Ozone Suppression in China Under High PM2.5 Conditions: A Two-Pollutant Control Strategy

Fine particulate matter (PM_2.5­) decreased by 30–40% across urban China over the five-year plan (2013–2017) of governmental Clean Air Action. However, surface ozone pollution worsened over the same period. After we resolve the effect of meteorological variability on the ozone trends, it shows increasing ozone trends of 1–3 ppbv a⁻¹ in megacity clusters of eastern China that we attribute to changes in anthropogenic emissions. By contrast, ozone decreased in some areas of southern China. Anthropogenic NO_x emissions in China are estimated to have decreased by 21% during 2013–2017, whereas volatile organic compounds (VOCs) emissions changed little. Decreasing NO_x would increase ozone under the VOC-limited conditions thought to prevail in urban China while decreasing ozone under rural NO_x-limited conditions.

However, simulations with the GEOS-Chem model indicate that a more important factor for ozone trends in the North China Plain is the ∼40% decrease of PM_2.5 over the 2013–2017 period, slowing down the aerosol sink of hydroperoxy (HO₂) radicals and thus stimulating ozone production. We further show observational evidence for this effect with joint summer data for ozone and PM_2.5­. The observations show suppression of ozone pollution at high PM_2.5 concentrations, consistent with a model simulation in which PM_{2.5­ ­}scavenging of HO₂ and NO_x depresses ozone concentrations by 25 ppb relative to PM_2.5-free conditions. PM_2.5­ chemistry makes ozone pollution less sensitive to NO_x emission controls, emphasizing the need for controlling emissions of VOCs which so far have not decreased in China. The 2018–2020 Clean Air Action calls for a 10% decrease in VOC emissions that should begin to reverse the long-term ozone increase even as PM_2.5 continues to decrease_­. Aggressive reduction of NO_x and aromatic VOC emissions should be particularly effective for decreasing both PM_2.5 and ozone.