Microphysical and optical properties of aerosol were studied during a mega-fire event in summer 2012 over Siberia using ground-based measurements of spectral solar radiation at the AERONET site in Tomsk and satellite observations. The data were analysed using multi-year (2003–2013) measurements of aerosol characteristics under background conditions and for less intense fires, differing in burning biomass type, stage of fire, remoteness from observation site, etc. (ordinary smoke). In June–August 2012, the average aerosol optical depth (AOD, 500 nm) had been 0.95 ± 0.86, about a factor of 6 larger than background values (0.16 ± 0.08), and a factor of 2.5 larger than in ordinary smoke. The AOD values were extremely high on 24–28 July and reached 3–5. A comparison with satellite observations showed that ground-based measurements in the region of Tomsk not only reflect the local AOD features, but are also characteristic for the territory of Western Siberia as a whole. Single scattering albedo (SSA, 440 nm) in this period ranged from 0.91 to 0.99 with an average of ∼ 0.96 in the entire wavelength range of 440–1020 nm. The increase in absorptance of aerosol particles (SSA(440 nm) = 0.92) and decrease in SSA with wavelength observed in ordinary smoke agree with the data from multi-year observations in analogous situations in the boreal zone of USA and Canada. Volume aerosol size distribution in extreme and ordinary smoke had a bimodal character with significant prevalence of fine-mode particles, but in summer 2012 the mean median radius and the width of the fine-mode distribution somewhat increased. In contrast to data from multi-year observations, in summer 2012 an increase in the volume concentration and median radius of the coarse mode was observed with growing AOD. The calculations of the average radiative effects of smoke and background aerosol are presented. Compared to background conditions and ordinary smoke, under the extreme smoke conditions the cooling effect of aerosol considerably intensifies: direct radiative effects (DRE) at the bottom (BOA) and at the top of the atmosphere (TOA) are −13, −35, and −60 W m⁻² and −5, −14, and −35 W m⁻² respectively. The maximal values of DRE were observed on 27 July (AOD(500 nm) = 3.5), when DRE(BOA) reached −150 W m⁻², while DRE(TOA) and DRE of the atmosphere were −75 W m⁻². During the fire event in summer 2012 the direct radiative effect efficiency varied in range: at the BOA it was −80–−40 W m⁻², at the TOA it was −50–−20 W m⁻² and in the atmosphere it was −35–−20 W m⁻².

An empirical model of the vertical profiles of aerosol optical characteristics is described. This model was developed based on data acquired from multi-year airborne sensing of optical and microphysical characteristics of the tropospheric aerosol over West Siberia. The main initial characteristics for the creation of the model were measurement data of the vertical profiles of the aerosol angular scattering coefficients in the visible wavelength range, particle size distribution functions and mass concentrations of black carbon (BC). The proposed model allows us to retrieve the aerosol optical and radiative characteristics in the visible and near-IR wavelength range, using the season, air mass type and time of day as input parameters. The columnar single scattering albedo and asymmetry factor of the aerosol scattering phase function, calculated using the average vertical profiles, are in good agreement with data from the AERONET station located in Tomsk. For solar radiative flux calculations, this empirical model has been tested for typical summer conditions. The available experimental database obtained for the regional features of West Siberia and the model developed on this basis are shown to be sufficient for performing these calculations.

We compare the time variations in monthly average concentrations of black carbon in the surface air layer obtained from field measurements ([BC]) and from MERRA-2 reanalysis ([BC]_M) at four monitoring sites located in the northern part of Russia (Tiksi Observatory, Pechora-Ilych Biosphere Reserve), Alaska (Barrow station), and Greenland (Summit station). It is shown that the MERRA-2 reanalysis data for the regions of Tiksi and Barrow do not completely reflect the [BC] variations during the year, in contrast to the Pechora-Ilych Biosphere Reserve, where the discrepancies are within 30–50%. The [BC]_M reanalysis products for the Summit monitoring site qualitatively agree with the measurements characterizing the BC content in the free troposphere, but they underestimate [BC] by more than a factor of two. On the whole, our analysis showed that the results of the MERRA-2 reanalysis of the monthly average indices of near-surface concentration of atmospheric black carbon can be used for climate assessments for hard-to-reach northern regions in the warm season with an error of about 30%. Possible causes for the discrepancies between [BC] and [BC]_M are discussed for different times of the year and observation sites.

We present the quantitative estimates of the vertical distribution of absorbed solar radiation and temperature effects in the background and extremely smoke-laden troposphere of Siberia, obtained...

We present model estimates of diurnally averaged direct radiative effects of smoke aerosol in the solar spectral range obtained using observations at the Tiksi station in the period of...

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.