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.
A Binned Approach to Cloud-Droplet Riming Implemented in a Bulk Microphysics Model