The balance between the cooling and warming effects of aerosol originating from open biomass burning (BB) critically depends on the ratio of its major absorbing and scattering components, such as elemental carbon (EC) and organic carbon (OC), but available direct measurements of this ratio in remote regions are limited and rather uncertain. Here, we propose a method to estimate the EC/OC mass ratio in BB aerosol using continuous observations of aerosol optical properties by the Aerosol Robotic

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... etwork (AERONET) and apply it to the data from two AERONET sites situated in Siberia. Our method exploits a robust experimental finding (that was reported recently based on laboratory analysis of aerosol from the combustion of wildland fuels) that the single scattering albedo of BB aerosol particles depends linearly on the EC/(EC + OC) mass ratio. We estimated that the mean value of the EC/OC ratio in BB aerosol observed in summer 2012 was 0.036 (±0.009), which is less than the corresponding value (0.061) predicted in our simulations with a chemistry transport model using the emission factors from the Global Fire Emissions Database 4 (GFED4) fire emission inventory. Based on results of our analysis, we propose a parameterization that allows constraining the EC/OC ratio in BB aerosol with available satellite observations of the absorption and extinction aerosol optical depths. secondary species from primary gaseous emissions [16] . Similarly, any observational constraints to the partitioning between EC and OC may be beneficial for improving model representations of organic aerosol mass, which (again unlike EC) is prone to significant changes during the atmospheric evolution of BB aerosol [17] [18] [19] [20] , in the context of air pollution modeling applications. (Note that the distinction made in the literature between EC and BC mostly reflects the differences between specific measurement techniques [7,21] and is not so important here. For consistency, the former term (EC) is mostly used throughout this paper, unless otherwise necessary in a given context). The partitioning between EC and OC in models simulating BB aerosol composition is usually based on data of fire emission inventories, such as, e.g., the Global Fire Emissions Database (GFED) [22], the Fire INventory from NCAR (FINN) [23] or the Global Fire Assimilation System (GFAS) [24] , which employ available in situ measurements of the emission factors to calculate emissions of individual species on regional and global scales. However, due to variations in combustion conditions and in fuel origin, moisture and geometry, different measurements yield a wide range of emission factor values, even when they are made in similar climate zones and types of environment [25] . As a result of large variations in the emission factors for EC and OC, the uncertainty of typical values of the EC/OC ratio may considerably exceed a factor of two for BB aerosol in extratropical forest and a factor of four in savanna and grassland [26, 27] . It seems reasonable to expect that the uncertainties in the emission factors for EC and OC assumed in the fire emission inventories are especially large in the case of fires in such a remote region as Siberia due to the lack of dedicated local measurements. Siberia represents one of the world's largest forested regions [28, 29] . Devastating Siberian fires are known to considerably affect the composition of the atmosphere over vast territories in Russia [30], with smoke plumes reaching Japan, Korea and even North America [31] [32] [33] . Emissions from Siberian fires are also found to provide a major source of EC deposited in the Arctic [34], thus contributing to rapid warming observed there [35] . As the on-going global warming is likely to lead to increases in the probability and intensity of fires in the boreal forest, there is a need to better understand and quantify emissions from Siberian fires and possible complex feedbacks between Siberian fires and their effects on regional and global climate [36] [37] [38] . In view of these challenges, it is particularly important to ensure that predictions of the composition and optical properties of BB aerosol originating from boreal fires by climate and chemistry transport models are sufficiently adequate. Consistent and abundant data on BB aerosol optical properties worldwide are available presently as inversion products from measurements performed by the ground-based Sun photometers at the Aerosol Robotic Network (AERONET) sites [39] . Several AERONET sites are located in the Siberian region. The aerosol absorption optical depth (AAOD) and single scattering albedo (SSA) retrieved from the AERONET measurements [40] are extensively used to evaluate and constrain the aerosol light absorption and atmospheric EC (BC) amounts simulated with models (e.g., [41] [42] [43] [44] [45] ). Another promising source of aerosol light absorption data that can be used to constrain the models is the AAOD retrievals from satellite measurements [46] . For example, the OMI (Ozone Monitoring Instrument) observations of AAOD at the 388-nm wavelength were proven to be useful in improving simulations with a global chemistry transport model and in reducing uncertainties in BC emissions in Southeast Asia [47] . A major obstacle in interpretation and addressing the discrepancies between the model and measurement AAOD data (note that the models were typically found to strongly underestimate AAOD) is that the modeled relationship between the atmospheric EC amounts and AAOD is very sensitive to the assumptions and simplifications regarding the mixing state of aerosol and intensive optical properties (such as, e.g., the absorption Ångström exponent and the refractive index) of its individual constituents, including EC and organic compounds [7, 48] . One of the available approaches to overcoming this obstacle involves constraining the contribution of EC to aerosol light absorption by using the multi-wavelength absorption measurements and taking into account the fact that the absorption Ångström exponent (AAE) for EC is different from AAE of other aerosol components (including BrC and dust). Multiple studies (e.g., [11, 15, [49] [50] [51] ) followed this approach in the analysis Atmosphere 2017, 8, 122 3 of 21 of field measurements and remote sensing aerosol absorption data by using different AAE-based methods. However, although such methods proved to be useful in constraining absorption of EC and BrC, they typically do not provide a direct inference on the EC/OC mass ratio in aerosol particles. Another measurement-based approach to constraining EC absorption involves parameterization of the aerosol absorption properties as a function of the aerosol chemical composition. Specifically, it was shown [9,10] that the imaginary part of the refractive index for organic BB aerosol can be parameterized as a growing function of the EC/OC ratio. More recently, as a result of the analysis of optical properties and the composition of BB aerosol formed from a wide range of wildland fuels from North America, it was demonstrated [52] that there is a linear relationship between the EC/(EC + OC) ratio and SSA at several visible wavelengths (405, 532 and 660 nm). In this study, we combine the AERONET multi-wavelength absorption measurements made in Siberia during the extreme summer fire season of 2012 [20, 30] and the empirical relationships [52] between SSA and the EC/OC ratio. Our primary goal is to obtain measurement-based estimates of the EC/OC ratio for BB aerosol in Siberia and to use them for the evaluation of the corresponding simulations performed with a regional chemistry transport model involving EC and OC emission factors used in the GFED inventory. Another goal of this study is to develop a parameterization enabling estimation of the EC/OC mass ratio using a combination of data from available satellite measurements, such as AAOD retrieved from the OMI measurements [46] and extinction aerosol optical depth (AOD) from the MODIS (Moderate Resolution Imaging Spectroradiometer) [53] measurements. The parameterization is intended to contribute to the methodological basis for possible prospective inverse modeling studies aimed at constraining EC emissions from boreal wildfires using satellite observations.