Ozone and its precursors in a high-elevation and highly forested region in central China: Origins, in-situ photochemistry and implications of regional transport

Atmospheric chemistry observation and modelling in alpine areas have strong indication of regional air quality. In this study, one-month continuous ozone (O3) measurement at a high-elevation and highly forested site in central China, in combination with observations at similar stations, indicated elevated O3 levels in the free troposphere (FT) over China. Uninterrupted O3 injuries to the old-growth forest in the study region were expected. FT O3 overwhelmed daytime photochemical formation at the site, and the transport of O3 from the north (e.g., Guanzhong Plain) was most significant. Air masses from the adjacent southwest and south regions (e.g., Chongqing) contained higher levels of nitric oxide, sulfur dioxide, fine particles and volatile organic compounds (VOCs) in association with vehicular and industrial emissions. In contrast, air masses from the northeast (e.g., North China Plain) were laden with combustion tracers. The in-situ photochemistry modelling confirmed weak O3 formation at the site. For the reasons, the insufficient nitrogen oxides suppressed transformation of peroxyl radicals to alkoxy and hydroxyl radicals, resulting in considerable losses of peroxyl radicals via self-reactions. VOCs showed little impact on in-situ O3 production, and accounted for net consumption of OH, with isoprene as the most predominant OH depleting species. This study fills the gaps in observation of O3 and its precursors and modelling of in-situ photochemistry in high-elevation regions of central China, and provides hints for the impacts of transport on air pollution in this region.