Parameterization of single-scattering albedo (SSA) and absorption Ångström exponent (AAE) with EC / OC for aerosol emissions from biomass burning

Abstract

Abstract. Single-scattering albedo (SSA) and absorption Ångström exponent (AAE) are two critical parameters in determining the impact of absorbing aerosol on the Earth's radiative balance. Aerosol emitted by biomass burning represent a significant fraction of absorbing aerosol globally, but it remains difficult to accurately predict SSA and AAE for biomass burning aerosol. Black carbon (BC), brown carbon (BrC), and non-absorbing coatings all make substantial contributions to the absorption coefficient of biomass burning aerosol. SSA and AAE cannot be directly predicted based on fuel type because they depend strongly on burn conditions. It has been suggested that SSA can be effectively parameterized via the modified combustion efficiency (MCE) of a biomass burning event and that this would be useful because emission factors for CO and CO2, from which MCE can be calculated, are available for a large number of fuels. Here we demonstrate, with data from the FLAME-4 experiment, that for a wide variety of globally relevant biomass fuels, over a range of combustion conditions, parameterizations of SSA and AAE based on the elemental carbon (EC) to organic carbon (OC) mass ratio are quantitatively superior to parameterizations based on MCE. We show that the EC ∕ OC ratio and the ratio of EC ∕ (EC + OC) both have significantly better correlations with SSA than MCE. Furthermore, the relationship of EC ∕ (EC + OC) with SSA is linear. These improved parameterizations are significant because, similar to MCE, emission factors for EC (or black carbon) and OC are available for a wide range of biomass fuels. Fitting SSA with MCE yields correlation coefficients (Pearson's r) of ∼ 0.65 at the visible wavelengths of 405, 532, and 660 nm while fitting SSA with EC / OC or EC / (EC + OC) yields a Pearson's r of 0.94–0.97 at these same wavelengths. The strong correlation coefficient at 405 nm (r = 0.97) suggests that parameterizations based on EC / OC or EC / (EC + OC) have good predictive capabilities even for fuels in which brown carbon absorption is significant. Notably, these parameterizations are effective for emissions from Indonesian peat, which have very little black carbon but significant brown carbon (SSA = 0.990 ± 0.001 at 532 and 660 nm, SSA = 0.937 ± 0.011 at 405 nm). Finally, we demonstrate that our parameterization based on EC / (EC + OC) accurately predicts SSA during the first few hours of plume aging with data from Yokelson et al. (2009) gathered during a biomass burning event in the Yucatán Peninsula of Mexico.

Black and brown carbon over central Amazonia: Long-term aerosol measurements at the ATTO site

“…We have found a trend where å abs increases with decreasing BC to OA mass ratio following an exponential function. These results are comparable to those presented by Pokhrel et al (2016) and Saleh et al (2014), with slightly lower å abs values in our study, however. This pattern could be related to a dominant presence of primary organic aerosol (POA) that has characteristically lower absorption wavelength dependence compared to SOA (Saleh et al, 2013).…”

“…The ratio BC to OA has been used before to parameterize å abs and ω 0 (Pokhrel et al, 2016;Saleh et al, 2014) to OA mass ratio during the North African LRT events in the wet season can be found in Fig. 11.…”

Relative importance of black carbon, brown carbon, and absorption enhancement from clear coatings in biomass burning emissions

“…These parameters include modified combustion efficiency (MCE), black carbon to organic aerosol ratio (BC / OA), and elemental to total carbon ratio ( EC EC+OC ) (Pokhrel et al, 2016;Lu et al, 2015;Saleh et al, 2014;McMeeking et al, 2014;. The current study demonstrates that the magnitude of total BrC absorption can be parameterized with the EC / OC ratio, a distinct result from Saleh et al (2014) who demonstrated that the intensive quantity of absorptivity, or imaginary refractive index, can be parameterized with the BC / OA ratio.…”

“…Further details of the PAS calibration using the CRDS can be found in Lack et al (2012b). Further details about the instrumental setup of the PAS and CRDS can be found in Pokhrel et al (2016). The uncertainty associated with the PAS measurements during FLAME-4 was found to be 7 % (1σ ) in the 660 and 532 nm channels and 18 % (1σ ) in the 405 nm channel (Pokhrel et al, 2016).…”

“…R. P. Pokhrel et al: Relative importance of black carbon, brown carbon, and absorption enhancement 5065 2 Materials and methods Measurements were made during the multi-investigator FLAME-4 experiment. Details of the fuels burned and overall experiment can be found in Stockwell et al (2014), and details of the experimental setup for optical measurements in Pokhrel et al (2016). The schematic of the instrumental setup during the FLAME-4 experiment is shown in Fig.…”