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Browsing Department of Atmospheric Science by Subject "aerosol"
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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 Characterizing the influence of anthropogenic emissions and transport variability on sulfate aerosol concentrations at Mauna Loa Observatory(Colorado State University. Libraries, 2013) Potter, Lauren E., author; Kreidenweis, Sonia, advisor; Maloney, Eric, committee member; Farmer, Delphine, committee member; Cooley, Daniel, committee memberSulfate aerosol in the atmosphere has substantial impacts on human health and environmental quality. Most notably, atmospheric sulfate has the potential to modify the earth's climate system through both direct and indirect radiative forcing mechanisms (Meehl et al., 2007). Emissions of sulfur dioxide, the primary precursor of sulfate aerosol, are now globally dominated by anthropogenic sources as a result of widespread fossil fuel combustion. Economic development in Asian countries since 1990 has contributed considerably to atmospheric sulfur loading, particularly China, which currently emits approximately 1/3 of global anthropogenic SO2 (Klimont et al., 2013). Observational and modeling studies have confirmed that anthropogenic pollutants from Asian sources can be transported long distances with important implications for future air quality and global climate change. Located in the remote Pacific Ocean (19.54°N, 155.58°W) at an elevation of 3.4 kilometers above sea level, Mauna Loa Observatory (MLO) is an ideal measurement site for ground-based, free tropospheric observations and is well situated to experience influence from springtime Asian outflow. This study makes use of a 14-year data set of aerosol ionic composition, obtained at MLO by the University of Hawaii at Manoa. Daily filter samples of total aerosol concentrations were made during nighttime downslope (free-tropospheric) transport conditions, from 1995 to 2008, and were analyzed for aerosol-phase concentrations of the following species: nitrate (NO3-), sulfate (SO42-), methanesulfonate (MSA), chloride (Cl-), oxalate, sodium (Na+), ammonium (NH4+), potassium (K+), magnesium (Mg2+), and calcium (Ca2+). An understanding of the factors controlling seasonal and interannual variations in aerosol speciation and concentrations at this site is complicated by the relatively short lifetimes of aerosols, compared with greenhouse gases which have also been sampled over long time periods at MLO. Aerosol filter data were supplemented with observations of gaseous radon (Rn222) and carbon monoxide (CO), used as tracers of long distance continental influence. Our study applied trajectory analysis and multiple linear regression to interpret the relative roles of aerosol precursor emissions and large-scale transport characteristics on observed MLO sulfate aerosol variability. We conclude that observed sulfate aerosol at MLO likely originated from a combination of anthropogenic, volcanic, and biogenic sources that varied seasonally and from year to year. Analysis of chemical continental tracer concentrations and HYSPLIT back trajectories suggests that non-negligible long distance influence from either the Asian or North American continents can be detected at MLO during all seasons although large interannual variability was observed. Possible influence of circulation changes in the Pacific Basin related to the El Niño-Southern Oscillation were found to be both species and seasonally dependent. We further found an increasing trend in monthly mean sulfate aerosol concentrations at MLO of 4.8% (7.3 ng m-3) per year during 1995-2008, significant at the 95% confidence level. Multiple linear regression results suggest that the observed trend in sulfate concentrations at MLO cannot reasonably be explained by variations in meteorology and transport efficiency alone. An increasing sulfate trend of 5.8 ng m-3 per year, statistically significant at the 90% confidence level, was found to be associated with the variable representing East Asian SO2 emissions. The results of this study provide evidence that MLO sulfate aerosol observations during 1995-2008 reflect, in part, recent trends in anthropogenic SO2 emissions which are superimposed onto the natural meteorological variability affecting transport efficiency.Item Open Access Climate and health impacts of particulate matter from residential combustion sources in developing countries(Colorado State University. Libraries, 2018) Kodros, John K., author; Pierce, Jeffrey R., advisor; Volckens, John, committee member; Kreidenweis, Sonia, committee member; Ravishankara, A. R., committee memberGlobally, close to 2.8 billion people lack access to clean cooking technology, while 1.8 billion people lack access to electricity altogether. As a means to generate energy for residential tasks, it is common in many developing countries to rely on combustion of solid fuels (wood, dung, charcoal, trash, etc.). Solid fuel use (SFU) can emit substantial amounts of fine particulate matter (PM2.5), often in or in close proximity to residences, creating concerns for human health and climate; however, large uncertainties exist in indoor and outdoor concentrations and properties, limiting our ability to estimate these climate and health impacts. This work explores the uncertainty space in estimates of premature mortality attributed to exposure to PM2.5 from residential SFU (e.g., cooking, heating, lighting) and makes the first estimates of health and radiative effects from combustion of domestic waste (i.e., trash burning). Next, we investigate key uncertain parameters (emission size distribution, black carbon mixing state, and size-resolved respiratory deposition) that drive uncertainties in health and radiative impacts from SFU, in order to improve model estimates of aerosol impacts from all sources. In many developing regions, combustion of solid fuels for cooking and heating is not the only aerosol source impacting air quality and climate. While uncontrolled combustion of domestic waste has been observed in many countries, this aerosol source is not generally included in many global emissions inventories. Using a global chemical-transport model, we estimate exposure to ambient PM2.5 from domestic-waste combustion to cause 270,000 (5th-95th percentile: 213,000 to 328,000) adult mortalities per year, most of which occur in developing countries. Regarding aerosol radiative effects, we estimate the globally averaged direct radiative effect (DRE) to range from -40 mW m-2 to +4 mW m-2 and the aerosol indirect effect (AIE) to range from -4 mW m-2 to -49 mW m-2. In some regions with significant waste combustion, such as India and China, the aerosol radiative effects exceed −0.4 W m−2.The sign and magnitude of the global-mean DRE is strongly sensitive to assumptions on how black carbon (BC) is mixed with scattering particles, while the AIE is strongly sensitive to the emission size distribution. To determine what factors dominate the uncertainty space in mortality estimates from SFU, we perform a variance-based sensitivity analysis on premature mortality attributed to the combined exposure to ambient and household PM2.5 from SFU. We find that uncertainty in the percent of the population using solid fuels for energy contributes the most to the uncertainty in mortality (53-56% of uncertainty across Asia and South America) with the concentration-response function the next largest contributor (40-50%). In the second half of this dissertation, we explore several key uncertainties in climate and health estimates of aerosol from residential sources in order to reduce overall model uncertainty of aerosol impacts from any source. To test the sensitivity of the AIE to treatment of aerosol size distributions in global models, we estimate the AIE due to anthropogenic emissions with prognostic sectional aerosol microphysics and compare this to the AIE calculated when the simulated aerosol mass of each species is remapped onto a prescribed size distribution. Simulations using the prognostic scheme yield a global mean anthropogenic AIE of −0.87 W m−2, while the simulations with the prescribed scheme predict −0.66 W m−2. These differences suggest that simulations with prescribed size‐distribution mapping are unable to capture regional and temporal variability in size‐resolved aerosol number and thus may lead to biases in estimates of the AIE.Item Open Access Examination of the potential impacts of dust and pollution aerosol acting as cloud nucleating aerosol on water resources in the Colorado River Basin(Colorado State University. Libraries, 2016) Jha, Vandana, author; Cotton, William R., advisor; Rutledge, Steven A., committee member; Pierce, Jeffrey, committee member; Ramirez, Jorge, committee memberIn this study we examine the cumulative effect of dust acting as cloud nucleating aerosol (cloud condensation nuclei (CCN), giant cloud condensation nuclei (GCCN), and ice nuclei (IN)) along with anthropogenic aerosol pollution acting primarily as CCN, over the entire Colorado Rocky Mountains from the months of October to April in the year 2004-2005; the snow year. This ~6.5 months analysis provides a range of snowfall totals and variability in dust and anthropogenic aerosol pollution. The specific objectives of this research is to quantify the impacts of both dust and pollution aerosols on wintertime precipitation in the Colorado Mountains using the Regional Atmospheric Modeling System (RAMS). In general, dust enhances precipitation primarily by acting as IN, while aerosol pollution reduces water resources in the CRB via the so-called “spill-over” effect, by enhancing cloud droplet concentrations and reducing riming rates. Dust is more episodic and aerosol pollution is more pervasive throughout the winter season. Combined response to dust and aerosol pollution is a net reduction of water resources in the CRB. The question is by how much are those water resources affected? Our best estimate is that total winter-season precipitation loss for for the CRB the 2004-2005 winter season due to the combined influence of aerosol pollution and dust is 5,380,00 acre-feet of water. Sensitivity studies for different cases have also been run for the specific cases in 2004-2005 winter season to analyze the impact of changing dust and aerosol ratios on precipitation in the Colorado River Basin. The dust is varied from 3 to 10 times in the experiments and the response is found to be non monotonic and depends on various environmental factors. The sensitivity studies show that adding dust in a wet system increases precipitation when IN affects are dominant. For a relatively dry system high concentrations of dust can result in over-seeding the clouds and reductions in precipitation. However, when adding dust to a system with warmer cloud bases, the response is non-monotonical, and when CCN affects are dominant, reductions in precipitation are found.Item Open Access Impacts of unconventional oil and gas development on atmospheric aerosol particles(Colorado State University. Libraries, 2017) Evanoski-Cole, Ashley R., author; Collett, Jeffrey L., advisor; Kreidenweis, Sonia M., committee member; Pierce, Jeffrey R., committee member; Ham, Jay, committee memberRising demands for global energy production and shifts in the economics of fossil fuel production have recently driven rapid increases in unconventional oil and gas drilling operations in the United States. Limited field measurements of atmospheric aerosol particles have been conducted to understand the impacts of unconventional oil and gas extraction on air quality. These impacts can include emissions of greenhouse gases, the release of volatile organic compounds that can be hazardous and precursors to tropospheric ozone formation, and increases in atmospheric aerosol particles. Aerosol particles can also contribute to climate change, degrade visibility and negatively impact human health and the environment. Aerosol formation can result from a variety of activities associated with oil and gas drilling operations, including emission of particles and/or particle precursors such as nitrogen oxides from on-site power generation, evaporation or leaking of fracking fluids or the produced fuel, flaring, the generation of road dust, and increases in traffic and other anthropogenic emissions associated with growing populations near drilling locations. The work presented here details how activities associated with unconventional oil and gas extraction impact aerosol particle characteristics, sources, and formation in remote regions. An air quality field study was conducted in the Bakken formation region during a period of rapid growth in oil production by unconventional techniques over two winters in 2013 and 2014. The location and time of year were chosen because long term IMPROVE network monitoring records show an increasing trend in particulate nitrate concentrations and haze in the Bakken region during the winter, strongly contrasting with sharp decreases observed across most of the U.S. The comprehensive suite of instrumentation deployed for the Bakken Air Quality Study (BAQS) included measurements of aerosol concentrations, composition, and scattering, gaseous precursors important for aerosol formation, volatile organic compounds, and meteorology. Regional measurements of inorganic aerosol composition were collected, with average concentrations of total inorganic PM2.5 between 4.78 – 6.77 µg m-3 and 1.99 – 2.52 µg m-3 for all sampling sites during the 2013 and 2014 study periods, respectively. The maximum inorganic PM2.5 concentration observed was 21.3 µg m-3 for a 48 hour filter sample collected at Fort Union National Historical Site, a site located within a dense area of oil wells. Organic aerosol measurements obtained during the second study at the north unit of Theodore Roosevelt National Park (THRO-N) featured an average concentration of 1.1 ± 0.7 µg m-3. While oil production increased from 2013 to 2014, the lower PM2.5 in 2014 can be explained by the meteorological differences. During the first study, increased snow cover, atmospheric stability, solar illumination, and differences in the dominant wind direction contributed to higher PM2.5. The enhanced concentrations of inorganic PM2.5 measured in the Bakken region were tied to regional oil and gas development. Elevated concentrations of PM2.5 were observed during periods of air mass stagnation and recirculation and were associated with VOC emissions aged less than a day, both indicating a predominant influence from local emissions. High PM2.5 concentrations occurred when low i-/n-pentane VOC ratios were observed, indicating strong contributions from oil and gas operations. The hourly measurements of gas and aerosol species in an extremely cold environment also provided a unique data set to investigate how well thermodynamic aerosol models represent the partitioning of ammonium nitrate. In general, during the coldest temperatures, the models overpredicted the formation of particulate nitrate. The formation of additional PM2.5 in this region is more sensitive to availability of N(-III) species during the coldest periods but increasingly sensitive to available N(V) when temperatures are relatively warmer and ammonia availability increases. These measurements and modeling results show that continued growth of oil and gas drilling operations in remote areas such as the Bakken region could lead to increased PM2.5 and impact haze formation in nearby federally protected lands.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 Measurement of low-altitude aerosol layers surrounding convective cold pool passage observed by uncrewed aircraft(Colorado State University. Libraries, 2024) Heffernan, Brian, author; Kreidenweis, Sonia, advisor; Perkins, Russell, advisor; Pierce, Jeffrey, committee member; Jathar, Shantanu, committee memberConvectively generated cold pools can have myriad impacts on local aerosol concentrations. Passage of cold pools may loft dust, pollen or other aerosols from the surface, and precipitation and humidity changes accompanying cold pools also impact local aerosols in several ways. The vertical profile of aerosols can have important effects on meteorology, however, the effects of cold pools on the vertical distribution of aerosol are largely unstudied. During the BioAerosol and Convective Storms (BACS) field campaigns in the Colorado plains in spring of 2022 and 2023, Uncrewed Aircraft (UA) were utilized to observe the vertical profile of aerosol, and how this vertical profile may be affected by the passage of cold pools. UAs with mounted aerosol and meteorological instrument packages were deployed in a vertical column to profile different atmospheric variables. Flights were conducted before, during, and after the passage of cold pools, and UA data were contextualized using radiosonde measurements and surface-based aerosol and meteorological instruments. A discussion of the challenges of UA-mounted aerosol sampling is presented. Validation experiments were conducted to assess the reliability of UA-mounted Optical Particle Counters (OPCs), and analyzed to show that UA-mounted OPCs can provide reliable data under certain circumstances. Two primary issues are discussed in detail: sensor drift and suppressed OPC sampling flow. A calibration procedure was developed and utilized to address the issue of sensor drift, while suppressed OPC sample flow was addressed by removing all data below a determined critical threshold flow rate. These methodologies lead to the creation of a robust data product for the measurement of aerosol vertical profiles using UA-mounted OPCs. Using these OPC data, an analysis of the vertical profiles observed during the BACS campaign is provided, up to 350m above the surface. We find that a common feature of a post cold pool environment is a layer of enhanced submicron aerosol concentration measured 120m above the surface. This feature and its evolution are examined in detail for several case studies, and different possible explanations are presented. Potential causes of this observed feature include pollen-rupture, low temperature inversions trapping aerosol in a low stable layer of elevated aerosol concentration, and emission and/or deposition of aerosols, but these explanations each appear to be insufficient. This feature appears to be caused by the dynamics of the cold pool, which can entrain and redistribute airmasses from different levels of the atmosphere.Item Open Access Model code for "Constraining nucleation, condensation, and chemistry in oxidation flow reactors using size-distribution measurements and aerosol microphysical modelling"(Colorado State University. Libraries, 2018-08) Hodshire, Anna; Bian, Qijing; Jimenez, Jose; Pierce, JeffreyOxidation flow reactors (OFRs) allow the concentration of a given atmospheric oxidant to be increased beyond ambient levels in order to study secondary organic aerosol (SOA) formation and aging over varying periods of equivalent aging by that oxidant. Previous studies have used these reactors to determine the bulk OA mass and chemical evolution. To our knowledge, no OFR study has focused on the interpretation of the evolving aerosol size distributions. In this study, we use size distribution measurements of the OFR and an aerosol microphysics model to learn about size-dependent processes in the OFR. Specifically, we use OFR exposures between 0.09–0.9 equivalent days of OH aging from the 2011 BEACHON-RoMBAS and the GoAmazon2014/5 field campaigns. We use simulations in the TOMAS (TwO-Moment Aerosol Sectional) microphysics box model to constrain the following parameters in the OFR: (1) the rate constant of gas-phase functionalization reactions of organic compounds with OH, (2) the rate constant of gas-phase fragmentation reactions of organic compounds with OH, (3) the reactive uptake coefficient for heterogeneous fragmentation reactions with OH, (4) the nucleation rate constants for three different nucleation schemes, and (5) an effective accommodation coefficient that accounts for possible particle diffusion limitations of particles larger than 60nm in diameter. We find the best model-to-measurement agreement when the accommodation coefficient of the larger particles (Dp>60nm) was 0.1 or lower (with an accommodation coefficient of 1 for smaller particles), which suggests a diffusion limitation in the larger particles. When using these low accommodation-coefficient values, the model agrees with measurements when using a published H2SO4-organics nucleation mechanism and previously published values of rate constants for gas-phase oxidation reactions. Further, gas-phase fragmentation was found to have a significant impact upon the size distribution, and including fragmentation was necessary for accurately simulating the distributions in the OFR. The model was insensitive to the value of the reactive uptake coefficient on these aging timescales. Monoterpenes and isoprene could explain 24–95% of the observed change in total volume of aerosol in the OFR, with ambient semivolatile and intermediate-volatility organic compounds (S/IVOCs) appearing to explain the remainder of the change in total volume. These results provide support to the mass-based findings of previous OFR studies, give insight to important size-distribution dynamics in the OFR, and enable the design of future OFR studies focused on new particle formation and/or microphysical processes.Item Open Access Potential indirect effects of aerosol on tropical cyclone development(Colorado State University. Libraries, 2010) Krall, Geoffrey Michael, author; Cotton, William R., advisor; van den Heever, Susan C., committee member; Eykholt, Richard Eric, 1956-, committee memberObservational and model evidence suggest that a 2008 Western Pacific typhoon (NURI) came into contact with and ingested elevated concentrations of aerosol as it neared the Chinese coast. This study uses a regional model with two-moment bin emulating microphysics to simulate the typhoon as it enters the field of elevated aerosol concentration. A continental field of cloud condensation nuclei (CCN) was prescribed based on satellite and global aerosol model output, then increased for further sensitivity tests. The typhoon was simulated for 96 hours beginning 17 August 2008, the final 60 of which were under varying CCN concentrations as it neared the Philippines and coastal China. The model was initialized with both global reanalysis model data and irregularly spaced dropsonde data from a 2008 observational campaign using an objective analysis routine. At 36 hours, the internal nudging of the model was switched off and allowed to evolve on its own. As the typhoon entered the field of elevated CCN in the sensitivity tests, the presence of additional CCN resulted in a significant perturbation of windspeed, convective fluxes, and hydrometeor species behavior. Initially ingested in the outer rainbands of the storm, the additional CCN resulted in an initial damping and subsequent invigoration of convection. The increase in convective fluxes strongly lag-correlates with increased amounts of supercooled liquid water within the storm domain. As the convection intensified in the outer rainbands the storm drifted over the developing cold-pools, affecting the inflow of air into the convective towers of the typhoon. Changes in the timing and amount of rain produced in each simulation resulted in differing cold-pool strengths and size. The presence of additional CCN increased resulted in an amplification of convection within the storm, except for the extremely high CCN concentration simulation, which showed a damped convection due to the advection of pristine ice away from the storm. This study examines the physical mechanisms that could potentially alter a tropical cyclone (TC) in intensity and dynamics upon ingesting elevated levels of CCN.Item Open Access Regional aerosol effects on precipitation: an observational study(Colorado State University. Libraries, 2011) Boyd, Kathryn J., author; Kummerow, Christian, advisor; van den Heever, Susan, committee member; Reising, Steven, committee memberThere have been a multitude of studies on the effects increased amounts of aerosols may have on clouds. The connection between increased cloud condensation nuclei (CCN) and cloud microphysics has been established by in situ observations as well as modeling studies. However, the impact on precipitation is less well established. Of the studies that have assessed aerosol effects on precipitation most have been limited to modeling studies or global studies using satellite data. The few observational studies that have examined these relationships have been mainly limited to data from short-lived field campaign, such as oceanic stratocumulus decks or biomass burning areas. This study attempts to examine regional aerosol effects on precipitation in areas not previously examined in field campaigns, using data from two different sites, one from an Atmospheric Radiation Measurement (ARM) Program permanent facility in Oklahoma and the other from a mobile facility located in the Azores. These two sites were chosen in order to illustrate the differences between a marine and a continental location. Meteorological conditions were taken into account in both locations through surface and sounding data and trends in precipitation were found with increasing aerosol concentrations. The marine site witnessed a suppression of precipitation, consistent with past studies and proposed theories of aerosol effects. This was not true for clouds with liquid water paths exceeding 200g/m2. These clouds appear to contain sufficient amounts of water to overcome the aerosol effect. The continental site, however, experienced an opposite trend, with enhancement of precipitation witnessed in all clouds examined in this study. This is thought to be due to a buffering mechanism in these types of clouds, as introduced by Stevens and Feingold (2009). Results were separated by season and cloud type using the horizontal variability of radar reflectivity at cloud top height. The seasonal results generally either were in line with the year round results or were too noisy to interpret. The results separated by cloud type give a concrete result, illustrating the fact that differing cloud dynamics may lead to opposing trends in precipitation with increasing aerosols. Competing effects of aerosols within clouds appear to dampen any effect on precipitation to the point that it is not detectable from the in-situ observations considered here.Item Open Access Remote continental aerosol characteristics in the Rocky Mountains of Colorado and Wyoming(Colorado State University. Libraries, 2013) Levin, Ezra JT, author; Kreidenweis, Sonia M., advisor; Collett, Jeffrey L., committee member; van den Heever, Susan C., committee member; Ham, Jay, committee memberThe Rocky Mountains of Colorado and Wyoming enjoy some of the cleanest air in the United States, with few local sources of particulate matter or its precursors apart from fire emissions, windblown dust, and biogenic emissions. However, anthropogenic influences are also present with sources as diverse as the populated Front Range, large isolated power plants, agricultural emissions, and more recently emissions from increased oil and gas exploration and production. While long-term data exist on the bulk composition of background fine particulate matter at remote sites in the region, few long-term observations exist of aerosol size distributions, number concentrations and size resolved composition, although these characteristics are closely tied to important water resource issues through the potential aerosol impacts on clouds and precipitation. Recent modeling work suggests sensitivity of precipitation-producing systems to the availability of aerosols capable of serving as cloud condensation nuclei (CCN); however, model inputs for these aerosols are not well constrained due to the scarcity of data. In this work I present aerosol number and volume concentrations, size distributions, chemical composition and hygroscopicity measurements from long-term field campaigns. I also explore the volatility of organic material from biomass burning and the potential impacts on aerosol loading. Relevant aerosol observations were obtained in several long-term field studies: the Rocky Mountain Atmospheric Nitrogen and Sulfur study (RoMANS, Colorado), the Grand Tetons Reactive Nitrogen Deposition Study (GrandTReNDS, Wyoming) and as part of the Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics & Nitrogen project (BEACHON, Colorado). Average number concentrations (0.04 < Dp < 20 μm) measured during the field studies ranged between 1000 - 2000 cm-3 during the summer months and decreased to 200 - 500 cm-3 during the winter. These seasonal changes in aerosol number concentrations were correlated with the frequency of events typical of new particle formation. Measured sub-micron organic mass fractions were between 70 - 90% during the summer months, when new particle formation events were most frequent, suggesting the importance of organic species in the nucleation or growth process, or both. Aerosol composition derived from hygroscopicity measurements indicate organic mass fractions of 50 - 60% for particles with diameters larger than 0.15 μm during the winter. The composition of smaller diameter particles appeared to be organic dominated year-round. High organic mass fractions led to low values of aerosol hygroscopicity, described using the κ parameter. Over the entire year-long BEACHON study, κ had an average value of 0.16 ± 0.08, similar to values determined during biologically active periods in tropical and boreal forests, and lower than the commonly assumed value of κcontinental = 0.3. There was also an observed increase in κ with size, due to external mixing of the fine mode aerosol. Incorrect representations of κ or its size dependence led to erroneous values of calculated CCN concentrations, especially for supersaturation values less than 0.3%. At higher supersaturations, most of the measured variability in CCN concentrations was captured by changes in total measured aerosol number concentrations. While data from the three measurement sites were generally well correlated, indicating similarities in seasonal cycles and in total number concentrations, there were some variations between measurements made at different sites and during different years that may be partly due to the effects of local emissions. The averaged data provide reasonable, observationally-based parameters for modeling of aerosol number size distributions and corresponding CCN concentrations. Field observations clearly indicated the episodic influence of wildfire smoke on particle number concentrations and compositions. However, the semi-volatile nature of the organic carbon species emitted makes it difficult to predict how much of the emitted organic mass will remain in the condensed phase downwind. To better constrain the volatility of organic species in smoke, emissions from laboratory biomass combustion experiments were subjected to quantified dilution, resulting in reduction of aerosol mass concentrations over several orders of magnitude and a corresponding volatilization response of the organic particles that was fit to the commonly-applied Volatility Basis Set. Organic emissions from all burns with initial organic aerosol concentrations greater than 1000 μg m-3 contained material with saturation concentration values ranging between 1 and 10,000 μg m-3, with most of the organic mass falling at the two extremes of this range. For most burns, a single distribution was able to capture the volatility behavior of the organic material, within experimental uncertainty, despite the considerable variability in fuel and fire characteristics, suggesting that a simplified two-product model of gas-aerosol partitioning may be adequate to describe the evolution of biomass burning organic aerosol in models.Item Open Access Simulating southwestern U.S. desert dust influences on severe, tornadic storms(Colorado State University. Libraries, 2012) Lerach, David Gregory, author; Cotton, William R., advisor; Rutledge, Steven A., committee member; Kreidenweis, Sonia M., committee member; Roesner, Larry A., committee memberIn this study, three-dimensional numerical simulations were performed using the Regional Atmospheric Modeling System (RAMS) model to investigate possible southwestern U.S. desert dust impacts on severe, tornadic storms. Initially, two sets of simulations were conducted for an idealized supercell thunderstorm. In the first set, two numerical simulations were performed to assess the impacts of increased aerosol concentrations acting as cloud condensation nuclei (CCN) and giant CCN (GCCN). Initial profiles of CCN and GCCN concentrations were set to represent "clean" continental and aerosol-polluted environments, respectively. With a reduction in warm- and cold-rain processes, the polluted environment produced a longer-lived supercell with a well-defined rear flank downdraft (RFD) and relatively weak forward flank downdraft (FFD) that produced weak evaporative cooling, a weak cold-pool, and an EF-1 tornado. The clean environment produced no tornado and was less favorable for tornadogenesis. In the second ensemble, aerosol microphysical effects were put into context with those of convective available potential energy (CAPE) and low-level moisture. Simulations initialized with greater low-level moisture and higher CAPE produced significantly stronger precipitation, which resulted in greater evaporation and associated cooling, thus producing stronger cold-pools at the surface associated with both the forward- and rear-flank downdrafts. Simulations initialized with higher CCN concentrations resulted in reduced warm rain and more supercooled water aloft, creating larger anvils with less ice mass available for precipitation. These simulated supercells underwent less evaporative cooling within downdrafts and produced weaker cold-pools compared to the lower CCN simulations. Tornadogenesis was related to the size, strength, and location of the FFD- and RFD-based cold-pools. The combined influence of low-level moisture and CAPE played a considerably larger role on tornadogenesis compared to aerosol impacts. However, the aerosol effect was still evident. In both idealized model ensembles, the strongest, longest-lived tornado-like vortices were associated with warmer and weaker cold-pools, higher CAPE, and less negative buoyancy in the near-vortex environment compared to those storms that produced shorter-lived, weaker vortices. A final set of nested grid simulations were performed to evaluate dust indirect microphysical and direct radiative impacts on a severe storms outbreak that occurred during 15-16 April 2003 in Texas and Oklahoma. In one simulation, neither dust microphysical nor radiative effects were included (CTL). In a second simulation, only dust radiative effects were considered (RAD). In a third simulation, both dust radiative and indirect microphysical effects were simulated (DST), where dust was allowed to serve as CCN, GCCN, and ice nuclei (IN). Fine mode dust serving as CCN reduced warm rain formation in the DST simulation. Thus, cloud droplets were transported into the mixed phase region, enhancing freezing, aggregation, and graupel and hail production. However, graupel and hail were of smaller sizes in the DST simulation due to reduced riming efficiencies. Dust particles serving as GCCN and IN played secondary roles, as these impacts were offset by other processes. The DST simulation yielded the lowest rainfall rates and accumulated precipitation, as much of the total water mass within the convective cells were in the form of aggregates and small graupel particles that were transported into the anvil region rather than falling as precipitation. The combined effects of warm rain efficiency, ice production, and hydrometeor size controlled the evolution of cold-pools and storm structure. The RAD and CTL simulations produced widespread cold-pools, which hindered the formation of long-lived supercells relative to the DST simulation. The DST convective line was associated with reduced rainfall and multiple long-lived supercells. Comparisons between the RAD and CTL simulations revealed that dust radiative influences played an important role in convective initiation. The increased absorption of solar radiation within the dust plume in the RAD simulation warmed the dust layer over time, which reduced the amount of radiation that reached the surface, resulting in slight cooling at the surface and increased atmospheric stability within the lowest 2 km. Dew points at low levels were slightly lower in the RAD simulation, due to reduced surface water vapor fluxes (latent heat fluxes) below the dust plume. With the presence of a stronger capping inversion but more available low-level moisture, the CTL simulation initially produced more widespread convection and precipitation, while the RAD simulation produced the strongest convective cores, including a long-lived supercell. The results from all three sets of simulations suggest that dust indirect microphysical and direct radiative impacts on severe convection may at times greatly influence the development of severe storms. In this study, dust often increased the potential for tornadogenesis. Additional modeling studies at horizontal grid spacing ≤100 m are needed in order to address the robustness of these results and better isolate potential dust influences on severe storms and tornadogenesis.Item Open Access TOMAS-VBS model for "More than emissions and chemistry: fire size, dilution, and background aerosol also greatly influence near-field biomass burning aerosol aging"(Colorado State University. Libraries, 2019) Hodshire, Anna; Pierce, Jeffrey; Bian, QijingBiomass burning emits particles (black carbon and primary organic aerosol; POA) and precursor vapors to the atmosphere that chemically and physically age in the atmosphere. This theoretical study explores the relationships between fire size (determining the initial plume width and concentration), dilution rate, and entrainment of background aerosol on particle coagulation, OA evaporation, and secondary organic aerosol (SOA) condensation in smoke plumes. We examine the impacts of these processes on aged smoke OA mass, geometric mean diameter (Dg), peak lognormal modal width (σg), particle extinction (E), and cloud condensation nuclei (CCN) concentrations. In our simulations, aging OA mass is controlled by competition between OA evaporation and SOA condensation. Large, slowly diluting plumes evaporate little in our base set of simulations, which may allow for net increases in mass, E, CCN, and Dg from SOA condensation. Smaller, quickly diluting fire plumes lead to faster evaporation, which favors decreases in mass, E, CCN, and Dg. However, the SOA fraction of the smoke OA increases more rapidly in smaller fires due to faster POA evaporation leading to more SOA precursors. Net mass changes for smaller fires depend on background OA concentrations; increasing background aerosol concentrations decrease evaporation rates. Although coagulation does not change mass, it can decrease the number of particles in large/slowly diluting plumes, increasing Dg and E, and decreasing σg. While our conclusions are limited by being a theoretical study, we hope they help motivate future smoke-plume analyses to consider the effects of fire size, meteorology, and background OA concentrations.