The relationship between tropospheric ozone and atmospheric circulation in Taiwan

The aim of the study is to investigate the relationship between variability in tropospheric ozone in Taiwan and the regional atmospheric circulation, paying particular attention to the influence of long-range transport on ozone pollution levels. The study period is 1994 to 2004. The data used in this study include air pollution data from the Taiwan Air Quality Monitoring Network and NCEP/NCAR reanalysis data. The spatial and temporal variations in weather types have been characterised using an objective circulation classification scheme and relationships with episodes of high ozone levels over Taiwan have been determined. The signature of the large-scale atmospheric circulation associated with high ozone pollution and the connection with long-range transport of ozone precursors and ozone have been identified using spatial composites and back-trajectory analysis. Trajectories were calculated using the Hybrid Single-Particle Integrated Trajectory model. The air pollution data analysis shows that Taiwan experiences a seasonal cycle in ozone levels, with maxima in spring and autumn and a minimum in summer. The spatial composite and back trajectory analyses indicate that long-range transport does play a role in increasing high ozone episodes in Taiwan. A link with the seasonal variation of the monsoon circulations at different times is shown, with a weaker summer monsoon and a stronger winter monsoon circulation associated with enchanced ozone levels over Taiwan. The characteristics of atmospheric circulation for ozone pollution episodes include enhanced north and northeasterly flows, originating from nearby polluted areas and suggest that mainland China, Korea and Japan are source regions of ozone and its precursors for Taiwan. Moreover, the transport pathways at the high level of 2000m show that the southern China is also prominent source region, which is a previously unidentified distant source of ozone pollution in Taiwan. It is suggested that the increase of ozone pollution in summer found in this study is caused by a weaker summer monsoon circulation in recent years. While it is emphasised that variability in long-range transport is the only factor affecting ozone pollution levels over Taiwan, the influence of global warming on the Asian monsoon circulation and, hence, long-range transport of ozone and its precursors warrants serious consideration.