Richard Arimoto

The atmospheric input of trace species to the world ocean

Over the past decade it has become apparent that the atmosphere is a significant pathway for the transport of many natural and pollutant materials from the continents to the ocean. The atmospheric input of many of these species can have an impact (either positive or negative) on biological processes in the sea and on marine chemical cycling. For example, there is now evidence that the atmosphere may be an important transport path for such essential nutrients as iron and nitrogen in some regions. In this report we assess current data in this area, develop global scale estimates of the atmospheric fluxes of trace elements, mineral aerosol, nitrogen species, and synthetic organic compounds to the ocean; and compare the atmospheric input rates of these substances to their input via rivers. Trace elements considered were Pb, Cd, Zn, Cu, Ni, As, Hg, Sn, Al, Fe, Si, and P. Oxidized and reduced forms of nitrogen were considered, including nitrate and ammonium ions and the gaseous species NO, NO2, HNO3, and NH3. Synthetic organic compounds considered included polychlorinated biphenyls (PCBs), hexachlorocyclohexanes (HCHs), DDTs, chlordane, dieldrin, and hexachlorobenzenes (HCBs). Making this assessment was difficult because there are very few actual measurements of deposition rates of these substances to the ocean. However, there are considerably more data on the atmospheric concentrations of these species in aerosol and gaseous form. Mean concentration data for 10° × 10° ocean areas were determined from the available concentration data or from extrapolation of these data into other regions. These concentration distributions were then combined with appropriate exchange coefficients and precipitation fields to obtain the global wet and dry deposition fluxes. Careful consideration was given to atmospheric transport processes as well as to removal mechanisms and the physical and physicochemical properties of aerosols and gases. Only annual values were calculated. On a global scale atmospheric inputs are generally equal to or greater than riverine inputs, and for most species atmospheric input to the ocean is significantly greater in the northern hemisphere than in the southern hemisphere. For dissolved trace metals in seawater, global atmospheric input dominates riverine input for Pb, Cd, and Zn, and the two transport paths are roughly equal for Cu, Ni, As, and Fe. Fluxes and basin-wide deposition of trace metals are generally a factor of 5-10 higher in the North Atlantic and North Pacific regions than in the South Atlantic and South Pacific. Global input of oxidized and reduced nitrogen species are roughly equal to each other, although the major fraction of oxidized nitrogen enters the ocean in the northern hemisphere, primarily as a result of pollution sources. Reduced nitrogen species are much more uniformly distributed, suggesting that the ocean itself may be a significant source. The global atmospheric input of such synthetic organic species as HCH,PCBs, DDT, and HCB completely dominates their input via rivers.

Dust emission from Chinese desert sources linked to variations in atmospheric circulation

Estimates of atmospheric dust deposition to five Asian/Pacific regions indicate that ∼800 Tg of Chinese desert dust is injected into the atmosphere annually; about 30% of this is redeposited onto the deserts, 20% is transported over regional scales, primarily within continental China. The remaining 50% of the dust is subject to long-range transport to the Pacific Ocean and beyond. Elemental tracers based on several dust-derived elements (Al, Fe, Mg, and Sc) reveal high-frequency variability in the contributions of the western desert sources versus northern high-dust and low-dust desert sources to eolian deposits from the center of the Loess Plateau. Comparisons of the patterns uncovered with climate signals from the remote North Atlantic region for the last glaciation show that shifts in source areas of Asian dust were synchronous with large-scale variations in atmospheric circulation.

Relationships among aerosol constituents from Asia and the North Pacific during PEM‐West A

Aerosol particle samples collected from Asia and the North Pacific were analyzed to investigate the relationships among atmospheric sea salt, mineral aerosol, biogenic emissions (methanesulfonate (MSA)), and several anthropogenic substances (sulfate, nitrate, and various trace elements). These studies specifically focused on the sources for aerosol SO4= and on the long-range transport of continental materials to the North Pacific. Ground-based aerosol sampling was conducted at four coastal-continental sites: Hong Kong, Taiwan, Okinawa, and Cheju; and at three remote Pacific islands, Shemya, Midway, and Oahu. Non-sea-salt (nss) SO4= and MSA were uncorrelated at the East Asian sites presumably because pollution sources overwhelm the biogenic emissions of nss SO4=. At the coastal-continental sites, marine biogenic emissions accounted for only 10 to <5% of the total nss SO4=. In contrast, over the ocean the concentrations of nss SO4= and MSA were correlated (Midway r = 0.70; Oahu r = 0.59), and higher percentages of biogenic nss SO4= occurred, 55 and 70% at Oahu and Midway, respectively. The concentrations of nss SO4= and NO3− were correlated at Cheju, Hong Kong, Taiwan, Okinawa, Midway, and Oahu, indicating some similarities in their sources and the processes governing their transport; however, differences in the nss SO4=/NO3− ratios among sites suggest regional differences in the pollution component of the aerosol. At Shemya the concentrations of MSA during the summer (100 ng m−3 or more) are about 2 orders of magnitude higher than those in winter. The dimethylsulfide-derived fraction of the nss SO4= is highest in the summer when the monthly median nss SO4=/MSA ratios range from 2.7 to 4.5, i.e., comparable to the ratios observed over Antarctica and other high-latitude locations. However, the monthly median nss SO4=/MSA ratios increase, reaching 50 to 200 in the winter as productivity nearly ceases, and the biogenic fraction of nss SO4= at Shemya decreases dramatically; this suggests a strong seasonal pollution component to the sulfate aerosol. The meteorological conditions favoring the long-range transport of Asian dust to the North Pacific also lead to transport of anthropogenic materials. At Oahu the correlation between NO3− and Al (dust) was highly significant (r = 0.75; p < 0.001), while the correlations between nitrate and Al at the continental sites were low. These differences indicate that the composition of the air sampled at the coastal-continental stations may be quite different from the air transported to the remote ocean. This phenomenon also appeared to affect the relationship between nss SO4= and antimony. The correlations between nss SO4= and Sb were weak at the Asian sites but strong at the open ocean sites where the nss SO4=/Sb ratios were higher than those over the continent.

Sources of Asian dust and role of climate change versus desertification in Asian dust emission

For long-term variations and elemental signatures of Asian dust aerosol, changes in mass, twenty elemental concentrations over the period 2001～2003 were assessed from five surface-based stations in western, northern, northeast deserts, the Loess Plateau and the coastal areas in China. Together with the back trajectory analyses and visibility observations, the elemental signatures of soil dust aerosol from different air-mass clusters were characterized for the dust storm (DS) and non-dust storm (N-DS)conditions, respectively.……

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

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

Trace elements in the atmosphere over the North Atlantic

The concentrations of trace elements in aerosol particles from the atmosphere over the North Atlantic Ocean were determined as part of a program designed to characterize the chemical climatology of the region. For these studies, which were part of the Atmosphere- Ocean Chemistry Experiment (AEROCE), 2 years of samples were collected at Tudor Hill, Bermuda (BTT), and at Ragged Point, Barbados (BAT); and 1 year of samples was collected at Mace Head, Ireland (MHT) and at the Izafia Observatory, Tenerife, Canary Islands (IZT). One major component of the aerosol was atmospheric dust, and the ranking for the median mineral dust concentrations as represented by aluminum was BAT > IZT > BTT > MHT. The A1 concentrations at BAT, IZT, and BTT ranged over 4 orders of magnitude, i.e., from 0.001 to 10 gg m -3. At MHT the maximum dust concentrations were about a factor of 10 lower than at the other sites, but the lower end of the range in dust concentrations was similar at all sites. The mineral dust concentrations generally were highest in summer, and the flux of atmospheric dust was dominated by sources in North Africa. The elements showing clear enrichments over the concentrations expected from sea salt or crustal sources were I, Sb, Se, V, and Zn. At Izafia, which is in the free troposphere (elevation -2360 m), the concentrations of Se and I were much lower than at the boundary layer sites; this difference between sites most likely results from the marine emissions of these elements. The impact of pollution sources on trace element concentrations was evident at all sites but varied with season and location. The concentrations of elements originating from pollution sources generally were low at Barbados. Analyses of trace element ratios indicate that there are large-scale differences in the pollution emissions from North America versus those from Europe and Africa. Emissions from pyrometallurgical industries, steel and iron manufacturing, and possibly biomass burning are more evident in the atmospheric samples influenced by transport from Europe and Africa.

Eolian dust and climate: relationships to sources, tropospheric chemistry, transport and deposition

Abstract The amount of atmospheric dust suspended in the atmosphere and subsequently deposited on land and in the oceans is largely controlled by processes related to climate; however, the dusts presence in the atmosphere also can affect climate and, as a result, feedbacks between dust and climate are possible. By scattering and absorbing incoming solar and outgoing infrared radiation, dust particles can alter the Earths radiative balance. The net effect of the Earths dust veil can be one of heating or cooling depending on a number of variables, especially the concentration, composition and physical properties of the dust; the albedo of the surface underlying the suspended dust; and the vertical distribution of dust in the atmosphere. Indirect feedbacks between dust and climate also are possible as a result of the dusts chemical reactivity and involvement in the biogeochemical cycles of other substances with links to climate. Studies of modern dust fluxes and their connections to climate are important, not only for developing methods for predicting the impacts of future climate change scenarios, but also for strengthening the underpinnings of paleoclimate reconstructions.

Unexpected high levels of NO observed at South Pole

Reported here are the first Austral summer measurements of NO at South Pole (SP). They arc unique in that the levels are one to two orders of magnitude higher (i.e., median, 225 pptv) than measured at other polar sites. The available evidence suggests that these elevated levels arc the result of photodenitrification of the snowpack, in conjunction with a very thin atmospheric mixing depth. Important chemical consequences included finding the atmospheric oxidizing power at SP to be an order of magnitude higher than expected.

Relationships between the dust concentrations over eastern Asia and the remote North Pacific

Dust storm reports as well as chemical data indicate that 1988 and 1989 were high-dust and low-dust years in eastern Asia, respectively. This pattern was not reflected in data from Midway, a remote island in the North Pacific, where the mineral aerosol concentrations during 1989 were as high as or higher than any year for which data are available. Analyses of meteorological data showed that the amount of precipitation, the strength of the westerly winds, and the position of large-scale meteorological features were related to the dust concentrations over eastern Asia. The observed disparity in the interannual trend in dust concentrations over Beijing and Midway was explained by differences in the source regions for the dust. Dust over eastern Asia evidently originates from source regions to the northeast of the Tibetan Plateau, while dust at Midway is ascribed to dust materials transported from the desert lands in western China. The Asian dust flux to the open ocean is influenced not only by conditions in or near the source regions but also by the large-scale atmospheric circulation patterns.

Atmospheric trace elements over source regions for Chinese dust: concentrations, sources and atmospheric deposition on the Loess plateau

Abstract The mass-particle size distributions of up to 17 trace elements in aerosol particle samples from dust storm and non-dust storm periods were determined for three sites in or near the source regions of Chinese dust. The mass of particulate material in the atmosphere at the sites is dominated by mineral aerosol particles. An absolute principal component analysis of the non-dust storm elemental data for the loess region allows the estimation of the mass contributions from two coarse-particle classes (soil dust and dust associated with pollutants), and two fine-particle classes (soil dust and anomalously enriched). For most elements (Al, Si, Ca, Fe, Ti, K, S and As), the mass-particle size distributions (MSDs) were approximately log-normal. The mass-median diameters (MMDs) of the soil-derived elements tended to decrease with distance from the desert region and when the dust storms subsided. Total dry deposition velocities were calculated by fitting a log-normal distribution to the aerosol data and calculating deposition rates for 100 particle-size intervals using a two-layer deposition model. The mean dry-deposition rates and fluxes were highest during dust storms over desert regions. In thloess region, the calculated dry deposition velocities of soil derived elements (Al, Si, Ca, Fe and Ti) during non-dust storm periods were from 3.1 to 3.7 cm s−1. From the estimated mass-particles size distributions, the coarser and finer mineral particles were found to benriched with Ca, Fe, Ti and K relative to Al or Si. On a yearly basis, the dry atmospheric input to the Loess Plateau was mainly attributable to normal transport processes, i.e. non-dust storm conditions. Wet deposition fluxes estimated from scavenging ratios indicate that dry deposition dominated the total atmospheric deposition of mineral aerosol. The deposition of aerosol particles associated with coal burning or other anthropogenic sources also was considerable on the Loess Plateau.