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<p>A new method is presented for estimating urban hydroxyl radical (OH) concentrations using the downwind decay of the ratio of nitrogen dioxide over carbon monoxide column-mixing ratios (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">XNO</mi><mn mathvariant="normal">2</mn></msub><mo>/</mo><mi mathvariant="normal">XCO</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="61pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="601b6f47d8119bdf298a09e595bd2151"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-16053-2022-ie00004.svg" width="61pt" height="14pt" src="acp-22-16053-2022-ie00004.png"/></svg:svg></span></span>) retrieved from the Tropospheric Monitoring Instrument (TROPOMI). The method makes use of plumes simulated by the Weather Research and Forecast model (WRF-Chem) using passive-tracer transport, instead of the encoded chemistry, in combination with auxiliary input variables such as Copernicus Atmospheric Monitoring Service (CAMS) OH, Emission Database for Global Atmospheric Research v4.3.2 (EDGAR) NO<span class="inline-formula">_<i>x</i></span> and CO emissions, and National Center for Environmental Protection (NCEP)-based meteorological data. NO<span class="inline-formula">₂</span> and CO mixing ratios from the CAMS reanalysis are used as initial and lateral boundary conditions. WRF overestimates NO<span class="inline-formula">₂</span> plumes close to the center of the city by 15 % to 30 % in summer and 40 % to 50 % in winter compared to TROPOMI observations over Riyadh. WRF-simulated CO plumes differ by 10 % with TROPOMI in both seasons. The differences between WRF and TROPOMI are used to optimize the OH concentration, NO<span class="inline-formula">_<i>x</i></span>, CO emissions and their backgrounds using an iterative least-squares method. To estimate OH, WRF is optimized using (a) TROPOMI <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">XNO</mi><mn mathvariant="normal">2</mn></msub><mo>/</mo><mi mathvariant="normal">XCO</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="61pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="bbcb85403938e904148cd326636d9a89"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-16053-2022-ie00005.svg" width="61pt" height="14pt" src="acp-22-16053-2022-ie00005.png"/></svg:svg></span></span> and (b) TROPOMI-derived XNO<span class="inline-formula">₂</span> only.</p> <p>For summer, both the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">2</mn></msub><mo>/</mo><mi mathvariant="normal">CO</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="b19836d38374afb215e63b5e5bdd1eb3"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-16053-2022-ie00006.svg" width="46pt" height="14pt" src="acp-22-16053-2022-ie00006.png"/></svg:svg></span></span> ratio optimization and the XNO<span class="inline-formula">₂</span> optimization increase the prior OH from CAMS by 32 <span class="inline-formula">±</span> 5.3 % and 28.3 <span class="inline-formula">±</span> 3.9 %, respectively. EDGAR NO<span class="inline-formula">_<i>x</i></span> and CO emissions over Riyadh are increased by 42.1 <span class="inline-formula">±</span> 8.4 % and 101 <span class="inline-formula">±</span> 21 %, respectively, in summer. In winter, the optimization method doubles the CO emissions while increasing OH by <span class="inline-formula">∼</span> 52 <span class="inline-formula">±</span> 14 % and reducing NO<span class="inline-formula">_<i>x</i></span> emissions by 15.5 <span class="inline-formula">±</span> 4.1 %. TROPOMI-derived OH concentrations and the pre-existing exponentially modified Gaussian function fit (EMG) method differ by 10 % in summer and winter, confirming that urban OH concentrations can be reliably estimated using the TROPOMI-observed <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M22" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">2</mn></msub><mo>/</mo><mi mathvariant="normal">CO</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="13d42e1590cc963490a7adec484359f2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-16053-2022-ie00007.svg" width="46pt" height="14pt" src="acp-22-16053-2022-ie00007.png"/></svg:svg></span></span> ratio. Additionally, our method can be applied to a single TROPOMI overpass, allowing one to analyze day-to-day variability in OH, NO<span class="inline-formula">_<i>x</i></span> and CO emission.</p>