Sources of Nitrate and Ozone in the Marine Boundary Layer of the Tropical North Atlantic

During the period April 1989 through December 1990, °3 concentrations in the marine boundary layer at Barbados, West Indies, show a pronounced seasonal cycle. Daily averaged values in the winter and spring often fall in the range of 25-35 ppbv for periods of several days, and they seldom fall below 20 ppbv. In contrast, during the summer, values typically fall in the range of 10-20 ppbv. During the winter-spring period, there is a very strong negative correlation between °3 and a number of aerosol species, including N03"' These anticorrelations appear to be driven by changing transport patterns over the North Atlantic as opposed to chemical reactions involving °3 and nitrogen species in the atmosphere. Analyses of isentropic trajectories clearly show that high °3 and low N03are associated with transport from higher latitudes and high altitudes. Conversely, high N03" and relatively low °3 are associated with transport from Africa. Our study suggests that North America and the middle troposphere (and stratosphere! are not strong sources for N03" over the tropical North Atlantic. The strong correlation of N03with 2 Orb