2006 TEXAQS/GoMACCS 

Comprehensive Chemical Analysis of TexAQS II data 

-108 -106 -104 -102 -100 -98 -96 -94 -92

Longitude

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28

30

32

34

36Latitude

Boundary-layer profilers

Serial rawinsondes

Full tropospheric profilers

Boundary-layer profilers

NWS Rawinsondes

NEXRAD

Existing Upper-air Network

TexAQS-II Enhancements

Combining Climate Change and Air Quality Research 

August 2007 

1
Comprehensive Chemical Analysis of TexAQS II Data 

I. Introduction 

In March 2007, TECQ requested that, in addition to the original twelve SIP-relevant 

questions addressed in the Rapid Science Synthesis Report, NOAA undertake to 

provide additional guidance to TCEQ regarding fourteen specific issues and areas of 

concern that are related to chemical sources and processes that could contribute to air 

quality issues in East Texas. These fourteen specific issues and areas of concern 

provide both additional TCEQ guidance concerning emphasis and focus to the original 

twelve SIP-relevant questions addressed in the Rapid Science Synthesis Report as well 

as new areas of TCEQ concern that require future analysis and interpretations. The 

fourteen additional areas of concern are: 

• Examine NO/NO2 ratios in point source plumes, in an effort to determine whether 

the NO/NO2 ratios in the emissions inventories are correct. Is NO2 underestimated 

in emission inventories? 

• Examine role of ethene, propene, 1,3-butadiene, and butenes, other highly volatile 

volatile organic compounds (HRVOCs), and less reactive volatile organic 

compounds (VOCs) in ozone formation in Houston, in an effort to identify whether 

there are additional compounds for which emission reductions could yield 

substantial (and disproportionally large) ozone reductions. 

• Attempt to determine whether ozone formation in various locations throughout 

greater Houston is limited by availability of NOX or of radicals. 

• Calculate ozone production efficiency in urban, power plant, and industrial plumes. 

• Calculate OH radical reaction rates, i.e., OH reactivity. 

• Perform sequential reactivity model analysis of PAN and VOC data. 

• Radical budget analysis—rates of radical initiation, propagation, destruction. 

• Aldehyde emission, production, and photolysis rates. 

• Top-down emissions estimates for isolated petrochemical facilities, and Houston 

Ship Channel (HSC). 

• NOX ship emission estimates. 

• Free tropospheric ozone distributions 

• Comparisons of emissions of isoprene estimated from aircraft observations to 

biogenic emissions of isoprene from inventories. 

• Hg source investigation (in collaboration with ANL site at Moody Tower). 

• Investigate NH3 source. 

The initial findings pertaining to this request for additional information were contained in 

the oral and poster presentations that were given at the TEXAQS II / GoMACCS Data 

Analysis Workshop that was held in Austin on May 29 – June 1, 2007 or have been 

gleamed from subsequent analysis of the finalized data. This report is a synopsis of the 

initial findings and the additional analysis. The findings for the fourteen areas are 

presented in the form of fourteen brief (two to four pages) summaries of findings from 

2
the analysis that we have carried out to date. However, additional analysis and 

interpretation of the data can be used to provide much better quantification of the existing 

findings. To this end, we have included a section that indicates additional analysis that 

can be undertaken by NOAA that will considerably improve the findings contained in 

this preliminary report, as well as, the prior analysis for the original twelve SIP-relevant 

questions documented in the Rapid Science Synthesis Report. 

3
II. NO/NO2 ratios in point source plumes. 

Background 

A representative NO/NO2 emissions ratio from large combustion point sources is 

generally believed to be roughly 20 moles/mole, but peer-reviewed literature values of 

this ratio are difficult to find. Changes in this ratio will directly affect the amount of 

atmospheric ozone formed from these emissions, as each molecule of directly emitted 

NO2 represents an additional molecule of O3 expected downwind, over and above the O3

formed over time from VOC-mediated photochemistry in the atmosphere. With some 

caveats, we can begin to quantify NO to NO2 emissions ratios from large combustion 

point sources using the data from the TexAQS field studies. 

Results 

In this preliminary analysis, we use data from the NOAA WP-3D research aircraft during 

the TexAQS 2006 field study taken in the Oklaunion power plant plume after local sunset 

on October 10, 2006. All data were reported as 1-second averages, giving roughly 100-

meter spatial resolution at typical aircraft flight speeds. 

Species Technique 1-sigma uncertainty Ref. 

NO O3-induced chemiluminescence (CL) ±(0.02 ppbv + 5%) 1 

NO2 narrow-band UV photolysis; CL (P-CL) ±(0.04 ppbv + 9%) 1 

NO2 pulsed cavity ring-down spectroscopy (CRDS) ±(0.20 ppbv + 5%) 2 

NOy Au-catalysis with added CO; CL ±(0.20 ppbv + 12%) 3 

O3 NO-induced CL ±(0.05 ppbv + 3%) 1 

Note that two different techniques – photolysis followed by CL, and CRDS - were used 

to separately measure NO2 (c.f., Fig. 1). This is especially useful in the present analysis, 

as retrieving accurate NO2 data concurrently during high ambient variability of NO 

mixing ratios in plumes is a difficult test of the P-CL technique. A comparisons is shown 

in Fig. 1, demonstrating quantitative agreement within the combined uncertainties of the 

Figure 1. (left) Simultaneous measurement of O3, NO, and NO2 (by two techniques) made 

during a nighttime crossing of the Oklaunion Power Plant (North central Texas) plume. 

(right) Correlation plot that shows the comparison of simultaneous NO2 measurements 

with two different techniques that were made during the plume crossing. 

50

40

30

20

10

0

NO, NO2, O3 (ppbv)

19:25:30

10/10/06

19:26:00 19:26:30

Local time

Oklaunion

60 40

 NO2 (CRDS)

 NO2 (P-CL)

 O3

 NO

50

40

30

20

10

0

CRDS, ppbv

0 20 40 60

photolysis-CL, ppbv

linear fit to data:

slope = 1.052

int. = -0.01 ppbv

NO2 comparison

all data

30

20

10

0

NO, ppbv

4
two NO2 measurement techniques, even at significant levels of NO in the downwind 

Oklaunion plume. 

Interpretation and Conclusions

Near-field plume data taken after local sunset are considered here, to minimize the effects 

of NOX (= NO + NO2) oxidation to PANs, HNO3, and other species that occur during the 

daytime. Negligible conversion of NOX by nitrate radical chemistry for these transects 

was confirmed by examining concurrent measurements (not shown) of NO3 and N2O5

from the aircraft. 

The nighttime plume data are still affected by rapid conversion of NO to NO2 by ambient 

ozone. This chemistry substantially alters the signature of the original NO2 to NO ratio 

between the time of emission and the time of measurement by the aircraft. The plot 

below (c.f. Figure 2) shows nearly complete depletion of ozone in the plume, accounting 

for roughly 42 ppbv of the 54 ppbv of NO2 observed at this transect. In the absence of 

directly emitted NO2, a plot of O3 versus NO2 for this transect should have a negative 

slope of 1.0. A linear fit to the measured data for this example transect gives a tight 

correlation (r2

 = 0.994) and a significantly shallower slope of -0.80±0.02. This difference 

may be interpretable as a measure of the amount of directly emitted NO2, and relative to 

the observed NO. This provides a snapshot of the emissions ratio for this source. 

50

40

30

20

10

0

NO, NO2, O3 (ppbv)

19:25:30

10/10/06

19:26:00 19:26:30

Local time

Oklaunion

O3+NO2

 NO2 (CRDS)

 O3

 NO

50

40

30

20

10

0

O3, ppbv

10 20 30 40 50

linear fit :

slope = -0.80

r^2=0.994

0

NO2, ppbv

O3 vs. NO2

expected -1.0 slope

from NO+O3 alone

Figure 2. (left) Simultaneous measurement of O3, NO, and NO2 (by two techniques) made during 

a nighttime crossing of the Oklaunion Power Plant (Northeast Texas) plume. (right) Correlation 

plot showing the comparison O3 a versus NO2 indicating the depletion of ozone in the plume 

relative to the formation of NO2. 

5
III. Examine role of ethene, propene, 1,3-butadiene, and butenes, other 

HRVOCs, and less reactive VOCs in ozone formation in Houston. 

Background 

In order to evaluate and predict air quality in the Houston area, it is important to 

understand the magnitude and variability of sources of Highly Reactive Volatile Organic 

Compounds in the area, and how these sources are evolving over time. HRVOCs were 

measured from the NOAA WP-3D aircraft using (1) a whole air sampler (WAS) and 

subsequent analysis by gas chromatography, (2) a proton-transfer-reaction mass 

spectrometer (PTR-MS), and (3) a laser photo-acoustic spectroscopy (LPAS) instrument 

for fast-response measurements of ethene. The data allow to identify the location of the 

main sources and to determine the emission fluxes. Average mixing ratios of ethene and 

other HRVOC? species were lower in 2006 than in 2000. The chemical removal of 

HRVOCs and the formation of secondary species were studied on several flights that 

made transects at several distances downwind from the ship channel. 

Results 

The aircraft flights of TexAQS 2000 and 2006 reveal a persistent feature over Houston: 

on all days conducive to photochemical ozone production, narrow intense plumes of 

ozone formed and were transported downwind from HSC. Figure 1 shows as an example 

measurements from a flight on a day when the wind was from the east-northeast. The top 

two graphs show the measurements of the ozone precursors. NOy represents the NOX

emissions plus their oxidation products, primarily PAN and nitric acid. Ethylene is an 

important HRVOC ozone precursor. The concentrations of these two primary emissions 

are highest over HSC, and define a narrow downwind plume with decreasing 

concentrations. It is apparent that the NOX and ethylene 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 is primarily due to dilution. It is clear 

that ethylene decreases more rapidly than NOY, due to its much faster removal through 

photochemical oxidation. The bottom two graphs of Figure 1 show the formation of two 

secondary photochemical products, ozone and formaldehyde, coincident with the plume 

of primary emissions. Formaldehyde, which is produced from the photochemical 

oxidation of ethylene and other HRVOCs, forms during the downwind transport as the 

HRVOCs are oxidized; formaldehyde itself is an important ozone precursor. Ozone is 

produced during transport as the precursors emitted from HSC area react, resulting in 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. 

 

6
Longitude

Figure 1. Measurements of four species plotted along the flight track of the NOAA WP-3D aircraft on 6

October 2006. The symbols are sized and colored 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 to better show the HSC plume evolution.) Latitude

The high temporal resolution of the ethylene measurements 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) provide detailed characterization of the 

HRVOC sources in HSC. The map in Figure 2 shows the ethylene measurements from 

these two systems downwind from six petrochemical facilities; the plumes of ethylene 

emissions from the facilities are clearly delineated. The four pie charts in Figure 2 

indicate the total OH reactivity of the VOCs in 4 of the WAS samples, and show how that 

reactivity is divided among 4 of the lightest HRVOC species, and the total other VOCs 

including alkanes, aromatics and biogenics. These 4 examples illustrate the general 

finding that the HRVOCs dominate the VOC reactivity immediately downwind of the 

petrochemical facilities, and are responsible for a large fraction of the high ozone 

production in the Houston area. The total reactivity and the fraction of the reactivity 

contributed by the HRVOCs decrease downwind in the plumes as expected from their 

very short lifetimes. 

7
The pie charts in Figure 2 do not include the aldehydes. These oxygenated VOCs, which 

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

Figure 2. The measurements of ethylene along the section of the NOAA WP-3D flight track are shown 

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

the indicted concentrations. The triangles indicate the 20-aecond average measurements by LPAS, and 

the circles indicate the measurements from WAS. The pie charts show the VOC OH reactivity 

contribution

contributions from various VOCs at different locations along the flight track.(was not 

able to include into figure caption) 

products of the oxidation of the HRVOCs. Thus, the contribution of the aldehydes to 

ozone production increases the fraction of the VOC reactivity in the HSC area that is due 

to the HRVOCs. 

8
Interpretation and Conclusions 

The highest (i.e. > 125 ppbv) ozone concentrations in the Houston Galveston Brazoria 

(HGB) area result from rapid and efficient ozone formation in relatively narrow, intense 

plumes, which transport HRVOCs and NOX co-emitted from petrochemical facilities. 

The Houston Ship Channel (HSC) is the source of the most concentrated plumes. The 

total reactivity and the fraction of the reactivity contributed by the HRVOCs decrease 

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

which is produced from the photochemical oxidation of ethylene and other HRVOCs, 

forms during the downwind transport as the HRVOCs are oxidized; formaldehyde itself 

is an important ozone precursor. The high temporal resolution of the ethylene 

measurements 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) provide a detailed characterization of the HRVOC sources in HSC. 

9
IV-a. Is ozone formation in various locations throughout greater 

Houston limited by availability of NOX or of radicals? (Daytime 

chemistry) 

Preface: As originally posed, we presume that this question was aimed at understanding 

the influence of NOx relative to VOCs in ozone formation associated with daytime 

photochemistry. Part IV-a (below) indicated some important new findings in this regard. 

However, our research discovered some very surprising nighttime radical chemistry 

associated with NOx that could important consequences subsequent daytime chemistry. 

Part IV-b (the succeeding section) indicates some of our findings in this regard.

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. 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 air quality community is still in the 

early stages of the evaluation process; a process that is limited both by resources for 

model development, and by the availability of observations with which to compare the 

models. 

The East 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. The concentrated plumes 

that determine the highest ozone values in the Houston area present a further challenge. 

Results 

Although it is not possible to provide a 

rigorous answer to the “direction of 

control” question, we have been able to 

apply observation and modeling based 

approaches to approximately establish 

how the sensitivity of ozone 

production can be considered. We feel 

this heuristic approach can provide 

some useful guidance to air quality 

managers. 

Wert et al. (2003) present a Lagrangian 

plume model designed to closely 

reproduce emissions, ozone formation, 

other secondary photochemical product 

formation and plume dispersion 

observed during the TexAQS 2000 

200

150

100

50

0

Ozone (ppbv

Measured

)

Model

0 5 10 15

CH2O (ppbv)

20 25 30

Figure 1. Relationship between ozone and 

formaldehyde concentrations observed from 

the Electra aircraft on 1 September 2000 and 

modeled by the Lagrangian plume model. 

10
study. Figure 1 shows that the model accurately reproduced the rapid production of 

ozone and formaldehyde that was measured in a concentrated plume originating from the 

Houston Ship Channel region. The model required only two HRVOC - ethene and 

propene. Two factors were both critical in successfully estimating the highest ozone 

levels: a large value for the ozone production efficiency as shown by observations, 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 

 flux by only about oneays during TexAQS 2000 and 2006. These 

emissions dilute and disperse. 

Figure 2 shows the modeled sensitivity 

of the ozone production to both NOx 

and VOC emission reductions for the 

plume illustrated in Figure 1. The 

upper panel indicates that NOx 

emission reductions lead to a more 

rapid ozone production, but a lower 

peak ozone concentration, while 

HRVOC reductions lead to both slower 

ozone production and a lower peak 

ozone concentration. The lower panel 

indicates 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 one-half, reduces 

the ozone

quarter. 

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. Combined with other of 

our results, the calculated plume flux in 

Figure 2 represents 45 to 80% of the 

total Houston flux determined on six d

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.

Figure 2. O3 sensitivity estimated by 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 reduction in emissions 

NOx and VOC, respectively. 

Conclusions and Interpretation 

We have attempted to provide a sample of a heuristic approach can provide some useful 

guidance to air quality managers concerning the relative response of O3 concentrations to 

future reductions in NOx and VOC emissions. A simple Lagrangian plume model 

designed to closely reproduce emissions, ozone formation, other secondary 

11
photochemical product formation and plume dispersion accurately reproduced the rapid 

production of ozone and formaldehyde that was measured in a concentrated plume 

originating from the Houston Ship Channel region. These calculations suggest that 

targeted reductions in either or both VOC and NOx emission could reduce the highest 

observed ozone levels. Hence, these initial modeling results indicate that the high ozone 

in the Houston/Galveston Bay area is sensitive to both VOC and NOx emission 

reductions. 

12
IV-b. Is ozone formation in various locations throughout greater 

Houston limited by availability of NOX or of radicals? (Nighttime 

chemistry) 

Preface: As originally posed, we presume that this question was aimed at understanding 

the influence of NOx relative to VOCs in ozone formation associated with daytime 

photochemistry. Part IV-a (the preceeding section) indicated some important new 

findings in this regard. However, our research discovered some very surprising nighttime 

radical chemistry associated with NOx that could important consequences subsequent 

daytime chemistry. Part IV-b (below) indicates some of our findings in this regard.

Background 

The diurnal variation in the atmospheric chemistry of nitrogen oxides depends on the 

reactions that are important in the presence or absence of sunlight. Daytime convective 

boundary layers over land generally mix surface emitted pollutants such as NOX (= NO + 

NO2) to altitudes of 1-2 km during summer. The daytime NOX emissions undergo 

photochemical cycling in the presence of VOCs and sunlight to generate regional-scale 

ozone pollution. At night, the mixing height over the surface is typically an order of 

magnitude shallower, leading to accumulation of nighttime emissions; mixing at all levels 

of the residual daytime boundary layer is also generally less efficient. Nocturnal nitrogen 

oxide chemistry occurring within this more stratified environment involves the formation 

of NO3 and N2O5 from the oxidation of NOX in the presence of O3.

During TexAQS II/GoMACCS, the nighttime P-3 flights were aimed at characterizing 

the nocturnal reactions, transport and loss of NOX, VOCs and O3, with an emphasis on 

the role of the nighttime nitrogen oxides, NO3 and N2O5. Because of the large emissions 

of NOX in this area, it can be possible to convert a large fraction of O3 to NO3 and N2O5. 

The NO3 that is formed is a radical and is a strong oxidant of reactive VOCs (e.g. 

alkenes). However, the association of NO3 with NO2 to form N2O5 can be followed by 

the reaction of N2O5 on wetted aerosol to form HNO3. This effectively removes the NOX

and O3 used to form the N2O5 from the atmospheric photochemistry. 

In addition, during TexAQS II/GoMACCS, NOAA scientists using chemical ionization 

mass spectrometry (CIMS) onboard NOAA R/V Ronald H. Brown (RHB) made the first 

measurements of ClNO2 in the ambient atmosphere. Nitryl chloride, ClNO2, is a known 

product of nighttime reaction of N2O5 on sea salt that is rapidly photo-dissociated by 

sunlight releasing atomic chlorine. These new findings have important consequences for 

our understanding of the relation between ozone formation and the availability of NOX

and radicals In the Houston area, 50 - 90% of NOX converted to NO3 + N2O5 overnight. 

Results 

NOX Radical Chemistry Measured from NOAA WP-3D

Preliminary analysis of the data shows that mixing ratios of NO3 and N2O5 were highly 

variable and occasionally very large, consistent with previous aircraft measurements of 

these compounds. Mixing ratios of NO3 reached 400 pptv in plumes from the Houston 

Ship Channel. At these levels, NO3 is capable of efficiently oxidizing highly reactive 

13
VOCs that might otherwise participate in ozone production on the following day. The 

nocturnal P-3 flights included multiple, low-level vertical profiles to address the issue of 

stratification, reduced mixing and their influence on chemical transformation of 

pollutants at night. 

One of the important finding from 

TexAQS II/GoMACCS was that 

during the nighttime P-3 flights, 

the N2O5 hydrolysis to form HNO3

was surprisingly slow. A sample 

of some of the data that was used 

to support this conclusion is shown 

in Figure 1. These data were taken 

on October 10, 2006 during a flight 

that investigated the plume 

downwind of the Oklaunion Power 

Plant Plume that is located in north 

Texas). Plotted are the time series 

of measurements made aboard the 

P3 of O3, NO2, N2O5, and HNO3, 

during one plume crossing. These 

measurements illustrate the loss of 

ozone that occurs when NO is 

converted to NO2, and NO2 is 

converted to NO3 and N2O5

followed by the subsequent 

production of HNO3 through the 

hydrolysis of N2O5 on aerosols. 

The data show the production of NO2 and N2O5 but show no evidence for the formation 

of HNO3 from N2O5 hydrolysis. Also, plotted is OX the sum of the mixing ratios of O3, 

NO2 N2O5 and HNO3. Finally, the sum of the species, OX, balances, thus, accounting for 

the loss of O3. Similar results were obtained from flights made downwind of the Parrish 

Power Plant located in the Houston metropolitan area. 

Figure 1. Plume-crossing downwind of the Oklaunion 

Power Plant (North Texas) taken during a nighttime 

flight (10/10/06). Plotted are time series of 

measurements of O3, NO2, N2O5, HNO3 and Ox (sum of 

the other four compounds). 

These results imply that during the nighttime hours the NOX is conserved in these power 

plant plumes that are NOX-rich and VOC-lean. Hence at sunrise with daytime heating the 

NOX that has been converted to NO3 and N2O5 will be converted back to NOX and be 

available to be involved in daytime ozone formation if larger missing ratios of VOCs are 

present. 

NOX Radical Chemistry Measured from NOAA RV Ronald H. Brown

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 is relatively stable at 

night, but photolyzes at sunrise to yield Cl atoms and NO2. These reactions link the 

nitrogen oxides to halogen activation. 

14
An episode of high ClNO2 was observed 

on September 2, 2006 when the R.H. 

Brown was in the Barbour’s Cut inlet 

located off Galveston Bay near HSC. 

Figure 2a (top panel) shows the 

measured N2O5 and ClNO2 along with 

the Cl atom production rate calculated 

from the measured ClNO2 and solar 

flux, and known ClNO2 UV absorption 

cross section. This Cl atom production 

rate reached almost 106 sec-1. This is 

significant since Cl atoms react with 

VOC up to 100 times more rapidly than 

OH radicals. Figure 2b (bottom panel) 

compares O3 measured on September 2 

with the average (±1 std deviation) of 

the O3 levels observed during all other 

times that the RH Brown was in was in 

the Houston Galveston area during the 

2006 study. The September 2 O3 showed 

a more rapid rise, and a higher peak 

concentration than the average. This observation, while not definitive, is consistent with 

a 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 employed the Master Chemical Mechanism 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 2b. 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 level at the end of the afternoon production period. These results amplify the 

need to add this Cl atom source to current regional photochemical models. 

Conclusions and Interpretation 

NOX Radical Chemistry Measured from NOAA WP-3D

Our measurements suggest that at night, in power plant plumes with low mixing ratios of 

VOCs, NOx and Ox could be effectively transported in the form of N2O5. By contrast in 

the plumes of petrochemical plants, NO3 could serve as an effective oxidant. In several 

cases, VOC loss rates of 0.5 - 4 ppbv hr-1 were observed in Ship Channel plumes. During 

nighttime, the chemistry of NO3/N2O5 could be strongly influenced by vertical 

stratification. This could produce shallow, concentrated NOX plumes and lead to large 

difference between surface and aloft. 

 

NOX Radical Chemistry Measured from NOAA RV Ronald H. Brown

Figure 2. N2O5, ClNO2 measured on September 

2, 2006, and calculated Cl atom production in 

the atmosphere (a); the measured O3 on 

15
Observations show that nitryl chloride, ClNO2, is formed within the nocturnal boundary 

layer when pollution and marine influences are both present. Following sunrise this 

species photolyzes to yield chlorine atoms, which may lead to more intense O3

production in the HGB region. The measurements show ClNO2 mixing ratios up to 1 

ppbv during periods of active N2O5 aerosol chemistry, and that submicron chloridecontaining non-sea salt aerosol is involved in ClNO2 formation. Preliminary laboratory 

results confirm that ClNO2 is produced from N2O5 uptake on the aerosol substrates 

{(NH4)2SO4, (NH4)HSO4} that contain chloride ion concentrations that were observed in 

the TexAQS II/GoMACCS measurements. ClNO2 photolyzes upon sunrise, producing 

chlorine atoms (Cl) and NO2. The impacts of this Cl atom source are far-reaching and 

include: more intense VOC-NOx photochemistry early in the day, enhanced oxidation in 

NOX plumes from ship emissions, and the preservation of NOX that would normally be 

lost through nighttime chemistry. 

16
V. Estimate the ozone production efficiency in urban, power plant, and 

industrial plumes. 

Background 

Ozone is formed over time in the troposphere by photochemical reactions involving the 

oxides of nitrogen, NO and NO2 (summed as NOX), and reactive volatile organic 

compounds (VOCs). However, ozone formation rates and yields are dependent on both 

the absolute concentrations of NOX and VOCs and the ratios of these species. Results 

from field studies have confirmed that substantial differences in the timing and 

magnitude of ozone production consistently occur in plumes downwind of different 

anthropogenic source types, characterized by different NOX and VOC emissions rates and 

the VOC/NOX ratios that result. Three anthropogenic source types with contrasting 

emissions rates and VOC/NOX ratios are fossil-fueled electric power plants, the 

transportation sources typical of urban areas, and the petrochemical industry.

Results 

Measurements obtained from the NCAR Electra in 2000 and the NOAA WP-3 in 2006 on 

daytime flights in the boundary layer under a variety of meteorological conditions have 

been used to determine the ozone production efficiency in plumes downwind from an

urban power plant and petrochemical industry plants in Houston. During TexAQS 2000, 

four Electra flights encountered ozone concentrations above 150 ppbv. Figure 1 shows 

the flight track segments where the highest ozone was observed. In each case, these 

plumes were traced back to industrial emission sources in the HSC area by trajectory 

analysis. Measured chemical characteristics of the plumes (simultaneous high NOx, SO2, 

CO2, and oxidation products of HRVOCs) confirm this source attribution. The 

Figure 1. 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. 

17
relationship between the transport times derived from the trajectory analysis and the 

observed enhancements in ozone provided a measure of the net average ozone production 

rates in the plumes. Figure 1 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. 

By contrast, Figure 2 presents a similar analysis for three NOAA WP-3D daytime flights 

and the RHB 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 trace back to industrial 

emission sources in the Houston Ship Channel area. Figure 2 also shows the relationship 

between ozone and the products of NOx oxidation in these plumes. 

Figure 2. Highest ozone observed by the WP-3D aircraft and RHB research vessel during TexAQS 2006. 

The figure is in the same format as Figure 1. The dotted rectangle indicates area covered in Figure 1. 

Interpretation and Conclusions 

A comparison of Figures 1 and 2 indicates 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 observed ozone 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-hand plots in Figures 1 and 2, is lower in 

2006 and the lower background leads to lower maximum concentrations. The lower 2006 

background may indicate lesser effects of stagnation that year. Finally, the maximum 

ozone concentrations are seen at greater distances from the ship channel in 2006 than in 

2000 (note difference in area covered in the two maps). This difference may be 

attributable to slower ozone production or greater transport speeds in 2006. 

18
VI. Calculate OH radical reaction rates, and OH reactivity for ozone 

precursors. 

Background 

Volatile Organic Compounds and NOX (= NO + NO2) are vital components in the 

photochemical production of ozone (O3). The first step in initiating the radical cycle 

responsible for O3 production is the reaction between •OH (produced in situ during the 

daytime) and VOCs. Along the Gulf coast of Texas there exists an immense variety of 

industrial, urban, and natural sources of VOCs. A wide spectrum of VOCs were 

measured aboard the NOAA R/V Ronald H. 

Brown during TexAQS/GoMACCS 2006 in 

an effort to fully characterize their sources 

and determine the key species responsible 

for the photochemical ozone production in 

the Houston/ Galveston area. 

Results 

The reactivity of the ozone precursors were 

investigated during TexAQS 2006 from 

measurement made aboard the NOAA 

research vessel Ronald H. Brown. The VOC 

measurements were made with a custom dual 

channel gas chromatograph (GC) 

incorporating both a Flame Ionization 

Detector (FID) associated with an 18m 

Aluminum oxide column (Al2O3) for the 

light hydrocarbons and a quadrupole mass 

spectrometer (MS) associated with a 20m 

DB-624 column for the heavier VOCs. 

Additional measurements of VOCs were 

made using a modified commercial proton 

transfer reaction mass spectrometer (PTRMS). Formaldehyde was measured using a 

quantum cascade tunable diode laser 

absorption spectrometer (QC-TDALS). The 

NO and NOx measurements were made with 

a chemiluminescence NO detector equipped 

with a UV photolytic converter to convert 

NO2 to NO. The accuracy for all compounds 

is ± 20%. 

During the study over two hundred VOCs were measured using the GC-FID/MS. Of 

these the eighty most abundant have been quantified and the measured mixing ratios that 

were observed in the Galveston Bay/Houston Ship Channel are summarized in the table 

on the right and compared with the measured mixing rations of these species that were 

measured at La Porte during the 2000 study. Also indicated in the table is the OH 

19
reactivity for these VOCs. These compounds account for approximately 90% of the 

VOC-OH reactivity. 

Interpretation and Conclusions 

Over all, it was found that the VOCs dominated the OH chemistry at all inland locations. 

This is illustrated in the Figure 1 that is shown on the right. However, the magnitude and 

variability of the VOC OH Loss rate 

depended strongly on: meteorology 

(wind speed and direction), time of day 

(day/night), location (sources and 

emission sources/rates), and VOC 

chemical processing (fresh vs. aged). 

VOC mixing ratios were typically 

highest during the nighttime. This is 

related to many factors including 

meteorology principally limiting 

vertical mixing and reduced chemical 

destruction. The nighttime VOC plumes 

also showed strong indications of 

industrial sources (e.g., monomers—

used in plastics production). There were 

large quantities of highly reactive VOCs present (alkenes, terpenes, monomers) through 

early morning that quickly reacted away as ozone production began. Late afternoon VOC 

composition were characterized by high mixing ratios of aldehydes associated with 

secondary photochemical formation. 

Fig. 1. OH reactivity for all Galveston Bay and 

Houston Ship Channel measurements made aboard 

the NOAA RV RH Brown. 

19 % 

VOCs 

73 % 

6 % 

NO2

2% 

CH4

CO 

In summary, these results indicated that in Galveston Bay and the Houston Ship Channel 

ethylene, propylene, isoprene (mostly from industrial as opposed to biogenic sources), 

formaldehyde, and acetaldehyde were the largest contributors (based on median values) 

to possible ozone production in the Galveston Bay/Houston area during TexAQS 2006. 

The magnitude and variability of the VOC/OH reactivity was dependent upon: 

• Location – Houston Ship Channel and Barbour’s Cut were strongly affected by 

industrial emissions; 

• Chemistry – Highly reactive VOCs can accumulate during night (slower loss rates) 

and react away during day (shorter chemical lifetimes); 

• Time of day – Meteorology and emission source rates (biogenic, urban, industrial) 

may vary over 24-hours. 

20
VII. Sequential reactivity model analysis of PAN and VOC data. 

Background 

The relationship between primary VOCs and their product compounds is intimately 

connected to the same photochemistry that produces O3 and to some extent particles. 

Complex Lagrangian and Eulerian numerical models are often used to examine these 

relationships, but their results are dependent on accurate knowledge of sources, transport, 

and reaction pathways: details that are often unknown or incomplete. A conceptually 

simple sequential reaction model has been successfully used to examine small pieces of 

the complex VOC photochemistry in instances where the parent and product VOC 

measurements are available, and the reaction pathways (rates, branching ratios) are 

known. The information that has resulted from this type of analyses includes: the time 

scales on which these process are occurring, the strong correlation of conversion of 

parent to product with O3 formation, and indications of additional chemical pathways not 

captured by the simple model. This last feature has been used to show the importance of 

isoprene chemistry to the production of PAN (and therefore O3) in data from several 

surfaces sites. An analysis of the TexAQS 2000 measurements at the La Porte site 

showed that isoprene chemistry was comparatively unimportant at this site. 

Results 

The preliminary aldehyde-PANs sequential reaction analysis carried out for the TexAQS 

2006 data acquired aboard the NOAA R/V Ronald H Brown is substantially different 

from the 2000 results. These differences seem to be due to differences in the 

acetaldehyde and to a lesser extent PPN measurements between the two years. At present, 

we are not sure what the causes are. 

Interpretation and Conclusions 

Further analysis is required to determine if the differences in data sets are due to different 

precursor or loss chemistry, time and location of the measurements, or analysis problems. 

21
VIII. Perform radical budget analysis including rates of radical 

initiation, propagation, destruction. 

Background 

An extensive set of chemical and physical measurements was made at the La Porte 

Airport in Houston during the 2000 Texas Air Quality Study. OH and HO2 mixing ratios 

were measured using the Penn State Ground-based Tropospheric Hydrogen Oxides 

Sensor (GTHOS). Total OH reactivity was measured by the Penn State Total OH Loss 

rate Measurements (TOHLM) method. Extensive measurements of other chemical 

species and physical parameters were made at the La Porte Airport site during TexAQS 

2000. 

By way of independent comparison, steady state [OH] and [HO2] were calculated 

simultaneously along with the concentrations of speciated RO2, NO3, N2O5, HNO2, and 

HNO4 by a box model with most other input variables constrained by measurements. A 

key assumption in the analysis was that OH, HO2, and the other calculated species were 

in steady state with each other and with the constrained measured compounds, which in 

turn assumes vertical and horizontal homogeneity in the vicinity of the La Porte ground 

site.

Results 

The observed hydroxyl and hydroperoxyl radical concentrations ([OH] and [HO2]) at La 

Porte were compared to those predicted by a steady state box model. The model is 

constrained by ambient measurements of ozone, reactive nitrogen compounds, carbon 

monoxide, volatile organic compounds, photolysis rate coefficients, and meteorological 

variables that were measured at the site. The photochemical production rate of ozone, 

P(O3), due to HO2 reactions were derived from the observed [HO2] and nitric oxide 

concentration ([NO]) and compared to the model. Budgets of OH and HOx were 

quantified using the model. Model-measurement comparisons of [OH], [HO2], 

[HO2]/[OH], and P(O3) were further evaluated by considering their dependence on 

variables such as [NO] and actinic flux. 

Interpretation and Conclusions 

The radical budgets indicate that chemistry at La Porte was dominated by light alkenes 

and their photochemical products (oxygenates). The results indicated the impact of 

nearby industrial and mobile sources, particularly refineries and polymer production 

plants. The model and measurements are in rough quantitative agreement throughout 

most of the daytime. However, in general, the model [OH] tends to be lower than the 

observations in the afternoon, while the model [HO2] tends to be higher than the 

measurements from mid-morning on. These tendencies lead to model overestimates of 

the [HO2]/[OH] ratio during the day. The observed radical sources, sinks, and cycling 

were in reasonable agreement with the model during periods when the highest 

concentration of reactive compounds were observed. From these results is appears that 

photochemistry can easily account for rapid O3 production seen at La Porte. It should be 

22
noted that some discrepancies were found between model simulations and measurements 

when the measured concentrations of ozone precursors were low. 

Resolution of the differences noted above in the in HOX measurements at La Porte in 

2000 and those inferred from the initial model simulations requires further analysis and 

additional data. Extensive analysis of the 2006 data and reanalysis of the 2000 data using 

improved modeling approaches are now being undertaken. 

23
IX. Estimate aldehyde emission, production, and photolysis rates. 

Background 

Formaldehyde and higher aldehydes including acetaldehyde are products of the 

atmospheric oxidation of hydrocarbons, a potential source of HOx, and a contributing 

species to determining how HOx is partitioned between OH and HO2. 

Results 

Formaldehyde Measurements from NOAA WP-3D

Airborne, 1s measurements of formaldehyde (CH2O) were performed using a tunable 

mid-IR difference frequency generation (DFG) laser spectrometer. The average detection 

limit (S/N=1) for all flights was 117 pptv (1-s average) and 21 pptv (60-s average), 

respectively. For most flights, the instrument was operated semi-autonomously with a 

duty cycle of 70% (60 s sample / 30 s background, incl. flushing) affording good spatial 

coverage and precision. The instrument operated with the quoted performance for over 94 

% of research flight time. Using an advanced DFG laser source, the present instrument 

performed with 2x better sensitivity, longer sampling times (60 s), and 70% higher duty 

cycle, compared to the liquid nitrogen cooled tunable lead-salt diode laser absorption 

spectrometer (TDLAS) employed during the TexAQS 2000 study. 

Results from these measurements are summarized in Figure 1. Compared to the study in 

2000 (TexAQS 2000), the median concentrations in the vertical distributions are 

significantly lower (50 %) and this may be due to different ambient sampling / 

photochemistry conditions (Sep./Oct. 2006 vs. Aug./Sep. 2000) during TexAQS 2006. 

Figure 1. Measurements of CH2O made aboard the NOAA WP-3D. The plot on the left shows the 

altitude profile for all the 1-sec. measurements that were made during the study. The table on the 

right compares measurement made in 2006 with those obtained in 2000 binned according to altitude. 

24
Downwind plumes from power plants, the Houston shipping channel and selected 

refineries have been intercepted and CH2O shows a clear signature and correlation with 

O3 and NOY. Persistent progression of plume indicates in-situ photochemical production 

of CH2O during the day. 

An important issue that was addressed during the 2006 study pertained to the evidence 

for primary sources of CH2O. However, because of the rapid formation of CH2O during 

the daytime the fingerprints of such primary sources of CH2O can only be retrieved from 

the data with more extensive analysis. 

During the nighttime however the 

signatures of primary source should be 

more apparent. An example of this 

type of data was afforded by data 

taken during the NOAA WP-3D flight 

on October 12th. The results shown in 

Figure 2 shows a missed approach at 

Montgomery County Airport. In the 

figure, each symbol represents a 1-s. 

measurement of CH2O color coded by 

the altitude of the aircraft. The strong 

correlation of CH2O with CO in 

measurements made below 200 m 

indicates the contribution of primary 

sources of CH2O. The line drawn 

though this portion of the correlation 

between CO and CH2O indicates an 

upper limit of 1.8 pptv of CH2O 

emitted for each ppbv of CO. Clearly 

there are other possible non-primary 

sources of CH2O that could be 

contributing to the observed CH2O. 

These would include CH2O remaining 

from the daytime photochemical formation and secondary CH2O produced in the 

reactions of oxidized and/or unsaturated VOC with ozone. 

Figure 2. Missed approach at Montgomery County 

Airport. Data from NOAA WP-3DP3 flight onf 

Oct. 12thSept. xx. Each symbol represents a 1-sec. 

measurement of CH2O color coded by the altitude 

of the aircraft. 

Formaldehyde Measurements from R/V Brown

Formaldehyde was measured aboard the NOAA RV Ronald H. Brown using pulsed 

quantum cascade tunable infrared differential absorption spectroscopy (pQC-TILDAS, 

Aerodyne Corp.) in the 5.6 μm (1765 cm-1) wavelength region. The in-the-field 

determination of the detection sensitivity resulted in 1-s rms levels of 250 pptv. The 

wavelength region used for determining CH2O also allowed an opportunistic 

measurement of formic acid. As was the case with the aircraft measurements, the daytime 

measurements of CH2O were strongly influenced by photochemical CH2O formation. 

This largely masked primary sources of CH2O.

25
In the Houston Ship Channel, the diurnally averaged peak concentration of CH2O was 

observed to precede the peak concentrations of O3 by 2.5 hours. 

Figure 3. This figure shows the simultaneous measurements of CO2 and CH2O 

that wese made in the plume of a large ship (Callsign: WDA8059). The ratio of 

CH2O relative to CO2 from this plot implies to an emission of 0.05 g of CH2O for 

each kg of fuel burned.

However, even during the 

daytime it was possible to 

observe the primary emission of 

CH2O from the exhaust of large 

ships. This is illustrated in 

Figure 3 that shows the 

intercept of the plume from a 

large ship that was loitering in 

the Houston Ship Channel. The 

figure shows the simultaneous 

measurements of CO2 and 

CH2O that were made in plume 

of this ship. The ratio of CH2O 

relative to CO2 from this plot 

implies an emission of 0.05 g of 

CH2O for each kg of fuel 

burned. Several such plume 

intercepts were made during the 

study and will afford the 

opportunity to better quantify 

Figure 4. Measurements of CH2O made in the turning 

basin during the night and early morning of August 

2nd. The measurements are color coded according to 

the intensity of sunlight (presented here as the 

measured photolysis rate for CH2O). The correlation 

indicates an upper limit for CH2O primary emission of 

3 pptv per ppbv of primary CO emissions. 

26
the emission of CH2O from these large diesel powered marine vessels. 

In addition to the determination of the emission of CH2O from single isolated sources, 

nighttime measurements of CH2O also afforded the opportunity to place an upper limit on 

the primary emissions of CH2O during the nighttime from sources. Figure 4 shows 

measurements of CH2O that were made in the turning basin during the night and early 

morning of August 2007. During this period the wind was steady from the west 

transporting emission from central Houston to the west of the ship channel to the location 

of the NOAA RV. In the figure, the measurements are color coded according to the 

intensity of sunlight (presented here as the measured photolysis rate for CH2O). The 

correlation indicates an upper limit for CH2O primary emission of 3 pptv per ppbv of 

primary CO emissions, which are predominately due to on-road vehicles. 

Interpretation and Conclusions 

During the TexAQS in 2006, CH2O was measured aboard two of the NOAA mobile 

research platforms, the WP-3D aircraft and the research vessel Ronald H. Brown. This 

data set will provide excellent opportunities to better understand the sources and 

processes involving CH2O in Houston and the surrounding area. In the Houston 

metropolitan area copious quantities of CH2O are produced photochemically during the 

day. Elevated mixing ratios of CH2O that are encountered in plumes can be attributed to 

the rapid photochemical formation of CH2O from the fast photochemistry between oddhydrogen free radicals, HRVOCs and NOX in the plumes. In general, this photochemical 

production masks the smaller primary emissions of CH2O. However, simultaneous 

measurements of CH2O and CO2 during plume intercepts of the emissions from large 

diesel marine vessel indicated that under certain conditions a few hundreds of a gram of 

CH2O night be emitted for each kilogram of fuel consumed. 

In addition, limited nighttime measurements of CH2O from both the aircraft and the 

research vessel indicated that the direct emission of CH2O is a few tenths of a percent or 

less of the direct emission of CO from the on-road vehicle fleet. However, this small 

amount of CH2O is an upper limit to the primary emissions, since it could possibly be 

attributed to residual CH2O remaining from the CH2O formed photochemically during 

the proceeding day or produced photochemically during the night by ozone or nitrate 

radical initiated oxidation of HRVOCs. 

27
X. Provide top-down emissions estimates for isolated petrochemical 

facilities, in and around the Houston Ship Channel.

Background 

Improving the quantitative understanding of the location, timing, and speciation of 

gaseous and aerosol emissions into the atmosphere is critical to the development and 

improvement of air quality management strategies and approaches. Known or suspected 

inaccuracies in current emissions inventories account for a substantial uncertainty in 

model simulations of air quality processes. While some inventories are reasonably 

accurate others may have substantial errors. In order to evaluate and improve emission 

inventories that are used for air quality modeling, several new strategies for top-down 

emission estimates have been developed by NOAA scientists and applied to the 

evaluation of inventories for emissions from specific industrial point sources. Because of 

the importance in the formation of ozone in and around the Houston metropolitan area, 

considerable attention during TEXAQS II was directed to providing top-down emissions 

estimates for isolated petrochemical facilities, and the Houston Ship Channel. 

Results

 Volatile Organic Compounds (VOCs) are a vital ingredient in the photochemical 

production of ozone. Along the gulf coast of Texas there is a variety of industrial, urban, 

and natural sources of VOCs. Volatile organic compounds (VOCs) were measured on 

the NOAA WP-3 aircraft using various techniques. These data provide the information 

necessary for estimating the flux of VOC emissions from individual industrial sources 

located in the Houston Ship Channel area. 

Figure 1. Data from the WP3D flight on Sept. 25, 2006. 

The map indicates the 

aircraft interception of the 

plume from the Mt. Belvieu 

petrochemical plant. The 

arrow shows wind direction 

and the flight track is colorcoded according to the 

concentration of ethene. The 

trace (right) indicates the 

mixing ratio of ethene 

measured during plume 

interception. 

prevailing 

wind 

28
Ethene is one of the most important of the VOCs in the photochemical formation of 

ozone. During the study we were able to directly measurement ethene with a newly 

developed technique, laser photo-acoustic absorption spectroscopy (LPAS). An example 

of the result is shown in Figure 1. The data were taken during the W-P3 flight on 

September 25, 2006. The map on the left indicates a portion of the flight track of the 

aircraft on this day where the aircraft intercepted the plume that analysis indicated was 

associated with emissions from the Mt. Belvieu petrochemical plant. During this day the 

wind was from the north and the flight track is color-coded according to the measured 

concentration of ethene to indicate where the aircraft encountered the plume. The trace 

on the left shows the measurement of the mixing ratio of ethene during the plume 

interception. The analysis of the mixing ratio of ethene measured during this encounter 

yielded the emission flux: 280 kg h-1. The estimated flux of ethene measured during the 

course of this study agreed within a factor of two with flux estimates made independently 

by ground based solar occultation flux (SOF) measurements made on this same day. 

During TexAQS II, volatile organic compounds (VOCs) were also measured on the 

NOAA research vessel Ronald H. Brown using various techniques. A GC-MS (Gas 

Chromatograph-Mass Spectrometer) and a (GC)-PIT-MS (Gas Chromatograph-Proton 

Ion Trap-Mass Spectrometer) were used to collect data on a wide variety of VOCs aboard 

the Ronald H. Brown during TexAQS/GoMACCS 2006 in an effort to identify and 

characterize their sources. The two examples below indicate the additional approaches 

that were used in the Houston Ship Channel area to infer if VOCs were primary or 

secondary and to place limits on these sources. 

The diurnal variations of ethene and formaldehyde at several locations in the Houston 

Ship Channel were studied. Figure 2 shows the data from Barbour’s Cut as an example. 

The primary hydrocarbons ethene and 

propene are high at night, when emissions 

accumulate in a shallow boundary layer and 

when photochemistry is absent. Mixing 

ratios are lower during the day, when the 

boundary layer expands and reactions with 

OH remove the alkenes. In contrast, HCHO 

is highest during the day, indicating that the 

source of this oxygenated VOC was 

predominantly photochemistry. 

Nighttime measurements onboard the Ronald 

H. Brown and the NOAA WP-3D provide a 

means to estimate the direct emissions of 

formaldehyde during the night. The left 

panel of Figure 3 shows the relationship of 

formaldehyde to CO measured on the Ronald 

H. Brown in the turning basin at the western 

end of the Ship Channel with winds coming 

from Houston. The data are color-coded by the measured photolysis rate of 

Figure 2. Diurnal variation of ethene, 

propene and formaldehyde measured from 

the Ronald H. Brown in Barbour’s Cut. 

29
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). 

From these observations we 

estimate the primary emissions of 

formaldehyde during the night from 

sources in the area to be 0.18-0.3 % 

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 earlier 

to at least a limited extent. 

Interpretation and Conclusions 

NOAA scientists have developed a 

variety of approaches to provide 

top-down emissions estimates of 

NOx, SO2, and a wide variety of VOC sources for isolated petrochemical facilities, in and 

around the Houston Ship Channel. In addition, the data can be used qualitatively to 

establish if compounds are primary emission from these sources or are secondary 

photochemical products. A great deal is yet to be learned from further analysis of the 

extensive data set taken during TexAQS I and TexAQS II. 

Figure 3. Relationships of HCHO versus CO 

obtained onboard the R/V Ronald H. Brown in the 

turning basin at the western end of the HSC on 8/2. 

30
XI. Estimates of NOX Emissions from Commercial Marine Shipping.

Background 

Commercial marine vessels range in size from small fishing boats (20-30 meters in 

length) to extremely large container ships (>300 meters in length). These ships almost 

without exception use diesel engines for propulsion and auxiliary power generation. The 

larger ships, comprising bulk carriers, tankers and container carriers, utilize diesel 

engines that produce power in the 10 MW to 100 MW range. Such engines typically 

consume heavy fuel oils, which are high in sulfur content (1%-4.5% by weight). These 

engines are also extremely efficient, converting essentially all of the carbon in the fuel to 

CO2, but also emitting NOX, CO, SO2, VOCs, and PM. 

Results 

During TexAQS 2006 our measurements on board the Ronald H. Brown characterized 

the emissions from a large number of commercial marine vessels. From this data, we 

have calculated mass-based emission factors for many of the compounds noted above. 

Table 1 presents the average derived NOx 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 for each of the two engine types. 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 

Engine type NOX CO SO2 CH2O 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.

Table 1. Summary of average marine vessel emission factors. 

 

accurate characterization of NOX emissions from underway vessels in the HGB region. 

The emission factors for CO are within 20% of the value reported in 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 earlier Lloyd's data. Measurements of CH2O emissions from 

ships show little distinction between MSD and SSD engines; emission of CH2O is less 

than 5% the emission of CO. Emission factors for SO2 vary with fuel sulfur content. In 

the HGB region the mean fuel sulfur 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 made. 

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, 

31
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. 

Interpretation and Conclusions 

Table 2 gives ship emissions of NOX, CO and SO2, relative to the emission of CO2, and 

compares it to emission ratios for electric power generating units (EGUs) from the 2004 

point source emission inventory, that was updated to 2006 with CEMS data. This 

comparison shows that ships emit 10 to 100 times more NOx (and only 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 due to 

technical reasons and complications arising from international law. 

Table 2. 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. 

In addition, during the TexAQS/GoMACCS 2006 field campaign, the NOAA R/V 

Ronald H. Brown often encountered small marine recreational craft while sailing close to 

the Texas coast, especially in Galveston Bay. Measurement of a suite of trace gases at 

high time resolution (1 Hz) allowed us to calculate emission factors (EFs), relative to 

carbon dioxide, for nitrogen oxides (NOX), sulfur dioxide (SO2) and carbon monoxide 

(CO) for distinct exhaust plumes from these vessels. As previously observed along the 

New England coast, gasoline-powered craft showed significantly higher NOX/CO2 and 

lower CO/CO2 EFs than current emissions inventories predict, although in agreement 

with the most recent published literature. These findings imply lower volatile organic 

compound emissions from these vessels, although this was not directly measured. 

32
XII. Investigate free tropospheric ozone distributions.

Background 

Tropospheric ozone is a central trace gas that determines the oxidizing capacity of the 

troposphere. Most of the tropospheric ozone burden is produced from anthropogenic 

emissions resulting in widespread ozone increases since the late 1800s. A recent 

development in our understanding of tropospheric ozone is the discovery of an O3

maximum during summertime within the upper tropospheric anticyclone above the 

southern United States. The ozone enhancement is believed to be the result of convective 

lofting of surface ozone, and in situ production involving anthropogenic NOX, lightning 

NOX, and in part HOX radicals produced from convectively lifted formaldehyde. The 

ozone could be produced over many days due to the trapping of the ozone precursors in 

the semi-permanent upper tropospheric anticyclone located above eastern North America 

during summer. 

Results 

Daily ozonesondes were launched from 14 

North American sites during August 2006 

(c.f., Figure 1A). These measurements 

provide the best set of free tropospheric 

ozone measurements ever gathered across 

the continent in a single season. The data 

reveal a distinct upper tropospheric ozone 

maximum above eastern North America, 

and centered over the southeastern USA 

(c.f., Figure 1A). Recurring each year, the 

location and strength of the ozone 

maximum is influenced by the summertime 

upper tropospheric anticyclone that traps 

convectively lofted ozone, ozone 

precursors and lightning NOX above the 

southeastern USA. Figure 1B shows the 

residual ozone that is left after the 

estimated ozone from the stratosphere has 

been subracted. In addition, this result is 

supported by calculations from a general 

circulation model that indicates that 

lightning NOX emissions led to the 

production of 25- 30 ppbv of ozone at 250 

hPa above the southern United States 

during the study period. 

A

B

Figure 1. Estimated O3 mixing ratios between

10-11 km during August 2006, measured at 14

sites. A) total ozone; and B) estimate ozone

derived only from tropospheric sources.

Interpretation and Conclusions 

The measurements made during the 

summer of 2006 provided 410 ozone profiles across North America. The study confirmed 

the hypothesis that the upper free tropospheric O3 maximum above eastern North 

33
America is a recurring summertime feature and that the exact location and strength varies 

inter-annually with the mean location and strength of the upper tropospheric anticyclone. 

Independently, modeling work indicates that lightning NOX led to the production of 25-

30 ppbv ozone in layers approximately 10-11 km altitude above the southern United 

States during August 2006. 

34
XIII. Compare emission estimates of isoprene derived from aircraft 

measurements to inventory-estimated biogenic emissions of isoprene. 

Background

In order to understand the photochemical formation of ozone on local to regional scales, 

it is necessary to have reliable estimates of the contribution of the natural emissions of 

VOCs, such as isoprene, to the total VOCs that are available in those areas. During 

TexAQS in 2006, volatile organic compounds (VOCs) were measured on the NOAA 

WP-3D aircraft using a PTR-MS instrument and from whole air samples (WAS) using 

various gas chromatographic techniques. Using the online measurements on the aircraft, 

isoprene emission fluxes were estimated and directly compared to the BEIS-3 inventory 

(EPA Biogenic Emissions Inventory System). The BEIS-3 data were also incorporated in 

a Lagrangian particle dispersion 

model (FLEXPART) and results 

were compared with measurements. 

Results 

An example of the results obtained 

by PTR-MS is given in Figure 1, 

which shows the measurements 

from a flight over an area with high 

isoprene emissions in northeast 

Texas. Isoprene mixing ratios are 

high and highly variable in the 

boundary layer, and quickly drop 

off to zero above the boundary 

layer. The monoterpene levels are 

relatively low at 60 pptv or less, 

whereas in other regions of the US, 

levels over 200 pptv have been 

observed from the NOAA WP-3D. 

The correlation between measured 

isoprene mixing ratios from all 

flights and the BEIS-3 emissions 

inventory (for nominal, standard 

meteorology) is shown in Figure 2. 

It can be seen that measured 

isoprene was generally higher 

above areas with higher emissions. 

An exception is an area to the 

southeast of Dallas-Fort Worth, 

where the highest emissions are 

expected according to the inventory, 

but where the NOAA WP-3D only measured moderately enhanced mixing ratios. 

Figure 1. Flight track of the NOAA WP-3D on 

September 16, 2006, color-coded by the measured 

mixing ratio of isoprene. The lower panel shows the 

time series of isoprene along with that of the 

monoterpenes and the flight altitude. 

35
Figure 2. Correlation between the BEIS-3 isoprene emission 

inventory and the isoprene mixing ratios measured from the NOAA 

WP-3D in 2006 and the NCAR Electra in 2000.

Measurements from the NCAR Electra during TexAQS 2000 are shown for comparison. 

They show higher isoprene in this area southeast of Dallas/Ft. Worth. 

36
Interpretation and Conclusions 

In general, the comparison between measurements and inventory showed an agreement 

within ±50%, somewhat dependent on the day and the location. The OH reactivity of the 

biogenic VOCs, which is important for the ozone production efficiency, was also 

compared to the OH reactivity from anthropogenic VOCs separated into urban and 

industrial VOCs. Before final conclusions are reached, several important issues must be 

investigated including the different drought stress on the vegetation within these two 

study years, and the influence of recalculating the BEIS-3 isoprene emissions with the 

actual meteorology specific to the time of the isoprene measurements. 

37
XIV. Hg Source Investigation. 

Background 

Gas-phase elemental mercury (Hg(0)), while long-lived in the atmosphere, is thought to 

be the primary source of mercury (II) and particulate mercury species, which are far more 

toxic, soluble, and bioavailable. Anthropogenic sources of Hg(0) to the atmosphere 

include emissions from coal-fired power plants, waste incineration, cement and steel 

manufacturing, smelting, petroleum refining, and industrial processes such as used in 

chlor-alkali plants. Gas-phase Hg(0) was measured aboard the NOAA research vessel 

Ronald H. Brown during the TexAQS II project in order to evaluate potential emissions 

sources of this compound along the Texas coastline and in industrialized harbor areas. 

Results 

Mercury data were reported at 1-second intervals for the TexAQS II RHB cruise, which 

sampled extensively in the Gulf, Galveston Bay, the Houston Ship Channel (HSC), for a 

full day in the Beaumont-Port Arthur area (BPA), and for several hours in Matagorda 

Bay. A modified Ohio Lumex instrument was used to detect atmospheric Hg(0) using 

absorption of the 253.7 nm emission line of a mercury lamp in a 10-m multipath sample 

cell. Specificity was afforded 

by high-frequency 

modulation of the emission 

lamp into three polarized 

Zeeman components, with the 

s

-

 line uniquely absorbed by 

atomic mercury. Instrument 

zeroes were regularly 

checked by sampling ambient 

air through an iodinated 

charcoal trap, and instrument 

calibration was routinely 

evaluated by adding a known 

standard of Hg(0) to the 

ambient sample. 1-sigma 

uncertainties of the 1-second 

data averaged ±(2 ng/m3

 + 

16%) of the reported value. 

40

30

20

10

0

Hg(0), ng/m^3

30.0

29.5

29.0

28.5

28.0

27.5

The RHB ship track in the TexAQS II study region is shown in Figure 1, scaled by 1-

utside of the industrialized source areas, the data show an average Hg(0) concentration 

minute averages of Hg(0) concentration measured aboard the ship. 

O

of 1.5 ± 1.4 ng/m3, broadly consistent with the expected background value of 1.7 ng/m3

for this season and latitude. No strong signature of Hg emissions is seen outside of the 

industrial areas, despite extensive sampling very close to oil and natural gas drilling and Latitude

-97.0 -96.0 -95.0 -94.0

Longitude

TexAQS 2006 Hg data

NOAA R/V Brown track

scaled by 1-min avgs. of Hg(0)

Figure 1. RHB ship track scaled by 1-minute averages of

Hg(0) data.

38
production platforms in the Gulf, and in the very concentrated plumes of large 

commercial vessels burning a variety of bunker fuels. 

Within the industrialized Houston Ship Channel and Beaumont-Port Arthur areas, the 

data show very concentrated, spatially narrow plumes, as shown in detail below. 

igure 2. Hg(0) data from industrial source regions during TexAQS II. Note changes in scales 

he Hg(0) plume in the Houston Ship Channel, shown in Figure 2, was encountered on 

and Conclusions

ed Hg(0) enhancements were completely uncorrelated 

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

F

between the two graphs; missing data are due to automated instrument calibration or zeroing. 

T

each of four transits in approximately the same location when local winds were southerly, 

suggesting a single source. No enhancement was seen when winds were northerly. Peak 

enhancements for this plume ranged from 250 to 15 ng/m3, with the variability 

potentially affected by differences in dilution, transport, and variability in the source 

strength, during each transit. A second plume (not shown) was encountered at a different 

geographic location in HSC, indicative of a separate source. The BPA plume was 

observed during the single RHB visit to Beaumont. Winds were steadily onshore during 

the visit to Matagorda Bay, precluding sampling the decommissioned chlor-alkali plant 

emissions by the instrument aboard the R/V Ronald H. Brown. 

 

Interpretation 

Most interesting is that all observ

with any of the very wide range of other measurements made on RHB; a list of these 

measurements is available here. This complete lack of correlation rules out most known 

source types, which can be expected to co-emit one or more of the suite of species 

measured concurrently aboard the Ronald H. Brown. Further, the RHB mercury 

observations are not consistent with the latest TCEQ, AIRS, EGRID, and TRI inventory 

source locations for known Hg emitters in this region. 

39
XV. Investigate NH3 source emissions. 

Background 

The presence of fine aerosol in the troposphere affects air quality by threatening human 

health and lowering visibility. In some regions of the United States, secondary formation 

of ammonium nitrate aerosol can account for a significant fraction of particle mass and 

reduce visibility. Ammonium nitrate aerosol is formed from the reaction of gas phase 

ammonia (NH3) and nitric acid (HNO3). Anthropogenic emissions of NH3 and NOX (= 

NO + NO2), which in sunlight can be oxidized to form HNO3, can result in observations 

of elevated ammonium nitrate levels. In the present study, the sources and processes 

associated with ammonium nitrate formation were observed from the NOAA WP-3D 

aircraft over Houston. To accomplish this the TexAQS 2006 study combined fastresponse measurements of NH3, HNO3, particle composition, and particle size 

distribution. 

Results

Sources of NH3 in the Houston area include automobiles, industrial facilities, and 

outlying agricultural activity. Figure 1 shows the altitude profile of all the 1-second 

measurements of NH3 mixing ratios that were made aboard the NOAA WP-3D during the 

2006 study. Typically, NH3 mixing 

ratios over the urban area ranged 

from 0.2 to 3 ppbv. In general, NH3

mixing ratios decreased with 

increasing altitude. In addition, 

several plumes with high NH3 levels 

were sampled on different flights and 

plumes with NH3 mixing ratios 

greater than 5 ppbv, though 

infrequent, were observed in the 

boundary layer below 1 km. The NH3

mixing ratios in these plumes ranged 

from 5 to over 50 ppbv. 

Figures 2 and 3 illustrate the 

observation of these plumes. Figure 

2 shows the flight track of NOAA 

WP-3D on October 6, 2006. The 

color-coding on the flight track (c.f., 

color bar on the right) indicates the 

NH3 mixing ratio, while the barbs 

indicate two-minute averaged wind 

speed and direction. As indicated the 

wind was reasonably steady from the east-southeast during the flight. As the color-coding 

on the flight tract indicates, during this flight plumes containing ammonia were 

intercepted during transects labeled 1 and 2 on the flight track. Also, identified on the 

Figure 1. NH3 measurements made during the 

2006 TexAQS as a function of altitude. 

40
map is Cedar Bayou, which is the location of the only significant NH3 point source 

upwind of the observed NH3 plume that is listed in the National Emission Inventory of 

1999 (NEI99v3). Figure 3 shows the times series of NH3, NOx, and CO2 measurements 

that were made during the transect labeled 1 in Figure 2. Though these high NH3 plumes 

were downwind of NEI99v3 

point source at Cedar Bayou, 

other simultaneously measured 

gas-phase species, suggest the 

influence from other sources too 

(or only other?). 

However, in all these plumes, 

the observed NH3 enhancement 

was found to be accompanied by 

a corresponding decrease in 

HNO3 mixing ratios and a 

corresponding increase in the 

particulate NO3

-

 concentration 

that indicated ammonium nitrate 

formation. In these plumes the 

magnitude of the observed 

HNO3 loss and NO3

- Figure 2. Flight track of NOAA WP3 on October 6, 

2006. The color-coding on the flight track (c.f., color bar 

 right) indicates the NH3 mixing ratio. The barbs 

indicate wind speed and direction. 

-

 formed is 

consistent with ammonium 

nitrate formation within a factor 

of 2. 

on the

Interpretation and Conclusions

During TexAQS 2006 the observed NH3 mixing ratios typically ranged from 0.1 to 3 

ppbv, though a few plumes with NH3 mixing ratios greater than 5 ppbv were also 

sampled. The sources were located in the Houston metropolitan area, around Beaumont 

and in St. James Parrish in 

Louisiana. In addition the to 

industrial plumes, NH3 plumes 

that were associated with 

biomass burning were also 

encountered. Many of the NH3

plumes that we encountered 

could not be traced to point 

sources in the NEI99v3 

inventory. This highlights the 

need to revise and update the 

NH3 inventories. Additional 

analysis will be directed toward 

the correlation of the NH3 plumes with other compounds that were measured aboard the 

WP-3D (e.g., VOCs). This will allow a better identification of the sources and estimation 

of the amount of NH3 that is emitted. 

Figure 3. Times series of NH3, NOx, and CO2

measurements made during the transect labeled 1 in

Figure 2.

41
With regard to processes that involve NH3, we observed coincident decreases in HNO3

mixing ratio and increases in aerosol NO3

-

 and fine particle volume indicating ammonium 

nitrate formation in these plumes. The magnitude of the observed HNO3 lost and NO3

-

formed is within a factor of 2 consistent with ammonium nitrate formation. Further 

analysis of the particle size distribution and aerosol composition data is needed to fully 

assess the quantitative agreement between the gas-phase and aerosol observations. 

42