Seasonal variation of fine- and coarse-mode nitrates and related aerosols over East Asia: synergetic observations and chemical transport model analysis

Abstract

We analyzed long-term fine- and coarse-mode synergetic observations of nitrate and related aerosols (SO42−, NO3−, NH4+, Na+, Ca2+) 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 (fSO42−, fNO3−, and fNH4+), numerical results explained the seasonal changes, and a sensitivity analysis excluding Japanese domestic emissions clarified the LRT fraction at Fukuoka (85 % for fSO42−, 47 % for fNO3−, 73 % for fNH4+). Observational data confirmed that coarse NO3− (cNO3−) made up the largest proportion (i.e., 40–55 %) of the total nitrate (defined as the sum of fNO3−, cNO3−, and HNO3) during the winter, while HNO3 gas constituted approximately 40 % of the total nitrate in summer and fNO3− peaked during the winter. Large-scale dust–nitrate (mainly cNO3−) outflow from China to Fukuoka was confirmed during all dust events that occurred between January and June. The modeled cNO3− was in good agreement with observations between July and November (mainly coming from sea salt NO3−). During the winter, however, the model underestimated cNO3− levels compared to the observed levels. The reason for this underestimation was examined statistically using multiple regression analysis (MRA). We used cNa+, nss-cCa2+, and cNH4+ as independent variables to describe the observed cNO3− levels; these variables were considered representative of sea salt cNO3−, dust cNO3−, and cNO3− accompanied by cNH4+), respectively. The MRA results explained the observed seasonal changes in dust cNO3− 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 cNO3−), 19 % (dust cNO3−), and 38 % (cNH4+ term). The MRA dust–cNO3− component had a high value during the dust season, and the sea salt component made a large contribution throughout the year. During the winter, cNH4+ term made a large contribution. The model did not include aerosol microphysical processes (such as condensation and coagulation between the fine anthropogenic aerosols NO3− and SO42− and coarse particles), and our results suggest that inclusion of aerosol microphysical processes is critical when studying observed cNO3− formation, especially in winter.