Abstract. Biomass burning is a major source of atmospheric brown
carbon (BrC), and through its absorption of UV/VIS radiation,
it can play an important role in the planetary radiative balance and
atmospheric photochemistry. The considerable uncertainty of BrC impacts is
associated with its poorly constrained sources, transformations, and
atmospheric lifetime. Here we report laboratory experiments that examined
changes in the optical properties of the water-soluble (WS) BrC fraction of
laboratory-generated biomass burning particles from hardwood
pyrolysis. Effects of direct UVB photolysis and OH oxidation in the aqueous
phase on molecular-weight-separated BrC were studied. Results indicated that the majority of
low-molecular-weight (MW)
BrC (<400 Da) was rapidly photobleached by both direct photolysis and OH
oxidation on an atmospheric timescale of approximately 1 h. High MW BrC
(≥400 Da) underwent initial photoenhancement up to ∼15 h,
followed by slow photobleaching over ∼10 h. The laboratory experiments
were supported by observations from ambient BrC samples that were collected
during the fire seasons in Greece. These samples, containing freshly emitted
to aged biomass burning aerosol, were analyzed for both water- and
methanol-soluble BrC. Consistent with the laboratory experiments, high-MW BrC
dominated the total light absorption at 365 nm for both methanol and
water-soluble fractions of ambient samples with atmospheric transport times
of 1 to 68 h. These ambient observations indicate that overall,
biomass burning BrC across all molecular weights has an atmospheric lifetime
of 15 to 28 h, consistent with estimates from previous field studies –
although the BrC associated with the high-MW fraction remains relatively
stable and is responsible for light absorption properties of BrC throughout most of its atmospheric lifetime. For
ambient samples of aged (>10 h) biomass burning emissions, poor linear
correlations were found between light absorptivity and levoglucosan,
consistent with other studies suggesting a short atmospheric lifetime for
levoglucosan. However, a much stronger correlation between light absorptivity
and total hydrous sugars was observed, suggesting that they may serve as more
robust tracers for aged biomass burning emissions. Overall, the results from
this study suggest that robust model estimates of BrC radiative impacts
require consideration of the atmospheric aging of BrC and the stability of
high-MW BrC.
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