Date of Award
Doctor of Philosophy (PhD)
Atmospheric aerosols directly affect the Earth’s radiative budget by absorbing and scattering solar radiation. Carbonaceous aerosols constitute 20-90% of the global aerosol mass burden and are recognized by the Intergovernmental Panel on Climate Change as important drivers of direct radiative forcing (DRF). Aerosol radiative impacts have been implicated in regional atmospheric warming in South Asia: changing Indian monsoon patterns, and accelerating melting of the Himalayan glaciers. There are systematic global discrepancies between estimates of aerosol absorption optical depths derived from observations and those from climate models. Over South Asia, models predict six times lower aerosol absorption than ground-based observations, leading to a low bias in modeled DRF. To resolve this bias, there is a need to (1) account for relevant emission source types, and associated emission rates, and (2) constrain aerosol optical properties: mass absorption cross-sections (MAC), single scattering albedo (SSA) and scattering directionality parameters (asymmetry parameter or upscatter fraction). To that end, two broad classes of light absorbing carbonaceous aerosols need to be separately dealt with: black carbon (BC) and brown carbon (BrC). BC is known to strongly absorb visible solar radiation and its optical properties have been characterized using both direct measurements and optical models. BC aerosols exhibit aggregate morphologies, with fractal dimensions of 1.8 and 2.6 for fresh and aged particles, respectively. As a simplification, current climate models usually approximate BC aerosols as volume-equivalent spheres and use analytical solutions (known as the Lorenz-Mie theory) of Maxwell’s equations for estimating their optical properties. Recent modeling studies employed the numerically-exact superposition transition-matrix method to compute optical cross-sections of fractal aggregates of varying sizes and fractal dimensions. These studies highlight the effect of morphology on BC optical behavior soot but their findings (expressed in terms of fractal properties) cannot be used directly by aerosol experimentalists and climate modelers. Exploiting the theoretical bases of aerosol sizing techniques, I determined empirical relationships between numerically-exact optical properties of fractal BC particles and their equivalent diameters, that can be measured by common aerosol instrumentation. In a related study, I reported improved relationships between scattering directionality parameters of BC aggregates, and compared them with the canonical equations which did not allow for treatment of particle morphology. The second branch of my thesis is concerned with light absorbing organic carbon (OC). OC is conventionally modeled as purely light scattering in radiative transfer calculations. However, this approach has been challenged by mounting observational evidence of a class of OC aerosols exhibiting strong absorption in the near ultra-violet wavelengths and little to no absorption in the near-infrared region. This wavelength dependence of absorption leads to a brownish appearance, hence the name brown carbon. Absorption properties of BrC depend on fuel properties and combustion phase (flaming/smoldering): their observed values are source-specific, spanning an order of magnitude in literature. The focus of this part of my research is on the largest source of OC emissions in South Asia: household biomass cookstoves. I conducted a field study in a household in central India in December 2015 and developed a dataset of emission rates for commonly used biomass fuels from various regions of India, which showed that (1) laboratory cookstove tests underestimated particulate mass emission factors by 2-4 times and (2) cookstove aerosol emissions were dominated by thermally stable OC, which is linked with stronger light absorption than volatile OC. To constrain the MAC values for cookstove OC emissions, I performed optical (transmission and reflection) measurements on filter samples of aerosols collected during the field study. Filter optical measurements are associated with artifacts arising from the interaction of the filter medium with light. Through a laboratory study of a wide variety of combustion aerosols, I developed correction schemes for estimating aerosol-phase light absorption from filter-based measurements. This aided the estimation of absorption characteristics of cookstove particulate emissions and their OC components. We found that light absorbing OC contributes roughly as much as BC to total absorption cross-sections of cookstove emissions at 550 nm wavelength, enhancing their direct forcing efficiency. We proposed values for key absorption characteristics of cookstove OC emissions for use within climate impact assessment and mitigation efforts.
Rajan K. Chakrabarty
Pratim B. Biswas, Jay R. Turner, Raymond E. Arvidson, Brent J. Williams,