trans-Pacific Transport Research Articles

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.

The impact of transpacific transport of mineral dust in the United States

We use a global chemical transport model (GEOS-Chem) to estimate the impact of transpacific transport of mineral dust on aerosol concentrations in North America during 2001. We have implemented two dust mobilization schemes in the model (GOCART and DEAD) and find that the best simulation of North American surface observations with GEOS-Chem is achieved by combining the topographic source used in GOCART with the entrainment scheme used in DEAD. This combination restricts dust emissions to year-round arid areas but includes a significant wind threshold for dust mobilization. The model captures the magnitude and seasonal cycle of observed surface dust concentrations over the northern Pacific. It simulates the free tropospheric outflow of dust from Asia observed in the TRACE-P and ACE-Asia aircraft campaigns of spring 2001. It reproduces the timing and distribution of Asian dust outbreaks in North America during April–May. Beyond these outbreaks we find persistent Asian fine dust (averaging 1.2 μg m −3) in surface air over the western United States in spring, with much weaker influence (0.25 μg m −3) in summer and fall. Asian influence over the eastern United States is 30–50% lower. We find that transpacific sources accounted for 41% of the worst dust days in the western United States in 2001.

Transpacific transport of Asian anthropogenic aerosols and its impact on surface air quality in the United States

We use satellite (MODIS) observations of aerosol optical depths (AODs) over the North Pacific, together with surface aerosol measurements at a network of remote U.S. sites (IMPROVE), to improve understanding of the transpacific transport of Asian aerosol pollution and assess the ability of a global 3‐D chemical transport model (GEOS‐Chem CTM) to quantify Asian aerosol enhancements in U.S. surface air. The MODIS observations show the strongest transpacific transport occurring in spring at 40–55°N. This transport in the model takes place mainly in the lower free troposphere (900–700 hPa) because of scavenging during transport either in the boundary layer or during lifting to the upper troposphere. The preferential altitude of aerosol transpacific transport results in direct impact on the elevated terrain of the NW United States. Sulfate observations in the NW United States in spring 2001 show higher concentrations on the days of model‐predicted maximum Asian influence (1.04 μg m−3) than seasonal mean values (0.69 μg m−3). No such Asian enhancements are observed for nitrate or for organic carbon (OC) aerosol. Distinct Asian sulfate episodes correlated with dust events are observed in the NW United States and simulated with the model. The mean Asian pollution enhancement in that region in spring is 0.16 μg m−3 with a 50% estimated uncertainty. This is higher than the estimated natural concentration of 0.09 μg m−3 presently used as objective for regulation of visibility in U.S. wilderness areas.

Asian dustfall in the St. Elias Mountains, Yukon, Canada

In April 2001, a major atmospheric dustfall event occurred in the St. Elias Mountains, Yukon Territory, Canada. Field samples were collected and analyzed for particle size, mineralogy, chemical composition and Sr, Nd and Pb isotopes. Dusts found above ∼3000 m had their source in the Gobi desert region of northern China and Inner Mongolia, and were transported to the Yukon following a series of major dust storms that took place in early April. Dusts found below 3000 m had local (Yukon) or mixed source(s). The Asian dusts had a modal volume diameter of ∼4 μm typical of far-traveled mineral aerosols. However larger (>10 μm) particles were also found at ∼5000 m, suggesting a very rapid trans-Pacific transport in the mid-troposphere. We estimate that the April 2001 event deposited from 5500 to 6335 tons of dust over an area of 21,000 km 2 in the southwestern Yukon, most of which probably fell within a week. Our findings are consistent with instrumental observations and model simulations of the April 2001 event. While the dust cloud was reportedly mixed with volatile pollutants from Asia, we found no evidence of metal pollution associated with the dustfall in the Yukon. Our findings contribute to clarify the dynamics and the geochemical impact of Asian dust long-range transport events, and to better estimate eolian fluxes of dustborne elements (e.g., Fe) to the Ocean associated with such events. They may also assist in identifying past Asian dust events in ice cores drilled from the St. Elias Mountains icefields, to develop a long-term record of their frequency, magnitude and source(s).

Characteristics of Asian Dust Transport Based on Synoptic Meteorological Analysis over Korea

Classification of synoptic patterns and their correlation with dust events over East Asia were performed by means of cluster analysis. The average linkage and K-means clustering techniques were used to identify two major weather types during Asian dust events (ADEs; total 26 ADEs with 47 dusty days) of six spring seasons from 1996 to 2001. The first weather type mainly influenced neighboring Asian countries and frequently occurred with ADEs (∼23% of ADE cases). It mostly occurred under a surface high (low)-pressure system over the west (east) of the Korean peninsula coupled with an upper-level trough and cutoff low passage over the center of the Korean peninsula. It showed strong advection in the middle/upper troposphere with both a high aerosol index and enhanced coarse particulate matter (PM) loading over Korea. In contrast, the second weather type was mostly associated with long distance or continental-scale transport and occurred less frequently with ADEs (∼15%). It appeared with an upper-level trough and a cutoff low vertically connected with a surface low system that was formed by a strong cyclonic vortex over the north of the Korean peninsula. There were weak advection, low aerosol index, and low coarse PM concentration over the Korean peninsula during the second weather type. In addition, it was found to be mostly associated with the trans-Pacific transport of Asian dust to the western coast of North America.

A Simulated Climatology of Asian Dust Aerosol and Its Trans-Pacific Transport. Part I: Mean Climate and Validation

Abstract The Northern Aerosol Regional Climate Model (NARCM) was used to construct a 44-yr climatology of spring Asian dust aerosol emission, column loading, deposition, trans-Pacific transport routes, and budgets during 1960–2003. Comparisons with available ground dust observations and Total Ozone Mapping Spectrometer (TOMS) Aerosol Index (AI) measurements verified that NARCM captured most of the climatological characteristics of the spatial and temporal distributions, as well as the interannual and daily variations of Asian dust aerosol during those 44 yr. Results demonstrated again that the deserts in Mongolia and in western and northern China (mainly the Taklimakan and Badain Juran, respectively) were the major sources of Asian dust aerosol in East Asia. The dust storms in spring occurred most frequently from early April to early May with a daily averaged dust emission (diameter d < 41 μm) of 1.58 Mt in April and 1.36 Mt in May. Asian dust aerosol contributed most of the dust aerosol loading in the troposphere over the midlatitude regions from East Asia to western North America during springtime. Climatologically, dry deposition was a dominant dust removal process near the source areas, while the removal of dust particles by precipitation was the major process over the trans-Pacific transport pathway (where wet deposition exceeded dry deposition up to a factor of 20). The regional transport of Asian dust aerosol over the Asian subcontinent was entrained to an elevation of <3 km. The frontal cyclone in Mongolia and northern China uplifted dust aerosol in the free troposphere for trans-Pacific transport. Trans-Pacific dust transport peaked between 3 and 10 km in the troposphere along a zonal transport axis around 40°N. Based on the 44-yr-averaged dust budgets for the modeling domain from East Asia to western North America, it was estimated that of the average spring dust aerosol (diameter d < 41 μm) emission of ∼120 Mt from Asian source regions, about 51% was redeposited onto the source regions, 21% was deposited onto nondesert regions within the Asian subcontinent, and 26% was exported from the Asian subcontinent to the Pacific Ocean. In total, 16% of Asian dust aerosol emission was deposited into the North Pacific, while ∼3% of Asian dust aerosol was carried to the North American continent via trans-Pacific transport.

A Simulated Climatology of Asian Dust Aerosol and Its Trans-Pacific Transport. Part II: Interannual Variability and Climate Connections

Abstract A 44-yr climatology of spring Asian dust aerosol emission, column loading, deposition, trans-Pacific transport routes, and budgets during 1960–2003 was simulated with the Northern Aerosol Regional Climate Model (NARCM). Interannual variability in these Asian dust aerosol properties simulated by the model and its climate connections are analyzed with major climatic indices and records in ground observations. For dust production from most of the source regions, the strongest correlations were with the surface wind speed in the source region and the area and intensity indices of the Asian polar vortex (AIAPV and IIAPV, respectively). Dust emission was negatively correlated with precipitation and surface temperatures in spring. The strength of the East Asian monsoon was not found to be directly related to dust production but rather with the transport of dust from the Asian subcontinent. The interannual variability of dust loading and deposition showed similar relations with various climate indices. The correlation of Asian dust loading and deposition with the western Pacific (WP) pattern and Atmospheric Circulation Index (ACI) exhibited contrasting meridional and zonal distributions. AIAPV and IIAPV were strongly correlated with the midlatitude zonal distribution of dust loading and deposition over the Asian subcontinent and the North Pacific. The Pacific–North American (PNA) pattern and Southern Oscillation index (SOI) displayed an opposite correlation pattern of dust loading and deposition in the eastern Pacific, while SOI correlated significantly with dust loading over eastern China and northeast Asia. The Pacific decadal oscillation (PDO) was linked to variations of dust aerosol and deposition not only in the area of the eastern North Pacific and North America but also in the Asian dust source regions. The anomalies of transport flux and its divergence as well as dust column loading were also identified for eight typical El Niño and eight La Niña years. A shift of the trans-Pacific transport path to the north was found for El Niño years, which resulted in less dust storms and dust loading in China. In El Niño years the deserts in Mongolia and western north China closer to the polar cold air regions contributed more dust aerosol in the troposphere, while in La Niña years the deserts in central and eastern north China far from polar cold regions provided more dust aerosol in the troposphere. On the basis of the variability of Asian dust aerosol budgets, the ratio of inflow to North America to the outflow from Asia was found to be correlated negatively with the PNA index and positively with the WP index.

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, ��, 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 ��, 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 �� 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.

Seasonal variations of CO and HCN in the troposphere measured by solar absorption spectroscopy over Poker Flat, Alaska

[1] Tropospheric partial column abundances of CO and HCN have been retrieved from infrared solar spectra observed with a ground-based spectrometer at Poker Flat Alaska (65°N, 147°W) over the time period from 2000 to 2004. From these data we report the transpacific transport induced inter-annual variability of tropospheric CO over Poker Flat. This is the first report of solar infrared data from the Poker Flat station, where the geographical location of the site means that remote sensing measurements are sampling the transport of transpacific air parcels going to Northern America from Eastern Siberia and Asia. The five year time-span of the data also show significant differences in year to year CO and HCN tropospheric column enhancements driven by changes in Siberian/Asian pollution sources.

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.

Analysis of seasonal and interannual variability in transpacific transport

The purpose of our analysis is both to evaluate the meteorological component of the seasonal and interannual variability of transpacific transport and to identify meteorological features that can be used to estimate transpacific transport. To accomplish this goal, we simulate the transport of nine continental tracers with uniform emissions and two‐week lifetimes using the global Model of Ozone and Related Tracers Version 2 (MOZART‐2) driven with NCEP reanalysis meteorology from 1991–2001. In addition, we define a transpacific “transport potential,” a measure of the quantity of a tracer transported from a particular region normalized by its total emissions from that region, across a meridional plane in the eastern Pacific at 130°W. We find that at midlatitudes, the east Asian and Indian tracers have the largest transport potentials, particularly in spring. The interannual variability of the transpacific transport potentials of most tracers is relatively high in winter and fall (particularly in February and September) but is low from April to August. At high latitudes the former Soviet Union, east Asian, and European tracers have the largest transpacific transport potentials, especially in late summer and fall, when the lowest interannual variability is observed. We find that El Niño winters are associated with stronger eastward transport of east Asian emissions in the subtropical eastern Pacific. Transport of the east Asian tracer in the central North Pacific is well correlated with the North Pacific Index. However, we find that the interannual variability of transport across the west coast of North America is mostly driven by local meteorology. We therefore created a new index based on meteorology over the eastern Pacific, which we call the Eastern Pacific Index (EPI). The EPI captures most of the interannual variability of transpacific transport at both middle‐ and high‐latitude regions across the west coast of North America.

Hexachlorocyclohexanes and Endosulfans in Urban, Rural, and High Altitude Air Samples in the Fraser Valley, British Columbia: Evidence for Trans-Pacific Transport

High-volume air samples collected over the period Aug. 14-30, 2001, in the Lower Fraser Valley, BC, Canada, were used to assess urban/rural differences of organochlorine pesticides (OCPs) for ground level samples and to attempt to directly measure events of trans-Pacific inputs through the mid-troposphere. Hexachlorocyclohexanes (alpha- and gamma-isomers; 2-25 pg m(-3)) and endosulfan1 and -2 (5-150 pg m(-3)) were detected in all ground level samples. Seven air samples were collected during mid-troposphere flights (approximately 4400 m altitude) over the Lower Fraser Valley. These flights occurred concurrently with ground level sampling. Trajectory analysis identified three events of substantial mid-troposphere, trans-Pacific flow where 10-day back trajectories stemmed from potential source regions in Asia. These events were also characterized by higher air concentrations of alpha-HCH at 4400 m as compared to the ground level stations. This represents the first event-based, aircraft measurement of advection inputs of OCPs in the mid-troposphere of the west coast of North America.

Seasonal and Spatial Variation of Polychlorinated Naphthalenes and Non-/Mono-Ortho-Substituted Polychlorinated Biphenyls in Arctic Air

Archived extracts of weekly air samples collected at remote arctic monitoring stations at Alert and Tagish, Canada, and Dunai Island, Russia, in 1994-1995 were combined into 4-week composites and analyzed for levels and seasonal trends of polychlorinated naphthalenes (PCNs) and non- and mono-ortho-substituted polychlorinated biphenyls (PCBs). Mean annual sigmaPCN concentrations were 0.69, 0.82, and 0.38 pg/m3 at Alert, Dunai, and Tagish, respectively. PCNs exhibited a seasonal trend at Alert and Dunai, with higher levels occurring during winter when air masses originating over Eurasia influence the high arctic and coincide with the haze period. Episodic, trans-Pacific transport impacted PCN concentrations at Tagish. A seasonal trend was not evident for the non-/mono-o-PCBs. The contrary PCN and non-/mono-o-PCB trends indicate that the sources of these two compound classes to arctic air differ, and that atmospheric transport from source regions has a greater influence on PCN levels than for non-/ mono-o-PCBs. PCNs apparently originating from combustion sources contribute to levels in winter, as indicated by the presence of combustion marker congeners, but evaporative emissions from source regions are likely the dominant source. PCNs contributed 71 and 75% of dioxin toxic equivalents (TEQ) relative to the non-/mono-o-PCBs at Alert and Dunai and 30% at Tagish during the winter months, demonstrating the toxicological importance of PCNs as a compound class relative to PCBs.

Long‐range transport of Asian pollution to the northeast Pacific: Seasonal variations and transport pathways of carbon monoxide

Continuous CO measurements were obtained at Cheeka Peak Observatory (CPO, 48.3°N, 124.6°W, 480 m), a coastal site in Washington state, between 9 March 2001 and 31 May 2002. We analyze these observations as well as CO observations at ground sites throughout the North Pacific using the GEOS‐CHEM global tropospheric chemistry model to examine the seasonal variations of Asian long‐range transport. The model reproduces the observed CO levels, their seasonal cycle and day‐to‐day variability, with a 5–20 ppbv negative bias in winter/spring and 5–10 ppbv positive bias during summer. Asian influence on CO levels in the North Pacific troposphere maximizes during spring and minimizes during summer, ranging from 91 ppbv (44% of total CO) to 52 ppbv (39%) along the Asian Pacific Rim and from 44 ppbv (30%) to 24 ppbv (23%) at CPO. Maximum export of Asian pollution to the western Pacific occurs at 20°–50°N during spring throughout the tropospheric column, shifting to 30°–60°N during summer, mostly in the upper troposphere. The model captures five particularly strong transpacific transport events reaching CPO (four in spring, one in winter) resulting in 20–40 ppbv increases in observed CO levels. Episodic long‐range transport of pollutants from Asia to the NE Pacific occurs throughout the year every 10, 15, and 30 days in the upper, middle, and lower troposphere, respectively. Lifting ahead of cold fronts followed by transport in midlatitude westerlies accounts for 78% of long‐range transport events reaching the NE Pacific middle and upper troposphere. During summer, convective injection into the upper troposphere competes with frontal mechanisms in this export. Most events reaching the NE Pacific lower troposphere below 2 km altitude result from boundary layer outflow behind cold fronts (for spring) or ahead of cold fronts (for other seasons) followed by low‐level transpacific transport.

PHOBEA/ITCT 2002 airborne observations of transpacific transport of ozone, CO, volatile organic compounds, and aerosols to the northeast Pacific: Impacts of Asian anthropogenic and Siberian boreal fire emissions

During the spring of 2002, vertical profiles of O3, CO, nonmethane volatile organic compounds (VOCs), and total aerosol scattering were collected over the northwestern coast of Washington State as part of the University of Washington's Photochemical Ozone Budget of the Eastern North Pacific Atmosphere (PHOBEA) research campaign. These observations coincided with NOAA's Intercontinental Transport and Chemical Transformation 2002 (NOAA‐ITCT 2K2) project. Thirteen research flights were conducted from 29 March through 23 May and several well‐defined polluted layers of varying thickness (∼0.2 to >3 km) were observed at altitudes between 0 and 6 km. These layers were characterized by correlated enhancements of O3, CO, VOCs, and particles. We observed rapid transpacific transport of polluted air masses on 15 April and 14, 17, and 23 May 2002, with ΔO3and ΔCO (where Δ refers to the enhancement over background) exceeding 30 and 60 ppbv, respectively, and total aerosol scattering of green light (“σsp (550 nm)”) exceeding 65 Mm−1. These episodes were efficient in transporting O3to the northeast (NE) Pacific troposphere, with ΔO3/ΔCO ratios in the pollution layers varying from 0.22 to 0.42 mol mol−1. In contrast, the average Δσsp (550 nm)/ΔCO ratio of the mid‐May events (0.66 ± 0.21 (1σ)) was more than twice that of the 15 April event (0.32 ± 0.05). The correlation between O3, CO, aerosols, and VOCs coupled with back‐trajectory analyses, satellite data, and the GEOS‐CHEM global chemical transport model indicate that the primary source of pollution observed on 15 April originated from a mixture of Asian anthropogenic and biomass‐burning emissions. For the May events, our analyses indicate that the early onset of the 2002 Siberian fire season was a significant source of the pollution episodes observed in May.

Sources of carbonaceous aerosols over the United States and implications for natural visibility

We use a global three‐dimensional model (GEOS‐CHEM) to better quantify the sources of elemental carbon (EC) and organic carbon (OC) aerosols in the United States through simulation of year‐round observations for 1998 at a network of 45 sites (Interagency Monitoring of Protected Visual Environments (IMPROVE)). Simulation with our best a priori understanding of sources, including global satellite data to constrain fire emissions, captures most of the variance in the observations (R2 = 0.84 for EC, 0.67 for OC) with a low bias of 15% for EC and 26% for OC. Multiple linear regression to fit the IMPROVE data yields best estimates of 1998 U.S. sources of 0.60 Tg year−1 EC and 0.52 Tg year−1 OC from fossil fuel; 0.07 Tg year−1 EC and 0.89 Tg year−1 OC from biofuel; 0.08 Tg year−1 EC and 0.60 Tg year−1 OC from wildfires; and 1.10 Tg year−1 OC from vegetation. We find that fires in Mexico and Canada contributed 40–70% of annual mean natural EC in the United States for 1998 and 20–30% of annual mean natural OC. Transpacific transport from Asian pollution sources amounted to less than 10% of the natural EC and less than 2% of the natural OC; in contrast to ozone, we find that intercontinental transport of anthropogenic carbonaceous aerosols does not enhance significantly the natural background. IMPROVE observations and model simulations for the summer of 1995 show that Canadian fire emissions can produce large events of elevated EC and OC in the southeastern United States. Our best estimates of mean natural concentrations of EC and OC in the United States, using a model simulation with climatological monthly mean fire emissions, are 2–3 times higher than the default values recommended by the U.S. Environmental Protection Agency for visibility calculations, except for OC in the eastern United States (16% lower).

Six ‘new’ episodes of trans-Pacific transport of air pollutants

Chemical measurements of CO, O 3, non-methane hydrocarbons, aerosol chemistry, and aerosol scattering in air masses arriving at the west coast of North America demonstrate that a variety of chemical species can be transported across the Pacific from the Eurasian continent. In this paper, we analyze data from several ground sites in the Pacific Northwest and from aircraft observations in the region to identify six ‘new’ (i.e. previously unreported) episodes of trans-Pacific transport that occurred between 1993 and 2001. Of the six new episodes identified, one consisted of mineral dust combined with industrial emissions, while the other five appeared to consist of predominantly industrial emissions. In addition to gas and aerosol measurements, we calculated the Ångström exponent, an indicator of the aerosol size distribution, to help identify the aerosol characteristics in 4 of the cases considered. Combining these data with previous reports, we find that there is a high degree of variability in these trans-Pacific episodes. In four episodes there were significant O 3 enhancement, with mixing ratios sometimes exceeding 80 ppbv . However, O 3 was only enhanced in episodes that were transported in the free troposphere and in the absence of mineral dust. In other words, transport in the boundary layer or transport of industrial emissions with mineral dust seems to preclude any significant O 3 enhancement. Clarifying such patterns increases our understanding of not only trans-Pacific transport of air pollutants, but also intercontinental transport in general.

Atmospheric organochlorine pesticides in the western Canadian Arctic: Evidence of transpacific transport

Concentrations of hexachlorocyclohexanes (HCHs), chlordane, and dichlorodiphenyltrichloroethane (DDT) were measured in ambient air samples on a weekly basis between December 1992 and January 1995 at Tagish Yukon, Canada. In winter, unusually high air concentrations of HCHs, DDT, and chlordanes at Tagish were predominantly influenced by transpacific long‐range atmospheric transport from eastern Asia that generally occurred within 5 days. HCH and heptachlor epoxide concentrations were correlated with the time that air spent over eastern Asia prior to arrival at Tagish. Chlordane and DDT, which also increase with transpacific transport, do not show a correlation with the time the upwind airshed included Asia as the composition of these pesticides in the atmosphere is affected by differences in usage patterns, application methods, variable composition of parent pesticides and metabolites in the soil, and rates of volatilization. Air masses originating from North America had the highest concentrations of HCHs and chlordanes when the 5‐day upwind airshed included the western United States. Concentrations of HCHs may also be influenced by lindane usage in Canada.

A comparative study of the chemical composition of the sardine (Sardinea cærulea) from California and British Columbia

Forcing factors and mechanisms underlying multidecadal variability in the production of the world’s major fish stocks are one of the great mysteries of the oceans. The Japanese and California sardine are species that exhibit the regime shifts. It is shown in the present work that during two periods of frequent Asian dust events over the last 100 years, sardines on opposite sides of the Pacific Ocean only flourished under a dust-active regime. The earlier such regime that peaked in the 1930s was strong, and it brought synchronous changes in the two species that were linked to the frequency of Asian dust events. However, there is an apparent mismatch in the rise and fall of abundance between the two species in the current dust-active regime. The massive increase in Japanese sardine stock in the 1970s was related to high levels of ocean precipitation and strong winter mixing, whereas the stock collapse since 1988 has been attributed to diminished winter mixing. High levels of ocean precipitation in the western North Pacific effectively cause wet deposition of Asian dust and enhance Japanese sardine stock, whereas it reduces dust flux that can be transported to the eastern North Pacific, delaying the increase of California sardine stock. Analysis further indicates that productivity of Japanese sardine stock is jointly controlled by wet deposition of Asian dust and winter mixing, which supplies macronutrients from depth. California sardine productivity is inversely related to precipitation in the western North Pacific and is positively affected by precipitation off western North America. This indicates that Asian dust influx dominates productivity of the species because of iron-limited ocean productivity in the California sardine ranges. The analysis suggests that dust regime shifts influence shifts in sardine productivity regimes and that iron input from Asian dust during trans-Pacific transport is directly responsible. It appears that in addition to enhancing phytoplankton growth and carbon sinks, atmospheric iron deposition can regulate the production of living ocean resources, which is an extension of Martin’s iron hypothesis.