The long-term trend and production sensitivity change in the US ozone pollution from observations and model simulations

• Main Authors: H. He, X.-Z. Liang, C. Sun, Z. Tao, D. Q. Tong
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-03-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/20/3191/2020/acp-20-3191-2020.pdf

<p>We investigated the ozone pollution trend and its sensitivity to key precursors from 1990 to 2015 in the United States using long-term EPA Air Quality System (AQS) observations and mesoscale simulations. The modeling system, a coupled regional climate–air quality model (CWRF-CMAQ; Climate-Weather Research Forecast and the Community Multiscale Air Quality), captured well the summer surface ozone pollution during the past decades, having a mean slope of linear regression with AQS observations of <span class="inline-formula">∼0.75</span>. While the AQS network has limited spatial coverage and measures only a few key chemical species, CWRF-CMAQ provides comprehensive simulations to enable a more rigorous study of the change in ozone pollution and chemical sensitivity. Analysis of seasonal variations and diurnal cycle of ozone observations showed that peak ozone concentrations in the summer afternoon decreased ubiquitously across the United States, up to 0.5&thinsp;ppbv&thinsp;yr<span class="inline-formula">⁻¹</span> in major non-attainment areas such as Los Angeles, while concentrations at certain hours such as the early morning and late afternoon increased slightly. Consistent with the AQS observations, CMAQ simulated a similar decreasing trend of peak ozone concentrations in the afternoon, up to 0.4&thinsp;ppbv&thinsp;yr<span class="inline-formula">⁻¹</span>, and increasing ozone trends in the early morning and late afternoon. A monotonically decreasing trend (up to 0.5&thinsp;ppbv&thinsp;yr<span class="inline-formula">⁻¹</span>) in the odd oxygen (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">O</mi><mi>x</mi></msub><mo>=</mo><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">3</mn></msub><mo>+</mo><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="69pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="c13b7ba31a73eaac2dac9773cc4bcd0b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-3191-2020-ie00001.svg" width="69pt" height="13pt" src="acp-20-3191-2020-ie00001.png"/></svg:svg></span></span>) concentrations are simulated by CMAQ at all daytime hours. This result suggests that the increased ozone in the early morning and late afternoon was likely caused by reduced NO–<span class="inline-formula">O₃</span> titration, driven by continuous anthropogenic <span class="inline-formula">NO_<i>x</i></span> emission reductions in the past decades. Furthermore, the CMAQ simulations revealed a shift in chemical regimes of ozone photochemical production. From 1990 to 2015, surface ozone production in some metropolitan areas, such as Baltimore, has transited from a VOC-sensitive environment (<span class="inline-formula">&gt;50</span>&thinsp;% probability) to a <span class="inline-formula">NO_<i>x</i></span>-sensitive regime. Our results demonstrated that the long-term CWRF-CMAQ simulations can provide detailed information of the ozone chemistry evolution under a changing climate and may partially explain the US ozone pollution responses to regional and national regulations.</p>