Ozone source apportionment (OSAT) to differentiate local regional and super-regional source contributions in the Pearl River Delta region, China

It is well-known that ground-level ozone is not just a local or regional air quality problem; emission sources from super-regional (sources outside the PRD region) scales are known to contribute significantly to local ozone concentrations. However, source apportionment studies differentiating the relative contributions of local, regional, and super-regional ozone precursors are still limited. In this paper, using the Pearl River Delta (PRD) as an example, we have conducted a detailed apportionment (by source categories and by source regions) study of surface ozone using photochemical model source apportionment tools. Our results show that, while the super-regional contribution is dominant under mean ozone conditions, elevated local and regional sources are the causative factor for the formation of high ozone episodes. In particular, the local and PRD regional contributions increase from about 30% during non-episode days to about 50% during high ozone episode days in the autumn (November 2006) and even up to about 70% during high ozone episodes in the summer (July 2006). These results suggest that local and regional controls of ozone precursors are still very important for ozone reduction, particularly for episodic events. Furthermore, our results show that mobile emission is by far the highest contributing source category to ozone levels in the PRD for episodic ozone events. Moreover, we find substantial seasonal variations in the way ozone precursors from neighboring areas affect ozone levels in any particular city, suggesting that regional collaborations are important for developing effective long-term strategies to reduce ozone over the PRD region.