Influence of Tropical Cyclones on Hong Kong Air Quality

Tropical cyclones（TCs） constitute one of the major atmospheric activities affecting the air quality of the Pearl River Delta region. In this study, the impact of TCs on air quality in Hong Kong during the TC active season（July–October） from 2000 to 2015 is investigated. It is found that 57.5% of days with concentration of particulate matter with an aerodynamic diameter 10 μm（PM（10）） above the 90th percentile are related to TC activity. TCs in three regions, located to the east, southeast, and southwest of Hong Kong, have obvious impacts on pollutant concentration. When TCs are located east of Hong Kong near Taiwan, 65.5%/38.7% of the days have high or extremely high PM（10）/ozone（O3） levels, which are associated with northerly wind, sinking motion, and relatively low precipitation. When TCs are located southeast of Hong Kong, 48.1%/58.2% of the days have high pollution levels, associated mainly with continental air mass transport. When TCs are south or west of Hong Kong, only 20.8%/16.9% of the days have high PM（10）/O3 levels, and the air quality in Hong Kong is generally good or normal due to TC-associated precipitation, oceanic air mass transport, and an enhanced rising motion. The higher chance of high O3 days when TCs are present between Hong Kong and Taiwan, possibly due to lower-than-normal precipitation along the east coast of China under TC circulation. The results in this study highlight the important influence of TC position and associated atmospheric circulations on the air quality in Hong Kong.

Intraseasonal Variation of Visibility in Hong Kong

Visibility is one of the parameters for indicating air pollution. In this study, visibility variation in Hong Kong during summer and winter is investigated. Visibility in Hong Kong has clear intraseasonai variation. Examination of different environmental parameters suggests that the intraseasonal component dominates the overall circulation anomalies in both summer and winter. Associated with the intraseasonal variation of environmental parameters, obvious variation in visibility impair- ment is found in both summer and winter. In summer, local visibility and air quality are found to be significantly affected by the （MJO） and the 10-30-day intraseasonal oscillation （ISO） through modulation of associated atmospheric circulations. In winter, the modulation effects appear to be weaker due to the southward shift of the associated convection. The results in this study highlight the importance of the ISO in contributing to the overall variation in visibility in Hong Kong, and provide useful implications for the development of possible mitigation strategies associated with visibility impairment and air pollution in Hong Kong.