Semiannual cycles of pollution at Bermuda

Abstract

To identify the sources and determine the transport pathways for aerosol during the Atmosphere/Ocean Chemistry Experiment (AEROCE), we examined the temporal variations of trace elements in daily aerosol samples collected at Bermuda from 1988 to 1994. Crustal (e.g., Al) and marine (e.g., Na) elements showed annual cycles with summer and winter maxima, respectively. In contrast, pollution‐derived elements (e.g., Sb) showed unusual semiannual cycles with strong spring maxima and weaker fall maxima, which to the best of our knowledge, have not been previously documented. The seasonality in trace element concentrations was mainly transport‐driven: The spring maxima of pollutants were caused by rapid westerly transport from North America, and the fall maxima were caused by North American air slowly transported to Bermuda by large high‐pressure systems that stagnated over the lower mid‐Atlantic states. Low concentrations of pollution elements in winter resulted from the southwestward extension of the Bermuda‐Azores high‐pressure system that brought marine air to Bermuda from the east or northeast while hindering transport from North America and Africa. The summer minima in pollutants were associated with air transported from the eastern Atlantic and Africa. The variations of the trace gases O 3 and CO and two naturally occurring radionuclides, 210 Pb and 7 Be, showed semiannual cycles similar to those of the pollution‐derived trace elements.