Atmospheric Aerosols over the Western Pacific Ocean during the R/V Mirai Cruises in 2002

Abstract

The optical and chemical properties of atmospheric aerosols were measured over the Pacific Ocean during two R/V Mirai cruises. One was the MR02-K02 cruise (winter cruise), crossing the western Pacific from Japan to 5°S between February 21 and March 31, 2002. The second was the MR02-K04 cruise (summer cruise), that covered the western Pacific Ocean, the Indian Ocean, and the seas around Indonesia from Japan to 10°S between June 25 and August 22, 2002. The surveys mainly encountered three kinds of air masses, maritime air masses (marine aerosol), air masses influenced by fossil fuel combustion (FF aerosol), and those influenced by biomass burning (BB aerosol). From 10°N to 20°N, marine aerosols were observed both in the summer and winter cruises with the value of the single scattering albedo (ω) above 0.98 in the summer and, 0.92 in the winter. The concentrations of anthropogenic components such as elemental carbon, vanadium, and arsenic were somewhat low. Sea salt particles play a dominant role in the variation of the scattering coefficients, and the optical thickness in this region. Between 30°N and 40°N, FF aerosols were observed on both cruises. With the absorptive aerosols, the mean ω in the winter cruise was 0.74, while with the transparent aerosols in the summer cruise, the mean ω was 0.92. This seasonal difference comes mainly from differences in the concentrations of elemental carbon. There were BB aerosols around Indonesia in the summer, with a mean ω of 0.72. This air mass contained the most absorptive aerosol measured on these cruises. The chemical compositions of BB particles were different from the other aerosols, with carbonous components and non-sea salt potassium in high concentrations and metal components, such as vanadium and arsenic, in low concentrations.