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.

This transport accounts for about 60% of the most part, reduced sulfur compounds act as the mean total concentration of each of the sinks for these oxidants although they do not anions at Barbados.
Our data, combined with play a significant role as such. The major those from previous studies, indicate that these oxidation products, HNO 3 and H2S04, are dust-related fractions are probably not derived substantial contributors to the acidity of from the Sahara soil material; they are more precipitation and particles in remote, as well as likely derived from anthropogenic sources. The polluted, regions. By reducing the pH of these nitrate-to-nss sulfate ratio in this dust-related components, they can cause the volatilization of fraction changes markedly from the summer to the weaker acids, inorganic as well as organic, and winter.
During the summer the general meteor-the partial dissolution of minerals, including ology and nitrate-to-nss sulfate mass ratio carbonates and aluminosilicate dust. Nitrate and (0.36) are consistent with Europe being the major sulfate readily form aerosols, which affect source region' during the winter the ratio climate and weather through their interaction increases by a factor of 4 to 1.44, which may be with solar and terrestrial radiation and their consistent with a source region over southern action as condensation nuclei. West Africa.
The ratio of the mean MSA concen-   Note that relatively high concentrations of nitrate and nss sulfate occurred during these same time periods.
However, a closer examination of the time series plots in Figure  2 reveals that the apparent relationship between the dust and the two major acid species is rather tenuous.
As an example, note peak number 2.
While high concentrations of the other species are evident during the dust peak, the concentrations were equally high or higher during the preceding week. Comparing peaks 1 and 2, the peak nitrate and nss sulfate concentrations are similar during the two periods, even though the dust concentration during peak 2 is a factor of 3 higher. Similar comparisons can be made with peaks 5 and 6, among others.
Note that for peak 6 the mineral dust is the highest concentration measured during June;     The long-range transport of pollutants with lies between that region and the Sahara during Saharan dust is favored by a number of factors. the summer.
Because of the strong vertical convection and A likely source of the dust-related nitrate lack of precipitation over North Africa, aerosol and nss sulfate during the winter is the region deposition over the arid desert region would be of West Africa south of the Sahara. During the greatly reduced. Gas-to-particle conversion winter the ITCZ shifts well to the south so that processes and other reactions may also be limited essentially all o f West Africa is because of the extremely low relative humidities meteorologically within the northern hemisphere and the resultant dryness of even hygroscopic circulation patterns. The southward transport of particles.
Consequently, many gaseous and hot, dry, dust-laden Saharan air produces the soparticulate species are likely to have much called harmattan periods over the south coastal longer lifetimes over the desert and, region of West Africa [Delmas, 1980;Kalu, 1979; subsequently, over the ocean within the Saharan Morales, 1986; d•Almeida, 1986]. At this time of air layer (SAL), which is largely confined year, the nitrate-to-nss sulfate mass ratio is between two inversions at 1.5 and 6 km altitude. about 1.44, far too high to be consistent with a Within the marine boundary layer over the ocean, European pollutant source. In contrast, the the region of the dust outbreaks is characterized emissions from south of the Sahara may well be by severely suppressed cloud conditions; compatible with this high nitrate:nss sulfate consequently, precipitation scavenging will also ratio. Major pollutant sources in that region be greatly reduced, except within the immediate include wood and biomass burning [Delmas, 1982] area of the easterly wave disturbances.
which release gases and particulates with far If one accepts the premise that the higher nitrogen-to-sulfur ratios than those continental source that supplies the major produced from the combustion of fossil fuels.