Evaluation of Transboundary Secondary Organic Aerosol in the Urban Air of Western Japan: Direct Comparison of Two Site Observations

Transboundary secondary organic aerosol (SOA) from the Asian continent may substantially influence the urban air quality of western Japan, where the local anthropogenic emissions are expected to be the major sources of airborne particulate organic matter. To better understand the influence of transboundary SOA, we conducted simultaneous field studies in March 2012 and December 2010 at sites in Fukuoka city and Fukue Island, representing an urban and a background site of the northern Kyushu region in western Japan, respectively. During the studies, high time resolution (10 min) measurements of organic aerosol (OA) and its carboxylate component (m/z 44) in fine particulate matter (PM1.0) were conducted using aerosol mass spectrometers. Independently, total suspended particulate matter was collected daily on filter media, and the concentration of the low-volatile water-soluble organic carbon (LV-WSOC) component, which was expected to include SOA, and its stable carbon isotope ratio (delta C-13) were analyzed by an elemental analyzer coupled with a high-precision isotope ratio mass spectrometer. Results showed that the concentrations of m/z 44 and LV-WSOC at Fukue were higher than those at Fukuoka, while the magnitude of OA at Fukue was comparable to or less than that at Fukuoka during the study periods. A comparison of the daily averaged concentrations of m/z 44 and LV-WSOC between the two sites showed high correlations during both study periods (r(2) higher than 0.72), while the OA concentrations were less correlated (r(2) higher than 0.47). Given that the OA, m/z 44, and LV-WSOC at Fukue were the transboundary transport origin, the high correlations imply the predominant contribution of m/z 44 and LV-WSOC from the transboundary origin in the Fukuoka urban air. A comparison between m/z 44 and LVWSOC concentrations showed high correlations (R-2 > 0.91) with similar slopes of linear regressions (ca. 0.3 mu g mu g(-1) of C) at both sites during the spring study. Furthermore, the delta C-13 plot of LV-WSOC as a function of the m/z 44 fraction in OA in the urban air showed a systematically increasing trend, which is evidence of predominant SOA in LV-WSOC and a binary mixture of primary and secondary OA in PM1.0. Although the same comparison for the winter study also displayed high correlations between LV-WSOC and m/z 44 at both sites (R-2 > 0.85), the slopes of the linear regressions were significantly different (0.58 and 0.22 mu g mu g(-1) of C for Fukuoka and Fukue, respectively) and the delta C-13 plot exhibited a random variation, indicating a complex mixture of OA and/or LV-WSOC. The series of findings suggest that SOA from the transboundary transport origin predominated LV-WSOC, even in the urban air of western Japan, where OA from the local origin was significant.