Synoptic weather patterns associated with NOx pollution episodes in Hong Kong urban areas

This study aims to find out the relationship between synoptic weather patterns and the air quality in HK urban areas. The air pollutants in HK are mainly vehicular emissions. Therefore, in this study, NOx (NO + NO2) was used as an indicator of the urban air quality. 3 years record of NOx concentration was examined and high NOx episodes were selected. 70% of high NOx pollution episodes were found to be related to calm or light wind conditions. Under low wind speed and stable conditions, both horizontal and vertical dispersion of local air pollutants are prohibited, hence air pollutants accumulate to harmful levels. The synoptic weather patterns on the episode days associated with calm conditions were investigated. We found that the existence of: (1) a tropical cyclone over the NW Pacific Ocean, and HK is under the influence of its periphery subsiding air; (2) a weak continental high-pressure system with central highest pressure less than 1030hPa; and (3) a low-pressure trough being close to HK, were likely to give rise to calm conditions and hence NOx episodes in urban areas.