Summertime characteristics and in-situ photochemistry of volatile organic compounds in a high-altitude region in central China: Implications of regional transport

The chemical characteristics and photochemistry of volatile organic compounds (VOCs) were investigated at Shennongjia (SNJ), a high-altitude site in central China, from July to August 2023. The mean mixing ratio of total VOCs (TVOCs) was 14.61 ± 1.64 ppbv, dominated by alkanes (35.4 %) and carbonyls (29.4 %). The northeastern air mass exhibited the highest VOC concentration (16.33 ± 7.62 ppbv), followed by the southeastern air mass (10.45 ± 3.25 ppbv). Analysis of potential source contribution function (PSCF) and concentration-weighted trajectory (CWT) identified central/eastern Hubei and the Sichuan Basin as major source regions for VOCs at SNJ. An observation-based photochemical box model was employed to examine VOC photochemistry and in-situ O₃ formation. Extremely low nitric oxide (NO) mixing ratios led to incomplete HOx cycling, and VOCs actively participated in the generation of free radicals. In-situ O₃ formation was governed by regionally transported air masses with distinct VOC compositions. Northwestern air masses, characterized by high temperature, high carbonyls, and low relative humidity (RH) despite lower TVOCs (8.10 ± 1.29 ppbv), yielded the highest net O₃ production rate (1.27 ± 0.33 ppbv/h), with formaldehyde photolysis contributing 13.3 % to hydroperoxyl radical (HO₂) generation. Conversely, the southeastern air masses, despite carrying the highest TVOC load, inhibited O₃ formation due to elevated RH. Formaldehyde and isoprene dominated photochemical reactivity. Our results highlight that VOC speciation, especially reactive carbonyls and biogenic VOCs, critically controls radical cycling and O₃ dynamics in remote regions.