Asian Outflow Research Articles

Brown Carbon Spheres in East Asian Outflow and Their Optical Properties

Atmospheric aerosols play a substantial role in climate change through radiative forcing. Combustion-produced carbonaceous particles are the main light-absorbing aerosols; thus, quantifying their optical properties is essential for determining the magnitude of direct forcing. By using the electron energy-loss spectrum in the transmission electron microscope, we quantified the optical properties of individual, submicrometer amorphous carbon spheres that are ubiquitous in East Asian-Pacific outflow. The data indicate that these common spheres are brown, not black, with a mean refractive index of 1.67 - 0.27i (where i = square root 1) at a wavelength of 550 nanometers. The results suggest that brown carbon aerosols should be explicitly included in radiative forcing models.

Reactive and particulate mercury in the Asian marine boundary layer

The variability of atmospheric mercury in elemental, reactive, and particulate forms has been studied at a remote site (Cape Hedo Observatory, CHO) at Okinawa Island (Japan) March 23 to May 2, 2004, downwind of the major Asian source regions. Under prevailing meteorological conditions, episodes of higher levels of atmospheric mercury and other atmospheric species are observed at CHO. The mean (±1 σ) concentrations of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particulate-bound mercury (PHg) are 2.04 ± 0.38 ng m −3, 4.5 ± 5.4 pg m −3 and 3.0 ± 2.5 pg m −3, respectively. In Asian outflow the combined contribution of RGM and PHg constitutes less than 1% of the GEM in the boundary layer, which indicates that most mercury export in the marine boundary layer is due to the GEM form, and direct outflow of RGM and PHg is very low. While the data from Okinawa suggest minimal export of RGM and PHg, this does not preclude greater export of these species at higher elevations. Based on the correlations of PHg and submicron aerosol mass (SAM), we found a ΔPHg/ΔSAM ratio of 0.20 μg g −1 ( R = 0.58, p < 0.01), which we believe to be characteristic of East Asian industrial aerosols during outflow. A diurnal variation is observed in RGM with a peak near noon. Using a rate constant of 9 × 10 −14 cm 3 molecule −1 s −1 for the OH oxidation rate (Sommar, J., Gardfeldt, K., Stromberg, D., Feng, X., 2001. A kinetic study of the gas phase reaction between the hydroxyl radical and atomic mercury. Atmospheric Environment 35, 3049–3054; Pal, B., Ariya, P.A., 2004a. Gas-phase HO center dot-initiated reactions of elemental mercury: kinetics, product studies, and atmospheric implications. Environmental Science and Technology 38 (21), 5555–5566.) and a typical OH concentration of 1–5 × 10 6 cm −3 would result in RGM production rates of 0.6–3.0 pg m −3 h −1. Although OH may not be the sole oxidant, this is consistent with the observed change in concentration during daytime of 1.4 ± 1.5 pg m −3 h −1. A significant correlation is found between GEM and CO; GEM and SAM; and PHg and SAM. Lower ΔSAM/ΔCO and ΔGEM/ΔCO are observed for transport events with rainfall and for air parcels remaining in the mixed layer. Back trajectory analysis along with the correlation study suggests that the air from China has a higher GEM concentration compared to the air coming from southern Japan.

Implications of the chemical transformation of Asian outflow aerosols for the long-range transport of inorganic nitrogen species

To improve our understanding of the chemical characteristics of aerosols transported from the Asian continent to the western North Pacific, an aerosol observation network has been established in Taiwan. From the measurements made during 2003–2005, it was found that the aerosol concentrations in the continental outflows were much higher than those of remote areas, evidently due to the long-range transport of air pollutants and dust from the Asian continent. Analysis on the chemical compositions of aerosols revealed that the Asian outflow aerosols underwent chemical transformation and, consequently, became more abundant in ammonium and nitrate when they mixed with air pollutants originating from Taiwan. The NH 4 +/SO 4 2− ratio in fine aerosols (PM2.5) increased from 1.55 at the Cape Fuguei, the northern tip of Taiwan, to 2.30 at Penghu, in the middle of the Taiwan Strait. The increased NH 4 +/SO 4 2− ratio implied that the acidity of the sulfate aerosols in Asian outflows was totally neutralized by ammonia as the aerosols traveled through the North Taiwan and its vicinity. In addition, the analysis indicated that the chlorine deficiency of sea salt aerosols was higher at the southern stations than at the Cape Fuguei. The chlorine deficiency was attributed to the heterogeneous reaction of NaCl and HNO 3(g), which means that the oxidation of SO 2 in sea spray droplets was inhibited. Moreover, uptake of secondary acids by the dust particles was observed. The results of this study suggested that the Asian outflow aerosols are important carriers of gaseous inorganic nitrogen species, particularly nitric acid and ammonia, in this region. Hence the atmospheric deposition of soluble inorganic nitrogen could become enhanced in the northern South China Sea, which is downwind of Taiwan during the periods of Asian winter monsoons.

Carbonyl sulfide (OCS): Large‐scale distributions over North America during INTEX‐NA and relationship to CO2

An extensive set of carbonyl sulfide (OCS) observations were made as part of the NASA Intercontinental Chemical Transport Experiment–North America (INTEX‐NA) study, flown from 1 July to 14 August 2004 mostly over the eastern United States and Canada. These data show that summertime OCS mixing ratios at low altitude were dominated by surface drawdown and were highly correlated with CO2. Although local plumes were observed on some low‐altitude flight legs, anthropogenic OCS sources were small compared to this sink. These INTEX‐NA observations were in marked contrast to the early springtime 2001 Transport and Chemical Evolution over the Pacific experiment, which sampled Asian outflow dominated by anthropogenic OCS emissions. To test the gridded OCS fluxes used in past models, the INTEX‐NA observations were combined with the sulfur transport Eulerian model (STEM) regional atmospheric chemistry model for a top‐down assessment of bottom‐up OCS surface fluxes for North America. Initial STEM results suggest that the modeled fluxes underestimate the OCS plant sink by more than 200%.

Regional climatology of particulate carbonaceous substances in the northern area of the east Asian Pacific rim

A year‐round observation of atmospheric aerosols and their associated species was conducted from March 2001 to May 2002 on Rishiri Island in the northern area of the east Asian Pacific rim region. Asian outflows brought continental air masses to this area during the period from the autumn to the spring although marine air masses from the high‐latitudinal ocean often broke into this area during the midwinter. In contrast, marine air masses were predominant over this area in the summer. Particulate elemental carbon (EC) would be mainly transported with biomass smoke particles, and seasonal variation in its concentrations was well correlated with the air mass alternation, showing higher concentrations during the period from the autumn to the spring with some decreases in the midwinter. The concentrations of particulate organic carbon (OC) showed a similar seasonal trend with those of the particulate EC, but relatively high concentrations were found in the summer due to photochemical secondary productions. The particulate OC that is vaporized by heating at higher temperatures (OCHT) would be mainly carried with the biomass smoke, and the particulate OC that is vaporized at lower temperatures (OCLT) would be mainly caused by secondary production processes. Summer enhancements of the secondary species, such as OC and nss‐SO42–, caused decreases in the ratio of EC/PM2.5, which would contribute to the high single scattering albedo of fine aerosols in the summer. Aerosols in the Asian outflows in this area are relatively “black” in the winter, although the aerosol mass loading increases in the spring.

Chemical characteristics of water‐soluble organic carbon in the Asian outflow

Semicontinuous measurements of water‐soluble organic carbon (WSOC) and organic carbon (OC) in PM2.5 were made at Gosan, Korea, in March‐April 2005. On average, the WSOC/OC mass ratio for all air masses observed at Gosan was 0.30 ± 0.12. WSOC correlated well with CO (r2 = 0.54) in Chinese outflow, suggesting that a major part of the observed WSOC and/or their precursors was of combustion origin. The relationship between the increase of WSOC and O3 suggests that the observed WSOC was mostly secondary product. To interpret the measured organic compounds, thermal analyses of organic standards were made in the laboratory. Thermograms of a single standard of water‐soluble organic species showed that carbon that evolved at high temperatures (600°–870°C) was generally associated with water‐soluble compounds having high molecular weights (MWs) on the order of hundreds, while carbon that evolved at low temperatures (&lt;300°C) generally had MWs of less than ∼180 g mol−1. Positive matrix factorization (PMF) analysis revealed three organic compound groups (low, medium, and highly refractory compounds) based on the OC thermograms. On average, highly and low refractory compound groups accounted for 79% and 21% of the WSOC mass, respectively, at Gosan. Highly refractory compound groups significantly contributed to WSOC regardless of air mass origin. The results of the laboratory experiments imply that a large fraction of these highly refractory compound groups was likely associated with high MW compounds. For water‐insoluble organic carbon(=OC–WSOC), medium and low refractory compound groups accounted for 60% and 40%, respectively, consistent with the results of the laboratory experiments.

Intercontinental transport of pollution and dust aerosols: implications for regional air quality

Abstract. We use the global model GOCART to examine the impact of pollution and dust aerosols emitted from their major sources on surface fine particulate matter concentrations at regional and hemispheric scales. Focusing on the North America region in 2001, we use measurements from the IMPROVE network in the United States to evaluate the model-simulated surface concentrations of the "reconstructed fine mass" (RCFM) and its components of ammonium sulfate, black carbon (BC), organic matter (OM), and fine mode dust. We then quantify the RCFM budget in terms of the RCFM chemical composition, source type, and region of origin to find that in the eastern U.S., ammonium sulfate is the dominant RCFM component (~60%) whereas in the western U.S., dust and OM are just as important as sulfate but have considerable seasonal variations, especially in the NW. On an annual average, pollution aerosol (defined as aerosols from fuel combustion for industrial and transportation uses) from North America accounts for 65–70% of the surface RCFM in the eastern U.S. and for a lower proportion of 30–40% in the western U.S.; by contrast, pollution from outside of North America contributes to just 2–6% (~0.2 μg m−3) of the total RCFM over the U.S. In comparison, long-range transport of dust brings 3 to 4 times more fine particles than the transport of pollution to the U.S. (0.5–0.8 μg m−3 on an annual average) with a maximum influence in spring and over the NW. Of the major pollution regions, Europe has the largest potential to affect the surface aerosol concentrations in other continents due to its shorter distance from receptor continents and its larger fraction of sulfate-producing precursor gas in the outflow. With the IPCC emission scenario for the year 2000, we find that European emissions increase levels of ammonium sulfate by 1–5 μg m−3 over the surface of northern Africa and western Asia, and its contribution to eastern Asia (≥0.2 μg m−3) is twice as much as the Asian contribution to North America. Asia and North America pollution emissions exert strong impacts on their neighboring oceans, but their influences over other continents are relatively small (≤10%) due to long traveling distances across the oceans and efficient removal during transport. Among the major dust source regions, Asia displays a significant influence over large areas in the northern hemisphere except over the North Atlantic and the tropics, where African dust dominates. We also notice that the African dust and European pollution can travel eastward through a pathway spanning across Asia and North Pacific to western North America; such a pathway is difficult to detect because these aerosols usually merge and travel together with Asian dust and pollution labeled as "Asian outflow".

Investigations of the Diurnal Cycle and Mixing State of Oxalic Acid in Individual Particles in Asian Aerosol Outflow

The mixing state of oxalic acid was measured in Asian outflow during ACE-Asia by direct shipboard measurements using an ATOFMS single-particle mass spectrometer. Oxalic and malonic acids were found to be predominantly internally mixed with mineral dust and aged sea salt particles. A persistent diurnal cycle of oxalic acid in mineral dust occurred for over 25 days in marine, polluted marine, and dust storm air masses. The preferential enrichment of diacids in mineral dust over carbonaceous particles and their diurnal behavior indicate a photochemical source of the diacids. Oxalate was only detected simultaneously with elevated aged dust particle counts. This suggests that the diurnal production of diacids most likely results from episodic atmospheric processing of the polluted dust aerosol. We propose a mechanism to explain these observations in which the photochemical oxidation of volatile organic compounds is followed by partitioning of the diacids and precursors to the alkaline Asian dust, with subsequent heterogeneous and aqueous oxidation. Our data indicate that the particulate diacids were produced over just a few hours close to the source; no significant production or destruction appears to have occurred during long-range transport to the ship. No evidence of extensive cloud processing of the sampled aerosol was found. This mixing state of diacids has important implications for the solubility and cloud nucleation properties of the dominant fraction of water-soluble organics and the bioavailability of iron in dust.

Water‐soluble species in the marine aerosol from the northern South China Sea: High chloride depletion related to air pollution

Dichotomous (PM2.5–10 and PM2.5 modes) and size‐resolved marine aerosols collected during the northeastern monsoon on two wintertime cruises in the subtropical South China Sea (SCS) were analyzed for water‐soluble ions. During the sampling periods the study region was under the influence of strong pollution originating primarily from the Asian continent. Elevated levels of non‐sea‐salt sulfate and ammonium ions of up to 4.5 and 1.2 μg/m3, respectively, were observed, indicating that the SCS is now substantially contaminated by massive amounts of air pollutants most likely from China and South/Southeast Asia. The non‐sea‐salt sulfate to nitrate mass ratios reaching 3.8 ± 1.9 are much larger than those (approximately 2) in and around East Asia and the western Pacific Ocean, suggesting that the Asian outflow aerosols measured in the SCS experienced different traveling history from those in the vicinity of source regions. High chloride depletion (Cl‐depletion) measured in the SCS marine aerosols was, on average, 30% for coarse‐mode particles and nearly 90% for fine‐mode particles. Cl‐depletion is size‐dependent, and maximizes in submicrometer particles (i.e., Cl has almost been completely lost). Acid displacement is responsible for the observed high Cl‐depletion: nitrate substitution accounts for the coarse‐mode depletion, whereas sulfate substitution accounts for the fine‐mode depletion. The acid displacement of sea salt aerosols may be related to a variety of factors, especially the substantial air pollution, which is discussed in detail in this paper. On cloudy/rainy days, fine‐mode aerosol samples have moderate Cl‐depletion (i.e., ∼40–50%), in contrast to nearly complete Cl loss on sunny days, presumably indicating that photochemical reactions would play a key role in the Cl‐deficit; however, it merits further investigation as the available samples were limited.

Simulation of atmospheric aerosols in East Asia using modeling system RAMS-CMAQ: Model evaluation

The modeling system RAMS-CMAQ is applied in this paper to East Asia to simulate the temporo-spatial concentration distributions of atmospheric aerosols. For evaluating its performances, modeled concentrations of aerosols such as sulfate, nitrate, ammonium, black carbon and organic carbon were compared with observations obtained in East Asia on board of two aircrafts in the springtime of 2001. The comparison showed generally good agreement, and, in particular, that the modeling system captured most of the important observed features, including vertical gradients of the aerosols of the Asian outflow over the western Pacific. The evaluation results provide us with much confidence for further use of the modeling system to investigate the transport and transformation processes of atmospheric aerosols over East Asia and to assess their impacts on the Earth's radiation budget.

Summertime influence of Asian pollution in the free troposphere over North America

We analyze aircraft observations obtained during INTEX‐A (1 July to 14 August 2004) to examine the summertime influence of Asian pollution in the free troposphere over North America. By applying correlation analysis and principal component analysis (PCA) to the observations between 6 and 12 km, we find dominant influences from recent convection and lightning (13% of observations), Asia (7%), the lower stratosphere (7%), and boreal forest fires (2%), with the remaining 71% assigned to background. Asian air masses are marked by high levels of CO, O3, HCN, PAN, C2H2, C6H6, methanol, and SO42–. The partitioning of NOy species in the Asian plumes is dominated by PAN (∼600 pptv), with varying NOx/HNO3 ratios in individual plumes, consistent with individual transit times of 3–9 days. Export of Asian pollution occurred in warm conveyor belts of midlatitude cyclones, deep convection, and in typhoons. Compared to Asian outflow measurements during spring, INTEX‐A observations display lower levels of anthropogenic pollutants (CO, C3H8, C2H6, C6H6) due to shorter summer lifetimes; higher levels of biogenic tracers (methanol and acetone) because of a more active biosphere; and higher levels of PAN, NOx, HNO3, and O3 reflecting active photochemistry, possibly enhanced by efficient NOy export and lightning. The high ΔO3/ΔCO ratio (0.76 mol/mol) in Asian plumes during INTEX‐A is due to strong photochemical production and, in some cases, mixing with stratospheric air along isentropic surfaces. The GEOS‐Chem global model captures the timing and location of the Asian plumes. However, it significantly underestimates the magnitude of observed enhancements in CO, O3, PAN and NOx.

Low‐level transpacific transport

The role of low‐level transpacific paths in bringing east Asian air to the North American west coast surface is determined using conditional transit‐time distributions. These distributions are computed with the MATCH transport model driven by NCEP reanalysis data. Transpacific paths that lie entirely below ∼2.2 km contribute negligibly to the west coast column burden of east Asian air. However, during summer such paths account for about half of the climatological west coast surface mixing ratio of east Asian air with transit times of less than ∼3 weeks. Summertime transport is characterized by distinct upper and lower level transport modes, which form by convective injection and boundary layer outflow into a summertime marine troposphere of high stability and weak baroclinicity. During the other seasons the climatological east Asian air plume organizes into distinct midlatitude and subtropical parts, particularly evident during spring and fall. The midlatitude plume is characterized by mixing along sloping isentropes, which transports low‐level air aloft over the western Pacific and brings midtropospheric air to the surface over the eastern Pacific. The subtropical part of the plume is formed by convectively injected air. Lagged, weighted composites show that summertime surface events are characterized by strong low‐level Asian outflow, while for the other seasons surface events are associated with enhanced mixing along sloping isentropes.

Atmospheric Outflow of Anthropogenic Semivolatile Organic Compounds from East Asia in Spring 2004

To estimate the emissions of anthropogenic semivolatile organic compounds (SOCs) from East Asia and to identify unique SOC molecular markers in Asian air masses, high-volume air samples were collected on the island of Okinawa, Japan between 22 March and 2 May 2004. Contributions from different source regions (China, Japan, the Koreas, Russia, and ocean/local) were estimated by use of source region impact factors (SRIFs). Elevated concentrations of hexachlorobenzene (HCB), hexachlorcyclohexanes (HCHs), dichlorodiphenyltrichloroethanes (DDTs), and particulate-phase polycyclic aromatic hydrocarbons (PAHs) were attributed to air masses from China. A large proportion of the variation in the current-use pesticides, gas-phase PAHs, and polychlorinated biphenyl (PCB) concentrations was explained by meteorology. Chlordanes showed a technical mixture profile and similar concentrations regardless of source region. alpha/gamma HCH and trans/cis chlordane ratios did not vary significantly with different source regions and had regional averages of 2.5 +/- 1.0 and 1.2 +/- 0.3, respectively. Particulate-phase PAH concentrations were significantly correlated (p value < 0.05) with other incomplete combustion byproduct concentrations, including elemental mercury (Hg0), CO, NOx, black carbon, submicrometer aerosols, and SO2. By use of measured PAH, CO, and black carbon concentrations and estimated CO and black carbon emission inventories, the emission of six carcinogenic particulate-phase PAHs was estimated to be 1518-4179 metric tons/year for Asia and 778-1728 metric tons/year for China, respectively. These results confirm that East Asian outflow contains significant emissions of carcinogenic particulate-phase PAHs.

The continuous field measurements of soluble aerosol compositions at the Taipei Aerosol Supersite, Taiwan

The characteristics of ambient aerosols, affected by solar radiation, relative humidity, wind speed, wind direction, and gas–aerosol interaction, changed rapidly at different spatial and temporal scales. In Taipei Basin, dense traffic emissions and sufficient solar radiation for typical summer days favored the formation of secondary aerosols. In winter, the air quality in Taipei Basin was usually affected by the Asian continental outflows due to the long-range transport of pollutants carried by the winter monsoon. The conventional filter-based method needs a long time for collecting aerosols and analyzing compositions, which cannot provide high time-resolution data to investigate aerosol sources, atmospheric transformation processes, and health effects. In this work, the in situ ion chromatograph (IC) system was developed to provide 15-min time-resolution data of nine soluble inorganic species (Cl −, NO 2 −, NO 3 −, SO 4 2−, Na +, NH 4 +, K +, Mg 2+ and Ca 2+). Over 89% of all particles larger than approximately 0.056 μm were collected by the in situ IC system. The in situ IC system is estimated to have a limit of detection lower than 0.3 μg m −3 for the various ambient ionic components. Depending on the hourly measurements, the pollutant events with high aerosol concentrations in Taipei Basin were associated with the local traffic emission in rush hour, the accumulation of pollutants in the stagnant atmosphere, the emission of industrial pollutants from the nearby factories, the photochemical secondary aerosol formation, and the long-range transport of pollutants from Asian outflows.

A regional air quality model: Evaluation and simulation of O 3 and relevant gaseous species in East Asia during spring 2001

A regional air quality model (RAQM) has been introduced, evaluated and applied to investigate the behaviors of tropospheric O 3 and relevant species in East Asia. Model simulation is performed with an up-to-date monthly emission inventory [Streets, D.G., Bond, T.C., Carmichael, G.R., Fernandes, S.D., Fu, Q., He, D., Klimont, Z., Nelson, S.M., Tsai, N.Y., Wang, M.Q., Woo, J.-H. and Yarber, K.F., 2003. An inventory of gaseous and primary emissions in Asia in the year 2000. Journal of Geophysical Research 108 (D21), 8809. doi:10.1029/2002JD003093] for March 2001 when extensive experimental data sets were available, including ground-level observations and the TRACE-P (TRAnsport and Chemical Evolution over the Pacific) aircraft experiments, which allow a comprehensive evaluation for this model system. Model validation demonstrates that RAQM is able to represent most of the major processes of chemical species in the troposphere, with model results agreeing with observations reasonably well. Statistical analysis has been conducted for both meteorological parameters (wind speed, temperature and humidity) and chemical components (10 key gaseous species) by comparing with the TRACE-P data sets. At altitudes <5 km, the correlations ( R) for meteorological parameters are all above 0.8, with temperature the highest (0.97). The model produces the highest R for O 3 (around 0.75), followed by CO (0.66–0.72) and HO 2 (0.7), and the lowest R for NO x (0.1–0.2), with R = 0.4–0.7 for the remaining species. The model successfully reproduces the major features in chemical transport associated with the Asian outflow and reasonably explains the observed fluctuation in species concentrations at downwind monitoring sites over the western Pacific.

Seasonal variations in SO 2 plume transport over Japan: Observations at the summit of Mt. Fuji from winter to summer

We undertook continuous measurements of SO 2 concentrations at the summit of Mt. Fuji, Japan (3776 m above sea level), during the period January to July, 2004. These data include the first data set that fills the previous gap in SO 2 time series for the springtime middle troposphere over Japan. The time series shows trends in high-SO 2 long-range transport events from winter to spring. Elevated SO 2 concentrations are always accompanied by elevated CO and 222Rn concentrations, and backward trajectories for such events indicate a continental origin. These data display clear sporadic signatures of Asian outflow over Japan to the North Pacific. Such high-SO 2 events decline in strength and frequency during spring. The observed seasonal variation of SO 2 events may in fact have nothing to do with seasonality in SO 2 emissions from the Asian continent, as seasonal patterns in SO 2 events show a similar phase shift to that of the prevailing wind direction and relative humidity (RH) observed on-site. The changes in air mass indicated by daily backward trajectory analysis may also explain the observed temporal changes in SO 2 events. A major factor appears to be the removal of SO 2 from the continental pollution plume during long-range transport. The SO 2-bearing polluted air is dry during the depths of winter; accordingly, little SO 2 is removed by cloud processes at this time of year. The nature of the air mass changes during spring when increasing humidity levels lead to the increased removal of SO 2 by cloud processes. RH data along the air mass travel path also support the importance of cloud processes in reducing SO 2 concentrations within the pollution plume.

Source identifications of PM10 aerosols depending on hourly measurements of soluble components characterization among different events in Taipei Basin during spring season of 2004

Continuous measurements of hourly PM10 soluble ions were performed by the in situ IC technology in order to assess the impact of Asian outflows on local air quality. The intensive aerosol observation was carried in Taipei from 11 February 22:00 to 7 April 19:00, 2004. Concentrations of the water-soluble ions (Cl −, NO 2 - , NO 3 - , SO 4 2 - , Na +, NH 4 + , K +, and Ca 2+) were measured in a total of 3300 samples. The characteristics of air pollutant events in Taipei Basin were classified as frontal dust, dust, northeast monsoon, south wind and sea/land breeze according to the hourly meteorology and air pollutant concentrations. Factor analysis was conducted based on hourly data for 13 variables to find the group of variables with similar behavior. According to the source characteristics of high loading species, the possible sources of PM10 aerosols in each group were identified. Three to four factors were identified for each event. The total variances of frontal dust, dust, northeast trade, south wind, and sea/land breeze events were explained about 85%, 86%, 76%, 77%, and 80%, respectively, indicating that the identified factors were satisfactory.

Analysis of the seasonal variation of ozone in the boundary layer in East Asia using the Community Multi-scale Air Quality model: What controls surface ozone levels over Japan?

Ozone (O 3) concentrations in East Asia were simulated using the Community Multi-scale Air Quality model and their reproducibility was confirmed by comparing with observation data at Japanese monitoring sites. The model can reproduce the seasonal variation of surface O 3 over Japan. In particular, for the southern parts of Japan, simulated surface O 3 concentrations are strongly correlated with observations. Surface O 3 distribution over East Asia varies dynamically from season to season according to the meteorological condition. In May and June, 2-month average O 3 concentrations in the boundary layer are highest, 55–70 ppbv over East China and Japan at 35–40°N. We estimated the seasonal variation of the contribution of chemically produced O 3 by east Asian regional emissions. In the summertime, the contribution of the regional Asian emissions to O 3 concentrations is around 40–70% over most of China, and the highest is 60–70% (<35 ppbv) at the mouth area of Yantze River. This suggests that O 3 concentrations over East Asia are strongly influenced by the chemical production by regional emissions during summer season. However, the inflow from outside of this model domain also contributes substantial O 3 concentration over East Asia and its contribution is predominant in winter and early spring. The seasonal change of O 3 concentrations over Japan is characterized by two peaks in spring and autumn and summer minimum. The spring peak event in 2002 consists of split peaks: the first peak in Mar–Apr is mainly influenced by the inflow from outside of this model domain, the second spring peak between May and June is mainly influenced by chemically produced O 3 by regional emissions. During the summer season, O 3 concentrations are the lowest over year because of the weak Asian outflow and northward penetration of the marine air mass.

Seasonality and weather‐driven variability of transpacific transport

We quantify transport from the industrialized regions of E Asia using the transit‐time probability density function, &amp;#55349;&amp;#56482;, to isolate the role of transport from any other factors, such as chemistry and deposition. Using the offline transport model MATCH driven by NCEP reanalyses, we calculate &amp;#55349;&amp;#56482;, which is the mass fraction of air that had its last contact with the E‐Asian source region during a given day, for each day of a three‐year period. Ensemble means of &amp;#55349;&amp;#56482; establish the climatological seasonal‐mean transport from E Asia. Export from the source region is most efficient in spring, with nearly all E‐Asian air involved in transpacific transport. In summer, E‐Asian air is transported aloft across the Pacific and, in nearly equal measure, west over SE Asia to the Middle East. Winter transport is similar to that of spring, except winter has low‐level transport to SE Asia. Fall transport is intermediate between that of summer and winter. For all seasons, the most probable transit times to N America are 6–8 days in the mid‐to‐upper troposphere and approximately one week (two for summer) longer at the surface. The surface signal of E‐Asian air over N America is strongest in spring. Daily variability of transpacific transport is quantified in terms of the transit‐time partitioned burden of E‐Asian air over western N America. The standard deviation of the transit‐time partitioned fluctuations has a nearly universal dependence on the corresponding seasonal‐mean burden. The standard deviation peaks several days before the burden at a transit time of ∼7 days. Lagged event and nonevent composites, based on the western N‐American burden of E‐Asian air, reveal that transport events are associated with dipolar wind perturbations over the eastern Pacific that are positioned and phased to receive enhanced Asian outflow. Surface‐pressure correlations are consistent with an associated strengthened Pacific High and weakened Aleutian Low.

Meteorological indices for Asian outflow and transpacific transport on daily to interannual timescales

We examine the variability of Asian pollution outflow to the western Pacific and its associated long‐range transport to the northeast (NE) Pacific between 1992 and 2002, using a global simulation of carbon monoxide (CO). We develop three new sea‐level pressure indices, which capture variations in transpacific transport on daily to interannual timescales. Our first index, LRT1 (standardized daily pressure anomalies in NE China), is related to the passage of mid‐latitude cyclones in East Asia and captures 14–22% of the variability in the outflow of Asian pollution to the western Pacific. A second index, LRT2 (standardized daily pressure anomalies North of Hawaii), captures 25–32% of the variability in transpacific transport during winter‐spring. Enhanced transpacific transport is characterized by a strong Pacific High displaced to the west of 140°W and a strong low over Alaska, resulting in strong zonal winds in the NE Pacific. The last index, LRT3 (based on monthly SLP anomalies over the North Pacific), identifies stronger than normal Pacific High and Aleutian Low pressure systems, which together result in unimpeded transport across the Pacific during winter and spring. LRT3 captures 42% and 55% of the monthly and interannual variance in transpacific transport, respectively. We find that 1992–1996 and 2002 were below‐average transpacific transport years, while 1998–2000 had unusually strong transpacific transport. By extending our LRT3 time series back to 1948, we find that the strength of transpacific transport projects strongly upon the Pacific/North American (PNA) index, the leading mode of month‐to‐month variability over the Pacific. There appears to be decadal variability in LRT3, with below‐average transpacific transport for 1948–1976 and above‐average transport since 1977.