Field Studies for Secondary Organic Aerosol in the Transboundary Air

Abstract:

To study formation of secondary organic aerosol (SOA) in the air outflowed from the Chinese continent and its fraction in an urban city located in downwind, we have conducted field studies at two background sites and one urban site in the western Japan: the Cape Hedo Aerosol and Atmospheric Monitoring Station (26.9˚N, 128.3˚E), the Fukue Atmospheric Monitoring Station (32.8˚N, 128.7˚E), and Fukuoka University (33.6˚N, 130.4˚E), respectively. During the studies, stable carbon isotope ratio (δ¹³C) of low-volatile water-soluble organic carbon (LV-WSOC) was measured in 24 h collected filter samples of total suspended particulate matter. Concentration of fine organic aerosol and the proportion of the signal at m/z 44 (ions from the carboxyl group) in the organic mass spectra (f₄₄) were also measured by Aerodyne aerosol mass spectrometers. Limited to the Fukue site only, mixing ratios of trace gas species, such as aromatic hydrocarbons, NO_x, and NO_y, were also measured using GC-FID and NO_x and NO_yanalyzers for estimation of photochemical age (t[OH]).

A case study in December 2010 showed that plots of δ¹³C versus f₄₄ showed systematic variations at Hedo and Fukue. However, their trends were opposite. At Fukue the trend was consistent in the plot of δ¹³C of LV-WSOC versus t[OH] estimated by the NO_x/NO_y or the hydrocarbon ratios, indicating influence of SOA. The systematic trends aforementioned qualitatively agreed with a binary mixture model of SOA with background LV-WSOC having the f₄₄ of ~0.06 and the δ¹³C of -17‰ or higher, implication of some influence of primary emission associated with C₄plants.

Given that the LV-WSOC at the urban Fukuoka site was a binary mixture, a mass balance for δ¹³C was constructed below. In the equation, δ¹³C_Mix, δ¹³C_Local, δ¹³C_Trans, and F_Local are δ¹³C of binary LV-WSOC mixture, δ¹³C of LV-WSOC from local emission origin, δ¹³C of LV-WSOC from transboundary pollution origin, and a fraction of LV-WSOC from local emission origin in the mixture, respectively. We applied d¹³C values at Fukuoka and Fukue to δ¹³C_Mix and δ¹³C_Trans, respectively, and δ¹³C of -27‰ for vehicular emission to δ¹³C_Local. The preliminary results demonstrated that, except some negative F_Local, the majority of calculated F_Local ranged from 4% to 86% and the lower F_Local, the higher LV-WSOC concentration.