Direct formation of HONO through aqueous-phase photolysis of organic nitrates

<p>Organic nitrates RONO<span class="inline-formula">₂</span> are secondary compounds whose fate is closely related to the transport and removal of NO<span class="inline-formula">_<i>x</i></span> in the atmosphere. Despite their ubiquitous presence in submicron aerosols, the photochemistry of RONO<span class="inline-formula">₂</span> has only been investigated in the gas phase, leaving their reactivity in condensed phases poorly explored. This work aims to address this gap by investigating, for the first time, the reaction products and the mechanisms of aqueous-phase photolysis of four RONO<span class="inline-formula">₂</span> i.e., isopropyl nitrate, isobutyl nitrate, <span class="inline-formula"><i>α</i></span>-nitrooxy acetone, and 1-nitrooxy-2-propanol. The results show that the reactivity of RONO<span class="inline-formula">₂</span> in the aqueous phase differs significantly from that in the gas phase. In contrast to the gas phase, where RONO<span class="inline-formula">₂</span> release NO<span class="inline-formula">_<i>x</i></span> upon photolysis, the aqueous-phase photolysis of RONO<span class="inline-formula">₂</span> leads primarily to the direct formation of nitrous acid HONO or HNO<span class="inline-formula">₂</span>, which was confirmed by quantum chemistry calculations. Hence, the aqueous-phase photolysis of RONO<span class="inline-formula">₂</span> represents both a NO<span class="inline-formula">_<i>x</i></span> sink and a source of atmospheric nitrous acid, a significant precursor of <span class="inline-formula">⋅</span> OH and <span class="inline-formula">⋅</span> NO. These secondary radicals <span class="inline-formula">⋅</span> OH and <span class="inline-formula">⋅</span> NO are efficiently trapped in the aqueous phase, leading to the formation of HNO<span class="inline-formula">₃</span> and functionalized RONO<span class="inline-formula">₂</span>. This reactivity can thus potentially contribute to the aging of secondary organic aerosol (SOA) and serves as an additional source of aqueous-phase SOA.</p>