We present some results of our numerical study of the radiative impact of Siberian biomass burning aerosol (SBBA) on clouds in the eastern Arctic. The numerical experiments were performed using the CHIMERE chemistry-transport model coupled with the WRF meteorological model. Based on an original numerical methodology, we examined the radiative impact of SBBA on characteristics of cloudiness, such as the cloud optical depth and cloud cover fraction for low, medium-level, and high clouds, as a function of the duration of the radiative forcing of SBBA. The results of the experiments show that SBBA-induced disturbances in radiative fluxes can lead to significant but variable changes in the cloudiness characteristics. The changes in the characteristics of low and medium-level clouds tend to have opposite signs and are caused predominantly by scattering components of the aerosol particles. Overall, this study contributes to a better understanding of the complex mechanisms of formation of the semi-direct radiative effect of SBBA in the Arctic.
We present the first results of our numerical study aimed at identifying the possible effects of the atmospheric evolution of biomass burning (BB) aerosol on the aerosol direct radiative forcing (DRF). We used a microphysical box model, which implements different parameterizations of the organic aerosol evolution within the volatility basis set framework, along with the Mie theory and radiative transfer calculations to simulate the dynamics of the physical, optical and radiative properties of polydisperse carbonaceous aerosol in an isolated BB smoke plume. We found that atmospheric transformations of the organic fraction of BB aerosol can result in major changes in the BB aerosol DRF. These changes are related to the underlying variations in the aerosol optical depth and single scattering albedo in a nonlinear manner and are especially strong in optically dense plumes.
Numerical experiments using a microphysical box model were performed to study the changes in the mass absorption efficiency (MAE) of biomass burning (BB) aerosol during its atmospheric aging in an isolated smoke plume. The model describes the oxidation and gas-particle partitioning processes within the volatility basis set (VBS) framework and involves a schematic representation of plume dispersion. The computations of MAE which is determined by the composition, size distribution, and mixing state of the particles were based on the Mie theory by assuming that the aerosol particles consist of an absorbing black carbon core and a non-absorbing organic shell. The results indicate that the atmospheric evolution of BB aerosol can be associated with considerable changes in MAE. The changes are found to depend in a nonlinear manner on both the initial parameters of the BB plume and the duration of the aerosol evolution. They are also found to be sensitive to how the oxidation processes are represented in the model.
A regional chemistry-transport model and remote sensing observations are used to study the absorption properties of organic carbon in biomass burning (BB) aerosol emitted from wildfires in Siberia. Using multi-wavelength retrievals of the absorption and extinction aerosol optical depths (AAOD and AOD) at Siberian sites of AERONET, we obtained an empirical relationship enabling the estimation of the contribution of brown carbon (BrC) to the BB aerosol absorption in a near-UV spectral region by using available satellite observations of AAOD and AOD. Application of this relationship to constrain our simulations of a large-scale episode of transport of BB plumes from Siberia towards Europe allowed us to parameterize an imaginary part of the refractive index of organic carbon in Siberian BB aerosol as a function of the BB aerosol photochemical age as well as to estimate the efficient mass fraction of BrC in the smoke plumes.
Carbonaceous aerosol originating from forest fires and other types of open biomass burning (BB) is known to have a strong impact on the radiative balance of the atmosphere but available model estimates of the corresponding climate effects are characterized by large uncertainties due to insufficient knowledge of its sources and atmospheric transformations (aging). Here we study the atmospheric aging of BB aerosol by analyzing data of satellite measurements of the absorption and extinction aerosol optical depths (AAOD and AOD) in BB plumes from wildfires in Siberia. Along with the aerosol measurements, our analysis involves satellite data for CO column amounts as well as the simulations of the BB aerosol photochemical age using a chemistry transport model. The analysis includes the two main stages. First, we study the BB aerosol evolution as a function of its photochemical age by using the observations over Siberia in the period from May to September of 2012. Second, we examine changes in the BB aerosol optical characteristics in the smoke plumes that had been transported from Siberia into the European part of Russia in July of 2016. The results indicate that photochemical aging strongly affects the behavior of both AAOD and AOD. The results also show that aged BB aerosol becomes less absorptive at the near UV wavelength range, indicating a depletion of brown carbon in particles. The BB aerosol aging effects detected at the both stages of the analysis are found to be consistent.
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