Size-segregated budgets of soil dust aerosols in Asia for spring 2001 during ACE-Asia were investigated using the NARCM model [Gong et al., 2003b]. Simulated mass size distributions of dust deposition showed a similar size distribution to the dust emission fluxes over the source regions and a decreased peak corresponding to a 1–3 μm diameter range over downwind regions. The simulations suggest that dry deposition was a dominant dust removal process near the source areas and the removal of dust particles by precipitation was the major process over the trans-Pacific transport pathway, where wet deposition exceeded dry deposition by up to a factor of 10. The Asian dust deposition from the atmosphere to the North Pacific Ocean was correlated not only with precipitation over the North Pacific but also with the dust transport patterns. Variations of monthly Asian dust outflow were identified with the latitudinal center of transport at 38°N in March, 42°N in April, and 47°N in May. The monthly trans-Pacific transport patterns of Asian dust in spring were characterized. The transport axis extended around 30°N and 40°N from the east Asian subcontinent to the North Pacific in March. A zonal transport pathway around 40°N was well developed in April over the North Pacific and reached North America. However, the transport in May was separated into two pathways: an eastward zonal path over the North Pacific and a meridional path from the source regions to the northeast Asian continent. On the basis of the averaged dust budgets during spring 2001, it was found that the major sources of Asian dust were located in the desert regions in China and Mongolia with an estimated dust emission of 21.5 tons km−2, and the regions from the Loess Plateau to the North Pacific were sinks of soil dust aerosols with the Loess Plateau as the main sink for Asian dust.
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 Because of the atmospheric and geochemical importance of Asian dust storms, an intensive field study was phased with Aerosol Characterization Experiment (ACE)-Asia during spring 2001. This included measurements of many types of aerosol particles of widely varying composition emitted by human activities and industrial sources, as well as wind-blown dust at a network of ground stations along the coast of China, Japan and Korea. These measurements were used to quantify the chemical, physical and radiative properties of aerosols in the ACE-Asia study area and assess their spatial and temporal (seasonal and inter-annual) variability. Surface measurements of soil dust were made to characterize aerosol properties in Chinese source regions as a part of ACE-Asia. These also provided size distributions of Asian dust emission to the numerical models [Zhang et al., 2003]. To understand the transport mechanism of Asian dust storms, a regional climate model with a size-distributed active aerosol algorithm (northern aerosol regional climate model (NARCM)) was used to simulate the production and transport of soil dust in east Asia and over the Northern Pacific Ocean [Gong et al., 2003b]. The model was driven by NCEP reanalyzed meteorological fields and included all relevant atmospheric aerosol processes (i.e., production, transport, growth, coagulation, dry and wet deposition and an explicit microphysical cloud module to treat aerosol-cloud interactions). A detailed soil texture data set and up-to-date desert distribution in China was introduced to drive the size-distributed dust emission module. Comparisons of NARCM-simulations with both ground based measurements in east Asia and North America, and satellite observations showed that the model captured most of the dust mobilization episodes during this period in China. It also produced reasonable simulations of the dust concentrations in source regions and downwind areas from eastern China to western North America. The vertical dust loading above 700 hPa correlated reasonably well with TOMS aerosol index (AI) observations [Gong et al., 2003b].
 NARCM simulation of ACE-Asia soil dust storms for spring 2001 yielded reasonable spatial and temporal distributions compared with observations. On the basis of the success of previous NARCM simulations [Gong et al., 2003b], we extend the focus here to the size distributed dust budgets including emission fluxes, and dry and wet deposition. The major objectives of this study are to (1) characterize the dust emissions over the source regions, (2) quantify dust deposition from east Asia, over the North Pacific to western North America, (3) to better quantify the budgets of the trans-Pacific dust transport, and (4) to present the meteorological transport pathways of dust outflow from east Asia to North America.
2. ACE-Asia Simulation
 In NARCM a size-segregated multicomponent aerosol mass conservation equation is expressed as follows [Gong et al., 2003a]:
In equation (1), the rate of change of mixing ratio of dry particle mass p in a size range i is divided into factor terms (or tendencies) for transport, sources, clear air, dry deposition, in-cloud and below cloud processes. The transport includes resolved motion as well as sub-grid turbulent diffusion and convection. The sources include (1) surface emission rate of both natural and anthropogeic aerosols and (2) production of secondary aerosols (i.e., airborne aerosol mass-produced by chemical transformation of their precursors). The latter together with partical nucleation, condensation and coagulation contribute to the clear-air process. Dry deposition of gases and particles affects the “dry” tendency. Scavenging in in-cloud and below-cloud processes is regarded as wet deposition of gases and particles.
 NARCM is composed of the Canadian regional climate model (RCM), a transport model, coupled with the Canadian aerosol module (CAM) [Gong et al., 2003a]. RCM uses the physics package from the Canadian global climate model (GCM) and a semi-Lagrangian and semi-implicit transport scheme for dynamics and passive tracers [Robert et al., 1985]. The meteorological boundary and initial conditions for RCM are driven with the NCEP reanalyzed meteorological data for spring 2001. NARCM runs on a stereographic projection with a horizontal resolution of 100 km at 60°N and 22 vertical levels on a Gal-Chen terrain following coordinate system from ground to about 30 km. The integration time step was 20 minutes. Twelve diameter classes from 0.01 to 40.96μm were used to represent the size distribution of soil dust (Table 1). All atmospheric aerosol processes including dust emission fluxes, concentrations and deposition were calculated for each size bin. A size distributed soil dust scheme [Alfaro and Gomes, 2001; Marticorena and Bergametti, 1995; Marticorena et al., 1997] in NARCM was modified and driven with a Chinese soil texture scheme that infers the size distribution with 12 categories and up-to-date desert distribution in China [Gong et al., 2003b]. Combined data sets for the desert disttribution/texture; satellite land-use/roughness length and observed soil moisture provide a coherent input parameter set for the soil dust emission scheme for deserts in east Asia. The model domain covered east Asia, the North Pacific and western North America (Figure 1). Five regions (A, B, C, D and E) were chosen in the domain to represent the source region and receptor regions in China, east Asia, North Pacific and western North Pacific, respectively. The southernmost edge of the Chinese receptor region was set at 28°N, beyond which no historical records of soil dust deposition were found [Zhang, 1984]. Region B was further divided into B1 and B2 representing the Loess Plateau and other parts in China. The NARCM-simulations with four mixed aerosols of soil dust, sulphate, sea-salt and black carbon were conducted for four months from 1 February to 31 May 2001. The detailed comparisons of NARCM-simulations with the surface observations and Total Ozone Mapping Spectrometer- Aerosol Index (TOMS-AI) are presented by Gong et al. [2003b]. They showed that the NARCM provided satisfactory simulations of spatial and temporal distributions of dust storms in source regions and long-range transport of Asian dust to the North Pacific and North America for spring 2001. Here simulations of March, April, and May are used to investigate Asian dust budgets including the size-distributed dust emission fluxes, dry and wet deposition and trans-Pacific transport.
Table 1. All 12 Size Bins and the Corresponding Aerosol Size in NARCM
Size Bin Number
Diameter Range, μm
3. Size-Segregated Soil Dust Emission Fluxes
Gong et al. [2003b] identified four major dust source regions in east Asia: (1) the Taklimakana desert in Xinjiang Province in west China, (2) Desert groups in west and middle Inner Mongolia of China, (3) the Onqin Daga and Horqin deserts in northeast Inner Mongolia of China and (4) the Gobi desert in Mongolia. Figure 2 shows the predicted distribution of total dust emission during spring 2001. This was calculated as the sum of emissions from all 12 size bins of soil dust. In addition to major Asian desert dust sources, there was also dust emissions included from coastal desert regions around the Pacific Rim, other arid areas on the Asian Continent and the desert areas of western North America. Major sources of Asian dust were located in the desert regions of China and Mongolia with an estimated dust emission of 21.5 tons km−2.
 A reasonable size distribution of the vertical dust flux is required to successfully simulate dust loading, transport and optical properties [Balkanski et al., 1996]. Ground based size-segregated aerosol observations (1994–2001) in the vicinity of the Chinese source regions were conducted using Battle type cascade impactors in the size ranges <0.25, 0.25 to 0.5, 0.5 to 1, 1 to 2,2 to 4, 4 to8, 8 to 16 and >16 μm in aerodynamic equivalent diameters. The impactors have a cutoff size of <0.25 μm for the background mode and >16 μm for the coarse mode. Between 0.25 and 16 μm, the mass size distributions were approximated by a lognormal fitting of two dust elements (Al and Si) from desert-sample data in China [Zhang et al., 2003]. Figure 3 shows the average size distributions of simulated vertical dust fluxes over Chinese deserts at three wind speeds. In this study, we have assumed that the size distributions of the vertical soil dust flux are the same as size distributions for surface soil dust measurements in the source regions. Therefore, the measured size distributions of surface dust emission are used as model input.
4. Dry and Wet Deposition
 Deposition is the major removal process for aerosol. In NARCM, deposition includes gas and particle dry and wet deposition with below-cloud and in-cloud scavenging [Gong et al., 2003a]. Figure 4 shows the averaged size distributions of dust deposition modeled over region A, B1, B2, C, D and E (Figure 1) for spring 2001. The dust deposition is size dependent (Figure 4). The resultant mass size distributions of dust deposition in source region A were similar to the size distribution of dust emission fluxes with a peak concentration around 5 μm in diameter. Over the dust transport pathway away from source region (region B, C, D and E) the size distributions of deposition showed decreased peaks corresponding to a 1–3 μm diameter. This agrees well with observations for locations remote from dust sources in Africa [Dulac et al., 1992], which might be expected to be reasonably representative of the global mean value.
Figure 5 shows the total dust deposition and ratio of wet to dry deposition of dust for 1 March to 31 May 2001 for all 12 size bins. The total dust mass deposition ranges between 0.05 and 500 tons km−2 over the NARCM domain. This range is of the same order of magnitude as that modeled by Tegen and Fung . Maximum deposition was over the dust source regions with most of the emitted dust redeposited in close proximity to the source. Over Asian source regions, which are arid with low precipitation (Figure 6), dry deposition is the dominant removal process. On the entire model domain total dry deposition was also greater than wet deposition with respect to dust mass deposition because a major part of dust deposition occurred over source areas (Table 1). Nevertheless, on the pathway of trans-Pacific dust transport, wet deposition exceeded dry deposition by a factor of ∼10 (Figure 5). Wet deposition as a function of precipitation is the major process of soil dust removal from the atmosphere to ocean in the North Pacific (Figure 5). Climatologically, precipitation increases from March to May in spring in the North Pacific. Because of this, wet deposition showed an increasing contribution to the total deposition from March to May. The modeled averaged fractions of wet deposition to total deposition over the North Pacific in the model domain (Figure 1) during spring 2001 rose from 74% in March, to 77% in April and to 81% in May. The geographic distribution of dust deposition (Figure 5) is similar to the pattern of precipitation, especially over the regions remote from dust sources (Figure 6). These results indicate that dry deposition is a dominant dust removal process near the source areas whereas the removal of dust particles by precipitation is the major process of dust deposition during the trans-Pacific transport of Asian dust.
 Dust storms in China are most frequent in April when approximately one third to one half of dust storms occur [Liu et al., 1985]. Similarly, the NARCM-simulation indicated that the monthly averaged dust emission over region A (Figure 1) was 5.36 tons km−2, 12.71 tons km−2 and 3.43 tons km−2 in March, April, and May 2001, respectively. However, the monthly averaged dust deposition in the North Pacific Ocean was 64.19 kg km−2 in March, 61.66 kg km−2 in April and 48.92 kg km−2 in May 2001. This difference between dust emission from the source regions and dust deposition in the North Pacific is caused by changes of dust transport patterns. These are governed by seasonal changes in the atmospheric general circulation, especially emergence of a trough in the mid-latitude westerlies extending from the Asian continent to the North Pacific [Gong et al., 2003b]. Asian dust deposition from the atmosphere into the North Pacific Ocean is therefore related not only to precipitation over the North Pacific, but also to the dust transport patterns. The trans-Pacific transport pattern of Asian dust will be discussed in the next section.
5. Asian Dust Budget and Trans-Pacific Transport
Table 2 presents modeled dust budgets (balance of emission and deposition) over regions A, B1, B2, C, D and E. During the months of March, April, and May 2001 a regionally averaged total of 21.5 tons km−2 of dust from region A was emitted into the model domain. Although more than half of the emitted dust particles were redeposited onto Region A (dry deposition of 11.72 tons km−2 and wet deposition of 1.35 tons km−2) Region A was the major Asian dust source. It had the largest contribution of dust loading to the atmosphere with a net dust export of 8.42 tons km−2. The regionally averaged dust emissions over region B1, B2, C and D were produced only from the local arid, semiarid or coast desert areas there. From averaged dust budgets, however, it is apparent that there were negative dust balances between emission and deposition over region B1, B2, C and D (from the Loess Plateau to the North Pacific). This indicates that these areas are net sinks for Asian soil dust. Region B1 (Loess Plateau) is the principal sink for Asian dust (largest negative dust balance). Asian dust storms originating mainly from region A (western and northern China and Mongolia), are widely considered to be the major sources for Asian dust contributing to loess matter deposited on the Loess Plateau and marine sediments in the North Pacific [Blank et al., 1985; Liu et al., 1985; Merrill et al., 1994; Merrill et al., 1989; Prospero, 1981; Zhang et al., 1996]. They also play an important role in trans-Pacific dust transport and biogeochemical links between land, atmosphere and ocean in the Northern Hemisphere. The desert areas in western North America (Figure 1), another major dust source in the North Hemisphere [Orgill and Sehmel, 1976], are responsible for the regionally averaged positive dust balance between emission and deposition in region E.
Table 2. Dust Emission and Removal by Dry and Wet Deposition in Spring 2001
Dry Deposition, ton/km2
Wet Deposition, ton/km2
 As the prevailing winds over the model domain in the mid-latitude free troposphere are westerly, most dust transport from east Asia is expected to be zonal. Therefore the product of dust concentration and zonal wind component U (i.e., zonal dust transport flux) can be used to estimate the amount and direction of atmospheric dust transport. Positive (negative) dust transport fluxes indicate eastward (westward) transport of dust aerosol. Figure 7 illustrates the monthly averaged dust exports from east Asia through 130°E from March to May. Most of the dust export occurred in the lower troposphere between 1 km and 3 km with the center of dust export moving from about 38°N in March (Figure 7a), to 42°N in April (Figure 7b) to 47°N in May (Figure 7c). This northward movement of maximum dust export corresponds with the northward motion of the westerly jet in the North Hemisphere from March to May. Figure 8 shows the altitude-latitude dependence for the averaged zonal dust transport fluxes in spring at 140°W entering North America. Simulated Asian dust transport peaks at around 6 km between 40°N and 45°N over the eastern North Pacific. Observations over North America during a particularly intense dust transport event in April 1998 indicate that the dust layer was concentrated between 5 and 10 km [Husar et al., 2001; Tratt et al., 2001]. The NARCM simulation for spring 2001 is therefore in broad agreement with previous observations of the vertical distribution of Asian dust over North America.
 Regional-scale transport of Asian desert is dominated by surface-level winds of the Asian winter monsoon [An et al., 1990]. The Asian winter monsoon is characterized by the invasion of dry cold air with strong surface winds over east Asia [Ding, 1994]. Before dust from Asian source regions reaches the Pacific Ocean or south China under the influence of northwesterly winter monsoon winds, it is lifted to the free troposphere and carried by the westerly winds that are typical of the northern mid-latitudes in the springtime (Figure 7 and 8). Trans-Pacific transport patterns are closely associated with the global-scale variations of atmospheric circulation, especially of westerlies in the mid-latitudes. Most trans-Pacific dust transport occurs in the middle troposphere, where the zonal dust transport fluxes are at a maximum. Figure 9 shows the modeled distributions of monthly averaged zonal dust concentration fluxes at 6000 m in the middle troposphere in March, April, and May. In March, the main axis of dust transport extended from around 30°N and 40°N from the east Asian subcontinent to the mid-Pacific (Figure 9a). Asian dust was transported eastward across the North Pacific, but not sufficiently far to reach North America, and deposited into the Pacific Ocean. This explains the high rate of deposition in the North Pacific in March. In April, zonal transport around 40°N was well developed over North Pacific (Figure 9b). Climatologically, stronger mid-latitude winds prevailed in the middle troposphere between 40°N and 45°N, and zonal circulation dominated between east Asia and the west Pacific. This is an ideal situation for trans-Pacific transport of Asian dust [Husar et al., 2001]. In May, the dust transport was separated into two pathways: an eastward zonal path over the North Pacific and a meridional path from China to the Northeast Asian continent (Figure 9c). The larger dust transport was on the meridional path. This transport pattern in May weakened the strength of dust transport and deposition over the North Pacific. It should be noted in Figure 9, that only Asian dust was engaged in this high level long-range transport across the Pacific; dust from the other emission sources in the model domain (Figure 2) only influenced the surface-level.
 Size segregated Asian dust emission, deposition and budgets associated with trans-Pacific transport during ACE-Asia 2001 were investigated with the NARCM transport model. The analyses lead to the following conclusions.
 1. The modeled average distribution of dust emission during 2001 ACE-Asia was in agreement with observations when the input size distribution of dust emission flux was derived from the observed size distribution of surface dust in Chinese deserts. Simulated mass size distributions of deposition in the source region were similar to the size distribution of dust emission fluxes. Over the dust transport pathway far from the source region, the size distributions of dust deposition showed a decreased peak corresponding to a diameter of 1–3 μm.
 2. Dry deposition is the dominant removal process over the source regions that are arid with low precipitation. On the pathway for trans-Pacific transport where precipitation is greater (far from the source regions and in the North Pacific), wet deposition exceeded dry deposition by up to a factor of 10. The geographical distribution of dust deposition is similar to the pattern of precipitation, especially over the regions downwind from the dust sources. The monthly averaged fractions of wet deposition in total deposition over the North Pacific during spring rose from about 74% in March to 81% in May. Wet deposition is the major process of soil dust removal from the atmosphere over the North Pacific.
 3. The monthly averaged Asian dust outflow from the east Asian subcontinent showed peak transport from 38°N in March, 42°N in April and 47°N in May, respectively. Dust was lifted from below 3 km in the lower troposphere in east Asia to around 6 km in the middle troposphere over the North Pacific and western North America. In March, the transport axis extended around 30°N and 40°N from east Asia to the North Pacific while in April a zonal transport pathway around 40°N was well developed over the North Pacific and reached North America. In May, the transport was separated into two pathways: an eastward zonal path over North Pacific and a meridional path from the source regions to the Northeast Asian continent.
 4. Budget-wise, the desert areas in western and northern China and Mongolia, where the regionally averaged dust emission was 21.5 tons km−2 with a net export of 8.42 tons km−2 were found to be the largest contributor to the dust transport. The regions from the Loess Plateau to the North Pacific are shown to be dust sinks with the Loess Plateau being the principal sink of Asian dust with a net dust receipt of 5.84 tons km−2.
 In view of the current state of numerical modeling studies on Asian dust transport [Kotamarthi and Carmicheal, 1993; Wang et al., 2000; Xiao et al., 1997], especially with respect to trans-Pacific transport, the NARCM modeling study described herein represents a first attempt to fully characterize Asian dust budgets in order to better understand trans-Pacific transport mechanisms. The success shown for the ACE-Asia period implies that NARCM could be used to simulate Asian dust transport, not only in operational mode, but also for past climatic conditions.
 The authors wish to thank the Canadian Foundation for Climate and Atmospheric Sciences (CFCAS) for their financial support for this research. This research was also supported by grants from Chinese Academy of Sciences (KZCX2-305), the National Key Project of Basic Research (G2000048703) and Nature Science Foundation of China (49825105, 40121303).