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

<p>In the polar regions, the usual OH radical formation pathway (ozone photolysis and reaction of O(¹D) with H₂O) is limited by the low water vapour concentration. However, gases emitted from the snowpack can be pre-cursors of HO<em>_x</em> radicals and ozone, thereby controlling the oxidising capacity of the lower atmosphere above remote snow-covered regions.</p> <p>Snowpack photolysis of nitrate and the resulting emissions of the reactive nitrogen species NO<em>_x</em> and HONO can lead to OH production through rapid cycling of RO₂ &#8594; HO₂ &#8594; OH and photolysis of HONO. Research into reactive nitrogen species in polar environments has focused on NO<em>_x</em>, with far fewer investigations into HONO. Previous studies of HONO in the polar boundary layer and snowpack interstitial air suggest a photolytic snowpack source but the exact mechanism for HONO production is poorly understood; photochemical models of HONO sources and sinks often cannot be reconciled with the measured HONO concentrations.</p> <p>A LOng Path Absorption Photometer (LOPAP) was used to investigate the net HONO flux density above snow in the Clean Air Sector at Halley VI Research Station in coastal Antarctica during Austral summer 2021/22. We present amount fraction measurements of HONO in ambient air, as well as measurements of the HONO flux density between the snow and atmosphere by the flux-gradient method. The potential snowpack reactions driving this HONO release are discussed, as well as the implications of these measurements for the HO<em>_x</em> budget. These findings help further our understanding of the atmospheric budget of reactive nitrogen and highlight the significant effects snow surfaces can have on the atmospheric chemistry in the boundary layer above.</p>