A meteorological analysis on a low tropospheric ozone event over Xining, North Western China on 26–27 July 1996

A pronounced low tropospheric ozone (O3) event was observed over the Tibetan plateau in Xining (XN) (101.45°E, 36.43°N, 2296 m a.s.l), Northwestern China by electrochemical concentration cell ozonesonde from 25–27 July 1996. The event featured very low O3 mixing ratio (<56 ppbv) below 15 km above ground and high relative humidity (30–100%). A comparison with the vertical distribution of tropospheric O3 over Lhasa (91.03°E, 29.40°N, 3650 m a.s.l) has suggested that the low tropospheric O3 mixing ratio over XN represented a common characteristic of tropospheric O3 over the Tibetan plateau. Meteorological measurements from the regional radiosonde network in China and satellites revealed that the low O3 event was associated with an anticyclone in the upper troposphere, which was characterized by an intensive upwelling transport. The dynamical processes of the transport were investigated using latitudinal cross-section analysis on the National Centre of Environmental Prediction data set and three-dimensional backward trajectories. The results showed that the O3-depleted air originated from Southeast Asia or the marine boundary layer of the South China Sea and had travelled across the Tibetan plateau where an ‘O3 valley’ was observed at the time during the long-range transport inland over XN.