AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Brown Carbon in Fresh and Aged Biomass Burning Emissions
RAWAD SALEH, Ellis Shipley Robinson, Daniel S. Tkacik, Adam Ahern, Shang Liu, Allison Aiken, Albert A. Presto, Ryan Sullivan, Manvendra Dubey, Neil Donahue, Allen Robinson, Carnegie Mellon University
Abstract Number: 695 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract To date, most climate forcing calculations treat black carbon (BC) and dust as the only particulate light absorbers. Numerous studies have shown that some organic aerosols (OA), referred to as brown carbon (BrC), also absorb light. BrC has been identified in biomass burning emissions, however, its light absorption properties are poorly constrained. Literature values of the imaginary part of the refractive indices of biomass burning OA (k$_(OA)) span two orders of magnitude. This variability, attributed to differences in fuel type and burning conditions, complicates the representation of biomass burning BrC in climate models. Proper accounting for BrC absorption in climate forcing calculations is of great importance, and can enhance the models’ performance and agreement with observations.
Here, we show that k$_(OA) of biomass burning OA, both fresh and aged, can be estimated as a function of a single property of the emissions, namely the BC-to-OA ratio. Specifically, kOA at wavelength (lambda) of 550 nm increases linearly with the BC-to-OA ratio, while the spectral-dependence, w, where k$_(OA) = k$_(OA,550nm)*(550/lambda)$^w, is inversely proportional to the BC-to-OA ratio. These correlations were determined by examining emissions from small scale laboratory burns of six fuels (black spruce, pendorosa pine, hay, rice straw, saw grass, and wire grass). The BC-to-OA ratios in the experiments were between 0.01 and 0.2. Aging of the emissions (photo-oxidation or dark ozonolysis) was performed in a smog chamber. k$_(OA) values were retrieved by performing optical closure, which combines Mie theory calculations with measurements of light absorption, and total and BC size distributions.