A Measurement of Gaseous and Particulate Biogenic Semi-Volatile Organic Compounds in the Forest Atmosphere with an Annular Denuder Sampling System

Atmospheric concentrations of biogenic semi-volatile organic compounds (SOCs) in both gas and particle phase have been measured in two experimental forests with an annular denuder sampling system. The sampling sites were an experimental forest of Hokkaido University at Moshiri in Hokkaido Island, northern Japan (from August 22 to 29, 2002) and an experimental forest of Duke University at Chapel Hill in North Carolina, east coast of the United States (from July 9 to 12, 2003). The compounds detected in the forest atmosphere are glycolaldehyde, hydroxyacetone, n-nonanal, n-decanal, glyoxal, methylglyoxal and 4-oxopental. Glycolaldehyde, hydroxyacetone and methylglyoxal are known as oxidation products of isoprene which has the largest emission rate of any hydrocarbon species. Nonanal and decanal are probably directly emitted from vegetation. 4-oxopentanal is an oxidation product of 6-methyl-5-hepten-2-one. This is an oxidation product of organic compounds which are abundant on plant surfaces. All of these compounds were detected in both gas and particle phases. Their concentrations ranged from the detection limit (approximately 1 pptv) to 154 pptv (630 ng m-3, 4-oxopentanal), and the particulate concentrations ranged from the detection limit (approximately 3 ng m-3) to 200 ng m-3 (4-oxopentanal) in Moshiri, 2002. The gaseous concentrations of these compounds showed a clear diurnal variation on fine days. Although their production processes are different each other, the variation showed the same pattern which was quite similar to the ambient temperature. On the other hand, partition ratios (particle/(gas+particle)) of these compounds suggested that the process of gas-to-particle conversion of these SOCs is different between water soluble compounds and insoluble compounds. The partition ratios of water soluble compounds such as glycolaldehyde, hydroxyacetone and 4-oxopentanal (their Henry_fs constant exceed 10000 mol kg-1 atm-1) depends on relative humidity at the sampling site. This result suggested that water soluble SOCs can convert into particles via dissolution into aerosols existing in the atmosphere, and this is a particular process for the soluble SOCs. In this presentation, we will compare the emission and removal processes of biogenic SOCs in the forest atmosphere of Northern Japan with those in the East Coast of the United States. The observed SOCs behavior at the Duke Forest site is compared with VOCs and aerosol chemical composition observed by other investigators during the CELTIC study.