On the Use of Brown Carbon Spectra as a Tool to Understand Their Broader Composition and Characteristics: A Case Study from Crop-residue Burning Samples.

ABSTRACT

This study proposes a novel approach to the use of brown carbon (BrC) absorption spectra as a tool to understand their broader composition and characteristics. The ratios of absorption coefficient (b abs) spectra over a wavelength range (310-600 nm) for water-soluble and methanol-soluble BrC were used to quantify the relative contribution of water-soluble and water-insoluble chromophores to total BrC. The same ratios for the samples collected during the day versus night were used to assess the diurnal variability in BrC composition and concentrations. Ratios of b abs at different wavelengths with respect to that at 365 nm were used to understand whether BrC is predominantly composed of one type of chromophore, that is, humic-like substances, or different chromophores (e.g., nitroaromatic compounds) with the understanding that different chromophores absorb predominantly at different wavelengths. As a case study, day/night pairs of PM2.5 samples collected from Patiala (30.33°N, 76.4°E) during paddy residue burning were used, and results are discussed. A majority of BrC from paddy residue burning were found to be water-insoluble, and the fraction of water-soluble BrC to total BrC showed a decreasing trend with increasing wavelength. During the burning period, night-time water-soluble nitrogenous organic species were found to be more absorbing than daytime water-soluble nitrogenous species. The proposed method will be very useful for BrC studies over the globe.