Light-absorption Properties of Secondary Organic Aerosol Emitted from Duff Burning
MUHAMMAD ABDURRAHMAN, Rawad Saleh, University of Georgia
Abstract Number: 643
Working Group: Carbonaceous Aerosols
Abstract
Previous studies have shown that secondary organic aerosol (SOA) that forms from the photochemical aging of biomass-burning emissions is light-absorbing and falls under the umbrella of brown carbon (BrC). However, retrieving the imaginary part of the refractive index of biomass-burning SOA (kSOA) is not straightforward. kSOA has previously been inferred from optical measurements of the aged aerosol, which is comprised of SOA and primary organic aerosol (POA).
This study presents direct quantification of the light-absorption properties of SOA derived from duff burning. We performed smoldering combustion of duff in an environmental chamber and photochemically aged the emissions in an oxidation flow reactor (OFR) under two configurations: (1) sending all the emissions (particles and vapors) into the OFR and (2) first filtering out the particles and sending only the vapors into the OFR to isolate the SOA. Mass concentrations, obtained from integrating size distributions measured using scanning mobility particle sizer, of the fresh and aged aerosol showed significant organic aerosol enhancement (SOA/POA = 0.82 ± 0.18). We performed optical closure, which involved fitting Mie theory calculations to absorption coefficients measured using a photoacoustic spectrometer, to retrieve k at 422 nm of POA (kPOA,422 = 0.0184 ± 0.0007 ), aged aerosol (kaged,422 = 0.0114 ± 0.001), and SOA (kSOA,422 = 0.0094 ± 0.0009).
These results confirm that biomass-burning SOA is light-absorbing, though to a lesser extent than POA. Furthermore, inferring the light-absorption properties of SOA from the aged aerosol measurements (assuming that the aged aerosol absorption is a combination of that of the POA and SOA) yields kSOA,422 = 0.0028 ± 0.0015, more than a factor of 3 smaller than kSOA,422 retrieved from direct SOA measurements. This discrepancy suggests that photochemical aging of POA leads to reduction in absorption due to photobleaching, as has been reported by previous studies.