Anthropogenic Sulfate Aerosols Research Articles

Seasonal variations of anthropogenic sulfate aerosol and direct radiative forcing over China

Regional climate model (RegCM2) and sulfur transport model (NJUADMS) were combined to simulate the distribution of anthropogenic sulfate aerosol burden over China, where a “look up table” method was applied to illustrate sulfate formation from SO2-oxidation. Direct radiative forcing of sulfate aerosol was further estimated using the scheme suggested by Charlson et al (1991). Investigations show that the annual average total sulfate column over mainland China is 2.01 mg/m2 with high value in East and Central areas (more than 7 mg/m2). The annual average direct radiative forcing of China is about −0.85 W/m2. The forcing can reach −7 W/m2 in Central and East China during the winter season. Total sulfate column shows significant seasonal variations with winter maximum-summer minimum in the Southern part of China and spring maximum-autumn minimum in the northern part of China. Strong seasonal cycles of direct radiative forcing are also found due to the influence of total sulfate column, cloud, relative humidity and the reflectivity of underlying surface

Regional temperature response due to indirect sulfate aerosol forcing: impact of model resolution

A regional atmospheric climate model, including an interactive module of the tropospheric sulfur cycle, has been used to conduct yearlong equilibrium simulations of the temperature response due to anthropogenic sulfate aerosol forcing on cloud albedo. A main purpose is to examine differences in the magnitudes as well as patterns of forcing and response between simulations conducted with high (0.4° × 0.4°, HR) and low (2.0° × 2.0°, LR) spatial resolutions. Averaged over the model domain, the annual mean indirect forcing differs by only 7% between HR and LR and there is no difference in the annual mean temperature response. The results thus indicate that it is not important to represent small-scale variability (≤2.8°) when the average indirect climate effect over Europe is considered. However, a notable difference in the geographical distributions of forcing and response is obtained when different resolutions are employed. In addition, a clear correspondence between the patterns of radiative forcing and temperature response is obtained when HR is used. The correspondence is less obvious in the LR simulation. It is interesting to compare the present results with those of Roeckner et al. 1999, who found a poor correspondence between the patterns of forcing and response in their simulations using a coarse resolution GCM.

DMS atmospheric concentrations and sulphate aerosol indirect radiative forcing: a sensitivity study to the DMS source representation and oxidation

Abstract. The global sulphur cycle has been simulated using a general circulation model with a focus on the source and oxidation of atmospheric dimethylsulphide (DMS). The sensitivity of atmospheric DMS to the oceanic DMS climatology, the parameterisation of the sea-air transfer and to the oxidant fields have been studied. The importance of additional oxidation pathways (by O3 in the gas- and aqueous-phases and by BrO in the gas phase) not incorporated in global models has also been evaluated. While three different climatologies of the oceanic DMS concentration produce rather similar global DMS fluxes to the atmosphere at 24-27 Tg S yr -1, there are large differences in the spatial and seasonal distribution. The relative contributions of OH and NO3 radicals to DMS oxidation depends critically on which oxidant fields are prescribed in the model. Oxidation by O3 appears to be significant at high latitudes in both hemispheres. Oxidation by BrO could be significant even for BrO concentrations at sub-pptv levels in the marine boundary layer. The impact of such refinements on the DMS chemistry onto the indirect radiative forcing by anthropogenic sulphate aerosols is also discussed.

Statistical separation of observed global and European climate data into natural and anthropogenic signals

Observed global and European spatiotemporal related fields of surface air temperature, mean-sea-level pressure and precipitation are analyzed statistically with respect to their response to external forcing factors such as anthropogenic greenhouse gases, anthropogenic sulfate aerosol, solar variations and explosive volcanism, and known internal climate mechanisms such as the El Nino-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). As a first step, a princi- pal component analysis (PCA) is applied to the observed spatiotemporal related fields to obtain spa- tial patterns with linear independent temporal structure. In a second step, the time series of each of the spatial patterns is subject to a stepwise regression analysis in order to separate it into signals of the external forcing factors and internal climate mechanisms as listed above as well as the residuals. Finally a back-transformation leads to the spatiotemporally related patterns of all these signals being intercompared. Two kinds of significance tests are applied to the anthropogenic signals. First, it is tested whether the anthropogenic signal is significant compared with the complete residual variance including natural variability. This test answers the question whether a significant anthropogenic cli- mate change is visible in the observed data. As a second test the anthropogenic signal is tested with respect to the climate noise component only. This test answers the question whether the anthro- pogenic signal is significant among others in the observed data. Using both tests, regions can be specified where the anthropogenic influence is visible (second test) and regions where the anthro- pogenic influence has already significantly changed climate (first test).

Tropical Rainfall Trends and the Indirect Aerosol Effect

An atmospheric global climate model coupled to a mixed layer ocean model is used to study changes in tropical rainfall due to the indirect effects of anthropogenic sulfate aerosol. The model is run to equilibrium for present-day (PD) and preindustrial (PI) sulfur emission scenarios. As in two other recent studies, the model generally gives a southward shift of tropical rainfall in the PD run relative to the PI run. This is largely due to a hemispheric asymmetry in the reduction of sea surface temperature (SST) induced by the perturbation of cloud albedo and lifetime. Observed precipitation trends over land for the period 1900‐98 show a complex pattern in the Tropics, but when zonally averaged, a southward shift similar to (but weaker than) the modeled shift is clearly evident. The zonally averaged tropical trends are significant at the 5% level in several latitude bands. The modeled presentday hemispheric contrast in cloud droplet effective radius (which affects cloud albedo) is well supported by one long-term satellite retrieval, but not by another. A third satellite retrieval, which only covers an 8-month period, does show a marked hemispheric contrast in effective radius. Both in the modeled changes and the observed trends, a prominent feature is the drying of the Sahel in North Africa. Modeled dynamical changes in this region are similar to observed changes that have been associated with Sahelian drought. Previous work has identified a near-global, quasi-hemispheric pattern of contrasting SST anomalies (cool in the Northern Hemisphere and warm in the Southern Hemisphere) associated with dry conditions in the Sahel. The present results, combined with this earlier finding, suggest that the indirect effects of anthropogenic sulfate may have contributed to the Sahelian drying trend. More generally, it is concluded that spatially varying aerosol-related forcing (both direct and indirect) can substantially alter low-latitude circulation and rainfall.

Multidecadal solar radiation trends in the United States and Germany and direct tropospheric aerosol forcing

In recent studies, anthropogenic aerosols have been recognized as important radiative forcing factors of climate because of their ability to scatter and/or absorb sunlight. At clear‐sky conditions the direct aerosol forcing at ground is negative and implies less solar heating of the surface because of aerosols. In this study, an intensified direct aerosol forcing of −7 to −8 W/m2 has been detected in the United States for the interval from 1960 to 1990. In Germany a weakened aerosol forcing of +3 W/m2 was observed during the same time period. Even though the aerosol forcing is stronger in the eastern United States compared to the western United States, the positive trend is almost equal. We attained these results by scrutinizing clear‐sky global solar radiation recordings for these regions and this time period. Additionally, the diurnal cycle and the direct to diffuse ratio of solar radiation were used for constraining the observed trends. Increased absorption and declined light scattering are presumably responsible for the intensified direct aerosol forcing in the United States. While at the same time in Germany, both aerosol absorption and scattering must have declined to explain the parallel weakened aerosol forcing and the increased direct/diffuse ratio. To estimate the possible anthropogenic portion of these observed changes, we compared the observational results with modeled aerosol forcing scenarios retrieved from the Goddard Institute for Space Studies general circulation model (GISS GCM). Modeled surface solar radiation, aerosol optical thickness, and single‐scattering albedo are derived from emission trends of anthropogenic sulfate and carbonaceous aerosols. The emission distributions are calculated from fossil fuel consumption databases. On the basis of these simulations we suspect that the declining trend of sulfate burden over Germany between 1960 and 1990 was stronger than estimated with the model. Over the United States the simulated small increase in the carbonaceous aerosol burden was exaggerated in order to explain the observed changes in surface solar radiation, diurnal cycle, and direct/diffuse ratio of surface solar radiation. In addition to emission changes from fossil fuel burning, other reasons explaining the solar radiation trends are also discussed.

Origins of Model–Data Discrepancies in Optimal Fingerprinting

Abstract Two approaches to distinguishing anthropogenic greenhouse gas and sulfate aerosol signals in the observed surface temperature record are compared. Both rely on a variant of general regression called “optimal fingerprinting.” One approach is equivalent to a stepwise regression procedure estimating, first, a greenhouse gas signal and, in a second step, the sulfate aerosol signal. This is different from multiple regression, under which both signals are estimated simultaneously and treated symmetrically. The stepwise regression approach is a more powerful means of detecting greenhouse gas influence in the presence of a small and possibly poorly simulated sulfate aerosol signal. However, when both signals are of comparable size, multiple regression provides estimates of the amplitude of the greenhouse and sulfate responses that are, in principle, independent of each other, making it generally simpler to interpret. It is shown that there is a simple linear transform relating the stepwise and multiple r...

Visibility impairment during Yellow Sand periods in the urban atmosphere of Kwangju, Korea

For continuous monitoring of atmospheric visibility in the city of Kwangju, Korea, a transmissometer system consisting of a transmitter and a receiver was installed at a distance of 1.91 km across the downtown Kwangju, Korea. At the transmitter site a nephelometer and an aethalometer were also installed to measure the scattering and absorption coefficients of the atmosphere, respectively. Unusually high number of Yellow Sand events had occurred in the Northeast Asia during the spring of 2000. Visibility in Kwangju under such conditions was greatly impaired over large area for a few days. In order to investigate the effects of Yellow Sand on visibility impairment, chemical and elemental analyses of aerosol samples were performed along with the optical measurement of visibility. Hourly averaged visual range decreased from 61.7 to 1.9 km when hourly averaged concentration of PM10 varied from 32.9 to 601.8 μg/m 3 during Yellow Sand periods. Fine particulate (<2.5 μm) concentrations were relatively lower than coarse particulate matters. Results of the data analyses show that mineral dusts originated from continental sources were simultaneously transported along with anthropogenic sulfate aerosols and marine aerosols. Total light extinction coefficient, b ext, proposed by the IMPROVE network showed poor correlation with b ext measured by transmissometer. Coarse mass scattering efficiency was classified into three categories; E NHSOc, E mineral, and E sea-salt, which were determined as ammonium sulfate combined with nss-sulfate of 1.0, sea-salt of 0.4, and mineral of 0.77 m 2/g, respectively. Mass fraction of NHSOc, sea-salt, and mineral dust was 0.20, 0.14, and 0.66, respectively.

Indirect sulphate aerosol forcing in a climate model with an interactive sulphur cycle

The effects of anthropogenic sulphate aerosol on cloud albedo and on precipitation efficiency (the first and second indirect effects, respectively) are investigated using a new version of the Hadley Centre climate model. This version includes a new cloud microphysics scheme, an interactive sulphur cycle, and a parameterization of the effects of sea salt aerosol. The combined global mean radiative impact from both indirect effects is estimated to be approximately −1.9 W m−2 in terms of the change in net cloud forcing, with the “albedo” effect dominating: we obtain values of −1.3 and −0.5 W m−2 for the first and second effects, respectively, when calculated separately. The estimate for the combined effect has at least a factor of 2 uncertainty associated with it: for example, alternative assumptions which affect the concentration of natural “background” sulphate aerosol reduce the forcing by over 25%, and different parameterizations of the autoconversion of cloud droplets to rainwater can double the forcing.

The response of the climate system to the indirect effects of anthropogenic sulfate aerosol

The indirect effects of anthropogenic sulfate aerosols on the albedo and lifetime of clouds may produce a significant impact on the climate system. A `state of the art' general circulation model (GCM) which includes an interactive sulfur cycle and a physically based cloud microphysics scheme is coupled to a mixed-layer ocean model in order to study the impact of the indirect effects on the coupled climate system. The linearity of the two indirect effects on the model response is also investigated by including each effect separately in the model. The response of the sea surface temperatures (SSTs) and sea ice is found to provide an important feedback on the cooling at high latitudes and the change in meridional SST gradient results in a southward shift of the inter-tropical convergence zone (ITCZ). The sensitivity of the model to the forcing from the indirect effects of sulfate aerosol is found to be similar to, but slightly weaker than that obtained from a doubling of CO2.

A physically based estimate of radiative forcing by anthropogenic sulfate aerosol

Estimates of direct and indirect radiative forcing by anthropogenic sulfate aerosols from an integrated global aerosol and climate modeling system are presented. A detailed global tropospheric chemistry and aerosol model that predicts concentrations of oxidants as well as aerosols and aerosol precursors, is coupled to a general circulation model that predicts both cloud water mass and cloud droplet number. Both number and mass of several externally mixed aerosol size modes are predicted, with internal mixing assumed for the different aerosol components within each mode. Predicted aerosol species include sulfate, organic and black carbon, soil dust, and sea salt. The models use physically based treatments of aerosol radiative properties (including dependence on relative humidity) and aerosol activation as cloud condensation nuclei. Parallel simulations with and without anthropogenic sulfate aerosol are performed for a global domain. The global and annual mean direct and indirect radiative forcing due to anthropogenic sulfate are estimated to be −0.3 to −0.5 and −1.5 to −3.0 W m−2, respectively. The radiative forcing is sensitive to the model's horizontal resolution, the use of predicted versus analyzed relative humidity, the prediction versus diagnosis of aerosol number and droplet number, and the parameterization of droplet collision/coalescence. About half of the indirect radiative forcing is due to changes in droplet radius and half to increased cloud liquid water.

Discriminants of Twentieth-Century Changes in Earth Surface Temperatures

An approach to identifying climate changes is presented that does not hinge on simulations of natural climate variations or anthropogenic changes. Observed interdecadal climate variations are decomposed into several discriminants, mutually uncorrelated spatiotemporal components with a maximal ratio of interdecadal-to-intradecadal variance. The dominant discriminants of twentieth-century variations in surface temperature exhibit large-scale warming in which, particularly in the Northern Hemisphere summer months, localized cooling is embedded. The structure of the large-scale warming is consistent with expected effects of increases in greenhouse gas concentrations. The localized cooling, with maxima on scales of 1000–2000 km over East Asia, eastern Europe, and North America, is suggestive of radiative effects of anthropogenic sulfate aerosols.

Estimates of the direct and indirect radiative forcing due to tropospheric aerosols: A review

This paper reviews the many developments in estimates of the direct and indirect global annual mean radiative forcing due to present‐day concentrations of anthropogenic tropospheric aerosols since Intergovernmental Panel on Climate Change [1996]. The range of estimates of the global mean direct radiative forcing due to six distinct aerosol types is presented. Additionally, the indirect effect is split into two components corresponding to the radiative forcing due to modification of the radiative properties of clouds (cloud albedo effect) and the effects of anthropogenic aerosols upon the lifetime of clouds (cloud lifetime effect). The radiative forcing for anthropogenic sulphate aerosol ranges from −0.26 to −0.82 W m−2. For fossil fuel black carbon the radiative forcing ranges from +0.16 W m−2 for an external mixture to +0.42 W m−2 for where the black carbon is modeled as internally mixed with sulphate aerosol. For fossil fuel organic carbon the two estimates of the likely weakest limit of the direct radiative forcing are −0.02 and −0.04 W m−2. For biomass‐burning sources of black carbon and organic carbon the combined radiative forcing ranges from −0.14 to −0.74 W m−2. Estimates of the radiative forcing due to mineral dust vary widely from +0.09 to −0.46 W m−2; even the sign of the radiative forcing is not well established due to the competing effects of solar and terrestrial radiative forcings. A single study provides a very tentative estimate of the radiative forcing of nitrates to be −0.03 W m−2. Estimates of the cloud albedo indirect radiative forcing range from −0.3 to approximately −1.8 W m−2. Although the cloud lifetime effect is identified as a potentially important climate forcing mechanism, it is difficult to quantify in the context of the present definition of radiative forcing of climate change and current model simulations. This is because its estimation by general circulation models necessarily includes some level of cloud and water vapor feedbacks, which affect the hydrological cycle and the dynamics of the atmosphere. Available models predict that the radiative flux perturbation associated with the cloud lifetime effect is of a magnitude similar to that of the cloud albedo effect.

Reflectivity variations off the Peru Coast: Evidence for indirect effect of anthropogenic sulfate aerosols on clouds

Using reflectivity measurements from the Total Ozone Mapping Spectrometer (TOMS), we show that over the months when stratocumulus clouds are prominent off the Peru Coast, the ultraviolet (UV) reflectance of two marine sites is consistently higher than that of the surroundings. The regions of reflectivity enhancement coincide with large anthropogenic sulfate aerosol emission sources, and the magnitude of the enhancement has a strong seasonal dependence that is related to the seasonal cloud movement. We propose the indirect aerosol effect as a plausible explanation for the reflectivity observations.

Indirect effect of sulfate and carbonaceous aerosols: A mechanistic treatment

The indirect effect of anthropogenic aerosols, whereby aerosol particles change cloud optical properties, is the most uncertain component of climate forcing over the past 100 years. Here we use a mechanistic treatment of droplet nucleation and a prognostic treatment of the number of cloud droplets to study the indirect aerosol effect from changes in carbonaceous and sulfate aerosols. Cloud droplet nucleation is parameterized as a function of total aerosol number concentration, updraft velocity, and an activation parameter, which takes into account the mechanism of sulfate aerosol formation. Where previous studies focussed either on sulfate aerosols or carbonaceous aerosols only, here we estimate the combined effect. The combined indirect aerosol effect amounts to −1.1 W m−2 for an internally mixed aerosol and −1.5 W m−2 for an externally mixed aerosol compared to −1.4 W m−2, which we obtained by empirically relating sulfate mass to cloud droplet number. In the case of an internally mixed aerosol, the contribution from increasing carbonaceous and sulfate aerosols is close to being additive as the individual simulations yield an indirect effect of −0.4 W m−2 due to anthropogenic sulfate aerosols and −0.9 W m−2 due to anthropogenic carbonaceous aerosols. The contribution of anthropogenic sulfate to the indirect effect is close to zero if an externally mixed aerosol is assumed, while the contribution of carbonaceous aerosols increases to −1.3 W m−2. The effect of sulfate in the external mixture approach is much smaller than that of carbonaceous aerosols because its burden only increases by a third of that of carbonaceous aerosols and because the mode radius of sulfate is much larger than that of black and organic carbon.

Detecting a global warming signal in hemispheric temperature series : A structural time series analysis

Non-stationary time series such as global and hemispheric temperatures, greenhouse gas concentrations, solar irradiance, and anthropogenic sulfate aerosols, may contain stochastic trends (the simplest stochastic trend is a random walk) which, due to their unique patterns, can act as a signal of the influence of other variables on the series in question. Two or more series may share a common stochastic trend, which indicates that either one series causes the behavior of the other or that there is a common driving variable. Recent developments in econometrics allow analysts to detect and classify such trends and analyze relationships among series that contain stochastic trends. We apply some univariate autoregression based tests to evaluate the presence of stochastic trends in several time series for temperature and radiative forcing. The temperature and radiative forcing series are found to be of different orders of integration which would cast doubt on the anthropogenic global warming hypothesis. However, these tests can suffer from size distortions when applied to noisy series such as hemispheric temperatures. We, therefore, use multivariate structural time series techniques to decompose Northern and Southern Hemisphere temperatures into stochastic trends and autoregressive noise processes. These results show that there are two independent stochastic trends in the data. We investigate the possible origins of these trends using a regression method. Radiative forcing due to greenhouse gases and solar irradiance can largely explain the common trend. The second trend, which represents the non-scalar non-stationary differences between the hemispheres, reflects radiative forcing due to tropospheric sulfate aerosols. We find similar results when we use the same techniques to analyze temperature data generated by the Hadley Centre GCM SUL experiment.

Anthropogenic and natural causes of twentieth century temperature change

We analyse spatio-temporal patterns of near-surface temperature change to provide an attribution of twentieth century climate change. We apply an “optimal detection” methodology to seasonal and annual data averaged over a range of spatial and temporal scales. We find that solar effects may have contributed significantly to the warming in the first half of the century although this result is dependent on the reconstruction of total solar irradiance that is used. In the latter half of the century, we find that anthropogenic increases in greenhouses gases are largely responsible for the observed warming, balanced by some cooling due to anthropogenic sulphate aerosols, with no evidence for significant solar effects.

Radiative forcing from tropospheric ozone calculated with a unified chemistry‐climate model

We have developed a global model for the study of chemistry‐climate interactions by incorporating a detailed simulation of tropospheric ozone‐NOx‐hydrocarbon chemistry within a general circulation model (GCM). We present a first application of the model to the calculation of radiative forcing from tropospheric ozone since preindustrial times. Longwave and shortwave radiation fluxes are computed every 5 hours in the GCM using the locally simulated ozone fields. In this manner, the model resolves synoptic‐scale correlations between ozone and meteorological variables. A simulation for present‐day conditions is compared to a preindustrial atmosphere (∼1800 A.D.) with no fossil fuel combustion, 10% of present‐day biomass burning, and 0.7 ppm methane. The two simulations use the same meteorological fields; the radiative forcing does not feed back into the GCM. The model reproduces well the observed distributions of ozone and its precursors in the present‐day atmosphere. Increases in ozone since preindustrial times are 20–200% depending on region and season. The global mean, instantaneous radiative forcing from anthropogenic ozone is 0.44 W m2 (0.35 longwave, 0.09 shortwave). The model reveals large shortwave forcings (0.3–0.7 W m2) over polar regions in summer. The total forcing is greater than 1.0 W m2 over large areas, including the Arctic, during Northern Hemisphere summer. The normalized radiative forcing per unit of added ozone column varies globally from −0.01 to 0.05 W m2. This variance can be explained in large part by the temperature difference between the surface and the tropopause; clouds are an additional factor, particularly at low latitudes. An off‐line radiative calculation using the same ozone fields but averaged monthly shows nearly identical forcings, with differences less than ±2% over most of the Earth. The similarity between the off‐line and on‐line simulations suggests that the common use of off‐line ozone fields is acceptable in radiative forcing calculations. Addition of the direct forcings from anthropogenic sulfate aerosol and tropospheric ozone computed with the same GCM shows compensating effects, with sulfate dominating at northern midlatitudes and ozone usually dominating elsewhere.

Model assessment of regional surface temperature trends (1949–1997)

Analyses are conducted to assess whether simulated trends in SST and land surface air temperature from two versions of a coupled ocean‐atmosphere model are consistent with the geographical distribution of observed trends over the period 1949–1997. The simulated trends are derived from model experiments with both constant and time‐varying radiative forcing. The models analyzed are low‐resolution (R 15, ∼4°) and medium‐resolution (R30, ∼2°) versions of the Geophysical Fluid Dynamics Laboratory (GFDL) coupled climate model. Internal climate variability is estimated from long control integrations of the models with no change of external forcing. The radiatively forced trends are based on ensembles of integrations using estimated past concentrations of greenhouse gases and direct effects of anthropogenic sulfate aerosols (G+S). For the regional assessment, the observed trends at each grid point with adequate temporal coverage during 1949–1997 are first compared with the R15 and R30 model unforced internal variability. Nearly 50% of the analyzed areas have observed warming trends exceeding the 95th percentile of trends from the control simulations. These results suggest that regional warming trends over much of the globe during 1949–1997 are very unlikely to have occurred due to internal climate variability alone and suggest a role for a sustained positive thermal forcing such as increasing greenhouse gases. The observed trends are then compared with the trend distributions obtained by combining the ensemble mean G+S forced trends with the internal variability “trend” distributions from the control runs. Better agreement is found between the ensemble mean G+S trends and the observed trends than between the model internal variability alone and the observed trends. However, the G+S trends are still significantly different from the observed trends over about 30% of the areas analyzed. Reasons for these regional inconsistencies between the simulated and the observed trends include possible deficiencies in (1) specified radiative forcings, (2) simulated responses to specified radiative forcings, (3) simulation of internal climate variability, or (4) observed temperature records.

Anthropogenic sulphate aerosol from India: estimates of burden and direct radiative forcing

A one-box chemical-meteorological model had been formulated to make preliminary estimates of sulphate aerosol formation and direct radiative forcing over India. Anthropogenic SO 2 emissions from India, from industrial fuel use and biomass burning, were estimated at 2.0 Tg S yr -1 for 1990 in the range of previous estimates of 1.54 and 2.55 Tg S yr -1 for 1987. Meteorological parameters for 1990 from 18 Indian Meteorological Department stations were used to estimate spatial average sulphate burdens through formation from SO 2 reactions in gas and aqueous phase and removal by dry and wet deposition. The hydrogen peroxide reaction was found dominating for undepleted oxidant-rich conditions. Monthly mean sulphate burdens ranged from 2–10 mg m -2 with a seasonal variation of winter–spring highs and summer lows in agreement with previous GCM studies. The sulphate burdens are dominated by sulphate removal rates by wet deposition, which are high in the monsoon period from June–November. Monthly mean direct radiative forcing from sulphate aerosols is high (−3.5 and −2.3 W m -2) in December and January, is moderate (−1.3 to −1.5 W m -2) during February to April and November and low (−0.4 to −0.6 W m -2) during May to October also in general agreement with previous GCM estimates. This model, in reasonable agreement with detailed GCM results, gives us a simple tool to make preliminary estimates of sulphate burdens and direct radiative forcing.