Carbonaceous aerosol over the Indian Ocean: OC/EC fractions and selected specifications from size‐segregated onboard samples

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Abstract
Concentrations and mass fractions of organic carbon, elemental carbon, and numerous organic species have been determined for submicrometer and supermicrometer particles in the Indian Ocean on the cruise of the R/V Ron Brown during INDOEX in February/March 1999. The total carbon (TC) concentration was low for air masses originating from the southern hemisphere of the Indian Ocean, where only small amounts of organic carbon (OC) could be detected in supermicrometer (sea salt) particles. The OC concentration was typically more than one order of magnitude higher in the northern Indian Ocean, where continental air masses were encountered, both for submicrometer and supermicrometer particles. Elemental carbon (EC) was mostly found in submicrometer particles, increasing up to more than two orders of magnitude in the northern part compared to the southern part of the Indian Ocean. The mass fraction of carbonaceous material was in the range of 6–15% and 2–12% for submicrometer and supermicrometer particles, respectively, with the higher values for more polluted air masses. A high degree of mixing with light scattering material is the main reason for high‐absorption coefficients. The absorption efficiency is higher for lower mass fractions of EC. Various short‐chain dicarboxylic and hydroxylated dicarboxylic acids have been determined. Dicarboxylic acids were mostly found in supermicrometer particles, whereas hydroxylated dicarboxylic acids showed up mainly in submicrometer particles. Alkanes and polycyclic aromatic hydrocarbons have been found in small amounts, leading to the conclusion that an important part of the organic material is of secondary origin. This is confirmed by the good correlation of the sum of carboxylic acids with OC, nitrate with a less pronounced correlation with sulfate. However, low OC/EC ratios indicate the existence of primary OC.