Asian Pacific Regional Aerosol Characterization Research Articles

Measurements of organic and elemental carbon in Asian outflow during ACE‐Asia from the NSF/NCAR C‐130

During the spring 2001 Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) intensive observation period, we measured elemental and organic carbon (EC and OC) from the National Science Foundation/National Center for Atmospheric Research (NSF/NCAR) C‐130 aircraft. Samples were collected on quartz filters using a Particle Concentrator–Brigham Young University Organic Sampling System (PC‐BOSS) to minimize positive and negative artifacts and analyzed using a Sunset Labs thermal/optical analyzer. Although short aircraft sampling times limited our sensitivity, total carbon (EC plus OC) was quantifiable on all but one level flight leg. We could not confidently apportion total carbon (TC) between EC and OC on about a quarter of the legs, mostly in the free troposphere (FT). About a third of the samples showed significant evaporation of collected aerosols from their quartz filter (a median of 20% on these samples), while a similar number had too little evaporation to detect. We found that TC and OC concentrations were generally higher in the boundary layer (medians of 7.6 μg TC/m3 and 5.8 μg OC/m3) than in the free troposphere (medians of 3.1 μg TC/m3 and 3.9 μg OC/m3, the latter from only a few samples). The same appears to be true for EC, but the free troposphere (FT) statistics for EC are also poor. Average concentrations were somewhat higher than the medians, reflecting the impact of a few more polluted samples. OC was much more variable than was EC. In the FT the TC to non‐sea‐salt sulfate ratio ranged from 3.2 to 6.0, so carbonaceous aerosols were considerably more concentrated than non‐sea‐salt sulfate in the upper troposphere. When dust mass was included, organic matter ranged from about 0.1 to 0.7 of the fine particle mass. TC/EC ratios ranged from 2 to 15, with medians of ∼4–5. Air in the Yellow Sea, the Korea Strait, and the Sea of Japan had generally higher concentrations of OC than that in the East China Sea or the Pacific south and east of Japan.

Spectral absorption of solar radiation by aerosols during ACE‐Asia

As part of the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia), the upward and downward spectral solar radiant fluxes were measured with the Spectral Solar Flux Radiometer (SSFR), and the aerosol optical depth was measured with the Ames Airborne Tracking Sunphotometer (AATS‐14) aboard the Center for Interdisciplinary Remotely‐Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft. In this paper, we examine the data obtained for two cases: a moderately thick aerosol layer, 12 April, and a relatively thin aerosol case, 16 April 2001. On both days, the Twin Otter flew vertical profiles in the Korean Strait southeast of Gosan Island. For both days we determine the aerosol spectral absorption of the layer and estimate the spectral aerosol absorption optical depth and single‐scattering albedo. The results for 12 April show that the single‐scattering albedo increases with wavelength from 0.8 at 400 nm to 0.95 at 900 nm and remains essentially constant from 950 to 1700 nm. On 16 April the amount of aerosol absorption was very low; however, the aerosol single‐scattering albedo appears to decrease slightly with wavelength in the visible region. We interpret these results in light of the two absorbing aerosol species observed during the ACE‐Asia study: mineral dust and black carbon. The results for 12 April are indicative of a mineral dust‐black carbon mixture. The 16 April results are possibly caused by black carbon mixed with nonabsorbing pollution aerosols. For the 12 April case we attempt to estimate the relative contributions of the black carbon particles and the mineral dust particles. We compare our results with other estimates of the aerosol properties from a Sea‐Viewing Wide Field‐of View Sensor (SeaWiFS) satellite analysis and aerosol measurements made aboard the Twin Otter, aboard the National Oceanic and Atmospheric Administration Ronald H. Brown ship, and at ground sites in Gosan and Japan. The results indicate a relatively complicated aerosol mixture of both industrial pollution (including black carbon) and mineral dust. This underscores the need for careful measurements and analysis to separate out the absorption effects of mineral dust and black carbon in the east Asia region.

Marine boundary layer dust and pollutant transport associated with the passage of a frontal system over eastern Asia

Aerosol chemical composition and number size distributions were measured aboard the R/V Ronald H. Brown during the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) from 14 March to 20 April 2001. This manuscript focuses on the prefrontal and postfrontal air masses sampled aboard the ship in the Sea of Japan between 6 and 15 April 2001 to illustrate the different chemical sources/mixtures off the coast of Asia resulting from the contrasting meteorological transport patterns. The prefrontal air masses had a dominant accumulation mode composed of pollution and volcanic aerosols. The aerosol was predominately ammonium sulfate and organic carbon. Minor amounts of dust were present in the marine boundary layer (MBL) as a result of subsidence from a pronounced Taklimakan dust aerosol layer aloft. The sea salt in both the submicron and supermicron modes was highly depleted in chloride from reaction with sulfuric and nitric acid vapors. The passage of a large low‐pressure center, surrounded by a widespread distribution of airborne dust, on 10 April brought elevated concentrations of submicron and supermicron Gobi desert dust to the ship. The supermicron dust particles contained high concentrations of sulfate, nitrate, organic, and elemental carbon. The MBL aerosol properties and controlling processes described here provide data to evaluate and refine chemical transport models.

Mercury in the atmosphere around Japan, Korea, and China as observed during the 2001 ACE‐Asia field campaign: Measurements, distributions, sources, and implications

We report on mercury in the atmosphere of east Asia (Japan, Korea, and China) as measured from sea level to ∼7000 m during 16 research flights in the National Science Foundation (NSF)/National Center for Atmospheric Research (NCAR) C‐130 aircraft during the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) campaign (http://saga.pmel.noaa.gov/aceasia/). The air at all altitudes contained concentrations of atmospheric mercury above the global background. The atmosphere was highly stratified with plumes originating from massive dust storms carried out of China, from local industrial pollution, from volcanoes, and, less well defined, from biomass burning. Most often the air masses were mixtures, e.g., dust layers contained anthropogenic emissions or volcanic plumes were embedded in anthropogenic pollution; thus the total data set showed no significant correlations of gaseous mercury with the most common anthropogenic pollutants in the area (CO and SO2), but good correlations were observed for identifiable plumes. Highest mixing ratios for gaseous elemental mercury (GEM) were found in industrial plumes exiting China (∼6.3 ng/m3), Korea (∼3 ng/m3), and Japan (∼3 ng/m3). The core of the plume from Miyake Jima volcano contained ∼3.7 ng/m3 of GEM. Crustal mercury was also present, emitted and subsequently deposited during the outbreak of the spring dust storms. Some of the nondust aerosols contained soluble mercury in highly variable ratios with the gas‐phase mercury contained in the same plume. Preliminary estimates for the export from China are 5–15 t of crustal mercury during the dust storms, ∼150 t/yr of gas‐phase mercury from biomass/biofuel combustion, and ∼600 t/yr from industrial sources, mostly from coal combustion. Gaseous mercury is a useful tracer for industrial, volcanic, and biomass‐burning sources, but in most cases, robust plume identification required one or more cotracers. Because of the inertness of GEM and the ease of its measurement it is well suited for the tracking of long‐range transport.

Continuous observations of Asian dust and other aerosols by polarization lidars in China and Japan during ACE‐Asia

Continuous observations of aerosols in China and Japan were made by polarization lidars during March to May 2001, corresponding with the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) field campaign period. Lidars in Beijing, Nagasaki, and Tsukuba were continuously operated regardless of weather conditions. Scatterers in the atmosphere were categorized for all vertical profiles, and occurrence frequencies of dust, spherical aerosols, and clouds up to 6 km were calculated. The frequency of dust was highest in Beijing for the whole height range. There was a peak of dust occurrence near the ground in Nagasaki. Dust was frequently detected in the free troposphere in Tsukuba. The contributions of dust and spherical aerosols to the total backscattering coefficient were estimated from the depolarization ratio with the assumption of the external mixture of both kinds of aerosols. Vertical profiles of backscattering by dust and by spherical aerosols represented the different characteristics of these aerosols. The monthly averaged backscattering coefficients by dust near the surface were 0.003/km/sr in Beijing, 0.001–0.002/km/sr in Nagasaki, and 0.0006/km/sr in Tsukuba. The backscattering coefficients by spherical aerosols near the surface were 0.002–0.004/km/sr at all three observatories. We compared the derived backscattering coefficients with aerosol mass concentrations calculated by a numerical model, Chemical Weather Forecasting System (CFORS). CFORS reproduced well the vertical structures of the tall dust events and the enhancements of spherical aerosols throughout the observation period. A specific dust event on 16–19 May 2001 was analyzed by using five lidars in Japan, and its fine structure is described.

Chemical composition of atmospheric aerosols from Zhenbeitai, China, and Gosan, South Korea, during ACE‐Asia

Studies were conducted as part of Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) to characterize the major ion and elemental composition of aerosol particle samples collected at Gosan, an ACE‐Asia supersite (GOS, Korea, total suspended particle or TSP samples) and at Zhenbeitai (ZBT, China, TSP and particles < 2.5 μm diameter or PM2.5 samples), a site closer to the sources for Asia dust. The concentrations of 24 elements in the ZBT PM2.5 samples were correlated with Al (an indicator of mineral dust), and the ratios of these elements to Al were similar to those in a loess certified reference material, but a second group of elements was enriched over crustal proportions most likely as a result of pollution emissions. The concentrations of various water‐soluble (WS) cations (Na+, K+, Ca2+, Mg2+) also were generally well correlated with Al in both the ZBT and GOS samples, with the exception being WS K+ at ZBT, where biomass burning may have had an effect. The percentage of calcium that was soluble approached 100% at ZBT versus ∼60% at GOS, and the ratio WS Ca2+/Al also was higher at ZBT. The molar ratio of sulfate to WS Ca2+ was ∼0.1 at ZBT but increased to near unity at GOS, where the aerosol nitrate/WS Ca2+ ratio was tenfold to hundredfold higher compared with ZBT, presumably because of anthropogenic influences. The observed differences in aerosol characteristics between sites can only be explained as the end product of different source contributions combined with complex processes involving gas‐particle conversion, size‐dependent fractionation, and aerosol mixing.

Meteorological events and transport patterns in ACE‐Asia

The Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) intensive phase took place from 30 March to 4 May 2001, involving extensive aerosol observations from aircraft, ship, and ground stations. Meteorological events of significance to the ACE‐Asia intensive observations made during this period were identified and analyzed using a combination of gridded meteorological analysis fields, observations, and analyses of precipitation, atmospheric radiation distributions, and other variables, and products prepared from these data. The observed flow characteristics were found to be close to the center of the range of interannual variation, with most anomalies spatially confined and small in magnitude. The occurrence and distribution of midlatitude cyclones which had an impact on the circulation and transport patterns affecting the area of the campaign were close to the climatological mean. Total aerosol scattering and the fraction caused by submicron particles, and trajectory analysis results from points along the flight track of the C‐130, were used to examine the relationship between the aerosol distributions and transport patterns and the meteorological situation. Notably strong coarse‐mode scattering was observed at low altitudes in post cold‐frontal circulations during dust outbreaks, and very strong mixed‐mode scattering occurred in plumes from polluted areas flowing offshore removed from important surface or upper air features.

Impacts of dust on regional tropospheric chemistry during the ACE‐Asia experiment: A model study with observations

A comprehensive regional‐scale chemical transport model, Sulfur Transport and Emissions Model 2001 (STEM‐2K1), is employed to study dust outflows and their influence on regional chemistry in the high‐dust Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) period, from 4–14 April 2001. In this period, dust storms are initialized in the Taklamagan and Gobi deserts because of cold air outbreaks, are transported eastward, and are often intensified by dust emitted from exposed soils as the front moves off the continent. Simulated dust agrees well with surface weather observations, satellite images, and the measurements of the C‐130 aircraft. The C‐130 aircraft observations of chemical constituents of the aerosol are analyzed for dust‐rich and low‐dust periods. In the submicron aerosol, dust‐rich air masses have elevated ratios of ΔCa/ΔMg, ΔNH4+/ΔSO42−, and ΔNO3−/ΔCO (Δ represents the difference between observed and background concentrations). The impacts of heterogeneous reactions on dust involving O3, NO2, SO2, and HNO3 are studied by incorporating these reactions into the analysis. These reactions have significant influence on regional chemistry. For example, the low O3 concentrations in C‐130 flight 6 can be explained only by the influence of heterogeneous reactions. In the near‐surface layer, the modeled heterogeneous reactions indicated that O3, SO2, NO2, and HNO3 are decreased by up to 20%, 55%, 20%, and 95%, respectively, when averaged over this period. In addition, NO, HONO, and daytime OH can increase by 20%, 30%, and 4%, respectively, over polluted regions. When dust encounters fresh pollutants, these heterogeneous reactions can lead to a series of complex responses of the photochemical system. In addition, these reactions can alter the chemical‐size distribution of the aerosol. Under heavy dust loadings, these reactions can lead to >20% of the sulfate and >70% of the nitrate being associated with the coarse fraction. The radiative influence of dust can also affect the photochemical system. For example, OH levels can decrease by 20% near surface. The dust radiative influence is shown to be weaker than the heterogeneous influence for most species.

Aerosol chemical, physical, and radiative characteristics near a desert source region of northwest China during ACE‐Asia

The Gobi desert in northwest China is an important source of mineral aerosols over both eastern Asia and the northern Pacific Ocean. In order to determine the chemical, physical, and radiative properties of aerosols originating from the Gobi desert source region, field measurements were performed in Yulin, China, in April 2001 as part of the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) campaign. The means and standard deviations of the measured aerosol light absorption coefficient σap, scattering coefficient σsp, and single‐scattering albedo ω are 6 Mm−1 (11 Mm−1), 158 Mm−1 (193 Mm−1), and 0.95 (0.05), respectively. A clear diurnal pattern is observed in both σap and σsp, resulting from diurnal changes in the mixing height as well as from local combustion sources in the morning and dust sources in the afternoon. Two distinct populations of aerosol mass scattering efficiencies Escat_2.5, one for aerosols dominated by desert dust (∼1.0 m2 g−1) and the other for aerosols composed primarily of local pollutants (∼3.0 m2 g−1), are observed. During the field study there were three significant dust events that occurred for, on average, several days at a time. The most significant dust storm resulted in a 24‐hour‐average PM2.5 concentration (mass concentration of particles having aerodynamic diameters less than 2.5 μm) of 453 μg m−3 and a peak σsp of 2510 Mm−1 on 8 April. The mean PM2.5 mass concentration during the dust storm periods is approximately 169 μg m−3, about 4 times greater than the mean value of 44 μg m−3 observed during local pollution periods. When local pollution is the dominant source of fine particulate mass, organic matter (OM) is the major chemical component, contributing 41% to the PM2.5 mass, followed by crustal material (29%), sulfate (17%), and elemental carbon (EC) (13%). During sand storm periods, ∼51% of PM2.5 mass is crustal material, followed by CO32− (11%) and OM (9.5%). The element enrichment factors indicate that coal combustion, biomass burning, and mobile source emissions are important local pollution sources. Overall, our results indicate that in addition to dust, local pollution also has a significant influence on aerosol properties in the region.

Molecular composition of the water‐soluble fraction of atmospheric carbonaceous aerosols collected during ACE‐Asia

During the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia), samples of carbonaceous aerosols were collected on board the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) De Havilland DHC‐6 Twin Otter aircraft. The samples were analyzed to determine their total carbon (TC) and water‐soluble organic compound (WSOC) contents as well as to identify the individual compounds comprising the WSOC fraction of the aerosol. The TC concentrations varied from 3.5 to 14.3 μg C m−3; the highest TC levels were observed for samples collected in pollution layers that originated over mainland China. WSOC concentrations ranged from 0.54 to 7.2 μg C m−3, with the WSOC fraction contributing from 10 to 50% of the carbon mass. About 50% of the carbonaceous aerosol mass in pollution layers could be attributed to WSOC. For samples collected in dust layers the WSOC fraction of TC was much lower than that observed in pollution layers. The sum of all the detected organic ions accounted for 6.9–19% of the WSOC. In the six samples collected by the Twin Otter during ACE‐Asia, of the organic ions identified in the WSOC fraction, oxalate had the highest concentration. Samples collected from pollution layers exhibited a slightly higher ratio of formate to oxalate as compared to the other samples. Two samples had a relatively high ratio of lactate to oxalate, which might be a signature of some currently unidentified source of carbonaceous aerosol. The sum of the masses of sulfate and nitrate ions exceeded the sum of the masses of the identified organic ions by a factor of 9 to 17. The chemical levoglucosan, a tracer for biomass burning, comprised from 0.1 to 0.4% of TC mass. Comparing this ratio to the ratio measured directly in wood‐burning studies it was determined that biomass burning may have represented from ≈2 to 10% of the carbonaceous aerosol collected during ACE‐Asia.

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

Although continental-scale plumes of Asian dust and pollution reduce the amount of solar radiation reaching the earth's surface and perturb the chemistry of the atmosphere, our ability to quantify these effects has been limited by a lack of critical observations, particularly of layers above the surface. Comprehensive surface, airborne, shipboard, and satellite measurements of Asian aerosol chemical composition, size, optical properties, and radiative impacts were performed during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) study. Measurements within a massive Chinese dust storm at numerous widely spaced sampling locations revealed the highly complex structure of the atmosphere, in which layers of dust, urban pollution, and biomass-burning smoke may be transported long distances as distinct entities or mixed together. The data allow a first-time assessment of the regional climatic and atmospheric chemical effects of a continental-scale mixture of dust and pollution. Our results show that radiative flux reductions during such episodes are sufficient to cause regional climate change.

Source signatures of carbon monoxide and organic functional groups in Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) submicron aerosol types

Atmospheric submicron particles were collected on Teflon filters downstream of a three‐stage concentrator aboard the National Center for Atmospheric Research C‐130 aircraft near Japan during the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia). Particle‐phase organic carbon (OC) was quantified using Fourier transform infrared (FTIR) transmission spectroscopy. Silicate, carbonate, alkane, alkene, aromatic, alcohol, carbonyl, amine, and organosulfate functional groups were identified and separated with a four‐solvent rinsing procedure. X‐ray fluorescence identified elemental composition. Total OC constructed from FTIR measurements agreed with simultaneous thermal‐optical OC measurements with a slope of 0.91 and an R2 value of 0.93. OC varied from 0.4 to 14.2 μg m−3, and organic mass varied from 0.6 to 19.6 μg m−3, representing on average 36% of the identified submicron aerosol mass. Measured carbon monoxide (CO) to OC slopes illustrate 10 groups of air from regions described by an Asian emissions inventory. The CO/OC slope is used to compare sources and their influence on organic composition. Fifty‐two percent of ACE‐Asia samples have CO/OC slopes indicative of biomass combustion. Unitless CO/OC slopes above 15 are associated with increased fractions of alcohol groups, unsaturated C‐H groups, and inorganic nitrate. Increased carbonyl carbon fractions in air originating over northern Asia are consistent with secondary OC formation. Case studies in the boundary layer demonstrate that aerosol compositions downwind of large Asian aerosol sources show clear regional composition signatures.

Spatial distributions of oxygenated organic compounds (dicarboxylic acids, fatty acids, and levoglucosan) in marine aerosols over the western Pacific and off the coast of East Asia: Continental outflow of organic aerosols during the ACE‐Asia campaign

Aerosol sampling for major oxygenated organic compounds (dicarboxylic acids, fatty acids, and levoglucosan) was conducted from 15 March to 19 April 2001 on board the National Oceanic and Atmospheric Administration R/V Ronald H. Brown over the western North Pacific, the East China Sea, and the Sea of Japan, as part of the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) campaign. During the cruise, aerosol particles were collected on quartz fiber filters using a high‐volume air sampler. Diacids were also collected using an annular denuder sampling system to assess their gas‐particle partitioning. Concentrations of total diacids, total fatty acids, and levoglucosan were found to be higher off the coasts of East Asia than in the remote Pacific. Contributions of these organic compounds to total organic carbon and total organic matter (OM) were calculated to be 9.8 ± 2.3% and 19.0 ± 4.8%, respectively, assuming OM to be organic carbon × 1.6. Diacid concentrations were highly correlated with nss‐SO42− and NO3−, which originate from anthropogenic sources over East Asia. The temporal variations and chain‐length distributions of fatty acids suggest that atmospheric transport of terrestrial organic materials is important off the coast of the continent. C2 − C5 diacids were present predominantly in the particulate phase. The relative abundances of C2 − C5 diacids stayed almost unchanged throughout the cruise, suggesting that deposition is more important than chemical decomposition as a sink of diacids and that they are relatively stable end products in the atmosphere.

Observations of elemental carbon and absorption during ACE‐Asia and implications for aerosol radiative properties and climate forcing

Measurements of elemental carbon (EC) during the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) show that significant amounts of EC were found in the coarse particle phase during yellow sand events. Coagulation during long‐range transport is consistent with this observation. The daily averaged specific mass absorption efficiencies of EC were calculated, yielding values of 12.6 ± 2.6 and 14.8 ± 2.3 m2/g for PM10 (particulate matter <10 μm diameter) and PM1, respectively. On a limited number of days, the absorption efficiency for the coarse particles only (PM10 − PM1) was determined to be 5.7 ± 1.6 m2/g during dust days and 2.0 ± 1.0 m2/g for nondust days. These measurements suggest that fine particulate EC was internally mixed and that the dust was possibly somewhat absorbing. Specific mass absorption efficiency was observed to be inversely related to EC mass concentration, a result that does not appear to reflect only air mass aging effects. We speculate that if this observation holds on a global scale, it would reduce the effectiveness of a strategy for mitigating climate change by reducing EC emissions. Model simulations of idealized nonspherical dust radiative properties predict that scattering is strongly (by nearly a factor of 3) dependent on geometry, while absorption is a very weak function of geometry. The net change in shortwave absorption by polluted dust layers due to coagulation of EC with dust is predicted by model calculations to range between −42% and +58%, depending on the assumption about the initial mixing state of the EC and the dust optical properties, with the observations supporting values in the range of −10 to −40%.

Wavelength‐dependent aerosol single‐scattering albedo: Measurements and model calculations for a coastal site near the Sea of Japan during ACE‐Asia

Wavelength‐dependent optical properties together with the size‐resolved elemental and chemical composition of the atmospheric aerosol were measured at Yasaka, Japan, near Wakasa Bay on the coast of the Sea of Japan. Observations were performed as part of the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) campaign during three weeks in March and April 2001. Information about the detailed chemical composition and the size distribution of the aerosol, including absorbing material, was used to model optical properties of the aerosol and compare them with directly measured properties. Continuous, size‐resolved measurements of aerosol scattering and backscattering coefficients were made at wavelengths of 450, 550, and 700 nm. Aerosol size distribution measurements were taken three times during the campaign for a period of 3–4 days each, using cascade impactors. One of the measurement periods was influenced by an intense yellow dust event originating on the Asian continent (20–25 March 2001), while the other two data sets were only weakly influenced by dust particles. The size‐resolved measurement of aerosol scattering coefficients and the chemical analysis showed that the dust was accompanied by fine accumulation‐mode particles during the dust event. The effects of the mineral dust particles on the optical and chemical properties of the aerosol are discussed, and data sets with and without desert dust influence are compared. Before the optical closure calculation a mass closure of the aerosol was performed, and the influence of the nonanalyzed fraction on the calculated optical properties of the aerosol is investigated. In general, good agreement between measured and modeled aerosol optical properties was found for the aerosol scattering and absorption coefficients, while less agreement was found for the backscattering coefficients, especially for the coarse mode of the aerosol. A distinct difference between the dust and nondust periods was found for the wavelength‐dependent single‐scattering albedo.

Chemical properties and outflow patterns of anthropogenic and dust particles on Rishiri Island during the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia)

Investigations of chemical properties and transport mechanisms of continental aerosols are necessary for estimating their influences on global radiative budget and on the global material cycle. Intensive measurements of atmospheric aerosols and the associated species on Rishiri Island, near the northern tip of Japan, were conducted from March to May 2001, in order to understand the chemical properties, source regions, transport pathways, and transport patterns of anthropogenic and mineral aerosols over the east Asian Pacific Rim region during the spring. Mean concentrations of nss‐SO42−, NO3−, NH4+, nss‐Ca2+ in aerosols were 2.48, 0.64, 0.72, and 0.17 μg m−3, respectively. Elemental carbon and organic carbon in fine particles (d < 2.5 μm) yielded mean concentrations of 0.25 and 0.80 μg m−3, respectively. The concentrations of these species frequently increased to higher values because of outbreaks of continental polluted air masses, whereas under background conditions, they decreased to lower values similar to those observed over the remote ocean. Our results demonstrate that nss‐SO42− and NH4+ coexist in fine particles, that NO3− and nss‐Ca2+ coexist in coarse particles, and that each set is transported in an alternate manner. Continentally derived NO3− is transported as coarse particle to the east Asian Pacific Rim region. Anthropogenic pollutants and dust particles are not necessarily transported together. It was often found that anthropogenic fine particles containing abundant nss‐SO42− appeared first and were then followed by large mineral particles that had absorbed NO3−. Short‐term intrusion of the air masses containing abundant particulate carbonaceous compounds, probably due to the influence of biomass burning, also often occurred during the outflow events of continental air masses. Atmospheric behaviors of sulfate, nitrate, and carbonaceous species are different from one another, although they are all derived mainly from combustion processes.

A model for the radiative forcing during ACE‐Asia derived from CIRPAS Twin Otter and R/V Ronald H. Brown data and comparison with observations

Vertical profiles of aerosol size, composition, and hygroscopic behavior from Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter and National Oceanic and Atmospheric Administration R/V Ronald H. Brown observations are used to construct a generic optical model of the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) aerosol. The model accounts for sulfate, black carbon, organic carbon, sea salt, and mineral dust. The effects of relative humidity and mixing assumptions (internal versus external, coating of dust by pollutants) are explicitly accounted for. The aerosol model is integrated with a Monte Carlo radiative transfer model to compute direct radiative forcing in the solar spectrum. The predicted regional average surface aerosol forcing efficiency (change in clear‐sky radiative flux per unit aerosol optical depth at 500 nm) during the ACE‐Asia intensive period is −65 Wm−2 for pure dust and −60 Wm−2 for pure pollution aerosol (clear skies). A three‐dimensional atmospheric chemical transport model (Chemical Weather Forecast System (CFORS)) is used with the radiative transfer model to derive regional radiative forcing during ACE‐Asia in clear and cloudy skies. Net regional solar direct radiative forcing during the 5–15 April 2001 dust storm period is −3 Wm−2 at the top of the atmosphere and −17 W m−2 at the surface for the region from 20°N to 50°N and 100°E to 150°E when the effects of clouds on the direct forcing are included. The model fluxes and forcing efficiencies are found to be in good agreement with surface radiometric observations made aboard the R.H. Brown. Mean cloud conditions are found to moderate the top of atmosphere (TOA) radiative forcing by a factor of ∼3 compared to clear‐sky calculations, but atmospheric absorption by aerosol is not strongly affected by clouds in this study. The regional aerosol effect at the TOA (“climate forcing”) of −3 Wm−2 is comparable in magnitude, but of opposite sign, to present‐day anthropogenic greenhouse gas forcing. The forcing observed during ACE‐Asia is similar in character to that seen during other major field experiments downwind of industrial and biomass black carbon sources (e.g., the Indian Ocean Experiment (INDOEX)), insofar as the primary effect of aerosol is to redistribute solar heating from the surface to the atmosphere.

Column closure studies of lower tropospheric aerosol and water vapor during ACE‐Asia using airborne Sun photometer and airborne in situ and ship‐based lidar measurements

We assess the consistency (closure) between solar beam attenuation by aerosols and water vapor measured by airborne Sun photometry and derived from airborne in situ and ship‐based lidar measurements during the April 2001 Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia). The airborne data presented here were obtained aboard the Twin Otter aircraft. Comparing aerosol extinction σep(550 nm) from four different techniques shows good agreement for the vertical distribution of aerosol layers. However, the level of agreement in absolute magnitude of the derived aerosol extinction varied among the aerosol layers sampled. The σep(550 nm) computed from airborne in situ size distribution and composition measurements shows good agreement with airborne Sun photometry in the marine boundary layer but is considerably lower in layers dominated by dust if the particles are assumed to be spherical. The σep(550 nm) from airborne in situ scattering and absorption measurements are about ∼13% lower than those obtained from airborne Sun photometry during 14 vertical profiles. Combining lidar and the airborne Sun photometer measurements reveals the prevalence of dust layers at altitudes up to 10 km with layer aerosol optical depth (from 3.5 to 10 km altitude) of ∼0.1 to 0.2 (500 nm) and extinction‐to‐backscatter ratios of 59–71 sr (523 nm). The airborne Sun photometer aboard the Twin Otter reveals a relatively dry atmosphere during ACE‐Asia with all water vapor columns <1.5 cm and water vapor densities ρw < 12 g/m3. Comparing layer water vapor amounts and ρw from the airborne Sun photometer to the same quantities measured with aircraft in situ sensors leads to a high correlation (r2 = 0.96), but the Sun photometer tends to underestimate ρw by 7%.

Regional chemical weather forecasting system CFORS: Model descriptions and analysis of surface observations at Japanese island stations during the ACE‐Asia experiment

The Chemical Weather Forecast System (CFORS) is designed to aid in the design of field experiments and in the interpretation/postanalysis of observed data. The system integrates a regional chemical transport model with a multitracer, online system built within the Regional Atmospheric Modeling System (RAMS) mesoscale model. CFORS was deployed in forecast and postanalysis modes during the NASA Global Tropospheric Experiment (GTE)‐Transport and Chemical Evolution over the Pacific (TRACE‐P), International Global Atmospheric Chemistry project (IGAC)‐International Geosphere‐Biosphere Programme (IGBP) Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia), and National Oceanic and Atmospheric Administration Intercontinental Transport and Chemical Transformation of Anthropogenic Pollution 2002 (ITCT 2K2) field studies. A description of the CFORS model system is presented. The model is used to help interpret the Variability of Maritime Aerosol Properties (VMAP) surface observation data. The CFORS model results help to explain the time variation of both anthropogenic pollutants (sulfate, black carbon, and CO) and natural constituents including radon and mineral dust. Time series and time‐height cross‐section analysis of gases and aerosols are presented to help identify key processes. Synoptic‐scale weather changes are found to play an important role in the continental‐scale transport of pollution in the springtime in East Asia. The complex vertical and horizontal structure of pollutants in these outflow events is also presented and discussed.