Nonlinear behavior of organic aerosol in biomass burning plumes: a microphysical model analysis
Authors: Konovalov Igor B. , Beekmann Matthias , Golovushkin Nikolai A. , Andreae Meinrat O. 
Published: October 2019

Abstract. Organic aerosol (OA) is a major component of smoke plumes
from open biomass burning (BB). Therefore, adequate representation of the
atmospheric transformations of BB OA in chemistry-transport and climate
models is an important prerequisite for accurate estimates of the impact of
BB emissions on air quality and climate. However, field and laboratory
studies of atmospheric transformations (aging) of BB OA have yielded a wide
diversity of observed effects. This diversity is still not sufficiently
understood and thus not addressed in models. As OA evolution is governed by
complex nonlinear processes, it is likely that at least a part of the
observed variability in the BB OA aging effects is due to the factors
associated with the intrinsic nonlinearity of the OA system. In this study,
we performed a numerical analysis in order to gain a deeper understanding of
these factors. We employ a microphysical dynamic model that represents
gas–particle partitioning and OA oxidation chemistry within the volatility
basis set (VBS) framework and includes a schematic parameterization of BB OA
dilution due to dispersion of an isolated smoke plume. Several VBS schemes
of different complexity, which have been suggested in the literature to
represent BB OA aging in regional and global chemistry-transport models, are
applied to simulate BB OA evolution over a 5 d period representative of
the BB aerosol lifetime in the dry atmosphere. We consider the BB OA mass
enhancement ratio (EnR), which is defined as the ratio of the mass
concentration of BB OA to that of an inert tracer and allows us to eliminate
the linear part of the dilution effects. We also analyze the behavior of the
hygroscopicity parameter, κ, that was simulated in a part of our
numerical experiments. As a result, five qualitatively different regimes of
OA evolution are identified, which comprise (1) a monotonic saturating
increase in EnR, (2) an increase in EnR followed by a decrease, (3) an
initial rapid decrease in EnR followed by a gradual increase, (4) an EnR
increase between two intermittent stages of its decrease, or (5) a gradual
decrease in EnR. We find that the EnR for BB aerosol aged from a few hours
to a few tens of hours typically increases for larger initial sizes of the
smoke plume (and therefore smaller dilution rates) or for lower initial OA
concentrations (and thus more organic gases available to form secondary OA – SOA).
However, these dependencies can be weakened or even reversed, depending on
the BB OA age and on the ratio between the fragmentation and
functionalization oxidation pathways. Nonlinear behavior of BB OA is also
exhibited in the dependencies of κ on the parameters of the plume.
Application of the different VBS schemes results in large quantitative and
qualitative differences between the simulations, although our analysis
suggests also that the main qualitative features of OA evolution simulated
with a complex two-dimensional VBS scheme can also be reproduced with a much
simpler scheme. Overall, this study indicates that the BB aerosol evolution
may strongly depend on parameters of the individual BB smoke plumes (such as
the initial organic aerosol concentration and plume size) that are typically
not resolved in chemistry-transport models.