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 4 2− , 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 4 2− , 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 4 2− , 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 4 2− and coarse particles), and our results suggest that inclusion of aerosol microphysical processes is critical when studying observed cNO 3 − formation, especially in winter.