Hourly organic carbon (OC) and elemental carbon (EC) concentrations in PM_2.5 were measured from June 2013 to May 2014 in Wanzhou, the second largest city in the Chongqing Municipality, in the southwest of China. Results show that the annual average concentrations of OC and EC were 13.16 ± 7.98 and 3.12 ± 1.51 μgC·m⁻³, respectively. Clear seasonal variations of OC and EC concentrations were observed, with their concentrations at minima in summer and maxima in winter. The diel concentration profile of OC and EC presented a bimodal pattern, which was attributed to the cooperative effects of local meteorological conditions and source emissions. The daily average OC/EC ratio ranged from 2.05 to 8.17 with an average of 4.15 for the whole study period. Strong correlations between OC and EC were found in winter and spring, indicating their common sources, while their correlations were poorer in summer and autumn, indicating that the influence of biogenic emissions and secondary organic carbon (SOC) were significant during those seasons. The estimated SOC concentrations were 2.19 ± 1.55, 7.66 ± 5.89, 5.79 ± 3.51, and 3.43 ± 2.26 μgC·m⁻³, accounting for 29.2%, 52.7%, 27.4%, and 30.5% of total organic carbon in summer, autumn, winter, and spring, respectively. The analysis of back trajectories suggested that high PM_2.5, OC, and EC concentrations were associated with air masses originating from or passing over several industrial centers and urban areas in western and northwestern China. Air trajectories from the southeast with short pathways were the dominant trajectories arriving at Wanzhou, indicating that local sources had a big influence on PM_2.5, OC, and EC concentrations.

In order to explore the characteristics of water-soluble inorganic ions (WSIIs) in the atmosphere of Wanzhou, a small mountainous city in Chongqing, four representative seasonal PM_2.5 samples and gaseous precursors (SO₂ and NO₂) were collected from April 2016 to January 2017. The WSIIs (including Cl⁻, NO₃⁻, SO₄²⁻, Na⁺, NH₄ ⁺, K⁺, Mg²⁺, and Ca²⁺) were analyzed by ion chromatography. During the sampling period, daily PM_2.5 concentration varied from 3.47 to 156.30 μg·m⁻³, with an average value of 33.38 μg·m⁻³, which was lower than the second-level annual limit of NAAQS-China. WSIIs accounted for 55.6% of PM_2.5, and 83.1% of them were secondary inorganic ions (SNA, including SO₄²⁻, NO₃⁻, and NH₄⁺). The seasonal variations of PM_2.5 and WSIIs were similar, with the minimum in summer and the maximum in winter. PM_2.5 samples were the most alkaline in summer, weakly alkaline in spring and winter, and close to neutral in fall. The annual average ratio of NO₃⁻/SO₄²⁻ was 0.54, indicating predominant stationary sources for SNA in Wanzhou. NO₃⁻, SO₄²⁻, and NH₄⁺ mainly existed in the form of (NH₄)₂SO₄ and NH₄NO₃. The results of the principal component analysis (PCA) showed that the major sources of WSIIs in Wanzhou were the mixture of secondary inorganic aerosols, coal combustion, automobile exhaust (49.53%), dust (23.16%), and agriculture activities (9.68%). The results of the backward trajectory analysis showed that aerosol pollution in Wanzhou was mainly caused by local emissions. The enhanced formation of SNA through homogeneous and heterogeneous reactions contributed to the winter PM_2.5 pollution event in Wanzhou.

Major water-soluble ions were analyzed for two-year precipitation samples in Wanzhou, a typical urban site of the hinterland of Chinese Three Gorges Reservoir. The pH values of the precipitation were in the range of 4.0 to 8.3, and the volume-weighted mean (VWM) value was 5.0. The concentration order of anions and cations was as follows: SO42->NO3->Cl->F- and NH4+>Ca2+>Na+>K+>Mg2+, respectively. Good correlations were found between SO42- and NH4+, SO42- and Ca²⁺, NO3- and NH4+, and NO3- and Ca²⁺, implying their co-occurrence in the precipitation, most likely as (NH₄)₂SO₄, (NH₄)HSO₄, NH₄NO₃, CaSO₄, and Ca(NO₃)₂. The sum of all measured ions was 416.4 μeq L⁻¹, indicating serious air pollution in Wanzhou. NH4+ and Ca²⁺ were the most important ions neutralizing the acidic compounds in the precipitation; their major sources included agricultural activity and crustal dust. Local anthropogenic activities, for example, coal burning and traffic related sources, contributed most of SO42- and NO3-. The equivalent concentration ratio of SO42-/NO3- was 4.5, indicating that excessive emission of sulfur was the main reason leading to the precipitation acidity in Wanzhou. However, this ratio was lower than the ratio (5.9) in 2000s in Wanzhou, indicating that the contribution of nitric acid to the acidity of precipitation was strengthening.