Spectroscopic comparison of water- and methanol-soluble brown carbon particulate matter

Abstract

It is now recognized that some organic components of ambient aerosols absorb light with a spectrum distinct from that of other absorbers such as black carbon and mineral components. The most common method for isolating this light-absorbing organic fraction, or “brown carbon,” is to collect particulate matter on filters and extract in a solvent, usually water or methanol. Here, we compare the absorption spectra of water-soluble (WS) and methanol-soluble (MS) extracts from ambient samples collected in Athens, Georgia. We find that despite syringe filtering the MS extracts, extinction by suspended particles is evident in the spectra leading to an overestimation of absorption by a factor of two on average. No such particle extinction is evident in the WS extracts. We demonstrate that it is possible to subtract the extinction contribution in the MS extracts by fitting the spectrum to the sum of two power-law functions, one describing the absorption spectrum and the other describing the extinction spectrum. With extinction thus removed, we find that integrated absorption (300–800 nm) by the MS brown carbon extract is highly correlated with the WS extract and is on average 1.55× larger. The wavelength dependence of the WS and MS spectra are also correlated and very similar with average absorption Ångström exponents of 6.1 (±0.7) and 6.7 (±1.1), respectively. This study demonstrates that for the samples collected: (1) brown carbon absorption can be overestimated if scattering in MS spectra is not accounted for, (2) there is no spectral evidence that the WS and MS chromophores are different, and (3) it may be possible to use WS spectra to represent total brown carbon absorption using a simple scaling factor. These findings may differ for other types of aerosol samples and analytical methods. © 2017 American Association for Aerosol Research.