Light-absorbing properties of polar- and non-polar brown carbon fractions of aerosols in Delhi

Abstract

In the present study, PM2.5 samples (n = 104) were collected during February 2020–March 2021 on diurnal basis in Delhi (a site of Indo-Gangetic Plain (IGP)), with a view to investigate seasonal and diurnal patterns of ambient brown carbon (BrC) with consideration of their potential sources. Given that the polarity of ambient aerosol influences their chemical and physical processing in the atmosphere, we compared the light absorption properties of brown carbon (BrC) in PM2.5 samples extracted with three different solvents i.e., water, methanol, and n-hexane. Methanol extracts showed much stronger light-absorbing ability followed by water and hexane extracts in the wavelength range of 300–600 nm. Peak values of mass absorption coefficient (babs-BrC-365nm) were observed during post-monsoon season in both polar- (water-and methanol-soluble) and non-polar (n-hexane soluble) fractions of BrC, mainly due to the influence of biomass-burning (BB) aerosols. This point is further supported by air mass back trajectories which are well correlated with fire-count data, and marker species such as EC, OC and non-sea-salt K+. Comparison of babs-BrC-365nm, mass absorption efficiency (MAE365nm), absorption Ångstorm exponent (AÅE) and simple forcing efficiency (SFE300–700) from this study with that in published datasets reveal the significant contribution of biomass-burning activities to BrC absorption and abundance in the ambient atmosphere. Furthermore, the optical properties of HULIS (humic-like substances) (n = 40, during day- and nighttime) in post-monsoon (October – November 2020) and winter aerosols (January – February 2021) were also characterised. Based on UV–Vis analysis, similar average babs-365nm of HULIS was observed for daytime and nighttime samples i.e., 9.96 ± 3.8 and 9.89 ± 3.3 Mm−1, respectively in post-monsoon samples. Opposingly in winter period, significantly high daytime value of babs-365nm of HULIS (7.24 ± 2.7 M m−1) compared to that of nighttime (4.45 ± 2.8 M m−1) was found. These results along with indicative E2/E3 (ratio of absorbance at 250 nm and 365 nm, respectively) and AÅE values in HULIS fraction suggest a more significant contribution from regional crop-residue burning emission in the formation of HULIS in post-monsoon. While in winter period local emissions and subsequent secondary formation of HULIS are important. To the best of our knowledge, this study is the first attempt to investigate light absorption properties of different polarity BrC extracts (methanol, water, n-hexane, HULIS) in aerosol samples collected throughout the year at a representative site in Indo-Gangetic Plain (IGP).