Systematic evaluation of ozone control policies using an Ozone Source Apportionment method

Ground-level ozone is a secondary pollutant that causes serious health and environmental damage. One of the main challenges in controlling ground-level ozone is to identify the categories of ozone precursor species that contribute the most to ozone levels, and the regions from which they are emitted. In this study, we apply a photochemical model source apportionment tool to identify the contributions of various precursors to seasonal ozone levels in Hong Kong (HK) and the Pearl River Delta (PRD) region, according to their source regions and categories. We establish a full precursor contribution matrix for the ozone concentrations in all cities within the HK/PRD region. This matrix can be used to rank the relative importance of precursors by source region/category for each city. It can also be used to show how the precursor emissions from any city/category affect ozone levels in neighboring cities. The results show substantial seasonal variation in the source contributions from the HK/PRD region. During ozone episodes, precursors from the region accounted for 68-80% of ozone concentrations in July and 35-55% in October, but only 19-32% in January and April. For all of the cities in the PRD, the top three precursor categories contributing to ozone episodes were mobile, point and area sources. Biogenic and point sources were more important in July than in October. In contrast, the ranking of contributing source regions showed considerable variation across cities and seasons, implying substantial variations in the ways in which precursors from one city affect ozone levels in neighboring cities. Our results show that source contribution and impact analysis can be very useful for evaluating the ozone control strategies of local governments. For the PRD region, a local reduction in mobile, area and point sources, together with regional collaborations on pollution control, are of particular importance in effectively reducing episodic ozone concentrations.