Seasonal Characteristics of Sulfate and Nitrate in Size-segregated Particles in Ammonia-poor and -rich Atmospheres in Chengdu, Southwest China

摘要

In order to determine the seasonal characteristics of water-soluble inorganic ions (WSIIs) in aerosols in urban atmospheres, size-segregated particulate matter (PM) samples were collected over a one-year period from February 2012 to January 2013 in a typical urban location, Chengdu in Southwest China, using an Andersen cascade impactor sampler. The PM mass concentrations, particularly the fine fraction, peaked during winter, and the WSIIs were more enriched in the fine fraction (21.7%) than the coarse fraction (9.2%). The sums of the equivalent ratios of cations (Na~(+), NH_(4)~(+), K~(+), Mg~(2+), and Ca~(2+)) to anions (SO_(4)~(2–), NO_(3)~(–), Cl~(–), and F~(–)) indicated that the fine particles (0.86) were more acidic than the coarse ones (1.60). The average NH_(4)~(+)/SO_(4)~(2–) molar ratio (A/S) in the fine fraction (1.79) was much higher during winter than the other three seasons (< 1.5), implying a generally NH_(3)-poor atmosphere in Chengdu; hence, the NO_(3)~(–) in the fine particles was principally formed through homogeneous reactions involving ammonia and nitric acid during winter, whereas it was heterogeneously formed during the other three seasons. Significant positive correlations were observed between the A/S and NO_(3)~(–) molar concentrations during spring and winter; therefore, the formation of particle-phase NO_(3)~(–) may be accelerated by increased A/S in both NH_(3)-poor and -rich atmospheres. Moreover, the A/S and NO_(3)~(–)/SO_(4)~(2–) molar ratios displayed negative and positive correlations during spring and winter, respectively, suggesting that the variation in atmospheric NH_(4)~(+) (or NH_(3)) during winter affected the formation of NO_(3)~(–) more strongly than that of SO_(4)~(2–), whereas more SO_(4)~(2–) than NO_(3)~(–) was formed in the NH3-poor atmosphere during spring, when most of the NO_(3)~(–) in the aerosols would be expected to form via heterogeneous reactions.