Sources and characteristics of water and methanol soluble brown carbon over a megacity: Insights from EEM-PARAFAC...

The fluorophoric composition of brown carbon (BrC) remains limited from where significant emissions of organic aerosols are reported such as megacities in the Indo-Gangetic Plain (IGP). The present work explores diurnal variation in the bulk absorption properties and fluorophoric composition of water- and methanol-soluble BrC in fine particulates (PM_2.5) over a megacity (Delhi) during the post-monsoon and winter seasons. To deconvolve the fluorophoric components of BrC, the excitation-emission matrix fluorescence spectroscopy and parallel factor analysis (EEM-PARAFAC) approach was used. Results revealed that the light absorption coefficient (b_abs) of BrC_methanol at 365 nm is about two times higher than that for BrC_water, which indicates a significant presence of insoluble BrC in ambient PM_2.5. It is also reflected in the magnitude of the BrC radiative forcing relative to elemental carbon (RRF_300-2500nm), which was 20% for BrC_water and around 30% for BrC_methanol. It further suggests that a substantial underestimation of the BrC forcing occurs if only water-soluble BrC is considered as a surrogate for total BrC. Further, through EEM-PARAFAC analysis, four fluorophoric components were identified: low-oxidized humic-like substances (LO-HULIS), highly oxidized HULIS (HO-HULIS), protein-like substances (PRLIS), and a mixed component. The multiple linear regression analysis shows that HO-HULIS accounted for more than 50% of the absorption in both fractions, with a greater contribution in the water-soluble fraction. Findings show that primary combustion sources are more dominant than secondary processing towards BrC abundance during the study period. These insights are crucial for refining regional estimates of BrC radiative forcing and for developing effective mitigation strategies in the IGP region. Such studies contribute to a growing body of work aimed at bridging the gap between the chemical and optical properties of atmospheric BrC aerosols, while introducing advanced spectroscopic tools to a previously underexplored regional context.