Volatile Organic Compound Measurements and Oxidant Modeling at a Remote Mountain Site in the western North Pacific

Anthropogenic pollution, including primary compounds such as nitrogen oxides (NOx) and organics, but also secondary pollutants such as ozone, are a threat to sensitive mountain ecosystems, which are already under continued threat of diminished area, due to climate change. This study analyzes two days of volatile organic compound (VOC) concentrations, including a limited set of carbonyls, at the Lulin Atmospheric Background Station (LABS; 2,862 meters asl), which is a well-known remote sub-tropical site that receives long-range transported air masses from various clean and polluted origins. VOCs were sampled in a canister and analyzed by GC-MS/FID, while carbonyls were trapped on Supelco DNPH-coated cartridges and quantified by HPLC-UV. Routine LABS measurements of ozone, CO, and PM_2.5 were also utilized; Copernicus Atmospheric Monitoring Service (CAMS) reanalysis NOx and HNO₃ concentrations were further incorporated into an oxidant modeling analysis. In comparing the two sampling days, the first day was characterized by hazier conditions (~3 times higher PM_2.5 concentrations) and 2-3 times higher VOC and CO concentrations and calculated OH reactivity values. Based on a HYSPLIT back-trajectory analysis, the first day was influenced by air that had a longer residence time at lower altitudes over South China, while the second day was characterized by higher level transport. Also influential at the site are subsidence events that bring drier air that can contain high ozone and NOy species. The measurement and reanalysis data were incorporated into a 0-dimensional modeling analysis (F0AM) of the gas-phase oxidation chemistry in order to further characterize the different air masses that influence this sensitive mountain area. While the first day also was simulated to have higher level of ozone production, the second day captured an afternoon spike in pollutants and ozone production that was likely driven by upslope mountain valley air sourced for lower altitudes in Taiwan. Thus, the oxidation potential in this environment is characterized by large swings driven by local upslope pollution, subsidence events, and long-range transported pollution from the Asian continent, which ultimately dictate the pollutant exposure for this sensitive ecosystem.