AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Light Absorption Properties of Brown Carbon from Fresh and Photo-chemically Aged Biomass Burning Emissions
RAWAD SALEH, Christopher Hennigan, Gavin McMeeking, Wayne Chuang, Hugh Coe, Neil Donahue, Allen Robinson, Carnegie Mellon University
Abstract Number: 421 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract To date, climate forcing calculations treat black carbon (BC) and dust as the only particulate light absorbers. Recent studies have shown that some organic aerosols, referred to as brown carbon (BrC), also absorb light. BrC has been identified in biomass burning emissions, however, its light absorption properties are poorly quantified. We determined imaginary refractive indices (absorptivity) of organics in fresh and photo-chemically aged biomass burning emissions from both household wood burning (oak) and fuels commonly consumed in wildland and prescribed fires in the US (pocosin pine and gallberry). A core (BC) – shell (organic) light absorption model based on Mie theory was used to determine the imaginary refractive index of the organic shell that best fits absorption coefficients measured using an aethalometer. The organic carbon in both primary emissions and aged (secondary) aerosol was brown, with absorption efficiencies comparable to BC at short visible wavelengths. This is the first direct demonstration that secondary organic aerosol produced by photo-oxidation of biomass burning emissions is brown. The values of imaginary refractive indices at 550 nm and their wavelength dependence were: 0.45 and lambda$^(-2.2) for primary oak, 0.3 and lambda$^(-1.55) for secondary oak, 0.1 and lambda$^(-3.5) for primary pocosin pine, 0.06 and lambda$^^(-2.9) for secondary pocosin pine, and 0.04 and lambda$^(-2.7) for primary gallberry. The common current assumption that biomass burning emissions consist of BC cores and non-absorbing (instead of brown) organic coatings may result in a considerable underestimation of their climate forcing.