Spatiotemporal Variability and Radiative Forcing of Secondary Brown Carbon over India

We used multiyear (2016–2020) measurements of equivalent black carbon (EBC) from the India Meteorological Department’s (IMD) aethalometer network (14 Indian stations) to estimate brown carbon (BrC) and secondary brown carbon (Sec_BrC) absorptions. The composite analysis shows BrC and Sec_BrC are greater in inland megacities (73.24 ± 30.25 Mm–1) compared to coastal megacities (30.40 ± 19.68 Mm–1) influenced by higher biomass burning. The relative contribution of Sec_BrC to total BrC was greater in coastal megacities (33.28%) compared to that of inland (25.68%), indicating an enhanced ratio of secondary to primary BrC, especially over coastal stations. Further analysis illustrates a strong positive association of BrC and Sec_BrC absorption with relative humidity and atmospheric stability, especially during nighttime. Direct radiative forcing (DRF) calculations show that BrC-induced warming of atmosphere values are 7.84 and 5.21 W/m2 over inland and coastal stations, respectively. These values are ∼25–50% of BC-induced warming of the atmosphere. Further, Sec_BrC accounts for ∼50 and ∼40% of BrC-induced warming over inland and coastal stations. As such, BrC-induced cooling at the top of the atmosphere is −3.21 and −2.56 W/m2, respectively, counteracting nearly half of BC’s warming effects. These findings emphasize the importance of improving BrC representation in climate models and implementing effective emission control strategies to mitigate its regional climate and air quality impacts.