Worsening urban ozone pollution in China from 2013 to 2017 – Part 2: The effects of emission changes and implications for multi-pollutant control

• Main Authors: Y. Liu, T. Wang
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-06-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/20/6323/2020/acp-20-6323-2020.pdf
• ISSN: 1680-7316; 1680-7324

<p>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 <span class="inline-formula">SO₂</span>, <span class="inline-formula">NO_<i>x</i></span>, and particulate matter (PM). However, surface ozone (<span class="inline-formula">O₃</span>) 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 <span class="inline-formula">O₃</span> trend in various regions of China. In this study, we evaluated the effect of changes in multi-pollutant emissions from anthropogenic activities on <span class="inline-formula">O₃</span> 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&thinsp;<span class="inline-formula">h</span> average (MDA8) <span class="inline-formula">O₃</span> mixing ratio in urban areas increased by 0.46&thinsp;ppbv per year (<span class="inline-formula">ppbv a⁻¹</span>) (<span class="inline-formula"><i>p</i>=0.001</span>) from 2013 to 2017. In contrast, a slight decrease in MDA8 <span class="inline-formula">O₃</span> by 0.17&thinsp;<span class="inline-formula">ppbv a⁻¹</span> (<span class="inline-formula"><i>p</i>=0.005</span>) in rural areas was predicted, mainly attributable to the <span class="inline-formula">NO_<i>x</i></span> emission reduction. The effects of changes in individual pollutant emissions on <span class="inline-formula">O₃</span> were also simulated. The reduction of <span class="inline-formula">NO_<i>x</i></span> emission increased the <span class="inline-formula">O₃</span> levels in urban areas due to the nonlinear <span class="inline-formula">NO_<i>x</i></span> and volatile organic compound (VOC) chemistry and decreasing aerosol effects; the slight increase in VOC emissions enhanced the <span class="inline-formula">O₃</span> levels; the reduction of PM emissions increased the <span class="inline-formula">O₃</span> levels by enhancing the photolysis rates and reducing the loss of reactive gases on aerosol surfaces; and the reduction of <span class="inline-formula">SO₂</span> emissions resulted in a drastic decrease in sulfate concentrations, which increased <span class="inline-formula">O₃</span> through aerosol effects. In contrast to the unfavorable effect of the above changes in pollutant emissions on efforts to reduce surface <span class="inline-formula">O₃</span>, the reduction of CO emissions did help to decrease the <span class="inline-formula">O₃</span> level in recent years. The dominant cause of increasing <span class="inline-formula">O₃</span> due to changes in anthropogenic emissions varied geographically. In Beijing, <span class="inline-formula">NO_<i>x</i></span> and PM emission reductions were the two largest causes of the <span class="inline-formula">O₃</span> increase; in Shanghai, the reduction of <span class="inline-formula">NO_<i>x</i></span> and increase in VOC emissions were the two major causes; in Guangzhou, <span class="inline-formula">NO_<i>x</i></span> reduction was the primary cause; in Chengdu, the PM and <span class="inline-formula">SO₂</span> emission decreases contributed most to the <span class="inline-formula">O₃</span> increase. Regarding the effects of decreasing concentrations of aerosols, the drop in heterogeneous uptake of reactive gases – mainly <span class="inline-formula">HO₂</span> and <span class="inline-formula">O₃</span> – was found to be more important than the increase in photolysis rates. The adverse effect of the reductions of <span class="inline-formula">NO_<i>x</i></span>, <span class="inline-formula">SO₂</span>, and PM emissions on <span class="inline-formula">O₃</span> abatement in Beijing, Shanghai, Guangzhou, and Chengdu would have been avoided if the anthropogenic VOCs emission had been reduced by 24&thinsp;%, 23&thinsp;%, 20&thinsp;%, and 16&thinsp;%, respectively, from 2013 to 2017. Our analysis revealed that the <span class="inline-formula">NO_<i>x</i></span> reduction in recent years has helped to contain the total <span class="inline-formula">O₃</span> production in China. However, to reduce <span class="inline-formula">O₃</span> levels in major urban and industrial areas, VOC emission controls should be added to the current <span class="inline-formula">NO_<i>x</i></span>-<span class="inline-formula">SO₂</span>-PM policy.</p>