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

Abstract

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.