Trace Gas Distributions Research Articles

Aerosols in the tropical and subtropical UT/LS: in-situ measurements of submicron particle abundance and volatility

Abstract. Processes occurring in the tropical upper troposphere (UT), the Tropical Transition Layer (TTL), and the lower stratosphere (LS) are of importance for the global climate, for stratospheric dynamics and air chemistry, and for their influence on the global distribution of water vapour, trace gases and aerosols. In this contribution we present aerosol and trace gas (in-situ) measurements from the tropical UT/LS over Southern Brazil, Northern Australia, and West Africa. The instruments were operated on board of the Russian high altitude research aircraft M-55 "Geophysica" and the DLR Falcon-20 during the campaigns TROCCINOX (Araçatuba, Brazil, February 2005), SCOUT-O3 (Darwin, Australia, December 2005), and SCOUT-AMMA (Ouagadougou, Burkina Faso, August 2006). The data cover submicron particle number densities and volatility from the COndensation PArticle counting System (COPAS), as well as relevant trace gases like N2O, ozone, and CO. We use these trace gas measurements to place the aerosol data into a broader atmospheric context. Also a juxtaposition of the submicron particle data with previous measurements over Costa Rica and other tropical locations between 1999 and 2007 (NASA DC-8 and NASA WB-57F) is provided. The submicron particle number densities, as a function of altitude, were found to be remarkably constant in the tropical UT/LS altitude band for the two decades after 1987. Thus, a parameterisation suitable for models can be extracted from these measurements. Compared to the average levels in the period between 1987 and 2007 a slight increase of particle abundances was found for 2005/2006 at altitudes with potential temperatures, Θ, above 430 K. The origins of this increase are unknown except for increases measured during SCOUT-AMMA. Here the eruption of the Soufrière Hills volcano in the Caribbean caused elevated particle mixing ratios. The vertical profiles from Northern hemispheric mid-latitudes between 1999 and 2006 also are compact enough to derive a parameterisation. The tropical profiles all show a broad maximum of particle mixing ratios (between Θ≈340 K and 390 K) which extends from below the TTL to above the thermal tropopause. Thus these particles are a "reservoir" for vertical transport into the stratosphere. The ratio of non-volatile particle number density to total particle number density was also measured by COPAS. The vertical profiles of this ratio have a maximum of 50% above 370 K over Australia and West Africa and a pronounced minimum directly below. Without detailed chemical composition measurements a reason for the increase of non-volatile particle fractions cannot yet be given. However, half of the particles from the tropical "reservoir" contain compounds other than sulphuric acid and water. Correlations of the measured aerosol mixing ratios with N2O and ozone exhibit compact relationships for the tropical data from SCOUT-AMMA, TROCCINOX, and SCOUT-O3. Correlations with CO are more scattered probably because of the connection to different pollution source regions. We provide additional data from the long distance transfer flights to the campaign sites in Brazil, Australia, and West-Africa. These were executed during a time window of 17 months within a period of relative volcanic quiescence. Thus the data represent a "snapshot picture" documenting the status of a significant part of the global UT/LS fine aerosol at low concentration levels 15 years after the last major (i.e., the 1991 Mount Pinatubo) eruption. The corresponding latitudinal distributions of the measured particle number densities are presented in this paper to provide data of the UT/LS background aerosol for modelling purposes.

Assessment of the breakup of the Antarctic polar vortex in two new chemistry‐climate models

Successful simulation of the breakup of the Antarctic polar vortex depends on the representation of tropospheric stationary waves at Southern Hemisphere middle latitudes. This paper assesses the vortex breakup in two new chemistry‐climate models (CCMs). The stratospheric version of the UK Chemistry and Aerosols model is able to reproduce the observed timing of the vortex breakup. Version 2 of the Goddard Earth Observing System (GEOS V2) model is typical of CCMs in that the Antarctic polar vortex breaks up too late; at 10 hPa, the mean transition to easterlies at 60°S is delayed by 12–13 days as compared with the ERA‐40 and National Centers for Environmental Prediction reanalyses. The two models' skill in simulating planetary wave driving during the October–November period accounts for differences in their simulation of the vortex breakup, with GEOS V2 unable to simulate the magnitude and tilt of geopotential height anomalies in the troposphere and thus underestimating the wave driving. In the GEOS V2 CCM the delayed breakup of the Antarctic vortex biases polar temperatures and trace gas distributions in the upper stratosphere in November and December.

A regional scale modeling analysis of aerosol and trace gas distributions over the eastern Pacific during the INTEX-B field campaign

Abstract. The Sulfur Transport and dEposition Model (STEM) is applied to the analysis of observations obtained during the Intercontinental Chemical Transport Experiment-Phase B (INTEX-B), conducted over the eastern Pacific Ocean during spring 2006. Predicted trace gas and aerosol distributions over the Pacific are presented and discussed in terms of transport and source region contributions. Trace species distributions show a strong west (high) to east (low) gradient, with the bulk of the pollutant transport over the central Pacific occurring between ~20° N and 50° N in the 2–6 km altitude range. These distributions are evaluated in the eastern Pacific by comparison with the NASA DC-8 and NSF/NCAR C-130 airborne measurements along with observations from the Mt. Bachelor (MBO) surface site. Thirty different meteorological, trace gas and aerosol parameters are compared. In general the meteorological fields are better predicted than gas phase species, which in turn are better predicted than aerosol quantities. PAN is found to be significantly overpredicted over the eastern Pacific, which is attributed to uncertainties in the chemical reaction mechanisms used in current atmospheric chemistry models in general and to the specifically high PAN production in the SAPRC-99 mechanism used in the regional model. A systematic underprediction of the elevated sulfate layer in the eastern Pacific observed by the C-130 is another issue that is identified and discussed. Results from source region tagged CO simulations are used to estimate how the different source regions around the Pacific contribute to the trace gas species distributions. During this period the largest contributions were from China and from fires in South/Southeast and North Asia. For the C-130 flights, which operated off the coast of the Northwest US, the regional CO contributions range as follows: China (35%), South/Southeast Asia fires (35%), North America anthropogenic (20%), and North Asia fires (10%). The transport of pollution into the western US is studied at MBO and a variety of events with elevated Asian dust, and periods with contributions from China and fires from both Asia and North America are discussed. The role of heterogeneous chemistry on the composition over the eastern Pacific is also studied. The impacts of heterogeneous reactions at specific times can be significant, increasing sulfate and nitrate aerosol production and reducing gas phase nitric acid levels appreciably (~50%).

Distributions of short-lived iodocarbons and biogenic trace gases in the open ocean and atmosphere in the western North Pacific

Biogenic trace gases, especially halomethanes, which are important with respect to atmospheric chemistry, are released from the ocean and carry halogens to the troposphere and stratosphere. The concentrations of 10 halocarbons and isoprene in seawater were measured during the spring of 2007 in the western North Pacific Ocean (37–43° N, 143–146° E). Sea–air fluxes of CH 3Cl, CH 3Br, CH 2ClI, and CH 2I 2 were also estimated based upon the atmospheric as well as oceanic measurement of these species. Temperature–salinity scatter diagram analyses divided the sampling stations into the Oyashio region, Tsugaru warm current region, and Kuroshio region. Mean (range) concentrations of the gases in the water columns (5–100 m) were 114 (56–150) pmol L − 1 CH 3Cl, 6.9 (4.1–19.4) pmol L − 1 CH 3Br, 1.7 (0.7–2.9) pmol L − 1 CH 3I, 1.9 (0.9–4.1) pmol L − 1 CH 2BrCl, 4.8 (3.2–8.1) pmol L − 1 CH 2Br 2, 1.0 (0.6–1.8) pmol L − 1 CHBrCl 2, 1.2 (0.7–2.0) pmol L − 1 CHBr 2Cl, 10.8 (4.7–24.5) pmol L − 1 CHBr 3, 1.7 (0.7–5.4) pmol L − 1 CH 2ClI, 3.0 (< 0.1–22.2) pmol L − 1 CH 2I 2, and 19.7 (3.8–68.2) pmol L − 1 isoprene. The maximum concentration of isoprene was observed in the Oyashio region, where concentrations of chlorophyll a (maximum: 2.94 µg L − 1 ) were highest in the present study. However, the peaks of CH 3Br, CH 2ClI, and CH 2I 2 were observed in the Tsugaru warm current region, where concentrations of chlorophyll a were not as high (maximum: 0.65 µg L − 1 ). The results of chlorophyll a size fractionation showed a high occurrence of halomethanes in the stations dominated by pico-sized phytoplankton. These results indicate the importance of picoplankton as a possible source of halocarbon production. Chlorophyll b and prasinoxanthin had a statistically significant positive correlation with CH 2I 2 ( r 2 = 0.69 and r 2 = 0.71, respectively) and with CH 2ClI ( r 2 = 0.87 and r 2 = 0.77, respectively). These results suggest that some species of prasinophytes might contribute to CH 2I 2 and CH 2ClI production. For other compounds, there was no peak in the vertical profile in seawater. In the depth profiles, the peak of CH 2ClI was observed above the peak of CH 2I 2; these profiles suggest that a photochemical reaction could yield CH 2ClI from CH 2I 2 in seawater. The mean mixing ratio and range of CH 3Cl, CH 3Br, and CH 2ClI in the air were measured as 548 (524–609), 12.1 (8.6–19.0), and 0.27 (0.03–0.90) pptv, respectively. CH 2I 2 was not detected in the atmosphere (< 1 pptv). The saturation anomaly of CH 3Br was positive at all stations (the sea surface temperature varied from 1.7 °C to 19 °C). The highest mixing ratio of CH 2ClI in air was also observed near the station at which the highest concentration of CH 2ClI was observed in seawater; the sea-to-air fluxes of CH 2ClI and CH 2I 2 were 3.8 and 1.6 nmol m − 2 day − 1 , respectively. These results suggest that the production of CH 2ClI and CH 2I 2 in seawater is an important source of organic iodine compounds in the remote atmosphere.

Seasonal variability in mixed layer height and its impact on trace gas distribution over a tropical urban site: Ahmedabad

Abstract Altitude profiles of virtual potential temperature ( θ v ) and specific humidity ( q ) derived from meteorological data obtained from balloon-borne radiosonde ascents are used to investigate the seasonal variations in mixed layer height over Ahmedabad (23.03°N, 72.54°E), an urban site located on the western part of India. A total of 82 balloon ascents were conducted fortnightly in the morning hours during a period of about four years spanning from April 2003 to July 2007. Analysis of the vertical profiles of θ v and q reveals a systematic seasonal variability in the mixed layer height (MLH), showing the decreasing trend from summer–monsoon to winter season. The MLH is observed to be maximum (∼ 1170 m) in summer–monsoon while a minimum (∼ 160 m) is observed during the winter months. In general, the mixed layer height was found to be highly variant during pre-monsoon and summer–monsoon seasons. This variability is observed to be comparatively lower in the post-monsoon and winter months. Effects of MLH have been investigated on the variations in surface ozone and MOPITT derived surface and vertical distributions of CO. The reverse trend is observed in surface ozone and CO with mixed layer seasonal variability. The impact of MLH over CO vertical distributions is observed up to an altitude of 3–4 km.

MIPAS reduced spectral resolution UTLS-1 mode measurements of temperature, O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;, HNO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;, N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O, H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O and relative humidity over ice: retrievals and comparison to MLS

Abstract. During several periods since 2005 the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat has performed observations dedicated to the region of the upper troposphere/lower stratosphere (UTLS). For the duration of November/December 2005 global distributions of temperature and several trace gases from MIPAS UTLS-1 mode measurements have been retrieved using the IMK/IAA (Institut für Meteorologie und Klimaforschung/Instituto de Astrofísica de Andalucía) scientific processor. In the UTLS region a vertical resolution of 3 km for temperaure, 3 to 4 km for H2O, 2.5 to 3 km for O3, 3.5 km for HNO3 and 3.5 to 2.5 km for N2O has been achieved. The retrieved temperature, H2O, O3, HNO3, N2O, and relative humidity over ice are intercompared with the Microwave Limb Sounder (MLS/Aura) v2.2 data in the pressure range 316 to 0.68 hPa, 316 to 0.68 hPa, 215 to 0.68 hPa, 215 to 3.16 hPa, 100 to 1 hPa and 316 to 10 hPa, respectively. In general, MIPAS and MLS temperatures are biased within ±4 K over the whole pressure and latitude range. Systematic, latitude-independent differences of −2 to −4 K (MIPAS-MLS) at 121 hPa are explained by previously observed biases in the MLS v2.2 temperature retrievals. Temperature differences of −4 K up to 12 K above 10.0 hPa are present both in MIPAS and MLS with respect to ECMWF (European Centre for Medium-Range Weather Forecasts) and are likely due to deficiencies of the ECMWF analysis data. MIPAS and MLS stratospheric volume mixing ratios (vmr) of H2O are biased within ±1 ppmv, with indication of oscillations between 146 and 26 hPa in the MLS dataset. Tropical upper tropospheric values of relative humidity over ice measured by the two instruments differ by ±20% in the pressure range ~146 to 68 hPa. These differences are mainly caused by the MLS temperature biases. Ozone mixing ratios agree within 0.5 ppmv (10 to 20%) between 68 and 14 hPa. At pressures smaller than 10 hPa, MIPAS O3 vmr are higher than MLS by an average of 0.5 ppmv (10%). General agreement between MIPAS and MLS HNO3 is within the range of −1.0 (−10%) to 1.0 ppbv (20%). MIPAS HNO3 is 1.0 ppbv (10%) higher compared to MLS between 46 hPa and 10 hPa over the Northern Hemisphere. Over the tropics at 31.6 hPa MLS shows a low bias of more than 1 ppbv (&gt;50%). In general, MIPAS and MLS N2O vmr agree within 20 to 40 ppbv (20 to 40%). Differences in the range between 100 to 21 hPa are attributed to a known 20% positive bias in MIPAS N2O data.

The benefit of limb cloud imaging for infrared limb sounding of tropospheric trace gases

Abstract. Advances in detector technology enable a new generation of infrared limb sounders to measure 2-D images of the atmosphere. A proposed limb cloud imager (LCI) mode will detect clouds with a spatial resolution unprecedented for limb sounding. For the inference of temperature and trace gas distributions, detector pixels of the LCI have to be combined into super-pixels which provide the required signal-to-noise and information content for the retrievals. This study examines the extent to which tropospheric coverage can be improved in comparison to limb sounding using a fixed field of view with the size of the super-pixels, as in conventional limb sounders. The study is based on cloud topographies derived from (a) IR brightness temperatures (BT) of geostationary weather satellites in conjunction with ECMWF temperature profiles and (b) ice and liquid water content data of the Consortium for Small-scale Modeling-Europe (COSMO-EU) of the German Weather Service. Limb cloud images are simulated by matching the cloud topography with the limb sounding line of sight (LOS). The analysis of the BT data shows that the reduction of the spatial sampling along the track has hardly any effect on the gain in information. The comparison between BT and COSMO-EU data identifies the strength of both data sets, which are the representation of the horizontal cloud extent for the BT data and the reproduction of the cloud amount for the COSMO-EU data. The results of the analysis of both data sets show the great advantage of the cloud imager. However, because both cloud data sets do not present the complete fine structure of the real cloud fields in the atmosphere it is assumed that the results tend to underestimate the increase in information. In conclusion, real measurements by such an instrument may result in an even higher benefit for tropospheric limb retrievals.

Analysis of Convective Transport and Parameter Sensitivity in a Single Column Version of the Goddard Earth Observation System, Version 5, General Circulation Model

Abstract Convection strongly influences the distribution of atmospheric trace gases. General circulation models (GCMs) use convective mass fluxes calculated by parameterizations to transport gases, but the results are difficult to compare with trace gas observations because of differences in scale. The high resolution of cloud-resolving models (CRMs) facilitates direct comparison with aircraft observations. Averaged over a sufficient area, CRM results yield a validated product directly comparable to output from a single global model grid column. This study presents comparisons of vertical profiles of convective mass flux and trace gas mixing ratios derived from CRM and single column model (SCM) simulations of storms observed during three field campaigns. In all three cases, SCM simulations underpredicted convective mass flux relative to CRM simulations. As a result, the SCM simulations produced lower trace gas mixing ratios in the upper troposphere in two of the three storms than did the CRM simulations. The impact of parameter sensitivity in the moist physics schemes employed in the SCM has also been examined. Statistical techniques identified the most significant parameters influencing convective transport. Convective mass fluxes are shown to be strongly dependent on chosen parameter values. Results show that altered parameter settings can substantially improve the comparison between SCM and CRM convective mass flux. Upper tropospheric trace gas mixing ratios were also improved in two storms. In the remaining storm, the SCM representation of CO2 was not improved because of differences in entrainment and detrainment levels in the CRM and SCM simulations.

Effects of the quasi‐biennial oscillation on low‐latitude transport in the stratosphere derived from trajectory calculations

The quasi‐biennial oscillation (QBO) of stratospheric zonal winds induces a secondary meridional circulation (SMC) consisting of QBO variations in meridional and vertical winds. In this work, we investigate how these instantaneous meridional circulation anomalies add over time to variations of stratospheric transport. To that end, we compute backward parcel trajectories on the basis of the output of a chemistry‐climate model (CCM). At the equator, the trajectories show the strongest vertical parcel displacement over a seasonal timescale when the QBO progresses toward easterly phase in the middle stratosphere. During the solstitial seasons a large number of parcels come from the summer hemisphere, causing in addition a QBO variation in the spread of the total ascent among equatorial parcels. A QBO effect on meridional transport is diagnosed from PV gradients during summer in the easterly phase of the QBO, which suggests a variation of the tropical‐subtropical barrier strength. Analyses of the parcel trajectories and CCM trace gas distributions confirm this finding. We suggest that this variation is due to the combined effects of QBO and annual variation in meridional advection and in wave‐induced eddy transport.

Direct observation of two dimensional trace gas distributions with an airborne Imaging DOAS instrument

Abstract. In many investigations of tropospheric chemistry information about the two dimensional distribution of trace gases on a small scale (e.g. tens to hundreds of metres) is highly desirable. An airborne instrument based on imaging Differential Optical Absorption Spectroscopy has been built to map the two dimensional distribution of a series of relevant trace gases including NO2, HCHO, C2H2O2, H2O, O4, SO2, and BrO on a scale of 100 m. Here we report on the first tests of the novel aircraft instrument over the industrialised South African Highveld, where large variations in NO2 column densities in the immediate vicinity of several sources e.g. power plants or steel works, were measured. The observed patterns in the trace gas distribution are interpreted with respect to flux estimates, and it is seen that the fine resolution of the measurements allows separate sources in close proximity to one another to be distinguished.

Accounting for the effect of horizontal gradients in limb measurements of scattered sunlight

Abstract. Limb measurements provided by the SCanning Imaging Absorption spectrometer for Atmospheric CHartographY (SCIAMACHY) on the ENVISAT satellite allow retrieving stratospheric profiles of various trace gases on a global scale, among them BrO for the first time. For limb observations in the UV/VIS spectral region the instrument measures scattered light with a complex distribution of light paths: the light is measured at different tangent heights and can be scattered or absorbed in the atmosphere or reflected by the ground. By means of spectroscopy and radiative transfer modelling these measurements can be inverted to retrieve the vertical distribution of stratospheric trace gases. The fully spherical 3-D Monte Carlo radiative transfer model "Tracy-II" is applied in this study. The Monte Carlo method benefits from conceptual simplicity and allows realizing the concept of full spherical geometry of the atmosphere and also its 3-D properties, which is important for a realistic description of the limb geometry. Furthermore it allows accounting for horizontal gradients in the distribution of trace gases. In this study the effect of horizontally inhomogeneous distributions of trace gases along flight/viewing direction on the retrieval of profiles is investigated. We introduce a tomographic method to correct for this effect by combining consecutive limb scanning sequences and utilizing the overlap in their measurement sensitivity regions. It is found that if horizontal inhomogenity is not properly accounted for, typical errors of 20% for NO2 and up to 50% for OClO around the altitude of the profile peak can arise for measurements close to the Arctic polar vortex boundary in boreal winter.

Trends and variability in stratospheric mixing: 1979–2005

Abstract. Changes in climate are likely to drive changes in stratospheric mixing with associated implications for changes in transport of ozone from tropical source regions to higher latitudes, transport of water vapour and source gas degradation products from the tropical tropopause layer into the mid-latitude lower stratosphere, and changes in the meridional distribution of long-lived trace gases. To diagnose long-term changes in stratospheric mixing, global monthly fields of Lyapunov exponents were calculated on the 450 K, 550 K, and 650 K isentropic surfaces by applying a trajectory model to wind fields from NCEP/NCAR reanalyses over the period 1979 to 2005. Potential underlying geophysical drivers of trends and variability in these mixing fields were investigated by applying a least squares regression model, which included basis functions for a mean annual cycle, seasonally dependent linear trends, the quasi-biennial oscillation (QBO), the solar cycle, and the El Niño Southern Oscillation (ENSO), to zonal mean time series of the Lyapunov exponents. Long-term positive trends in mixing are apparent over southern middle to high latitudes at 450 K through most of the year, while negative trends over southern high latitudes are apparent at 650 K from May to August. Wintertime negative trends in mixing over northern mid-latitudes are apparent at 550 K and 650 K. Over low latitudes, within 40° of the equator, the QBO exerts a strong influence on mixing at all three analysis levels. This QBO influence is strongly modulated by the annual cycle and shows a phase shift across the subtropical mixing barrier. Solar cycle and ENSO influences on mixing are generally not significant. The diagnosed long-term changes in mixing should aid the interpretation of trends in stratospheric trace gases.

Geophysical validation of MIPAS-ENVISAT operational ozone data

Geophysical validation of MIPAS-ENVISAT operational ozone data

Remote sensing retrievals of fine mode aerosol optical depth and impacts on its correlation with CO from biomass burning

] Globally significant amounts of atmospheric aerosolsand trace gases, including carbon monoxide (CO), areemitted from different types of biomass burning across theglobe ranging from human-induced burning of savanna andgrassland for agriculture and land-clearing to wildfires inboreal regions of the northern hemisphere. Biomass burningsmoke has adverse impacts on air quality and can affect theEarth’s radiative balance [Andreae et al., 1998]. Despite thegreat interest in the subject, quantifying the amount of gasesand particulates emitted from the burning of biomassremains a challenging problem. The challenges stem fromthe fact that amounts of CO and pyrogenic aerosols varystrongly depending on fuel type, oxygen availability, andburning phase. Chemical transport models (CTMs) capableof treating trace gases and aerosol particles are beingincreasingly used to improve our understanding of biomassburning emissions and transport [e.g., Heald et al., 2003].However, these models typically rely on bottoms-up emis-sion inventories with large uncertainties, assumptions onfuel type and burning efficiencies [Andreae and Merlet,2001], and on surface measurement networks with insuffi-cient coverage, particularly in the developing world. Giventhe inherent problems in ground-based measurements ofbiomass burning and a growing number of satellite sensorssuited to observing biomass burning emissions, it would bedesirable to utilize satellite remote sensing to the full extentpossible to improve our understanding of regional andglobal distributions of biomass burning aerosols and tracegases as well as their impacts.[

Global cloud and precipitation chemistry and wet deposition: tropospheric model simulations with ECHAM5/MESSy1

Abstract. The representation of cloud and precipitation chemistry and subsequent wet deposition of trace constituents in global atmospheric chemistry models is associated with large uncertainties. To improve the simulated trace gas distributions we apply the new submodel SCAV, which includes detailed cloud and precipitation chemistry and present results of the atmospheric chemistry general circulation model ECHAM5/MESSy1. A good agreement with observed wet deposition fluxes for species causing acid rain is obtained. The new scheme enables prognostic calculations of the pH of clouds and precipitation, and these results are also in accordance with observations. We address the influence of detailed cloud and precipitation chemistry on trace constituents based on sensitivity simulations. The results confirm previous results from regional scale and box models, and we extend the analysis to the role of aqueous phase chemistry on the global scale. Some species are directly affected through multiphase removal processes, and many also indirectly through changes in oxidant concentrations, which in turn have an impact on the species lifetime. While the overall effect on tropospheric ozone is relatively small (&lt;10%), regional effects on O3 can reach ≈20%, and several important compounds (e.g., H2O2, HCHO) are substantially depleted by clouds and precipitation.

Evolution of ozone, particulates, and aerosol direct radiative forcing in the vicinity of Houston using a fully coupled meteorology‐chemistry‐aerosol model

A new fully coupled meteorology‐chemistry‐aerosol model is used to simulate the urban‐ to regional‐scale variations in trace gases, particulates, and aerosol direct radiative forcing in the vicinity of Houston over a 5 day summer period. Model performance is evaluated using a wide range of meteorological, chemistry, and particulate measurements obtained during the 2000 Texas Air Quality Study. The predicted trace gas and particulate distributions were qualitatively similar to the surface and aircraft measurements with considerable spatial variations resulting from urban, power plant, and industrial sources of primary pollutants. Sulfate, organic carbon, and other inorganics were the largest constituents of the predicted particulates. The predicted shortwave radiation was 30 to 40 W m−2 closer to the observations when the aerosol optical properties were incorporated into the shortwave radiation scheme; however, the predicted hourly aerosol radiative forcing was still underestimated by 10 to 50 W m−2. The predicted aerosol radiative forcing was larger over Houston and the industrial ship channel than over the rural areas, consistent with surface measurements. The differences between the observed and simulated aerosol radiative forcing resulted from transport errors, relative humidity errors in the upper convective boundary layer that affect aerosol water content, secondary organic aerosols that were not yet included in the model, and uncertainties in the primary particulate emission rates. The current model was run in a predictive mode and demonstrates the challenges of accurately simulating all of the meteorological, chemical, and aerosol parameters over urban to regional scales that can affect aerosol radiative forcing.

The atmospheric chemistry general circulation model ECHAM5/MESSy1: consistent simulation of ozone from the surface to the mesosphere

Abstract. The new Modular Earth Submodel System (MESSy) describes atmospheric chemistry and meteorological processes in a modular framework, following strict coding standards. It has been coupled to the ECHAM5 general circulation model, which has been slightly modified for this purpose. A 90-layer model setup up to 0.01 hPa was used at spectral T42 resolution to simulate the lower and middle atmosphere. With the high vertical resolution the model simulates the Quasi-Biennial Oscillation. The model meteorology has been tested to check the influence of the changes to ECHAM5 and the radiation interactions with the new representation of atmospheric composition. In the simulations presented here a Newtonian relaxation technique was applied in the tropospheric part of the domain to weakly nudge the model towards the analysed meteorology during the period 1998–2005. This allows an efficient and direct evaluation with satellite and in-situ data. It is shown that the tropospheric wave forcing of the stratosphere in the model suffices to reproduce major stratospheric warming events leading e.g. to the vortex split over Antarctica in 2002. Characteristic features such as dehydration and denitrification caused by the sedimentation of polar stratospheric cloud particles and ozone depletion during winter and spring are simulated well, although ozone loss in the lower polar stratosphere is slightly underestimated. The model realistically simulates stratosphere-troposphere exchange processes as indicated by comparisons with satellite and in situ measurements. The evaluation of tropospheric chemistry presented here focuses on the distributions of ozone, hydroxyl radicals, carbon monoxide and reactive nitrogen compounds. In spite of minor shortcomings, mostly related to the relatively coarse T42 resolution and the neglect of inter-annual changes in biomass burning emissions, the main characteristics of the trace gas distributions are generally reproduced well. The MESSy submodels and the ECHAM5/MESSy1 model output are available through the internet on request.

The Vertical Distribution and Trends of the Trace Gases in Stratosphere over China

AbstractIn order to provide some useful data for studies on radiative and chemical processes in the stratosphere, the vertical distribution of trace gases (NO, NO2, HCl, HF, CH4, H2O and O3) and their variation trends in the stratosphere over China have been analyzed by using the HALOE data from 1992 to 2005. Besides the mixing ratio of the traces averaged over China, the values averaged over Qinghai‐Xizang Plateau are also given.The analyses show that the mixing ratios for various trace gases distribute vertically in different patterns in stratosphere, the distribution and seasonal variation of the traces near tropopause are much different from those in stratosphere, The analyses also show that the seasonal variation, the quasi‐biennial oscillation (QBO) signals and the long‐term trends are clear for all the traces, and the trends of the traces gases in various levels of stratosphere are different. In middle stratosphere, the mixing ratios of NO, NO2, HCl and H2O are all increased before 1998 and then decreased after 1998, while it is contrary for mixing ratio of O3 which decreased evidently before 1998 and the decrease trend is not significant after 1998. That means the decrease of those traces lead to the ozone depletion retarded in middle stratosphere in recent years. While in the lower stratosphere, the ozone depletion is still significant.

Tomographic multiaxis-differential optical absorption spectroscopy observations of Sun-illuminated targets: a technique providing well-defined absorption paths in the boundary layer

A novel experimental procedure to measure the near-surface distribution of atmospheric trace gases by using passive multiaxis differential absorption optical spectroscopy (MAX-DOAS) is proposed. The procedure consists of pointing the receiving telescope of the spectrometer to nonreflecting surfaces or to bright targets placed at known distances from the measuring device, which are illuminated by sunlight. We show that the partial trace gas absorptions between the top of the atmosphere and the target can be easily removed from the measured total absorption. Thus it is possible to derive the average concentration of trace gases such as NO(2), HCHO, SO(2), H(2)O, Glyoxal, BrO, and others along the line of sight between the instrument and the target similar to the well-known long-path DOAS observations (but with much less expense). If tomographic arrangements are used, even two- or three-dimensional trace gas distributions can be retrieved. The basic assumptions of the proposed method are confirmed by test measurements taken across the city of Heidelberg.

Air quality remote sensing from space

Recent advances in tropospheric remote sensing have opened the way for measuring, monitoring, and understanding processes that lead to atmospheric pollution. As part of an integrated observing strategy, satellite measurements provide a context for localized observations and help to extend these observations to continental and global scales. The challenge for future space‐borne missions will be directly accessing the local scale and facilitating the use of remotely sensed information for improving local‐ and regional‐scale air quality (AQ) forecasts. Achieving this goal could provide important societal dividends for public health, for policy applications related to managing national AQ, and for assessing the impact of daily human activity on the distributions of important trace gases and aerosols and their short‐timescale variability—known as ‘chemical weather’—as well as on climate.