Modeling study of the hygroscopic, optical, and cloud-activation properties of aerosols from biofuel combustion
Residential combustion of biofuel is an important source of aerosols and gases in the atmosphere. Emissions from biofuel combustion can affect the climate due to the presence of warming species that absorb radiation like black carbon and greenhouse gases, or cooling due to aerosols that scatter radiation back to space and affect cloud characteristics. The climate impact of emissions from biofuel combustion is uncertain in part due uncertainties in the physical, chemical, and optical properties of biofuel burning aerosols. This dissertation is divided into three investigations that address this gap in knowledge by providing information on the size distribution, mixing state, hygroscopicity, optical properties, CCN activity, and total quantity of aerosols emitted from biofuel combustion. The change in cloud characteristics due to biofuel burning aerosols is uncertain, in part, due to the uncertainty in the added number of cloud condensation nuclei (CCN) from biofuel burning. In the first investigation of this dissertation, I provide estimates of the CCN activity of biofuel burning aerosols by explicitly modeling plume dynamics (coagulation, condensation, chemical reactions, and dilution) in a young biofuel burning plume from emission until the plume reaches ambient temperature and specific humidity through entrainment. I found that aerosol-scale dynamics affect the average aerosol size and hygroscopicity only during the first few seconds of evolution, after which they reach a constant value. Homogenizing factors of the aerosol population mixing state are co-emission of semi-volatile organic compounds (SVOCs) or emission at small aerosol sizes. SVOC co-emission can be the main factor determining CCN activity for hydrophobic or small aerosols. Coagulation limits emission of CCN to about 1016 per kg of fuel. Depending on emission factor, aerosol size, and composition, some of these aerosols may not activate at low supersaturation. Hygroscopic Aitken mode aerosols can contribute to CCN through self-coagulation, but have a small effect on the CCN activity of accumulation-mode aerosols, regardless of composition differences. Simple models (monodisperse coagulation and average hygroscopicity) can be used to estimate plume-exit CCN within about 20% if aerosols are unimodal and have homogeneous composition, or when aerosols are emitted in the Aitken mode even if they are not homogeneous. On the other hand, if externally-mixed aerosols are emitted in the accumulation mode without SVOCs, an average hygroscopicity would overestimate emitted CCN by up to a factor of 2. This work identified conditions under which aerosol populations become more homogeneous during plume processes. This homogenizing effect requires the components to be truly co-emitted, rather than sequentially emitted. In the second investigation of this dissertation I provide information on the hygroscopicity and optical properties of Organic Carbon (OC). OC is a ubiquitous component of ambient aerosols, and the major component of biofuel burning aerosols. The hygroscopic and optical properties of OC are important to include in atmospheric radiative transfer models, but OC absorption at high relative humidity is still uncertain. I provided a model that describes measured changes in aerosol size and optical properties of light-absorbing or “brown” OC from wood pyrolysis at relative humidity (RH) ranging from 40% to 95%. Extinction, scattering, and absorption were modeled using Mie-Lorenz theory, which requires aerosol size and refractive index at all RHs as inputs. Diameter growth factors as a function of RH were measured with a Hygroscopic-Tandem Differential Mobility Analyzer (H-TDMA) and fitted with the κ-Köhler model. I explored several mixing rules to model the aerosol mixing state and estimate the effective refractive index of the humidified OC aerosol. There was agreement between modeled and measured extinction, scattering, and absorption within the uncertainties of the system for nigrosin aerosols up to 92% RH. There was agreement in extinction and scattering of OC, but its absorption was too small to obtain an accurate measurement. Different models of the effective refractive index predict an increase in OC absorption of 10% to 30% at 90% RH for a light wavelength of 467 nm, and no change in absorption with RH at 530 nm and 660 nm. In the third investigation of this dissertation I assessed approaches to reduce the uncertainty in measurements of aerosol mass concentration obtained from light scattering. The aerosol mass emitted from a combustion event is a parameter needed in models describing the evolution of aerosols, hence it is typically measured in field measurements. Light scattering is widely used as proxy of aerosol mass concentration in commercial instruments. A main uncertainty in this approach is in the factor used to convert from scattering to aerosol mass, i.e., the mass scattering cross-section (MSC), which is obtained from a calibration aerosol, whose properties might differ from the aerosol measured. Using Mie-Lorenz theory, I assessed that MSC can vary between 0.04 and 10.8 m2/g for the range of size distributions and refractive index of biofuel burning aerosols. Within a single combustion event, MSC can vary by up to an order of magnitude between different combustion phases due to changes in aerosol size, hence a fixed MSC value would incorrectly apportion aerosol mass to each combustion phase. To reduce the uncertainty in the inferred aerosol mass concentration, I evaluated using a look-up table approach with the wavelength and angular dependence of scattered light to constrain MSC. I estimated that this approach could reduce the uncertainty in MSC by 79% on average. Due to the sensitivity of scattered light to aerosol morphology, this approach may underestimate the mass of soot aerosols by more than a factor of two due to their fractal structure.
aerosol, biofuel, hygroscopic, activation, CCN, organic carbon, light absorption, aerosol mass concentration, scattering
University of Illinois - Urbana-Champaign
Networked Digital Library of Theses and Dissertations
University of Illinois at Urbana-Champaign