Microsoft Word-Size-segregated_PM_manuscript_rev1

1 Seasonal characteristics of sulfate and nitrate in size-segregated 1 particles in ammonia-poor and -rich atmosphere at Chengdu, 2 Southwest China 3 4 Qinkai Li1,2, Zhou Yang3, Xiaodong Li1,2*, Shiyuan Ding1,2, Feng Du4 5 6 1Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China 7 2Tianjin Key Laboratory of Earth Critical Zone Science and Sustainable Development in Bohai 8 Rim, Tianjin University, Tianjin 300072, China 9 3College of Tourism and Geographical Sciences, Tongren University, Tongren 565300, China 10 4State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu 11 University of Technology, Chengdu 610059, China 12 13 Abstract 14 15 In order to understand the seasonal characteristics of water-soluble inorganic ions (WSIIs) in 16 aerosols in the urban atmosphere, size-segregated particulate matter (PM) samples were collected 17 over a one-year period from February 2012 to January 2013 in a typical urban location, Chengdu 18 in Southwest China using an Andersen Cascade Impactor sampler. PM mass concentrations, 19 particularly the fine fraction, peaked during winter, and WSIIs were more enriched in fine fraction 20 (21.7%) compared to that in the coarse fraction (9.2%). The sums of equivalent ratios of cations 21 (Na+, NH4+, K+, Mg2+ and Ca2+) to anions (SO42−, NO3−, Cl− and F−) indicated that fine particles 22 (0.86) were more acidic than coarse particles (1.60). Average NH4+/SO42− molar ratio (A/S) in fine 23 fraction (1.79) was much higher in winter than in other three seasons (below 1.5), implied a general 24 NH3-poor atmosphere in Chengdu, and hence, the NO3− in fine particles were principally formed 25 through homogeneous reactions involving ammonia and nitric acid in winter, yet heterogeneously 26 formed during the other three seasons. Significant positive correlations were observed for A/S and 27 NO3− molar concentrations in spring and winter, thus the formation of particle NO3− could be 28 promoted by the increase of A/S under both NH3-poor and -rich atmosphere. Moreover, the A/S 29 and NO3−/SO42− molar ratio displayed negative and positive correlations in spring and winter, 30 respectively, suggested that the formation of NO3− was more sensitive than that of SO42− to the 31 variations in atmospheric NH4+ (or NH3) in winter, whereas more SO42− could be formed than NO3− 32 in NH3-poor atmosphere during spring, when most of the aerosol NO3− were expected to be 33 heterogeneously formed. 34 35