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

Abstract

Abstract. 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 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 −0.04 ∘CT50 h−1 of sunlight at ground level. In other words, the ice nucleation activity of photooxidized DOM can require up to 4 ∘C 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 CO2. 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.Highlights. 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 d in the atmosphere. A photomineralization mechanism involving the loss of organic carbon and the production of organic acids, CO and CO2 explains the observed changes and affects the liquid-water-to-ice ratio in clouds.

Macromolecular fungal ice nuclei in <i>Fusarium</i>: effects of physical and chemical processing

“…These results demonstrate a high stability of Fusarium IN under oxidizing and nitrating conditions. This is in contrast to other biological IN, e.g., bacterial IN (Snomax ® ) (Kunert et al, 2018), birch and alder pollen (Gute and Abbatt, 2018), and dissolved organic matter (Borduas-Dedekind et al, 2019), where exposure to oxidizing agents reduced the IN activity.…”

Aging induced changes in ice nucleation activity of combustion aerosol as determined by near edge X-ray absorption fine structure (NEXAFS) spectroscopy

“…In addition, we recognize that realistic atmospheric aging is further complicated by other factors such as multiphase chemistry involving gas to particle partitioning and photochemical processes in the cloud water and/or aqueous aerosol phase, largely determining the aqueous chemistry of organic aqueous species, that can affect the aerosol aging 107,108 and associated ice nucleation properties. 109 The largest challenge remains to further disentangle the contribution of changes from the physical properties versus those from chemical properties of the soot particles upon different aging processes, and the degree of their contribution to the enhanced ice nucleation ability. Certainly, one caveat of our analysis is that upon nebulizing soot particles from the bulk solutions, the particles can change their morphology.…”

Development of the drop Freezing Ice Nuclei Counter (FINC), intercomparison of droplet freezing techniques, and use of soluble lignin as an atmospheric ice nucleation standard

“…Lignin is not the most active INM in the sea surface microlayer, in sea spray, or in bioaerosols (Steinke et al, 2019), and so we were therefore surprised to notice the similarity in slope. However, organic matter n m data from several other studies are inconsistent with this slope, highlighting the difficulties in predicting the ice nucleating ability of organic matter (Borduas-Dedekind et al, 2019;Pummer et al, 2015;O'Sullivan et al, 2014;McCluskey et al, 2017).…”

“…Lignin could therefore be termed a nano-INP (O Sullivan et al, 2015), as an INM (Pummer et al, 2015) and operationally defined as soluble (Borduas-Dedekind et al, 2019). The ability of lignin to be chemically recalcitrant further demonstrates its value as a standard.…”

“…INPs typically include solid surfaces such as dust and cellular material which template ice, but recently reported ice nucleating macromolecules (INMs) are also capable of freezing supercooled water droplets (Pummer et al, 2012). INMs are defined here as operationally dissolved organic matter passing through a 0.2 µL filter (Borduas-Dedekind et al, 2019). In the absence of INPs and INMs, cloud droplets with an average radius of 10 µm remain liquid until instantaneous (< 1 s) homogeneous nucleation at approximately −38 • C (Kanji et al, 2017).…”