The influence of solvent and pH on determination of the light absorption properties of water-soluble brown carbon

- Effects of concentration, pH, and solvent on WSOC optical properties were studied.
- pH and solvent can affect the optical properties of water-soluble brown carbon.
- pH of HULIS should be adjusted for determining contribution to light absorption.
- A particle:water ratio of 0.25 mg/mL is proposed for the extraction of WSOC.

Brown carbon (BrC) is a class of unidentified organic compounds that efficiently absorb solar radiation in the ultraviolet (UV) wavelengths, and its effects on climate are poorly understood. Measurement of the light absorption properties of BrC in liquid extracts is a commonly used BrC analytical method, but the optical characteristics of water-soluble BrC may be affected by pH and solvent. In this work, we investigated the effects of concentration, pH, and solvent on water-soluble BrC from ambient aerosols, biomass burning, diesel exhaust, and a humic substance standard. The results showed that pH can affect the light absorption properties of water-soluble BrC, whereas concentration had little effect, except low concentrations dissolved in methanol. Therefore, the pH of humic-like substances (HULIS) should be adjusted to the same value as water-soluble carbon (WSOC) for calculating the light-absorption contribution of HULIS to WSOC. The light absorptivity of water-soluble BrC dissolved in methanol was higher than that in water. Considering the pH and concentration effects, extraction of WSOC with a particle: water ratio of 0.25 mg/mL is proposed as well as to get a reference pH for light absorption analysis.