Ultraviolet and visible complex refractive indices of secondary organic material produced by photooxidation of the aromatic compounds toluene and m-xylene

Secondary organic material (SOM) produced by the oxidation of anthropogenic volatile organic compounds can be light-absorbing (i.e., brown carbon). Spectral data of the optical properties, however, are scarce. The present study obtained the continuous spectra of the real and imaginary refractive indices (m = n-i k) in the ultraviolet (UV)-to-visible region using spectroscopic ellipsometry for n and UV–visible spectrometry for k. Several different types of SOM were produced in an oxidation flow reactor by photooxidation of toluene and m-xylene for variable concentrations of nitrogen oxides (NOx). The results show that the k values of the anthropogenically derived material were at least 10 times greater than those of the biogenically derived material. The presence of NOx was associated with the production of organonitrogen compounds, such as nitro-aromatics and organonitrates, which enhanced light absorption. Compared with the SOM derived from m-xylene, the toluene-derived SOM had larger k values, as well as a greater NOx-induced enhancement, suggesting different brown-carbon-forming potentials of different aromatic precursor compounds. The results imply that anthropogenic SOM produced around urban environments can have an important influence on ultraviolet irradiance, which might consequently influence photochemical cycles of urban pollution.