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 (SO2 4 , NO3 , NHC4 , NaC, Ca2C) 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 (fSO2 4 , fNO3 , and fNHC4 ), numerical results explained the seasonal changes, and a sensitivity analysis excluding Japanese domestic emissions clarified the LRT fraction at Fukuoka (85% for fSO2 4 , 47% for fNO3 , 73% for fNHC4 ). 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 cNaC, nsscCa2 C, and cNHC4 as independent variables to describe the observed cNO3 levels; these variables were considered representative of sea salt cNO3 , dust cNO3 , and cNO3 accompanied by cNHC4 ), 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% (cNHC4 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, cNHC4 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 SO2 4 and coarse particles), and our results suggest that inclusion of aerosol microphysical processes is critical when studying observed cNO3 formation, especially in winter.