Responses of sulfate and nitrate to anthropogenic emission changes in eastern China - in perspective of long-term variations.

We investigate responses of sulfate (SO42-) and nitrate (NO3-) to anthropogenic emission changes in 2006-2017 by fixing meteorology at the 2009 level using nested 3D chemical transport model GEOS-Chem. We find that sulfate concentration decreases following SO2 emissions, but with a relatively smaller reduction rate (by 16 % in North China Plain (NCP) and 28 % in Yangtze River Delta (YRD)) due to larger sulfur oxidation ratio (SOR) at lower SO2 level. SOR follows a power law with SO2 emissions in general except in winter in NCP, when and where both SO2 emission reduction and atmospheric oxidation capacity are critical to the inter-annual variations of SOR. Nitrate concentration ([pNO3-]) decreases along with NOx emission reduction in summer, but increases slightly in winter in 2011-2017. Equilibrium with gas phase HNO3, NO3- in particle phase (pNO3-) is determined by total HNO3 (TN = [pNO3-] + [gHNO3]) oxidized from NO2 and gas-particle partitioning (ε(NO3-) = [pNO3-]/TN). TN is decreasing faster in summer (~33 %) than in winter (~25 %) in 2011-2017. In contrast, ε(NO3-) changes marginally in summer (within 5 %) but increases by 36 % in NCP and by 51 % in YRD in winter in 2006-2017. The increasing of ε(NO3-) in winter is attributed to the strong reduction of [pSO42-], which increases the relative abundance of NH3 and thus favors partitioning of NO3- to the particle phase. The effect of increasing ε(NO3-) overcomes that of decreasing TN in winter. We suggest reduce SO2 emissions to further reduce [pSO42-] in eastern China. In addition, we recommend reduce NOx emissions in summer, and reduce atmospheric oxidation capacity and relative abundance of NH3 in winter to reduce [pNO3-].