Model Of Atmospheric Transport And Chemistry Research Articles

Lifetime dependent flux into the lowermost stratosphere for idealized trace gases of surface origin

The flux of idealized trace gases across the thermal tropopause is quantified as a function of their chemical lifetime using the Model of Atmospheric Transport and Chemistry (MATCH) driven by National Centers for Environmental Prediction (NCEP) reanalyses. The flux is computed in the limit of instant stratospheric chemical loss, and tropospheric chemistry is idealized as decay with a constant lifetime, τc. Emissions are idealized as time independent, with either a generic anthropogenic pattern or a uniform ocean source. We find that the globally averaged flux into the stratosphere normalized by surface emissions is ∼1% for τc= 8 days and ∼30% for τc ∼ 140 days, slowly approaching the long‐lived limit of balance between stratospheric sinks and surface sources. The qualitative τc dependence of the globally averaged flux is captured by a simple one‐dimensional model. The flux patterns computed with MATCH for the NCEP reanalyses are insensitive to τc and reveal preferred pathways into the stratosphere: The divergent circulation feeding isentropic cross‐tropopause transport, storm tracks in the winter hemisphere, and isentropic transport to high latitudes.

Impact of climate change on ozone-related mortality and morbidity in Europe

Ozone is a highly oxidative pollutant formed from precursors in the presence of sunlight, associated with respiratory morbidity and mortality. All else being equal, concentrations of ground-level ozone are expected to increase due to climate change. Ozone-related health impacts under a changing climate are projected using emission scenarios, models and epidemiological data. European ozone concentrations are modelled with the model of atmospheric transport and chemistry (MATCH)-RCA3 (50×50 km). Projections from two climate models, ECHAM4 and HadCM3, are applied under greenhouse gas emission scenarios A2 and A1B, respectively. We applied a European-wide exposure-response function to gridded population data and country-specific baseline mortality and morbidity. Comparing the current situation (1990-2009) with the baseline period (1961-1990), the largest increase in ozone-associated mortality and morbidity due to climate change (4-5%) have occurred in Belgium, Ireland, the Netherlands and the UK. Comparing the baseline period and the future periods (2021-2050 and 2041-2060), much larger increases in ozone-related mortality and morbidity are projected for Belgium, France, Spain and Portugal, with the impact being stronger using the climate projection from ECHAM4 (A2). However, in Nordic and Baltic countries the same magnitude of decrease is projected. The current study suggests that projected effects of climate change on ozone concentrations could differentially influence mortality and morbidity across Europe.

Simulation and validation of the aerosol optical thickness over China in 2006

The Model of Atmospheric Transport and Chemistry (MATCH) developed by the US National Center for Atmospheric Research (NCAR) was used to calculate the aerosol optical thickness (AOT) over China in 2006, with emission source data of the Intercontinental Chemical Transport Experiment Phase B (INTEX-B) and NCEP/NCAR reanalysis data as inputs. The simulation results of AOT were then validated with observational data from the Moderate Resolution Imaging Spectroradiometer (MODIS), Chinese Sun Hazemeter Network (CSHNET), Aerosol Robotics Network (AERONET), and China Aerosol Remote Sensing Network (CARSNET) at more than 30 stations over China. The comparison results indicated that the high values of AOT in the areas such as the Sichuan basin and East and South China and the low values of AOT over the Tibetan Plateau and Northwest and Northeast China were reasonably simulated by the MATCH. This model tended to underestimate the AOT values in high-aerosol-loading areas but overestimate the AOT values in less polluted areas because there are still large uncertainties in the expression of emission sources, the description of the optical properties of aerosols, the treatment of cloud and precipitation, and the selection of grid resolution. The modeling results were consistent with the CSHNET, CARSNET, AERONET, and MODIS data in most parts of China, and the correlation coefficient of the monthly mean AOT between the model and the observation was 0.79 with CSHNET data at 23 stations, 0.51 with MODIS data, and 0.88 with data at 3 CARSNET stations and 2 other stations. All of them passed the significance test with α < 0.0001. The results demonstrated that the MATCH has the ability to simulate the characteristics of the AOT distribution and its seasonal variation over China.

Atmospheric three-dimensional inverse modeling of regional industrial emissions and global oceanic uptake of carbon tetrachloride

Abstract. Carbon tetrachloride (CCl4) has substantial stratospheric ozone depletion potential and its consumption is controlled under the Montreal Protocol and its amendments. We implement a Kalman filter using atmospheric CCl4 measurements and a 3-dimensional chemical transport model to estimate the interannual regional industrial emissions and seasonal global oceanic uptake of CCl4 for the period of 1996–2004. The Model of Atmospheric Transport and Chemistry (MATCH), driven by offline National Center for Environmental Prediction (NCEP) reanalysis meteorological fields, is used to simulate CCl4 mole fractions and calculate their sensitivities to regional sources and sinks using a finite difference approach. High frequency observations from the Advanced Global Atmospheric Gases Experiment (AGAGE) and the Earth System Research Laboratory (ESRL) of the National Oceanic and Atmospheric Administration (NOAA) and low frequency flask observations are together used to constrain the source and sink magnitudes, estimated as factors that multiply the a priori fluxes. Although industry data imply that the global industrial emissions were substantially declining with large interannual variations, the optimized results show only small interannual variations and a small decreasing trend. The global surface CCl4 mole fractions were declining in this period because the CCl4 oceanic and stratospheric sinks exceeded the industrial emissions. Compared to the a priori values, the inversion results indicate substantial increases in industrial emissions originating from the South Asian/Indian and Southeast Asian regions, and significant decreases in emissions from the European and North American regions.

Optimal estimation of the surface fluxes of methyl chloride using a 3-D global chemical transport model

Abstract. Methyl chloride (CH3Cl) is a chlorine-containing trace gas in the atmosphere contributing significantly to stratospheric ozone depletion. Large uncertainties in estimates of its source and sink magnitudes and temporal and spatial variations currently exist. GEIA inventories and other bottom-up emission estimates are used to construct a priori maps of the surface fluxes of CH3Cl. The Model of Atmospheric Transport and Chemistry (MATCH), driven by NCEP interannually varying meteorological data, is then used to simulate CH3Cl mole fractions and quantify the time series of sensitivities of the mole fractions at each measurement site to the surface fluxes of various regional and global sources and sinks. We then implement the Kalman filter (with the unit pulse response method) to estimate the surface fluxes on regional/global scales with monthly resolution from January 2000 to December 2004. High frequency observations from the AGAGE, SOGE, NIES, and NOAA/ESRL HATS in situ networks and low frequency observations from the NOAA/ESRL HATS flask network are used to constrain the source and sink magnitudes. The inversion results indicate global total emissions around 4100 ± 470 Gg yr−1 with very large emissions of 2200 ± 390 Gg yr−1 from tropical plants, which turn out to be the largest single source in the CH3Cl budget. Relative to their a priori annual estimates, the inversion increases global annual fungal and tropical emissions, and reduces the global oceanic source. The inversion implies greater seasonal and interannual oscillations of the natural sources and sink of CH3Cl compared to the a priori. The inversion also reflects the strong effects of the 2002/2003 globally widespread heat waves and droughts on global emissions from tropical plants, biomass burning and salt marshes, and on the soil sink.

The Path Density of Interhemispheric Surface-to-Surface Transport. Part II: Transport through the Troposphere and Stratosphere Diagnosed from NCEP Data

Abstract Interhemispheric transport from the earth’s surface north of 32.4°N (region ΩN) to the surface south of 32.4°S (region ΩS) is quantified using the path-density diagnostic developed in Part I of this study. The path density is computed using the Model of Atmospheric Transport and Chemistry (MATCH) driven by NCEP reanalyses. The structure of both the ΩN → ΩS and ΩN → ΩN zonally averaged path densities is examined in detail for air that had last ΩN contact (“ΩN air”) during January and July. The path density provides the joint probability that ΩN air will make its next surface contact with either ΩN or ΩS, that it will have surface-to-surface transit time τ ∈ (τ, τ + dτ), and that it can be found in volume element d 3r during its surface-to-surface journey. The distribution of surface-to-surface transit times, the probability of ΩN air making next contact with ΩS, and the probability of finding ΩN air destined for ΩS in the stratosphere are computed from suitable integrations of the path density. Approximately one-third of the ΩN air undergoes interhemispheric transport to ΩS, with a ∼20% probability of being found in the stratosphere during its surface-to-surface journey. The stratospheric fraction has about equal contributions from the part of the stratosphere that is isentropically isolated from the troposphere and from the part that is isentropically connected to the troposphere (i.e., from the overworld and the stratospheric middleworld in the terminology of Hoskins). The flow rate through the stratospheric middleworld is about twice as large as the flow rate through the overworld.

A Modeling Study with MATCH on the Aerosol Optical Depth Distribution over China in 2006

The Model of Atmospheric Transport and Chemistry(MATCH) is used in this work to simulate the spatial distribution and seasonal variation of AOD(λ=550nm) over China in 2006,by taking NCEP/NCAR reanalysis data as its meteorological input.The AOD obtained by MATCH is then compared with the MODIS(MODerate resolution Imaging Spectro-radiometer) satellite data and the CSHNET(the Chinese Sun Hazemeter Network) observational data.The comparison of the results in this work with MOD08_M3 indicates that MATCH has the basic ability to simulate the AOD variability over China region.The correlation coefficient between the simulation and the observational data is 0.693(with α0.0001).The simulated results in this work show that the AOD over China varies greatly with different regions and seasons.The higher value of AOD occurred in North China,Central China and South China,whereas the lower values of AOD are mainly located in Northwest China,Northeast China and Tibetan Plateau.The AOD over China is mainly generated by sulfate aerosol,secondly by organic aerosol,and the least by black carbon and sea-salt aerosol.

Estimation of regional emissions of nitrous oxide from 1997 to 2005 using multinetwork measurements, a chemical transport model, and an inverse method

Nitrous oxide (N2O) is an important ozone‐depleting gas and greenhouse gas with multiple uncertain emission processes. Global nitrous oxide observations, the Model of Atmospheric Transport and Chemistry (MATCH) and an inverse method were used to optimally estimate N2O emissions from twelve source regions around the globe. MATCH was used with forecast center reanalysis winds at T62 resolution (192 longitude by 94 latitude surface grid, and 28 vertical levels) from 1 July 1996 to 30 June 2006. The average concentrations of N2O in the lowest four layers of the model were then compared with the monthly mean observations from four national/international networks measuring at 65 surface sites. A 12‐month‐running‐mean smoother was applied to both the model results and the observations, due to the fact that the model was not able to reproduce the very small observed seasonal cycles. The inverse method was then used to solve for the time‐averaged regional emissions of N2O for two time periods (1 January 1997 to 31 December 2001 and 1 January 2002 to 31 December 2005). The best estimate inversions assume that the model stratospheric destruction rates, which lead to a global N2O lifetime of 125 years, are correct. It also assumes normalized emission spatial distributions within each region from Bouwman et al. (1995). We conclude that global N2O emissions with 66% probability errors are 16.3−1.2+1.5 and 15.4−1.3+1.7 TgN (N2O) a−1, for 1997–2001 and 2001–2005 respectively. Emissions from the equator to 30°N increased significantly from the initial Bouwman et al. (1995) estimates while emissions from southern oceans (30°S–90°S) decreased significantly. The quoted uncertainties include both the measurement errors and modeling uncertainties estimated using a separate flexible 12‐box model. We also found that 23 ± 4% of the N2O global total emissions come from the ocean, which is slightly smaller than the Bouwman et al. (1995) estimate. For the estimation of emissions from the twelve model regions, we conclude that, relative to Bouwman et al. (1995), land emissions from South America, Africa, and China/Japan/South East Asia are larger, while land emissions from Australia/New Zealand are smaller. Our study also shows a shift of the oceanic sources from the extratropical to the tropical oceans relative to Bouwman et al. (1995). Between the periods 1997–2001 and 2002–2005, emissions increased in China/Japan/South East Asia, 0°–30°N oceans, and North West Asia and decreased in Australia/New Zealand, 30°S–90°S oceans, 30°N–90°N oceans, and Africa. The lower tropical ocean emissions in 1997–2001 relative to 2002–2005 could result from the effects of the 1997–1998 El Nino in the earlier period.

Regional pollution potentials of megacities and other major population centers

Abstract. Megacities and other major population centers represent large, concentrated sources of anthropogenic pollutants to the atmosphere, with consequences for both local air quality and for regional and global atmospheric chemistry. The tradeoffs between the regional buildup of pollutants near their sources versus long-range export depend on meteorological characteristics which vary as a function of geographical location and season. Both horizontal and vertical transport contribute to pollutant export, and the overall degree of export is strongly governed by the lifetimes of pollutants. We provide a first quantification of these tradeoffs and the main factors influencing them in terms of "regional pollution potentials", metrics based on simulations of representative tracers using the 3-D global model MATCH (Model of Atmospheric Transport and Chemistry). The tracers have three different lifetimes (1, 10, and 100 days) and are emitted from 36 continental large point sources. Several key features of the export characteristics emerge. For instance, long-range near-surface pollutant export is generally strongest in the middle and high latitudes, especially for source locations in Eurasia, for which 17–34% of a tracer with a 10-day lifetime is exported beyond 1000 km and still remains below 1 km altitude. On the other hand, pollutant export to the upper troposphere is greatest in the tropics, due to transport by deep convection, and for six source locations, more than 50% of the total mass of the 10-day lifetime tracer is found above 5 km altitude. Furthermore, not only are there order of magnitude interregional differences, such as between low and high latitudes, but also often substantial intraregional differences, which we discuss in light of the regional meteorological characteristics. We also contrast the roles of horizontal dilution and vertical mixing in reducing the pollution buildup in the regions including and surrounding the sources. For some regions such as Eurasia, dilution due to long-range horizontal transport governs the local and regional pollution buildup; however, on a global basis, differences in vertical mixing are dominant in determining the pollution buildup both around and further downwind of the source locations.

Sensitivities of gas‐phase dimethylsulfide oxidation products to the assumed mechanisms in a chemical transport model

The gas‐phase products of dimethylsulfide (DMS) oxidation are simulated in the global 3D Model of Atmospheric Transport and Chemistry (MATCH). The focus is on the sensitivities to the assumed mechanisms of DMS oxidation chemistry in large‐scale models, and hence volcanic and anthropogenic sources of SO2 are ignored. Four representations of DMS chemistry are considered, including two comprehensive mechanisms (about 50 reactions) and two parameterized schemes (four and five reactions). The gas‐phase yields of DMS, SO2, methanesulfonic acid (MSA) and H2SO4 are compared between these four cases as a measure of the sensitivity to uncertain DMS chemistry. Among the four cases, DMS is largely invariant, while SO2 using parameterized chemistry is three times higher than its levels using comprehensive chemistry in the tropical upper troposphere. For MSA and H2SO4, there are order‐of‐magnitude inter‐mechanistic differences at high and low altitudes in the extratropics, respectively. The differences are attributed to fixed branching yields and the absence of important intermediate species and pathways in the parameterized mechanisms. Regional budgets are also analyzed within the remote Southern Ocean and central tropical Pacific. While the DMS budget varies primarily between the regions, the SO2, MSA and H2SO4 budgets vary strongly between the regions and mechanisms. The DMS oxidation products, therefore, are as sensitive to the assumed mechanisms as to the external conditions between the tropics and extratropics. To distinguish between the mechanism cases, the MATCH simulations are also compared to campaign measurements. The simulated values of DMS and SO2 are found to approximate the observations, but do not provide mechanism differentiation. The gas‐phase H2SO4 and MSA simulations differ more extensively from the observations, yet the comprehensive chemistry cases give a better fit to the MSA measurements. Better fits to the MSA observations are also achieved for the two mechanisms that consider dimethylsulfoxide (DMSO) as an MSA precursor.

Improvement of the vertical humidity distribution in the chemistry-transport model MATCH through increased evaporation of convective precipitation

[1] We compare the vertical monthly mean distributions of model water vapor from the Model of Atmospheric Transport and Chemistry (MATCH) to tropospheric in situ sonde profiles and to upper tropospheric water vapor climatologies from the Microwave Limb Sounder (MLS). We perform two model runs with and without increased evaporation of precipitation in midlevel downdrafts and subsidence regions of convective systems. We demonstrate that higher evaporation rates significantly improve the modelled vertical distribution of specific and relative humidity with respect to what has been observed, by moistening of mid and lower tropospheric levels in regions of strong convection. We further demonstrate that the more efficient evaporation increases the modelled water vapor residence time, reducing the gap to observed mean water-vapor residence times from synergistic use of the Global Ozone Monitoring Experiment (GOME) water-vapor fields and data from the Global Precipitation Climatology Project (GPCP).

Evaluation of the hydrological cycle of MATCH driven by NCEP reanalysis data: comparison with GOME water vapor measurements

Abstract. This study examines two key parameters of the hydrological cycle, water vapor (WV) and precipitation rates (PR), as modelled by the chemistry transport model MATCH (Model of Atmospheric Transport and Chemistry) driven by National Centers for Environmental Prediction (NCEP) reanalysis data (NRA). For model output evaluation we primarily employ WV total column data from the Global Ozone Monitoring Experiment (GOME) on ERS-2, which is the only instrument capable measuring WV on a global scale and over all surface types with a substantial data record from 1995 to the present. We find that MATCH and NRA WV and PR distributions are closely related, but that significant regional differences in both parameters exist in magnitude and distribution patterns when compared to the observations. We also find that WV residual patterns between model and observations show remarkable similarities to residuals observed in the PR when comparing MATCH and NRA output to observations comprised by the Global Precipitation Climatology Project (GPCP). We conclude that deficiencies in model parameters shared by MATCH and NRA, like in the surface evaporation rates and regional transport patterns, are likely to lead to the observed differences. Monthly average regional differences between MATCH modelled WV columns and the observations can be as large as 2 cm, based on the analysis of three years. Differences in the global mean WV values are, however, below 0.1 cm. Regional differences in the PR between MATCH and GPCP can be above 0.5 cm per day and MATCH computes on average a higher PR than what has been observed. The lower water vapor content of MATCH is related to shorter model WV residence times by up to 1 day as compared to the observations. We find that MATCH has problems in modelling the WV content in regions of strong upward convection like, for example, along the Inter Tropical Convergence Zone, where it appears to be generally too dry as compared to the observations. We discuss possible causes for this bias and demonstrate the value of the GOME WV record for model evaluation.

A global simulation of tropospheric ozone and related tracers: Description and evaluation of MOZART, version 2

We have developed a global three‐dimensional chemical transport model called Model of Ozone and Related Chemical Tracers (MOZART), version 2. This model, which will be made available to the community, is built on the framework of the National Center for Atmospheric Research (NCAR) Model of Atmospheric Transport and Chemistry (MATCH) and can easily be driven with various meteorological inputs and model resolutions. In this work, we describe the standard configuration of the model, in which the model is driven by meteorological inputs every 3 hours from the middle atmosphere version of the NCAR Community Climate Model (MACCM3) and uses a 20‐min time step and a horizontal resolution of 2.8° latitude × 2.8° longitude with 34 vertical levels extending up to approximately 40 km. The model includes a detailed chemistry scheme for tropospheric ozone, nitrogen oxides, and hydrocarbon chemistry, with 63 chemical species. Tracer advection is performed using a flux‐form semi‐Lagrangian scheme with a pressure fixer. Subgrid‐scale convective and boundary layer parameterizations are included in the model. Surface emissions include sources from fossil fuel combustion, biofuel and biomass burning, biogenic and soil emissions, and oceanic emissions. Parameterizations of dry and wet deposition are included. Stratospheric concentrations of several long‐lived species (including ozone) are constrained by relaxation toward climatological values. The distribution of tropospheric ozone is well simulated in the model, including seasonality and horizontal and vertical gradients. However, the model tends to overestimate ozone near the tropopause at high northern latitudes. Concentrations of nitrogen oxides (NOx) and nitric acid (HNO3) agree well with observed values, but peroxyacetylnitrate (PAN) is overestimated by the model in the upper troposphere at several locations. Carbon monoxide (CO) is simulated well at most locations, but the seasonal cycle is underestimated at some sites in the Northern Hemisphere. We find that in situ photochemical production and loss dominate the tropospheric ozone budget, over input from the stratosphere and dry deposition. Approximately 75% of the tropospheric production and loss of ozone occurs within the tropics, with large net production in the tropical upper troposphere. Tropospheric production and loss of ozone are three to four times greater in the northern extratropics than the southern extratropics. The global sources of CO consist of photochemical production (55%) and direct emissions (45%). The tropics dominate the chemistry of CO, accounting for about 75% of the tropospheric production and loss. The global budgets of tropospheric ozone and CO are generally consistent with the range found in recent studies. The lifetime of methane (9.5 years) and methylchloroform (5.7 years) versus oxidation by tropospheric hydroxyl radical (OH), two useful measures of the global abundance of OH, agree well with recent estimates. Concentrations of nonmethane hydrocarbons and oxygenated intermediates (carbonyls and peroxides) generally agree well with observations.

Springtime trans‐Pacific atmospheric transport from east Asia: A transit‐time probability density function approach

The atmosphere is known to episodically transport dust, aerosols, and gaseous pollutants from industrialized east Asia, the Gobi desert, and Siberian wild fires to western North America. We give a novel characterization of the climatological springtime transport from these regions and of the probability of transport “events,” that is, long‐range transport of high concentrations with minimal dispersion. Our primary transport diagnostic is the transit‐time probability density function (pdf), , which is a tracer‐independent measure of the flow that allows us to isolate the role of transport from other factors such as source variability and chemistry. The pdf approach, unlike typical back‐trajectory analyses, captures transport due to all possible paths and accounts for both resolved advection and subgrid processes. We use a numerical model of the global atmosphere (Model of Atmospheric Transport and Chemistry (MATCH)) driven by National Centers for Environmental Prediction reananlysis data to establish the springtime statistics of daily averages of . A suitably defined average of quantifies the climatological mass fraction of air from the source region per interval of transit times, or ages. Over the North American west coast, this fraction peaks at transit times of ∼8 days in the upper troposphere (∼6 days later at the surface) for the dust and pollution regions and at 12–14 days for the Siberian region. An analysis of the variability of at fixed transit time allows us to identify transport events and to estimate their probability of occurrence. This is illustrated for transport events to the “Pacific Northwest” (PNW) region of North America, defined as (43.8°–53.3°N) × (115.3°–124.7°W). Correlations between averaged over the PNW and the winds at any point in the atmosphere identify large‐scale anomaly structures of the flow that correspond to favorable transport to the PNW.

Stratospheric transport in a three‐dimensional isentropic coordinate model

A new version of the Model of Atmospheric Transport and Chemistry (MATCH) is developed, which uses a coordinate that is terrain‐following near the surface and becomes isentropic in the upper troposphere. Starting from a simulation of age of air and water vapor in a standard version of National Center for Atmospheric Research's Middle Atmospheric Community Climate Model (MACCM), we use the meteorological fields to drive two versions of the off‐line MATCH model: the standard hybrid‐pressure coordinate (MATCH) and the new hybrid‐isentropic coordinate (IMATCH). An analysis of the age of air estimates from MACCM, MATCH, and IMATCH and observations shows that the hybrid‐isentropic model, IMATCH, is better able to capture the observed ages than either the original online MACCM or the hybrid‐pressure MATCH model. Comparisons of water vapor distributions in the tropical lower stratosphere in the three different model versions and observations suggest that IMATCH produces a better propagation of the seasonal cycle than the other models. Analysis of the model results and vertical velocities suggests that the improvements in the age of air and the water vapor simulations are largely due to the reduced numerical vertical diffusion in IMATCH in the lower tropical stratosphere region. In this region, diabatic vertical motions are much smaller than reversible adiabatic motions, making simulations less susceptible to numerical errors if the transport is calculated on isentropic surfaces. Because the lower tropical stratosphere acts as the source region for transport into the stratosphere, correct simulation of the transport in the lower tropical stratosphere is crucial for improving model simulations of the stratosphere.

Simulating aerosols using a chemical transport model with assimilation of satellite aerosol retrievals: Methodology for INDOEX

A system for simulating aerosols has been developed using a chemical transport model together with an assimilation of satellite aerosol retrievals. The methodology and model components are described in this paper, and the modeled distribution of aerosols for the Indian Ocean Experiment (INDOEX) is presented by Rasch et al. [this issue]. The system generated aerosol forecasts to guide deployment of ships and aircraft during INDOEX. The system consists of the Model of Atmospheric Transport and Chemistry (MATCH) combined with an assimilation package developed for applications in atmospheric chemistry. MATCH predicts the evolution of sulfate, carbonaceous, and mineral dust aerosols, and it diagnoses the distribution of sea salt aerosols. The model includes a detailed treatment of the sources, chemical transformation, transport, and deposition of the aerosol species. The aerosol forecasts involve a two‐stage process. During the assimilation phase the total column aerosol optical depth (AOD) is estimated from the model aerosol fields. The model state is then adjusted to improve the agreement between the simulated AOD and satellite retrievals of AOD. During the subsequent integration phase the aerosol fields are evolved using meteorological fields from an external model. Comparison of the modeled AOD against estimates of the AOD from INDOEX Sun photometer data show that the differences in daily means are −0.03±0.06. Although the initial application is limited to the Indian Ocean, the methodology could be extended to derive global aerosol analyses combining in situ and remotely sensed aerosol observations.

Atmospheric response time of cosmogenic 14CO to changes in solar activity

Carbon‐14 monoxide is an unique tracer for certain important aspects of atmospheric chemistry and transport. Particularly, it may provide an independent means of testing model‐based OH distributions and seasonality. The cosmogenic source strength of 14CO is inversely correlated with the solar activity. This time dependence demands a rescaling of 14CO measurements or model results to comparable conditions regarding the solar cycle, in order to achieve information on global distributions and trends of OH. For this rescaling, a two‐parameter filter is derived using the three‐dimensional global Model of Atmospheric Transport and Chemistry (MATCH). The filter function defines the atmospheric response time of the 14CO concentration to changes in the total cosmogenic 14CO source strength. The latitude dependence of the response time in addition to the atmospheric lifetime of the tracer provides useful information about the model's transport properties, especially about the role of stratosphere‐troposphere exchange. It may serve as a direct indicator for possible solutions of the unresolved hemispheric asymmetry of the atmospheric 14CO concentration. The general concept can be applied as a diagnostic test for coupled changes in atmospheric transport and chemistry in models. Time series of 14CO measurements can provide this information about the real atmosphere.

Simulations of cosmogenic 14CO using the three‐dimensional atmospheric model MATCH: Effects of 14C production distribution and the solar cycle

Most atmospheric 14CO is produced by cosmic rays in the lowermost stratosphere and upper troposphere. The main removal process for 14CO is oxidation by OH radicals. Assuming that the spatial distribution of OH is well known, 14CO can be useful as a test‐tracer for the transport properties of a three‐dimensional chemical model. Conversely, if the transport schemes of the model are sufficiently realistic, in particular with respect to stratosphere‐troposphere exchange, the OH distribution can be evaluated. In either case, it has to be assumed that the source of 14CO is known in sufficient detail. Two presently available distributions of cosmogenic 14C production are implemented into the Model of Atmospheric Transport and Chemistry (MATCH). The tropospheric 14CO concentrations that are obtained are relatively insensitive to the source differences. The calculations for one source distribution are performed for solar minimum and solar maximum conditions. The spatial distribution of 14CO is almost unaffected by the solar activity, and the absolute concentration levels can be scaled to the actual solar cycle conditions.

Transport of 222radon to the remote troposphere using the Model of Atmospheric Transport and Chemistry and assimilated winds from ECMWF and the National Center for Environmental Prediction/NCAR

The Model of Atmospheric Transport and Chemistry (MATCH) is used to simulate the transport of 222Rn using both European Centre for Medium‐Range Weather Forecasts (ECMWF) winds and National Center for Environmental Prediction/National Center for Atmospheric Research (hereafter referred to as NCEP) reanalysis winds. These winds have the advantage of being based on observed winds but have the disadvantage that the subgrid‐scale transport processes are not routinely archived. MATCH derives subgrid‐scale mixing rates for the boundary layer using a nonlocal scheme and for moist convective mixing using one of two parameterizations (Tiedtke [1989] or Pan and Wu [1997]). This paper describes the ability of the model to recreate mixing rates of 222Rn using the forecast center winds. Radon 222 is a species with a continental crust source and a simple sink involving radioactive decay with an e‐folding timescale of 5.5 days. This atmospheric constituent is therefore a good tracer for testing the vertical transport in the chemical transport model, as well as the horizontal transport from continental regions to remote oceanic regions. The various simulations of 222Rn are compared with observations as well as with each other, allowing an estimate of the uncertainty in transport due to uncertainties in the winds and subgrid‐scale processes. The calculated vertical profiles over the western United States are somewhat similar to observed, and the upper tropospheric concentrations compare reasonably well in their spatial distribution with data collected during Tropospheric Ozone II (TROPOZ II), although the model values tend to be higher than observed values, especially in the upper troposphere. The model successfully simulates specific observed pollution events at Cape Grim. It has more difficulty at sites farther from continental source regions, although the model captures the seasonal structure of the pollution events at these sites (Macquarie Island, Amsterdam Island, Kerguelen Island, and Crozet Island). Inclusion of a moist convective mixing scheme in MATCH increases 222Rn concentrations in the upper troposphere by 50% compared to not having moist convective mixing, while surface concentrations do not appear to be very sensitive to moist convection. In addition, differences between the upper tropospheric concentrations of radon predicted using the ECMWF and NCEP winds can be 30% for large areas of the globe, due to either differences in the forecast center winds themselves or the moist convective mixing schemes used in conjunction with them. This has implications for model simulations of radiatively and chemically important trace species in the atmosphere.

Representations of transport, convection, and the hydrologic cycle in chemical transport models: Implications for the modeling of short‐lived and soluble species

We compare chemical transport simulations performed in a model using archived meteorological data (an off‐line transport model) to those performed in a model in which the meteorological data are predicted every time step (an on‐line model). We identify the errors associated with using data sampled at timescales much longer than those operating in the atmosphere or in the on‐line model, and strategies for ameleorating those errors. The evaluation is performed in the context of a global off‐line chemical transport model called the Model of Atmospheric Transport and Chemistry (MATCH) for three test problems: (1) the passive advection of blobs initially concentrated in the lower and upper troposphere; (2) the surface emission of radon and its decay to lead; and (3) the removal of lead from the atmosphere by wet and dry deposition processes. These problems exercise the important processes of transport by resolved scale winds, rapid transport by smaller scale convection processes, and wet removal (which depends on the representation of the hydrologic cycle). We show that the errors in off‐line model simulations (compared to the on‐line simulations) can be made small when the sampling interval is order 6 hours or less. We also show that one can accurately reproduce the subgrid‐scale processes within the off‐line model, rather than needing to archive the results of those processes as input to the off‐line model. This suggests that for the spatial and temporal scales treated in global models it is possible to treat many problems nearly as accurately in an off‐line mode as one can with an on‐line treatment.