Browsing by Author "Pierce, Jeffrey, advisor"
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Item Open Access Aerosol size distribution changes in FIREX-AQ biomass burning plumes: the role of plume concentration on coagulation and OA condensation/evaporation(Colorado State University. Libraries, 2022) June, Nicole, author; Pierce, Jeffrey, advisor; Kreidenweis, Sonia, committee member; Jathar, Shantanu, committee memberThe evolution of organic aerosols and aerosol size distributions within smoke plumes are uncertain due to the variability in rates of coagulation and organic aerosol (OA) condensation/evaporation across different smoke plumes and potentially in different locations within a single plume. We use aircraft data from the western US portion of the FIREX-AQ campaign to evaluate differences in aerosol size distribution evolution (growing by 10s to over 100 nm in several hours), OA mass, and Oxygen to Carbon ratios (O:C) under different concentrations and amounts of dilution. The observations show diameter increasing more quickly in more concentrated plumes despite these plumes generally having more OA evaporation than in the less concentrated plumes. Initial observations of OA and O:C suggest that evaporation and/or secondary OA formation between emission and the first measurement is also influenced by plume concentration. We estimate the isolated role of coagulation on size changes using model simulations, and we estimate the role of OA condensation/evaporation on size changes using the observed time evolution of the observed OA enhancement. We find that coagulation alone explains the majority of the diameter growth in the transect averages, with more growth occurring in plumes with higher initial number and OA concentrations. Overall, for each of the smoke plumes analyzed, including OA evaporation/condensation has a relatively minor impact on the simulated diameter compared to the changes due to coagulation. Additionally, we examine differences in evolution between the dilute and concentrated sections of the plume based on CO concentration to expand the range of plume concentrations represented in the observations. To determine if these in-plume concentration gradients could be used to understand smoke plumes outside of the range of the sampled average concentration, we simulate the dilute and concentrated plume regions independently (no mixing). In these simulations of each smoke plume region, the model underestimates particle growth in the less-concentrated regions of the plume and overestimates particle growth in the more-concentrated regions. This poor comparison suggests that turbulent mixing between the more- and less-concentrated regions is occurring on timescales too fast for the regions to evolve independently, but slow enough that aerosol size differences are still seen between the regions. The mixing in the plume limits the ability for our conclusions on variations in growth and condensation/evaporation within a plume to be applied to other plumes of a similar concentration. Overall, we conclude that coagulation dominates growth with plume concentrations being important in determining how much coagulational growth is observed.Item Open Access An observational and theoretical investigation of the evolution of biomass burning aerosol size distributions(Colorado State University. Libraries, 2015) Sakamoto, Kimiko M., author; Pierce, Jeffrey, advisor; Kreidenweis, Sonia, committee member; Volckens, John, committee memberBiomass-burning aerosols contribute to aerosol radiative forcing on the climate system. The magnitude of this effect is partially determined by aerosol size distributions, which are functions of source fire characteristics (e.g. fuel type, MCE) and in-plume microphysical processing (occurring on a GCM sub-grid scale). The uncertainties in biomass-burning emission number size-distributions in climate model inventories lead to uncertainties in the CCN concentrations and forcing estimates derived from these models. This emphasizes the need for observational and modelling studies to better represent effective biomass-burning size-distributions in larger-gridbox models. The BORTAS-B measurement campaign was designed to sample boreal biomass-burning outflow over Eastern Canada in the summer of 2011. Using these BORTAS-B data, we implement plume criteria to isolate the characteristic size-distribution of aged biomass-burning emissions (aged ~ 1 - 2 days) from boreal wildfires in Northwestern Ontario. The composite median size-distribution yields a single dominant accumulation mode with Dpm = 230 nm (number-median diameter), σ = 1.5, which are comparable to literature values of other aged plumes of a similar type. The organic aerosol enhancement ratios (ΔOA/ΔCO) along the path of Flight b622 show values of 0.05-0.18 μg m⁻³ ppbv⁻¹ with no significant trend with distance from the source. This lack of enhancement ratio increase/decrease with distance suggests no detectable net OA production/evaporation within the aged plume over the sampling period. A Lagrangian microphysical model was used to determine an estimate of the freshly emitted size distribution and flux corresponding to the BORTAS-B aged size-distributions. The model was restricted to coagulation and dilution processes only based on the insignificant net OA production/evaporation derived from the ΔOA/ΔCO enhancement ratios. We estimate that the fresh-plume median diameter was in the range of 59-94 nm with modal widths in the range of 1.7-2.8 (the ranges are due to uncertainty in the entrainment rate). Thus, the size of the freshly emitted particles is relatively unconstrained due to the uncertainties in the plume dilution rates. Expanding on the fresh-plume coagulational modelling of the BORTAS-B plumes, a coagulation-only parameterization for effective biomass-burning size-distributions was developed using the SAM-TOMAS plume model and a gaussian emulator. Under a range of biomass-burning conditions, the SAM-TOMAS simulations showed increasing Dpm and decreasing σ (converging to 1.2) with distance from the emission source. Final Dpm also shows a strong dependence on dM/dx (Mass flux x Fire area/vg), with larger values resulting in more rapid coagulation and faster dDpm/dt. The SAM-TOMAS simulations were used to train the Gaussian Emulation Machine for Sensitivity Analysis (GEM-SA) to build a Dpm and σ parameterization based on seven inputs. The seven inputs are: emission Dpm0, emission σ0, mass flux, fire area, mean boundary layer wind (vg), time, and plume mixing depth (dmixing). These inputs are estimated to account for 81% of the total variance in the final size distribution Dpm, and 87% of the total variance in the final σ. The parameterization performs very well against non-training modelled SAM-TOMAS size-di stributions in both final Dpm (slope = 0.92, R² = 0.83, NMBE=-0.06) and final σ (slope = 0.91, R² = 0.93, NMBE = 0.01). These final size distribution parameters are meant to be inserted as effective biomass-burning aerosol size-distributions (single lognormal mode) into larger-scale atmospheric models.Item Open Access Blending model output with satellite-based and in-situ observations to produce high-resolution estimates of population exposure to wildfire smoke(Colorado State University. Libraries, 2016) Lassman, William, author; Pierce, Jeffrey, advisor; Fischer, Emily, committee member; Schumacher, Russ, committee member; Magzamen, Sheryl, committee member; Pfister, Gabriele, committee memberIn the western US, emissions from wildfires and prescribed fire have been associated with degradation of regional air quality. Whereas atmospheric aerosol particles with aerodynamic diameters less than 2.5 μm (PM 2.5 ) have known impacts on human health, there is uncertainty in how particle composition, concentrations, and exposure duration impact the associated health response. Due to changes in climate and land-management, wildfires have increased in frequency and severity, and this trend is expected to continue. Consequently, wildfires are expected to become an increasingly important source of PM 2.5 in the western US. While composition and source of the aerosol is thought to be an important factor in the resulting human health-effects, this is currently not well-understood; therefore, there is a need to develop a quantitative understanding of wildfire-smoke-specific health effects. A necessary step in this process is to determine who was exposed to wildfire smoke, the concentration of the smoke during exposure, and the duration of the exposure. Three different tools are commonly used to assess exposure to wildfire smoke: in-situ measurements, satellite-based observations, and chemical-transport model (CTM) simulations, and each of these exposure-estimation tools have associated strengths and weakness. In this thesis, we investigate the utility of blending these tools together to produce highly accurate estimates of smoke exposure during the 2012 fire season in Washington for use in an epidemiological case study. For blending, we use a ridge regression model, as well as a geographically weighted ridge regression model. We evaluate the performance of the three individual exposure-estimate techniques and the two blended techniques using Leave-One-Out Cross-Validation. Due to the number of in-situ monitors present during this time period, we find that predictions based on in-situ monitors were more accurate for this particular fire season than the CTM simulations and satellite-based observations, so blending provided only marginal improvements above the in-situ observations. However, we show that in hypothetical cases with fewer surface monitors, the two blending techniques can produce substantial improvement over any of the individual tools.Item Open Access Effects of near-source coagulation of biomass burning aerosols on global predictions of aerosol size distributions and implications for aerosol radiative effects(Colorado State University. Libraries, 2018) Ramnarine, Emily, author; Pierce, Jeffrey, advisor; Kreidenweis, Sonia, committee member; Jathar, Shantanu, committee memberBiomass burning is a significant global source of aerosol number and mass. In fresh biomass burning plumes, aerosol coagulation reduces aerosol number and increases the median size of aerosol size distributions, impacting aerosol radiative effects. Near-source biomass burning aerosol coagulation occurs at spatial scales much smaller than the grid boxes of global and many regional models. To date, these models ignore sub-grid coagulation and instantly mix fresh biomass burning emissions into coarse grid boxes. A previous study found that the rate of particle growth by coagulation within an individual smoke plume can be approximated using the aerosol mass emissions rate, initial size distribution median diameter and modal width, plume mixing depth, and wind speed. In this thesis, we use this parameterization of sub-grid coagulation in the GEOS-Chem-TOMAS global aerosol microphysics model to quantify the impacts on global aerosol size distributions, the direct radiative effect, and the cloud-albedo aerosol indirect effect. We find that inclusion of biomass burning sub-grid coagulation reduces the biomass burning impact on the number concentration of particles larger than 80 nm (a proxy for CCN-sized particles) by 37% globally. This CCN reduction causes our estimated global biomass burning cloud-albedo aerosol indirect effect to decrease from -76 to -43 mW m−2. Further, as sub-grid coagulation moves mass to sizes with more efficient scattering, including it increases our estimated biomass burning all-sky direct effect from -224 to -231 mW m−2 with assumed external mixing and from -188 to -197 mW m−2 with assumed internal mixing with core-shell morphology. However, due to differences in fire and meteorological conditions across regions, the impact of sub-grid coagulation is not globally uniform. We also test the sensitivity of the impact of sub-grid coagulation to two different biomass burning emission inventories, to various assumptions about the fresh biomass burning aerosol size distribution, and to two different timescales of sub-grid coagulation. The impacts of sub-grid coagulation are qualitatively the same regardless of these assumptions.Item Open Access Inorganic gas-aerosol partitioning in and around animal feeding operation plumes in northeastern Colorado in late summer 2021(Colorado State University. Libraries, 2023) Li, En, author; Pierce, Jeffrey, advisor; Fischer, Emily, advisor; Jathar, Shantanu, committee member; Sullivan, Amy, committee memberAmmonia (NH3) from animal feeding operations (AFOs) is an important source of reactive nitrogen in the US, but despite its ramifications for air quality and ecosystem health, its near-source evolution remains understudied. To this end, Phase I of the Transport and Transformation of Ammonia (TRANS2Am) field campaign was conducted in the northeastern Colorado Front Range in summer 2021 and characterized atmospheric composition downwind of AFOs during 10 research flights. Airborne measurements of NH3, nitric acid (HNO3), and a suite of water-soluble aerosol species collected onboard the University of Wyoming King Air (UWKA) research aircraft present a unique opportunity to investigate the sensitivity of particulate matter (PM) formation to AFO emissions. We couple the observations with thermodynamic modeling to predict the seasonality of ammonium nitrate (NH4NO3) formation. We find that during TRANS2Am northeastern Colorado is consistently in the NH3-rich and HNO3-limited NH4NO3 formation regime. Further investigation using the Extended Aerosol Inorganics Model (E-AIM) reveals that summertime temperatures (mean: 23 ˚C) of northeastern Colorado, especially near the surface, inhibit NH4NO3 formation despite high NH3 concentrations (max: ≤ 114 ppbv). Lastly, we model and winter conditions to explore the seasonality of NH4NO3 formation and find that cooler temperatures could support substantially more NH4NO3 formation. Whereas summertime NH4NO3 only exceeds 1 µg m-3 ~10% of the time in summer, modeled NH4NO3 would exceed 1 µg m-3 61% (88%) of the time in spring/autumn (winter), with a 10°C (20°C) temperature decrease relative to the campaign.Item Open Access Look up: probing the vertical profile of new particle formation and growth in the planetary boundary layer with models and observations(Colorado State University. Libraries, 2022) O'Donnell, Samuel, author; Pierce, Jeffrey, advisor; Jathar, Shantanu, committee member; Kreidenweis, Sonia, committee memberThe processes of new particle formation (NPF) and growth are important contributors to cloud condensation nuclei (CCN) concentrations, and CCN are important for climate from their impact on planetary radiative forcing. While the general ubiquity and importance of NPF is understood, the vertical extent and governing mechanisms of NPF and growth in the lower troposphere are uncertain. We present a two-part analysis of the vertical profile of NPF during the HI-SCALE field campaign at the Southern Great Plains observatory in Oklahoma, USA. Firstly, we analyzed airborne and ground-based observations of four NPF events. Secondly, we used a column aerosol chemistry and microphysics model, along with the observations, to probe factors that influence the vertical profile of NPF. During HI-SCALE, we found several instances of enhanced NPF occurring several hundred meters above the surface; however, the spatio-temporal characteristics of the observed NPF made comparisons between airborne- and ground-based observations difficult. For six unique events, the model represented the observed NPF (or lack of NPF) and particle growth at the surface to within 10 nm. The model predicted enhanced NPF rates in the upper mixed layer, and this enhancement is primarily due to the temperature dependence in the NPF schemes. The simulations were sensitive to the initial vertical profile of gas-phase species from HI-SCALE, such that vertical mixing in the model either enhanced or suppressed NPF rates, aerosol number concentrations, and particle growth rates at the surface. Finally, our analysis provides insights for future field campaigns and modeling efforts investigating the vertical profile of NPF.Item Open Access The validation of emission rate estimation methods(Colorado State University. Libraries, 2015) Wells, Bradley, author; Collett, Jeffrey L., advisor; Pierce, Jeffrey, advisor; Ham, Jay, committee memberOil and natural gas production throughout the United States has been dramatically increasing in recent years, due in large part to hydraulic fracturing processes and horizontal drilling techniques that allow for extraction from unconventional wells. The rise in well drilling and completion activities raises concern over potential air quality impacts on nearby communities. Methane, other volatile organic compounds (VOCs), and nitrogen oxides (NOₓ) may be emitted into the atmosphere during well development and production activities. Methane is a greenhouse gas, VOCs and NOₓ act as ozone precursors, and some VOCs are classified as air toxics. For these reasons, there is a need to accurately quantify the rate of emissions of these gases into the atmosphere from oil and gas development and production. One such emission rate estimation technique is the tracer ratio method (TRM). The TRM requires access to a well site and involves the release of a passive tracer gas as close to the source of emissions as possible. This known emission rate is multiplied by the ratio of the downwind concentrations of emission gas to the tracer gas (both in excess of background) to derive an estimate of the emission gas emission rate. Another technique, recently developed by the Environment Protection Agency, utilizes a simplified point source Gaussian plume (PSG) dispersion model. This approach requires only one mobile downwind measurement location for both concentration and meteorological measurements, without the need for site access; it does not require a tracer gas. In order to evaluate the effectiveness of these techniques, a series of experiments were conducted at Christman airfield in Fort Collins, Colorado. These experiments involved releasing both acetylene, as a tracer gas, and methane (to simulate an emission source) at controlled flow rates to compare the predicted emission rate of methane to its actual emission rate. A vehicle equipped with a PICARRO methane and acetylene analyzer traversed or remained stationary within the gas plume to provide real-time concentration measurements of both gases. A 3-D sonic anemometer was used to characterize local meteorological conditions. The TRM is evaluated using both a mobile transect and a stationary approach. There is an overall positive bias in both cases. Our best results are obtained when sources are co-located during a stationary analysis and changes in background methane concentrations are determined and corrected. In these cases the mean bias is +9% with σ=22% (standard deviation about the mean bias). The separation of tracer and emission gas sources in the mobile transect analysis is the largest cause for uncertainty. The mean bias when sources are separated is +83% (σ=99%), as opposed to transect analyses of co-located sources which have a mean bias of +33% (σ=31%). The PSG technique, which involves a 20 minute stationary analysis, contains more inconsistent results compared to the stationary approach performed by the TRM (mean bias of methane emission rate prediction +34%, σ=123%). Most interesting of note is that for nearly every sample the bias in the prediction of the emission rate of acetylene is more negative than the bias in methane emission rate predictions (mean bias of acetylene emission rate prediction -19%, σ=128%). This suggests possible biases in the acetylene release rate or concentration measurement; however, at this time the issue cannot be located. Regardless, a stationary TRM technique produces the best results, and its use is recommended when site access is available for tracer release.Item Open Access Uncertainties in global aerosols and climate effects due to biofuel emissions(Colorado State University. Libraries, 2015) Kodros, John Kelly, author; Pierce, Jeffrey, advisor; Volckens, John, committee member; Kreidenweis, Sonia, committee memberAerosol emissions from biofuel combustion impact both health and climate; however, while reducing emissions through improvements to combustion technologies will improve health, the net effect on climate is largely unconstrained. In this study, we examine sensitivities in global aerosol concentration, direct radiative climate effect, and cloud-albedo aerosol indirect climate effect to uncertainties in biofuel emission factors, optical mixing-state, and model nucleation and background SOA. We use the Goddard Earth Observing System global chemical-transport model (GEOS-Chem) with TwO Moment Aerosol Sectional (TOMAS) microphysics. The emission factors include: amount, composition, size and hygroscopicity, as well as optical mixing-state properties. We also evaluate emissions from domestic coal use, which is not biofuel but is also frequently emitted from homes. We estimate the direct radiative effect assuming different mixing states (internal, core-shell, and external) with and without absorptive organic aerosol (brown carbon). We find the global-mean direct radiative effect of biofuel emissions ranges from -0.02 to +0.06 W m-2 across all simulation/mixing state combinations with regional effects in source regions ranging from -0.2 to +1.2 W m-2. The global-mean cloud-albedo aerosol indirect effect ranges from +0.01 to -0.02 W m-2 with regional effects in source regions ranging from -1.0 to -0.05 W m-2. The direct radiative effect is strongly dependent on uncertainties in emissions mass, composition, emissions aerosol size distributions and assumed optical mixing state, while the indirect effect is dependent on the emissions mass, emissions aerosol size distribution and the choice of model nucleation and secondary organic aerosol schemes. The sign and magnitude of these effects have a strong regional dependence. We conclude that the climate effects of biofuel aerosols are largely unconstrained, and the overall sign of the aerosol effects is unclear due to uncertainties in model inputs. This uncertainty limits our ability to introduce mitigation strategies aimed at reducing biofuel black carbon emissions in order to counter warming effects from greenhouse-gases. To better understand the climate impact of particle emissions from biofuel combustion, we recommend field/laboratory measurements to narrow constraints on: 1) emissions mass, 2) emission size distribution, 3) mixing state, and 4) ratio of black carbon to organic aerosol.