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

Abstract

<p><strong>Abstract.</strong> An organic aerosol particle has a lifetime of approximately one week in the atmosphere during which it will be exposed to sunlight. Yet, 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 ability to form mixed-phase clouds, by acting as cloud condensation nuclei (CCN) and by acting as ice nucleating particles (INPs). We find that photochemical processing, equivalent to 4.6 days 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 −0.04°C<sub>T50</sub> h<sup>−1</sup> of sunlight at ground level. In other words, the ice nucleation activity of photooxidized DOM can require up to 4 degrees colder temperatures for 50 % 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 CO<sub>2</sub>. This mechanism explains and predicts the observed increase in CCN and decrease in INP efficiencies. Indeed, we show that photochemical processing can be a dominant atmospheric aging process, impacting CCN and INP 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.</p>

Kunert et al., Biogeoschiences, 2019, reviewer comments

“…The null effect of chemical processing with O3 and NO2 was somewhat surprising. Based on (Borduas-Dedekind et al, 2019;Gute and Abbatt, 2018;Kunert et al, 2018), I would have expected to see oxidation of the proteinaceous material and thus decrease in IN ability. A discussion involving a hypothesis to the resistance of the strains to oxidation is warranted in light of these studies.…”

Molecular level insights into the direct health impacts of some organic aerosol components

“…Fulvic acid (FA) was selected to represent OAs because: 1) it mimics both the aliphatic and aromatic structures in the polycarboxylic acid fraction of OAs, 61 2) it displays the functionality and behavior of organic PM fragments, 62 3) 6-11% of the total organic mass extracted from dust aerosols are composed of humic and fulvic acids, 63 and 4) it has been accepted as a suitable surrogate for highly oxidized OAs, 64 and a proxy for atmospheric PMs 65,66 . Among various FA chemical structures, the model structure selected and geometry optimized by Sadhu et al 67 was adopted.…”

The effect of (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> on the freezing properties of non-mineral dust ice nucleating substances of atmospheric relevance

“…Humic like substances contain multiple organic compounds that are abundant in soils and have been identified in atmospheric aerosol (Graber and Rudich, 2006;White, 2009). The ice nucleating abilities of humic and fulvic acid have been characterized in previous studies (Knopf et al, 2010;O'Sullivan et al, 2014;Pratt et al, 2009;Shilling et al, 2006;Borduas-Dedekind et al, 2019). A Suwanee River fulvic acid standard and a Leonardite humic acid standard were obtained from the International Humic Substances Society, and 0.1 wt % solutions were prepared using ultrapure water.…”