Characteristics of atmospheric ice nucleating particles associated with biomass burning in the US: prescribed burns and wildfires
Date
2013
Authors
McCluskey, Christina S., author
Kreidenweis, Sonia, advisor
DeMott, Paul, advisor
Pierce, Jeffrey, committee member
Volckens, John, committee member
Journal Title
Journal ISSN
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Abstract
Insufficient knowledge regarding the sources and number concentrations of atmospheric ice nucleating particles (INP) leads to large uncertainties in understanding the interaction of aerosols with cloud processes, such as cloud life time and precipitation rates. An increasingly important source of aerosol in the United States is biomass burning, particularly in the form of prescribed burns and wildfires in the southeastern and western U.S., respectively. Prior field and laboratory observations have suggested that biomass burning can be a source of INP. However, emissions from biomass burning are complex, varying with combustion efficiency, fuel type, plume age and dilution. Thus, this potentially important source of INP is poorly characterized. This study utilizes measurements of INP from a diverse set of biomass burning events to better understand INP associated with biomass burning in the U.S. Prescribed burns in Georgia and Colorado, two Colorado wildfires and two laboratory burns were monitored for INP number concentrations (nINP) using the Colorado State University continuous-flow diffusion chamber (CFDC) to activate INP in the condensation/immersion freezing nucleation mode. Additional measurements included total particle number concentrations, number concentrations of particles with diameters larger than 500 nm, aerosol mass concentrations, carbon monoxide concentrations and chemically-speciated bulk aerosol filter samples. Additionally, activated INP were collected onto TEM grids downstream of the CFDC, isolating INP for single particle chemical and morphological analyses. These fires varied by fuel type, including wiregrass, longleaf pine and ponderosa pine, and also varied by combustion efficiency, ranging from highly flaming to a mixture of flaming and smoldering. Additionally, plume histories were different between the fires including aged plumes from the wildfires and freshly emitted smoke from the prescribed and laboratory burns. The relationship between nINP and total particle number concentrations, evident within prescribed burning plumes, was degraded within aged smoke plumes from the wildfires, limiting the utility of this relationship for comparing laboratory and field data. Larger particles, represented by n500nm, are less vulnerable to plume processing and have previously been evaluated for their relation to nINP. Our measurements indicated that for a given n500nm, nINP associated with the wildfires were nearly an order of magnitude higher than nINP found in prescribed fire emissions. That is, nINP represented a much larger fraction of n500nm in wildfires as compared with prescribed fires. Further, an existing parameterization for "global" nINP that relates INP abundance to n500nm largely under-predicted and over-predicted nINP emitted from wildfires and prescribed burns, respectively. Reasons for the differences between INP characteristics in these emissions were explored, including variations in combustion efficiency, fuel type, transport time and environmental conditions. Combustion efficiency and fuel type were eliminated as controlling factors by comparing samples with contrasting combustion efficiencies and fuel types. Transport time was eliminated because the expected impact would be to reduce n500nm, thus resulting in the opposite effect from the observed change. Bulk aerosol chemical composition analyses support the potential role of elevated soil dust particle concentrations during the fires, contributing to the population of INP, but the bulk analyses do not target INP composition directly. Predictions from the Naval Aerosol Analysis and Prediction System model further indicate elevated dust mass concentrations during the wildfire periods, suggesting impact of mineral dust from long-range transport (LRT). It is hypothesized that both hardwood burning and soil lofting are responsible for the elevated production of INP in the Colorado wildfires in addition to LRT of mineral dust. The chemical compositions of INP were probed directly via TEM imaging. Single particle analyses of residual INP showed that they comprised various C-containing particle types, but with a higher abundance of mineral and metal oxide containing INP in emissions from flaming phase combustion. Fractal soot was found as an INP type comprising up to 60% of collected INP in young smoke emissions from the Georgia prescribed burns. In a series of laboratory combustion experiments, the use of a new instrumental set up, pairing the CFDC with a single particle soot photometer, revealed up to a 60% decrease in active INP after the removal of refractory black carbon from smoke aerosol emitted from a highly flaming burn of wiregrass, supporting that soot particles serve as INP in fire emissions. The presence of soil minerals was clearly evident in TEM images of samples taken during the wildfires in addition to tarballs, carbon balls most commonly associated with aged smoke plumes. These results demonstrate that the ice nucleating particles observed in the wildfires were influenced by other factors not represented in the smoke emitted from the laboratory or prescribed burns. Finally, an INP parameterization was developed based on the temperature dependent relationship between nINP and n500nm, following methods used by previous studies. This parameterization is likely only representative of the Hewlett and High Park wildfires due to the apparent impact of non-biomass-burning aerosol. However, all wildfires are typically associated with vigorous localized convection and arid soils, required for the lofting of the soils and dusts similar to these wildfires. It will be useful to compare future wildfires in various regions to the proposed parameterization.
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Subject
atmospheric ice nucleation
biomass burning