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. Organic aerosol (OA) is a major component of smoke plumesfrom open biomass burning (BB). Therefore, adequate representation of theatmospheric transformations of BB OA in chemistry-transport and climatemodels is an important prerequisite for accurate estimates of the impact ofBB emissions on air quality and climate. However, field and laboratorystudies of atmospheric transformations (aging) of BB OA have yielded a widediversity of observed effects. This diversity is still not sufficientlyunderstood and thus not addressed in models. As OA evolution is governed bycomplex nonlinear processes, it is likely that at least a part of theobserved variability in the BB OA aging effects is due to the factorsassociated with the intrinsic nonlinearity of the OA system. In this study,we performed a numerical analysis in order to gain a deeper understanding ofthese factors. We employ a microphysical dynamic model that representsgas–particle partitioning and OA oxidation chemistry within the volatilitybasis set (VBS) framework and includes a schematic parameterization of BB OAdilution due to dispersion of an isolated smoke plume. Several VBS schemesof different complexity, which have been suggested in the literature torepresent BB OA aging in regional and global chemistry-transport models, areapplied to simulate BB OA evolution over a 5 d period representative ofthe BB aerosol lifetime in the dry atmosphere. We consider the BB OA massenhancement ratio (EnR), which is defined as the ratio of the massconcentration of BB OA to that of an inert tracer and allows us to eliminatethe linear part of the dilution effects. We also analyze the behavior of thehygroscopicity parameter, κ, that was simulated in a part of ournumerical experiments. As a result, five qualitatively different regimes ofOA evolution are identified, which comprise (1) a monotonic saturatingincrease in EnR, (2) an increase in EnR followed by a decrease, (3) aninitial rapid decrease in EnR followed by a gradual increase, (4) an EnRincrease between two intermittent stages of its decrease, or (5) a gradualdecrease in EnR. We find that the EnR for BB aerosol aged from a few hoursto a few tens of hours typically increases for larger initial sizes of thesmoke plume (and therefore smaller dilution rates) or for lower initial OAconcentrations (and thus more organic gases available to form secondary OA – SOA).However, these dependencies can be weakened or even reversed, depending onthe BB OA age and on the ratio between the fragmentation andfunctionalization oxidation pathways. Nonlinear behavior of BB OA is alsoexhibited in the dependencies of κ on the parameters of the plume.Application of the different VBS schemes results in large quantitative andqualitative differences between the simulations, although our analysissuggests also that the main qualitative features of OA evolution simulatedwith a complex two-dimensional VBS scheme can also be reproduced with a muchsimpler scheme. Overall, this study indicates that the BB aerosol evolutionmay strongly depend on parameters of the individual BB smoke plumes (such asthe initial organic aerosol concentration and plume size) that are typicallynot resolved in chemistry-transport models.

Nonlinear behavior of organic aerosol in biomass burning plumes: a microphysical model analysis