New Analytical Paradigm\nto Determine Concentration\nof Brown Carbon and Its Sample-by-Sample Mass Absorption Efficiency

Brown carbon (BrC) has a substantial direct radiative\neffect, but\ncurrent estimates of its impact on radiative balance are highly uncertain\ndue to a lack of measurements of its light-absorbing properties, such\nas mass absorption efficiency (MAE). Here, we present a new analytical\nparadigm based on a Bayesian inference (BI) model that takes multiwavelength\naethalometer measurements and total carbon data to resolve the concentrations\nof black carbon and BrC, and MAEs of BrC on a sample-by-sample basis.\nHourly MAEs, unattainable in previous studies, can now be calculated,\nenabling the first-time observation of the darkening-bleaching dynamics\nof BrC in response to photochemical transformation. We demonstrate\nthe application of this BI model to analyze measurements collected\nover one year (2021–2022) in Hong Kong. Diel variations in\nMAE<sub>370 nm</sub> of BrC reveal a darkening-to-bleaching transition\noccurring between 8 and 10 O’clock when the solar irradiance\nranges from 30 to 400 W m<sup>–2</sup>. Furthermore, we consistently\nobserve an increase in MAE<sub>370 nm</sub> of BrC with nitrogen\noxide concentrations, suggesting the enhanced formation of nitrogenous\norganics. This BI model-based data analysis would bring forth a breakthrough\nin amassing observation data of BrC and its MAEs in diverse ambient\nenvironments and with high time resolution.