ACE-ASIA: Regional Climatic and Atmospheric Chemical Effects of Asian Dust and Pollution

367MARCH 2004AMERICAN METEOROLOGICAL SOCIETY | N o other region on Earth is as large and diverse a source of aerosols (and trace gases) as the Asian continent. In spring, when storm and frontal ac- tivity in Asia is most prevalent, industrial pollution, biomass burning, and mineral dust outflows produce an extraordinarily complex regional aerosol mix, composed of inorganic compounds (such as salts of sulfates and nitrates), organic carbon (OC), black car- bon (BC or soot), mineral dust, and water. Yet there are few data on the chemical, physical, and optical properties of Asian dust and pollution aerosols, espe- cially as a function of altitude above the surface. Sev- eral important questions relevant to the chemical and climatic effects of Asian aerosol outflow remain to be ACE-ASIA Regional Climatic and Atmospheric Chemical Effects of Asian Dust and Pollution BY JOHN H. SEINFELD, GREGORY R. CARMICHAEL, RICHARD ARIMOTO, WILLIAM C. CONANT, FREDERICK J. BRECHTEL, TIMOTHY S. BATES, THOMAS A. CAHILL, ANTONY D. CLARKE, SARAH J. DOHERTY, PIOTR J. FLATAU, BARRY J. HUEBERT, JIYOUNG KIM, KRZYSZTOF M. MARKOWICZ, PATRICIA K. QUINN, LYNN M. RUSSELL, PHILIP B. RUSSELL, ATSUSHI SHIMIZU, YOHEI SHINOZUKA, CHUL H. SONG, YOUHUA TANG, ITSUSHI UNO, ANDREW M. VOGELMANN, RODNEY J. WEBER, JUNG-HUN WOO, AND XIAO Y. ZHANG A large international field experiment and use of transport modeling has yielded physical, chemical, and radiative properties of the abundant aerosols originating from Asia. AFFILIATIONS: SEINFELD AND CONANT—Departments of Chemical Engineering and Environmental Science and Engineer- ing, California Institute of Technology, Pasadena, California; CARMICHAEL, SONG, TANG, AND WOO—College of Engineering, University of Iowa, Iowa City, Iowa; ARIMOTO—CEMRC, New Mexico State University, Las Cruces, New Mexico; BRECHTEL— Brechtel Manufacturing, Inc., Hayward, California; BATES AND QUINN—NOAA Pacific Marine and Environmental Laboratory, Seattle, Washington; CAHILL—Department of Chemical Engineering, University of California, Davis, Davis, California; C LARKE, HUEBERT, AND SHINOZUKA—Department of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii; DOHERTY— Joint Institute for the Study of the Atmosphere and Oceans, University of Washington, Seattle, Washington; F LATAU—Naval Research Laboratory, Monterey, California; KIM—Meteorology Research Institute/KMA, Seoul, Korea; MARKOWICZ—Institute of Geophysics, Warsaw University, Warsaw, Poland; L. M. RUSSELL AND VOGELMANN—Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California; P. B. RUSSELL—NASA Ames Research Center, Moffett Field, California; SHIMIZU—National Institute for Environmental Studies, Tsukuba, Japan; UNO—Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan; W EBER—School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia; ZHANG—Chinese Academy of Science, Beijing, China CORRESPONDING AUTHOR: Dr. John H. Seinfeld, Mail Code 210-41, California Institute of Technology, Pasadena, CA 91125 E-mail: seinfeld@caltech.edu DOI: 10.1175/BAMS-85-3-367 In final form 19 September 2003 ©2004 American Meteorological Society

368 MARCH 2004| answered. For example: to what extent are dust and pollution particles internally mixed, and what impact does this have on their optical and hygroscopic prop- erties? To what extent does dust serve as a surface for uptake of gases like SO 2 and HNO 3? Can chemical transport models simulate the optical properties and layering of different aerosol types sufficiently well to compute their chemical and radiative impacts? From 5 to 15 April 2001, a massive dust storm oc- curred over the Asian mainland (Fig. 1) (Holden 2001). Dust storms in east Asia occur most frequently in the spring as a result of the combined effects of low rainfall, increased occurrence of high winds associ- ated with cold fronts, and freshly tilled soil for spring planting. The dust outbreak of 5–15 April 2001 was among the largest on record, interrupting transpor- tation schedules and disrupting daily activities. The position of the polar jet over midlatitudes, the asso- ciated strong surface winds, and the accompanying de- velopment of a midlatitude cyclone spawned the dust storm. The maximum sur- face winds during this time period occurred near 45°N, one of the major source re- gions for Asian dust. At the time of this dust outbreak, the Asian Pacific Regional Aerosol Char- acterization Experiment (ACE-Asia) field studies were being conducted (in- formation available online at http://saga.pmel.noaa. gov/aceasia/ ) (Huebert et al. 2003). While previous field experiments were conducted in the western Pacific to study the chem- istry and aerosols coming off the Asian continent (Arimoto et al. 1996; Carmichael et al. 1996; Hayami and Carmichael 1998; Kim et al. 1998a,b), this international experi- ment, involving multiple aircraft, ships, satellites, and surface sites, obtained the most comprehensive measurements ever of hemispheric aerosol emis- sion and transport. The data acquired during ACE- Asia allow a first-time assessment of the regional cli- matic and atmospheric chemical effects of a conti- nental-scale mixture of dust and pollution. This article reviews what information is required to assess aerosol impacts over a continental or hemispheric scale, describes how ACE-Asia addressed the issues, and summarizes what the multidisciplinary studies showed. PHYSICAL, CHEMICAL, AND OPTICAL PROPERTIES OF ASIAN AEROSOL. Figure 2 shows estimated dust emissions (black areas) from eastern Asia over the period from 1 to 20 April 2001. Mineral particles are emitted into the atmosphere as a result of high surface winds and can be transported aloft through boundary layer convection and vertical motions associated with frontal boundaries. Natural FIG. 1. The Sea-viewing Wide Field-of-View Sensor (SeaWiFS) image for 7 Apr 2001 illustrates the complexity of the aerosol distributions during large dust storms. Dust (shown in yellowish brown) is transported, along with the intense low pressure system, horizontally and vertically into the center of the low. Ahead of the front, urban pollution (brownish gray area south of the dust) is transported across Korea and Japan. During this period, widespread fires were burning in Southeast Asia, and carbonaceous aerosols associated with these fires were transported away from the continent and toward Japan in the warm sector ahead of this large cold front. Over the Pacific, these aerosols are higher than the clouds. The composite TOMS aerosol index (AI) for 7–9 Apr 2001 (upper left corner) is indicative of absorbing aerosol (either dust or black car- bon). The high values of AI (indicated by dark red) over east Asia are due to dust, while those in Southeast Asia are due to biomass burning. The TOMS image shows clearly the two waves of dust associated with the storm.

369MARCH 2004AMERICAN METEOROLOGICAL SOCIETY | dust emission areas are defined as desert and semi- desert areas in the United States Geological Survey (USGS) vegetation database [based on Advanced Very High Resolution Radiometer (AVHRR) data obtained in 1992/93]. By using this method, large parts of the Gobi and Taklimakan Deserts are indexed as source regions. The Loess Plateau and the Nei Monggol’s small desert were further assigned as dust source ar- eas based on Total Ozone Mapping Spectrophotom- eter (TOMS) aerosol index (AI) climatologies. Snow cover data are used to mask predicted emission areas (e.g., many parts of the Himalayan mountain range), providing a seasonal element to dust emissions. Com- plete details of the algorithm used in estimating the dust emissions can be found in Uno et al. (2003). Approximately 50 teragrams (Tg) of dust is esti- mated to have been emitted during this period from the arid and semiarid regions of western China, north- ern China and Mongolia, the loess regions west of Beijing, China, and the drought-stricken regions in Liaoning Province (northeast of Beijing) (Uno et al. 2003). The BC emissions from biomass burning (red areas in Fig. 2) during this same period are estimated as ~ 0.05 Tg, with an additional 0.1 Tg from fossil and biofuel combustion. Emission rates of BC from bio- mass burning were based on regional estimates of bio- mass burned and AVHRR fire counts. Fossil and biofuel emission estimates are based on Streets et al. (2001). Sulfur emissions from combustion are esti- mated as 0.95 Tg (S). Aerosols collected in the dust source region near Zhenbeitai, China (38°37¢N, 109°46¢E; see Fig. 2), es- tablished the elemental composition of the dust. These samples were collected during April 2001 over nomi- nal 24-h intervals using an Interagency Monitoring of Protected Visual Environments (IMPROVE)-type sampler, operating at a flow rate of ~ 11.5 l min -1. Acid digests (HF, HCl, and HNO3) of the GN-4 Metricel fil- ters (Pall Gelman Sciences, Inc.) were analyzed by in- ductively coupled plasma mass spectroscopy. The ra- tios of 25 elements (Al, Ba, Ca, Ce, Co, Dy, Er, Eu, Fe, Gd, K, La, Li, Mg, Mn, Na, Nd, Pr, Sc, Si, Sm, Sr, Th, Ti, and U) to Al in the aerosol samples were within 30% of those certified or measured in a loess refer- ence material (Nishikawa et al. 2000), demonstrating a chemical connection between the dust and eolian FIG. 2. Dust emissions fluxes (black) and AVHRR fire counts (red) over the period 1–20 Apr 2001. Sur- face measurement sites discussed in the text are shown, along with the location of the R/V Ronald H. Brown, and the flight tracks of the C-130 and Twin Otter aircraft on 8 and 12 Apr. Numbers next to the tracks indicate Apr dates. Near-surface three-dimensional trajectories were calculated by the RAMS meteorological model, as described in Uno et al. (2003). At the time that the R/V Ronald H. Brown ob- served the dust storm on 11 Apr it was positioned near the tip of South Korea, where trajectories were very similar to those at Gosan.

370 MARCH 2004| sedimentary material. In contrast, a second group of elements (Ag, Bi, Cd, Cr, Cu, Hg, Mo, Ni, Pb, Sb, Tl, V, and Zn) was uncorrelated with Al, and these ele- ments displayed ratios to Al ranging from 6 to 100 times those expected from the Asian mineral dust standard, most likely reflecting pollutant emissions (Nriagu and Pacyna 1988). The detection of Tl is es- pecially noteworthy because Tl is emitted by coal-fired power plants, smelting operations, and cement plants, all of which are important sources of pollutants in Asia. These elemental data indicate that even in the dust source region itself, significant quantities of pol- lutants can become mixed with the mineral dust. Lidar images of the normalized aerosol backscat- ter, extinction coefficient, and depolarization ratio at Beijing show the advance of the storm over 6– 15 April (three left panels of Fig. 3). Automated Mie scattering lidar (Sugimoto et al. 2003) was used to de- termine vertical profiles of backscattering intensities and depolarization ratios. Extinction coefficients for 532 nm were derived using Fernald’s method (Fernald 1984). Lidar measurements of atmospheric depolarization can be used to distinguish between spherical and nonspherical particles. The depolariza- tion ratio of a linearly polarized laser beam backscattered from spherical particles is zero, so the magnitude of the ratio is indicative of the amount of nonspherical (in this case, dust) particles. The early April dust storm consisted of two distinct events. Dust (as indicated by the region of enhanced depo- larization) arrived over Beijing in a layer between 2 and 4 km on 6 April, where it overrode a sulfate-rich pollution layer (as evidenced by high backscatter and extinction in this near-surface layer). The dust layer descended to the surface during 7–8 April when op- tical extinctions reached values up to 0.3 km -1. After the dust-overriding pollution layers passed on 8 April, a new pollution layer, rich in sulfate and car- bonaceous aerosol, was transported from southwest- ern China to Beijing (as seen by the high extinction and backscatter early on 9 April). The second dust wave reached Beijing on 9 April. Dust in the leading edge of this front extended up to 6 km; behind the front the dust was confined largely to heights below 2–3 km, where it was mixed with polluted air, as shown by the model results where the enhanced ob- served extinction in the boundary layer on 10 April is attributed to high levels of BC, sulfate, and dust. During ACE-Asia, several global and regional chemical transport models were used in the field to help in mission planning. One such model was the Chemical Weather Forecast System (CFORS; Uno FIG. 3. (left) Lidar images over Beijing. (right) Time–height profiles of calculated mass distributions of (bottom) dust, (middle) sulfate, and (top) black carbon for Beijing using the CFORS model. Abcissa shows data (UTC) indicating day in Apr 2001.

371MARCH 2004AMERICAN METEOROLOGICAL SOCIETY | et al. 2003). CFORS is a multitracer, online system built within the Regional Atmospheric Modeling Sys- tem (RAMS) mesoscale meteorological model (Pielke et al. 1992). In CFORS multiple tracers are run online in RAMS, so that all the online meteorological infor- mation, such as 3D winds, boundary layer turbulence, surface fluxes, and precipitation amount, are available at every time step to produce high-time-resolution 3D tracer fields. CFORS treats size-resolved mineral dust using 12 particle bins (ranging from 0.1 to 20 mm in radius) and a wide variety of tracers: 1) important anthropogenic species (SO 2/SO4, CO, black carbon, organic carbon, fast and slow reacting hydrocarbons, and NOx), 2) species of natural origin (yellow sand, sea salt, radon, volcanic SO2), and 3) markers for bio- mass burning (CO, black carbon, and organic car- bon). The three right-hand panels of Fig. 3 show the vertical distributions of black carbon, sulfate, and dust over Beijing as predicted by the CFORS chemical transport model. The two events composing this dust storm stand out in the 3-day composite TOMS aero- sol index insert in Fig. 1. Over the course of the next 3–4 days, the wave of dust moved eastward away from the coast of China and was sampled aloft by aircraft and at numerous surface sites. During the 9–13 April dust storm, several research flights were conducted by the National Science Foun- dation C-130 and U.S. Navy Twin Otter to character- ize the vertical profiles of aerosol extinction, mass, and size. On 8 April, over the Sea of Japan, during the same dust storm observed at Zhenbetai and Beijing, the C-130 repeatedly flew in and out of an elevated river of dust rich in calcium, situated above a sulfate- rich pollution plume located near the surface down- wind of the Korean peninsula. These two aerosol fields were separated by an intermediate pollution layer near the temperature inversion at ~ 2.5 km. The ob- served complex structure was not uncommon and is best visualized in 3D plots of key parameters during the flight (Fig. 4); the complexity also demonstrates the challenges faced by three-dimensional atmo- spheric chemical transport models. Total extinction of incoming radiation by aerosols is the sum of scattering and absorption. The aerosol- scattering data in Fig. 4a obtained over the Sea of Japan reveal highest values near 4-km altitude to the north of the region, while aerosol absorption (Fig. 4b) is at a maximum near the surface to the south. The aerosol single scattering albedo (SSA; the ratio of in- cident radiation scattered to total extinction) is the commonly used measure of the relative contribution of absorbing aerosol to extinction and is a key vari- able in assessing the climatic effect of the aerosol. Its value depends both on the concentration and size dis- tribution of absorbing substances and how these are mixed with nonabsorbing aerosol material (Jacobson 2000). The strongest particulate absorber is BC; iron compounds present in dust also absorb shortwave radiation. The effective refractive index for average Asian dust was constrained by the asymptotic value of coarse absorption per unit volume as coarse vol- umes approached maximum values. This assessment included size-resolved corrections for soot associated with the coarse mode and for impactor transmission efficiency for a nominal size cut at 0.75 mm. The re- sulting dust refractive index was determined to be 1.53 – 0.0007i (± 0.0002) (Y. Shinozuka 2003, per- sonal communication). The range of dust distribu- tions measured during ACE-Asia yield calculated SSA values larger than 0.99 for submicrometer dust and about 0.97 ± 0.02 for coarse dust at 550-nm wave- length. The latter is also consistent with bulk SSA values inferred from differences between measured total and submicrometer scattering and absorption. The SSA (Fig. 4) approaches 0.97 in the upper lev- els of the north, where total extinction is completely dominated by scattering associated with dust, and about 0.85 near the surface to the south, where south- westerly transport of carbonaceous aerosol from re- gions impacted by biomass burning leads to a signifi- cant absorbing component. This high SSA for the dust-dominated aerosol aloft is consistent with recent similar values for Asian dust plumes crossing the Pa- cific and has been observed being entrained into the marine boundary layer (Clarke et al. 2001). These and similar cases observed aloft on the C-130 indicate that lower SSA values often found in the presence of Asian dust measured near the surface are generally a result of absorption by BC present in the accumulation (0.1– 1.0- mm diameter) mode. The ratio of the particle wet diameter at a given relative humidity (RH) to its dry diameter [denoted as f(RH)] is a measure of the amount of water that is taken up by the aerosol as RH increases. This prop- erty is important because particle swelling associated with increasing RH can strongly affect the scattering of solar radiation as the aerosol moves through dif- ferent ambient conditions. This measured hygro- scopic growth factor f(RH) from RH = 40% to RH = 80% (Fig. 4d) ranges from near unity aloft, where it indicates negligible growth due to water uptake by dust, to a maximum of 2.5 near the surface, indica- tive of strongly hygroscopic aerosol (e.g., NaCl). The elevated layer to the north exhibited high aerosol Ca 2+ (Fig. 4e), associated with large, coarse aerosol volumes (or mass) (Fig. 4g), both indicative of dust. In con-

372 MARCH 2004| trast, the highest aerosol sulfate is evident near the surface to the south where submicrometer aerosol volume (or mass) is at a maximum (Fig. 4h). The el- emental composition measured at 4 km by the C-130 on 8 April shows submicrometer aerosol composition, similar to that collected near Zhenbeitai, with nine elements (Al, Ca, Fe, K, Mg, Mn, Na, Sr, and Ti) within 30% of the loess- certified reference material. A con- trasting group of six elements (Cr, Cu, Ni, Pb, V, and Zn) indicated the presence of pollutants with ratios relative to Al that were enhanced over this dust composition by factors of 3 or more. The data in Fig. 4 reveal the close coupling between aerosol physical chemistry and associated optical ef- fects. The upper-level dust layer, with up to 1200 mg m -3 of dust (Fig. 4g; from 600 mm3 m-3, assum- ing a density of 2 g cm-3), has a total dry scattering intensity of near 200 Mm-1 (Fig. 4a). In contrast, the lower-altitude pollution plume has a pronounced accumulation mode volume (Fig. 4g) with a mass of about 30 mg m-3, including about 10 mg m-3 of soluble sulfate (Fig. 4f) and other as- sociated components, and exhibits dry scattering near 100 Mm -1 (Fig. 4a). However, when the differences in hygroscopic growth at the same RH are taken into account, the lower- level soluble aerosol produces an ambient scattering close to 200 Mm -1, similar to that of the up- per-level dust layer. Even within the elevated dust layer, sampling at 4-km altitude re- vealed the significant presence of submicrometer carbonaceous aero- sol. The organic carbon was domi- nated by saturated alkyl groups with an organic mass–to–organic carbon ratio of 1.3, resulting from absorp- tion by oxygenated groups. Organic mass–to–organic carbon ratios are based on composite analysis of or- ganic functional groups in submi- crometer particles by Fourier trans- form infrared spectroscopy (Maria et al. 2002). Elemental composition was determined by x-ray fluores- FIG. 4. Measurements aboard the NCAR C-130 on 8 Apr in the dust cloud over the Sea of Japan. This eight-panel plot is arranged into four panels of measured aerosol optical characteristics at 550 nm [(a) to- tal aerosol light scattering, (b) total aerosol light absorption, (c) aero- sol single scattering albedo, (d) f(RH), see text] and (e)–(h) four pan- els of the physiochemical properties that give rise to these charac- teristics. Observed aerosol (e) Ca 2+ and (f) SO4 2----- as a function of alti- tude along the flight track are based on rapid and continuous mea- surements with a new instrument (PILS) that captures particles (£££££1.5- mmmmmm diameter) directly into a small flow of purified water (Weber et al. 2001). Although the predominance of calcium occurred in larger particles (total calcium reached 19,000 pptv versus only 1600 pptv in particles £££££1.5 mmmmmm), the PILS data are still a reliable indication of dust because the tail of the dust distribution goes below 1.5 mmmmmm. (g) Coarse and (h) fine aerosol components, separated at aerodynamic particle diameter of 1 mmmmmm, are shown here as “dry” volumes as measured.

373MARCH 2004AMERICAN METEOROLOGICAL SOCIETY | cence of submicrometer particles on stretched Teflon filters. Trajectory analysis suggests this aerosol origi- nated from biomass burning in Southeast Asia. The lower-altitude carbonaceous aerosol was less aged and more closely correlated with local combustion- derived emissions of sulfate. In the above case, the river of dust near 4 km was largely isolated from the pollution layer. When in- termingling occurs, complex interactions are pos- sible. For example, high SO 2 and favorable conditions have resulted in estimated sulfuric acid production in the range from 0.2 to 1 × 10 6 cm -3 s -1 over the Yellow Sea (Weber et al. 2003). Under certain con- ditions, the H2SO4 can lead to production of new par- ticles. The H 2SO4 also rapidly deposits on particles, and such fluxes are consistent with observed mea- surements of aerosol growth. Sulfate and nitrate coat- ings on dust sampled in Beijing and Qingdao have been found to change the properties of dust, such as its hygroscopicity. Evidence of nitrate coatings on dust has also been observed in this region (Weber et al. 2001; Orsini et al. 2003). In the absence of dust, gas-to-particle conversion of sulfates and nitrates will occur on accumulation-mode particles; thus, when dust is present, gas-to-particle conversion affects both sub- and supermicrometer aerosol modes by re- directing much of the deposition that would normally occur on the accumulation mode to the larger dust particles. Shipboard measurements on the R/V Ronald H. Brown in the Sea of Japan (see Ronald H. Brown track in Fig. 2) complemented those of the C-130 during this episode (Fig. 5). Aerosol optical thickness (AOT, the integral of the aerosol extinction coefficient from the surface to the top of the atmosphere) exceeded 1.4 on 10 April (top left panel). On this day, emissions from the Miyakejima volcano (located 200 km south of Tokyo) were transported into the Sea of Japan, and elevated sulfate in this plume caused the enhanced extinction below 1000 m shown in the extinction panel in Fig. 5. Above 4 km, dust emitted from the Taklimakan region passed over the ship and contrib- uted to the elevated aerosol optical thickness. (The SO 2 emissions from the Miyakejima volcano are esti- mated to be 1.5 × 1010 g S day-1 during this period, an amount that was 50% of total anthropogenic sulfur emissions in East Asia.) At the sea surface, the SSA was 0.98, and the aerosol was dominated by sulfates (up to 30 mg m-3) and organics. The arrival of the sec- ond dust peak on 11 April at the ship was revealed by increases in total aerosol Al and Ca, peaking at ~ 13 and 7 mg m -3, respectively, on 12 April (top right panel). In this low-level outflow, the submicrometer portion of the dust, as determined by the measured ratio of sub- to supermicrometer Al and Ca, was sig- nificant (15%–30%). The two lower panels of Fig. 5 show aerosol extinction and total dust at the location of the ship, as predicted by the CFORS model. Surface observations of particle number size dis- tributions at Gosan, Korea (Fig. 6), also reveal the eastward progression of the dust event that arrived in Beijing early on 10 April. For descriptions of the in- strumentation used see Brechtel and Buzorius (2001) and Chun et al. (2001). Just before 0000 UTC 11 April, a striking increase in supermicrometer aerosol num- ber concentration marks the arrival of the dust storm at Gosan, which lasted 3 days. The same two extinc- tion maxima observed at Beijing on 8 and 10 April can be seen in the two-course mode maxima in the size distribution at Gosan on 10 and 12 April. The trans- ported aerosol reflected a mixture of pollution and dust with increases in super- and submicrometer mode area concentrations of 164% and 45%, respec- tively, compared to study averages and significant changes to ambient light extinction. The ACE-Asia project design established a net- work of surface sites that, with additional collabora- tors, covered approximately 40% of the earth’s cir- cumference. Size-resolved particle composition was measured at 15 sites, with 3-h time resolution. A se- quence of data at the Tango site, north of Kyoto, Japan, on a peninsula jutting into the Sea of Japan (see Fig. 2) from the 10-day period spanning the dust event (Fig. 7) complements the data presented above. Samples were analyzed for mass (soft beta ray), opti- cal transmission versus wavelength (320–820 nm), and elements (Na and heavier) by synchrotron x-ray fluorescence. Air trajectories arriving at Tango are similar to those shown in Fig. 2 for Gosan, Korea. Early in the event the air masses at Tango swept over central Japan and then abruptly switched direction and speed to reflect rapid transport of dust from the Taklimakan Desert. During the mid-April dust event, observations at numerous surface sites along the path of the dust storm demonstrate that optically impor- tant aerosols and coated dust particles, as well as pri- mary and secondary anthropogenic aerosol, cross the study region into the North Pacific and were detected as far away as Oregon (VanCuren and Cahill 2002). Heterogeneous reactions involving SO 2, NO2, and O3 on metal oxides and Chinese loess soils, in addi- tion to direct deposition of H 2SO4 and HNO 3, have been observed in the laboratory (Michel et al. 2002). As a result of the reaction of HNO 3 with Ca2+, the size of the aerosol nitrate was well correlated with that of soluble Ca during the postfrontal outflow of this dust

374 MARCH 2004| storm. Over the course of the project, just over 75% of the nitrate was on particles larger than 0.8 mm (B. Huebert 2002, unpublished manuscript). While some sulfate (up to 30%, but usually less than 5%) was also found in the larger particles, its size distribution typically peaked in the submicrometer range, where it was most strongly correlated with ammonium. This aerosol state apparently evolves from both the uptake of recent photochemically formed acidic gases and the progression of the urban aerosol to a more stable ther- modynamic state when mixed with alkaline dust. Measurements show that fresh urban plumes, iden- tified by high SO 2 concentrations, when blended with the dust in the postfrontal outflow, contained fine particles of ammonium sulfate and nitrate salts, and coarse alkaline dust (CaCO 3), with some Ca(NO 3)2. However, outside these fresh plumes, the more aged dust and pollution plumes still contain fine ammonium sulfate salts, but lack fine nitrate, suggesting that Ca(NO 3)2 was formed through the evaporation of the semivolatile ammonium nitrate. Thus, the different size distributions of sulfates and nitrates are attribut- able in part to the fact that SO 2 reactive uptake coef- ficients on metal oxides are roughly 0.1–0.01 of those for HNO3, especially under dry conditions (Grassian 2001), and that over time the plumes evolve to the most stable chemical forms. The reaction of nitric acid with calcareous dust particles is important because it provides a pathway to the formation of calcium nitrate, which is hygroscopic. Water uptake by dust particles increases their interaction with solar radiation and ren- ders them effective cloud condensation nuclei (CCN). Large quantities of aerosols are transported out of the region by these dust events. We estimate, using a three-dimensional model (Uno et al. 2003) that ap- proximately 14 Tg of dust, 0.4 Tg of sulfates, and 0.5 Tg of carbonaceous aerosol were transported east- ward across 130°E during the 5–15 April period, rep- resenting 28% and 42% of the total dust and sulfate emissions, respectively, in the region. These percent- ages reflect the preferential removal of dust relative to sulfates, which is consistent with the larger size of the dust particles. In Beijing alone, we estimate that ~ 67,000 tons of dust were deposited over the 11-day period of the dust storm. The flux of dust deposited to the ocean surface is estimated as 2.4 Tg during 5– FIG. 5. (top left) Measurements of AOT on the R/V Ronald H. Brown over 5–15 Apr show values exceeding 1.0 during the height of the dust storm. (top right) Al and Ca, measured onboard, also peak at this time. In most cases model predictions follow these data closely. (bottom) Predicted vertical distributions of (left) aerosol ex- tinction and (right) dust concentrations are shown corresponding to the Ronald H. Brown cruise track.

375MARCH 2004AMERICAN METEOROLOGICAL SOCIETY | 15 April and 8.7 Tg during all of March and April. Based on the measured dust composition of 4% Fe at Zhenbeitai, 0.1 Tg (Fe) (100,000 tons) was present in the dust during the dust storm and 0.35 Tg (Fe) for the spring of 2001. RADIATIVE ENERGY BALANCE OVER THE WESTERN PACIFIC. Unlike the long-lived greenhouse gases (GHGs), which tend to spread uni- formly over the globe, tropospheric aerosols have a lifetime of about 1 week, resulting in a distribution that is spatially and temporally inhomogeneous. In further contrast to GHGs, perturbations to the earth’s energy balance (radiative forcing) resulting from aerosols largely occur over and downwind of indus- trialized areas of the Northern Hemisphere, with sig- nificant impacts on regional energy budgets, evapo- ration, and precipitation. Regional aerosol forcings are often one order of magnitude larger than those of the GHGs (Ramanathan et al. 2001). For aerosols that exclusively scatter solar radiation, the increase in the reflected solar flux at the top of the atmosphere (TOA) is virtually identical to its reduc- tion at the surface, and, as a result, both the surface and the atmosphere cool. For particles that both scat- ter and absorb solar radiation, such as black carbon and mineral dust, the amount of solar radiation reach- ing the earth’s surface is likewise reduced, but the absorption of energy by these aerosols can heat the at- mosphere as well. Absorption of radiation by aerosols in the lower atmosphere, at the expense of absorption at the surface, may affect the temperature profile, at- mospheric stability, and relative humidity, all of which influence atmospheric dynamics and cloud formation. FIG. 6. (top) Electrical mobility and (bottom) optical particle (OPC) number size distributions measured at Gosan. Data are shown as timelines with particle size on the ordinate, time on the abscissa, and color contours repre- senting the number concentration at a given size. Three distinct dust events on 10–13, 19, and 24–25 Apr are evident in the coarse mode OPC data, with simultaneous decreases in submicrometer number concentrations, especially for particles larger than 150 nm and smaller than 30 nm. The noted behavior in the submicrometer range may be attributable to precipitation scavenging of particles in air heavily influenced by cloud processing in the frontal boundary transporting the dust.

376 MARCH 2004| Vogelmann et al. (2001) provide details on the radia- tive transfer code. The solar spectrum is divided into 38 wavelength bands from 0.2 to 4.0 mm. Aerosol optical properties used in MCRTM were determined from available observations of aerosol physical, opti- cal, and chemical properties made during ACE-Asia combined with standard aerosol optical properties (Hess et al. 1998; Kaufman et al. 2001). The single- scatter albedo of pure dust at 500 nm is 0.98, derived from ACE-Asia in situ measurements. Calculations represent a mean from 5 to 15 April for the region 20°–50°N, 100°–150°E. These calculations corroborate two major findings from other recent field campaigns conducted in the North Atlantic (Russell et al. 1999; Raes et al. 2000), Brazil (Eck et al. 1998), and the north Indian Ocean (Ramanathan et al. 2001; Satheesh and Ramanathan 2000). First, aerosol cooling can be comparable to or greater than human-induced greenhouse gas warm- ing of 2–3 W m -2 over vast regions downwind of an- thropogenic aerosol sources. Second, and more im- portantly, aerosols can exert a far greater influence on the surface and atmospheric energy budgets than on the TOA radiation budget because of their strong light-absorbing component (mainly black carbon). When BC is present, the mixing state of the atmo- spheric aerosol can have a profound effect on its ra- diative properties. Black carbon, which comprises approximately 6% of the submicrometer aerosol mass, may remain entirely separate from other par- ticles, or it may be mixed into them. This includes mixing in predominantly sulfate/OC aerosols and/or in mineral dust particles. To examine the sensitivity of radiative forcing to the state of aerosol mixing, we assume either that BC is separate or that 96% of the BC is internally mixed with sulfate/OC and 4% is in- ternally mixed with dust (Table 1). The coating of dust is calculated based on electron microscope analyses of Asian dust (Gao and Anderson 2001) that find a prominent “coated” mode of dust particles FIG. 7. Aerosol composition measured in several of the eight size ranges at the Tango site on the north coast of Japan. Air trajectory analyses show that five major source regions contributed to the observed data: 6–9 Apr slow trajectories over the Japanese inland sea; 10–11 Apr, same as the previous case plus the erupting volcano on Miyakejima, south of Tokyo; 12 Apr, a fast trajectory (6000 km in 4 days) from Siberia over the Taklimakan Desert in western China, and then over much of northern China; 13–14 Apr, somewhat slower trajectories from the Gobi Desert over Manchuria and Korea; and 15–16 Apr, slow trajectories from Beijing and Qingdao. (top) Calcium in three size modes, with the smallest size mode multiplied by 10.(middle) Calcium closely associated with sulfur in the same size mode, with relatively little silicon is a fingerprint for the Miyakejima volcano. The sharp increase in chlorine on 12 Apr resulted from sea salt NaCl transported in the high-wind regime to the site 6 km inland. Gobi dust arrived with anthropogenic elements and the highest accumulation mode sulfur seen in the entire 42-day experiment. (bottom) Highly correlated elements in the very fine mode (0.29–0.09 mmmmmm) previ- ously seen in studies of coal-fired power plants in the western United States. Radiative transfer calculations, using a Monte Carlo radiative transfer model (MCRTM) based on the predicted CFORS aerosol distributions, measured SSA, and clima- tological cloud conditions indicate that during the period of the dust outbreak aerosols cool the surface by 14 W m -2 and heat the atmosphere by 11 W m -2, resulting in a net climate forcing of –3 W m-2 over the region spanning 20°– 50°N, 100°–150°E (Table 1). Podgorny et al. (2000) and

377MARCH 2004AMERICAN METEOROLOGICAL SOCIETY | having a S:Si ratio of 2:50. Thus, each dust particle is coated with a 2.7% (by mass) layer of sulfate, organic carbon, and black carbon. Given that Si is assumed to comprise 26% of the dust mass, sulfur is assumed to be ammonium bisulfate, and OC and BC in the coating are taken from an assumed mass proportion of sulfate:OC:BC equal to 32:18:3. Under cloud-free conditions, when the aerosol is internally mixed, the predicted impact is an increase of aerosol absorption by 3.5 W m -2 relative to the externally mixed case (see Table 1), resulting in a change in surface flux of –2.2 W m -2 and a net change in TOA forcing of 1.3 W m-2. Clouds moderate the TOA forcing by aerosol scat- tering, and can either magnify or reduce the heating by aerosol absorption, depending on surface reflectivity and the vertical profile of aerosols relative to clouds. Climatological satellite data were used to constrain the regional albedo and the vertical distri- bution of clouds in MCRTM. The clouds present are predicted to moderate the TOA forcing by a factor of ~ 3 compared to clear-sky calculations, whereas at- mospheric absorption remains virtually unchanged. The predicted surface-to-TOA forcing ratio increases from 2.2 in the cloud-free case to 4.7 in the cloud- inclusive case. Ship and satellite observations of clear-sky radia- tive forcing support the regional predictions described above. Radiative forcing can be expressed as the prod- uct of two measured quantities: aerosol optical thick- ness and aerosol forcing efficiency (change in radia- tive flux per unit optical depth at 500 nm). Shipboard (R/V Ronald H. Brown) observations of these quan- tities show that the aerosol radiative forcing efficiency at the ocean surface varied from –50 to below –80 W m -2. The forcing is very sensitive to variations in aerosol composition—the days with the strongest forcing efficiencies were those with highest BC con- tent (lowest SSA). Mean 500-nm AOT in the Sea of Japan (7–13 April) was 0.43. The MCRTM/CFORS model is consistent with these observations, predict- ing a mean AOT of 0.45 and a clear-sky surface- forcing efficiency of –62 ± 5 W m -2 over the ship’s track in the Sea of Japan. Together, ship and satellite observations [from the Clouds and Earth’s Radiant Energy System (CERES) instrument on the Terra sat- ellite] show that the ratio of the surface-to-clear-sky TOA forcing is 2.1 ± 0.2, compared with the clear- sky-predicted value of 2.2. Studies of aerosol effects on the earth’s energy bud- get usually consider only the cooling effects at short (solar) wavelengths, but studies from the Ronald H. Brown demonstrate that they also have an important warming effect at thermal infrared (IR) wavelengths. Unique, high-resolution spectra obtained during the cruise were used to determine the aerosol greenhouse effect (IR radiative forcing) at the surface (Vogelmann et al. 2003) (Fig. 8). They found that the surface green- house effects are often a few watts per square meter and can reach almost 10 W m -2 for large aerosol load- ings. An IR model, developed using independent aerosol and scattering measurements, confirmed this result, by producing good agreement with these ob- servations, pyrgeometer downwelling fluxes, and IR satellite (CERES) measurements (Markowicz et al. 2003). This model was used to relate the IR aerosol forcing to the solar forcing; the surface infrared aero- sol radiative forcing is between 10% and 25% of the shortwave aerosol forcing, and at the top of the atmo- sphere is up to 19%. It is interesting to compare the radiative forcing measured during ACE-Asia to that seen during the Indian Ocean Experiment (INDOEX), which stud- ied industrial and biomass emissions from south Asia as they advect over the north Indian Ocean during the Asian winter monsoon (Satheesh and Ramanathan 2000). The surface forcing observed during ACE-Asia of –14 W m -2 is similar to that dur- ing INDOEX, but has significantly different control- ling factors. In comparison to the south Asian aero- sol plumes, the east Asian plume is characterized by increased radiative scattering by dust, more pro- Dust –9.3 3.8 –5.5 Sulfate –3.6 0.3 –3.3 Organic carbon –3.9 1.7 –2.2 Black carbon –4.1 4.5 0.4 Sea salt –0.4 0.0 –0.4 Internal mixture –2.2 3.5 1.3 Thermal IR 3.0 –2.3 0.7 Total forcing (clear sky) –20.5 11.5 –9.0 Total forcing (with clouds) –14.0 11.0 –3.0 T ABLE 1. Calculated TOA and surface radiative forcing during 5–15 Apr 2001 over east Asia (20°–50°N, 100°–150°E). Forcing (W m -----2) Surface Atmosphere TOA

378 MARCH 2004| nounced layering due to midlatitude frontal systems, and a greater radiative influence of high- and midlevel clouds. The mid- and upper-level clouds over east Asia reduce aerosol absorption, as opposed to the predominant low clouds that enhanced aero- sol absorption during INDOEX. CONCLUSIONS. Climatic effects of massive re- gional outbreaks of dust and anthropogenic aerosol are not currently known quantitatively but are likely to be both significant and complex, reflecting the op- posing influences of large reductions in radiative flux reaching the earth’s surface and atmospheric heating, owing to the presence of absorbing substances (such as soot). Dust provides surfaces for uptake of gas- phase species, and serves as a carrier for pollution- derived species, including sulfates, nitrates, trace metals, and carbonaceous components. Because dust reacts with sulfur dioxide and nitric acid and becomes more hygroscopic, it may play a role in regional cloud formation. This interaction of pollutant emissions and mineral dust aerosol is also expected to lead to in- creased solubility of Fe and other nutrients, with a potential impact on oceanic productivity when depos- ited to the ocean surface. Bishop et al. (2002) report a near doubling of biomass in the ocean mixed layer over a 2-week period after the passage of this dust storm, attributed to a biotic response to the natural iron fertilization by the dust. Trace metals show evi- dence for the long-range transport of dust-derived minerals like Al and Fe, in addition to fossil fuel com- bustion indicators, such as Pb and V. Carbonaceous components play a key role in the absorbing proper- ties of the dust, providing a proportionately stronger impact on atmospheric heating than submicrometer black carbon plays at lower altitudes, where its resi- dence time is shorter. Application of a state-of-the- art atmospheric chemical transport model to the dust episode of 5–15 April 2001 shows the ability to cap- ture both atmospheric vertical heterogeneity and the reduction in solar radiative fluxes at the surface. ACE- Asia can, therefore, be viewed as an important step in climate assessment, in which comprehensive re- mote and in situ measurements of atmospheric trace gases and aerosols were combined with models to quantify the anthropogenic alterations of atmospheric composition and properties that serve as a stimulus for climate change. ACKNOWLEDGMENTS. This work was funded by the National Science Foundation (the lead agency for ACE- Asia), the Office of Naval Research, and NOAA. This re- search is a contribution to the International Global Atmo- spheric Chemistry (IGAC) Core Project of the International Geosphere Biosphere Program (IGBP), and is part of the IGAC Aerosol Characterization Experiments (ACE). REFERENCES Arimoto, R., and Coauthors, 1996: Relationships among aerosol constituents from Asia and the North Pacific during PEM-West A. J. Geophys. Res., 101, 2011– 2023. Bishop, J. K. B., R. E. Davis, and J. T. Sherman, 2002: Robotic observations of dust storm enhancement of carbon biomass in the North Pacific. Science, 298, 817–820. Brechtel, F. J., and G. Buzorius, 2001: Airborne obser- vations of recent new particle formation over two urban areas in the U.S. J. Aerosol Sci., 32, S115–116. Carmichael, G. R., Y. Zhang, L.-L. Chen, M.-S. Hong, and H. Ueda, 1996: Seasonal variation of aerosol composition at Cheju Island, Korea. Atmos. Environ., 30, 2407–2416. Chun, Y., J.-Y. Kim, J. C. Choi, K. O. Boo, S. N. Oh, and M. Lee, 2001: Characterization of number size dis- FIG. 8. Comparison between (a) the surface and (b) the TOA shortwave and IR aerosol forcing. The positive values correspond with IR forcing and negative values are for solar aerosol forcing.

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