Alternative Approach for the In Situ Measurement of Absorption Enhancement of Atmospheric Black Carbon Due to Atmospheric Mixing

The significant uncertainty associated with black carbon (BC) radiative forcing estimation is mainly due to discrepancies related to its mixing state. The in situ measurement-based understanding of absorption properties is limited to only a few locations worldwide, primarily as a result of the unavailability of sophisticated instrumentations for absorption enhancement (Eabs) measurements resulting from mixing with non-BC chemicals. Therefore, we have proposed an alternative approach for a more robust in situ measurement of absorption enhancement using a thermal-optical carbon analyzer. In the present study, the absorption spectra during different stages of thermal-optical carbon analysis were used to estimate the absorption coefficients of mixed and pure BC aerosols. Moreover, we have also explored the possibility of apportioning light absorption by the BC core and absorbing organics (brown carbon). The present method was applied on a few ground-based aerosol samples collected at two distinct Indo-Gangetic Plain (IGP) sampling stations. Eabs at 808 nm was observed to be approximately 1.2 at both of the sampling sites. Interestingly, the absorbing brown carbon chromophores showed a wide range of absorption in the ultraviolet to near-infrared wavelengths with minimum absorption at 635 nm. Thus, the present study suggests that the absorption of organics in near-infrared wavelengths cannot be neglected.

Automated summary

This study proposes an alternative approach for the in situ measurement of absorption enhancement using a thermal-optical carbon analyzer to address the uncertainties associated with black carbon radiative forcing estimation. By analyzing the absorption spectra and coefficients, the absorption properties of mixed and pure black carbon aerosols are estimated, and the role of absorbing organics (brown carbon) in light absorption is explored. The findings suggest that absorption enhancement is approximately 1.2 at the sampling sites in the Indo-Gangetic Plain, and organic absorption in near-infrared wavelengths should not be neglected.