Internally mixed black carbon in the Indo-Gangetic Plain and its effect on absorption enhancement

We present the systematic analysis of individual black carbon (BC) mixing state and its impact on radiative forcing from an urban Indian city, Kanpur, located in Indo-Gangetic Plain (IGP). Simultaneous measurements using Single Particle Soot Photometer (SP2), Photo-Acoustic Soot Spectrometer (PASS-3) and High-Resolution Time-of-Flight Aerosol Mass Spectrometer (AMS) were conducted from 8 January 2015 to 28 February 2015 at Kanpur. BC mass and number concentrations varied between 0.7 and 17 μg/m³ and 277–5866 #/cm³ with a mean of 4.06 μg/m³ and 1314 #/cm³, respectively. The diurnal variation of BC mass concentration showed a traffic hour peak during both the morning and late night. The mean fraction of “thickly coated BC” particles (fTC_BC) was found to be 61.6%, indicating that a large fraction of BC particles was internally mixed. The fTC_BC increased after sunrise with a peak at about noontime, indicating that the formation of secondary organic aerosol under active photochemistry can enhance organic coating on a core of black carbon. High-resolution positive matrix factorization (HR-PMF) factors showed distinct characteristics with fTC_BC. While primary organic aerosols like cooking organic aerosols (COA) and biomass burning organic aerosols (BBOA) were negatively correlated with fTC_BC (r = − 0.78 and − 0.51, respectively), aged low volatile oxygenated organic aerosol (LVOOA) was forming a coating over BC (r = 0.6). Similar positive correlation of fTC_BC with inorganic species like ammonium (r = 0.58) and nitrate (r = 0.47) further suggested that BC appears to be largely coated with LVOOA, ammonium, and nitrate. A positive correlation between the fTC_BC and the mass absorption cross-section at 781 nm (MAC₇₈₁) was also observed (r = 0.58). Our results suggest that the observed fTC_BC could amplify the MAC₇₈₁ approximately by a factor of 1.8, which may catalyze the positive radiative forcing in the IGP.