Aerosol measurements over the Pacific Ocean in support of the IR aerosol backscatter program

The major efforts under NASA contract NAG8-841 included: (1) final analyses of the samples collected during the first GLOBE survey flight that occurred in November 1989 and collections and analysis of aerosol samples during the second GLOBE survey flight in May and June 1990. During the first GLOBE survey flight, daily samples were collected at four stations (Midway, Rarotonga, American Samoa, and Norfolk Island) throughout the month of November 1989. Weekly samples were collected at Shemya, Alaska, and at Karamea, New Zealand. During the second GLOBE survey flight, daily samples were collected at Midway, Oahu, American Samoa, Rarotonga, and Norfolk Island; weekly samples were collected at Shemya. These samples were all analyzed for sodium (sea-salt), chloride, nitrate, sulfate, and methanesulfonate at the University of Miami and for aluminum at the University of Rhode Island (under a subcontract). (2) Samples continued to be collected on a weekly basis at all stations during the periods between and after the survey flights. These weekly samples were also analyzed at the University of Miami for the suite of water-soluble species. (3) In August 1990, the results obtained from the above studies were submitted to the appropriate personnel at NASA Marshall Space Flight Center to become part of the GLOBE data base for comparison with data from instruments used aboard the aircraft. In addition, the data will be compared with data previously obtained at these stations as part of the Sea-Air Exchange (SEAREX) Program. This comparison will provide valuable information on the representativeness of the periods in terms of the longer term aerosol climatology over the Pacific Ocean. (4) Several publications have been written using data from this grant. The data will continue to be used in the future as part of a continuing investigation of the long-term trends and interannual variations in aerosol species concentrations over the Pacific Ocean

A global three-dimensional model of tropospheric sulfate

A three-dimensional model is used to simulate the global tropospheric distributions of dimethylsulfide (DMS), SO2, SO42−, and methanesulfonic acid (MSA). The model uses meteorological input from a general circulation model (GCM) developed at the Goddard Institute of Space Studies (GISS) with 4° × 5° horizontal resolution, nine layers in the vertical, and a time resolution of 4 hours. Model results are compared with observations from surface sites, ships, and aircraft. The model reproduces generally to within 30% the observed SO2 and SO42− concentrations over the United States and Europe; these concentrations are highly sensitive to the supply of H2O2 as an in-cloud SO2 oxidant. Sulfate concentrations and wet deposition fluxes observed at remote marine sites can be accounted for using a global DMS source of 22 Tg S yr−1 in the model. However, this source overestimates DMS air concentrations by a factor of 2 unless we assume the presence of another DMS oxidant besides OH and NO3. Inclusion of another DMS oxidant in our model also improves the simulation of the MSA to SO42− concentration ratio in marine air. Simulated SO42− concentrations in the northern hemispheric free troposphere are much lower than in previous global models and are more consistent with the few observations available. The difference reflects in part our accounting of efficient scavenging of SO2 and SO42− in wet convective updrafts. Global mean tropospheric lifetimes computed in our model are 1.0 days for DMS, 1.2 days for SO2, 3.9 days for SO42−, and 6.2 days for MSA. Fossil fuel combustion and industrial activities represent 68% of global non-sea-salt sulfur emissions. About 50% of SO2 globally is converted to SO42− aerosol (principally by in-cloud oxidation) while the remainder is removed by deposition (30% by dry, 20% by wet). In-cloud oxidation of SO2 represents 85% of the global SO42− source.Engineering and Applied Science

Nitrate And Non-Sea-Salt Sulfate Aerosols Over Major Regions Of The World Ocean: Concentrations, Sources, And Fluxes

Over 1000 bulk aerosol samples were analyzed to determine the geographical and temporal distributions of the concentrations of nitrate and NSS sulfate in the tropospheric boundary layer over major regions of the world ocean. These data were used in combination with several hundred previously published results to estimate the fluxes of nitrate and NSS sulfate to the world ocean.The area weighted average nitrate and NSS sulfate concentrations over the world ocean are .29 and .67 (mu)g/SCM, respectively. However, the mean concentrations vary substantially (factors of 30 to 50) from one region to another. Mean nitrate and NSS sulfate concentrations range from .09 and .20 (mu)g/SCM, respectively, over the pristine regions of the southern oceans to 3 and 9 (mu)g/SCM, respectively, over the Mediterranean Sea. These substantial geographical variations are primarily a consequence of the regional variations in the quantities derived from continental sources. However, over the relatively pristine areas of the ocean, much of the variation may result from the natural variability of the oceanic and/or atmospheric sources.In many areas, the concentrations of nitrate and NSS sulfate exhibit well-defined seasonal trends. The most dramatic seasonal change occurs over the Arabian Sea where the concentrations during the summer monsoon season are factors of five to six lower than during the remainder of the year. Seasonal variations of factors of two to three are evident for both constituents at Bermuda; Broome, Australia; and Miami, Florida; and for NSS sulfate over the North Pacific Ocean.The total deposition fluxes of nitrate and NSS sulfate to the world ocean are estimated to be 39 and 73 Tg/yr, respectively. The fluxes to the ocean are far from uniform, ranging over a factor of 20 from one region to another. Much of this variation simply reflects the wide variations in the mean concentrations. However, because wet deposition accounts for an average of 80% of the nitrate flux and 95% of the NSS sulfate flux, geographical variations in total rainfall are also important. Over the relatively pristine areas of the ocean, the nitrate fluxes are compatible with lightning and downmixing from the stratosphere as the major sources of NO(,x). The NSS sulfate fluxes in the same regions are consistent with the homogeneous oxidation of marine background levels of SO(,2)

Long‐term record of nss‐sulfate and nitrate in aerosols on Midway Island, 1981–2000: Evidence of increased (now decreasing?) anthropogenic emissions from Asia

Increasing anthropogenic emissions from Asia, especially from regions undergoing rapid industrial development, have raised interest in the outflow of chemically and radiatively important gases and aerosols. Previous studies have shown that high concentrations of Asian pollution spread over a broad region of the North Pacific every spring. Here we report on studies of aerosol concentrations at Midway Island (28°13′N, 177°22′W) in the central North Pacific over the period 1981–2000. Using a relatively simple procedure we estimate the natural and anthropogenic fractions of sulfate and nitrate aerosol and show that the estimated anthropogenic component almost doubled from 1981 to the mid‐1990s. This increase closely parallels estimates of increased emissions of SO2 from China. However, measurements in the late 1990s suggest that sulfate and nitrate concentrations have stabilized and perhaps decreased. Thus over the longer term pollution emissions from Asia and concentrations over the North Pacific may be less than earlier projections, a factor which has implications for the assessment of future climate trends

Non-sea-salt sulfate and nitrate in trade wind aerosols at Barbados: Evidence for long-range transport

From mid-May 1984 through December 1987, more than 1100 daily high-volume bulk aerosol samples were collected during onshore trade winds at Barbados, West Indies. All of these have been analyzed to determine the concentrations of particulate non-sea-salt (nss) sulfate, nitrate, and Saharan dust; 91 of the samples were also analyzed for methanesulfonate (MSA). The mean concentrations (in μg m−3) during the period were nitrate, 0.509 (s = 0.389); nss sulfate, 0.751 (s = 0.602); mineral dust, 16.0 (s = 21.1); and MSA, 0.0207 (s = 0.0093). The concentrations of both nitrate and nss sulfate are significantly correlated with those of Saharan dust, indicating that substantial fractions of both are transported across the tropical North Atlantic in association with the dust. This transport accounts for about 60% of the mean total concentration of each of the anions at Barbados. Our data, combined with those from previous studies, indicate that these dust-related fractions are probably not derived from the Sahara soil material; they are more likely derived from anthropogenic sources. The nitrate-to-nss sulfate ratio in this dust-related fraction changes markedly from the summer to the winter. During the summer the general meteorology and nitrate-to-nss sulfate mass ratio (0.36) are consistent with Europe being the major source region; during the winter the ratio increases by a factor of 4 to 1.44, which may be consistent with a source region over southern West Africa. The ratio of the mean MSA concentration to that of the nss sulfate that is not related to dust transport is 0.066, a value similar to the ratio of MSA to total nss sulfate at relatively pristine stations in the Pacific: Fanning Island and American Samoa. The similarity suggests that this fraction of the nss sulfate at Barbados may be derived predominantly from oxidation of reduced sulfur compounds emitted from the ocean. The mean concentration of nitrate that is not related to dust transport is 0.22 μg m−3, about double the total mean concentration over the tropical South Pacific (0.11 μg m−3) and 40% higher than that over the equatorial Pacific (0.16 μg m−3). The major source (or sources) of this nondust-related nitrate is still very uncertain

Non-sea-salt sulfate and nitrate in trade wind aerosols at Barbados: Evidence for long-range transport

From mid-May 1984 through December 1987, more than 1100 daily high-volume bulk aerosol samples were collected during onshore trade winds at Barbados, West Indies. All of these have been analyzed to determine the concentrations of particulate non-sea-salt (nss) sulfate, nitrate, and Saharan dust; 91 of the samples were also analyzed for methanesulfonate (MSA). The mean concentrations (in μg m−3) during the period were nitrate, 0.509 (s = 0.389); nss sulfate, 0.751 (s = 0.602); mineral dust, 16.0 (s = 21.1); and MSA, 0.0207 (s = 0.0093). The concentrations of both nitrate and nss sulfate are significantly correlated with those of Saharan dust, indicating that substantial fractions of both are transported across the tropical North Atlantic in association with the dust. This transport accounts for about 60% of the mean total concentration of each of the anions at Barbados. Our data, combined with those from previous studies, indicate that these dust-related fractions are probably not derived from the Sahara soil material; they are more likely derived from anthropogenic sources. The nitrate-to-nss sulfate ratio in this dust-related fraction changes markedly from the summer to the winter. During the summer the general meteorology and nitrate-to-nss sulfate mass ratio (0.36) are consistent with Europe being the major source region; during the winter the ratio increases by a factor of 4 to 1.44, which may be consistent with a source region over southern West Africa. The ratio of the mean MSA concentration to that of the nss sulfate that is not related to dust transport is 0.066, a value similar to the ratio of MSA to total nss sulfate at relatively pristine stations in the Pacific: Fanning Island and American Samoa. The similarity suggests that this fraction of the nss sulfate at Barbados may be derived predominantly from oxidation of reduced sulfur compounds emitted from the ocean. The mean concentration of nitrate that is not related to dust transport is 0.22 μg m−3, about double the total mean concentration over the tropical South Pacific (0.11 μg m−3) and 40% higher than that over the equatorial Pacific (0.16 μg m−3). The major source (or sources) of this nondust-related nitrate is still very uncertain

Dust vertical distribution in the Caribbean during the Puerto Rico dust experiment

Geophysical Research Letters, Vol. 29, Issue 7, 55-1 - 55-4.The article of record as published may be located at http://dx.doi.org/10.1029/2001GL01409

Nitrate in the atmospheric boundary layer of the tropical South Pacific: Implications regarding sources and transport

Weekly bulk aerosol samples collected at Funafuti, Tuvalu (8°30′S, 179°12′E), American Samoa (14°15′S, 170°35′W), and Rarotonga (21°15′S, 159°45′W), from 1983 through most of 1987 have been analyzed for nitrate and other constituents. The mean nitrate concentration is about 0.11 μg m-3 at each of these stations: 0.107±0.011 μg m-3 at Funafuti; 0.116±0.008 at American Samoa; and 0.117±0.010 at Rarotonga. Previous measurements of mineral aerosol and trace metal concentrations at American Samoa are among the lowest ever recorded for the near-surface troposphere and indicate that this region is minimally affected by transport of soil material and pollutants from the continents. Consequently, the nitrate concentration of 0.11 μg m-3 can be regarded as the natural level for the remote marine boundary layer of the tropical South Pacific Ocean. In contrast, over the tropical North Pacific which is significantly impacted by the transport of material from Asia and North America, the mean nitrate concentrations are about three times higher, 0.29 and 0.36 μg m-3 at Midway and Oahu, respectively. The major sources of the nitrate over the tropical South Pacific are still very uncertain. A very significant correlation between the nitrate concentrations at American Samoa and the concentrations of 210Pb suggests that transport from continental sources might be important. This continental source could be lightning, which occurs most frequently over the tropical continents. A near-zero correlation with 7Be indicates that the stratosphere and upper troposphere are probably not the major sources. A significant biogenic source would be consistent with the higher mean nitrate concentrations, 0.16 to 0.17 μg m-3, found over the equatorial Pacific at Fanning Island (3°55′N, 159°20′W) and Nauru (0°32′S, 166°57′E). The lack of correlation between nitrate and nss sulfate at American Samoa does not necessarily preclude an important role for marine biogenic sources. © 1989 Kluwer Academic Publishers

Atmospheric selenium; geographical distribution and ocean to atmosphere flux in the Pacific

Approximately 700 weekly aerosol samples from seven island sites in the North Pacific have been analyzed for their selenium content. A much more limited set of selenium aerosol analysis is also presented for two South Pacific island sites. Selenium concentrations (0.25+ or - 0.16 ng m (super -3) ) in the high-latitude North Pacific atmosphere were a factor of 2 greater and much more variable than those found at mid-North Pacific basin sites. Aerosol concentrations at five sites in the mid- and southwestern North Pacific were uniformly low, with mean concentrations of 0.10-0.13 ng Se m (super -3) . Seasonal trends in the data were not evident at these locations. Near the equator, in the productive upwelling regime of the central North Pacific, mean particulate selenium concentrations were a factor of 3-4 greater than those observed in oligotrophic midocean regions. Effects of the 1982-1983 El Nino Southern Oscillation were also apparent in the aerosol record in this region. Consideration of aerosol selenium, nitrate, and non-sea-salt sulfate data from the Pacific suggests that source functions of aerosol selenium and excess sulfate may be similar, while removal mechanisms of selenium and nitrate may be analogous. The geographical distribution of aerosol selenium appears to be more closely related to primary productivity of the surrounding waters rather than to transport of continentally derived material into the central ocean basin. We estimate an ocean-to-atmosphere vapor phase selenium flux of 5-8 X 10 (super 9) g Se yr (super -1) . The anomalous enrichment of aerosol selenium which is observed in the marine atmosphere may be explained by the gas-to-particle conversion of this naturally produced vapor phase