Identifying and Resolving Artifacts Associated with Measurement and Characterization of Light Absorbing Organic Aerosols
Date of Award
Doctor of Philosophy (PhD)
Atmospheric aerosols constitute a major short-lived climate pollutant which affects the Earth’s radiative budget by scattering and absorbing solar radiation. Carbonaceous aerosol (CA) emissions -especially those from fossil fuel and biomass combustion- contribute 20-90% of the total fine particulate mass and can absorb light in the visible and near ultraviolet (UV) wavelengths, underscoring their importance in climate forcing. CA emissions comprise of organic aerosols (OA) and black carbon (BC) – both co-emitted during a combustion event. Both constituents have significantly different optical properties owing to their formation mechanism. Further, they could exist in a variety of mixing states in the atmosphere which complicates the separation of optical contributions from each individual component to net forcing calculations. In order to quantify the individual contributions of OA and BC to direct radiative forcing, we need (1) better estimates of OA optical properties, and (2) characterization of organic coating-induced BC absorption enhancements. To that end, I have attempted at addressing parts of these issues over three broad sections in my thesis: characterizing artifacts in traditional OA optical measurement techniques, studying biases in BC absorption enhancement measurements due to low volatility OA, and quantifying the optical properties and chemical composition of OA emitted during western US wildfires.
The traditional methodology for measuring optical properties of OA include sampling bulk particles on a filter substrate and extracting the organic compounds into a solvent. The absorbance of organic chromophores dissolved in the solvent is measured and subsequently converted to particle-phase absorption coefficients using theoretical correction factors obtained from Mie theory. This technique separates any contribution from aerosol constituents which are insoluble in a particular solvent: BC and dust. I characterized the differences between absorption coefficients measured using the traditional solvent-extraction method and those obtained from first-principle photoacoustic spectroscopy. I discovered that the theoretical correction factors could be off by a factor ranging from 0.5 to 7.5 and the difference increased as the BC fraction of the aerosol increased. I hypothesized that the increase in bias with the BC fraction was due to co-emission of solvent-insoluble organics with BC. I also established a linear relationship between the BC to total carbon mass ratio and the single scattering albedo of the biomass burning aerosol emissions. The optical properties for nascent BC are well documented in the literature; however, BC exists in an internally mixed state along with OA in the atmosphere. A BC aggregate coated with OA may exhibit enhanced light absorption, and quantifying this enhancement is necessary to accurately estimate the contribution of BC to radiative forcing. For the second section of my dissertation, I focused on the biases associated with quantifying light absorption enhancement for BC coated with OA (Eabs) while using a thermodenuder. I measured the light absorption enhancement for BC aggregates with mild coatings – generated by combusting wood pellets in a biomass cookstove. We observed differences in Eabs obtained from experiments and the literature with the difference being attributed to extremely low-volatility organics which did not evaporate at 300 0C. We performed discrete dipole approximation calculations and estimated that these low-volatility organic compounds could have imaginary refractive index values as high as 0.32 at a wavelength of 375 nm.
The final part of my dissertation brings closure to the insights gained in my laboratory studies by investigating emissions from real-world wildfires across the western United States as part of the 2019 FIREX-AQ field campaign. Forests in the western US are responsible for up to 40% of the total US carbon sequestration, and an increase in the frequency of wildfires in recent decades could lead to enhanced carbon dioxide and CA emissions. As part of the field campaign, we quantified the complex refractive index of low-volatility organics emitted across wildfires from three different states using electron energy loss spectroscopy. We discovered that highly absorbing low volatility OA could comprise up to 90% of the total particulate organic mass emitted from these fires. We parallelly measured the chemical composition of the aerosol emissions and correlated them with OA optical properties obtained using solvent-extraction based methods. I found that the nitrogen-containing compounds, along with the BC fraction, correlated well with the OA optical properties. During this field campaign, we corroborated our laboratory findings by identifying both the optical properties as well as the chemical composition of the low-volatility organics which result in artifacts during optical characterization of CA emissions.
Available for download on Thursday, September 21, 2023