Photomineralization mechanism changes the ability of dissolved organic matter to activate cloud droplets and to nucleate ice crystals

• Main Authors: N. Borduas-Dedekind, R. Ossola, R. O. David, L. S. Boynton, V. Weichlinger, Z. A. Kanji, K. McNeill
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-10-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/19/12397/2019/acp-19-12397-2019.pdf

<p>An organic aerosol particle has a lifetime of approximately 1 week in the atmosphere during which it will be exposed to sunlight. However, the effect of photochemistry on the propensity of organic matter to participate in the initial cloud-forming steps is difficult to predict. In this study, we quantify on a molecular scale the effect of photochemical exposure of naturally occurring dissolved organic matter (DOM) and of a fulvic acid standard on its cloud condensation nuclei (CCN) and ice nucleation (IN) activity. We find that photochemical processing, equivalent to 4.6&thinsp;d in the atmosphere, of DOM increases its ability to form cloud droplets by up to a factor of 2.5 but decreases its ability to form ice crystals at a loss rate of <span class="inline-formula">−0.04</span>&thinsp;<span class="inline-formula">^∘</span>C<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><msub><mi/><mrow><msub><mi>T</mi><mn mathvariant="normal">50</mn></msub></mrow></msub></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="13pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="8b240bec1e343973563bdbfa5af70aba"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-12397-2019-ie00001.svg" width="13pt" height="10pt" src="acp-19-12397-2019-ie00001.png"/></svg:svg></span></span>&thinsp;h<span class="inline-formula">⁻¹</span> of sunlight at ground level. In other words, the ice nucleation activity of photooxidized DOM can require up to 4&thinsp;<span class="inline-formula">^∘</span>C colder temperatures for 50&thinsp;% of the droplets to activate as ice crystals under immersion freezing conditions. This temperature change could impact the ratio of ice to water droplets within a mixed-phase cloud by delaying the onset of glaciation and by increasing the supercooled liquid fraction of the cloud, thereby modifying the radiative properties and the lifetime of the cloud. Concurrently, a photomineralization mechanism was quantified by monitoring the loss of organic carbon and the simultaneous production of organic acids, such as formic, acetic, oxalic and pyruvic acids, CO and <span class="inline-formula">CO₂</span>. This mechanism explains and predicts the observed increase in CCN and decrease in IN efficiencies. Indeed, we show that photochemical processing can be a dominant atmospheric ageing process, impacting CCN and IN efficiencies and concentrations. Photomineralization can thus alter the aerosol–cloud radiative effects of organic matter by modifying the supercooled-liquid-water-to-ice-crystal ratio in mixed-phase clouds with implications for cloud lifetime, precipitation patterns and the hydrological cycle.<br/><br/><strong>Highlights.</strong> During atmospheric transport, dissolved organic matter (DOM) within aqueous aerosols undergoes photochemistry. We find that photochemical processing of DOM increases its ability to form cloud droplets but decreases its ability to form ice crystals over a simulated 4.6&thinsp;d in the atmosphere. A photomineralization mechanism involving the loss of organic carbon and the production of organic acids, CO and <span class="inline-formula">CO₂</span> explains the observed changes and affects the liquid-water-to-ice ratio in clouds.</p>