Hydrogen peroxide photoformation in particulate matter and its contribution to S(IV) oxidation during winter in Fairbanks, Alaska

<p>The high levels of sulfate in wintertime particles in Fairbanks, Alaska, are a subject of keen research interest and regulatory concern. Recent results from the 2022 Alaska Layered Pollution And Chemical Analysis (ALPACA) field campaign indicate that roughly 40 % of wintertime sulfate in Fairbanks is secondary, with hydrogen peroxide (HOOH) the dominant oxidant. Since formation of <span class="inline-formula">HOOH</span> in the gas phase should be negligible during ALPACA because of high levels of <span class="inline-formula">NO_<i>x</i></span>, we examined whether reactions within particles could be a significant source of <span class="inline-formula">HOOH</span>. To test this, we collected particulate matter (PM) samples during the ALPACA campaign, extracted them, illuminated them with simulated sunlight, and measured <span class="inline-formula">HOOH</span> production. Aqueous extracts showed significant light absorption, a result of brown carbon (BrC) from sources such as residential wood combustion. Photoformation rates of <span class="inline-formula">HOOH</span> in the PM extracts (PMEs; normalized to Fairbanks winter sunlight) range from 6 to 71 <span class="inline-formula">µM h⁻¹</span>. While light absorption is nearly independent of pH, <span class="inline-formula">HOOH</span> formation rates decrease with increasing pH. Extrapolating to the concentrated conditions of aerosol liquid water (ALW) gives an average rate of in-particle <span class="inline-formula">HOOH</span> formation of <span class="inline-formula">∼</span> 0.1 <span class="inline-formula">M h⁻¹</span>. Corresponding rates of sulfate formation from particle-produced <span class="inline-formula">HOOH</span> are 0.05–0.5 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">µ</mi><mi mathvariant="normal">g</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">3</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><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="53pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="bbefb9113aea0e66c18f7309e2e62feb"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-5087-2025-ie00001.svg" width="53pt" height="15pt" src="acp-25-5087-2025-ie00001.png"/></svg:svg></span></span>, accounting for a significant portion of the secondary sulfate production rate. Our results show that <span class="inline-formula">HOOH</span> formed in particles makes an important contribution to sulfate formation in ambient wintertime particles, even under the low actinic flux conditions typical of winter in subarctic locations like Fairbanks.</p>