Aside from emission sources, prevailing meteorological conditions constitute another important factor affecting air quality. Tropical cyclones, one of the major atmospheric activities in summer and autumn, have an important influence on air quality. Dr. Eric C. H. CHOW, Dr. Richard C. Y. LI and Associate Professor Wen ZHOU, from the City University of Hong Kong, used 16 years of global meteorological data, local pollutant concentrations and tropical cyclone data to study their relationship. They found that the pollution level in Hong Kong is mainly determined by the locations of tropical cyclones. However, the impact of tropical cyclones on PM10 (relatively coarse particulate matter) and ozone is different.