Molecular composition of the water‐soluble fraction of atmospheric carbonaceous aerosols collected during ACE‐Asia

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Abstract
During the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia), samples of carbonaceous aerosols were collected on board the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) De Havilland DHC‐6 Twin Otter aircraft. The samples were analyzed to determine their total carbon (TC) and water‐soluble organic compound (WSOC) contents as well as to identify the individual compounds comprising the WSOC fraction of the aerosol. The TC concentrations varied from 3.5 to 14.3 μg C m−3; the highest TC levels were observed for samples collected in pollution layers that originated over mainland China. WSOC concentrations ranged from 0.54 to 7.2 μg C m−3, with the WSOC fraction contributing from 10 to 50% of the carbon mass. About 50% of the carbonaceous aerosol mass in pollution layers could be attributed to WSOC. For samples collected in dust layers the WSOC fraction of TC was much lower than that observed in pollution layers. The sum of all the detected organic ions accounted for 6.9–19% of the WSOC. In the six samples collected by the Twin Otter during ACE‐Asia, of the organic ions identified in the WSOC fraction, oxalate had the highest concentration. Samples collected from pollution layers exhibited a slightly higher ratio of formate to oxalate as compared to the other samples. Two samples had a relatively high ratio of lactate to oxalate, which might be a signature of some currently unidentified source of carbonaceous aerosol. The sum of the masses of sulfate and nitrate ions exceeded the sum of the masses of the identified organic ions by a factor of 9 to 17. The chemical levoglucosan, a tracer for biomass burning, comprised from 0.1 to 0.4% of TC mass. Comparing this ratio to the ratio measured directly in wood‐burning studies it was determined that biomass burning may have represented from ≈2 to 10% of the carbonaceous aerosol collected during ACE‐Asia.

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