Elucidating Contributions of Anthropogenic Volatile Organic Compounds and Particulate Matter to Ozone Trends over China.

In China, emissions of ozone (O 3 )-producing pollutants have been targeted for mitigation to reduce O 3 pollution. However, the observed O 3 decrease is slower than/opposite to expectations affecting the health of millions of people. For a better understanding of this failure and its connection with anthropogenic emissions, we quantify the summer O 3 trends that would have occurred had the weather stayed constant by applying a numerical tool that "de-weathers" observations across 31 urban regions (123 cities and 392 sites) over 8 years. O 3 trends are significant ( p < 0.05) over 234 sites after de-weathering, contrary to the directly observed trends (only 39 significant due to high meteorology-induced variability). The de-weathered data allow categorizing cities in China into four different groups regarding O 3 mitigation, with group 1 exhibiting steady O 3 reductions, while group 4 showing significant ( p < 0.05) O 3 increases. Analysis of the relationships between de-weathered odd oxygen and nitrogen oxides illustrates how the changes in NO x , in anthropogenic volatile organic compounds (VOCs), and reductions in fine particulate matter (PM 2.5 ) affect the O 3 trends differently in these groups. While this analysis suggests that VOC reductions are the main driver of O 3 decreases in group 1, groups 3 and 4 are primarily affected by decreasing PM 2.5 , which results in enhanced O 3 formation. Our analysis demonstrates both the importance of and possibility for isolating emission-driven changes from climate and weather for interpreting short-term air quality observations.