Synergistic effect in absorption properties of brown carbon and elemental carbon over IGP during weak south-west monsoon

Recent ground-based measurements reveal that model-based observations are underestimating absorption properties and direct radiative forcing (DRF) due to carbonaceous species by a factor of 2–3 over South and East Asia. Thus, to better constraint these parameters associated with carbonaceous species, seasonal variability records through ground-based measurements are very essential. In this context, we report herein the absorption properties of water-extractable brown carbon (BrC), elemental carbon (EC), and BrC + EC and DRF of BrC and BrC + EC (relative to EC) over the Indo-Gangetic Plain (IGP) during a weak monsoon season [July–September 2015; influenced by El Niño and PDO (Pacific Decadal Oscillation)]. PM2.5 (particulate matter with aerodynamic diameter ≤ 2.5 μm) samples (n = 31) have been assessed from central IGP location at Kanpur. Absorption coefficient at 365 nm (b abs-365) of BrC, EC, and BrC + EC centers at 3.6, 8.1, and 11.4 Mm−1, respectively. Strong linear regression correlation (R = 0.8) of b abs-365 of BrC with sea-salt and mineral dust corrected potassium ( $${\text{K}}^{ + }_{\text{BB}}$$ K BB + ) indicates biomass burning as the predominant source of BrC over the region in this study. Synergistic effect in b abs of BrC + EC (relative to that of EC) increases conspicuously (enhanced by a factor  varying from 1.05 to 1.21) with an increase in wavelength from UV (365 nm) to visible region (660 nm). DRF (relative to EC) of BrC during monsoon season ranges from 3.9 to 23.8 (13.0 ± 5.0) %, whereas total DRF (BrC + EC) ranges from 111.0 to 148.2 (126.6 ± 10.0) %. Individual contribution of BrC and EC to total DRF has been estimated as 10 and 79.5%, respectively. The remaining contribution (10.5%) to total DRF has been attributed to synergism in absorption properties (i.e., b abs) of BrC + EC.