AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
MOUDI Size-Resolved Measurements of Elemental and Brown Carbon and Their Contributions to Light Absorption Based on Mie Theory Calculations
JIUMENG LIU, Michael Bergin, Rodney Weber, Georgia Institute of Technology
Abstract Number: 216 Working Group: Urban Aerosols
Abstract Black carbon is traditionally regarded as the primary light absorber at visible wavelengths, while brown carbon, defined as light absorbing organic matter with a strong wavelength-dependent absorption in UV and UV-Visible ranges, is thought to mainly impact atmospheric photochemistry. Size-resolved atmospheric aerosol samples were collected using Micro-Orifice Uniform Deposition Impactors (MOUDI) at a number of sites in Georgia. The samples were analyzed for water-soluble organic carbon (WSOC), organic carbon (OC) and elemental carbon (EC), as well as bulk light absorption spectra of both water and methanol extracts. Concentrations of OC and WSOC both exhibited bimodal distributions, with a small coarse mode (3.2-5.6 micro-meter) peak and a large fine mode (0.56-1 micro-meter) peak, while EC was found to be enriched in particles of smaller size (0.18-0.56 micro-meter). Bulk light absorption of the methanol extract at 365nm wavelength had a similar size distribution to that of water-soluble brown carbon with peaks in the fine mode between 0.32 to 1 micro-meter, but the methanol extract was generally a factor of two more light absorbing. Methanol extracts had generally smaller absorption Angstrom exponents, with a value of 5.1±0.3, compared with 6.7±0.6 for water extracts averaged over all particle sizes, indicating more light absorption to higher wavelengths than the water extract. The bulk light absorption measurements at a number of wavelengths were used to determine the refractive index of OC, and simple Mie theory was applied to predict the light absorption of brown carbon. This method was validated by an EC light absorption closure between MOUDI and online absorption measurements that were made by MAAP and multi-wavelength Aethalometer. Comparisons of predicted light absorption by brown carbon to total light absorption show that water and methanol-soluble brown carbon can be a significant fraction at 365nm.