Estimation of the Elemental to Organic Carbon Ratio in Biomass Burning Aerosol Using AERONET Retrievals

Abstract

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 Network (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.

Estimation of black carbon emissions from Siberian fires using satellite observations of absorption and extinction optical depths

“…This assumption is based on the results of the Fourth Fire Laboratory at Missoula Experiment (FLAME-4) (Pokhrel et al, 2016), where an almost linear relationship between SSA and the elemental to total carbon was found for fresh BB aerosol from a wide variety 25 of biomass fuels. Furthermore, this assumption has been corroborated by the analysis of aircraft observations of aging BB aerosol (Pokhrel et al, 2016;Konovalov et al, 2017a). Accordingly, based on this assumption, we evaluate EC/(EC+OC) in the atmospheric BB aerosol column using the following empirical relationship: wavelengths (405, 532, and 660 nm), which are rather close to minus and plus unity, respectively, have been reported by Pokhrel et al (2016).…”

“…The method used in this study to evaluate AAOD as a function of the modeled BB aerosol composition and AOD was introduced in Konovalov et al (2017a). The main assumption underlying our method is that the dependence of the SSA on the ratio of elemental to total carbon, EC/(EC+OC), in the aerosol can be approximated by a linear function.…”

“…The empirical coefficients a 675 and b 675 were 5 evaluated using the power-law extrapolation from the absolute values reported by Pokhrel et al (2016) at 532 and 660 nm and were found to be -1.12 (±0.04) and 0.99 (±0.004), respectively. Note that using a more sophisticated analysis, Konovalov et al (2017a) derived EC/(EC+OC) from the AERONET observations at 870 nm. However, we found that the impact of the differences between the EC/(EC+OC) estimates corresponding to the two wavelengths on the empirical parameterization discussed below was negligible, and so we opted for a simpler and more transparent approach in this study.…”

“…where κ 1 and κ 2 are the regression coefficients, which were estimated in Konovalov et al (2017a) to be 2.05 (±0.86) and 0.014 (±0.028) (the confidence intervals are given in terms of the 90 th percentile). Note that unlike the standard least-squares 20 method which disregards errors in a predictor variable, the ODR method takes into account random errors in both dependent variables.…”

“…In view of this fact, explicit modeling of AAOD in the case of BB aerosol as a function of its composition would inevitably involve major uncertainties associated with the assumptions regarding the magnitude of the 20 imaginary part of the refractive index for organic carbon (OC) and the mixing state of aerosol particles; these characteristics are likely strongly variable, depending on sources and atmospheric processing of BB aerosol (Lack et al, 2012, Saleh et al, 2013Wong et al, 2017). To overcome this difficulty, we follow an empirical approach (Konovalov et al, 2017a) that involves parameterization of AAOD as a function of the EC/OC (elemental carbon to organic carbon) ratio and the aerosol extinction optical depth (AOD). This parameterization is based on the analysis of experimental relationships between the 25 single scattering albedo (SSA) of BB aerosol particles and the EC/OC ratio and is fitted to the retrievals of aerosol optical properties from the multi-wavelength measurements made at the Aerosol Robotic Network (AERONET) sites in Siberia in summer 2012.…”