Parameterization of single-scattering albedo (SSA) and absorption ??ngstr??m exponent (AAE) with ECa?ˉ/a?ˉOC for aerosol emissions from biomass burning

摘要

pstrongAbstract./strong Single-scattering albedo (SSA) and absorption ??ngstr??m 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 COsub2/sub, 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 ECspan class="thinspace"/spana??span class="thinspace"/spanOC ratio and the ratio of ECspan class="thinspace"/spana??span class="thinspace"/span(ECspan class="thinspace"/span+span class="thinspace"/spanOC) both have significantly better correlations with SSA than MCE. Furthermore, the relationship of ECspan class="thinspace"/spana??span class="thinspace"/span(ECspan class="thinspace"/span+span class="thinspace"/spanOC) 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 ir/i) of a??a??a??span class="thinspace"/span0.65 at the visible wavelengths of 405, 532, and 660span class="thinspace"/spannm while fitting SSA with ECspan class="thinspace"/span/span class="thinspace"/spanOC or ECspan class="thinspace"/span/span class="thinspace"/span(ECspan class="thinspace"/span+span class="thinspace"/spanOC) yields a Pearson's ir/i of 0.94a??0.97 at these same wavelengths. The strong correlation coefficient at 405span class="thinspace"/spannm (ir/ia??=a??span class="thinspace"/span0.97) suggests that parameterizations based on ECspan class="thinspace"/span/span class="thinspace"/spanOC or ECspan class="thinspace"/span/span class="thinspace"/span(ECspan class="thinspace"/span+span class="thinspace"/spanOC) 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 (SSAspan class="thinspace"/spana??=a??span class="thinspace"/span0.990span class="thinspace"/span?±span class="thinspace"/span0.001 at 532 and 660span class="thinspace&q