UK surface NO2 levels dropped by 42 % during the COVID-19 lockdown: Impact on surface O3

We report changes in surface nitrogen dioxide (<span classCombining double low line"inline-formula">NO2</span>) across the UK during the COVID-19 pandemic when large and rapid emission reductions accompanied a nationwide lockdown (23 March-31 May 2020, inclusively), and compare them with values from an equivalent period over the previous 5 years. Data are from the Automatic Urban and Rural Network (AURN), which forms the basis of checking nationwide compliance with ambient air quality directives. We calculate that <span classCombining double low line"inline-formula">NO2</span> reduced by <span classCombining double low line"inline-formula">42 %±9.8 %</span> on average across all 126 urban AURN sites, with a slightly larger (<span classCombining double low line"inline-formula">48 %±9.5 %</span>) reduction at sites close to the roadside (urban traffic). We also find that ozone (<span classCombining double low line"inline-formula">O3</span>) increased by 11 % on average across the urban background network during the lockdown period. Total oxidant levels (<span classCombining double low line"inline-formula"><math xmlnsCombining double low line"http://www.w3.org/1998/Math/MathML" idCombining double low line"M9" displayCombining double low line"inline" overflowCombining double low line"scroll" dspmathCombining double low line"mathml"><mrow><mrow classCombining double low line"chem"><msub><mi mathvariantCombining double low line"normal">O</mi><mtext mathvariantCombining double low line"italic">x</mtext></msub></mrow><mo>Combining double low line</mo><mrow classCombining double low line"chem"><msub><mi mathvariantCombining double low line"normal">NO</mi><mn mathvariantCombining double low line"normal">2</mn></msub></mrow><mo>+</mo><mrow classCombining double low line"chem"><msub><mi mathvariantCombining double low line"normal">O</mi><mn mathvariantCombining double low line"normal">3</mn></msub></mrow></mrow></math><span><svg:svg xmlns:svgCombining double low line"http://www.w3.org/2000/svg" widthCombining double low line"73pt" heightCombining double low line"13pt" classCombining double low line"svg-formula" dspmathCombining double low line"mathimg" md5hashCombining double low line"2b548850d308edc75bc21be3fb575c2b"><svg:image xmlns:xlinkCombining double low line"http://www.w3.org/1999/xlink" xlink:hrefCombining double low line"acp-20-15743-2020-ie00001.svg" widthCombining double low line"73pt" heightCombining double low line"13pt" srcCombining double low line"acp-20-15743-2020-ie00001.png"/></svg:svg></span></span>) increased only slightly on average (<span classCombining double low line"inline-formula">3.2 %±0.2 %</span>), suggesting the majority of this change can be attributed to photochemical repartitioning due to the reduction in <span classCombining double low line"inline-formula">NOx</span>. Generally, we find larger, positive <span classCombining double low line"inline-formula">Ox</span> changes in southern UK cities, which we attribute to increased UV radiation and temperature in 2020 compared to previous years. The net effect of the <span classCombining double low line"inline-formula">NO2</span> and <span classCombining double low line"inline-formula">O3</span> changes is a sharp decrease in exceedances of the <span classCombining double low line"inline-formula">NO2</span> air quality objective limit for the UK, with only one exceedance in London in 2020 up until the end of May. Concurrent increases in <span classCombining double low line"inline-formula">O3</span> exceedances in London emphasize the potential for <span classCombining double low line"inline-formula">O3</span> to become an air pollutant of concern as <span classCombining double low line"inline-formula">NOx</span> emissions are reduced in the next 10-20 years.