Aqueous‐Phase Secondary Processes and Meteorological Change Promote the Brown Carbon Formation and Transformation During Haze Events

Abstract The evolution and the impacts of meteorological conditions on brown carbon (BrC) absorption are not understood, which hinders the assessment of BrC radiative forcing. To address this issue, 1‐hr time‐resolved PM2.5 samples collected during three haze events in the North China Plain prior to the COVID‐19 pandemic were used to measure the optical properties of BrC. By coupling excitation‐emission matrix spectroscopy, chemical tracer analysis with multiple model analysis including positive matrix factorization (PMF) and a deweather‐random forest model, we found that a higher proportion of highly oxidized chromophoric components was present in water‐soluble BrC than in methanol‐soluble BrC, indicating the conversion of low‐oxidized water‐insoluble BrC into highly oxidized water‐soluble BrC during the day. The results of the PMF and the deweather‐random forest model showed that aqueous secondary processes were the major contributor to the BrC absorption (68% ± 38%), and the changes in meteorological conditions such as relative humidity (RH) could significantly lead to the changes in the light‐absorbing capacity of BrC, especially the enhancement for water‐soluble BrC and bleaching for methanol‐soluble BrC during the noon and afternoon. We further found that the BrC absorption capacity increased as RH increases to a maximum of ∼65%, and then decreased when RH >65%, highlighting the important role of RH in the generation of water‐soluble BrC.