Global Ozone Chemistry Aerosol Radiation And Transport Research Articles

Antecedent Soil Moisture Conditions Influenced Vertical Dust Flux: A Case Study in Iran Using WRF-Chem Model

Soil moisture is one of the most important parameters affecting dust emission flux. This study was conducted to investigate the effects of soil moisture on vertical dust flux in the central plateau region of Iran. In this study, the WRF-Chem (Weather Research and Forecast with Chemistry) model, with the GOCART (Global Ozone Chemistry Aerosol Radiation and Transport) scheme, was used to estimate the dust emission flux during a major storm from 19 to 21 July 2015, and to discriminate between dust sources. The results showed that the Kyrgyz deserts in Turkmenistan, the Arabian deserts in Saudi Arabia, the deserts of Iraq, and the Helmand region in Afghanistan are sources of foreign dust. Additionally, the central desert plain was identified as an internal dust source, where the dust level reached 7000 µg m−2 s−1. The results of WRF-Chem simulation were verified with reanalysis data from MERRA2 and AERONET data from Natanz station, which showed good agreement with the simulation. Based on the GLDAS reanalysis, soil moisture content varied between 2.6% and 34%. Linear and nonlinear regression of vertical dust flux values and soil moisture showed nonlinear behavior following the exponential function, with a correlation coefficient of 0.8 and a strong negative association between soil moisture and vertical dust flux.

Evaluation of drag coefficients under medicane conditions: Coupling waves, sea spray and surface friction

This work represents an initial step toward the coupled ocean-atmosphere-aerosols modeling of severe storms in the Mediterranean basin. This is realized extending the classical Charnock formulation of surface roughness over sea and consequently the determination of air-sea momentum fluxes in numerical models. To improve the forecast of severe Mediterranean storm systems, like the so-called Medicanes, it is essential to consider the wave-state and sea spray effects on the sea surface roughness explicitly. This is implemented in the present work by coupling the third-generation WAVEWATCH III model (WWIII) and the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) model configured with the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) aerosol package. The communication between the two models is realized through an offline coupling scheme, which utilizes a regridding algorithm between the numerical domains of the two models. In this context, the wave peak frequency is passed from WWIII to WRF-Chem model permitting the calculation of the wave-age and the sea spray source functions as well as the modification of the overseas drag coefficients. Two different surface roughness parameterizations are tested and compared with ERA5 reanalysis data from the ECMWF - Copernicus Climate Change Service. A single case study is performed considering the cyclone Rolf that occurred between 5 and 9 November 2011 in the western Mediterranean basin. It is considered one of the longest-lasting and most intense tropical-like cyclones in the Mediterranean with more than 48-h occurrence of tropical-like features and a wind speed peak reaching 30 m s−1 in its mature phase. The role of sea spray is evaluated considering two basic strategies, namely the limited-saturation layer approach and the “sea spray force” on the momentum balance. It is demonstrated that (i) the formulation based on the limited-saturation layer approach is better suited for describing the drag in the cyclonic region, where velocities are highest and consequently there is a very high concentration of sea-spray spume droplets; (ii) the momentum drag level-off at the highest wind speed. Our results give a contribution on the understanding of the air-sea flux parameterizations for a range of wind speed above 25–35 m s−1, indicating that sea-spray generation should be accounted for in coupled numerical models for ocean and atmosphere.

Spatial-Temporal Variation Characteristics of Vertical Dust Flux Simulated by WRF-Chem Model with GOCART and AFWA Dust Emission Schemes (Case Study: Central Plateau of Iran)

Dust and sand storms are among the major threats to central Iran. These phenomena pose irreparable risks to natural ecosystems and human societies, including effects on health. In this study, the spatial and temporal pattern of vertical dust flux (VDF) was used to identify dust sources as well as areas with high potential for dust generation. To simulate VDF, two intense dust storms, from 21 February 2015 and 14 February 2018, were selected using synoptic data and Moderate Resolution Imaging Spectroradiometer (MODIS) images. These dust storms were identified as responsible for a reduction of horizontal visibility to less than 1000 m, using remote sensing tools and Ackerman Dust Index. MODIS images show that these two storms covered most of Central Plateau of Iran. The Weather Research and Forecasting model with chemistry (WRF-Chem) was used to simulate the storms, with either the Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) or Air Force Weather Agency (AFWA) scheme to calculate VDF. Modeled vertical dust fluxes in both events indicate that the Arabian deserts in Saudi Arabia and in southwestern Iran can be identified as main sources of the dust in the central Iranian plateau. The other source of dust is the Hirmand Basin, located in the country of Afghanistan and in the southeast of Iran. The results of VDF simulations indicate that central southeast Iran could be the main dust source of internal origin. Additionally, over seasonal wetlands in Iran, the amount of VDF was simulated to be sometimes over 4000 μg/(m2s), an indication that these areas are sensitive to wind erosion in dry conditions and can be a source of dust. The WRF-Chem results were compared with the horizontal visibility measured in synoptic stations in the area. The results showed that the coefficients of determination of GOCART results with the measured horizontal visibility on 21 February 2015 and 14 February 2018 were 0.72 and 0.76, respectively, while the coefficient values from the simulations with AFWA scheme on 21 February 2015 and 14 February 2018 with the measured horizontal visibility were lower, 0.44 and 0.50, respectively. Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA2) re-analysis data also showed timing of peak dust levels consistent with the GOCART scheme.

Parameterization schemes on dust deposition in northwest China: Model validation and implications for the global dust cycle

Accurate estimation of dust deposition is of significance for modelling global radiation and the biochemical carbon cycle in the earth system. However, the paucity of dust deposition data precludes our ability to adequately verify estimations of dust deposition. Based on the environmental monitoring records in Xinjiang Province, northwest China, we conducted a numerical simulation of dust deposition using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) and compared observed and modelled deposition during the spring dust season (March–May). The performance of WRF-Chem on modelling dust deposition was tested and evaluated with adoption of Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) and Shao et al. (2011a) dust emission schemes. Our results indicate that the dry deposition schemes have the capability to predict size-resolved dust deposition. However, modelled and measured dust deposition differed by more than one order of magnitude. The modelled dust dry deposition does not satisfactorily agree well with field measurements. This study suggests significant distinctions exist among these two dust emission schemes when simulating mineral dust dry deposition in northwest China. Uncertainties in estimating the dry dust deposition are in a range of 77–96%. These uncertainties imply that parameterization in the current dust deposition schemes need to be further improved. We found that the estimation of dust deposition is highly underestimated by the Global Climate Model and Regional Climate Model (GCM/RCM). Thus global dust cycles and dust deposition may exceed our current estimates.

Analysis of dust wet deposition in the mid-latitudes of the Northern Hemisphere

Wet deposition is the efficient removal process for fine dust aerosol. Dust wet deposition in the mid-latitudes of the Northern Hemisphere is investigated in this study by analyzing the dust simulations with the Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model, measurements from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), and the meteorological and hydrological fields from Modern Era-Retrospective Analysis for Research and Applications (MERRA) reanalysis. The dust aerosol optical depth (AOD) and dust extinction coefficient from CALIPSO observation and GOCART simulation, in conjunction with the wind field from MERRA, show that the dust plume extending eastward from Asia to Pacific in the mid-latitudes becomes strongest in spring, while Taklamakan, Sahara, and Gobi dust are the main components and distribute vertically in the upper, middle, and lower parts of the dust layer across North Pacific, respectively. The wet deposition of dust in the mid-latitudes is mainly in the large-scale wet removal process, which becomes strongest in spring. The occurrence of wet deposition is accompanied by dust loading or transport. The comparison of wet deposition from GOCART simulation with the cloud water mixing ratio and precipitation production rate from MERRA indicated that wet deposition is mainly related to the water amount in ice cloud, and has a positive relationship with the precipitation in ice cloud layer. On the other hand, over arid and semiarid regions in central and eastern Asia with high dust loading, the absence of cloud water caused by the semidirect effect of dust (Huang et al., Geophys Res Lett 33(19), 2006b), can lower the amount of wet deposition. The comparison of wet deposition from GOCART simulation with the cloud water mixing ratio and vertical pressure velocity from MERRA demonstrates that a large-scale dynamic process, the ascending motion in the subpolar low-pressure system over the North Pacific, can increase the water amount in cloud and cause much more wet deposition of dust, which explains the occurrence of the largest wet deposition over the North Pacific.

Impact of long-range desert dust transport on coastal East Asia: analysis of urban dust concentration and wet deposition with model simulation

Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model simulations for dust in 2007 are applied in this study to investigate the impact of long-range desert dust transport on the dust concentration and wet deposition in coastal cities of East Asia: Shanghai and Hong Kong. The temporal variation of dust concentrations in both Shanghai and Hong Kong exhibits a maximum in spring (May), with a majority of Gobi dust, which accounts for 88 % in Shanghai and 58 % in Hong Kong in the lower troposphere. Correspondingly, the peaks of convective and large-scale wet deposition in May in Shanghai are controlled by Gobi dust, with a contribution of 74–83 %. The wet deposition of dust in summer is affected mainly by strong convective precipitation and dominated by Taklamakan dust, with a contribution of 31–47 % in Shanghai and 19–37 % in Hong Kong, followed by Arabian and Karakum–Kavir dusts. The wet deposition in spring (April) is caused mainly by large-scale precipitation, with a majority of Gobi dust, which accounts for 57 % in Shanghai and 71 % in Hong Kong, followed by Sahara and Taklamakan dusts. The temporal variation of wet deposition of dust in Shanghai is affected mainly by the dust concentration, while the variation in Hong Kong is controlled by both dust concentration and convective precipitation.

Effect of different emission inventories on modeled ozone and carbon monoxide in Southeast Asia

Abstract. In order to improve our understanding of air quality in Southeast Asia, the anthropogenic emissions inventory must be well represented. In this work, we apply different anthropogenic emission inventories in the Weather Research and Forecasting Model with Chemistry (WRF-Chem) version 3.3 using Model for Ozone and Related Chemical Tracers (MOZART) gas-phase chemistry and Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) aerosols to examine the differences in predicted carbon monoxide (CO) and ozone (O3) surface mixing ratios for Southeast Asia in March and December 2008. The anthropogenic emission inventories include the Reanalysis of the TROpospheric chemical composition (RETRO), the Intercontinental Chemical Transport Experiment-Phase B (INTEX-B), the MACCity emissions (adapted from the Monitoring Atmospheric Composition and Climate and megacity Zoom for the Environment projects), the Southeast Asia Composition, Cloud, Climate Coupling Regional Study (SEAC4RS) emissions, and a combination of MACCity and SEAC4RS emissions. Biomass-burning emissions are from the Fire Inventory from the National Center for Atmospheric Research (NCAR) (FINNv1) model. WRF-Chem reasonably predicts the 2 m temperature, 10 m wind, and precipitation. In general, surface CO is underpredicted by WRF-Chem while surface O3 is overpredicted. The NO2 tropospheric column predicted by WRF-Chem has the same magnitude as observations, but tends to underpredict the NO2 column over the equatorial ocean and near Indonesia. Simulations using different anthropogenic emissions produce only a slight variability of O3 and CO mixing ratios, while biomass-burning emissions add more variability. The different anthropogenic emissions differ by up to 30% in CO emissions, but O3 and CO mixing ratios averaged over the land areas of the model domain differ by ~4.5% and ~8%, respectively, among the simulations. Biomass-burning emissions create a substantial increase for both O3 and CO by ~29% and ~16%, respectively, when comparing the March biomass-burning period to the December period with low biomass-burning emissions. The simulations show that none of the anthropogenic emission inventories are better than the others for predicting O3 surface mixing ratios. However, the simulations with different anthropogenic emission inventories do differ in their predictions of CO surface mixing ratios producing variations of ~30% for March and 10–20% for December at Thai surface monitoring sites.

The contribution of different aerosol sources to the Aerosol Optical Depth in Hong Kong

The contribution of major aerosol components emitted from local and remote regions to Hong Kong's Aerosol Optical Depth (AOD) in 2007 is quantitatively determined using the chemical transport model GOCART (Global Ozone Chemistry Aerosol Radiation and Transport). Of the major aerosol components, sulphur has the largest influence (68%) on Hong Kong, followed by organic carbon (OC, 13%) and dust (11%), and the influences of black carbon (BC, 5%) and sea salt (3%) are the lowest. The highest AOD is seen in September 2007 and is composed mainly of sulphur aerosols (85%). The high AOD values in March and April 2007 are caused by sulphur and OC. OC has a relative contribution of 39% in March and 30% in April.The anthropogenic sulphur, BC, and OC emitted from every continent, as well as from China and South China, are considered respectively. In summer, South China's contribution of sulphur aerosols from anthropogenic SO2 emissions to the total sulphur AOD in Hong Kong is more than 20%. In other seasons, sulphur aerosols from anthropogenic SO2 emissions in Rest China (all of China except South China) accounts for more than 25%. Anthropogenic BC from South China accounts for more than 20% of total BC AOD in Hong Kong in summer. The contribution of anthropogenic BC from Rest China exceeds 40% in autumn and winter. Anthropogenic BC from Rest Asia (all of Asia except China) accounts for more than 30% in summer and autumn. The contribution of anthropogenic OC from Rest China is more than 35% in autumn and winter. The contribution of anthropogenic OC from Rest Asia exceeds 20% in summer.Gobi dust accounts for more than 40% of the total dust AOD in winter, and its impact appears mainly in the Atmospheric Boundary Layer (ABL), where it is responsible for 50% of the dust concentration. The contribution of Sahara dust to the dust AOD in spring exceeds 35%, and its contribution to the dust concentration in the free atmosphere (40%) is larger than that in the ABL (10%). More than 35% of the dust AOD in summer and autumn comes from Taklamakan dust, which exists mainly in the free atmosphere, where it accounts for 40% of the dust concentration.

Synthesis of information on aerosol optical properties

In a previous study (Liu et al., 2005) obtained are global scale estimates of aerosol optical depth at 0.55 μm based on spatial and temporal variation patterns from models and satellite observations, regulated by the Aerosol Robotic Network (AERONET) measurements. In this study an approach is developed to obtain information on global distribution of the single scattering albedo (ω0), the asymmetry parameter (g), and the normalized extinction coefficient over shortwave (SW) spectrum. Since space observations of ω0 are in early stages of development and none are available for g, first an approach was developed to infer them from relevant information from the Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model, Moderate Resolution Imaging Spectroradiometer (MODIS) and AERONET retrievals. The single scattering albedo is generated by extending GOCART ω0 at 0.55 μm to the entire SW spectrum using spectral dependence derived from AERONET retrievals. The asymmetry parameter over the solar spectrum is derived from the MODIS Ångström wavelength exponent, utilizing a relationship based on AERONET almucantar observations. The normalized extinction coefficient is estimated from the MODIS Ångström wavelength exponent. The methodology was implemented as a “proof of concept” with one year of data. The approach described here is a step in preparedness for utilizing information from new observing systems (e.g., MISR, A‐Train constellation) when available. The impact of the newly derived information on the quality of satellite based estimates of surface radiative fluxes was evaluated and is presented by Liu and Pinker (2008).

A global view of aerosols from merged transport models, satellite, and ground observations

Growing recognition of the importance of natural and anthropogenic aerosols in climate research led to numerous efforts to obtain information on aerosols based on model simulations, satellite remote sensing, and ground observations. This study describes an approach to combine information from independent sources that complement each other in their capabilities to achieve a global characterization of monthly mean clear‐sky daytime aerosol optical depth. The following sources of information have been used: simulations from the Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model; retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on the Terra satellite; and measurements from the Aerosol Robotic Network (AERONET). Leading empirical orthogonal functions (EOFs) are used to represent the significant variation signals from model and satellite results; the EOFs are fitted to the ground observations to propagate the AERONET information at a global scale. The methodology is implemented with a 2‐year time record when collocated data from all three sources are available.

Direct radiative effect of aerosols as determined from a combination of MODIS retrievals and GOCART simulations

The aerosol direct solar effect under clear sky is assessed by (1) combining multiple aerosol characterizations and (2) using the satellite‐retrieved land surface albedo. The aerosol characterization is made through an integration of the MODerate resolution Imaging Spectroradiometer (MODIS) retrievals and the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model simulations. The spectral and bidirectional albedo of land surface is derived from MODIS. On a global average, the solar forcing at the top of atmosphere (TOA) DFTOA is −4.5 Wm−2, of which about 1/3 is contributed by a sum of natural and anthropogenic sulfate and carbonaceous aerosols. Though the optical depth is about 50% higher over land than over ocean, no significant land‐ocean contrast in this TOA forcing is observed. It is reduced by larger aerosol absorption and higher surface albedo over land. As a result of absorption by soot and dust, a much larger surface cooling and substantial atmospheric absorption coexist over land and adjacent oceans. Globally, the surface cooling DFSFC is about −9.9 Wm−2, and the atmospheric absorption DFAIR is about 5.4 Wm−2, suggesting that more than half of the surface cooling results from the atmospheric absorption. Sensitivity tests show that an inclusion of MODIS‐derived anisotropy of land surface reflection reduces the diurnal variation of TOA solar forcing, because of aerosol‐induced changes in the fraction of direct beam and hence in the effective reflection from the surface. Constraining the GOCART dust absorption with recent measurements reduces DFAIR and DFSFC by 1.3 Wm−2 and 0.9 Wm−2, respectively, and increases the TOA cooling by 0.4 Wm−2.

A global aerosol model forecast for the ACE‐Asia field experiment

We present the results of aerosol forecast during the ACE‐Asia field experiment in spring 2001, using the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model and the meteorological forecast fields from the Goddard Earth Observing System Data Assimilation System (GEOS DAS). The model provides direct information on aerosol optical thickness and concentrations for effective flight planning, while feedbacks from measurements constantly evaluate the model for successful model improvements. We verify the model forecast skill by comparing model‐predicted aerosol quantities and meteorological variables with those measured by the C‐130 aircraft. The GEOS DAS meteorological forecast system shows excellent skills in predicting winds, relative humidity, and temperature, with skill scores usually in the range of 0.7–0.99. The model is also skillful in forecasting pollution aerosols, with most scores above 0.5. The model correctly predicted the dust outbreak events and their trans‐Pacific transport, but it constantly missed the high dust concentrations observed in the boundary layer. We attribute this “missing” dust source to desertification regions in the Inner Mongolia Province in China, which have developed in recent years but were not included in the model during forecasting. After incorporating the desertification sources, the model is able to reproduce the observed boundary layer high dust concentrations over the Yellow Sea. We demonstrate that our global model can not only account for the large‐scale intercontinental transport but also produce the small‐scale spatial and temporal variations that are adequate for aircraft measurements planning.

Long-term simulation of global dust distribution with the GOCART model: correlation with North Atlantic Oscillation

Global distribution of aeolian dust is simulated from 1981 to 1996 with the Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model. The results are compared with in situ measurements and satellite data. An index is calculated from the model results and the satellite viewing angles to allow quantitative comparison with the Total ozone mapping spectrometer (TOMS) absorbing aerosol index. The annual budget over the different continents and oceans are analyzed. The simulated annual emission varies from a minimum of 1950 Tg in 1996 to a maximum of 2400 Tg in 1988. Of these emissions, 65% is from North Africa and 25% from Asia. It is found that North America received twice as much dust from other continents than it emits per year. There is no significant trend over the 16-year simulation. The inter-annual variability of dust distribution is analyzed over the North Atlantic and Africa. It is found that in winter a large fraction of the North Atlantic and Africa dust loading is correlated with the North Atlantic Oscillation (NAO) index. It is shown that a controlling factor of such correlation can be attributed to dust emission from the Sahel. The Bodele depression is the major dust source in winter and its inter-annual variability is highly correlated with the NAO. However, the long record of dust concentration measured at Barbados indicates that there is no correlation with the NAO index and surface concentration in winter. Longer simulation should provide the information needed to understand if the effects of the NAO on dust distribution is rather limited or Barbados is at the edge of the affected region.

Annual cycle of global distributions of aerosol optical depth from integration of MODIS retrievals and GOCART model simulations

The Moderate‐Resolution Imaging Spectroradiometer (MODIS) instrument onboard the Earth Observing System (EOS) satellites provides an unprecedented opportunity to study aerosols from space with high accuracy and on a nearly global scale. However, difficulty with highly reflective arid and snow‐covered lands introduces significant gaps in global or regional coverage that must be filled by some other means. This study provides a complete global coverage of an annual cycle of aerosol optical depth by combining the MODIS retrievals and Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) simulations weighted with the uncertainties in each product. The assimilated aerosol optical depths over land are better correlated with the ground‐based Aerosol Robotic Network (AERONET) measurements than are either the MODIS retrievals or the GOCART simulations alone. The gaps in the MODIS retrievals are filled with values that are generally consistent with the AERONET aerosol climatology. The assimilated aerosol optical depths are in good agreement with the Advanced Very High Resolution Radiometer (AVHRR) aerosol climatology over the Atlantic and North Indian Oceans. In spring, large discrepancies between the MODIS retrievals in 2001 and the AVHRR climatology over the North Pacific are likely a result of extremely active transcontinental transport of Asian dust/pollutants to North America in the year 2001. Large model–satellite differences in the South Pacific and South Indian Oceans may be attributable to missing or underestimated sources in the model and/or cloud, whitecap, and glint contamination in satellite retrievals.

Effects of nonsphericity on mineral dust modeling

The dependency of nonsphericity on gravitational settling of mineral dust particles is parameterized for prolate ellipsoids and Reynolds number lower than 2. The settling speed is numerically solved from the momentum equation as a function of particle diameter and aspect ratio. The reduction of settling speed due to nonsphericity is included in the Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model to simulate dust size distribution for April 2001. Two numerical schemes for solving sedimentation are compared. For particles of diameter greater than 5 μm, the simulated size distribution is sensitive to the numerical sedimentation scheme. Changing the particle shape from spherical to nonspherical with λ = 2 makes little difference to the simulated surface concentration and size distribution except at the periphery of the dust sources. However, when very elongated particles (λ = 5) are simulated, the differences between nonspherical and spherical particles are significant. With limited in situ measurements reporting most frequent λ around 1.5, the overall effects on global modeling is rather negligible and the essential benefit is to relax the CFL condition of Eulerian settling schemes.

Tropospheric Aerosol Optical Thickness from the GOCART Model and Comparisons with Satellite and Sun Photometer Measurements

The Georgia Institute of Technology‐Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model is used to simulate the aerosol optical thickness t for major types of tropospheric aerosols including sulfate, dust, organic carbon (OC), black carbon (BC), and sea salt. The GOCART model uses a dust emission algorithm that quantifies the dust source as a function of the degree of topographic depression, and a biomass burning emission source that includes seasonal and interannual variability based on satellite observations. Results presented here show that on global average, dust aerosol has the highest t at 500 nm (0.051), followed by sulfate (0.040), sea salt (0.027), OC (0.017), and BC (0.007). There are large geographical and seasonal variations of t, controlled mainly by emission, transport, and hygroscopic properties of aerosols. The model calculated total ts at 500 nm have been compared with the satellite retrieval products from the Total Ozone Mapping Spectrometer (TOMS) over both land and ocean and from the Advanced Very High Resolution Radiometer (AVHRR) over the ocean. The model reproduces most of the prominent features in the satellite data, with an overall agreement within a factor of 2 over the aerosol source areas and outflow regions. While there are clear differences among the satellite products, a major discrepancy between the model and the satellite data is that the model shows a stronger variation of t from source to remote regions. Quantitative comparison of model and satellite data is still difficult, due to the large uncertainties involved in deriving the t values by both the model and satellite retrieval, and by the inconsistency in physical and optical parameters used between the model and the satellite retrieval. The comparison of monthly averaged model results with the sun photometer network [Aerosol Robotics Network (AERONET)] measurements shows that the model reproduces the seasonal variations at most of the sites, especially the places where biomass burning or dust aerosol dominates.

Sources and distributions of dust aerosols simulated with the GOCART model

The global distribution of dust aerosol is simulated with the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model. In this model all topographic lows with bare ground surface are assumed to have accumulated sediments which are potential dust sources. The uplifting of dust particles is expressed as a function of surface wind speed and wetness. The GOCART model is driven by the assimilated meteorological fields from the Goddard Earth Observing System Data Assimilation System (GEOS DAS) which facilitates direct comparison with observations. The model includes seven size classes of mineral dust ranging from 0.1–6 μm radius. The total annual emission is estimated to be between 1604 and 1960 Tg yr−1 in a 5‐year simulation. The model has been evaluated by comparing simulation results with ground‐based measurements and satellite data. The evaluation has been performed by comparing surface concentrations, vertical distributions, deposition rates, optical thickness, and size distributions. The comparisons show that the model results generally agree with the observations without the necessity of invoking any contribution from anthropogenic disturbances to soils. However, the model overpredicts the transport of dust from the Asian sources to the North Pacific. This discrepancy is attributed to an overestimate of small particle emission from the Asian sources.

Atmospheric sulfur cycle simulated in the global model GOCART: Model description and global properties

The Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model is used to simulate the atmospheric sulfur cycle. The model uses the assimilated meteorological data from the Goddard Earth Observing System Data Assimilation System (GEOS DAS). Global sulfur budgets from a 6‐year simulation for SO2, sulfate, dimethylsulfide (DMS), and methanesulfonic acid (MSA) are presented in this paper. In a normal year without major volcanic perturbations, about 20% of the sulfate precursor emission is from natural sources (biogenic and volcanic), and 80% is anthropogenic; the same sources contribute 33% and 67%, respectively, to the total sulfate burden. A sulfate production efficiency of 0.41–0.42 is estimated in the model, an efficiency which is defined as a ratio of the amount of sulfate produced to the total amount of SO2 emitted and produced in the atmosphere. This value indicates that less than half of the SO2 entering the atmosphere contributes to the sulfate production, the rest being removed by dry and wet depositions. In a simulation for 1990 we estimate a total sulfate production of 39 Tg S yr−1, with 36% and 64% from in‐air and in‐cloud oxidation, respectively, of SO2. We also demonstrate that major volcanic eruptions, such as the Mount Pinatubo eruption in 1991, can significantly change the sulfate formation pathways, distributions, abundance, and lifetime. Comparison with other models shows that the parameterizations for wet removal or wet production of sulfate are the most critical factors in determining the burdens of SO2 and sulfate. Therefore a priority for future research should be to reduce the large uncertainties associated with the wet physical and chemical processes.

Atmospheric sulfur cycle simulated in the global model GOCART: Comparison with field observations and regional budgets

We present a detailed evaluation of the atmospheric sulfur cycle simulated in the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model. The model simulations of SO2, sulfate, dimethylsulfide (DMS), and methanesulfonic acid (MSA) are compared with observations from different regions on various timescales. The model agrees within 30% with the regionally averaged sulfate concentrations measured over North America and Europe but overestimates the SO2 concentrations by more than a factor of 2 there. This suggests that either the emission rates are too high, or an additional loss of SO2 which does not lead to a significant sulfate production is needed. The average wintertime sulfate concentrations over Europe in the model are nearly a factor of 2 lower than measured values, a discrepancy which may be attributed largely to the sea‐salt sulfate collected in the data. The model reproduces the sulfur distributions observed over the oceans in both long‐term surface measurements and short‐term aircraft campaigns. Regional budget analyses show that sulfate production from SO2 oxidation is 2 to 3 times more efficient and the lifetimes of SO2 and sulfate are nearly a factor of 2 longer over the ocean than over the land. This is due to a larger free tropospheric fraction of SO2 column over the ocean than over the land, hence less loss to the surface. The North Atlantic and northwestern Pacific regions are heavily influenced by anthropogenic activities, with more than 60% of the total SO2 originating from anthropogenic sources. The average production efficiency of SO2 from DMS oxidation is estimated at 0.87 to 0.91 in most oceanic regions.