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45observation-based models to determine how each of the key chemicals %u2013 NOx and VOCs %u2013 affects the formation of ozone in Hong Kong. Prof. Guo%u2019s team simultaneously conducted studies in more than 10 other Chinese cities. They also developed analytical techniques for analysing trace-level VOCs in the air %u2013 chemicals that are crucial for fully understanding ozone pollution. To improve air quality in China, in 2013 the Chinese government rolled out its Air Pollution Prevention and Action Plan (the Action Plan). From 2013 to 2017, Prof. Wang and his team investigated how ozone levels changed in response to the lower emissions of air pollutants resulting from the implementation of the Action Plan. Using statistical and computer models, they analysed data from the national environmental monitoring network. Their analysis found that significant declines in ambient concentrations of primary pollutants followed the implementation of the Action Plan. However, these declines were accompanied by higher concentrations of ozone in many urban centres. The team quantified the impact of both emission changes and meteorological conditions on ozone levels and identified several explanations for this unexpected increase in ozone concentrations.TAKE A DEEP BREATH