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We analyzed long-term fine- and coarse-mode synergetic observations ofnitrate and related aerosols (SO42−, NO3−,NH4+, Na+, Ca2+) at Fukuoka(33.52° N, 130.47° E) from August 2014 to October 2015. AGoddard Earth Observing System chemical transport model (GEOS-Chem) includingdust and sea salt acid uptake processes was used to assess the observedseasonal variation and the impact of long-range transport (LRT) from theAsian continent. For fine aerosols (fSO42−, fNO3−,and fNH4+), numerical results explained the seasonal changes,and a sensitivity analysis excluding Japanese domestic emissions clarifiedthe LRT fraction at Fukuoka (85 % for fSO42−, 47 % forfNO3−, 73 % for fNH4+). Observational dataconfirmed that coarse NO3− (cNO3−) made up thelargest proportion (i.e., 40–55 %) of the total nitrate (defined as thesum of fNO3−, cNO3−, and HNO3) during thewinter, while HNO3 gas constituted approximately 40 % of thetotal nitrate in summer and fNO3− peaked during the winter.Large-scale dust–nitrate (mainlycNO3−) outflow from China to Fukuoka was confirmed during alldust events that occurred between January and June. The modeledcNO3− was in good agreement with observations between July andNovember (mainly coming from sea salt NO3−). During the winter,however, the model underestimated cNO3− levels compared to theobserved levels. The reason for this underestimation was examinedstatistically using multiple regression analysis (MRA). We usedcNa+, nss-cCa2+, and cNH4+ as independentvariables to describe the observed cNO3− levels; these variableswere considered representative of sea salt cNO3−, dustcNO3−, and cNO3− accompanied bycNH4+), respectively. The MRA results explained the observedseasonal changes in dust cNO3− and indicated that thedust–acid uptake schemereproduced the observed dust–nitrate levels even in winter. The annualaverage contributions of each component were 43 % (sea saltcNO3−), 19 % (dust cNO3−), and 38 %(cNH4+ term). The MRA dust–cNO3− component had ahigh value during the dust season, and the sea salt component made a largecontribution throughout the year. During the winter, cNH4+ termmade a large contribution. The model did not include aerosol microphysicalprocesses (such as condensation and coagulation between the fineanthropogenic aerosols NO3− and SO42− and coarseparticles), and our results suggest that inclusion of aerosol microphysicalprocesses is critical when studying observed cNO3− formation,especially in winter

atmospheric chemistry and physics

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