We present the first box model simulation results aimed at identification of possible effects of the atmospheric photochemical evolution of the organic component of biomass burning (BB) aerosol on the aerosol radiative forcing (ARF) and its efficiency (ARFE). The simulations of the dynamics of the optical characteristics of the organic aerosol (OA) were performed using a simple parameterization developed within the volatility basis set framework and adapted to simulate the multiday BB aerosol evolution in idealized isolated smoke plumes from Siberian fires (without dilution). Our results indicate that the aerosol optical depth can be used as a good proxy for studying the effect of the OA evolution on the ARF, but variations in the scattering and absorbing properties of BB aerosol can also affect its radiative effects, as evidenced by variations in the ARFE. Changes in the single scattering albedo (SSA) and asymmetry factor, which occur as a result of the BB OA photochemical evolution, may either reduce or enhance the ARFE as a result of their competing effects, depending on the initial concentration OA, the ratio of black carbon to OA mass concentrations and the aerosol photochemical age in a complex way. Our simulation results also reveal that (1) the ARFE at the top of the atmosphere is not significantly affected by the OA oxidation processes compared to the ARFE at the bottom of the atmosphere, and (2) the dependence of ARFE in the atmospheric column and on the BB aerosol photochemical ages almost mirrors the corresponding dependence of SSA.

The content and evolution of brown carbon (BrC) in smokes from Siberian forest fires were analyzed using measurements of the aerosol absorption optical depth (AAOD) at three Russian AERONET stations located in Tomsk, Zvenigorod, and Yekaterinburg. Estimates are obtained of the relative contribution of BrC in fine aerosol particles to the absorption of solar radiation at a wavelength of 440 nm (η_BrC) and, in particular, for an anomalous episode of long-range transport of smokes from Siberia to the European part of Russia in summer 2016. A considerable BrC content is found in smokes in Tomsk and Zvenigorod (where η_BrC is estimated to be 15 and 18% on average). It is noteworthy that no significant η_BrC values were revealed during passage of smokes from Siberian fires over Yekaterinburg. The η_BrC values were found to decrease with aerosol aging under sunlit conditions on the characteristic timescale of about 30 h. At the same time, the measurements in Zvenigorod indicate that the absorption properties of the organic component of smoke aerosol increase during a much longer evolution.

A bulk of evidence from in situ observations and lab experiments suggests that brown carbon (light-absorbing organic compounds in particles) can provide a significant yet highly variable contribution to the overall light absorption by aerosol particles from biomass burning (BB). Partly stemming from the complexity of the atmospheric evolution of organic aerosol (OA), the variability in brown carbon (BrC) absorption makes it difficult to partition the radiative effects of BrC and black carbon (BC) in atmospheric and climate models; as such, there are calls for satellite-based methods that could provide a statistical characterization of BrC absorption and its evolution in different regions of the world, especially in remote BB regions, such as Siberia. This study examined the feasibility of the statistical characterization of the evolution of BrC absorption and related parameters of BB aerosol in smoke plumes from intense wildfires in Siberia through the analysis of a combination of data from three satellite instruments: OMI (Ozone Monitoring Instrument), MISR (Multi-Angle Imaging SpectroRadiometer), and MODIS (Moderate Resolution Imaging Spectroradiometer). Using a Monte Carlo method, which related the satellite retrievals of the absorption and extinction aerosol optical depths to Mie theory calculations of the optical properties of BB aerosol, we found that the BrC absorption, as well as the imaginary refractive index for the OA, decreased significantly in Siberian BB smoke plumes during about 30 h of the daylight evolution, nevertheless remaining considerable until at least 70 h of the daylight evolution. Overall, the study indicated that the analysis of multi-platform satellite observations of BB plumes can provide useful insights into the atmospheric evolution of BrC absorption and the partitioning of BrC and BC contributions to the total light absorption by BB aerosol.

A good quantitative knowledge of regional sources and sinks of atmospheric carbon dioxide (CO₂) is essential for understanding the global carbon cycle. It is also a key prerequisite for elaborating cost-effective national strategies to achieve the goals of the Paris Agreement. However, available estimates of CO₂ fluxes for many regions of the world remain uncertain, despite significant recent progress in the remote sensing of terrestrial vegetation and atmospheric CO₂. In this study, we investigate the feasibility of inferring reliable regional estimates of the net ecosystem exchange (NEE) using column-averaged dry-air mole fractions of CO₂ (XCO₂) retrieved from Orbiting Carbon Observatory-2 (OCO-2) observations as constraints on parameters of the widely used Vegetation Photosynthesis and Respiration model (VPRM), which predicts ecosystem fluxes based on vegetation indices derived from multispectral satellite imagery. We developed a regional-scale inverse modeling system that applies a Bayesian variational optimization algorithm to optimize parameters of VPRM coupled to the CHIMERE chemistry transport model and which involves a preliminary transformation of the input XCO₂ data that reduces the impact of the CHIMERE boundary conditions on inversion results. We investigated the potential of our inversion system by applying it to a European region (that includes, in particular, the EU countries and the UK) for the warm season (May–September) of 2021. The inversion of the OCO-2 observations resulted in a major (more than threefold) reduction of the prior uncertainty in the regional NEE estimate. The posterior NEE estimate agrees with independent estimates provided by the CarbonTracker Europe High-Resolution (CTE-HR) system and the ensemble of the v10 OCO-2 model intercomparison (MIP) global inversions. We also found that the inversion improves the agreement of our simulations of XCO₂ with retrievals from the Total Carbon Column Observing Network (TCCON). Our sensitivity test experiments using synthetic XCO₂ data indicate that the posterior NEE estimate would remain reliable even if the actual regional CO₂ fluxes drastically differed from their prior values. Furthermore, the posterior NEE estimate is found to be robust to strong biases and random uncertainties in the CHIMERE boundary conditions. Overall, this study suggests that our approach offers a reliable and relatively simple way to derive robust estimates of CO₂ ecosystem fluxes from satellite XCO₂ observations while enhancing the applicability of VPRM in regions where eddy covariance measurements of CO₂ fluxes are scarce.

We consider the results of simulating the smoke aerosol radiative forcing (ARF) at the top and bottom of the atmosphere depending on the photochemical evolution of its organic component, illumination conditions, and underlying surface types (water, mixed forest, and snow/ice). At the top of the atmosphere, an increase in the surface albedo and a decrease in the aerosol optical depth can lead to the transformation of the cooling into a heating effect. An increase in absorptance of aerosol particles may cause the ARF to increase at the top of the atmosphere, which is manifested most significantly over snow/ice surfaces; while at the bottom of the atmosphere, ARF decreases. As the solar zenith angle grows, the ARF decreases in absolute value when the smoke plume propagates over weakly reflecting surfaces; however, with growing surface albedo, this dependence becomes nonmonotonic. It is shown that, when the transformations of the optical characteristics of organic aerosol are neglected, the aerosol radiative forcing at the top of the atmosphere can be overestimated or underestimated severalfold (by a few tens of W/m²), and this can also cause an error of determining the ARF sign.

The effects of atmospheric aerosol, including smoke, on cloudiness are of great interest for understanding the current climate changes. However, the radiative effect of Siberian biomass burning aerosol (SBBA) on clouds in the Arctic has been little studied. Based on simulation with the CHIMERE chemistry transport model and WRF meteorological model, we analyze the processes responsible for origination of the semidirect radiative effect (SDRE) of smoke from Siberian wildfires over snow–ice surfaces in the Arctic, including time- and space-averaged changes in the radiative fluxes, cloud parameters in different cloud levels, and some meteorological characteristics associated with cloud generation due to the radiative effect of (SBBA). The results show that solar radiation scattering by SBBA particles increases the static stability of the atmosphere at altitudes of 2–4 km and suppresses vertical turbulent motions, which decreases the rate of water condensation, the optical thickness of clouds, and the mixing ratio in the medium- and partly lower-level clouds. The decrease in the optical thickness of clouds, in turn, causes origination of positive SDRE of SBBA at the top and bottom of the atmosphere. Radiation absorption by SBBA particles does not play an essential role in these processes, although additionally changes meteorological characteristics. Our results expand fundamental knowledge of aerosol–cloud interactions in the Arctic and can be used for improving climate models.