Worsening urban ozone pollution in China from 2013 to 2017 – Part 2: The effects of emission changes and implications for multi-pollutant control
Authors: Liu Yiming , Wang Tao 
Published: June 2020

Abstract. The Chinese government launched the Air Pollution Prevention and
Control Action Plan in 2013, and various stringent measures have since been
implemented, which have resulted in significant decreases in emissions and
ambient concentrations of primary pollutants such as SO2, NOx, and
particulate matter (PM). However, surface ozone (O3) concentrations
have still been increasing in urban areas across the country. In a previous
analysis, we examined in detail the roles of meteorological variation during
2013–2017 in the summertime surface O3 trend in various regions of
China. In this study, we evaluated the effect of changes in multi-pollutant
emissions from anthropogenic activities on O3 levels during the same
period by using an up-to-date regional chemical transport model (WRF-CMAQ)
driven by an interannual anthropogenic emission inventory. The Community Multiscale Air
Quality (CMAQ) model
was improved with regard to heterogeneous reactions of reactive gases on
aerosol surfaces, which led to better model performance in reproducing the
ambient concentrations of those gases. The model simulations showed that the
maximum daily 8 h average (MDA8) O3 mixing ratio in urban areas
increased by 0.46 ppbv per year (ppbv a−1) (p=0.001) from 2013 to
2017. In contrast, a slight decrease in MDA8 O3 by 0.17 ppbv a−1
(p=0.005) in rural areas was predicted, mainly attributable to the
NOx emission reduction. The effects of changes in individual pollutant
emissions on O3 were also simulated. The reduction of NOx emission
increased the O3 levels in urban areas due to the nonlinear
NOx and volatile organic compound (VOC) chemistry and decreasing aerosol
effects; the slight increase in VOC emissions enhanced the O3 levels;
the reduction of PM emissions increased the O3 levels by enhancing the
photolysis rates and reducing the loss of reactive gases on aerosol
surfaces; and the reduction of SO2 emissions resulted in a drastic
decrease in sulfate concentrations, which increased O3 through aerosol
effects. In contrast to the unfavorable effect of the above changes in
pollutant emissions on efforts to reduce surface O3, the reduction of
CO emissions did help to decrease the O3 level in recent years. The
dominant cause of increasing O3 due to changes in anthropogenic
emissions varied geographically. In Beijing, NOx and PM emission
reductions were the two largest causes of the O3 increase; in Shanghai,
the reduction of NOx and increase in VOC emissions were the two major
causes; in Guangzhou, NOx reduction was the primary cause; in
Chengdu, the PM and SO2 emission decreases contributed most to the
O3 increase. Regarding the effects of decreasing concentrations of
aerosols, the drop in heterogeneous uptake of reactive gases – mainly
HO2 and O3 – was found to be more important than the increase in
photolysis rates. The adverse effect of the reductions of NOx,
SO2, and PM emissions on O3 abatement in Beijing, Shanghai,
Guangzhou, and Chengdu would have been avoided if the anthropogenic VOCs
emission had been reduced by 24 %, 23 %, 20 %, and 16 %,
respectively, from 2013 to 2017. Our analysis revealed that the NOx
reduction in recent years has helped to contain the total O3 production
in China. However, to reduce O3 levels in major urban and industrial
areas, VOC emission controls should be added to the current
NOx-SO2-PM policy.