Snowpack nitrate photolysis drives the summertime atmospheric nitrous acid (HONO) budget in coastal Antarctica

<p>Measurements of atmospheric nitrous acid (<span class="inline-formula">HONO</span>) amount fraction and flux density above snow were carried out using a long-path absorption photometer at Halley station in coastal Antarctica between 22 January and 3 February 2022. The mean <span class="inline-formula">±1<i>σ</i></span> <span class="inline-formula">HONO</span> amount fraction was (2.1 <span class="inline-formula">±</span> 1.5) <span class="inline-formula">pmol mol⁻¹</span> and showed a diurnal cycle (range of 1.0–3.2 <span class="inline-formula">pmol mol⁻¹</span>) with a maximum at solar noon. These <span class="inline-formula">HONO</span> amount fractions are generally lower than have been observed at other Antarctic locations. The flux density of <span class="inline-formula">HONO</span> from the snow, measured between 31 January and 1 February 2022, was between 0.5 and <span class="inline-formula">3.4×10¹²</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">s</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="39pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="7aa9599418446b482f97cb2b6fbb5c99"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-5533-2023-ie00001.svg" width="39pt" height="13pt" src="acp-23-5533-2023-ie00001.png"/></svg:svg></span></span> and showed a decrease during the night. The measured flux density is close to the calculated <span class="inline-formula">HONO</span> production rate from photolysis of nitrate present in the snow. A simple box model of <span class="inline-formula">HONO</span> sources and sinks showed that the flux of <span class="inline-formula">HONO</span> from the snow makes a <span class="inline-formula">&gt;10</span> times larger contribution to the <span class="inline-formula">HONO</span> budget than its formation through the reaction of <span class="inline-formula">OH</span> and <span class="inline-formula">NO</span>. Ratios of these <span class="inline-formula">HONO</span> amount fractions to <span class="inline-formula">NO_<i>x</i></span> measurements made in summer 2005 are low (0.15–0.35), which we take as an indication of our measurements being comparatively free from interferences. Further calculations suggest that <span class="inline-formula">HONO</span> photolysis could produce up to 12 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">pmol</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">mol</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">h</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="73pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="c0553d142508f72f22d8e6a455d687cf"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-5533-2023-ie00002.svg" width="73pt" height="15pt" src="acp-23-5533-2023-ie00002.png"/></svg:svg></span></span> of <span class="inline-formula">OH</span>, approximately half that produced by ozone photolysis, which highlights the importance of <span class="inline-formula">HONO</span> snow emissions as an <span class="inline-formula">OH</span> source in the atmospheric boundary layer above Antarctic snowpacks.</p>