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

摘要

pstrongAbstract./strong We analyzed long-term fine- and coarse-mode synergetic observations of nitrate and related aerosols (SOsub4/subsup2a??/sup, NOsub3/subsupa??/sup, NHsub4/subsup+/sup, Nasup+/sup, Casup2+/sup) at Fukuoka (33.52?°span class="thinspace"/spanN, 130.47?°span class="thinspace"/spanE) 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 (fSOsub4/subsup2a??/sup, fNOsub3/subsupa??/sup, and fNHsub4/subsup+/sup), numerical results explained the seasonal changes, and a sensitivity analysis excluding Japanese domestic emissions clarified the LRT fraction at Fukuoka (85span class="thinspace"/span% for fSOsub4/subsup2a??/sup, 47span class="thinspace"/span% for fNOsub3/subsupa??/sup, 73span class="thinspace"/span% for fNHsub4/subsup+/sup). Observational data confirmed that coarse NOsub3/subsupa??/sup (cNOsub3/subsupa??/sup) made up the largest proportion (i.e., 40a??55span class="thinspace"/span%) of the total nitrate (defined as the sum of fNOsub3/subsupa??/sup, cNOsub3/subsupa??/sup, and HNOsub3/sub) during the winter, while HNOsub3/sub gas constituted approximately 40span class="thinspace"/span% of the total nitrate in summer and fNOsub3/subsupa??/sup peaked during the winter. Large-scale dusta??nitrate (mainly cNOsub3/subsupa??/sup) outflow from China to Fukuoka was confirmed during all dust events that occurred between January and June. The modeled cNOsub3/subsupa??/sup was in good agreement with observations between July and November (mainly coming from sea salt NOsub3/subsupa??/sup). During the winter, however, the model underestimated cNOsub3/subsupa??/sup levels compared to the observed levels. The reason for this underestimation was examined statistically using multiple regression analysis (MRA). We used cNasup+/sup, nss-cCasup2+/sup, and cNHsub4/subsup+/sup as independent variables to describe the observed cNOsub3/subsupa??/sup levels; these variables were considered representative of sea salt cNOsub3/subsupa??/sup, dust cNOsub3/subsupa??/sup, and cNOsub3/subsupa??/sup accompanied by cNHsub4/subsup+/sup), respectively. The MRA results explained the observed seasonal changes in dust cNOsub3/subsupa??/sup and indicated that the dusta??acid uptake scheme reproduced the observed dusta??nitrate levels even in winter. The annual average contributions of each component were 43span class="thinspace"/span% (sea salt cNOsub3/subsupa??/sup), 19span class="thinspace"/span% (dust cNOsub3/subsupa??/sup), and 38span class="thinspace"/span% (cNHsub4/sub