Simulating the impact of sea salt on global nss sulphate aerosols

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D16, 4516, doi: /2002jd003181, 2003 Simulating the impact of sea salt on global nss sulphate aerosols S. L. Gong and L. A. Barrie 1 Meteorological Service of Canada, Downsview, Ontario, Canada Received 17 November 2002; revised 5 February 2003; accepted 26 March 2003; published 27 August [1] The Canadian Aerosol Module coupled with the Canadian third generation Global Climate Model was used to simulate the global distributions of size-segregated sea salt and sulphate aerosols of both anthropogenic and natural origins in the atmosphere. A sectional model of 12 size bins was used to treat the size distribution of sea salt and sulphate, which is assumed to be internally mixed in each size bin. The spatial and temporal distributions predicted by the model compare reasonably well with observations. The mixed aerosol simulations yield number and volume size distributions in the marine boundary layer (MBL) comparable with observations. Sea salt particles redistribute the mass and number distributions of sulphate aerosols by serving as a quenching agent to nucleation and as an additional surface area for condensation and by changing the cloud properties in the MBL. By differential simulations of global sea salt and sulphate it is found that the presence of sea salt increases the mass mean diameter of sulphate aerosols by up to a factor of 2 over the MBL with high sea salt concentrations and reduces the global sulphate aerosol mass in the surface MBL layer from 5 to 75% depending on the sea salt distributions. The high impacts are in the midlatitudes of both Northern and Southern Hemispheres with a minimum in the equatorial regions. In the polluted anthropogenic regions of North Pacific and Atlantic, sea salt reduces the sulphate concentration from 10 to 30%. The peak reductions of 50 75% occur in the roaring 40s of the Southern Hemisphere in spring and fall. The impact of sea salt on the annual global mass and number loading is estimated to be 9.13 and 0.76%, respectively. A reduction of 20 60% in the marine cloud droplet number concentrations (CDNC) was predicted because of the presence of sea salt, with greatest reductions in the roaring 40s south (40 70%) and in the midlatitude north (20 40%) where the sea salt concentrations were high. Along the equatorial regions some enhancement of total CDNC was simulated because of the presence of sea salt aerosols. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 3339 Meteorology and Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504); KEYWORDS: impact of global sea salt, sulphate, climate, aerosol-cloud interactions Citation: Gong, S. L., and L. A. Barrie, Simulating the impact of sea salt on global nss sulphate aerosols, J. Geophys. Res., 108(D16), 4516, doi: /2002jd003181, Introduction [2] Simulation of global sulphate aerosols has been carried out extensively in the past ten years to study the direct and indirect effects of anthropogenic sulphate on climate. From 1994 to 2000, there were at least 17 published estimates of sulphate radiative forcing using global sulphate models [Intergovernmental Panel on Climate Change (IPCC), 2001]. The direct and indirect radiative forcing of sulphate aerosols is estimated to be in the range 0.2 to 0.8 W m 2 and 0 to 1.5 W m 2, respectively. This forcing is opposite in sign and comparable in magnitude to 1 Now at Environment Division, Atmospheric Research and Environment Program, World Meteorological Organization, Geneva, Switzerland. Copyright 2003 by the American Geophysical Union /03/2002JD that of anthropogenic greenhouse gases. The range of uncertainty in forcing is in part due to the inability of large-scale sulphate models to accurately simulate the spatial and temporal distribution of aerosols and in part due to uncertainties in simulating clouds and their interactions with aerosols. Sulphates are a major aerosol component. An international comparison of ten global sulphate aerosol models [Barrie et al., 2001] found that there were large differences in the spatial and temporal distributions of sulphate predicted by the models. One of the sources of these differences is the lack of size-distributed process parameterizations in many of the models. That is, only the bulk mass of sulphate was simulated without attempting to model the aerosol size distribution. Furthermore, as the most abundant background aerosol in the atmosphere, sea salt aerosol impacts on sulphur aerosols have been neglected in these global sulphur models. The assumption of a sulphateonly atmosphere may lead to an overestimate of the occur- AAC 4-1

2 AAC 4-2 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE rence of sulphate aerosols and biases in estimates of their influence on radiative forcing. [3] Both observations [Quinn et al., 1999, 1998, 1996] and model simulations [Gong et al., 2003] show that sea salt is an important contributor to both total aerosol loading and optical thickness over the world oceans. In the eastern Pacific along longitude 140 W from 55 N to 70 S where RITS 93 and RITS 94 field campaigns [Covert et al., 1996; Quinn et al., 1996] were carried out, the fraction of the measured ionic mass that was sea salt averages 90% for total mass and 55% for the submicron mass. This is even larger than that for non-sea salt (nss) sulphate aerosols. Sea salt aerosols not only contribute to the total loading to the marine atmosphere but also provide a particulate background with which other aerosols and their gaseous precursors interact. Using aircraft measurements in the North Atlantic and eastern Pacific, O Dowd et al. [1999] found that, for a given sulphate mass concentration in the atmosphere, sea salt reduces the cloud droplet number density and increases the rate of change of cloud droplet number as a function of sulphate mass concentration compared to the case without sea salt. [4] In the absence of sea salt aerosols, the condensable H 2 SO 4 vapour will increase in vapour pressure until they reach a critical point for nucleation to occur producing new particles. These newly formed particles are usually in the nanometer size ranges. Through the life cycle of the sulphate particles in the atmosphere by coagulation among themselves, by condensation of H 2 SO 4 and H 2 O molecules and by dry and wet depositions, they form a characteristic number or mass size distribution depending on the age of the aerosol mass. They peak around mm for newly formed aerosols (nucleation mode) and 0.1 mm for aged aerosols (accumulation mode). This bimodal distribution is typical of polluted continental lower atmosphere [Pandis et al., 1995]. [5] In contrast, in the MBL in the presence of large amounts of sea salt aerosol surface area, when condensable sulphuric acid vapour is produced because of oxidation of biogenic DMS or anthropogenic SO 2 by OH radicals, both nucleation and condensation will occur [O Dowd et al., 1997]. The condensation rate depends on the available aerosol concentration and size distributions. In the marine atmosphere where large amounts of sea salt aerosol particles are released by breaking waves, there is enough surface area to enhance condensation. As a consequence, sea salt aerosols attract condensable sulphates to larger size ranges. The residence time of sulphate aerosols is influenced by this. Therefore neglect of sea salt aerosols in global sulphate models may result in large uncertainties in assessing aerosol radiative forcing. This paper presents an application of the Canadian Aerosol Module (CAM) in the Canadian Global Circulation/Climate Model (GCM) to simulate interactions between sea salt and nss sulphate aerosols as well as the condensable sulphate species in the atmosphere. The aim is to better understand the impact of sea salt on nss sulphate. The global sea salt mass budget has been published in a separate paper [Gong et al., 2002]. 2. CAM Essentials and GCM Configuration 2.1. Mass Balance Equation [6] The full details of development of CAM by a Canadian community effort are given in a previous paper [Gong et al., 2003]. The aerosol mixtures of sea salt and sulphate are divided into 12 size bins (r = mm) and assumed to be internally mixed in each bin. The mass balance equation of each species in each bin is expressed as ip ip ip CLEAR ip IN BELOW CLOUDS DRY In equation (1), the aerosol mass mixing ratio (dry) change has been divided into tendencies for dynamics, sources, clear air, dry deposition, in-cloud and below-cloud processes. The dynamics includes resolved motion as well as subgrid turbulent diffusion and convection. The sources include: (a) surface emission rate of both natural and anthropogenic aerosols and their precursors which serve as boundary conditions for the model and (b) production of secondary aerosols, i.e., airborne aerosols by chemical transformation. While source (a) is included as the SOURCES tendency in equation (1), source (b) is treated as a CLEAR AIR tendency term in equation (1). Particle nucleation, condensation and coagulation are also included in clear air process. The BELOW-CLOUD term calculates the scavenging removal of aerosols by both rain and snow while IN-CLOUD term includes the processes of aerosol activation, cloud evaporation and cloud oxidation of SO 2. The subscript i indicates the aerosol particle whose dry radius is within the size mode or section i and subscript p the type of aerosols in the size section. [7] Large-scale and subgrid transport of aerosols is carried out by the host model to which CAM is coupled. This is the process indicated in equation 1 by DYNAMICS tendency. In this application, the host model is the Canadian third generation GCM (GCMiii) [McFarlane et al., 1992]. The transport of aerosols in GCMiii includes a semi-lagrangian advection, vertical diffusion and convection. The aerosols are carried in the GCM as active constituents interacting with clouds and radiation. The stratiform cloud scheme in the GCM has been replaced by an explicit microphysical cloud [Lohmann and Roeckner, 1996] and the cloud droplet number concentrations are driven by total sulphate mass. The sulphur chemistry used in CAM has been given by Gong et al. [2003] Sulphate Source [8] There are two major types of sources relevant to sulphate aerosols: (1) primary sulphate (SO 2 4 ) emission from anthropogenic activities; (2) precursor gas emissions of SO 2 from anthropogenic, volcanic and biogenic sources and subsequent oxidations to SO 2 4 in clear sky by OH and in clouds by H 2 O 2 and O 3. GEIA (Global Emission Inventory Activity) data sets of 1 1 resolution for anthropogenic SO 2 and SO 2 4 (version 1-B) for 1985, biogenic H 2 S and DMS over continents [Benkovitz et al., 1994] and DMS over ocean [Kettle et al., 1999] have been incorporated into CAM to compute the source of sulphate aerosol. The volcanic emissions of SO 2 are included using the emission data of Graf et al. [1997]. ð1þ

3 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE AAC 4-3 [9] For the GEIA sulphur data set (version 1-B), the source is divided into area and elevated point (> 100 m) contributions. The area source is added to the first model layer while the point source is added to a higher layer, depending on the vertical resolution of the model. For primary sulphate aerosols, the size distribution of the source is required. Present knowledge of the size distribution of primary sulphates is very limited. McElroy et al. [1982] observed that coal burning yields aerosol emissions in the size range of the nuclei mode. Whitby [1978] gives two modes for sulphate aerosols from a large power plant and several near-source situations. The modal radius and geometric standard deviation are mm and 1.6 for nuclei mode and mm and 2.0 for accumulation mode. The fractional volumes in these two modes are assumed to be 0.17 and 0.83, respectively. Chan et al. [1983] measured the size distribution of sulphate aerosols in the stack plume from two smelters in Ontario, Canada and found that the mass mean radius of sulphate particles are in the range of mm. It is apparent that the size distribution of primary sulphate aerosols depends on both the source type and age. In view of these uncertainties, a lognormal modal distribution is assumed with the mass mean radius r equal to 0.1 mm and s g equal to 1.5. [10] The second source of sulphate aerosols is the oxidation of the gaseous precursors such as SO 2, H 2 S and DMS in cloud-free air by OH or NO 3 radicals to form condensable sulphuric acid vapour and of dissolved SO 2 in clouds by O 3 and H 2 O 2 to form sulphate in cloud droplets. Once it is formed from gas phase reactions, H 2 SO 4 is subject to two processes: nucleation and condensation to become aerosols. Sulphate formed by in-cloud oxidation of SO 2 is released as aerosols when clouds evaporate. Both stratiform and convective clouds are considered in our model [von Salzen et al., 2000]. Over the open oceans, most of the sulphate produced in cloud droplets was formed on sea salt aerosols since they provide the primary source of cloud nuclei. It should be pointed out that to calculate oxidation rates the concentrations of the oxidants OH, NO 3,O 3 and H 2 O 2 are needed. These can either be generated within the model or imported from a separate atmospheric chemistry model. The former is preferred but numerical overhead is high. In this study, prescribed 3-D daily averaged concentrations of OH, NO 3,O 3 and H 2 O 2 from MOZART model [Brasseur et al., 1998] were used to drive the oxidation chemistry. 3. Results and Discussions [11] The results were obtained using the model with a linear physical transform (Gaussian) grid of T47 (3.75 longitude), vertical resolution of 28 levels (from surface to 12 hpa pressure level) and 20-minute integration time step. The simulation was done for 2.5 years with the first 6 months as a spin-up time. Monthly mean values were obtained from the last 2-years of simulation. [12] In this study, a differential simulation method was used to investigate the impact of sea salt on sulphate. A stand alone sulphate simulation was done first to obtain the benchmark for sulphate aerosols (hereafter is refereed as case A). Then the sea salt aerosol is included with sulphate (case B). The mixed aerosol results were compared with the benchmark results and the difference is seen as the impact of sea salt on sulphate Global nss Sulphate Distributions [13] Compared with the previous results of sea salt aerosols [Gong et al., 2002], no significant effect of sulphates on the mass budget of sea salt aerosols in the atmosphere was found in the current mixed aerosol simulation, except for a slight decrease in the number concentrations of the submicron sea salt due to coagulations with sulphate aerosols. This is because the relative source strength of sea salt (10100 Tg yr 1 ) greatly exceeds that of total sulphate (340 Tg yr 1 ). Consequently, this paper is dedicated to the global nss sulphate aerosol and the impact of sea salt on it. The global distributions of seasonal average sulphate mass mixing ratio in the surface layer (50 m) are shown in Figures 1 and 2 for four seasons of case A and case B, respectively. For both cases, the spatial sulphate distributions are closely associated with the global anthropogenic emission patterns, especially in the Northern Hemisphere where the source of sulphate aerosols is predominantly anthropogenic. Similar patterns are obtained by most model contributions to an international sulphate model intercomparison study (COSAM) [Barrie et al., 2001]. For the midlatitudes of the Northern Hemisphere, no apparent seasonal variations are found over the oceans. However, for the regions of heavy anthropogenic sulphur emissions such as the eastern North America, East Asia, and Europe, there exist strong seasonal variations. In the Southern Hemisphere and high-latitude Northern Hemisphere, there is also a strong seasonal variation, reflecting the seasonal dependence of biogenic activities and long-range transport. This is consistent with observations and previous simulation results of most global sulphate models [Barrie et al., 2001]. [14] The impact of sea salt on nss sulphate mass mixing ratios is clearly evident in Figures 1 and 2, especially for the Southern Hemispheric MBL where sea salt is abundant. Figure 3 shows the global spatial and temporal distributions of the relative impact (RI) of sea salt aerosols on sulphate mass mixing ratios for four seasons, which is defined as: RI ¼ CaseA CaseB 100% CaseA For the roaring 40s in the southern oceans, RI is as high as 75%. In the tropics where sea salt aerosols are less abundant RI is around 10% while in the midlatitude northern oceans it is as high as 50%. In terms of the temporal distributions, RI peaks in spring (MAM) and fall (SON) for the southern oceans and in spring (MAM) and winter (DJF) for the northern oceans. It is apparent that the RI distribution strongly correlates with the global sea salt distributions. [15] RIs over continents where sea salt is low are not significantly affected. This is even true for the point comparisons of sulphate aerosols with COSAM sites, which are located over island or coastal locations where sea salt is much lower than over the open oceans. For both cases A and B, little difference was found for the scatterplots of model simulations and observations. Figure 4 ð2þ

4 AAC 4-4 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE Figure 1. Global seasonal average sulphate mass mixing ratios simulated without sea salt (case A) in the surface layer. Figure 2. Global seasonal average sulphate mass mixing ratios simulated with sea salt (case B) in the surface layer.

5 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE AAC 4-5 Figure 3. Seasonal percentages of relative impact (RI) between case A and case B. shows the seasonal results for case B. Generally, the model predictions and observations agreed within a factor of two for most locations. Figure 5 shows the detailed point-bypoint comparisons (a) for Northern Atlantic transect and (b) high-altitude sites for summer (JJA) and winter (DJF). As can be seen from Figure 5 that the point comparisons of total sulphate mass concentrations with observations are reasonable for the Northern Atlantic transect sites while the comparisons at high-altitude sites are less satisfactory (Figure 5b), especially for the wintertime (DJF). [16] The comparisons of SO 2 with observations are shown in Figure 6. Like most global sulphate models [Barrie et al., 2001], our results exhibit an over estimation of SO 2 concentrations. There are many factors contributing the discrepancy between model and observations: the emission inventory, removal parameterizations, climate model that provides the meteorology and, to some extend, the observations. Barrie et al. [2001] attributed the general 2 tendency to overpredict SO 2 while predicting SO 4 reasonably well to a problem with unrealistically high long range transport out of source regions in the models. This may also partially explain the high concentrations predicted by our model for high SO 2 4 altitudes by longrange transport from source regions (Figure 5b). Another possible source of bias is in the representativeness of the 1985 SO 2 emissions for which the observations are available (mainly 1980s to mid 1990s). According to Barrie et al. [2001], since most SO 2 observations are on the periphery of Europe and North America, an examination of emission differences would explain the models being high by 20 to 40%. [17] The impact of sea salt on global sulphate mass concentrations is primarily on the marine surface layer as indicated by Figure 3 but not limited to it. Figure 7 shows the vertical profiles of number and mass concentrations of sulphate in winter (DJF) with and without sea salt averaged over three regions in the world s oceans: (1) the North Atlantic, (2) the roaring 40 of the South Pacific and (3) the tropical Pacific. Along each profile, a vertical profile of RI is also shown. It is apparent that the impact of sea salt generally declines as the altitude increases and is influenced by the vertical profiles of sea salt aerosols. In the North Atlantic region and the roaring 40s south, a steady decrease of RIs was observed with altitudes (Figures 7a, 7c, 7d, and 7e) for both number and mass concentrations. However, there are some variations of the vertical distributions of RIs for some regions (Figures 7b and 7f ) with increasing RI or fluctuating RI with altitudes. To a large extend, these kinds of variations of the RI vertical profiles are governed by the variations of vertical distributions of sea salt aerosols and cloud processes. For example, it is found that because of the strong upmixing of air mass over the Intertropical Convergence Zone (ITCZ) [Gong et al., 2002], sea salt aerosols have a longer residence time than at middle or high latitudes and extends higher in the free troposphere. This high sea salt vertical profile in the ITCZ reflects the higher impact of sea salt on sulphate mass (Figure 7f ). Since the aerosol concentrations are very low above 20 km and the difference may not be statistically significant, we only showed the RIs up to 20 km for this study. [18] In order to characterize the impact of sea salt on total global sulphate budgets, the annual global sulphate column

6 AAC 4-6 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE Figure 4. Scatterplots of predicted and observed sulphate mass mixing ratios at observational sites used in COSAM [Barrie et al., 2001]. mass and number loadings for case B together with their respective RIs were shown in Figure 8. The spatial pattern of the mass loading generally correlates with the anthropogenic emissions of sulphur species and concentrates in the Northern Hemisphere. Long-range transports of sulphate across the Pacific and Atlantic oceans are reflected in the column loading (Figure 8a). The global averaged mass loading is estimated to be 7.15 mg m 2 (Table 1). The relative impact (RI) for the sulphate loading is generally positive except for few regions and ranges from a few percent up to 25% (Figure 8c). Comparing to Figure 3, RIs for the column mass loading are much weaker than for the surface mass mixing ratios. This implies that the ability of sea salt in reducing the column sulphate loading is not as strong as in reducing the surface mass mixing ratio. The spatial distribution pattern of number column loading generally coincides with the global sulphur emissions but shows a band of high loadings in the tropics along the equator (Figure 8b). Since the number concentration is usually dominated by the nucleation mode of sulphate, the long lifetime of sulphur dioxide and high OH concentrations in these regions may contribute to the high loadings. RIs for the column number loadings show more fluctuations than those for the mass loadings. In both hemispheres where sea salt concentrations are high, the RIs ranges from 2 to 15%. However, there are more regions with negative RIs compared to mass loadings, implying that the existence of sea salt increases the sulphate number loadings in these regions. The mechanism for the impact is very complicated and not fully understood. It is possible that the inclusion of sea salt changes the climate model simulation and thus the cloud dynamics and ph values at some grid points, resulting in more sulphate production. [19] The global averaged seasonal loadings of sulphate mass and number for case A and case B are listed in Table 1 together with the relative impacts (RIs). The RIs for global mass loading are positive for all seasons and have an average value of about 9%. There exist both positive and negative RIs for number loadings but the annual averaged RI is much smaller than that for mass loadings. For the mass loading, the RIs peak in spring (MAM) and winter (DJF) with a value up to 13% The nss Sulphate Size Distributions [20] Mass size distributions are simulated for mixed sea salt and sulphate aerosols and the number and volume size

7 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE AAC 4-7 Figure 5. Comparisons of predicted and observed sulphate mass mixing ratios for (a) Northern Atlantic transect and (b) for high-altitude locations. distributions are calculated from the mass distribution. Figure 9 shows the contour plots of the nss sulphate mass size distributions as a function of latitude along the longitude of 140 W where measurements were made in field campaigns of RITS 93 and RITS 94 [Covert et al., 1996; Quinn et al., 1996]. The impact of sea salt on the nss sulphate mass size distribution is clearly shown from the comparisons of case A and case B. Without sea salt aerosols (Figure 9a), the nss sulphate mass size distribution peaks around 0.2 mm with very little in the supermicron size range. With the introduction of sea salt aerosols, another peak for the nss sulphate mass size distribution appears around 1.92 mm corresponding to the peak of sea salt mass size distribution. These kinds of bimodal distributions have been observed during the RITS 93 and RITS 94 campaigns for nss sulphate (Figure 9c). Compared to the observations, the stand alone sulphate case apparently underestimates the mass size distribution in the supermicron size range. For the same latitudinal section at 140, the sulphate number size distribution is presented in Figure 10 for 40 N, 0 N and 40 S for both cases A and B. [21] Reductions and modifications in nss sulphate number size distribution caused by sea salt were investigated by comparing averages along longitudinal transects down the mid-pacific (Figure 10, 140 W) and mid-atlantic (Figure 11, 40 W). In the midlatitudes (40 ) of Southern and Northern Hemispheres, the sea salt impact reduces the sulphate aerosol number and modifies the shape of the distributions. This is especially evident for sulphate aerosol of less than 0.1 mm where the number concentration is relatively high. There is little impact in the equatorial regions where sea salt production is lowest because of low surface winds. For case B simulations (Figure 10f ), the number size distributions are reduced in open distributions that are more comparable in both shape and magnitudes with observed [Covert et al., 1996; Quinn et al., 1996]. This Open distribution was defined by Covert et al. [1996] as the distribution that has a significant concentration or even a maximum in concentrations smaller than 20 nm diameter. At 0 S and 140 W, the model predicts the observed bimodal number size distributions with one peak centred at 0.2 mm and one at 0.06 mm. These bimodal number size distributions have been observed by Quinn et al. [1996] during RITS 93 and RITS 94 [Quinn et al., 1996, Figures 4 and 5]. The impact of sea salt on sulphate number distributions is most pronounced over the Atlantic Ocean points (Figure 11). The reductions in total number and open shape distributions are strongly shown in midlatitudes (40 ) of Southern and Northern Hemispheres. In addition to the sea salt influence in converting the open shape to bimodal distributions, the bimodal distribution is also associated with stable air masses. Long residence time of this type of air mass enables the mixing and coagulation of ultra fine sulphate and the formation of bimodal distribution. [22] Figure 12 shows the total volume size distributions of (a) sulphate only and (b) sulphate/sea salt mixture for the mid-pacific longitudinal transects. It is evident that the sulphate only volume size distribution is unrealistic (Figure 12a) and the combined volume size distributions (Figure 12b) are more close to reality. Compared with observations (Figure 13) in the same regions [Quinn et al., 1996], Figure 12b predicts the shapes and magnitudes of the volume size distributions. The supermicron peaks at around 2 mm were obtained from both simulation and observation, reflecting the sea salt contribution to the total volume. The two submicron peaks centred around 0.06 mm

8 AAC 4-8 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE Figure 6. Scatterplots of predicted and observed SO 2 mass mixing ratios at observational sites used in COSAM [Barrie et al., 2001]. and 0.2 mm are consistent among model predictions (Figure 12b) and observations. Since the sea salt source only supplied particles greater than 0.3 mm, these two peaks are exclusively attributed to sulphate aerosols. [23] The global distribution of nss sulphate mass mean diameter (MMD) in spring for cases A and B are compared in Figure 14. The aerosol is in equilibrium with ambient water. The presence of sea salt aerosol generally increases the MMD of sulphate aerosols from 0.3 to 0.7 mm downwind of both polluted North American and East Asian continents. Early reviews of atmospheric sulphate size distributions [Milford and Davidson, 1987; Whitby, 1978] concluded that sulphate mass is present largely in the accumulation mode with a mass mean diameter of about mm. Recent measurement during INDOEX [Venkataraman et al., 2002] showed that sulphate aerosol over a coastal city Mumbai, India has a trimodal distribution with a dominant MMD of 0.6 mm. The simulated MMD of 0.7 mm for mixed sea salt and sulphate aerosols agrees with the observations better than sulphate-only MMD of 0.3 mm. [24] The simulation results of mixed sea salt and sulphate aerosols indicate that the presence of sea salt aerosols reduces the total number and mass concentrations and alters the sulphate mass and number size distributions by shifting mass from submicron to supermicron size ranges. This modification of sulphate characteristics changes the residence time and cycling frequencies and hence the global spatial distributions Impact Mechanisms by Sea Salt [25] The fate of sulphate aerosol and its precursors is governed by a series of chemical and physical processes. In the polluted regions, anthropogenic SO 2 is oxidized by OH to form the condensable H 2 SO 4 vapour while in the clean marine atmosphere the biogenic emission of DMS is oxidized by OH first into SO 2 which is then oxidized to condensable H 2 SO 4 vapour. This H 2 SO 4 vapour undergoes two competitive processes to form sulphate aerosols: nucleation to form new tiny particles and condensation to distribute to existing aerosol particles. Over the MBL, SO 2 can also be dissolved in sea salt aerosols and oxidized into sulphate. [26] In the presence of clouds, the major production of sulphate is through the oxidation of SO 2 in clouds by H 2 O 2 and O 3. Because of the presence of sea salt in the cloud

9 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE AAC 4-9 Figure 7. Vertical profiles of (top) number and (bottom) mass concentrations in three oceanic regions (a) and (c) North Atlantic: longitude , latitude ; (b) and (d) roaring 40s: longitude , latitude 45 to 35 ; and (c) and (f ) tropical Pacific: longitude , latitude 23.5 to The relative impact (RI) is also shown on the right side of each profile. Figure 8. Global annual (a) mass and (b) number loadings and their respective RIs.

10 AAC 4-10 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE Table 1. Seasonal and Annual Averaged Loadings of Sulphate Mass and Number for Case A and Case B Number, m 2 Case A Case B RI MAM % JJA % SON % DJF % Annual % Mass, mg m 2 Case A Case B RI MAM % JJA % SON % DJF % Annual % droplets and its modification of ph values of clouds, the sulphate production is also influenced as well Impacts Under Clear Skies [27] Since sea salt particles have a large size distribution, the net result of the existence of sea salt particles is to shift sulphate aerosol mass to large particles. This phenomenon is clearly shown in Figure 10. With the presence of sea salt aerosols, the mass size distribution is shifted from 0.2 mm to the large size range with a second peak around 1.92 mm formed. This is the peak size for sea salt aerosol distributions [Gong et al., 2002]. The same impacts on number size distributions are also shown in Figures 11 and 12 for mid Pacific and Atlantic locations. Even though the impacts are not identical for different locations as other processes such as vertical mixing, dry and wet deposition patterns vary, general trend prevails. The number size distributions are reduced in the regions where the sea salt concentration is large. Since large sea salt particles have shorter lifetime than nucleation or accumulation mode sulphate particles, the removal of sulphate aerosols mixed with sea salt is faster than those without sea salt. This leads to a lower sulphate concentration with the same fluxes of DMS and oxidation conditions in the MBL. The percent difference of the mass mixing ratios between case A and case B (Figure 3) illustrates the global distributions of this impact. Up to 70% decreases were evident in the high latitudes in spring and fall. [28] The simulated results of number size distributions indicate that the existence of sea salt aerosol surface area quenches the formation of new particles by competitively converting the oxidized H 2 SO 4 to sulphate aerosols through the condensation process in the MBL. In a box model study, Raes [1995] explained the lack of ultrafine particles in the remote MBL to be due to the entraining free tropospheric (FT) aerosol in the MBL. They argued that the entrained FT aerosol effectively acts as a seed aerosol which scavenges Figure 9. Contour plots of the nss sulphate mass size distributions (a) without and (b) with sea salt aerosols at 140 W and (c) the observed distributions from RITS 93 [Quinn et al., 1996]. The unit is mg m 3 /dlogd p.

11 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE AAC 4-11 Figure 10. The nss sulphate number size distributions for latitudinal points from north to south at 140 W over Pacific Ocean for case A and case B. Each curve in the plot represents a size distribution averaged over a 3-day simulation period in April. H 2 SO 4 molecules and prevents the build up of high H 2 SO 4 molecule concentrations that entails nucleation. Since sea salt aerosol was not explicitly simulated in their model, the competitive conversion of H 2 SO 4 vapour into sulphate was not considered. The fact that sea salt aerosol dominates the mass fraction of total measured ionic mass and volume size distributions [Quinn et al., 1996] in the MBL indicates that sea salt aerosol are more effective in quenching nucleation than the entrained FT aerosols. The predicted number size distributions in Figure 12 clearly illustrate the impact of sea salt aerosols as a quenching agent in the MBL as the only difference between cases A and B is the presence of sea salt aerosols. [29] We should point out that the oxidation of SO 2 in sea salt aerosols is not included in this version of study. Some modeling study [Katoshevski et al., 1999] has indicated that oxidation of dissolved SO 2 in liquid sea salt particles was a larger pathway than H 2 SO 4 condensation for nss sulphate

12 AAC 4-12 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE Figure 11. The nss sulphate number size distributions for latitudinal points from north to south at 40 W over Atlantic Ocean for case A and case B. Each curve in the plot represents a size distribution averaged over a 3-day simulation period in April. aerosol production. The cloud processing of SO 2 contributed to the largest fraction of sulphate production in the marine atmosphere except for a case of a large wind speed at 17 m s 1 (sea salt production) where the oxidation of SO 2 in the inactivated sea salt aerosols dominated the sulphate production. Under clear sky situations, both nucleation/ condensation and sea salt oxidation of SO 2 will occur Impacts Via Clouds [30] The role of sea salt in the production of marine clouds has been well documented. Since the largest pathway for sulphate production is aqueous reaction followed by cloud drop resuspension, the impact of sea salt through the cloud oxidation would be large. This cloud processing due to the presence of sea salt would be to increase the sulphate production and hence results in negative RIs. [31] In a close look at Figure 3 and Figure 8d, we did notice that there exist regions with negative RI, implying that the presence of sea salt enhances the concentrations of nss sulphate aerosols. An increase of more than 20% in in-cloud sulphate production due to additional sea salt particles and higher ph associated with newly formed sea salt-nucleated cloud droplets compared to sulphate-only

13 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE AAC 4-13 Figure 12. Total volume (sea salt + sulphate) size distributions for latitudinal points from north to south at 140 W of Pacific Ocean for case A and case B. Each curve in the plot represents a size distribution averaged over a 3-day simulation period in April. clouds has been reported [Lowe et al., 1995; O Dowd et al., 1997]. This increase in sulphate was shown in the regions where sulphate cloud production dominates the quenching mechanism by sea salt. Depending on the seasons, some other locations such as Mediterranean and North Sea also show some degree of sulphate enhancements. [32] However, the impact of sea salt on the sulphate concentrations and loadings through clouds is much more complicated than what we discussed in this paper. Sea salt can change the cloud droplet number concentration and hence the precipitation strength and patterns. The changes in precipitation also impact the removal rate of sulphate in the atmosphere Impact of Sea Salt on Global Cloud Droplet Number Concentration (CDNC) [33] The impact of sea salt on sulphate also affects cloud condensation nuclei (CCN) and potentially indirect radiative forcing. Using aircraft measurements over the North Atlantic and east Pacific and a 1-D parcel model, O Dowd et al. [1999] found that the presence of sea salt CCN reduces the influence of nss sulphate aerosol on cloud droplet concen-

14 AAC 4-14 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE Figure 13. Comparison of (a) the total volume (sea salt + sulphate) size distributions for latitudinal points from north to south at 140 W of Pacific Ocean to (b) the observation results in the same region [Quinn et al., 1996]. Each curve in Figure 13a represents a size distribution averaged over a 3-day simulation period in April. The two vertical lines in each plot mark the agreement of the two peak diameters in both simulations and observations. trations. Parameterizations were developed from the multiple 1-D parcel model simulations with mixed sea salt and sulphate aerosols to yield the aerosol-droplet relationship: D ¼ A 1 1 e A2A : ð3þ 8 >< A 2 ¼ >: 2 3:327 0:00682U þ 0:0051U :481 0:04922U þ 0:00527U 1000 w ¼ 0:1ms 1 w ¼ 0:175ms 1 ð5þ where A is the subcloud aerosol number concentrations (cm 3 ) and D is the cloud droplet number concentration (cm 3 ). This treatment represents the cloud droplet concentration as a function of sulphate concentration and wind speed (sea salt). The parameters A 1 and A 2 were given for two updraught velocities (w) as functions of wind speed (U, m s 1 ): 8 < 225:70 1:909U 0:164U 2 w ¼ 0:1ms 1 A 1 ¼ : 365:16 2:038U 0:438U 2 w ¼ 0:175ms 1 ð4þ [34] This parameterization was applied to case A and case B simulation results. Since the wind speed in the parameterization is an indication of the sea salt strength over the marine atmosphere, we have used an empirical formula to convert the sea salt concentrations from the model to wind speed (U) in order to correctly account for the impact of sea salt on the continental cloud droplet numbers. The spatial distribution of average seasonal CDNC calculated by using equation (3) from case B of mixed sea salt and sulphate number concentrations at a height of about 400 m for w = m s 1 is shown in Figure 15. The most striking feature is the well-known sharp contrast between maritime Figure 14. Sulphate mass mean diameters (MMD) in spring (MAM) for two cases of simulation.

15 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE AAC 4-15 Figure 15. Average cloud droplet number concentrations predicted from the mixed aerosol simulations (case B) for a height at about 400 m above the sea level for four seasons. and continental CDNC. Cloud droplet concentrations over continental regions are more than two to four times the values in clouds over maritime areas, which is consistent with the satellite derived data [Han et al., 1998]. This contrast is directly linked to the expected effect of higher concentrations of sulphate aerosols on clouds. A substantial seasonal variation in CDNC was also predicted from the mixed sea salt and sulphate simulations. CDNC was highest in northern spring and lowest in northern summer. [35] The average seasonal CDNC from case A and case B was used to calculate the relative impact (RI) defined by equation 2 (Figure 16). It is evident that the dominant impact of the sea salt aerosols in the atmosphere is to reduce the CDNC through suppression of the peak supersatuation achieved within the cloud [O Dowd et al., 1999]. Over the midlatitude marine boundary layer, a reduction of 20 to 60% in CDNC is predicted with greatest reductions in the roaring 40s of the Southern Hemisphere (40 to 60%) and midlatitude Northern Hemisphere (20 to 40%) where the sea salt concentrations were high. The impact on the continental CDNC was insignificant. The spatial distributions of the RI are consistent with those for surface sulphate mass mixing ratios (Figure 3). It should be emphasized that the RI for CDNC takes the impacts of sea salt on both sulphate aerosol number concentration and on aerosol activation into consideration since in case B simulations the sulphate aerosol has been impacted already by sea salt. [36] Figure 16 also shows some regions with negative RIs, indicating the enhancement of sea salt aerosols on the total CDNC. If one looks at equations (3), (4), and (5) carefully and considers a fact that A in equation (3) represents total aerosol number (sea salt and sulphate), there would be a threshold of sea salt and sulphate number concentrations where the reduction and enhancement of total CDNC would be equal. For wider updraught velocities and sulphate concentrations, an earlier study [Ghan et al., 1998] on the competition between sea salt and sulphate particles as cloud condensation nuclei has found that under all conditions increasing sea salt reduces the number of activated sulphate aerosols. However, the total number of droplets nucleated can either increase or decrease with increasing sea salt concentrations. This implies that under relative low sulphate number concentrations and strong updraught velocities, the total number concentration of activated cloud droplets may increase because of the activation of accumulation mode sea salt particles [Ghan et al., 1998]. Figure 16 clearly demonstrates this in a global simulation of sea salt impact on total CDNC. To realistically predict the total CDNC in a climate model, parameterizations for the impact of sea salt on the total number concentration of activated cloud droplets in a mixed sea salt and sulphate environment for wider updraught velocities are needed Implication to Air Quality and Climate Changes [37] From this analysis, it is evident that sea salt can serve as a natural cleansing agent for condensable pollutants such as sulphate and certain organic species. For example, the surface polluted aerosol plume (dominantly sulphate) arriving in Japan from China across the Yellow Sea and Sea of Japan is reduced by 10 to 20% by the presence of sea salt (Figure 3). The surface sulphate concentration in the North

16 AAC 4-16 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE Figure 16. Relative impact (RI, %) of sea salt aerosols on the CDNC for four seasons between case A and case B. See equation (2) for a definition of RI. Pacific is also reduced by 20 30% even though the reduction in the free troposphere may not be as large as on the surface. Thus when the polluted plume crosses the open ocean surface, sea salt is able to scavenge condensable species and by being more rapidly deposited reduce their concentrations. [38] The implication of sea salt impact on global sulphate allows us to address the biogenic sulphate contribution to the aerosol radiative forcing of climate. The CLAW hypothesis, suggested by Charlson et al. [1987], predicted that increased ocean temperatures from global warming would lead to increased production of dimethyl sulphide (DMS) and CCN, which would increase cloud albedo and thereby cool the Earth through increased reflection of incoming solar radiation, sea salt aerosols were not considered in the cycle. [39] These results indicate that due the presence of sea salt aerosols, the biogenic sulphate aerosols produced from DMS are significantly reduced (50 70%) in the high DMS regions. The feedback may not be as strong as previously thought. The results of the Aerosol Characterization Experiment (ACE-1) and other MBL experiments also indicate that the proposed CLAW hypothesis may not hold true. Because of the high concentration of sea salt particles in the MBL and their high ph [Keene et al., 1998], they can scavenge most of the SO 2 produced by the oxidation of DMS, and further convert it into nss sulphate [Sievering et al., 1999]. Since this nss sulphate is in the coarse aerosol mode, it is lost faster from the marine atmosphere through wet and dry deposition. Our global results combined with the sea salt aerosol chemistry scheme [Sievering et al., 1999] imply that the biogenic DMS aerosols will not counteract the effects of anthropogenically induced global warming as effectively as first suggested. [40] The impact of sea salt on air quality and climate is further envisaged by its reduction in the CDNC over the marine atmosphere. A reduced CDNC and hence an enlarged cloud effective radii will change the cloud albedo and its radiative forcing, which can have a significant climate impact. It also enhances precipitation by counteracting the precipitation suppression effect [Rosenfeld, 2000] of anthropogenic aerosols from the continents. A recent observation during INDOEX campaign has proved such impact [Rosenfeld et al., 2002]. Because of the precipitation enhancement by sea salt, an indirect cleansing effect of sea salt is obtained by more removals of pollutants from the atmosphere. 4. Conclusions [41] A global simulation of mixed sea salt and sulphate aerosols was carried out using the Canadian Aerosol Module in Canadian GCMiii. Reasonable agreements were found between model predictions and observations in spatial and temporal sulphate concentrations, number and volume size distributions. The point comparison of model predicted and observed sulphate concentrations are generally within a factor of two. There is an overestimate of SO 2 concentrations. [42] A differential simulation of the mixed aerosols shows a significant impact of sea salt on sulphate concentrations and size distributions. Sea salt aerosols increase the mass mean diameter of sulphate aerosols by up to a factor of two over the MBL with high sea salt concentrations and reduce

17 GONG AND BARRIE: IMPACT OF SEA SALT ON NSS SULPHATE AAC 4-17 the global sulphate aerosol mass in the MBL from 5 to 75% depending on the sea salt mass distributions. The strongest impact of sea salt on sulphate is in the southern roaring 40s with a value of 50 to 75%. In the downwind-polluted North Pacific and Atlantic Oceans, the sulphate concentrations are reduced by 20 to 30%. Surface sulphate reduction of 10 to 20% in Japan could attribute to the presence of sea salt aerosols. Very little impact was found in the equatorial regions where sea salt is at a minimum. [43] Quenching nucleation, enhancing condensation in the MBL and modifying cloud production of sulphate, sea salt aerosols play an important role in regulating the impact of biogenic sulphur on climate. The reduction of 50 to 75% in sulphate concentrations by sea salt aerosols in the MBL suggests that biogenic sulphur feedback of the CLAW hypothesis is weaker because of sea salt. The impact of sea salt on the global annual sulphate mass and number loadings is estimated to be 9.13% and 0.76%, respectively. [44] The indirect impact of sea salt is also included in this study as it has the potential to affect the sulphate activation to form cloud condensation nuclei and its size distributions [Pszenny et al., 1998]. It is found that over the marine boundary layer, a reduction of 7 to 60% in cloud droplet number concentrations was predicted because of the presence of sea salt with greatest reductions in the roaring 40s south of 40 to 60% and in midlatitude north of 20 to 40% where the sea salt concentrations were high. The impact on the continental cloud droplet number concentrations was not significant. 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