bluuebunny/crossref_metadata_2025 huggingface dataset:
Alternative methods for specification of observed forcing in single‐column models and cloud system models
Authors: Randall David A. , Cripe Douglas G.
Published: September 2002

We discuss alternative methods for prescribing advective tendencies in single‐column models (SCMs) and cloud system models. These include “revealed forcing,” in which the total advective tendency is prescribed from observations; “horizontal advective forcing,” in which the horizontal advective tendencies are prescribed, together with the observed vertical motion which is combined with the predicted sounding to determine the tendencies due to vertical advection; and “relaxation forcing,” in which the horizontal advective tendencies are computed by relaxing the sounding toward the observed upstream sounding, with a relaxation timescale determined by the time required for the wind to carry parcels across the grid column. When relaxation forcing is used, the horizontal advective tendencies can be diagnosed from the model output and compared with the corresponding observed tendencies. We present SCM results to illustrate these three forcing methods, based on data from several field experiments in both the tropics and the midlatitudes. Each method is shown to have its strengths and weaknesses. Overall, the results presented here do not show unambiguous differences between revealed forcing and horizontal advective forcing. The two methods appear to be generally comparable. Revealed forcing may therefore be preferred for its simplicity. Relaxation forcing guarantees realistic soundings of the state variables but can produce large errors in parameterized processes which are driven by rates (e.g., fluxes) rather than states. In particular, relaxation forcing gives large errors in the precipitation rate in this model. We demonstrate that relaxation forcing leads to unrealistically high (low) precipitation in versions of the model which tend to produce unrealistically dry (humid) soundings. The observed horizontal advective tendencies in the tropics are so weak, especially for temperature, that small absolute errors in the diabatic tendencies diagnosed with relaxation forcing can lead to large relative errors in the diagnosed horizontal advective tendencies.
Distribution and fate of selected oxygenated organic species in the troposphere and lower stratosphere over the Atlantic
Authors: Singh H. , Chen Y. , Tabazadeh A. , Fukui Y. , Bey I. , Yantosca R. , Jacob D. , Arnold F. , Wohlfrom K. , Atlas E. , Flocke F. , Blake D. , Blake N. , Heikes B. , Snow J. , Talbot R. , Gregory G. , Sachse G. , Vay S. , Kondo Y.
Published: September 2002

A large number of oxygenated organic chemicals (peroxyacyl nitrates, alkyl nitrates, acetone, formaldehyde, methanol, methylhydroperoxide, acetic acid and formic acid) were measured during the 1997 Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) airborne field campaign over the Atlantic. In this paper, we present a first picture of the distribution of these oxygenated organic chemicals (Ox‐organic) in the troposphere and the lower stratosphere, and assess their source and sink relationships. In both the troposphere and the lower stratosphere, the total atmospheric abundance of these oxygenated species (ΣOx‐organic) nearly equals that of total nonmethane hydrocarbons (ΣNMHC), which have been traditionally measured. A sizable fraction of the reactive nitrogen (10–30%) is present in its oxygenated organic form. The organic reactive nitrogen fraction is dominated by peroxyacetyl nitrate (PAN), with alkyl nitrates and peroxypropionyl nitrate (PPN) accounting for <5% of total NOy. Comparison of observations with the predictions of the Harvard three‐dimensional global model suggests that in many key areas (e.g., formaldehyde and peroxides) substantial differences between measurements and theory are present and must be resolved. In the case of CH3OH, there appears to be a large mismatch between atmospheric concentrations and estimated sources, indicating the presence of major unknown removal processes. Instrument intercomparisons as well as disagreements between observations and model predictions are used to identify needed improvements in key areas. The atmospheric chemistry and sources of this group of chemicals is poorly understood even though their fate is intricately linked with upper tropospheric NOx and HOx cycles.
Modeling OH, HO2, and RO2 radicals in the marine boundary layer: 2. Mechanism reduction and uncertainty analysis
Authors: Carslaw N. , Jacobs P. J. , Pilling M. J.
Published: September 2002

An observationally constrained box model has been constructed to investigate radical chemistry at the Mace Head Atmospheric Observatory, a remote marine location on the west coast of Ireland. The primary aim of the model has been to model concentrations of the hydroxyl (OH), hydroperoxy (HO2), and the sum of peroxy ∑([HO2]+[RO2]) radicals measured by in situ instruments at this location. The model used in these studies consists of about 1670 reactions and 500 species, and model predictions of radical concentrations have been evaluated against field data. In order to further understand the chemistry, the model has been reduced using sensitivity analysis on both a clean and a semipolluted day. For reduced mechanisms that predict the concentrations of OH and HO2 to within 5% of the full mechanism, the semipolluted day can be represented using 279 species and 986 reactions, and the clean day using 249 species and 894 reactions. A further reduction has been applied whereby the reduced mechanisms predict concentrations of OH and HO2 to within 20% of the full mechanism for the daytime hours. In this way, the OH and HO2 concentrations on the semipolluted day can be represented by 42 species and 64 reactions, and the clean day by 17 species and 25 reactions. We show that these reduced mechanisms are generally applicable for this location under broadly similar conditions. Simple steady state expressions have also been derived to represent the chemistry at this location, allowing the concentrations of OH and HO2 to be deduced analytically. The expressions are based on the reduced mechanisms and on a further analysis of the reaction rates. Finally, an uncertainty analysis has been carried out to quantify the effects of propagation of uncertainties in the rate parameters and constrained concentrations through to the calculated radical concentrations in the model. For model concentrations of OH, HO2 and ∑([HO2]+[RO2]) radicals, the 2σ uncertainties are 31, 21, and 25%, respectively for clean air, and 42, 25, and 27% for semipolluted air.
Improved electric field measurements with the T‐28 armored research airplane
Authors: Mo Qixu , Feind Rand E. , Kopp Fred J. , Detwiler Andrew G.
Published: September 2002

The South Dakota School of Mines and Technology armored T‐28 airplane has been used to collect electric field data inside thunderstorms since 1986. However, the derivation of the Ex field component in the flight direction has been problematic, and the accuracy of other field components derived has been suspect when heavy precipitation was encountered. The problems are due to (1) poor understanding of when and where corona ions are emitted from the airplane and how they affect the measurements, (2) lack of a complete and advanced calibration of the electric field meters' response to the ambient electric field components and airplane charge, and (3) poor understanding of this particular airplane's charging characteristics in the heavy precipitation and strong electric field environment of severe thunderstorms. By using data obtained during intercomparison/formation flights of the T‐28 with another well‐calibrated airplane in 1997, a complete calibration matrix for the T‐28 is presented in this paper. The calibration matrix provides improved and more robust estimates of all electric field components Ex, Ey and Ez. The data from the intercomparison flights are used to characterize the corona emission phenomena and its effects on the meter measurements. A methodology to detect emissions and their effect on the measurements is presented. Findings from this work and work from the Special Test Vehicle for Atmospheric Research (SPTVAR) at New Mexico Institute of Mining and Technology indicate that electric meters should not be located downstream of any airplane's propeller, the most likely source of corona ion emissions in large ambient electric fields. Alternative computations for field components and techniques for calculating airplane charge are also presented. In addition, we show that when flying through heavy precipitation, the effect of precipitation charging on a field meter's reading is small in comparison to the effect of typical ambient electric fields found in thunderstorms.
Infrared complex refractive indices of supercooled liquid HNO3/H2O aerosols
Authors: Norman M. L. , Qian J. , Miller R. E. , Worsnop D. R.
Published: September 2002

Complex refractive indices have been retrieved for the first time directly from the Fourier transform infrared extinction spectra of supercooled liquid HNO3/H2O aerosols at 220 K. Aerosol compositions were determined by simultaneously monitoring equilibrium HNO3 and H2O vapor pressures with a tunable diode laser. The new optical constants are compared with previous results from room temperature bulk solutions and <160 K amorphous solid thin films, and the differences are explained primarily in terms of the temperature‐dependent dissociation of molecular HNO3 into its constituent ions. Mie scattering analysis of aerosol extinction spectra suggests that using either the room temperature liquid or the amorphous solid refractive indices may result in inaccurate particle size determination. These new data sets are directly applicable to the characterization of polar stratospheric clouds through spectroscopic remote sensing.
A meteorological overview of the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) period
Authors: Fuelberg Henry E. , Hannan John R. , van Velthoven Peter F. J. , Browell Edward V. , Bieberbach George , Knabb Richard D. , Gregory Gerald L. , Pickering Kenneth E. , Selkirk Henry B.
Published: September 2002

Meteorological conditions are described during NASA's Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) that was conducted over the North Atlantic Flight Corridor (NAFC) during October and November 1997 to study the impact of aircraft emissions on atmospheric concentrations of NOx and ozone. The SONEX period exhibited frequent closed cyclones and anticyclones, as well as high‐amplitude troughs and ridges. These flow patterns often caused aircraft exhaust from the NAFC to follow broad looping north‐south trajectories, instead of more easterly routes that would have occurred if the flow had been more zonal. Mean meteorological conditions during SONEX include a pronounced long wave trough over the East Coast of the United States, as well as weaker low pressure over middle‐latitude portions of the Atlantic Ocean. Conversely, a well‐developed ridge was apparent over the North Atlantic near Iceland. Cloudiness exceeded climatology off the East Coast and the subtropical North Atlantic, with abundant lightning in these regions. There was less than average cloud cover over the middle latitudes between Newfoundland and central Europe. The tropopause was higher than climatology over much of the SONEX region, and the jet stream was located north of its typical position. These circulation features during SONEX are consistent with typical year‐to‐year variations. Meteorological conditions during individual SONEX flights also are described. Upper tropospheric flow patterns, 5‐day backward trajectories from the flight tracks, tropopause heights, lightning data, and differential absorption lidar ozone imagery are employed. Effects of aircraft were observed on numerous flights. Stratospheric conditions were encountered during many flights, sometimes because the DC‐8 passed through a tropopause fold. SONEX flight tracks frequently were downwind of regions of lightning, especially during flights from Bangor and the Azores. Finally, trajectories indicated that continental pollution signatures observed during some flights had originated over the United States.
A simple method to derive the water activities of highly supersaturated binary electrolyte solutions from ternary solution data
Authors: Chan Chak K. , Ha Zhanyao
Published: September 2002

Using mixing rules in electrolyte models for estimating the thermodynamic properties of multicomponent mixtures requires knowledge of data for each binary solution. Very often, a ternary solution in a single‐particle levitation experiment can achieve a higher ionic strength as compared with the ionic strengths derived from any of the binary solutions alone. Using water activity data of ternary solutions and the Zdanovskii‐Stokes‐Robinson and the Kusik and Meissner (KM) equations, water activities of binary solutions of (NH4)2SO4, NaCl, and NH4Cl at supersaturations higher than any previously reported are calculated. With available data from the literature on aqueous supersaturated droplets of NH4NO3‐(NH4)2SO4 ternary solutions and letovicite at ambient temperatures, water activities (>0.32) of (NH4)2SO4 solutions for ionic strengths up to 112 mol kg−1 are obtained. The new water activity relation enables the use of the KM and Pitzer models at higher ionic strengths than in previous research. Applying the same method to the data on (NH4)2SO4‐NaCl and NH4NO3‐NaCl systems yields water activities (>0.28) of NaCl solutions for ionic strengths up to 25 mol kg−1. Similarly, water activities (>0.34) of NH4Cl solutions up to 42 mol kg−1 are calculated from the measured data on NH4Cl‐NH4NO3 and NH4Cl‐(NH4)2SO4 systems. New water activity measurements for the NH4Cl‐NH4NO3, NH4Cl‐(NH4)2SO4, and NH4NO3‐NaCl systems are also reported.
Characterization of water‐soluble organic compounds in atmospheric aerosol: A new approach
Authors: Decesari Stefano , Facchini Maria Cristina , Fuzzi Sandro , Tagliavini Emilio
Published: September 2002

A new methodological approach is proposed to characterize aerosol water‐soluble organic compounds (WSOC). Real aerosol and fog water samples were subjected to a procedure based on a combination of Chromatographic separations, functional group investigation by proton nuclear magnetic resonance (HNMR), and total organic carbon determination. The complex mixture of aerosol/fog WSOC was separated by a Chromatographic procedure into three main classes of compounds: (1) neutral/basic compounds; (2) mono‐ and di‐carboxylic acids; (3) polyacidic compounds. Characterization by HNMR spectroscopy showed that fraction 1 is mainly composed of polyols or polyethers, fraction 2 is mainly composed of hydroxylated aliphatic acidic compounds, while fraction 3 is composed of highly unsaturated polyacidic compounds of predominantly aliphatic character, with a minor content of hydroxyl‐ groups. Quantitative data on the three classes of compounds were then derived from total organic carbon analysis, showing that the three separated fractions together account for 77% (in terms of C) of the total WSOC concentration of a fog water sample. Further quantitative information on the functional groups present in the three separated fractions can be obtained from HNMR spectra. This newly proposed approach to aerosol WSOC characterization provides comprehensive and synthetic information on aerosol organic composition which can be helpful for modeling purposes and is also particularly useful when aerosol chemical mass closure is pursued.
An Atlantic meridional transect of surface water dimethyl sulfide concentrations with 10–15 km horizontal resolution and close examination of ocean circulation
Authors: Belviso Sauveur , Morrow Rosemary , Mihalopoulos Nikolaos
Published: September 2002

Underway measurements of dimethyl sulfide (DMS) in the Atlantic surface waters have been made during the ALBATROSS campaign from 65°N to 45°S along about 30°W. The main patterns of DMS variability in subtropical waters of both hemispheres were the existence of (1) a poleward negative gradient of DMS (0.04 nM/°latitude) paralleling the temperature and salinity meridional trends and opposite to that of chlorophyll a (chl a) and particulate DMSP (pDMSP), and (2) sharp DMS enhancements, up to twenty fold the background levels, coinciding almost systematically with thermohaline frontal zones. We observed that DMS concentrations and TOPEX/Poseidon sea level anomalies (SLAs) were clearly in opposition of phase in the subtropical and tropical waters of the Atlantic. Neither meridional changes in pDMSP nor in chl a concentrations account for these large‐scale (15°–20° latitude) DMS variations. It is suggested that the spatial distribution of DMS is highly sensitive to the upper ocean dynamics. The tropical Atlantic is a zone of contrasted DMS levels with two broad maxima associated (1) with the cyclonic circulations generated by the North Equatorial currents and (2) with the South Equatorial Current, a situation very much resembling the autumnal meridional distribution of surface pC02. A close examination of the South Atlantic subtropical front (38°–43°S) show that DMS and in situ validated satellite chl a have a distinct spatial distribution suggesting important spatial segregation of biogeochemical processes in the frontal zones. These observations at different spatial scales provide indications for the existence of a DMS‐climate link through frontogenesis and surface ocean circulation in the Atlantic.
Effects of turbulence and heterogeneous emissions on photochemically active species in the convective boundary layer
Authors: Krol Maarten C. , Molemaker M. Jeroen , de Arellano Jordi Vilà Guerau
Published: September 2002

Photochemistry is studied in a convective atmospheric boundary layer. The essential reactions that account for the ozone formation and depletion are included in the chemical mechanism which, as a consequence, contains a wide range of timescales. The turbulent reacting flow is modeled with a large‐eddy simulation (LES) code. The deviations from chemical equilibrium that are caused by turbulent motions are investigated in terms of the intensity of segregation. For the studied cases it is found that the volume‐averaged concentrations calculated with the LES code agree well with the concentrations calculated with a box model. The reaction rate between RH (a generic hydrocarbon emitted at the surface) and OH is most strongly affected (3% slower than in the box model). However, if RH is emitted nonuniformly at the surface, or if the RH‐OH reaction rate is increased, the volume‐averaged RH destruction by OH may be slowed down by as much as 30% compared to a box model. Sensitivity studies showed that the intensity of segregation between RH and OH not only depends on the strength and spatial distribution of the RH emissions but also on the way NO is emitted in the model atmosphere. The results obtained indicate that the assumption that localized emissions of reactive hydrocarbons, for example, isoprene or terpenes, are instantaneously mixed may lead to an underestimation of their atmospheric lifetime.
A nonurban ozone air pollution episode over eastern China: Observations and model simulations
Authors: Luo C. , St. John J. C. , Xiuji Zhou , Lam K. S. , Wang T. , Chameides W. L.
Published: September 2002

Air quality data gathered from five nonurban sites in China over a 12‐month period from August 1994 to August 1995, along with meteorological observations from the same region and period, are used to identify and characterize a nonurban ozone (O3) pollution episode in China. Because of the influence of the Asian Monsoonal Circulation, high O3 concentrations were not observed at the nonurban sites during the summer months. However, enhanced O3 concentrations were observed during the other seasons, especially the fall and early winter. A more detailed inspection of the O3 data during the period from October 15, 1994, to January 15, 1995, indicated the occurrence of a multipleday episode in late October/early November when high O3 concentrations were observed at all four monitoring sites located in eastern China. Meteorological conditions during the episode were characterized by the presence of a strong and stationary high‐pressure ridge over eastern China; synoptic conditions quite similar to those observed during regional O3 pollution episodes over the United States, Canada, and Europe. An updated version of the Regional Acid Deposition Model (RADM) driven by meteorological fields derived from the Regional Climate Model (RegCM) and spatially disaggregated anthropogenic emissions prepared by the Chinese Academy of Meteorological Sciences is used to simulate 3 months of the observed O3 data from China. Comparisons between observations and model calculations indicate that the model is able to reproduce some of the key features of the O3 distribution and its relationship to the concentration of one primary pollutant (i.e., sulfur dioxide) provided the comparison is made using averaging times of several days or more. However, simulation of day‐to‐day variations in O3 at a given site was poorly correlated with observations. Model simulations suggest that peak O3 concentrations during this episode would respond to changes in NOx and VOC emissions in a spatially inhomogeneous manner. In general, rural areas in southern China tend to be NOx‐limited, but rural areas in northern China tend to be VOC‐limited. The Yangtze Delta region, where the highest O3 concentrations were observed and predicted to occur, was found to be transitional between VOC and NOx limitation.
The quasi 16‐day oscillations in the mesosphere and lower thermosphere at Saskatoon (52°N, 107°W), 1980–1996
Authors: Luo Yi , Manson Alan H. , Meek Chris E. , Meyer Christian K. , Forbes Jeffrey M.
Published: September 2002

The daily mean winds observed by the Saskatoon MF radar are used to investigate the quasi 16‐day oscillations in the mesosphere and lower thermosphere. Based on the wind data (every 3 km from 58 to 105 km, hourly from 1980 to 1996), the wave amplitudes and phases of all periods (2–24 days) as a function of height and time of year are available. Among them the 16‐day wave occurs there mostly in winter with a maximum at ∼60–65 km and covers a large range of altitudes (up to 100 km); in summer, however, the 16‐day wave is much weaker and confined to ∼85 km and above. The vertical wavelength in winter tends to be very long, but in summer it is a little shorter. After further investigation we found that the 16‐day waves are extremely sensitive to the background mean winds; this explains their penetration, preferably in the westerly flow, from the lower atmosphere to the mesosphere in winter, or through a presumed ducting from the Southern Hemisphere to the Northern Hemisphere in Saskatoon summer. The year‐by‐year variations of the 16‐day wave in this paper show little correlation with the winter stratospheric warming events but imply certain associations with the equatorial quasi‐biennial oscillation both in winter and in summer. The global scale wave model has been run to get the annual (12 months) 16‐day wave simulations from the ground to 200 km. The comparison with the observations at Saskatoon indicates good agreement in winter months but significant differences in summer.
Modification of mineral dust particles by cloud processing and subsequent effects on drop size distributions
Authors: Wurzler Sabine , Reisin Tamir G. , Levin Zev
Published: September 2002

Dust particles coated with soluble materials, such as sulfate, are frequently observed in the Mediterranean. Thus far, the processes responsible for the sulfate coating of dust particles have still not been identified. One possible explanation is that the formation of the sulfate‐coated aerosols is related to cloud processing of dust particles. In this process the scavenging of aerosol particles and gases, such as SO2, O3, and H2O2, by the droplets and the subsequent impaction scavenging of mineral dust particles followed by evaporation could release into the atmosphere dust particles coated with soluble materials. These modified particles can serve as giant cloud condensation nuclei and thus can have a significant impact on the microphysical development of other clouds. Using an air parcel model with detailed microphysics, it is shown that cloud processing of dust particles is a possible effective pathway to form soluble coatings on these particles. Furthermore, the simulations show that after one or two cycles of particles through convective clouds the contribution of gas uptake by drops and subsequent liquid phase oxidation add considerable mass of soluble material to particles in the size range of 0.05 μm. On the other hand, this process adds about 1 order of magnitude less mass to the larger particles as compared to the contribution made by coagulation of drops containing soluble aerosols.
Estimation of water uptake by organic compounds in submicron aerosols measured during the Southeastern Aerosol and Visibility Study
Authors: Dick William D. , Saxena Pradeep , McMurry Peter H.
Published: September 2002

In situ measurements of size‐dependent water uptake by atmospheric particles made with a tandem differential mobility analyzer (TDMA) and size‐resolved chemical composition of aerosol samples collected with cascade impactors in the Smoky Mountains have been examined in order to ascertain the influence of organic carbon compounds on aerosol hygroscopicity. Particles were dried to ∼5% relative humidity (RH) before entering the TDMA, leading us to believe that salts of ammonium and sulfate were in crystalline states for relative humidities below their expected deliquescent points. TDMA‐measured water content was found to be in excess of the sulfate‐associated water modeled using laboratory data for binary aqueous solutions and the method of Zdanovskii‐Stokes‐Robinson for multicomponent solutions over a wide range of humidities (RH = 5–85%). Furthermore, excess water was observed to increase in proportion to the organic fraction of mass associated with each examined size in the range 0.05 to 0.4 μm. These data are used to obtain an empirical relationship between the amount of water associated with particulate organics and relative humidity. This analysis shows that organic‐associated water content is considerably less than that of sulfate compounds, on a volume basis, for high RH, but comparable or greater for low RH. These results are consistent with laboratory data for water absorption by a range of organics vis‐à‐vis ammonium salts of sulfate.
Photochemistry of HOx in the upper troposphere at northern midlatitudes
Authors: Jaeglé L. , Jacob D. J. , Brune W. H. , Faloona I. , Tan D. , Heikes B. G. , Kondo Y. , Sachse G. W. , Anderson B. , Gregory G. L. , Singh H. B. , Pueschel R. , Ferry G. , Blake D. R. , Shetter R. E.
Published: September 2002

The factors controlling the concentrations of HOx radicals (= OH + peroxy) in the upper troposphere (8–12 km) are examined using concurrent aircraft observations of OH, HO2, H2O2, CH3OOH, and CH2O made during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) at northern midlatitudes in the fall. These observations, complemented by concurrent measurements of O3, H2O, NO, peroxyacetyl nitrate (PAN), HNO3, CH4, CO, acetone, hydrocarbons, actinic fluxes, and aerosols, allow a highly constrained mass balance analysis of HOx and of the larger chemical family HOy (= HOx + 2 H2O2 + 2 CH3OOH + HNO2 + HNO4). Observations of OH and HO2 are successfully simulated to within 40% by a diel steady state model constrained with observed H2O2 and CH3OOH. The model captures 85% of the observed HOx variance, which is driven mainly by the concentrations of NOx (= NO + NO2) and by the strength of the HOx primary sources. Exceptions to the good agreement between modeled and observed HOx are at sunrise and sunset, where the model is too low by factors of 2–5, and inside cirrus clouds, where the model is too high by factors of 1.2–2. Heterogeneous conversion of NO2 to HONO on aerosols (γNO2 = 10−3) during the night followed by photolysis of HONO could explain part of the discrepancy at sunrise. Heterogeneous loss of HO2 on ice crystals (γice_HO2 = 0.025) could explain the discrepancy in cirrus. Primary sources of HOx from O(1D)+H2O and acetone photolysis were of comparable magnitude during SONEX. The dominant sinks of HOy were OH+HO2 (NOx<50 parts per trillion by volume (pptv)) and OH+HNO4 (NOx>50 pptv). Observed H2O2 concentrations are reproduced by model calculations to within 50% if one allows in the model for heterogeneous conversion of HO2 to H2O2 on aerosols (γHO2 = 0.2). Observed CH3OOH concentrations are underestimated by a factor of 2 on average. Observed CH2O concentrations were usually below the 50 pptv detection limit, consistent with model results; however, frequent occurrences of high values in the observations (up to 350 pptv) are not captured by the model. These high values are correlated with high CH3OH and with cirrus clouds. Heterogeneous oxidation of CH3OH to CH2O on aerosols or ice crystals might provide an explanation (γice_CH3OH ∼ 0.01 would be needed).
The effects of mixing on tracer relationships in the polar vortices
Authors: Plumb R. Alan , Waugh Darryn W. , Chipperfield Martyn P.
Published: September 2002

We review recent analyses of tracer observations within the Arctic vortex, showing anomalous tracer‐tracer relationships which indicate the mixing of outside air into the vortex. Similar anomalous relationships have previously been observed in vortex fragments, following breakdown of the Arctic vortex, and interpreted as indicative of mixing within the filamentary fragments. In the light of other observations we conclude that this interpretation is incorrect: the mixing took place into the vortex prior to breakdown. We discuss the effects of mixing on tracer relationships within the vortex, illustrated by results both from a simple conceptual model and from two three‐dimensional chemical transport models. These models indicate that continuous mixing across the vortex edge produces entirely changed tracer relations inside the vortex and that separate, compact vortex relationships can develop. In general, tracer relationships should not be used to estimate denitrification or chemical ozone loss unless the effects of mixing can be quantified; this may be possible using information from other tracers, but one cannot assume that the effects of anomalous mixing are manifested as simple mixing lines on tracer‐tracer diagrams.
Trajectory hunting: A case study of rapid chlorine activation in December 1992 as seen by UARS
Authors: Danilin M. Y. , Santee M. L. , Rodriguez J. M. , Ko M. K. W. , Mergenthaler J. M. , Kumer J. B. , Tabazadeh A. , Livesey N. J.
Published: September 2002

Trajectory hunting (i.e., a technique to find air parcels sampled at least twice over the course of a few days) is applied to analyze Upper Atmosphere Research Satellite (UARS) measurements in conjunction with the Atmospheric and Environmental Research, Inc. (AER) photochemical box model. As a case study, we investigate rapid chlorine activation in the Arctic lower stratosphere on December 29, 1992 associated with a polar stratospheric cloud (PSC) event. Eleven air parcels that have been sampled several times along 5‐day trajectories at the 465 K (∼46 hPa), 520 K (∼31 hPa), and 585 K (∼22 hPa) levels were investigated. For the first time, the latest versions of the Cryogenic Limb Array Etalon Spectrometer (CLAES, version 9) and Microwave Limb Sounder (MLS, version 5) data sets are analyzed, and their consistency is assessed. A detailed sensitivity study with the AER photochemical box model along these trajectories leads to the conclusion that for the December 24–29, 1992 episode (1) the individual CLAES version 9 ClONO2 and MLS version 5 ClO measurements are self‐consistent within their uncertainties; and (2) most of the time, UARS measurements of ClO, ClONO2, HNO3, and aerosol extinction at 780 cm−1 agree within the range of their uncertainties with the model calculations. It appears that the HNO3 and aerosol extinction measurements for four parcels at 520 K look more supportive for the nitric acid trihydrate (NAT) scheme. However, the uncertainties in the individual UARS measurements and U.K. Meteorological Office temperature do not allow a definite discrimination between the NAT and supercooled ternary solution (STS) PSC schemes for this chlorine activation episode in December 1992.
Measurement of atmospheric optical parameters on U.S. Atlantic coast sites, ships, and Bermuda during TARFOX
Authors: Smirnov A. , Holben B. N. , Dubovik O. , O'Neill N. T. , Remer L. A. , Eck T. F. , Slutsker I. , Savoie D.
Published: September 2002

The Aerosol Robotic Network (AERONET) of automatic Sun/sky radiometers collected data on U.S. Atlantic coast sites, ships, and Bermuda in 1996 during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). Spatial and temporal analysis of Sun photometry data was supported by synoptic analysis of air mass evolution. The spatial distribution of aerosol optical depth is presented. In several cases the aerosol size distributions deduced from sky almucantar measurements and solar disk attenuation measurements at the various coastal sites yielded similar results within the same air masses. Ship‐based measurements in the Atlantic Ocean showed significant maritime aerosol optical property variations which for the most part could be attributed to the influence of continental sources and Saharan dust events. The Bermuda data (optical depths and Ängström parameter values) illustrated changes in atmospheric optical properties for various air masses and trajectories. Almost no correlation was observed between aerosol optical depth and water vapor content when the data from all stations and ship measurements were considered together. In the case of individual stations or ship transects, different degrees of correlation could be observed. In continental conditions on the east coast, optical depth and water vapor are well correlated, while in a maritime environment, optical depth can be relatively small despite high water vapor contents.
Nonmethane hydrocarbon mixing ratios in continental outflow air from eastern North America: Export of ozone precursors to Bermuda
Authors: Milne Peter J. , Prados Ana I. , Dickerson Russell R. , Doddridge Bruce G. , Riemer Daniel D. , Zika Rod G. , Merrill John T. , Moody Jennie L.
Published: September 2002

High surface ozone levels at Bermuda during springtime are associated with the transport of large scale frontal systems from the U.S. east coast over the North Atlantic. An objective of the AEROCE III study was to adopt a meteorologically informed sampling strategy of the chemical characteristics of air masses both in advance and behind eastward progressing cold fronts in order to differentiate between vertical sources of these elevated tropospheric ozone signals. Concentrations of hydrocarbons in air sampled over the eastern US and in offshore flights varied in a complex manner with sample altitude and prevailing meteorological situation. In several aircraft flights, the presence at altitude of distinct layers of air of elevated concentrations of NMHC's attested to the dynamic mixing of lower and upper air masses associated with springtime frontal activity. Layers of mid‐tropospheric air of high O3 (140 ppbv) and low background NMHC mixing ratios (1.44 ppbv ethane, 0.034 ppbv propene, 0.247 ppbv propane, 0.034 ppbv isobutene, 0.041 ppbv n‐butane, 0.063 ppbv benzene, 0.038 ppbv toluene) mixing ratios were indicative of descending, stratospherically influenced air on a flight to the east of Norfolk, VA on April 24 (alt 4600 m). Layers of high O3 (60–70 ppbv) and elevated NMHC concentrations (1.88 ppbv ethane, 0.092 ppbv propene, 0.398 ppbv propane, 0.063 ppbv isobutene, 0.075 ppbv n‐butane, 0.106 ppbv benzene, 0.102 ppbv toluene) observed on a flight to the west of Bermuda on April 28 (alt. 4100 m) indicated upwardly lifting convective activity of continentally sourced air masses. A meteorologically informed sampling strategy employed in the study was valuable in providing a framework with which to both optimize NMHC sampling decisions and to provide a context with which to interpret their observed mixing ratios.
Trajectory mapping: A tool for validation of trace gas observations
Authors: Morris Gary A. , Gleason James F. , Ziemke Jerald , Schoeberl Mark R.
Published: September 2002

We investigate the effectiveness of trajectory mapping (TM) as a data validation tool. TM combines a dynamical model of the atmosphere with trace gas observations to provide more statistically robust estimates of instrument performance over much broader geographic areas than traditional techniques are able to provide. We present four detailed case studies selected so that the traditional techniques are expected to work well. In each case the TM results are equivalent to or improve upon the measurement comparisons performed with traditional approaches. The TM results are statistically more robust than those achieved using traditional approaches since the TM comparisons occur over a much larger range of geophysical variability. In the first case study we compare ozone data from the Halogen Occultation Experiment (HALOE) with Microwave Limb Sounder (MLS). TM comparisons appear to introduce little to no error as compared to the traditional approach. In the second case study we compare ozone data from HALOE with that from the Stratospheric Aerosol and Gas Experiment II (SAGE II). TM results in differences of less than 5% as compared to the traditional approach at altitudes between 18 and 25 km and less than 10% at altitudes between 25 and 40 km. In the third case study we show that ozone profiles generated from HALOE data using TM compare well with profiles from five European ozonesondes. In the fourth case study we evaluate the precision of MLS H2O using TM and find typical precision uncertainties of 3–7% at most latitudes and altitudes. The TM results agree well with previous estimates but are the result of a global analysis of the data rather than an analysis in the limited latitude bands in which traditional approaches work. Finally, sensitivity studies using the MLS H2O data show the following: (1) a combination of forward and backward trajectory calculations minimize uncertainties in isentropic TM; (2) although the uncertainty of the technique increases with trajectory duration, TM calculations of up to 14 days can provide reliable information for use in data validation studies; (3) a correlation coincidence criterion of 400 km produces the best TM results under most circumstances; (4) TM performs well compared to (and sometimes better than) traditional approaches at all latitudes and in most seasons; and (5) TM introduces no statistically significant biases at altitudes between 22 and 40 km.
Erythemally weighted UV trends over northern latitudes derived from Nimbus 7 TOMS measurements
Authors: Ziemke J. R. , Chandra S. , Herman J. , Varotsos C.
Published: September 2002

This study examines the distribution of long‐term trends in ground level erythemally weighted ultraviolet (UV) exposures in the northern latitudes for the period 1979–1991 using measurements from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) instrument. A new erythemal UV data set (now available to the public via World Wide Web) was produced recently by NASA and has been tested by NASA at the Goddard Space Flight Center against a previous NASA erythemal UV product, which was used in a former study that included similar adjustments for aerosols and clouds but not aerosol absorption. Zonal mean erythemal UV data from both products show similar, ∼3–7% per decade, increases in the midlatitudes to high latitudes. The detection of regional patterns in trends in erythemal UV favors summer months when surface UV is strongest and noise factors such as clouds and aerosols are not as influential. Analysis of the zonal patterns in trends around summer months indicates that most of the regional increases (exceeding 6% per decade) in the Northern Hemisphere in the latitude range 30°N–40°N originate from the Pacific and Atlantic oceanic regions. Increases (also exceeding 6% per decade) in latitudes 40°–60°N appear to originate from the North American and Asian continents and also central Europe. Trends over the east Asian continent in high latitudes indicate increases exceeding 10% per decade for May‐August. The important conclusion is that positive trends in the northern subtropical latitudes originate mostly over oceanic regions, whereas positive trends at higher latitudes originate mostly over landmasses. Some of the increases in erythemal UV over central Europe and the east Asian continent in summer months can be attributed to decadal decreases in cloudiness for the 1979–1991 time period.
The influence of changing UVB radiation in near‐surface ozone time series
Authors: Brönnimann Stefan , Voigt Stefan , Wanner Heinz
Published: September 2002

UVB radiation plays an important role in tropospheric photochemistry since it determines the rate of ozone photolysis J(O1D) and subsequent formation of OH radicals. Consequently, changes of UVB radiation, for example due to changes of the stratospheric ozone amount, could alter the concentration of reactive tropospheric gases including ozone. An observation‐based attempt is made to quantify the effect of changing UVB radiation on surface ozone peaks on a day‐to‐day scale using a time series of measurements at a Swiss mountain site. Seven years data of ozone, NO, NOx, and meteorological measurements from Chaumont (1140 m above sea level (asl)), total ozone and UVB measurements from Arosa (1847 m asl), and surface albedo from satellite observations are investigated. The study is restricted to fair weather days with moderately high NOx concentrations. Multiple regression analysis is performed using chemical, meteorological, and UV dependent variables to predict afternoon ozone peaks. From autumn to spring, positive deviations of ozone peaks are clearly connected with positive UVB deviations. The relation is statistically significant only in part of the seasonal data subsets; however, it is consistent with model studies. The estimated net effect on ozone peaks is normally within a range of 4 ppb, a range of about 6 ppb is predicted for large UVB changes. Applying the coefficients for the large interannual variability of the stratospheric ozone layer observed in spring in the last 10 years results in a range of variation of at most 1 to 1.5 ppb for monthly mean ozone peaks. For trends of J(O1D) from 1970 to 1990, a trend bias of surface ozone peaks on polluted fair weather days of less than 0.12 ppb/yr is calculated. Although the numbers are rather small, they may play a role in certain circumstances.
Simulations of water vapor in the lower stratosphere and upper troposphere
Authors: Gettelman Andrew , Holton James R. , Douglass Anne R.
Published: September 2002

A three‐dimensional Chemical Tracer Model is used to simulate the distribution of water vapor in the upper troposphere (UT) and lower stratosphere (LS). The simulations include only the large scale advection of water vapor and the restriction that the air be at or below saturation at all times. This simulation is used to help distinguish among those processes that control water vapor in the UT/LS. The exchange of water vapor across the tropopause is analyzed in detail. The simulation qualitatively reproduces the water vapor distribution given by satellite and radiosonde observations on both sides of the tropopause, confirming the importance of the global temperature distribution and advection in controlling the water vapor distribution. Quantitative differences with observations of water vapor are ascribed to input temperature biases. Water vapor enters the stratosphere at all longitudes in the tropics. Details of subgrid‐scale dynamic and convective processes are not necessary to explain the entry of water vapor into the stratosphere, but these are found to be important for controlling the water vapor distribution in the upper troposphere.
Parameterization of OH for efficient computation in chemical tracer models
Authors: Duncan Bryan , Portman David , Bey Isabelle , Spivakovsky Clarissa
Published: September 2002

We present a parameterization for the tropospheric concentration of the hydroxyl radical (OH) which can be used to overcome the costs of solving kinetic equations in chemical tracer models. This parameterization accurately represents OH predicted by a full chemical mechanism. The 24‐hour average concentration of OH is represented as a set of high‐order polynomials in variables such as temperature, latitude, declination, and the concentrations of ozone, water vapor, carbon monoxide, nitrogen oxides (as a family), and hydrocarbons. The parameterization of OH consists of computer‐written FORTRAN functions for an efficient computation of the polynomials. The parameterization of OH is publicly available.
Minor flux adjustment near mixing ratio extremes for simplified yet highly accurate monotonic calculation of tracer advection
Authors: Walcek Chris J.
Published: September 2002

A simplified but very accurate method for calculating advection of mixing ratios in a mass conservative and absolutely monotonic manner in divergent or nondivergent multidimensional flows is presented. This scheme uses a second‐order‐accurate, upstream approximation with monotone limiters and additionally adjusts fluxes at two cell edges around local extremes of a tracer distribution to significantly improve overall advection calculations. The minor flux adjustment slightly aggregates mass around local peaks in a manner which counters the inherent numerical diffusion associated with most numerical advection algorithms when advecting poorly resolved features. When advecting tracer shapes which are resolved by fewer than 10–20 grid cells, this scheme is significantly more accurate than higher‐order algorithms for a wide range of test problems. For well‐resolved tracer distributions this algorithm is very accurate and usually preserves local peak and minimum values almost perfectly. The scheme is positive‐definite, but negative values can be advected with no modifications. A generalized algorithm and FORTRAN subroutine is presented for advecting mixing ratios or other conservative quantities through variable‐spaced grids of one to three dimensions, including deformational flows. One‐ and two‐dimensional tests are presented and compared with other higher‐order algorithms. The computational requirements of this algorithm are significantly lower than those of other higher‐order and less accurate schemes.
Absorption of solar radiation by water vapor in clear and cloudy skies: Implications for anomalous absorption
Authors: Hill Christian , Jones Roderick L.
Published: September 2002

We present ground‐based measurements of integrated atmospheric absorption obtained using a zenith‐pointing spectrometer for the wavelength region 550–1000 nm. A nonlinear, least squares fitting routine is used to fit the structured and unstructured components of the spectra, including the water vapor bands at 720, 820, and 940 nm. The structured component is found to be well‐accounted for by line‐by‐line calculations based on known absorptions of molecular H2O and O2. No other significant structured features are observed, apart from those due to Fraunhofer lines. A recently published theoretical cross section for H2O dimer is also assessed in this wavelength region. Lower limit calculations of the dimer abundance based on the fitted monomer amounts suggest that this cross section is incorrect and would overestimate the potential absorption due to the dimer. Overall, the structured absorption of solar radiation under both clear‐ and cloudy‐sky conditions is found to be well reproduced by known absorption mechanisms.