The impact of relative humidity on the size distribution and chemical processes of major water-soluble inorganic ions in the megacity of Chongqing, China

To investigate the impacts of relative humidity (RH) on the size distribution and evolution of water-soluble inorganic ions (WSIIs) in Chongqing, China, size-resolved aerosols were collected using nine-stage viable Andersen cascade impactors bi-weekly from June 2012 to May 2014. The WSIIs (SO42 −, NO3−, NH4+, K+, Na+, Cl−, Ca2 +, Mg2 + and F−) were analyzed. The SO42 −, NO3− and NH4+ peaks shifted from 0.43-0.65 μm particles at 40-60% RH to 0.65-1.1 μm particles at 70-90% RH. For K+, Na+ and Cl−, the peak concentrations were in the size range of 0.65-2.1 μm and increased as the RH increased from 40 to 90%. However, the Ca2 +, Mg2 + and F− peaks at all RH levels were mainly concentrated in the coarse mode, and their concentrations did not increase as regularly as those of SO42 −, NO3− and NH4+. All of the results suggest that RH was likely an important factor in determining the size distributions of the WSIIs. SO42 −, NO3− and NH4+ increased linearly in all size ranges as a function of RH (40-90%). Additionally, SO42 − presented the highest rate of mass increase at 1.8 μg/m3/10% RH in 1.1 < Dp ≤ 2.1 μm particles. For NO3−, the ratio of NO3− in the WSIIs in 1.1 < Dp ≤ 2.1 μm particles increased more with increasing RH than that in the other size ranges. The NO3− in 0.43 < Dp ≤ 2.1 μm particles increased by approximately 8% when the RH increased from 40-60% to 60-90%. The NH4+ in 1.1 < Dp ≤ 2.1 μm particles increased at the highest rate with increasing RH, and the proportion of NH4+ in the WSIIs in 0.43 < Dp ≤ 1.1 μm particles was higher than that in the other size ranges. Aerosol acidity in the fine-mode particles increased with increasing RH but decreased with the increase in particle size.