Seasonal variation of fine- and coarse-mode nitrates and related aerosols over East Asia: synergetic observations and chemical transport model analysis
We analyzed long-term fine- and coarse-mode synergetic observations of nitrate and related aerosols (SO 42−, NO 3−, NH 4+, Na +, Ca 2+) at Fukuoka (33.52° N, 130.47° E) from August 2014 to October 2015. A Goddard Earth Observing System chemical transport model (GEOS-Chem) including dust and sea salt acid uptake processes was used to assess the observed seasonal variation and the impact of long-range transport (LRT) from the Asian continent. For fine aerosols (fSO 42−, fNO 3−, and fNH 4+), numerical results explained the seasonal changes, and a sensitivity analysis excluding Japanese domestic emissions clarified the LRT fraction at Fukuoka (85 % for fSO 42−, 47 % for fNO 3−, 73 % for fNH 4+). Observational data confirmed that coarse NO 3− (cNO 3−) made up the largest proportion (i.e., 40–55 %) of the total nitrate (defined as the sum of fNO 3−, cNO 3−, and HNO 3) during the winter, while HNO 3 gas constituted approximately 40 % of the total nitrate in summer and fNO 3− peaked during the winter. Large-scale dust–nitrate (mainly cNO 3−) outflow from China to Fukuoka was confirmed during all dust events that occurred between January and June. The modeled cNO 3− was in good agreement with observations between July and November (mainly coming from sea salt NO 3−). During the winter, however, the model underestimated cNO 3− levels compared to the observed levels. The reason for this underestimation was examined statistically using multiple regression analysis (MRA). We used cNa +, nss-cCa 2+, and cNH 4+ as independent variables to describe the observed cNO 3− levels; these variables were considered representative of sea salt cNO 3−, dust cNO 3−, and cNO 3− accompanied by cNH 4+), respectively. The MRA results explained the observed seasonal changes in dust cNO 3− and indicated that the dust–acid uptake scheme reproduced the observed dust–nitrate levels even in winter. The annual average contributions of each component were 43 % (sea salt cNO 3−), 19 % (dust cNO 3−), and 38 % (cNH 4+ term). The MRA dust–cNO 3− component had a high value during the dust season, and the sea salt component made a large contribution throughout the year. During the winter, cNH 4+ term made a large contribution. The model did not include aerosol microphysical processes (such as condensation and coagulation between the fine anthropogenic aerosols NO 3− and SO 42− and coarse particles), and our results suggest that inclusion of aerosol microphysical processes is critical when studying observed cNO 3− formation, especially in winter.