Abstract
Abstract. Light absorbing organic carbon, often called brown carbon, has the potential to significantly contribute to the visible light-absorption budget, particularly at shorter wavelengths. Currently, the relative contributions of particulate brown carbon to light absorption, as well as the sources of brown carbon, are poorly understood. With this in mind size-resolved direct measurements of brown carbon were made at both urban (Atlanta), and rural (Yorkville) sites in Georgia. Measurements in Atlanta were made at both a representative urban site and a road-side site adjacent to a main highway. Fine particle absorption was measured with a multi-angle absorption photometer (MAAP) and seven-wavelength Aethalometer, and brown carbon absorption was estimated based on Mie calculations using direct size-resolved measurements of chromophores in solvents. Size-resolved samples were collected using a cascade impactor and analyzed for water-soluble organic carbon (WSOC), organic and elemental carbon (OC and EC), and solution light-absorption spectra of water and methanol extracts. Methanol extracts were more light-absorbing than water extracts for all size ranges and wavelengths. Absorption refractive indices of the organic extracts were calculated from solution measurements for a range of wavelengths and used with Mie theory to predict the light absorption by fine particles comprised of these components, under the assumption that brown carbon and other aerosol components were externally mixed. For all three sites, chromophores were predominately in the accumulation mode with an aerodynamic mean diameter of 0.5 μm, an optically effective size range resulting in predicted particle light absorption being a factor of 2 higher than bulk solution absorption. Mie-predicted brown carbon absorption at 350 nm contributed a significant fraction (20 to 40%) relative to total light absorption, with the highest contributions at the rural site where organic to elemental carbon ratios were highest. Brown carbon absorption, however, was highest by the roadside site due to vehicle emissions. The direct size-resolved measurement of brown carbon in solution definitively shows that it is present and optically important in the near-UV range in both a rural and urban environment during the summer when biomass burning emissions are low. These results allow estimates of brown carbon aerosol absorption from direct measurements of chromophores in aerosol extracts.
Highlights
Light absorbing components of atmospheric aerosols influence the planetary radiation budget and climate by direct absorption of radiation and through secondary routes that include influences on cloud optics and dynamics, and snow or ice albedo (Bond et al, 2013)
By performing size-resolved measurements of brown carbon and using the bulk solution lightabsorption measurements to estimate the absorbing refractive index, we investigate the optical importance of water and methanol-soluble brown carbon absorption predicted with the Mie theory and compare it to measurements of Black carbon (BC) made with standard optical instruments
Measurements were not made simultaneously at the different sites, the comparisons between sites show differences expected for primary and secondary aerosol components. Both elemental carbon (EC) and Organic carbon (OC) concentrations were highest at the RoadSide site (RS) site, compared to Jefferson Street (JST) and YRK, which tended to be closer in magnitude
Summary
Light absorbing components of atmospheric aerosols influence the planetary radiation budget and climate by direct absorption of radiation and through secondary routes that include influences on cloud optics and dynamics, and snow or ice albedo (Bond et al, 2013). Uncertainties associated with light absorbing aerosol forcing contribute to uncertainty in climate change estimates (IPCC, 2007). Black carbon (BC), largely comprised of elemental carbon (EC), is the strongest of the light-absorbing aerosol components (Bond, 2001; Moosmuller et al, 2009, and references therein). Other absorbing components include mineral dust (e.g., hematite). Preferentially absorb radiation in the lower visible to nearUV wavelength range due to a wavelength-dependent imaginary component of the refractive index. As a group, lightabsorbing organic aerosols have been referred to as brown carbon (Andreae and Gelencser, 2006)
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