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. An organic aerosol particle has a lifetime of approximately 1 week in theatmosphere during which it will be exposed to sunlight. However, the effect ofphotochemistry on the propensity of organic matter to participate in theinitial cloud-forming steps is difficult to predict. In this study, wequantify on a molecular scale the effect of photochemical exposure ofnaturally occurring dissolved organic matter (DOM) and of a fulvic acidstandard on its cloud condensation nuclei (CCN) and ice nucleation (IN)activity. We find that photochemical processing, equivalent to 4.6 d inthe atmosphere, of DOM increases its ability to form cloud droplets by up toa factor of 2.5 but decreases its ability to form ice crystals at a lossrate of −0.04 ∘CT50 h−1 of sunlight at ground level.In other words, the ice nucleation activity of photooxidized DOM can requireup to 4 ∘C colder temperatures for 50 % of the droplets to activateas ice crystals under immersion freezing conditions. This temperature changecould impact the ratio of ice to water droplets within a mixed-phasecloud by delaying the onset of glaciation and by increasing the supercooledliquid fraction of the cloud, thereby modifying the radiative properties andthe lifetime of the cloud. Concurrently, a photomineralization mechanism wasquantified by monitoring the loss of organic carbon and the simultaneousproduction of organic acids, such as formic, acetic, oxalic and pyruvicacids, CO and CO2. This mechanism explains and predicts the observedincrease in CCN and decrease in IN efficiencies. Indeed, we show thatphotochemical processing can be a dominant atmospheric ageing process,impacting CCN and IN efficiencies and concentrations. Photomineralizationcan thus alter the aerosol–cloud radiative effects of organic matter bymodifying the supercooled-liquid-water-to-ice-crystal ratio in mixed-phaseclouds with implications for cloud lifetime, precipitation patterns and thehydrological cycle.Highlights.During atmospheric transport, dissolved organic matter (DOM) within aqueousaerosols undergoes photochemistry. We find that photochemical processing ofDOM increases its ability to form cloud droplets but decreases its abilityto form ice crystals over a simulated 4.6 d in the atmosphere. Aphotomineralization mechanism involving the loss of organic carbon and theproduction of organic acids, CO and CO2 explains the observed changesand affects the liquid-water-to-ice ratio in clouds.

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