Production of particulate brown carbon during atmospheric aging of residential wood-burning emissions

We investigate the optical properties of light-absorbing organic carbon (brown carbon) from domestic wood combustion as a function of simulated atmospheric aging. At shorter wavelengths (370-470 nm), light absorption by brown carbon from primary organic aerosol (POA) and secondary organic aerosol (SOA) formed during aging was around 10% and 20 %, respectively, of the total aerosol absorption (brown carbon plus black carbon). The mass absorption cross section (MAC) determined for black carbon (BC, 13.7 m(2) g(-1) at 370 nm, with geometric standard deviation GSD = 1.1) was consistent with that recommended by Bond et al. (2006). The corresponding MAC of POA (5.5 m(2) g(-1); GSD = 1.2) was higher than that of SOA (2.4 m(2) g(-1); GSD = 1.3) at 370 nm. However, SOA presents a substantial mass fraction, with a measured average SOA / POA mass ratio after aging of similar to 5 and therefore contributes significantly to the overall light absorption, highlighting the importance of wood-combustion SOA as a source of atmospheric brown carbon. The wavelength dependence of POA and SOA light absorption between 370 and 660 nm is well described with absorption Angstrom exponents of 4.6 and 5.6, respectively. UV-visible absorbance measurements of water and methanol-extracted OA were also performed, showing that the majority of the lightabsorbing OA is water insoluble even after aging.