraft

 Automated Aircraft Reports 

 Profiles at Airports 

o Multi-Agency Profiler

o Radiometer

o Satellite Wind

 GOES Operational 3-Hour 

 GOES Experimental 1-Hour 

o Satellite Sounding

 NOAA POES 

o Satellite Radiance

 NOAA POES 

o Snow

• Grids 

o RSAS Surface Analyses

TCEQ Air Pollution Events 

http://www.tceq.state.tx.us/compliance/monitoring/air/monops/sigevents06.html

The TCEQ Air Pollution Events web pages provide preliminary analyses of large-scale high 

ozone and/or particulate events in Texas. The analyses include satellite imagery, webcam 

imagery, ozone contour animations, ozone plume animations, backward air trajectories, upper air 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 138 31 August 2007 

data graphs, and pollution data time series graphs. The discussions describe the intensity and 

geographic coverage of each event. The discussions also report any transport related aspects to 

the pollution, if appropriate, and provide an estimate of background levels and local add-on for 

ozone cases. 

EDAS (NCEP grid reanalysis) 

http://www.cdc.noaa.gov/cdc/reanalysis/reanalysis.shtml

It is a website for the NCEP/NCAR Reanalysis Project at the NOAA/ESRL Physical Sciences 

Division 

This page points you to information on the NCEP/NCAR Reanalysis project and the 

implementation of a netCDF-based, internet-accessible, data service at NOAA/ESRL PSD for 

this set of data products. 

 * The 6-hourly and daily data currently available on-line. 

 * The monthly and other derived data currently available on-line. 

This site also has links to other reanalysis project sites (e.g., ECMWF). 

NCEP/NCAR Reanalysis 

http://dss.ucar.edu/pub/reanalysis/

This site includes the following NCEP/NCAR REANALYSIS databases. 

 * DSS Reanalysis archives. 

Project Overview 

 * Project Description - 

 The project motivation and objectives, cooperative arrangement between NCEP and 

NCAR, and other published documentation are outlined 

 * Model Description 

 * Project Status 

 * Other Related Sites 

Data Product Description 

More than 20 different data products are output from the Reanalysis data assimilation, model 

run, and model forecast. These products are defined in terms of the NCAR archive names, 

physical variables, resolutions (temporal and spatial), and media storage size. CDROMS are also 

used to distribute selected reanalysis products. 

This web site includes much detail on all of the data bases used, etc. 

 * 2006AUG10 --All 1948-2006JUL pgb.f00 and grb2d files are now available on line for 

registered users. 

 * 2006AUG10 --JUL 2006 data files are released. All 1948-2006JUL reanalysis files are 

available. 

 * 2006APR20 --Public (non-restricted) version of 200309-200602 prepqm files are released. 

 * 2006APR11 --2005 annual cdrom is released. All 1950-2005 reanalysis annual cdroms are 

available. 

 * 2006MAR28 --2005OCT-2006FEB reanalysis forecasts are released. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 139 31 August 2007 

 * 2005Apr20 --2004OCT-2004DEC reruns to fix sea-ice problems are released. 

 * 2005Apr19 --2004AUG and 2004SEP reruns to fix sea-ice problems are released. 

 * 2005Apr08 --There will be a rerun from 2004080100 to 2005032212 due to sea-ice data 

problem. The 200501 and 200502 results are in. 

 * 2003Aug04 --NCEP-NCAR Reanalysis Temperature Change Plots, 1948-2002 

 * DSS Reanalysis archives. 

Project Overview 

 * Project Description - 

 The project motivation and objectives, cooperative arrangement between NCEP and 

NCAR, and other published documentation are outlined 

 * Model Description 

 * Project Status 

 * Other Related Sites 

Data Product Description 

More than 20 different data products are output from the Reanalysis data assimilation, model 

run, and model forecast. These products are defined in terms of the NCAR archive names, 

physical variables, resolutions (temporal and spatial), and media storage size. CDROMS are also 

used to distribute selected reanalysis products. 

This web site includes much detail on all of the data bases used, etc. 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 140 31 August 2007 

APPENDIX 3. INSTITUTIONAL AFFILIATIONS AND E-MAIL ADDRESSES FOR 

MEMBERS OF THE RAPID SCIENCE SYNTHESIS WORKING GROUPS AND 

OTHER COOPERATORS 

Dave Allen, University of Texas at Austin allen@che.utexas.edu 

Wayne Angevine, NOAA Earth System Research Laboratory wayne.m.angevine@noaa.gov 

Elliot Atlas, University of Miami e.atlas@miami.edu 

Bob Banta, NOAA Earth System Research Laboratory robert.banta@noaa.gov 

Tim Bates, NOAA Pacific Marine Environmental Laboratory tim.bates@noaa.gov 

Carl Berkowitz, DOE Pacific Northwest Laboratory carl.berkowitz@pnl.gov 

Veronique Bouchet, Meteorological Service of Canada veronique.bouchet@aec.gc.ca 

Kevin Bowman, NASA Jet Propulsion Laboratory, Caltech kevin.bowman@jpl.nasa.gov 

Pete Breitenbach, Texas Commission on Environmental Quality pbreiten@tceq.state.tx.us 

Chuck Brock, NOAA Earth System Research Laboratory charles.a.brock@noaa.gov 

Steve Brown, NOAA Earth System Research Laboratory steven.s.brown@noaa.gov 

Daewon Byun, University of Houston daewon.byun@mail.uh.edu 

Greg Carmichael, University of Iowa gregory-carmichael@uiowa.edu 

Bill Carter, University of California, Riverside carter@mail.cert.ucr.edu 

Ellis Cowling, North Carolina State University ellis_cowling@ncsu.edu 

Lisa Darby, NOAA Earth System Research Laboratory lisa.darby@noaa.gov 

Joost de Gouw, NOAA Earth System Research Laboratory joost.degouw@noaa.gov 

Basil Dimitriades, North Carolina State University basildi@hotmail.com 

Bright Dornblaser, Texas Commission on Environmental Quality bdornbla@tceq.state.tx.us 

Mark Estes, Texas Commission on Environmental Quality mestes@tceq.state.tx.us 

Alan Fried, National Center for Atmospheric Research fried@ucar.edu 

Maedeh Faraji, University of Texas at Austin mfaraji@mail.utexas.edu 

Cari Furiness, North Carolina State University cari_furiness@ncsu.edu 

Noor Gillani, University of Alabama in Huntsville gillani@nsstc.uah.edu 

Wanmin Gong, Meteorological Service of Canada wanmin.gong@ec.gc.ca 

Georg Grell, NOAA Forecast Systems Laboratory georg.a.grell@noaa.gov 

Mike Hardesty, NOAA Earth System Research Laboratory mike.hardesty@noaa.gov 

Scott Herndon, Aerodyne Inc. herndon@aerodyne.com 

John Holloway, NOAA Earth System Research Laboratory john.s.holloway@noaa.gov 

Harvey Jeffries, University of NC at Chapel Hill harvey@unc.edu 

John Jolly, Texas Commission on Environmental Quality jjolly@tceq.state.tx.us 

Dick Karp, Texas Commission on Environmental Quality dkarp@tceq.state.tx.us 

Siwan Kim, NOAA Earth System Research Laboratory siwan.kim@noaa.gov 

Susan Kemball-Cook, Environ Corp skemballcook@environcorp.com 

Bryan Lambeth, Texas Commission on Environmental Quality blambeth@tceq.state.tx.us 

Barry Lefer, University of Houston blefer@uh.edu 

Deborah Luecken, US Environmental Protection Agency luecken.debora@eps.gov 

John McHenry, Baron Advanced Meteorological Systems john.mchenry@baronams.com 

Stuart McKeen, NOAA Earth System Research Laboratory stuart.a.mckeen@noaa.gov 

Wallace McMillan, University of Maryland, Baltimore County mcmillan@umbc.edu 

Jeffery McQueen, NOAA National Weather Service jeff.mcqueen@noaa.gov 

Johan Mellqvist, University of Göteborg, Sweden johan.mellqvist@rss.chalmers.se 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 141 31 August 2007 

Ann Middlebrook, NOAA Earth System Research Laboratory ann.m.middlebrook@noaa.gov 

Gary Morris, Valparaiso University gary.morris@valpo.edu 

John Nielsen-Gammon, Texas A&M University n-g@tamu.edu 

Jonathan Andy Neuman, NOAA Earth System Research Laboratory andy.neuman@noaa.gov 

John Nowack, NOAA Earth System Research Laboratory john.nowak@noaa.gov 

Hans Osthoff, NOAA Earth System Research Laboratory hans.osthoff@noaa.gov 

David Parrish, NOAA Earth System Research Laboratory david.d.parrish@noaa.gov 

Ryan Perna, University of Houston ryan.perna@mail.uh.edu 

Brad Pierce, NOAA Satellite and Information Service robert.b.pierce@nasa.gov 

Particia Quinn, NOAA Pacific Marine Environmental Laboratory particia.k.quinn@noaa.gov 

Bernhard Rappenglück, University of Houston brappenglueck@uh.edu 

James Roberts, NOAA Earth System Research Laboratory james.m.roberts@noaa.gov 

Ted Russell, Georgia Institute of Technology trussell@ce.gatech.edu 

Tom Ryerson, NOAA Earth System Research Laboratory thomas.b.ryerson@noaa.gov 

Ken Schere, US Environment Protection Agency schere.kenneth@epa.gov 

Christoph Senff, NOAA Earth System Research Laboratory christoph.senff@noaa.gov 

David Sullivan, University of Texas at Austin sullivan231@mail.utexas.edu 

Michael Trainer, NOAA Earth System Research Laboratory michael.k.trainer@noaa.gov 

Youhua Tang, University of Iowa youhua.tang@noaa .gov 

Sara Tucker, NOAA Earth System Research Laboratory sara.tucker@noaa.gov 

William Vizuete, University of North Carolina at Chapel Hill vizuete@email.unc.edu 

Carsten Warneke, NOAA Earth System Laboratory carsten.warneke@noaa.gov 

Allen White, NOAA Earth System Research Laboratory allen.b.white@noaa.gov 

Jim Wilczak, NOAA Earth System Research Laboratory james.m.wilczak@noaa.gov 

Eric Williams, NOAA Earth System Research Laboratory eric.j.williams@noaa.gov 

David Winker, NASA Langley Research Center d.m.winker@larc.nasa.gov 

Yulong Xie, DOE Pacific Northwest Laboratory yulong.xie@pnl.gov 

Greg Yarwood, Environ Corp gyarwood@environcorp.com 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 142 31 August 2007 

APPENDIX 4. GUIDELINES FOR FORMULATION OF SCIENTIFIC FINDINGS TO 

BE USED FOR POLICY PURPOSES 

The following guidelines in the form of checklist questions were developed by the NAPAP 

Oversight Review Board to assist scientists in formulating presentations of research results to be 

used in policy decision processes. 

1) IS THE STATEMENT SOUND? Have the central issues been clearly identified? Does 

each statement contain the distilled essence of present scientific and technical understanding 

of the phenomenon or process to which it applies? Is the statement consistent with all 

relevant evidence – evidence developed either through NAPAP [or TexAQS] research or 

through analysis of research conducted outside of NAPAP [or TexAQS II]? Is the statement 

contradicted by any important evidence developed through research inside or outside of 

NAPAP [or TexAQS II] Have apparent contradictions or interpretations of available 

evidence been considered in formulating the statement of principal findings? 

2) IS THE STATEMENT DIRECTIONAL AND, WHERE APPROPRIATE, 

QUANTITATIVE? Does the statement correctly quantify both the direction and magnitude 

of trends and relationships in the phenomenon or process to which the statement is relevant? 

When possible, is a range of uncertainty given for each quantitative result? Have various 

sources of uncertainty been identified and quantified, for example, does the statement include 

or acknowledge errors in actual measurements, standard errors of estimate, possible biases in 

the availability of data, extrapolation of results beyond the mathematical, geographical, or 

temporal relevancy of available information, etc. In short, are there numbers in the statement? 

Are the numbers correct? Are the numbers relevant to the general meaning of the statement? 

3) IS THE DEGREE OF CERTAINTY OR UNCERTAINTY OF THE STATEMENT 

INDICATED CLEARLY? Have appropriate statistical tests been applied to the data used in 

drawing the conclusion set forth in the statement? If the statement is based on a mathematical 

or novel conceptual model, has the model or concept been validated? Does the statement 

describe the model or concept on which it is based and the degree of validity of that model or 

concept? 

4) IS THE STATEMENT CORRECT WITHOUT QUALIFICATION? Are there 

limitations of time, space, or other special circumstances in which the statement is true? If the 

statement is true only in some circumstances, are these limitations described adequately and 

briefly? 

5) IS THE STATEMENT CLEAR AND UNAMBIGUOUS? Are the words and phrases used 

in the statement understandable by the decision makers of our society? Is the statement free 

of specialized jargon? Will too many people misunderstand its meaning? 

6) IS THE STATEMENT AS CONCISE AS IT CAN BE MADE WITHOUT RISK OF 

MISUNDERSTANDING? Are there any excess words, phrases, or ideas in the statement 

which are not necessary to communicate the meaning of the statement? Are there so many 

caveats in the statement that the statement itself is trivial, confusing, or ambiguous? 

7) IS THE STATEMENT FREE OF SCIENTIFIC OR OTHER BIASES OR 

IMPLICATIONS OF SOCIETAL VALUE JUDGMENTS? Is the statement free of 

influence by specific schools of scientific thought? Is the statement also free of words, 

phrases, or concepts that have political, economic, ideological, religious, moral, or other 

personal-, agency-, or organization-specific values, overtones, or implications? Does the 

choice of how the statement is expressed rather than its specific words suggest underlying 

biases or value judgments? Is the tone impartial and free of special pleading? If societal 

value judgments have been discussed, have these judgments been identified as such and 

described both clearly and objectively? 
Final Rapid Science Synthesis Report: Findings from TexAQS II 

 143 31 August 2007 

8) HAVE SOCIETAL IMPLICATIONS BEEN DESCRIBED OBJECTIVELY? 

Consideration of alternative courses of action and their consequences inherently involves 

judgments of their feasibility and the importance of effects. For this reason, it is important to 

ask if a reasonable range of alternative policies or courses of action have been evaluated? 

Have societal implications of alternative courses of action been stated in the following general 

form?: 

 "If this [particular option] were adopted then that [particular outcome] would be expected." 

9) HAVE THE PROFESSIONAL BIASES OF AUTHORS AND REVIEWERS BEEN 

DESCRIBED OPENLY? Acknowledgment of potential sources of bias is important so that 

readers can judge for themselves the credibility of reports and assessments.