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A. A. N. Patrinos, M. J. Leach, R. M. Brown, R. L. Tanner, and F. S. Binkowski

Abstract

A field study in the Washington, D.C. area explored the impact of urban emissions and mesoscale meteorology on precipitation chemistry. The study was a follow-up to an earlier, considerably more industrialized, study in the Philadelphia area; emissions along the Delaware Valley were found to affect the deposition of nitrate and sulfate on the urban mesoscale. The Washington studies were designed to complement and enhance the earlier study with an expanded sampling domain, sequential precipitation sampling and airborne measurements. Four storms were sampled successfully between October 1986 and April 1987. Results appear to confirm the conclusions of the Philadelphia study, although the upwind-downwind contrast in nitrate and sulfate deposition is not as pronounced. This difference is attributed to the area's widely distributed emission patterns and to the prevailing theories regarding the production of nitric acid and sulfuric acid on the relevant time and space scales. The importance of mesoscale meteorology and hydrogen peroxide availability is highlighted in at least two of the sampled storms.

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R. J. Weber, P. H. McMurry, F. L. Eisele, and D. J. Tanner

Abstract

Atmospheric measurements of expected homogeneous nucleation precursors and aerosols were made at the Mauna Loa Observatory, Hawaii, from 28 June to 27 July 1992. Large molecular clusters and gas phase species including sulfuric acid (H2SO4), methane sulfonic acid (MSA), and OH were measured with a mass spectrometer. Aerosol measurements consisted of ultrafine particle concentrations (∼3–4 nm diameter) and fine particle size distributions (15–500 nm diameter). The altitude of the observatory (average pressure 680 mb) and the presence of mountain–valley winds permitted sampling of both up- and downslope air. Downslope air was found to have the highest concentrations of MSA but low ultrafine concentrations, whereas, upslope air had the highest H2SO4 and ultrafine concentrations. Episodes of substantial increase in ultrafine particles were observed during periods of rapid increase in H2SO4 concentrations. Total aerosol surface area and H2SO4 concentrations had the strongest influence on ultrafine particle concentrations. It is concluded that for the conditions at Mauna Loa, H2SO4 was a vapor precursor of the newly formed particles, and MSA contributed little to new particle formation. Low concentrations of ultrafine particles were ubiquitous in upslope air and may indicate a widespread, low nucleation rate, source of new particles. The data were also used for estimating particle nucleation and growth rates. Measurements taken suggest that nucleation rates exceed values predicted by the classic theory for binary H2SO4–H2O nucleation.

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