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Abstract:
We use a global chemical transport model (GEOSChem)
to interpret observed light-absorbing aerosols in
Amazonia during the wet season. Observed aerosol properties,
including black carbon (BC) concentration and light
absorption, at the Amazon Tall Tower Observatory (ATTO)
site in the central Amazon have relatively low background
levels but frequently show high peaks during the study period
of January–April 2014. With daily temporal resolution
for open fire emissions and modified aerosol optical properties,
our model successfully captures the observed variation
in fine/coarse aerosol and BC concentrations as well as
aerosol light absorption and its wavelength dependence over
the Amazon Basin. The source attribution in the model indicates
the important influence of open fire on the observed
variances of aerosol concentrations and absorption, mainly
from regional sources (northern South America) and from
northern Africa. The contribution of open fires from these
two regions is comparable, with the latter becoming more
important in the late wet season. The analysis of correlation
and enhancement ratios of BC versus CO suggests transport
times of < 3 days for regional fires and 11 days for African
plumes arriving at ATTO during the wet season. The model
performance of long-range transport of African plumes is
also evaluated with observations from AERONET, MODIS,
and CALIOP. Simulated absorption aerosol optical depth
(AAOD) averaged over the wet season is lower than 0.0015
over the central Amazon, including the ATTO site. We find
that more than 50%of total absorption at 550 nm is from BC,
except for the northeastern Amazon and the Guianas, where
the influence of dust becomes significant (up to 35 %). The
brown carbon contribution is generally between 20 and 30 %.
The distribution of absorption Ångström exponents (AAE)
suggests more influence from fossil fuel combustion in the
southern part of the basin (AAE 1) but more open fire and
dust influence in the northern part (AAE > 1.8). Uncertainty
analysis shows that accounting for absorption due to secondary
organic aerosol (SOA) and primary biogenic aerosol
(PBA) particles could result in differences of < 8 and 5–40% in total absorption, respectively.
