Organic aerosols
affect the planet’s radiative
balance by
absorbing and scattering light as well as by activating cloud droplets.
These organic aerosols contain chromophores, termed brown carbon (BrC),
and can undergo indirect photochemistry, affecting their ability to
act as cloud condensation nuclei (CCN). Here, we investigated the
effect of photochemical aging by tracking the conversion of organic
carbon into inorganic carbon, termed the photomineralization mechanism,
and its effect on the CCN abilities in four different types of BrC
samples: (1) laboratory-generated (NH4)2SO4-methylglyoxal solutions, (2) dissolved organic matter isolate
from Suwannee River fulvic acid (SRFA), (3) ambient firewood smoke
aerosols, and (4) ambient urban wintertime particulate matter in Padua,
Italy. Photomineralization occurred in all BrC samples albeit at different
rates, evidenced by photobleaching and by loss of organic carbon up
to 23% over a simulated 17.6 h of sunlight exposure. These losses
were correlated with the production of CO up to 4% and of CO2up to 54% of the initial organic carbon mass, monitored by gas chromatography.
Photoproducts of formic, acetic, oxalic and pyruvic acids were also
produced during irradiation of the BrC solutions, but at different
yields depending on the sample. Despite these chemical changes, CCN
abilities did not change substantially for the BrC samples. In fact,
the CCN abilities were dictated by the salt content of the BrC solution,
trumping a photomineralization effect on the CCN abilities for the
hygroscopic BrC samples. Solutions of (NH4)2SO4-methylglyoxal, SRFA, firewood smoke, and ambient Padua
samples had hygroscopicity parameters κ of 0.6, 0.1, 0.3, and
0.6, respectively. As expected, the SRFA solution with a κ of
0.1 was most impacted by the photomineralization mechanism. Overall,
our results suggest that the photomineralization mechanism is expected
in all BrC samples and can drive changes in the optical properties
and chemical composition of aging organic aerosols.