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+, NH4+, K+, Mg2+, and Ca2+) to anions (SO42–, NO3–, Cl–, and F–) indicated that the fine particles (0.86) were more acidic than the coarse ones (1.60). The average NH4+/SO42– 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 NH3-poor atmosphere in Chengdu; hence, the NO3– 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 NO3– molar concentrations during spring and winter; therefore, the formation of particle-phase NO3– may be accelerated by increased A/S in both NH3-poor and -rich atmospheres. Moreover, the A/S and NO3–/SO42– molar ratios displayed negative and positive correlations during spring and winter, respectively, suggesting that the variation in atmospheric NH4+ (or NH3) during winter affected the formation of NO3– more strongly than that of SO42–, whereas more SO42– than NO3– was formed in the NH3-poor atmosphere during spring, when most of the NO3– in the aerosols would be expected to form via heterogeneous reactions.