Path of coordinated control of PM2.5 and ozone in China

Since the Air Pollution Prevention and Control Action Plan was issued in 2013, air quality in China has significantly improved. But the concentration of fine particulate matter (PM2.5) remains 3 times higher than the guideline value of the WHO (World Health Organization, 2015), and the rising concentration of ozone (O-3) that has caused premature death and economic losses neutralized the environmental benefits brought in by partial reductions of PM2.5. The coordinated control of PM2.5 and O-3, hence, has been put at the top of the agenda for China's air pollution prevention and control. Given nitrogen oxides (NOx) and volatile organic compounds (VOCs) are major precursors of both PM2.5 and O-3, reducing the emissions of NOx and VOCs is considered as the key to the coordinated control of PM2.5 and O-3. Focused on the common precursors of PM(2.)5 and O-3, our research conducted qualitative analysis of the effects of reduced emissions of NOx and VOCs, and suggested China's path to coordinated control of PM2.5 and O-3. The results are expected to be adopted as scientific bases by administrative authorities for relevant policy-making. Our research was based on the WRF-CAMx model and scenario analysis was used. Different scenarios where NOx and VOCs emissions were reduced to different degrees were set to simulate the changes of concentrations of PM2.5 and O-3 across China and in key areas. The indicator of sensitivity was introduced to quantitively evaluate the responsiveness of PM2.5 and O-3 concentrations to the reduced emissions of NOx and VOCs. The effects of reduced emissions of NOx and VOCs on the concentrations of PM2.5 and O-3 were revealed directly in the research. As suggested by simulation results, the reduction of NOx or VOCs emission can contribute to the decrease of the concentration of PM2.5 nationwide and in key areas. If at the same ratio of reduction, the reduction of NOx emission can lead to a larger decrease of PM2.5 concentration. The change of O-3 concentration nationwide or in key areas varies under different NOx and VOCs emission reduction scenarios. From the perspective of the whole country, a small ratio of NOx emission reduction may lead to the rise of O3 concentration in some areas, but the O-3 concentration declines on the whole, and the rate of decline rises with the further reduction of NOx emission. The reduction of VOCs emission can decrease the O-3 concentration nationwide, but the effect is less obvious than the reduction of NOx emission. For the Pearl River Delta area, the reduced emission of VOCs is more effective for the decrease of O-3 concentration. For the "2+26" cities, the Yangtze River Delta, and the Fenwei Plain, if on a small scale, the reduction of VOCs is more effective for the decrease of O-3 concentration than that of NOx. But due to the existence of natural VOCs, the proportion of artificial VOCs to the total emission of VOCs reduces as the artificial VOCs continues to decline, and hence, the benefit of the reduction of VOCs becomes less obvious than that of NOx. Sensitivity results were in agreement with the concentration changes. Based on the abovementioned results, to realize the balance of the benefits of NOx and VOCs emissions reduction between the local and global and between the short term and the long term, we suggest that the continuous reduction of NOx emission should be regarded as the focus of coordinated control of PM2.5 and O-3 in terms of both the whole country and key areas and the reduction of VOCs emission is the key to the improvement of O-3 pollutions in key areas in the short term,