Urban and background ozone trend in 1984-1999 at subtropical Hong Kong, South China

There has been a growing concern for the impact of increasing Asian pollutant emissions on climate, atmospheric chemistry and air quality of the downwind region: In this study, we examined the long-term records of surface ozone (O-3) and its precursors (NO and NO2) measured in Hong Kong from the early 1980s to 2000. The urban O-3 concentration has shown more than two-fold increase since the early 1980s and the increase was especially apparent after the late 1980s. The O-3 increases led to frequent O-3 pollution episodes and caused deterioration of local air quality. Such increases were not accompanied by local NO and NO2 concentration changes in Hong Kong but coincided with the increase of NOx emissions from Asia. We derived the background O-3 concentration for the South China region using the early morning (1:00-5:00 a.m. local standard time) data and linked the transport of Asian emissions and background O-3 change using back air trajectory and local meteorology. The derived background O-3 concentration shows an increase rate of 1.5 % per year over the 15-year period from 1984 to 1999, which is close to that in the midlatitudes of East Asia at Japan. The sharp O-3 increase is related to the regional O-3 built-up in South China and the East Asia region due to the increases in the pollutant emissions as a result of rapid urban and industrial development in the region.