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

Uno, Itsushi; Osada, Kazuo; Yumimoto, Keiya; Wang, Zhe; Itahashi, Syuichi; Pan, Xiaole; Hara, Yukari; Kanaya, Yugo; Yamamoto, Shigekazu; Fairlie, Thomas Duncan

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.

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Uno, Itsushi / Osada, Kazuo / Yumimoto, Keiya / et al: Seasonal variation of fine- and coarse-mode nitrates and related aerosols over East Asia: synergetic observations and chemical transport model analysis. 2017. Copernicus Publications.

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