Measurement report: Elevated atmospheric ammonia may promote particle pH and HONO formation – insights from the COVID-19 pandemic

<p>HONO plays a crucial role as a precursor to <span class="inline-formula">OH</span> radicals in the tropospheric atmosphere. The incongruity between HONO concentration and <span class="inline-formula">NO_<i>x</i></span> emissions during the COVID-19 pandemic remains puzzling. Here, we show evidence from field observations of 10 sites in China where there was a noticeable increase in <span class="inline-formula">NH₃</span> concentrations during the COVID-19 pandemic. In addition to the meteorological conditions, the significant decrease in sulfate and nitrate concentrations enhanced the conversion of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="dd01b19a584d9ff35339d41174090f98"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-9885-2024-ie00001.svg" width="24pt" height="15pt" src="acp-24-9885-2024-ie00001.png"/></svg:svg></span></span> to <span class="inline-formula">NH₃</span>. Sensitivity analysis indicated that the decrease in anion concentrations (especially sulfate and nitrate) and the increase in cation concentrations during the COVID-19 pandemic led to an increase in particle pH. In other words, changes in the excess ammonia drove changes in particle pH that may consequently have impacted the rate of HONO formation. The calculation of reaction rates indicates that during the epidemic, the increase in pH may promote the generation of HONO by facilitating redox reactions, which highlights the importance of coordinating the control of <span class="inline-formula">SO₂</span>, <span class="inline-formula">NO_<i>x</i></span>, and <span class="inline-formula">NH₃</span> emissions.</p>