Characteristics of air pollutants and greenhouse gases at a regional background station in Southwestern China

The characteristics of air pollutants and greenhouse gases at regional background sites are critical to assessing the impact of anthropogenic emissions on the atmospheric environment, ecosystems and climate change. However, observational studies are still scarce at such background sites. In this study, continuous hourly observations of air pollutants (O₃, CO, SO₂, NO_x, PM_2.5 and PM₁₀) and greenhouse gases (CO₂, CH₄ and N₂O) were performed for one year (from January 1 to December 31, 2017) at the Gongga Mountain background station (GGS; 101°97′E, 29°55′N; elevation: 3541 m) in southwestern China. The concentrations and variations of these air pollutants and greenhouse gases were determined, and the effect of transboundary atmospheric transport on the air pollution at the study site was investigated. The results showed that the average annual concentrations (mixing ratios) of the O₃, CO, SO₂, NO₂, CO₂, CH₄, N₂O, PM_2.5 and PM₁₀ were 74.7 ± 22.0 µg m^–3, 0.3 ± 0.2 mg m^–3, 0.5 ± 0.6 µg m^–3, 1.7 ± 1.3 µg m^–3, 406.1 ± 9.5 ppm, 1.941 ± 0.071 ppm, 324.5 ± 14.8 ppb, 6.5 ± 6.2 µg m^–3 and 10.6 ± 11.2 µg m^–3, respectively. The concentrations (mixing ratios) of the abovementioned substances at the GGS are comparable to those at other background sites in China and around the world. The slight differences among concentrations at different sites may be mainly attributable to the impacts of anthropogenic emissions near the background sites and meteorological conditions. High values of O₃ were observed in spring and summer, while SO₂ and PM_2.5 showed higher concentrations in summer than in autumn. Relatively high CO, NO₂ and PM₁₀ values were mostly observed in spring and winter. Relatively low CO₂ concentrations were observed in summer due to the vigorous summertime photosynthesis of vegetation. The lowest concentrations for CH₄ were recorded in summer, whereas the levels in the other three seasons were similar to each other; by contrast, the highest N₂O concentrations were observed in summer due to enhanced microbial activity resulting from high ambient summer temperatures. A diurnal variation in O₃ was observed, with early morning minima and afternoon maxima. CO and NO₂ displayed higher concentrations in the daytime than in the nighttime. A slight increase in both PM_2.5 and PM₁₀ concentrations was also recorded in the daytime. These patterns were closely related to scattered anthropogenic emissions and regional atmospheric transport. Nevertheless, CO₂ exhibited lower concentrations in the daytime than in the nighttime, although CH₄ showed no obvious diurnal variation. The N₂O concentration peaked between 10:00 and 12:00 (local time), which can be ascribed to the enhancement of microbial activity due to the increased soil temperature. The results based on the relationship between the wind and the concentrations of air pollutants and greenhouse gases were almost consistent with those based on the potential contribution source function. It appears that O₃ and its precursors in parts of Inner Mongolia and Gansu, Ningxia, Sichuan, Chongqing and Hubei Provinces as well as adjacent areas of Hunan, Guizhou and Guangxi Provinces contributed to the increase in O₃ at the study site. The potential source areas for CO and SO₂ were similar and mainly distributed in India and Pakistan and areas of Inner Mongolia and Gansu and Guizhou Provinces in China. Potential source areas for NO₂, PM_2.5 and PM₁₀ were found in neighboring countries of South Asia in addition to domestic regions, including Inner Mongolia, Gansu Province and the Cheng-Yu economic region. Furthermore, parts of Yunnan Province (China) as well as India and Pakistan were potential source areas for CO₂, CH₄ and N₂O.