Surface ozone in southeast Tibet: variations and implications of tropospheric ozone sink over a highland.

Environment context: One-year-long on-line measurements of surface O₃ and CO mixing ratios were performed on the southeast Tibetan Plateau to examine O₃ behaviour. During the daytime, the O₃ mixing ratio was strongly affected by vertical air exchange. The O₃ mixing ratio was high in the afternoon and decreased at night, indicating a sink of tropospheric O₃. The upper limit of the tropospheric O₃ sink averaged from 4.5 to 5.5 ppb h⁻¹. Rationale: Ozone (O₃) behaviour over the Tibetan Plateau has attracted attention in recent decades. However, few long-term measurements have been performed in the region. Methodology: Field observations were conducted at a mountain site on the southeastern Tibetan Plateau from June 2014 to July 2015 in order to understand the behaviour of surface O₃ and its influencing factors. Backward trajectory cluster analysis was applied to understand long-range transport sources and their relative contributions. Results: The monthly average O₃ ranged from 22.1 to 48.6 ppb with a common high spring ozone concentration phenomenon. The O₃ diurnal variation exhibited a similar pattern to those in polluted areas but the cause was different. The O₃ mixing ratio was significantly positively correlated with mixed-layer depth and wind speed, and negatively with temperature and relative humidity, indicating strong vertical air exchange. Approximately 50% of air mass trajectories originated from the northeastern Bengal Bay region, with fairly low O₃ (CO) mixing ratios and high humidity. Others originated from the north Indian subcontinent (28%) and the Middle East (18%), with fairly high O₃ (and CO) and low humidity. Discussion: The average relative contributions of different air masses to surface O₃ and CO were small and scattered but large for trajectories arriving at 14:00 hours when vertical air exchange was close to its strongest for the day. The tropospheric O₃ sink may be common in the highlands, indicating a negative greenhouse effect there. The O₃ sink at Linzhi was estimated in the range of 4.5–5.5 ppb h⁻¹ at maximum. Summary. One-year-long on-line measurements of surface O₃ and CO mixing ratios were performed on the southeast Tibetan Plateau to examine O₃ behaviour. During the daytime, the O₃ mixing ratio was strongly affected by vertical air exchange. The O₃ mixing ratio was high in the afternoon and decreased at night, indicating a sink of tropospheric O₃. The upper limit of the tropospheric O₃ sink averaged from 4.5 to 5.5 ppb h⁻¹. [ABSTRACT FROM AUTHOR]