Abstract. Long-range transport of biomass burning (BB) aerosol from
regions affected by wildfires is known to have a significant impact on the
radiative balance and air quality in receptor regions. However, the changes
that occur in the optical properties of BB aerosol during long-range
transport events are insufficiently understood, limiting the adequacy of
representations of the aerosol processes in chemistry transport and climate
models. Here we introduce a framework to infer and interpret changes in the
optical properties of BB aerosol from satellite observations of multiple BB
plumes. Our framework includes (1) a procedure for analysis of available
satellite retrievals of the absorption and extinction aerosol optical depths
(AAOD and AOD) and single-scattering albedo (SSA) as a function of the BB
aerosol photochemical age and (2) a representation of the AAOD and AOD
evolution with a chemistry transport model (CTM) involving a simplified
volatility basis set (VBS) scheme with a few adjustable parameters. We apply
this framework to analyze a large-scale outflow of BB smoke plumes from
Siberia toward Europe that occurred in July 2016. We use AAOD and SSA data
derived from OMI (Ozone Monitoring Instrument) satellite measurements in the
near-UV range along with 550 nm AOD and carbon monoxide (CO) columns
retrieved from MODIS (Moderate Resolution Imaging Spectroradiometer) and
IASI (Infrared Atmospheric Sounding Interferometer) satellite observations,
respectively, to infer changes in the optical properties of Siberian BB
aerosol due to its atmospheric aging and to get insights into the processes
underlying these changes. Using the satellite data in combination with
simulated data from the CHIMERE CTM, we evaluate the enhancement ratios
(EnRs) that allow isolating AAOD and AOD changes due to oxidation and
gas–particle partitioning processes from those due to other processes,
including transport, deposition, and wet scavenging. The behavior of EnRs
for AAOD and AOD is then characterized using nonlinear trend analysis. It is
found that the EnR for AOD strongly increases (by about a factor of 2)
during the first 20–30 h of the analyzed evolution period, whereas the
EnR for AAOD does not exhibit a statistically significant increase during
this period. The increase in AOD is accompanied by a statistically
significant enhancement of SSA. Further BB aerosol aging (up to several
days) is associated with a strong decrease in EnRs for both AAOD and AOD.
Our VBS simulations constrained by the observations are found to be more
consistent with satellite observations of strongly aged BB plumes than
“tracer” simulations in which atmospheric transformations of BB organic
aerosol were disregarded. The simulation results indicate that the upward
trends in EnR for AOD and in SSA are mainly due to atmospheric processing of
secondary organic aerosol (SOA), leading to an increase in the mass
scattering efficiency of BB aerosol. Evaporation and chemical fragmentation
of the SOA species, part of which is assumed to be absorptive (to contain
brown carbon), are identified as likely reasons for the subsequent decrease
in the EnR for both AAOD and AOD. Hence, our analysis reveals that the
long-range transport of smoke plumes from Siberian fires is associated with
major changes in BB aerosol optical properties and chemical composition.
Overall, this study demonstrates the feasibility of using available
satellite observations for evaluating and improving representations in
atmospheric models of the BB aerosol aging processes in different regions of
the world at much larger temporal scales than those typically addressed in
aerosol chamber experiments.
Spectral optical properties of long-range transport Asian dust and pollution aerosols over Northeast Asia in 2007 and 2008
