Post Date: 12 July 2022

Investigation of the long-term trend of surface ozone and ozone-precursor relationship in Hong Kong

Abstract:

As a highly urbanized city in East Asia, Hong Kong has suffered from severe ozone (O3) pollution for many years. A comprehensive understanding of the current pollution characteristics is vital to mitigate the O3 problem. This study systematically investigated the long-term trends of surface O3 in the last 10 years based on measurement from the selected 11 Air Quality Monitoring Stations (AQMS). The annual average and daily maximum 8-hours average (DMA8) O3 presented upward trends from 2011 to 2020 at the rate of 0.66 ppb year-1 (p<0.05) and 0.72 ppb year-1 (p=0.06), respectively. Especially in the urban area, significant upward trends of DMA8 O3 were found at five urban stations in the range of 0.92-1.18 ppb year-1. Meanwhile, significant decreases in surface NO2 were found at all 11 stations (-0.14 to -1.77 ppb year-1), resulting in the decline of total oxidants (Ox, O3+NO2). The inverse trends of O3 and Ox imply the rise of O3 in urban areas was attributed to the weakened titration effect of nitrogen oxide (NOx) emission.

Additionally, observation-based model was applied to understand the O3-precursors relationships. Results showed that the O3 formation was still VOC-limited in Hong Kong, and the influence of NOx on O3 formation decreased in recent years. In terms of abatement strategies, apart from aromatics, we highlight the importance of carbonyl compounds due to their high sensitivity to O3 production. Because of the strict emission control measures implemented by the government of Hong Kong and adjacent areas, the locally produced Ox has been unsurprisingly alleviated, and the regional background Ox appeared to level off. However, the regional background O3 still contributed a large fraction to O3 concentrations and the nighttime O3 peak formation, calling for further regional pollution control collaboration between the Hong Kong government and the adjacent Guangdong province.