The influence of air origin on the chemical composition and size distribution of tropospheric aerosols

Abstract

Chemical and size distribution data and the corresponding air mass trajectories were obtained for aerosols collected at Death Valley, California and over the Pacific Ocean 250 km west of Santa Barbara, California. For air masses from the western Pacific, vertical profiles of Si, an element associated with continental sources, showed virtually no change in concentration with altitude at the offshore location. At this location the concentrations of Si and Cl in the lowest kilometer of an air mass which had recently passed over land indicated that modification had taken place. Differences in the profiles of Cl concentration over Death Valley and over the Pacific were consistent with the model of Toba for modification of sea salt profiles over land. At altitudes higher than 2.5 km, the concentrations of all of the elements measured were nearly constant. In the lowest kilometer at the ocean and Death Valley locations, size distributions of the aerosols were not greatly different in air masses of different origin. Modification of the chemical composition of aerosol at 300 m altitude in a maritime air mass was observed in measurements made at successive distances from the ocean to 180 km inland over the Orinoco Delta. In general, the relative concentration of particles in the size range 0.5 μm ≤ r ≤ 2 μm was larger at altitudes above 2.5 km than at lower altitudes. The experimental results and qualitative arguments based on the mechanics of aerosols indicate a significant relationship between aerosol properties and air mass only near the surface.