On the local anthropogenic source diversities and transboundary transport for urban agglomeration ozone mitigation

Abstract Mitigation of ozone in urban agglomerations of developing countries remains challenging, due partly to inadequate knowledge of the relative importance of local sources versus transboundary transport, aggravated by lack of synchronous measurements of ozone and precursors. Here we investigate the spatial-temporal characteristics of ozone and its precursors measured synchronously at five cities of Wuhan City Cluster (WCC) in Central China during the high-ozone months of May–June 2018, facilitated by multiple models of different complexities. We find substantial cross-city diversities in the measured maximum daily averaged 8-h (MDA8) ozone, nitrogen oxides (NOx = NO + NO2), volatile organic compounds (VOCs), VOCs/NOx ratios, and ozone formation potentials of VOCs, which are tied to distinctive local emission sources. GEOS-Chem simulations suggest that local anthropogenic emissions contribute 26.8–29.5% of ozone in each city averaged over May–June 2018, with higher values during ozone episodes (up to 32.0%). Transboundary transport from non-WCC Asian regions (18.5–19.2%) contributes much more than cross-city transport within WCC (2.5–3.1%) to ozone concentrations in the five cities. The contributions of background ozone from non-Asian anthropogenic emissions and global natural sources reach 48.9–51.6%. Thus local mitigation actions alone can substantially reduce city-level ozone; whereas the majority of ozone must be mitigated by country-wide or worldwide collaborations. Collaborations within an urban agglomeration alone may not effectively mitigate ozone.