Abstract. Understanding the sources of light-absorbing organic (brown) carbon (BrC)
and its interaction with black carbon (BC) and other non-refractory
particulate matter (NR-PM) fractions is important for reducing uncertainties
in the aerosol direct radiative forcing. In this study, we combine multiple
filter-based techniques to achieve long-term, spectrally resolved, source-
and species-specific atmospheric absorption closure. We determine the mass
absorption efficiency (MAE) in dilute bulk solutions at 370 nm to be equal
to 1.4 m2 g−1 for fresh biomass smoke, 0.7 m2 g−1 for
winter-oxygenated organic aerosol (OA), and 0.13 m2 g−1 for other less absorbing OA.
We apply Mie calculations to estimate the contributions of these fractions
to total aerosol absorption. While enhanced absorption in the near-UV has
been traditionally attributed to primary biomass smoke, here we show that
anthropogenic oxygenated OA may be equally important for BrC absorption
during winter, especially at an urban background site. We demonstrate that
insoluble tar balls are negligible in residential biomass burning
atmospheric samples of this study and thus could attribute the totality of
the NR-PM absorption at shorter wavelengths to methanol-extractable BrC. As
for BC, we show that the mass absorption cross-section (MAC) of this
fraction is independent of its source, while we observe evidence for a
filter-based lensing effect associated with the presence of NR-PM
components. We find that bare BC has a MAC of 6.3 m2 g−1 at 660 nm and an absorption Ångström exponent of 0.93 ± 0.16,
while in the presence of coatings its absorption is enhanced by a factor of
∼ 1.4. Based on Mie calculations of closure between observed
and predicted total light absorption, we provide an indication for a
suppression of the filter-based lensing effect by BrC. The total absorption
reduction remains modest, ∼ 10 %–20 % at 370 nm, and is
restricted to shorter wavelengths, where BrC absorption is significant.
Overall, our results allow an assessment of the relative importance of the
different aerosol fractions to the total absorption for aerosols from a
wide range of sources and atmospheric ages. When integrated with the solar
spectrum at 300–900 nm, bare BC is found to contribute around two-thirds of
the solar radiation absorption by total carbonaceous aerosols, amplified by
the filter-based lensing effect (with an interquartile range, IQR, of 8 %–27 %), while the IQR of the contributions by particulate BrC is 6 %–13 %
(13 %–20 % at the rural site during winter). Future studies that will
directly benefit from these results include (a) optical modelling aiming at
understanding the absorption profiles of a complex aerosol composed of BrC,
BC and lensing-inducing coatings; (b) source apportionment aiming at
understanding the sources of BC and BrC from the aerosol absorption
profiles; (c) global modelling aiming at quantifying the most important
aerosol absorbers.
Searching for Gravitationally Lensed Gamma-Ray Bursts with Their Afterglows