Wintertime simultaneous measurement and model analysis of fine- and coarse-mode sulfateand nitrate over East Asia

Abstract:

Sulfate and nitrate aerosols are major component of PM_2.5. In East Asia, it is well known that sulfate aerosols are widely distributed due to large-scale trans-boundary air pollution. For nitrate aerosols, it is considered that fine-mode nitrate can be converted into coarse-mode nitrate on reactions with sea-salt aerosols over East China Sea when transported from China to Japan, however, contributions of trans-boundary air pollution on fine-mode nitrate is not clarified. We observed sulfate and nitrate aerosols on the basis of ACSA (Aerosol Chemical Speciation Analyzer; KIMOTO Electric Co., LTD.) in high-temporal resolution (1 hr) on fine- and coarse-mode at Dazaifu, Fukuoka located in western-part of Japan. Two-way nested (81-27 km) simulation in East Asia by WRF-CMAQ modeling system was developed to investigate the source contributions of fine- and coarse-mode nitrate when the trans-boundary air pollution was occurred. Observed monthly mean concentration in January 2014 at Dazaifu site were 3.41 μg/m³ for fine-mode nitrate, 1.51 μg/m³ for coarse-model nitrate, and 3.81 μg/m³ for fine-mode sulfate. Simulation in fine scale resolution can reproduce the observed concentration with 4.79 μg/m³ (r = 0.60) for fine-mode nitrate, 1.24 μg/m³ (r = 0.51) for coarse-model nitrate, and 3.36 μg/m³ (r = 0.46) for fine-mode sulfate. At urban site in Japan, local-scale contributions are also considered to attribute fine-mode nitrate, and in order to further examine the contributions of trans-boundary air pollution, emission sensitivity analysis with switching off the anthropogenic emissions from Japan was conducted. This sensitivity simulation calculated that monthly mean concentration of 1.19 μg/m³ for coarse-mode nitrate and 3.26 μg/m³ for fine-mode sulfate, suggesting the dominance of trans-boundary air pollution for coarse-mode nitrate and fine-mode sulfate. On the other hand, monthly mean concentration was 1.80 μg/m³ for fine-mode nitrate on sensitivity simulation. That means the contribution from trans-boundary air pollution and local-scale air pollution were respectively 37.6% and 62.4% on monthly mean basis. Temporal variations of contribution of
trans-boundary/local-scale air pollution during the transportation from China to western-part of Japan are further studied, and presented.