Observational study of ozone and carbon monoxide at the summit of mount Tai (1534 m a.s.l.) in central-eastern China

Abstract

We report measurement results of ozone (O3) and carbon monoxide (CO) obtained from 5 July to 23 November 2003 at the summit of Mount Tai (1534 m a.s.l, 36.25°N, 117.10°E) in the Shandong Peninsula of China. The study was carried out to gain insights into regional O3 pollution and air-mass transport in the highly populated North China (Huabei) Plains. The average mixing ratio was 58 (±16) ppbv for O3 and 393 (±223) ppbv for CO during the study period. The monthly variations in O3 and CO exhibited a similar pattern, i.e. high in summer and low in autumn. While such a seasonal pattern for O3 is a common phenomenon in many rural areas in the Northern Hemisphere and is mainly due to seasonal changes in solar radiation and temperature, the higher CO levels in summer were attributed to the difference in dynamic transport and the evolution of PBL heights in summer and autumn over this region. An examination of diurnal variations in O3 indicated evident daytime photochemical production, especially in summer. The evolution of PBL heights and mountain-valley breezes also had a large impact on the diurnal patterns of O3 and CO. O 3 and CO showed a moderately good positive correlation (r=0.60) in July with a slope of 0.08 ppbv/ppbv, which is much lower than the slope (0.3-0.4 ppbv/ppbv) observed in North America. A back trajectory analysis showed that air masses mainly originated from the North China Plains or were re-circulating over the Shandong Peninsula, collectively accounting for 76% of the air masses sampled. Summertime air traveling in the lower troposphere over northern China had the highest concentrations of O3 and CO, revealing that the lower troposphere in northern China was significantly polluted in summer due to strong convection transporting PBL pollution to the free troposphere. The trajectories also showed notable contributions from eastern China, central China, and countries in northeast Asia such as Korea and Japan.