A machine learning examination of hydroxyl radical differences among model simulations for CCMI-1
Nicely, Julie M. ; Duncan, Bryan N. ; Hanisco, Thomas F. ; Wolfe, Glenn M. ; Salawitch, Ross J. ; Deushi, Makoto ; Haslerud, Amund S. ; Jöckel, Patrick ; Josse, Béatrice ; Kinnison, Douglas E. ; Klekociuk, Andrew ; Manyin, Michael E. ; Marécal, Virginie ; Morgenstern, Olaf ; Murray, Lee T. ; Myhre, Gunnar ; Oman, Luke D. ; Pitari, Giovanni ; Pozzer, Andrea ; Quaglia, Ilaria ; Revell, Laura E. ; Rozanov, Eugene ; Stenke, Andrea ; Stone, Kane ; Strahan, Susan ; Tilmes, Simone ; Tost, Holger ; Westervelt, Daniel M. ; Zeng, Guang
<strong>Abstract.</strong> The hydroxyl radical (OH) plays critical roles within the troposphere, such as determining the lifetime of methane (<span class="inline-formula">CH<sub>4</sub></span>), yet is challenging to model due to its fast cycling and dependence on a multitude of sources and sinks. As a result, the reasons for variations in OH and the resulting methane lifetime (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">?</mi><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="22pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="05ab45943081ffb3661d7ec9bd6fac87"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-1341-2020-ie00001.svg" width="22pt" height="12pt" src="acp-20-1341-2020-ie00001.png"/></svg:svg></span></span>), both between models and in time, are difficult to diagnose. We apply a neural network (NN) approach to address this issue within a group of models that participated in the Chemistry-Climate Model Initiative (CCMI). Analysis of the historical specified dynamics simulations performed for CCMI indicates that the primary drivers of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">?</mi><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="22pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="6cff8140caa772a7f53e45699d7c8dd3"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-1341-2020-ie00002.svg" width="22pt" height="12pt" src="acp-20-1341-2020-ie00002.png"/></svg:svg></span></span> differences among 10 models are the flux of UV light to the troposphere (indicated by the photolysis frequency <span class="inline-formula"><i>J</i>O<sup>1</sup>D</span>), the mixing ratio of tropospheric ozone (<span class="inline-formula">O<sub>3</sub></span>), the abundance of nitrogen oxides (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mi>x</mi></msub><mo>?</mo><mi mathvariant="normal">NO</mi><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="85pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="219e22fa86429804f8520706e3902cdc"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-1341-2020-ie00003.svg" width="85pt" height="13pt" src="acp-20-1341-2020-ie00003.png"/></svg:svg></span></span>), and details of the various chemical mechanisms that drive OH. Water vapour, carbon monoxide (CO), the ratio of <span class="inline-formula">NO:NO<sub><i>x</i></sub></span>, and formaldehyde (HCHO) explain moderate differences in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">?</mi><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="22pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="3ed8c39ea81cfa47f8a86470c8c7d2ad"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-1341-2020-ie00004.svg" width="22pt" height="12pt" src="acp-20-1341-2020-ie00004.png"/></svg:svg></span></span>, while isoprene, methane, the photolysis frequency of <span class="inline-formula">NO<sub>2</sub></span> by visible light (<span class="inline-formula"><i>J</i>NO<sub>2</sub></span>), overhead ozone column, and temperature account for little to no model variation in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">?</mi><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="22pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="e27a3d7e184809f515d99dd29b4c3358"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-1341-2020-ie00005.svg" width="22pt" height="12pt" src="acp-20-1341-2020-ie00005.png"/></svg:svg></span></span>. We also apply the NNs to analysis of temporal trends in OH from 1980 to 2015. All models that participated in the specified dynamics historical simulation for CCMI demonstrate a decline in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">?</mi><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="22pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="5416400bfdb425bfb041111ba9583291"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-1341-2020-ie00006.svg" width="22pt" height="12pt" src="acp-20-1341-2020-ie00006.png"/></svg:svg></span></span> during the analysed timeframe. The significant contributors to this trend, in order of importance, are tropospheric <span class="inline-formula">O<sub>3</sub></span>, <span class="inline-formula"><i>J</i>O<sup>1</sup>D</span>, <span class="inline-formula">NO<sub><i>x</i></sub></span>, and <span class="inline-formula">H<sub>2</sub>O</span>, with CO also causing substantial interannual variability in OH burden. Finally, the identified trends in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">?</mi><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="22pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="d40ce9ef1b678a5d0b2ee7f985bacba6"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-1341-2020-ie00007.svg" width="22pt" height="12pt" src="acp-20-1341-2020-ie00007.png"/></svg:svg></span></span> are compared to calculated trends in the tropospheric mean OH concentration from previous work, based on analysis of observations. The comparison reveals a robust result for the effect of rising water vapour on OH and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M18" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">?</mi><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="22pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="3c638831574feeb93ae0b6f1a98f1be1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-1341-2020-ie00008.svg" width="22pt" height="12pt" src="acp-20-1341-2020-ie00008.png"/></svg:svg></span></span>, imparting an increasing and decreasing trend of about 0.5 % decade<span class="inline-formula"><sup>?1</sup></span>, respectively. The responses due to <span class="inline-formula">NO<sub><i>x</i></sub></span>, ozone column, and temperature are also in reasonably good agreement between the two studies.</p>
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