10th International Aerosol Conference
September 2 - September 7, 2018
America's Center Convention Complex
St. Louis, Missouri, USA

Abstract View


Heterogeneous Oxidation of Brown Carbon Aerosol Diminishes Light Absorption

BENJAMIN SUMLIN, Apoorva Pandey, Michael Walker, Robert Pattison, Brent Williams, Rajan K. Chakrabarty, Washington University in St. Louis

     Abstract Number: 401
     Working Group: Oxidation Flow Reactor: Development, Characterization, and Application to Aerosols

Abstract
Brown carbon (BrC) aerosols are a class of light-absorbing organic aerosols, predominantly emitted from smoldering biomass combustion [1, 2]. BrC aerosols have been shown to absorb near-UV and short visible solar wavelengths which significantly impacts Earth’s radiative energy balance. The potential impact of atmospheric processing on the absorptivity of such particles is understudied, and satellite retrieval algorithms and climate models typically treat the optical properties of organic aerosols as constant throughout their atmospheric lifecycle.

Using a Potential Aerosol Mass (PAM) oxidation flow reactor [3], we investigated the effects of photochemical oxidation on the 375-532 nm scattering and absorption characteristics of primary BrC aerosols emitted from smoldering combustion of Alaskan peat, a common fuel for sustained biomass smoldering. Parallel plate activated carbon denuders removed gas phase combustion products, minimizing secondary aerosol formation in the PAM, and a PM1 cyclone removed particles larger than 1μm. Simultaneous size distribution measurements by a Scanning Mobility Particle Sizer along with a novel Lorenz-Mie theory inversion algorithm [4] facilitated the retrieval of the complex refractive index (m=n+ik), and an Aerosol Mass Spectrometer (AMS) was used to obtain the hydrogen-carbon (H:C) and oxygen-carbon (O:C) molar ratios, quantities commonly used to parameterize bulk aerosol composition [5, 6].

Upon oxidation up to approximately 4.5 equivalent atmospheric days, the 375 nm imaginary refractive index and absorption coefficients of BrC particles were found to decrease by ~36% and ~46%, respectively, and the single scattering albedo increased from 0.852±0.005 to 0.898±0.005. From the AMS measurements, we observed an increase in O:C from 0.34±0.01 to 0.40±0.01, suggesting that a transition from functionalization to fragmentation reactions with increasing photooxidation may contribute to the changes in optical behavior. We conclude with simple radiative forcing efficiency calculations that show the effects of atmospheric photooxidation on atmospheric warming attributed to BrC aerosols.

1. Chakrabarty, R.K., et al., Brown carbon aerosols from burning of boreal peatlands: microphysical properties, emission factors, and implications for direct radiative forcing. Atmospheric Chemistry and Physics, 2016. 16(5): p. 3033-3040.
2. Chakrabarty, R.K., et al., Brown carbon in tar balls from smoldering biomass combustion. Atmospheric Chemistry and Physics, 2010. 10(13): p. 6363-6370.
3. Kang, E., et al., Introducing the concept of Potential Aerosol Mass (PAM). Atmospheric Chemistry and Physics, 2007. 7: p. 5727-5744.
4. Sumlin, B.J., W.R. Heinson, and R.K. Chakrabarty, Retrieving the aerosol complex refractive index using PyMieScatt: A Mie computational package with visualization capabilities. Journal of Quantitative Spectroscopy and Radiative Transfer, 2018. 205: p. 127-134.
5. Williams, B.J., et al., The First Combined Thermal Desorption Aerosol Gas Chromatograph—Aerosol Mass Spectrometer (TAG-AMS). Aerosol Science and Technology, 2014. 48(4): p. 358-370.
6. Kroll, J.H., et al., Carbon oxidation state as a metric for describing the chemistry of atmospheric organic aerosol. Nat Chem, 2011. 3(2): p. 133-139.