Vertical profile of ambient VOCs in background region of Southwest China from Mt. Fanjing observation

Abstract

Abstract Atmospheric circulation and local meteorology strongly influence the transport and evolution of volatile organic compounds (VOCs) in mountainous environments, yet their vertical distribution and controlling factors in southwest China remain poorly characterized. Here, we conducted synchronous online measurements of VOCs and related air pollutants at the foot (550 m a.s.l.) and top (2119 m a.s.l.) of Mt. Fanjing during autumn and winter 2024, providing a rare side-by-side characterization of VOCs under contrasting atmospheric conditions in the background region of the Yunnan–Guizhou Plateau. The mean VOC concentration at the mountain top was 14.1 ± 4.8 ppbv, 17.5% lower than that at the mountain foot. Source apportionment showed that vehicles, biomass burning, and industrial-related sources together contributed more than 50% of VOCs at the foot site, whereas the relative contributions of aged traffic plume and solvent evaporation increased at the mountain top, reflecting the combined influence of transport and atmospheric processing. Elevated VOC species at both sites were mainly associated with emissions from the border region of Guizhou, Hunan, and Chongqing, in agreement with the emission inventory analysis. Mt. Fanjing was primarily affected by easterly and westerly background winds. Under westerly background conditions, air masses passing through the Sichuan Basin, Chongqing, and Yunnan enhanced the vertical gradient of VOCs at Mt. Fanjing and increased near-surface VOC accumulation, particularly during periods with pronounced diurnal evolution of local circulation. These results demonstrate that the vertical contrast in VOCs over Mt. Fanjing is jointly controlled by regional transport, background wind regimes, and thermally driven local circulation. The findings highlight that elevated mountain sites cannot be regarded simply as clean background environments and have important implications for regional air-quality assessment, ozone and secondary aerosol formation, and pollutant exposure in mountainous and ecologically sensitive areas.