Asian Pacific Regional Aerosol Characterization Research Articles

Characterization of light-absorbing carbon particles at three altitudes in East Asian outflow by transmission electron microscopy

Abstract. The morphology, microstructure, and composition of the submicron fraction of individual light-absorbing carbon (LAC) particles collected by research aircraft during the ACE-Asia (Asian Pacific Regional Aerosol Characterization Experiment) project above the Yellow Sea at altitudes of 120, 450 and 1500 m are investigated by transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). Two types of carbonaceous particles, small spherule soot with graphitic spherules and amorphous carbonaceous spheres (brown carbon), are found at all altitudes in high concentration. For soot particles, emphasis of the study is on the component subparticles (spherules). The nanoscopic structures of the small spherule soot show no significant difference at three altitudes although the size distribution of primary spherules showed that 70% of the total volume lies in the ranges 30–50, 50–85 and 30–50 nm, respectively. For the amorphous carbonaceous spheres, 70% of the total volume from three altitudes lies in the range 200–350, 160–470 and 150–320 nm, respectively. Within the size fraction studied (submicron, with most particles in the range 50 to 500 nm) the number concentration ratios of the amorphous carbonaceous spheres to primary spherules in soot at altitudes of 120, 450 and 1500 m are about 1, 1.5 and 10, respectively and their volume ratios are about 260, 50 and 1400. Lower relative concentrations of large spherule soot with intermediate graphitic structure were observed at 120 m. Also, low relative number concentrations of carbon cenospheres were observed at 120 and 1500 m. A key result of the study is that in vertically stratified outflow from East Asia, the character of LAC may have strong variance with altitude thus resulting in optical characteristics that vary with altitude. Also, apparent "aging" of LAC deduced from samples at multiple ground stations may instead reflect differences in the original carbon aerosols.

Single-Particle Characterization of Atmospheric Aerosols Collected at Gosan, Korea, during the Asian Pacific Regional Aerosol Characterization Experiment Field Campaign Using Low-Z (Atomic Number) Particle Electron Probe X-ray Microanalysis

ABSTRACT A quantitative energy-dispersive electron probe X-ray microanalysis (ED-EPMA), namely low-Z (atomic number) particle EPMA, was used to characterize the chemical compositions of the individual aerosol particles collected at the Gosan supersite, Jeju Island, Korea, as a part of the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia). On 4–10 April 2001 just before a severe dust storm arrived, seven sets of aerosol samples were obtained by a seven-stage May cascade impactor with a flow rate of 20 L/min. Overall 11,200 particles on stages 1–6 with cutoff diameters of 16, 8, 4, 2, 1, and 0.5 μm, respectively, were examined and classified based on their secondary electron images and X-ray spectra. In general, sea salt particles were the most frequently encountered, followed by mineral dust, organic carbon (OC)-like, (NH4)2SO4/NH4HSO4-containing, elemental carbon (EC)-like, Fe-rich, and K-rich particles. Sea salt and mineral dust particles had a higher relative abundance on stages 1–5, whereas OC-like, (NH4)2SO4/NH4HSO4-containing, Fe-rich, and K-rich particles were relatively abundant on stage 6. The analysis on relative number abundances of various particle types combined with 72-hr backward air mass trajectories indicated that a lot of reacted sea salt and reacted mineral dust (with airborne NOx and SO2 or their acidic products) and OC-like particles were carried by the air masses passing over the Yellow Sea (for sample “10 April”) and many NH4HSO4/(NH4)2SO4-containing particles were carried by the air masses passing over the Sea of Japan and Korea Strait (for samples “4–9 April”). It was concluded that the atmosphere over Jeju Island was influenced by anthropogenic SO2 and NOx, organic compounds, and secondary aerosols when Asian dust was absent. IMPLICATIONS A quantitative energy-dispersive electron probe X-ray microanalysis, called low-Z particle EPMA, was used to characterize aerosol particles collected at Jeju Island, Korea, on 4–10 April of 2001 during the intensive observation period of the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia). The particle size distribution and chemical compositions of individual atmospheric particles over Jeju Island were obtained, and many environmentally important atmospheric particles containing sulfate, nitrate, sea salt, alminosilicate, and carbonaceous species in the size range of 0.5–16 μm were characterized. It was found that anthropogenic SO2 and NOx made great impacts on compositions of aerosol particles. The results helped better understand how air pollutants interacted with aerosol particles in the Asia-Pacific region.

황사의 대기환경영향

ACE-Asia (Asian Pacific Regional Aerosol Characterization Experiment) in spring 2001 was a turning point to trigger international research interest on Asian dust (AD). This basically arose from the lack of research on AD, the important natural phenomenon to characterize the Northeast Asia. In contrast, health risk related to AD has been the primary concern in Korea. Although due in part to severe AD events of 2001~2002, research and mitigation measures on AD in Korea are typically based on the assumption that AD events are increasing in frequency and severity. However, contrary to the trend in Korea, the number of AD days and emission amounts of AD in China have decreased since 1980s. This paper reviews the impacts of AD on ambient air quality and variations in toxic substances during the AD event, reflecting the concern in Korea. While a great amount of dust particles flow into Korea during the AD event, a considerable amount of dust particles are locally generated as well. Since dust particles are mixed with toxic substances as they pass over polluted areas, the levels of toxic substances become lower in dust particles that do not pass over polluted areas. Broadening the research scope on AD is warranted to understand the AD as an important natural phenomenon, irrespective of its associated health risk.

Photochemical histories of nonmethane hydrocarbons inferred from their stable carbon isotope ratio measurements over east Asia

The first airborne measurements of stable carbon isotope ratios (δ13C) of nonmethane hydrocarbons (NMHCs) were made over east Asia and its downwind regions as part of the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia). The measured δ13C values for ethane, n‐butane, and n‐pentane varied from approximately −30‰ to −20‰. In contrast, acetylene showed much higher δ13C with a wide variation (−10‰ to +20‰). These are consistent with the high δ13C values of combustion‐derived acetylene and a significant isotopic fractionation due to photochemical removal process. Vertical profiles of δ13C‐derived photochemical ages of NMHCs differed from one NMHC to another: less reactive ethane and acetylene showed linear increases in age with altitude (∼8 days below 1 km to ∼20 days at about 6 km altitude), whereas more reactive n‐butane and n‐pentane (∼4 days) had no age gradient. This suggests that less reactive NMHCs in high‐altitude air are transported from upwind source regions and mixed with fresh emissions from east Asia, while reactive NMHCs, even in the free troposphere, have recently been emitted. Thus vertical profiles are caused by the mixing of fresh emissions with aged air masses containing reactivity‐determined amounts of photochemically aged NMHCs. This mixing causes the difference in the photochemical ages calculated by two methods (the “hydrocarbon clock” method using n‐butane/ethane ratios and the “isotopic hydrocarbon clock” method using δ13C values of ethane).

Characterizing the transport pathways of Asian dust

Backward trajectories, synoptic analyses, and regional dust models are applied to analyze the aerosol particles obtained from aircraft measurements during major dust events in the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) to understand the controlling mechanism of Asian dust transport. The selected high concentrations of aerosol measurements are traced back to their source areas, and the summarized eleven representative trajectories of dust particles are categorized into three types, including the upper‐level (U type), lower‐level (L type), and descending (D type) transport. The characteristics of the dust transport during different stages for each of the three types are inspected. It is found that the transport distance of a dust event strongly depended on the source areas relative to the synoptic conditions. For source areas ahead of a trough, the dust particles can be uplifted into the free troposphere. If the uplifted dust particles move to the ascending areas of a trough or ascend due to other processes in the later stage, then the dust particles likely travel longer distances. When dust particles ascend in an area too close to the trough, their subsidence may be expected as sinking motion could occur after trough passage, and thus the transport distance is limited. Uplifted dust particles are typically found at a height of 400–700 hPa (2–4.5 km above ground level) over source regions. For dust particles that are generated behind a trough, the vertical motion is limited to a layer around 700 hPa, and the transport distance is also shorter.

Dust characteristics over the North Pacific observed through shipboard measurements during the ACE-Asia experiment

To examine the diversity of chemical and physical properties of aerosol particles, in particular dust, over the North Pacific, aerosols were collected along ∼32°N latitude between 140°E and 170°W longitude aboard the NOAA R/V Ronald H. Brown during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) in the spring 2001. A total of 11,482 aerosol particles were examined through individual-particle analysis. Results indicate that dust particles over this region were dominated primarily by Si-rich particles, including aluminosilicates that contain Fe. Fe is also present as separate Fe-rich particles. Additional common particle types include Ca- and S-rich particles; many of the later appear to represent soil-derived calcium carbonate and its reaction products whereas the former are predominantly reaction products of sea salt and sulfate. Particles are often aggregates of different types including pollution-derived substances and highly heterogeneous, both internally and externally. Dust particles are non-spherical, having circularities from 1.0 up to 4.5, suggesting the high degree of complexity of particle shape. The majority of dust particles were dominated by particles with median diameters from 0.67 to 1.26 μm. However, dust particles with diameters of 5 μm or even larger do exist associated with those events of dust originated from Asian desert areas. The existence of soot and Fe-rich particles over this region indicates the influence of fossil fuel sources in Asia. Aerosol Fe from both Asian desert and fossil fuel combustion may contribute to the nutrient Fe in the surface waters of the North Pacific basin. Therefore, the transport of Asian dust associated with species of fossil fuel burning in the spring may play an important role in altering the natural composition of aerosols over the North Pacific.

Top-down estimate of mercury emissions in China using four-dimensional variational data assimilation

An inverse modeling method using the four-dimensional variational data assimilation approach is developed to provide a top-down estimate of mercury emission inventory in China. The mercury observations on board the C130 aircraft during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) campaign in April 2001 are assimilated into a regional chemical transport model, STEM. Using a 340 Mg of elemental mercury emitted in 1999, the assimilation results in an increase in Hg 0 emissions for China to 1140 Mg in 2001. This is an upper limit amount of the elemental mercury required in China. The average emission-scaling factor is ∼3.4 in China. The spatial changes in the mercury emissions after the assimilation are also evaluated. The largest changes are estimated on the China north-east coastal areas and the areas of north-center China. The influences of the observation and inventory uncertainties and the initial and boundary conditions on the emission estimates are discussed. Increasing the boundary conditions of Hg from 1.2 to 1.5 ng m −3, results in a top-down estimate of Hg 0 emissions for China of 718 Mg, and leads the average scaling factor from 3.4 to 2.1.

Number size distribution of atmospheric aerosols during ACE-Asia dust and precipitation events

Measurements of size-resolved particle number concentrations during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) field campaign were made at the Gosan super-site, South Korea. In East Asia, dust and precipitation phenomena play a crucial role in atmospheric environment and climate studies because they are major sources and sinks of atmospheric aerosols, especially in the springtime. Total Ozone Mapping Spectrometer (TOMS) Aerosol Index and backward trajectories are analyzed to investigate the spatial and temporal evolution of dust storms. The size distributions between dust and non-dust periods and times with and without precipitation are compared. In order to understand the temporal evolution of the aerosol size distribution during dust and precipitation events, a simple aerosol dynamics model is employed. The model predicted and observed size distributions are compared with the measured data. The results show that the coarse mode particle number concentrations increase by a factor of 10–16 during dust events. During precipitation, however, particles in the coarse mode are scavenged by impaction mechanism. It is found that the larger particles are more efficiently scavenged. The degree of scavenged particle varies depending on the rainfall rate, raindrop size distribution and aerosol size distribution.

Chemical apportionment of shortwave direct aerosol radiative forcing at the Gosan super-site, Korea during ACE-Asia

Shortwave direct aerosol radiative forcing (DARF) at the surface as well as aerosol optical depth (AOD) were estimated and chemically apportioned on the basis of ground-based aerosol and radiation measurements at the Gosan super-site in Korea during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) in April 2001. An aerosol optical model and a radiative transfer model (RTM) were employed to calculate the aerosol extinction coefficient and radiative flux at the surface, respectively. The calculated scattering and absorption coefficients for D p<10 μm aerosols agreed well with measured scattering and absorption coefficients with root mean square errors (RMSEs) of 23.6 and 3.0 Mm −1, respectively. The modeled direct and diffuse irradiances at the surface were also in good agreement with the measured direct and diffuse irradiances. In this study we found that the 17-day mean aerosol radiative forcing of −38.3 W m −2 at the surface is attributable to mineral dust (45.7%), water-soluble components (sum of sulfate, nitrate, ammonium, and water-soluble organic carbon (WSOC)) (26.8%), and elemental carbon (EC) (26.4%). However, sea salt does not play a major role. For the cases of Asian dust and smoke episodic events on 26 April 2001, a diurnal averaged forcing of −36.2 W m −2 was contributed by mineral dust (−18.8 W m −2), EC (−6.7 W m −2), and water-soluble components (−10.7 W m −2). The results of this study suggest that water-soluble and EC components as well as a mineral dust component are responsible for a large portion of the aerosol radiative forcing at the surface in the continental outflow region of East Asia.

Regional distribution and emissions of mercury in east Asia: A modeling analysis of Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) observations

Mercury tracer model simulations are performed to support the analysis of the total gas phase mercury (TGM) observations on 15 flights of the NSF/NCAR C‐130 aircraft during the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) campaign conducted during April 2001. Ninety‐seven percent of the observed mercury concentrations are greater than 1.2 ng/m3. The mean value of the observations is 1.9 ng/m3. No simple relationships between Hg‐CO and Hg‐total sulfur are found; however, strong correlations are observed in individual flight segments. Backward trajectory analysis is used to help characterize regional of Hg/CO ratios, and these are compared with emission‐based ratios from the various sectors. Model predictions using new Hg emission estimates are presented and compared to the observations. Under the condition of a mercury background level of 1.2 ng/m3 the tracer model underestimates the observations when the flights were over the Yellow Sea but is consistent with the observations when the flights were over the Japan Sea. Further analysis suggests that Hg emissions are underestimated in north central China and the eastern coastal areas of China. Model sensitivity studies indicate that the emissions of China may be underestimated by 80–200%.

Impacts of Asian megacity emissions on regional air quality during spring 2001

Measurements from the Transport and Chemical Evolution over the Pacific (TRACE‐P) and Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) field experiments obtained during the period of March–April 2001 are used to evaluate the impact of megacity emissions on regional air quality in east Asia. A classification method built upon back trajectory analysis and sensitivity runs using the Sulfur Transport and Emissions Model 2001 (STEM‐2K1) regional chemical transport model are used to identify the aircraft observations that were influenced by megacity emissions. More than 30% of measurement points are classified as urban points, with a significant number of plumes found to have originated from Shanghai, Qingdao, Beijing, Taiyuan, Tianjin and Guiyang, Seoul, and Pusan. These data are then analyzed, and chemical characteristics of these megacities are compared. Emission estimates for the megacities are also presented and discussed in the context of expected similarities and differences in the chemical signals in the ambient air impacted by these cities. Comparisons of the observation‐based ratios with emission‐based estimates are presented and provide a means to test for the consistency of the emission estimates. The observation‐based ratios are shown to be generally consistent with the emissions ratios. The megacity emissions are used in the STEM‐2K1 model to study the effects of these emissions on criteria and photochemical species in the region. Over large portions of the Japan Sea, Yellow Sea, western Pacific Ocean, and the Bay of Bengal, megacity emissions contribute in excess of 10% of the near‐surface ambient levels of O3, CO, SO2, H2SO4, HCHO, and NOz. The megacity emissions are also used to study ozone levels in Asia under a scenario where all cities evolve their emissions in a manner such that they end up with the same VOC/NOx emission ratio as that for Tokyo. Monthly mean ozone levels are found to increase by at least 5%.

Adjoint inverse modeling of black carbon during the Asian Pacific Regional Aerosol Characterization Experiment

An adjoint model is used for inverse modeling of black carbon during the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia). We use the four‐dimensional variational data assimilation (4D‐Var) approach to optimally recover spatially resolved anthropogenic and biomass‐burning emissions and initial and boundary conditions of black carbon. Boundary conditions and biomass‐burning emissions are assigned daily scaling factors. Anthropogenic emissions are scaled by a combination of daily and monthly scaling factors. Simulation results are compared to various observations of black carbon concentrations during the campaign. Measurements at five islands and on board the research vessel Ronald H. Brown are used for inverse modeling. Different levels of constraints are examined for inversion, and a case with 62% reduction in the total square errors is chosen. The assimilated results are compared with the observations on board the Twin Otter aircraft that were not used for assimilation. Among the scaled variables, anthropogenic emissions are the most significant, followed by the boundary conditions. The domain‐wide emissions inventory does not change significantly as a result of the assimilation, but sizable changes occur on the subregional level. Most noticeably, anthropogenic emissions over southeastern China are reduced while those in northeast China and Japan are increased.

Aerosol distribution in the Northern Hemisphere during ACE‐Asia: Results from global model, satellite observations, and Sun photometer measurements

We analyze the aerosol distribution and composition in the Northern Hemisphere during the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) field experiment in spring 2001. We use the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model in this study, in conjunction with satellite retrieval from the Moderate‐Resolution Imaging Spectroradiometer (MODIS) on EOS‐Terra satellite and Sun photometer measurements from the worldwide Aerosol Robotic Network (AERONET). Statistical analysis methods including histograms, mean bias, root‐mean‐square error, correlation coefficients, and skill scores are applied to quantify the differences between the MODIS 1° × 1° gridded data, the daytime average AERONET data, and the daily mean 2° × 2.5° resolution model results. Both MODIS and the model show relatively high aerosol optical thickness (τ) near the source regions of Asia, Europe, and northern Africa, and they agree on major features of the long‐range transport of aerosols from their source regions to the neighboring oceans. The τ values from MODIS and from the model have similar probability distributions in the extratropical oceans and in Europe, but MODIS is approximately 2–3 times as high as the model in North/Central America and nearly twice as high in Asia and over the tropical/subtropical oceans. Comparisons with the AERONET measurements in the Northern Hemisphere demonstrate that in general the model and the AERONET data have comparable values and similar probability distributions of τ, whereas MODIS tends to report higher values of τ over land, particularly North/Central America. The MODIS high bias is primarily attributed to the difficulties in land algorithm dealing with surface reflectance over inhomogeneous and bright land surfaces, including mountaintops, arid areas, and areas of snow/ice melting and with land/water mixed pixels. The model estimates that on average, sulfate, carbon, dust, and sea salt comprise 30%, 25%, 32%, and 13%, respectively, of the 550‐nm τ in April 2001 in the Northern Hemisphere, with ∼46% of the total τ from anthropogenic activities and 66% from fine mode aerosols.

Transport and transformation of sulfur compounds over East Asia during the TRACE-P and ACE-Asia campaigns

On the basis of the recently estimated emission inventory for East Asia with a resolution of 1×1°, the transport and chemical transformation of sulfur compounds over East Asia during the period of 22 February through 4 May 2001 was investigated by using the Models-3 Community Multi-scale Air Quality (CMAQ) modeling system with meteorological fields calculated by the regional atmospheric modeling system (RAMS). For evaluating the model performance simulated concentrations of sulfur dioxide (SO2) and aerosol sulfate (SO42−) were compared with the observations on the ground level at four remote sites in Japan and on board aircraft and vessel during the transport and chemical evolution over the Pacific and Asian Pacific regional aerosol characterization experiment field campaigns, and it was found that the model reproduces many of the important features in the observations, including horizontal and vertical gradients. The SO2 and SO42− concentrations show pronounced variations in time and space, with SO2 and SO42− behaving differently due to the interplay of chemical conversion, removal and transport processes. Analysis of model results shows that emission was the dominant term in regulating the SO2 spatial distribution, while conversion of SO2 to SO42− in the gas phase and the aqueous phase and wet removal were the primary factors that controlled SO42− amounts. The gas phase and the aqueous phase have the same importance in oxidizing SO2, and about 42% sulfur compounds (∼25% in SO2) emitted in the model domain was transported out, while about 57% (∼35% by wet removal processes) was deposited in the domain during the study period.

Environmental snapshots from ACE‐Asia

On five occasions spanning the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) field campaign in spring 2001, the Multiangle Imaging Spectroradiometer spaceborne instrument took data coincident with high‐quality observations by instruments on two or more surface and airborne platforms. The cases capture a range of clean, polluted, and dusty aerosol conditions. With a three‐stage optical modeling process, we synthesize the data from over 40 field instruments into layer‐by‐layer environmental snapshots that summarize what we know about the atmospheric and surface states at key locations during each event. We compare related measurements and discuss the implications of apparent discrepancies, at a level of detail appropriate for satellite retrieval algorithm and aerosol transport model validation. Aerosols within a few kilometers of the surface were composed primarily of pollution and Asian dust mixtures, as expected. Medium‐ and coarse‐mode particle size distributions varied little among the events studied; however, column aerosol optical depth changed by more than a factor of 4, and the near‐surface proportion of dust ranged between 25% and 50%. The amount of absorbing material in the submicron fraction was highest when near‐surface winds crossed Beijing and the Korean Peninsula and was considerably lower for all other cases. Having simultaneous single‐scattering albedo measurements at more than one wavelength would significantly reduce the remaining optical model uncertainties. The consistency of component particle microphysical properties among the five events, even in this relatively complex aerosol environment, suggests that global, satellite‐derived maps of aerosol optical depth and aerosol mixture (air‐mass‐type) extent, combined with targeted in situ component microphysical property measurements, can provide a detailed global picture of aerosol behavior.

Composition and major sources of organic compounds of aerosol particulate matter sampled during the ACE‐Asia campaign

The organic compound tracers of atmospheric particulate matter, as well as organic carbon (OC) and elemental carbon (EC), have been characterized for samples acquired during the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) from Gosan, Jeju Island, Korea, from Sapporo, Japan, and from Chichi‐jima Island in the western North Pacific, as well as on the National Oceanic and Atmospheric Administration R/V Ronald H. Brown. Total extracts were analyzed by gas chromatography–mass spectrometry to determine both polar and aliphatic compounds. Total particles, organic matter, and lipid and saccharide compounds were high during the Asian dust episode (early April 2001) compared to levels at other times. The organic matter can be apportioned to seven emission sources and to significant oxidation‐producing secondary products during long‐range transport. Terrestrial natural background compounds are vascular plant wax lipids derived from direct emission and as part of desert sand dust. Fossil fuel utilization is obvious and derives from petroleum product and coal combustion emissions. Saccharides are a major polar (water‐soluble) carbonaceous fraction derived from soil resuspension (agricultural activities). Biomass‐burning smoke is evident in all samples and seasons. It contributes up to 13% of the total compound mass as water‐soluble constituents. Burning of refuse is another source of organic particles. Varying levels of marine‐derived lipids are superimposed during aerosol transport over the ocean. Secondary oxidation products increase with increasing transport distance and time. The ACE‐Asia aerosols are composed not only of desert dust but also of soil dust, smoke from biomass and refuse burning, and emissions from fossil fuel use in urban areas.

Aerosol optical properties measured on board the Ronald H. Brown during ACE‐Asia as a function of aerosol chemical composition and source region

During the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) intensive field campaign conducted in the spring of 2001, aerosol properties were measured on board the R/V Ronald H. Brown to study the effects of the Asian aerosol on atmospheric chemistry and climate in downwind regions. Aerosol properties measured in the marine boundary layer included chemical composition; number size distribution; and light scattering, hemispheric backscattering, and absorption coefficients. In addition, optical depth and vertical profiles of aerosol 180° backscatter were measured. Aerosol within the ACE‐Asia study region was found to be a complex mixture resulting from marine, pollution, volcanic, and dust sources. Presented here as a function of air mass source region are the mass fractions of the dominant aerosol chemical components, the fraction of the scattering measured at the surface due to each component, mass scattering efficiencies of the individual components, aerosol scattering and absorption coefficients, single scattering albedo, Ångström exponents, optical depth, and vertical profiles of aerosol extinction. All results, except aerosol optical depth and the vertical profiles of aerosol extinction, are reported at a relative humidity of 55 ± 5%. An overdetermined data set was collected so that measured and calculated aerosol properties could be compared, internal consistency in the data set could be assessed, and sources of uncertainty could be identified. By adjusting the measured size distribution to take into account nonsphericity of the dust aerosol, calculated and measured aerosol mass and scattering coefficients agreed within overall experimental uncertainties. Differences between measured and calculated aerosol absorption coefficients were not within reasonable uncertainty limits, however, and may indicate the inability of Mie theory and the assumption of internally mixed homogeneous spheres to predict absorption by the ACE‐Asia aerosol. Mass scattering efficiencies of non‐sea‐salt sulfate aerosol, sea salt, submicron particulate organic matter, and dust found for the ACE‐Asia aerosol are comparable to values estimated for ACE 1, Aerosols99, and the Indian Ocean Experiment (INDOEX). Unique to the ACE‐Asia aerosol were the large mass fractions of dust, the dominance of dust in controlling the aerosol optical properties, and the interaction of dust with soot aerosol.

Spatial distribution and size evolution of particles in Asian outflow: Significance of primary and secondary aerosols during ACE‐Asia and TRACE‐P

Quasi‐Lagrangian aircraft measurements above the Yellow Sea, East China Sea, and Sea of Japan revealed synoptic‐scale secondary aerosol formation and condensational growth during the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) and Transport and Chemical Evolution over the Pacific (TRACE‐P) experiment. This occurred in the presence of pollution and mineral dust aerosol surface areas as high as 1200 μm2 cm−3. Concentrations of sulfuric acid generally appeared insufficient for binary nucleation, but observations, models, and theory are consistent with a ternary nucleation mechanism involving H2SO4‐H2O‐NH3. Growth rates of ∼2 nm h−1 can be explained by the condensation of sulfuric acid at a rate of 2 ± 1 × 106 molecules cm−3 s−1. Aerosol volatility suggested increasing neutralization of the aerosol during growth. Size distribution measurements suggest that weak (mean condensation nuclei (CN) 3–13 nm ≈ 500 cm−3) new particle production was a common occurrence in the region. However, new particle production was enhanced by ∼1 order of magnitude (mean CN 3–13 nm ≈ 5000 cm−3) in postfrontal air masses associated with offshore flow during cloud‐free conditions. Fog and clouds appear to be regionally important in modulating nucleation events through scavenging of secondary aerosol and through depletion of gas‐phase precursors through enhanced heterogeneous chemistry. Our results indicate that only 10–30% of the total aerosol population consists of aged secondary aerosols after ∼2 days of transport from source regions. In spite of their high production during nucleation events, secondary aerosols advected out over the Pacific Ocean will have a small impact upon indirect forcing and a negligible impact upon direct forcing compared to primary aerosol emissions and the species that condense upon them.

Estimation of aerosol optical thickness over northeast Asia from Sea‐Viewing Wide Field‐of‐View Sensor (SeaWiFS) data during the 2001 ACE‐Asia intensive observation period

Radiative forcing by aerosol is one of the greatest sources of uncertainty in climate change modeling. One of the main objectives of the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) was to assess the spatial and temporal variability of aerosol properties in northeast Asia. Aerosol optical parameters are retrieved from the Sea‐Viewing Wide Field‐of‐View Sensor (SeaWiFS) satellite data to study aerosol characteristics over northeast Asia during the ACE‐Asia intensive observation period 2001. The aerosol optical thickness (AOT) and Ångström exponent are retrieved over land and ocean at a spatial scale of 1 × 1 km2 by using the Bremen aerosol retrieval (BAER) technique. SeaWiFS‐derived AOT data were compared with those ground‐measured AERONET AOT. The validation between SeaWiFS and Sun photometer data showed good correlation coefficients (r &gt; 0.89) during the ACE‐Asia IOP. The SeaWiFS‐retrieved AOT showed a high AOT value of around 0.8 and an Ångström exponent of 0.51 during an Asian dust case of 13 April 2001 over Gosan. Aerosol parameters retrieved from SeaWiFS data by using the BAER technique showed promising results to monitor atmospheric aerosol loading and estimate its optical properties.

Aerosol composition and size versus altitude measured from the C‐130 during ACE‐Asia

During the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) intensive experiment in the spring of 2001 we used a total aerosol sampler (TAS) and a micro‐orifice impactor (MOI) to collect dust and pollution aerosols for ion chromatographic analysis. An aerodynamic particle sizer (APS) was used to estimate the total coarse‐mode volume. We conducted postexperiment passing efficiency measurements on the APS, the MOI, and their delivery tubing to constrain the inevitable (and sometimes large) artifacts associated with sampling supermicron particles from an aircraft. We have combined TAS and corrected MOI data to estimate ambient coarse and fine sulfate, ammonium, nitrate, calcium, sodium, chloride, potassium, magnesium, and oxalate. We found significant differences between aerosol composition in the free troposphere (FT) and boundary layer (BL). The molar ratio of nitrate to soluble calcium averaged 1.8 in the BL, but only 0.2 in the FT. Nitrate and calcium frequently had identical coarse size distributions, while sulfate and ammonium often had identical fine distributions. Dust clearly directs NOy toward coarse‐mode nitrate. Sulfate in the FT was closest to ammonium bisulfate (half neutralized), while non‐sea‐salt sulfate (NSS) in the BL was usually completely neutralized to ammonium sulfate. In the presence of dust, up to half the NSS was found in the coarse mode, probably the result of SO2 uptake by CaCO3 in the dust. Soluble calcium averaged 5–8% of the coarse dust mass inferred from the APS. BL aerosol chemistry was seldom a good indicator of ionic composition in the FT.