As part of this CAREER development plan, we comprehensively integrated the basic science necessary to understand the intensive optical properties of the morphologically-complex and multi-component aerosol types emitted from controlled (or laboratory-scale) combustion of commonly occurring biomass fuels. A total of 45 peer-reviewed publications and a patent application have resulted till date from this project. A total of 9 PhDs, 4 Masters, and 8 undergraduate students along with 3 post-doctoral scholars received training as part of this project.

First, we designed a comprehensive study to address one of the grand challenges in atmospheric climate science, which is quantitative estimation of enhancement in light absorption by atmospheric black carbon (BC) aerosol. Atmospheric BC has been identified as the second most important global warming agent after greenhouse gases. Field observations carried out worldwide have shown that greater than 75% of BC in the atmosphere exists as internal mixture with organic and non-refractory materials. Coating of nascent BC with these materials results in enhancement, by as much as three times, of its mass absorption cross-section (MAC_BC). MAC_BC is a key parameter in radiative transfer models to estimate the impact of BC aerosol on Earth?s radiative budget. Our objective was to identify and establish universal power-law expressions between MAC_BC, its enhancement, and BC mixing ratios. We showed that MAC_BC evolves with increasing internal mixing ratios in simple power-law exponents of 1/3. Remarkably, MAC_BC remains inversely proportional to wavelength at any mixing ratio. When mixing states are represented using mass-equivalent core-shell spheres, as is done in current climate models, it results in significant under prediction of MAC_BC. To demonstrate the universality of our findings, we consolidated observational BC light absorption dataset collected globally, over USA, UK and Asia, and showed that atmospheric BC accurately follows the scaling laws predicted by our model.

Second, we investigated the effects of atmospheric processing on Brown Carbon (BrC) aerosol light absorption reduction and clear-sky Direct Forcing Efficiency (DRF) efficiency. At 4.5 equivalent days of aging, the BrC Babs at 375 nm decreased by ~46%. The radiative forcing effect over snow (as well as other bright surfaces such as low-level clouds) is vital to climate models, especially since this type of surface is characteristic of regions over which boreal forest fire emissions are likely to be found. BrC aerosol contributes to positive forcing (warming) over bright terrain throughout the atmospheric aging time scales investigated in this work. However, with increased atmospheric residence time from 0 to 4.5 equivalent days, the integrated DRF efficiency decreased by approximately 27%. Corresponding decrease in DRF efficiency over ground was ~5%, from -4.04 ? 0.03 W m-2 to -4.25 ?0.05 W m‑2 for particle aging from fresh to 4.5 equivalent days.

Next, we investigated the impacts of biomass cookstoves, the largest source of fine particulate emissions and ambient particulate pollution in South Asia, over regional atmospheric warming. We found that cookstove emitted BrC contributes roughly as much as BC to total absorption cross-sections in the mid-visible wavelengths and should be classified as a ?strongly absorbing? type of organic carbon in climate models. Our findings highlighted the strong near-ultraviolet wavelength absorption characteristics of emitted BrC aerosol from household stove combustion of nationally-representative biomass fuels.

Two major technological innovations arising from this project are the design and development of i) a portable small-angle light scattering (PLS) instrument capable of in situ and real-time measurement of the aerosol scattering intensity, which is interchangeable with the phase function S11 with a multiplication factor, in the scattering angle range 0.3 to 180 degrees for a total of 1024 angles, and thus calculates the asymmetry parameter. Additionally, one can determine the fractal dimension or morphology of the particles using this device; and ii) the Python Mie Scattering package, or PyMieScatt, which is a complete tool for both forward and inverse Mie theory calculations. In addition to over twenty functions for Mie theory calculations, it contains an implementation of a highly visual method for solving the inverse Mie problem for the complex refractive index, given known or assumed size parameter and optical measurements. It is open-source, fully documented, and now being used by researchers from over 50 Institutes world wide.

Finally, on the outreach and broader impacts section of this project, we co-developed middle school course modules on weather, climate, and energy concepts and applications. We participated in teacher professional development from local school districts around these modules. Our modules were tested at two middle school districts in Saint Louis. Approximately 700 middle school students enrolled in four middle schools were administered the modules as part of their earth sciences curriculum. Both formative and summative evaluations suggested overwhelmingly positive response. Pre- and post-tests results showed the students showed a better grasp of key scientific concepts associated with the modules.

Last Modified: 11/16/2021
Modified by: Rajan K Chakrabarty