Abstract
Abstract Individual aerosol particles were collected in the middle troposphere, at 5–6 km altitude, over the western Pacific Ocean in the Pacific Atmospheric Chemistry Experiment campaign, between 38°S and 38°N, in January 1994. Particles were collected on electron microscope grids, over approximately 1.5° latitude bands using a 1-mm diameter jet impactor and were examined with an energy-dispersive X-ray (EDX) analyzer and transmission electron microscope. Based on the EDX analysis and morphological identification, particles were classified into various types, including sea salt, mineral, sulfate, and sulfuric acid. For the radius range of 0.1–1 μm, sulfuric acid and ammonium sulfate were found across most of the observed regions and are considered important background aerosols. As well, sea salt particles were abundant over the tropical regions (present at up to 67%), in association with vertical transport by convective clouds. North of 20°N, mineral particles were dominant, suggesting transport of Asian dust–storm particles from China in the westerly air stream. Evidence for the modification of mineral particles, prior to their collection in the free troposphere, is also presented and discussed.
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