Complex refractive index of secondary organic aerosol generated from isoprene/NO_x photooxidation in the presence and absence of SO₂

Abstract

Abstract We report the first measurements of the complex refractive index (RI) at 375, 405, 532, and 781 nm for secondary organic aerosol (SOA) generated from isoprene/NO x photooxidation. At all wavelengths studied, slightly greater real components of the RI were observed for the SOA generated in the absence of SO 2 compared with those generated in its presence. Considering the chemical properties, the differences in the oxidation state and/or ratio of particle density to molecular weight of compounds in the SOA are considered to be the main factors determining the real components. The imaginary components at ≤532 nm were found to increase with increasing initial SO 2 concentration. The highly conjugated oligomers are suggested to be plausible chromophore candidates. This study suggests that when large amounts of SOA are generated after mixing of isoprene with NO x and SO x , light absorption of these SOAs may compete with that of black carbon, especially at ultraviolet wavelengths.