VTechWorks Repository :: Browsing by Author "Isaacman-VanWertz, Gabriel"

Browsing by Author "Isaacman-VanWertz, Gabriel"

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An Analysis of Short-Term Load Forecasting on Residential Buildings Using Deep Learning Models
Suresh, Sreerag (Virginia Tech, 2020-07-07)
Building energy load forecasting is becoming an increasingly important task with the rapid deployment of smart homes, integration of renewables into the grid and the advent of decentralized energy systems. Residential load forecasting has been a challenging task since the residential load is highly stochastic. Deep learning models have showed tremendous promise in the fields of time-series and sequential data and have been successfully used in the field of short-term load forecasting at the building level. Although, other studies have looked at using deep learning models for building energy forecasting, most of those studies have looked at limited number of homes or an aggregate load of a collection of homes. This study aims to address this gap and serve as an investigation on selecting the better deep learning model architecture for short term load forecasting on 3 communities of residential buildings. The deep learning models CNN and LSTM have been used in the study. For 15-min ahead forecasting for a collection of homes it was found that homes with a higher variance were better predicted by using CNN models and LSTM showed better performance for homes with lower variances. The effect of adding weather variables on 24-hour ahead forecasting was studied and it was observed that adding weather parameters did not show an improvement in forecasting performance. In all the homes, deep learning models are shown to outperform the simple ANN model.
Atmospheric Impact of Biogenic Volatile Organic Compounds: Improving Measurement and Modeling Capabilities
Panji, Namrata Shanmukh (Virginia Tech, 2024-08-23)
Biogenic volatile organic compounds (BVOCs) are naturally occurring organic compounds emitted by plants, trees, and ecosystems, exerting a profound influence on the Earth's atmosphere, air quality, climate, and ecosystem dynamics. This research project aims to advance our understanding of BVOC emissions and their implications through a comprehensive and multi-faceted investigation. We investigate the dynamics of BVOCs in the atmosphere through three key objectives. First, we introduce a novel enriching inlet that uses selective permeation to preconcentrate reactive organic gases in small sample flows for atmospheric gas sampling, enhancing the sensitivity and detection limits of analytical instruments. Enrichments between 4640% and 111% were measured for major reactive atmospheric gases at ultra low flow rates and roughly several hundred percent for ambient samples at moderately low flow rates. Second, we constrain light-dependency in BVOC emissions models by comparing modeled and long-term observed BVOC concentrations measured at a mid-canopy monitoring site in a southeastern US forest. The Model of Emissions of Gases and Aerosols from Nature (MEGAN) and the Framework for 0-D Atmospheric Modeling (F0AM) were utilized to simulate emissions and chemical transformations, respectively to disentangle the time- and species-specificity of light dependency for various BVOC (α-pinene, camphene, and α-fenchene are completely light-independent and limonene, β-thujene, sabinene, and γ-terpinene are seasonally light-dependent). Finally, we examine these models deeper to investigate uncertainties and highlight current limitations due to variability in planetary boundary layer height (PBLH) datasets. We highlight the significance of simultaneous PBLH and BVOC measurements for improving the accuracy of BVOC concentration models. We show that a lack of co-located measurements is a large source of uncertainty in modeling BVOC concentrations. The successful completion of these objectives contributes to a better understanding of the complex interactions between BVOC emissions and atmospheric chemistry.
An autonomous remotely operated gas chromatograph for chemically resolved monitoring of atmospheric volatile organic compounds
McGlynn, Deborah F.; Panji, Namrata Shanmukh; Frazier, Graham; Bi, Chenyang; Isaacman-VanWertz, Gabriel (Royal Society Chemistry, 2023-01)
Volatile organic compounds (VOCs) range in their reaction rates with atmospheric oxidants by several orders of magnitude. Therefore, studying their atmospheric concentrations across seasons and years requires isomer resolution to fully understand their impact on oxidant budgets and secondary organic aerosol formation. An automated gas chromatograph/flame ionization detector (GC-FID) was developed for hourly sampling and analysis of C-5-C-15 hydrocarbons at remote locations. Samples are collected on an air-cooled multibed adsorbent trap for preconcentration of hydrocarbons in the target volatility range, specifically designed to minimize dead volume and enable rapid heating and sample flushing. Instrument control uses custom electronics designed to allow flexible autonomous operation at moderate cost, with automated data transfer and processing. The instrument has been deployed for over two years with samples collected mid-canopy from the Virginia Forest Laboratory located in the Pace research forest in central Virginia. We present here the design of the instrument itself, control electronics, and calibration and data analysis approaches to facilitate the development of similar systems by the atmospheric chemistry community. Detection limits of all species are in the range of a few to tens of ppt and the instrument is suitable for detection of a wide range of biogenic, lightly oxygenated, and anthropogenic (predominantly hydrocarbon) compounds. Data from calibrations are examined to provide understanding of instrument stability and quantify uncertainty. In this work, we present challenges and recommendations for future deployments, as well as suggested adaptions to decrease required maintenance and increase instrument up-time. The presented design is particularly suitable for long-term and remote deployment campaigns where access, maintenance, and transport of materials are difficult.
Composition, concentration, and oxidant reactivity of sesquiterpenes in the southeastern US
Frazier, Graham; McGlynn, Deborah F.; Barry, Laura E.; Lerdau, Manuel; Pusede, Sally E.; Isaacman-VanWertz, Gabriel (Royal Society of Chemistry, 2022-09-15)
Biogenic volatile organic compounds (BVOCs) contribute the majority of reactive organic carbon to the atmosphere and lead to aerosol formation through reaction with atmospheric oxidants including ozone and hydroxyl radicals. One class of BVOCs, sesquiterpenes, have a high reactivity with ozone but exist at lower concentrations compared to other BVOCs, and there are relatively few measurements of their concentrations in different environments or their importance in the atmospheric oxidant budget. To help close this knowledge gap, we examine concentrations of isomer-resolved sesquiterpene concentrations collected hourly at two sites in Virginia that are representative of different ecosystems in the southeastern US. Sesquiterpene concentrations are presented and discussed in relation to their diurnal patterns and used to estimate their contribution to reactivity with common gas-phase oxidants. Twenty-four sesquiterpenes were identified at the sites, eleven of which were observed at both sites. Total sesquiterpene concentrations were found to range between 0.8 and 2 ppt with no single isomer dominating throughout. Hydroxyl activity is similarly diverse, with no particular isomer dominating activity at either site. Ozone reactivity, however, was found to be dominated (similar to 3/4 total reactivity) by beta-caryophyllene and humulene despite these compounds representing roughly only 10% of total sesquiterpene mass, highlighting their importance as the major driver of sesquiterpene-ozone reactivity. Average reaction rate constants for sesquiterpenes with ozone and hydroxyl radicals were calculated for both sites as a method to simplify future atmospheric modelling concerning sesquiterpenes. This work provides broad insight into the composition and impacts of sesquiterpenes, suggesting that sesquiterpene composition is relatively similar between sites. Furthermore, while the calculated average sesquiterpene-ozone reaction rate constants are at least an order of magnitude higher than that of more prevalent BVOC classes (isoprene and monoterpenes), their low concentrations suggest their impacts on atmospheric reactivity are expected to be limited to periods of high emissions.
Comprehensive detection of analytes in large chromatographic datasets by coupling factor analysis with a decision tree
Kim, Sungwoo; Lerner, Brian M.; Sueper, Donna T.; Isaacman-VanWertz, Gabriel (Copernicus, 2022-09-05)
Environmental samples typically contain hundreds or thousands of unique organic compounds, and even minor components may provide valuable insight into their sources and transformations. To understand atmospheric processes, individual components are frequently identified and quantified using gas chromatography-mass spectrometry. However, due to the complexity and frequently variable nature of such data, data reduction is a significant bottleneck in analysis. Consequently, only a subset of known analytes is often reported for a dataset, and large amounts of potentially useful data are discarded. We present an automated approach of cataloging and potentially identifying all analytes in a large chromatographic dataset and demonstrate the utility of our approach in an analysis of ambient aerosols. We use a coupled factor analysis-decision tree approach to deconvolute peaks and comprehensively catalog nearly all analytes in a dataset. Positive matrix factorization (PMF) of small subsections of multiple chromatograms is applied to extract factors that represent chromatographic profiles and mass spectra of potential analytes, in which peaks are detected. A decision tree based on peak parameters (e.g., location, width, and height), relative ratios of those parameters, peak shape, noise, retention time, and mass spectrum is applied to discard erroneous peaks and combine peaks determined to represent the same analyte. With our approach, all analytes within the small section of the chromatogram are cataloged, and the process is repeated for overlapping sections across the chromatogram, generating a complete list of the retention times and estimated mass spectra of all peaks in a dataset. We validate this approach using samples of known compounds and demonstrate the separation of poorly resolved peaks with similar mass spectra and the resolution of peaks that appear in only a fraction of chromatograms. As a case study, this method is applied to a complex real-world dataset of the composition of atmospheric particles, in which more than 1100 unique chromatographic peaks are resolved, and the corresponding peak information along with mass spectra are cataloged.
Contributions of biomass-burning, urban, and biogenic emissions to the concentrations and light-absorbing properties of particulate matter in central Amazonia during the dry season
de Sa, Suzane S.; Rizzo, Luciana V.; Palm, Brett B.; Campuzano-Jost, Pedro; Day, Douglas A.; Yee, Lindsay D.; Wernis, Rebecca A.; Isaacman-VanWertz, Gabriel; Brito, Joel; Carbone, Samara; Liu, Yingjun J.; Sedlacek, Arthur; Springston, Stephen R.; Goldstein, Allen H.; Barbosa, Henrique M. J.; Alexander, M. Lizabeth; Artaxo, Paulo; Jimenez, Jose L.; Martin, Scot T. (European Geophysical Union, 2019-06-18)
Urbanization and deforestation have important impacts on atmospheric particulate matter (PM) over Amazonia. This study presents observations and analysis of PM1 concentration, composition, and optical properties in central Amazonia during the dry season, focusing on the anthropogenic impacts. The primary study site was located 70 km downwind of Manaus, a city of over 2 million people in Brazil, as part of the GoAmazon2014/5 experiment. A high-resolution time-of-flight aerosol mass spectrometer (AMS) provided data on PM1 composition, and aethalometer measurements were used to derive the absorption coefficient b(abs,BrC) of brown carbon (BrC) at 370 nm. Non-refractory PM1 mass concentrations averaged 12.2 mu g m(-3) at the primary study site, dominated by organics (83 %), followed by sulfate (11 %). A decrease in b(abs,BrC) was observed as the mass concentration of nitrogen-containing organic compounds decreased and the organic PM1 O : C ratio increased, suggesting atmospheric bleaching of the BrC components. The organic PM1 was separated into six different classes by positive-matrix factorization (PMF), and the mass absorption efficiency E-abs associated with each factor was estimated through multivariate linear regression of b(abs,BrC) on the factor loadings. The largest E-abs values were associated with urban (2.04 +/- 0.14 m(2) g(-1)) and biomass-burning (0.82 +/- 0.04 to 1.50 +/- 0.07 m(2)g(-1)) sources. Together, these sources contributed at least 80 % of b(abs,BrC) while accounting for 30 % to 40 % of the organic PM1 mass concentration. In addition, a comparison of organic PM1 composition between wet and dry seasons revealed that only part of the 9-fold increase in mass concentration between the seasons can be attributed to biomass burning. Biomass-burning factor loadings increased by 30-fold, elevating its relative contribution to organic PM1 from about 10 % in the wet season to 30 % in the dry season. However, most of the PM1 mass (> 60 %) in both seasons was accounted for by biogenic secondary organic sources, which in turn showed an 8-fold seasonal increase in factor loadings. A combination of decreased wet deposition and increased emissions and oxidant concentrations, as well as a positive feedback on larger mass concentrations are thought to play a role in the observed increases. Furthermore, fuzzy c-means clustering identified three clusters, namely "baseline", "event", and "urban" to represent different pollution influences during the dry season. The baseline cluster, representing the dry season background, was associated with a mean mass concentration of 9 +/- 3 mu g m(-3). This concentration increased on average by 3 mu g m(-3) for both the urban and the event clusters. The event cluster, representing an increased influence of biomass burning and long-range transport of African volcanic emissions, was characterized by remarkably high sulfate concentrations. The urban cluster, representing the influence of Manaus emissions on top of the baseline, was characterized by an organic PM1 composition that differed from the other two clusters. The differences discussed suggest a shift in oxidation pathways as well as an accelerated oxidation cycle due to urban emissions, in agreement with findings for the wet season.
Correcting bias in log-linear instrument calibrations in the context of chemical ionization mass spectrometry
Bi, Chenyang; Krechmer, Jordan E.; Canagaratna, Manjula R.; Isaacman-VanWertz, Gabriel (2021-10-11)
Quantitative calibration of analytes using chemical ionization mass spectrometers (CIMSs) has been hindered by the lack of commercially available standards of atmospheric oxidation products. To accurately calibrate analytes without standards, techniques have been recently developed to log-linearly correlate analyte sensitivity with instrument operating conditions. However, there is an inherent bias when applying log-linear calibration relationships that is typically ignored. In this study, we examine the bias in a log-linear-based calibration curve based on prior mathematical work. We quantify the potential bias within the context of a CIMS-relevant relationship between analyte sensitivity and instrument voltage differentials. Uncertainty in three parameters has the potential to contribute to the bias, specifically the inherent extent to which the nominal relationship can capture true sensitivity, the slope of the relationship, and the voltage differential below which maximum sensitivity is achieved. Using a prior published case study, we estimate an average bias of 30 %, with 1 order of magnitude for less sensitive compounds in some circumstances. A parameter-explicit solution is proposed in this work for completely removing the inherent bias generated in the log-linear calibration relationships. A simplified correction method is also suggested for cases where a comprehensive bias correction is not possible due to unknown uncertainties of calibration parameters, which is shown to eliminate the bias on average but not for each individual compound.
Coupling a gas chromatograph simultaneously to a flame ionization detector and chemical ionization mass spectrometer for isomer-resolved measurements of particle-phase organic compounds
Bi, Chenyang; Krechmer, Jordan E.; Frazier, Graham O.; Xu, Wen; Lambe, Andrew T.; Claflin, Megan S.; Lerner, Brian M.; Jayne, John T.; Worsnop, Douglas R.; Canagaratna, Manjula R.; Isaacman-VanWertz, Gabriel (2021-05-27)
Atmospheric oxidation products of volatile organic compounds consist of thousands of unique chemicals that have distinctly different physical and chemical properties depending on their detailed structures and functional groups. Measurement techniques that can achieve molecular characterizations with details down to functional groups (i.e., isomer-resolved resolution) are consequently necessary to provide understandings of differences of fate and transport within isomers produced in the oxidation process. We demonstrate a new instrument coupling the thermal desorption aerosol gas chromatograph (TAG), which enables the separation of isomers, with the high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS), which has the capability of classifying unknown compounds by their molecular formulas, and the flame ionization detector (FID), which provides a near-universal response to organic compounds. The TAG-CIMS/FID is used to provide isomer-resolved measurements of samples from liquid standard injections and particle-phase organics generated in oxidation flow reactors. By coupling a TAG to a CIMS, the CIMS is enhanced with an additional dimension of information (resolution of individual molecules) at the cost of time resolution (i.e., one sample per hour instead of per minute). We found that isomers are prevalent in sample matrix with an average number of three to five isomers per formula depending on the precursors in the oxidation experiments. Additionally, a multi-reagent ionization mode is investigated in which both zero air and iodide are introduced as reagent ions, to examine the feasibility of extending the use of an individual CIMS to a broader range of analytes with still selective reagent ions. While this approach reduces iodide-adduct ions by a factor of 2, [M - H](-) and [M + O-2](-) ions produced from lower-polarity compounds increase by a factor of 5 to 10, improving their detection by CIMS. The method expands the range of detected chemical species by using two chemical ionization reagents simultaneously, which is enabled by the pre-separation of analyte molecules before ionization.
Coupling of organic and inorganic aerosol systems and the effect on gas-particle partitioning in the southeastern US
Pye, Havala O. T.; Zuend, Andreas; Fry, Juliane L.; Isaacman-VanWertz, Gabriel; Capps, Shannon L.; Appel, K. Wyat; Foroutan, Hosein; Xu, Lu; Ng, Nga L.; Goldstein, Allen H. (European Geophysical Union, 2018-01-12)
Several models were used to describe the partitioning of ammonia, water, and organic compounds between the gas and particle phases for conditions in the southeastern US during summer 2013. Existing equilibrium models and frameworks were found to be sufficient, although additional improvements in terms of estimating pure-species vapor pressures are needed. Thermodynamic model predictions were consistent, to first order, with a molar ratio of ammonium to sulfate of approximately 1.6 to 1.8 (ratio of ammonium to 2 x sulfate, R-N/2S approximate to 0.8 to 0.9) with approximately 70% of total ammonia and ammonium (NHx) in the particle. Southeastern Aerosol Research and Characterization Network (SEARCH) gas and aerosol and Southern Oxidant and Aerosol Study (SOAS) Monitor for AeRosols and Gases in Ambient air (MARGA) aerosol measurements were consistent with these conditions. CMAQv5.2 regional chemical transport model predictions did not reflect these conditions due to a factor of 3 overestimate of the nonvolatile cations. In addition, gas-phase ammonia was overestimated in the CMAQ model leading to an even lower fraction of total ammonia in the particle. Chemical Speciation Network (CSN) and aerosol mass spectrometer (AMS) measurements indicated less ammonium per sulfate than SEARCH and MARGA measurements and were inconsistent with thermodynamic model predictions. Organic compounds were predicted to be present to some extent in the same phase as inorganic constituents, modifying their activity and resulting in a decrease in [H+](air) (H+ in mu g m(-3) air), increase in ammonia partitioning to the gas phase, and increase in pH compared to complete organic vs. inorganic liquid-liquid phase separation. In addition, accounting for nonideal mixing modified the pH such that a fully interactive inorganic-organic system had a pH roughly 0.7 units higher than predicted using traditional methods (pH = 1.5 vs. 0.7). Particle-phase interactions of organic and inorganic compounds were found to increase partitioning towards the particle phase (vs. gas phase) for highly oxygenated (O : C >= 0.6) compounds including several isoprene-derived tracers as well as levoglucosan but decrease particle-phase partitioning for low O : C, monoterpene-derived species.
Enabling Routine Chemical Composition and Volatility Distribution Measurements of Aerosols
Kumar, Purushottam (Virginia Tech, 2025-01-09)
Traditional online measurements of the chemical composition and other physicochemical properties (such as volatility and oxygenation) of particulate matter have relied on expensive and complex research-grade instrumentation based on mass spectrometry and/or chromatography. However, routine monitoring requires lower-cost alternatives that can be operated autonomously, and such tools are lacking. Routine monitoring of particulate matter, especially organic aerosol, relies instead on offline techniques such as filter collection that require significant operator effort. To address this gap, first, we built a new online semi-continuous aerosol chemical composition monitor, the "ChemSpot", that provides information on volatility-resolved organic carbon and degree of oxygenation along with sulfur content at relatively moderate costs. Autonomous operation of the ChemSpot instrument was demonstrated for four weeks alongside a mass spectrometer (an Aerosol Chemical Speciation Monitor, or ACSM), and the results of the comparison were encouraging. Mean absolute percentage errors (MAPE) were estimated to be 21% and 27% for aerosol organic carbon and equivalent sulfate (equivalent amount of sulfate for ChemSpot measured sulfur content). Chemspot-measured oxygen-to-carbon ratio (O:C) compared well with ACSM-measured O:C for moderate aerosol loadings. Second, we extended the capability of the ChemSpot instrument to provide volatility distributions of organic aerosols. A thermogram-based method was developed for the ChemSpot for volatility calibration and the calculation of volatility distributions. This work also highlighted the need for better observational constraints on vapor pressure values from structure-activity relationship based models. Finally, the ChemSpot was deployed at a biomass-burning experiment (Georgia Wildfire Simulation Experiment, G-WISE) to show the utility of this instrument in studying changes in volatility distributions of Biomass Burning Organic Aerosols (BBOA) produced from different biomass fuel types (samples from Blue Ridge and Coastal Plains eco-regions of the state of Georgia), different burn conditions (prescribed burning vs. wild burning) and simulated atmospheric aging. Significant changes in the volatility distributions of organic carbon were observed for the two biomass fuel types studied. Prescribed burning led to the formation of some higher volatility organic compounds in the aerosols compared to the wild burning case. A similar but more pronounced observation of the formation of higher volatility organics was observed after the simulated atmospheric aging of the BBOA samples. The formation of these higher volatility organics could be because of the presence of higher moisture content during the prescribed burning conditions. The successful completion of these objectives provides confidence that the ChemSpot could be a viable tool for long-term data collection of aerosol composition and volatility and in turn advancing aerosol science and helping policymakers devise strategies to curb air pollution.
Estimated timescales for wet deposition of organic compounds as a function of Henry's law constants
Bi, Chenyang; Isaacman-VanWertz, Gabriel (Royal Society Chemistry, 2022-11)
Atmospheric organic compounds may deposit to Earth's surfaces via dry deposition, driven by concentration gradients, and wet deposition, driven by the scavenging of compounds by precipitation. Their removal by deposition has downstream impacts on concentrations of secondary organic aerosol by removing potential aerosol precursors that would otherwise oxidize to form particulate matter. The impact of deposition processes can consequently be considered as competition between rates of oxidation and deposition, but timescales for deposition are not well constrained. While timescales for dry deposition have been estimated and experimentally validated in the past, understanding of wet deposition of organics is still very limited. In this work, we estimate the wet deposition timescale for gas-phase organic compounds in the atmosphere as a function of Henry's law constants, H, using real-world precipitation frequency and size distributions at five globally-distributed sites. The wet deposition timescale decreases significantly with the increase of H until reaching a stable minimum for compounds with H > 10(5) M per atm. We estimate that the median wet deposition timescale for highly soluble gases is approximately 5 hours during a continuous rain event for all sites. However, median estimated timescales ranged from 80 to 200 hours, depending on location. Timescales are found to depend primarily on the frequency and duration of precipitation events rather than their intensity or size characteristics. Based on these data, we demonstrate that timescales for wet deposition of gases can be estimated at any given location using only basic precipitation information, without detailed or high-precision measurements.
Evaluation of Impacts Resulting from Home Heating Oil Tank Discharges
Weiner, Ellen Rebecca (Virginia Tech, 2018-07-25)
Diesel #2 is used to heat nearly 400,000 dwellings in Virginia. Home heating oil released from leaking underground tanks located adjacent to homes and residing in unsaturated soil adjacent to houses poses a potentially serious health risk. Specifically, the migration of hazardous vapors into buildings, known as vapor intrusion, can negatively impact indoor air quality in homes and public buildings (USEPA 2015). In this look-back study, we assessed the potential for petroleum vapor intrusion by sampling soil vapor at 25 previously remediated spill sites. Residual contaminants, in particular total petroleum hydrocarbons (TPH) and naphthalene, were detected in approximately 1/3 of the samples. Concentration levels were correlated to site variables (building type, remediation time, physiographic region) including previous abatement measures. Spill category as assigned by the remediation contractor was investigated in conjunction with these three site variables. Remediation time was the most promising predictive site variable, with visible trends downward in DEQ Category 2 sites with increased remediation time. Higher contaminant concentrations were found near basement-style dwellings, which we hypothesize is due to the wall of the basement blocking horizontal migration of contaminants and the flow of oxygen to the release source zone. We found that many sites exceeded the sub-slab risk target threshold in naphthalene concentration, which has negative implications on previous abatement strategy efficacy.
Hydroxyl Radical Production via Acoustic Cavitation in Ultrasonic Humidifier Systems
Altizer, Chase Duncan (Virginia Tech, 2018-06-12)
Ultrasonic humidifiers use sound vibrations at frequencies higher than can be heard by humans (> 20,000 Hz) to generate aerosolized water also have potential for inducing sonochemical reactions for chemicals present in water. This research focuses on examining oxidants formed within ultrasonic humidifiers, as well as the oxidants effects of contaminants in water used in the systems. Hydroxyl radicals were found using DMPO as a spin trap. Caffeine and 17β-estradiol, as pharmaceutical contaminants of drinking water, were both emitted from the humidifier when present in the water reservoir and would enter breathing air. Emitted 17β-estradiol was found at 60% of the initial concentration filled in the ultrasonic humidifier after 480 minutes. Caffeine exhibited less degradation than 17β-estradiol. Degradation of both pharmaceuticals was attributed to ultrasonic processes, most likely oxidation with hydroxyl radicals produced. Bromide as a contaminant of the fill water was found to remain constant over time.
Instrument development for exploring the influence of interfacial chemistry on aerosol growth, aging, and partitioning of gases
Amick, Cecilia Lynn (Virginia Tech, 2019-12-04)
Investigation of aerosol chemistry and growth under atmospheric conditions in a novel rotating aerosol suspension chamber with cavity ring-down spectroscopy provided key insight into the effect of pollutants and other vapors on the overall atmospheric lifetime of particulate matter. The Atmospheric Cloud Simulation Instrument (ACSI) creates a well-defined and controllable atmosphere of suspended particles, analyte gases, and background gas molecules, which remains stable up to several days. Preliminary studies have shown that monodisperse polystyrene latex (dp = 0.994 µm) and polydisperse ammonium sulfate (CMD dp = 100 nm) particles remain suspended for at least 22 hours while the chamber rotates at 2 RPM. Further investigation into the aerosol dynamics showed the coagulation efficiency of high concentration particle suspensions (>10^6 particles/cm3) depends on particle phase state and composition. The coagulation efficiency decreased with increased humidity in the model atmosphere and with increased ion concentrations in the aerosols. The decrease in efficiency is attributed to repulsive forces from like-charges on the particle surfaces. In addition to humidity, the spectroscopy integrated into the main chamber monitors the real-time response to a perturbation in the model atmosphere, such as the introduction of a gas-phase reactant. Cavity ring-down spectroscopy, performed in situ along the center axis, records mid-infrared spectra (1010 cm-1 to 860 cm-1) to identify gas species evolved from gas-particle heterogeneous chemistry. In accord with previous studies, my results show that a known reaction between monomethyl amine and ammonia occurs readily on suspended ammonium sulfate particles in >50% RH and the extent of the reaction depends on the humidity of the model atmosphere. Acidic ammonium bisulfate aerosols also produced a detectable amount of ammonia upon exposure to monomethyl amine in a model atmosphere with >50% RH. Overall, the new ACSI approach to atmospheric science provides the opportunity to study the influence of interfacial chemistry on particle growth, aging, and re-admission of gas-phase compounds.
Interactions Between Dust and Ecosystem, and Landscape at Multiple Scales
Huang, Xinyue (Virginia Tech, 2024-09-05)
Atmospheric dust is the largest contributor to global aerosols from land. Dust emissions rate and properties are influenced by meteorological conditions, parent soil, and landscape, and in turn, it affects impacts on climate, ecosystems, and human societies through various pathways. This dissertation aims to explore the coupled dynamics of dust particle emissions and their essential properties in relation to topography, ecosystem, and atmospheric conditions by integrating information across multiple scales. Specifically, three research projects are pursued. First, the modulation of dust emissions by non-photosynthetic vegetation (NPV) is evaluated by implementing a satellite-based total vegetation dataset, which includes NPV, into a regional atmospheric chemistry model. Simulations of the entire year 2016 over the conterminous United States demonstrate that NPV reduces dust emissions by 10-70% from most dust sources in the southwest, particularly in spring. Second, the relationship between topographic wind conditions (i.e., speed and direction with respect to surface slope) and dust particle size distribution is investigated using a decade's worth of dust reanalysis data covering North Africa. Findings indicate that the fraction of coarse dust in emissions increases with wind speed and slope, particularly under uphill winds, the latter highlighting the role of topography in enhancing vertical transport for larger particles. These positive correlations weaken during the afternoon and summer events, suggesting that turbulence associated with haboob events suspends coarse particles. Finally, a series of air samples collected in Tenerife, Spain is revisited for a detailed study on the associated dust plume characteristics, which would facilitate the understanding of how environmental factors during transport influence airborne microbial assemblages. Using back trajectory analysis and dust optical depth reanalysis data, air samples impacted by African dust are identified. Seasonal variations in trajectories and associated environmental conditions reveal highly variable trans-Atlantic airflows. Elevated altitudes, higher temperatures, and lower relative humidity (RH) along summer trajectories implied the presence of Saharan Air Layer, whereas the frequent occurrence of higher RH (> 40%) and light precipitation in spring indicate more deposition of dust and associated microbes during transport. Overall, this work highlights the importance of accurately representing of various environmental elements that interact with the dust cycle, such as vegetation and topographic winds, which improves our ability to predict and manage the impacts of dust as well as other components of the Earth system.
Measurement report: Variability in the composition of biogenic volatile organic compounds in a Southeastern US forest and their role in atmospheric reactivity
McGlynn, Deborah F.; Barry, Laura E. R.; Lerdau, Manuel T.; Pusede, Sally E.; Isaacman-VanWertz, Gabriel (2021-10-22)
Despite the significant contribution of biogenic volatile organic compounds (BVOCs) to organic aerosol formation and ozone production and loss, there are few longterm, year-round, ongoing measurements of their volume mixing ratios and quantification of their impacts on atmospheric reactivity. To address this gap, we present 1 year of hourly measurements of chemically resolved BVOCs between 15 September 2019 and 15 September 2020, collected at a research tower in Central Virginia in a mixed forest representative of ecosystems in the Southeastern US. Mixing ratios of isoprene, isoprene oxidation products, monoterpenes, and sesquiterpenes are described and examined for their impact on the hydroxy radical (OH), ozone, and nitrate reactivity. Mixing ratios of isoprene range from negligible in the winter to typical summertime 24 h averages of 4-6 ppb, while monoterpenes have more stable mixing ratios in the range of tenths of a part per billion up to similar to 2 ppb year-round. Sesquiterpenes are typically observed at mixing ratios of < 10 ppt, but this represents a lower bound in their abundance. In the growing season, isoprene dominates OH reactivity but is less important for ozone and nitrate reactivity. Monoterpenes are the most important BVOCs for ozone and nitrate reactivity throughout the year and for OH reactivity outside of the growing season. To better understand the impact of this compound class on OH, ozone, and nitrate reactivity, the role of individual monoterpenes is examined. Despite the dominant contribution of alpha-pinene to total monoterpene mass, the average reaction rate of the monoterpene mixture with atmospheric oxidants is between 25% and 30% faster than alpha-pinene due to the contribution of more reactive but less abundant compounds. A majority of reactivity comes from alpha-pinene and limonene (the most significant low-mixing-ratio, high-reactivity isomer), highlighting the importance of both mixing ratio and structure in assessing atmospheric impacts of emissions.
Minor contributions of daytime monoterpenes are major contributors to atmospheric reactivity
McGlynn, Deborah F.; Frazier, Graham; Barry, Laura E. R.; Lerdau, Manuel T.; Pusede, Sally E.; Isaacman-VanWertz, Gabriel (Copernicus, 2023-01-04)
Emissions from natural sources are driven by various external stimuli such as sunlight, temperature, and soil moisture. Once biogenic volatile organic compounds (BVOCs) are emitted into the atmosphere, they rapidly react with atmospheric oxidants, which has significant impacts on ozone and aerosol budgets. However, diurnal, seasonal, and interannual variability in these species are poorly captured in emissions models due to a lack of long-term, chemically speciated measurements. Therefore, increasing the monitoring of these emissions will improve the modeling of ozone and secondary organic aerosol concentrations. Using 2 years of speciated hourly BVOC data collected at the Virginia Forest Research Lab (VFRL) in Fluvanna County, Virginia, USA, we examine how minor changes in the composition of monoterpenes between seasons are found to have profound impacts on ozone and OH reactivity. The concentrations of a range of BVOCs in the summer were found to have two different diurnal profiles, which, we demonstrate, appear to be driven by light-dependent versus light-independent emissions. Factor analysis was used to separate the two observed diurnal profiles and determine the contribution from each emission type. Highly reactive BVOCs were found to have a large influence on atmospheric reactivity in the summer, particularly during the daytime. These findings reveal the need to monitor species with high atmospheric reactivity, even though they have low concentrations, to more accurately capture their emission trends in models.
A new approach for measuring the carbon and oxygen content of atmospherically relevant compounds and mixtures
Hurley, James F.; Kreisberg, Nathan M.; Stump, Braden; Bi, Chenyang; Kumar, Purushottam; Hering, Susanne V.; Keady, Pat; Isaacman-VanWertz, Gabriel (2020-09-18)
Due to its complexity, gas- and particle-phase organic carbon in the atmosphere is often classified by its bulk physicochemical properties. However, there is a dearth of robust, moderate-cost approaches to measure the bulk chemical composition of organic carbon in the atmosphere. This is particularly true for the degree of oxygenation, which critically affects the properties and impacts of organic carbon but for which routine measurement approaches are lacking. This gap has limited the understanding of a wide range of atmospheric components, including particulate matter, the mass of which is monitored worldwide due to its health and environmental effects but the chemical characterization of which requires relatively high capital costs and complex operation by highly trained technical personnel. In this work, we demonstrate a new approach to estimate the mass of carbon and oxygen in analytes and mixtures that relies only on robust, moderate-cost detectors designed for use with gas chromatography. Organic compounds entering a flame ionization detector were found to be converted with approximately complete efficiency to CO2, which was analyzed downstream using an infrared detector to measure the mass of carbon analyzed. The ratio of the flame ionization detector (FID) signal generated to CO2 formed (FID=CO2) was shown to be strongly correlated (R-2 = 0.89) to the oxygen-to-carbon ratio (O=C) of the analyte. Furthermore, simple mixtures of analytes behaved as the weighted average of their components, indicating that this correlation extends to mixtures. These properties were also observed to correlate well with the sensitivity of the FID estimated by structure activity relationships (quantified as the relative effective carbon number). The relationships between measured FID=CO2, analyte O=C, and FID sensitivity allow the estimation of one property from another with < 15% error for mixtures and < 20% error for most individual analytes. The approach opens the possibility of field-deployable, autonomous measurement of the carbon and oxygen content of particulate matter using time-tested, low-maintenance detectors, though such an application would require some additional testing on complex mixtures. With some instrumental modifications, similar measurements on gas-phase species may be feasible. Moreover, the potential expansion to additional gas chromatography detectors may provide concurrent measurement of other elements (e.g., sulfur, nitrogen).
New Methods for Measuring Spatial, Temporal and Chemical Distributions of Volatile Organic Compounds
Hurley, James Franklin (Virginia Tech, 2023-01-20)
Volatile organic compounds (VOCs) are those chemical species having sufficiently high vapor pressures to exist largely or entirely in the gaseous phase, whereas reactive organic carbon (ROC) encompasses all organics except methane. ROC can be emitted biogenically and anthropogenically, usually in a pure hydrocarbon form that is susceptible to reaction with common atmospheric oxidants such as hydroxyl and ozone in the initial steps to the formation of particulate matter, the criteria pollutant most strongly implicated in human mortality. The diversity of both the emitted VOCs and their possible atmospheric reactions yields countless different compounds existing in the atmosphere with a correspondingly wide range of volatility, solubility, reactivity, etc.. Moreover, the temporal and spatial variability of a given analyte is often large. Real-time chemical characterization of gaseous and particulate organic compounds can be achieved by instrumentation utilizing chromatographic and/or mass spectrometric techniques, but these methods are expensive, often logistically challenging, and require high levels of skills for both operation and data analysis. Conversely, filter-based measurements for organic particulates are inexpensive and straightforward, but do not give real-time data and analytes may be lost or transformed before analysis. There is a niche for robust, low-maintenance, moderate-cost instrumentation that offers chemical information on atmospheric carbon. Presented here are two projects that develop and validate instrumentation for measuring ROC. The first combines flame ionization detection (FID) with a CO2 detector to estimate the O/C ratios of sampled gases and particulates. O/C ratios are a particularly valuable piece of chemical information as higher ratios give lower volatility and higher solubility, meaning increased propensity to partition into the condensed phase. The second project utilizes portable VOC samplers with sorbent tubes that trap and protect analytes for detailed analysis. The samplers' portability and programmable microcontrollers offers the investigator great flexibility, both spatially and temporally. A third project analyzed the chemical composition of commercially available fragrance mixtures and modeled their emissions' impact on oxidant reactivity. It was observed that terpenes, despite their low mole fractions in the mixtures, represent the vast majority of emitted reactivity and are quantitatively evolved from the mixtures in a matter of hours.
Observations of sesquiterpenes and their oxidation products in central Amazonia during the wet and dry seasons
Yee, Lindsay D.; Isaacman-VanWertz, Gabriel; Wernis, Rebecca A.; Meng, Meng; Rivera, Ventura; Kreisberg, Nathan M.; Hering, Susanne V.; Bering, Mads S.; Glasius, Marianne; Upshur, Mary Alice; Be, Ariana Gray; Thomson, Regan J.; Geiger, Franz M.; Offenberg, John H.; Lewandowski, Michael; Kourtchev, Ivan; Kalberer, Markus; de Sa, Suzane S.; Martin, Scot T.; Alexander, M. Lizabeth; Palm, Brett B.; Hu, Weiwei; Campuzano-Jost, Pedro; Day, Douglas A.; Jimenez, Jose L.; Liu, Yingjun; McKinney, Karena A.; Artaxo, Paulo; Viegas, Juarez; Manzi, Antonio; Oliveira, Maria B.; de Souza, Rodrigo; Machado, Luiz A. T.; Longo, Karla; Goldstein, Allen H. (European Geophysical Union, 2018-07-23)
Biogenic volatile organic compounds (BVOCs) from the Amazon forest region represent the largest source of organic carbon emissions to the atmosphere globally. These BVOC emissions dominantly consist of volatile and intermediate-volatility terpenoid compounds that undergo chemical transformations in the atmosphere to form oxygenated condensable gases and secondary organic aerosol (SOA). We collected quartz filter samples with 12 h time resolution and performed hourly in situ measurements with a semi-volatile thermal desorption aerosol gas chromatograph (SV-TAG) at a rural site ("T3") located to the west of the urban center of Manaus, Brazil as part of the Green Ocean Amazon (GoAmazon2014/5) field campaign to measure intermediate-volatility and semi-volatile BVOCs and their oxidation products during the wet and dry seasons. We speciated and quantified 30 sesquiterpenes and 4 diterpenes with mean concentrations in the range 0.01-6.04 ng m(-3) (1670 ppq(v)). We estimate that sesquiterpenes contribute approximately 14 and 12% to the total reactive loss of O-3 via reaction with isoprene or terpenes during the wet and dry seasons, respectively. This is reduced from similar to 50-70% for within-canopy reactive O-3 loss attributed to the ozonolysis of highly reactive sesquiterpenes (e.g., beta-caryophyllene) that are reacted away before reaching our measurement site. We further identify a suite of their oxidation products in the gas and particle phases and explore their role in biogenic SOA formation in the central Amazon region. Synthesized authentic standards were also used to quantify gas-and particle-phase oxidation products derived from beta-caryophyllene. Using tracer-based scaling methods for these products, we roughly estimate that sesquiterpene oxidation contributes at least 0.4-5% (median 1 %) of total submicron OA mass. However, this is likely a low-end estimate, as evidence for additional unaccounted sesquiterpenes and their oxidation products clearly exists. By comparing our field data to laboratory-based sesquiterpene oxidation experiments we confirm that more than 40 additional observed compounds produced through sesquiterpene oxidation are present in Amazonian SOA, warranting further efforts towards more complete quantification.

Browsing by Author "Isaacman-VanWertz, Gabriel"

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