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46Impact of emission reductions on ozone levels%u201cFirst, when there are fewer NOx emissions, the air contains fewer substances to counteract the formation of ozone,%u201d explains Prof. Wang. %u201cThis can lead to higher ozone concentrations. Second, the Action Plan mostly targeted particulate matter (PM). When fewer particles are suspended in the air, more sunlight reaches the earth%u2019s surface. The increased solar radiation triggers the reaction between NOx and VOCs to form ozone,%u201d he continues. %u201cThird, PM absorbs certain substances that contribute to the formation of ozone. When there is less PM in the air, fewer of these ozone-forming substances are absorbed, which can lead to increased ozone formation.%u201d In the view of the team, controlling the level of VOCs together with NOx and PM would have avoided this unintended consequence of reducing the level of PM.Influence of meteorological conditions on ozone pollutionProf. Wang%u2019s team also found that variations in chemical conditions alone were not sufficient to explain the observed year-to-year changes in ozone pollution levels. Variations in meteorological conditions also contributed significantly, highlighting the need to consider changes in meteorological conditions when evaluating the efficacy of emission control measures on ozone pollution.TAKE A DEEP BREATHHok Tsui air monitoring supersite