Estimation of light absorption by secondary brown carbon during agricultural residues burning

Measurements of absorption optical properties (σ ap,λ , AAE) and PM 10 concentrations were conducted near the rice fields of L'Albufera Natural Park (Eastern Spain) with the aim of assessing light absorption by secondary BrC generated from the burning of rice straw residues. Sampling was performed in autumn 2023. The measurement period was divided into three sub-periods (P 1 , P 2 and P 3 ) depending on the number of burning spots and the prevailing weather conditions. P 2 was the period most affected by biomass burning (BB) events. During this period, hourly values of σ ap,λ and AAE were above 100 Mm −1 and 2.0 and BrC absorption was higher than BC absorption in the UV spectral range. Secondary BrC concentrations were estimated using the EC-tracer method, and the R-squared (MRS) and 25 % percentile (P25) approaches were applied to the determination of (σ ap,λ /BC 880 ) pri . Secondary BrC concentrations calculated by the P25 method were significantly higher than those obtained by the MRS approach for P 2 , which lead to σ BrC-SEC values four times higher when the P25 method was used. The differences in the results obtained by both procedures were attributed to the formation of secondary BrC generated from the rapid oxidation of semi-volatile organic aerosols emitted by BB. The contribution of this secondary BrC to light absorption by BrC at 370 nm was 47 % for P 2 .