Sea-salt particles in the upper tropical troposphere

Abstract

Aircraft observation of aerosol particles was carried out at an altitude of 11.2 km over the tropical Pacific Ocean. Individual particles were collected on electron microscopic grids using an impactor, together with measurement of aerosol number concentrations using an optical particle counter. High concentrations of aerosols were observed intermittently with space, synchronized with the presence of deep convective clouds. In this region, sea-salt particles were collected in large number fractions of submicron particles; that is, the fractions were evaluated to be 0.42 for particles of 0.05–0.1 μm radius and 0.64 for particles of 0.1–1 μm radius. Elemental composition of the particles was examined with an energy-dispersive X-ray analyzer (EDX). The EDX analysis showed that the weight ratios of Cl/Na in sea-salt particles tended to decrease with increasing excess sulfur. Also, the Cl deficiency (loss) was large in smaller sea-salt particles. Moreover, the present research also suggests that sea-salt particles play an important role in the heterogeneous formation of sulfuric acid particles in the tropical free troposphere.