Date of Award

Winter 2-2018

Embargo Period

2-14-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Advisor(s)

Allen Robinson

Abstract

Anthropogenic gas- and particle-phase emissions affect the climate by absorbing and scattering radiation, and have been linked to adverse health effects. Black carbon (BC), a by-product of incomplete combustion, is the most potent light-absorbing component of atmospheric aerosols, with a top-of-the atmosphere direct radiative forcing estimated to be only second to CO2. However, there is a large uncertainty associated with BC’s total direct and indirect radiative forcings due to uncertain source emissions and optical properties and complex interactions with clouds. In this dissertation we investigate the direct radiative impact of two of the most important sources of BC particles: biofuel combustion and vehicles. Together these sources contribute around 40% of the global atmospheric BC burden. Recently, both of these energy sources are undergoing rapid technology changes, and the climate impacts from the emissions of these newly adopted technologies remain uncertain. We also investigate the role of atmospheric processing on the optical properties and growth rates of particles. This dissertation first assesses the climate impacts of aerosol emissions of two rapidly emerging technologies: improved cookstoves and gasoline direct injection (GDI) vehicles. We performed extensive measurements of gas- and particle emissions and optical properties of emissions from both these sources. Our data suggests that improved rocket cookstoves have, on average, a factor of two lower particulate matter (PM) emissions compared to traditional cookstoves but only a 4% climate benefits associated with their emissions. In contrast, we estimated a 30% climate benefit from switching traditional cookstoves to gasifier ones. Of all the stoves tested, charcoal stoves had the lowest emissions and climate impacts. Our data suggests the widespread deployment of improved cookstoves to replace existing, inefficient, traditional cookstoves will likely result in health and climate co-benefits. Similarly, we estimated that the rapid adoption of GDI vehicles to replace existing port fuel injection (PFI) vehicles will likely result in reduced warming from emissions. This is due to the higher fuel economy of GDI engines; we measured an average CO2 reduction of 57 g/mi, from switching engine technologies. GDI engine emissions had higher PM emissions compared to PFI engines, similar to previous findings. In addition, our data suggests that newer GDI engines have a factor of two lower PM emissions compared to older GDI engines. These improvements in emissions may enable GDI-equipped vehicles to meet the new Federal Tier 3 PM standard of 3.0 mg/mi without gasoline particulate filters (GPF, which would reduce their fuel economy). To better constrain the large uncertainty of radiative forcing associated with cookstove emissions, this dissertation examines emissions and optical properties from several cookstove and fuel combinations. We performed extensive laboratory measurements of the optical properties of fresh cookstove emissions using the newly developed firepower sweep protocol. Current model treatments of the optical properties of cookstove emissions assume: (1) complete internal mixture between BC and non-BC material and (2) absorption properties of organics based on parametrizations developed for biomass burning emissions. These assumptions do not accurately represent optical properties of fresh cookstove emissions. We developed new parametrizations of optical properties (BC-mass absorption cross section (MACBC), absorption angstrom exponent (AAE), and single scattering albedo (SSA)) of aerosol emissions from cookstoves as a function of the BC-to-PM mass ratio. These parametrizations are designed for use in climate models to more rigorously assess the global climate implications from adoption of improved stove technologies. Upon entering the atmosphere aerosol emissions undergo complex chemical transformations. Aerosol optical properties depend on their atmospheric processing which controls the amount of coating the particles accumulate and their lifetime. To assess the effects of coating on the optical properties, we performed targeted experiments using real world, size selected, BC particles emitted from a rocket improved cookstove, and coated with biogenic secondary organic aerosol (SOA) material. These experiments explicitly target to evaluate measurements and modeling using simple formulation like Mie theory. Measurements of MACBC and the mass scattering cross section (MSC) of coated BC particles were in good agreement with Mie predictions when the organic-to-BC mass ratio>5. Scattering (but not absorption) was sensitive to BC fractal-like morphology; Mie theory under-predicted measured scattering of fresh emissions. Our data suggest that Mie theory can be used in climate models to approximate the optical properties of coated BC particles emitted from cookstoves, if the mixing-state of BC particles is known. In this dissertation, we present initial evidence that particle growth rates depend on seed composition and gas-phase supersaturation. Current models do not account for seed-dependent growth rates. We conducted experiments to investigate the growth of diesel and biogenic SOA particles. Both seeds were exposed to the same gas-phase supersaturation, which allows us to accurately retrieve differences in growth rates and decouple the effects of surface activity and accommodation coefficients. We estimated that the accommodation coefficients of condensing material was 10% to 30% lower on the diesel particles compared to the SOA particles. Moreover, we measured larger surface activity of condensing material on the diesel particles, potentially due to less-miscible condensing vapors in the diesel particles compared to the SOA particles. Our data suggest that growth of BC (diesel) particles in the atmosphere is likely slower compared to SOA particles. Accurately representing these processes is important to estimate the lifetime and absorption enhancement from coated BC particles, as they compete with other particles for condensable vapors.

Share

COinS