Alternative Approach for the in-situ Measurement of Absorption Enhancement of Atmospheric Black Carbon Due to Atmospheric Mixing

ASHISH SONI, Tarun Gupta, Indian Institute of Technology Kanpur

     Abstract Number: 264
     Working Group: Carbonaceous Aerosol

Abstract
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 due to the unavailability of sophisticated instrumentations for absorption enhancement (E_abs) measurements resulting from mixing with non-BC chemicals. Therefore, we have proposed a novel approach for the 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 proposed a more robust approach for apportioning light absorption by BC core and absorbing organics (brown carbon). The present method was applied to ground-based aerosol samples collected at two distinct Indo-Gangetic Plain (IGP) sampling stations. Amongst one is dominated by the primary emissions, while the secondary emissions dominate another station.

The significant outputs and their implications are mentioned below:

(1) The E_abs at 808 nm were observed to be approximately 1.2 at both the IGP stations, which is identical to the previously estimated E_abs using sophisticated instrumentations in the neighboring IGP station. The result depicted the utilization of the proposed simplified approach for more ground-based aerosol samples will be helpful to reduce the uncertainty of BC absorption due to atmospheric mixing in the earth system models.

(2) The complex mixture of 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.

Note: It is a part of our ongoing research, small portion was also published in ACS Earth and Space Chemistry journal (Soni and Gupta 2022, DOI: acsearthspacechem.1c00362).