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.