About

Zachary Barkley is an Assistant Research Professor in the Department of Meteorology and Atmospheric Science at Penn State. His research specialty is Atmospheric Chemistry and Pollution. Barkley's educational background includes a B.S. in Meteorology with honors and an M.S. in Meteorology from Penn State, obtained in 2014 and 2016 respectively. His academic journey began with a fascination for data, which he developed as a child by observing and performing mathematical operations on numbers displayed on household devices. This early interest led him to pursue meteorology, a field centered on data collection and interpretation. During his undergraduate studies, Barkley worked under Dr. Chris Forest, focusing on large database analysis related to the world's oceans. After earning his honors B.S., he immediately pursued a master's degree under the guidance of Ken Davis and Thomas Lauvaux, where he calculated methane emissions from natural gas infrastructure in northeastern Pennsylvania. This work addressed a practical problem with global implications, particularly related to climate change and local socioeconomic impacts. Currently, Barkley is engaged in NASA’s ACT America project, applying techniques he developed to estimate methane emissions in the Marcellus region and other parts of the U.S., contributing to understanding atmospheric pollution and climate-related issues.

Research topics

• Environmental science
• Geology
• Meteorology
• Atmospheric sciences
• Chemistry
• Geography
• Oceanography
• Climatology
• Environmental engineering

Selected publications

• Evaluation of a Decade of Methane Observations from a Tower Network in Indianapolis, Indiana

  2026-02-21

  article1st authorCorresponding

  Methane (CH₄) is a critical near-term climate mitigation target, yet urban sources remain poorly constrained. Indianapolis hosts one of the world’s longest and densest urban CH₄ datasets, with continuous measurements from a tall-tower network spanning more than a decade. We leverage this record in a Bayesian inversion framework to quantify citywide CH₄ emissions for 2014–2023, evaluate temporal variability, and reconcile estimates with prior inventories and independent campaigns. Emissions from the South Side Landfill (SSLF), the city’s largest point source, are estimated separately using a novel statistical approach applied to sorted tower enhancements. Across the decade, total Indianapolis CH₄ emissions averaged 2900–3700 kg hr⁻¹, of which 1370–1760 kg hr⁻¹ originated from the SSLF. Emissions exhibited remarkable temporal stability, with annual variations within 14% of the mean. Seasonal differences were modest: winter emissions were 13% higher than summer, smaller than seasonal swings reported for other U.S. cities. Total emissions agree with aircraft mass-balance estimates and an independently developed bottom-up inventory but are difficult to compare with EPA inventory values because of conflicting emission estimates from two different landfill reporting methodologies in addition to large discrepancies in diffuse citywide emissions. This work finds diffuse emissions likely arise from leaks in the natural-gas distribution system and equal 1.6–2.4% of city consumption, despite leak-tight plastic infrastructure, suggesting a predominantly post-meter origin. These results demonstrate that long-term, high-density tower observations robustly quantify total and source-specific urban CH₄ emissions, helping reconcile top-down and bottom-up estimates and inform effective mitigation strategies.

  PublisherDOI

• Evaluating GHGSat plume strength estimates in an urban environment using nested WRF- LES simulations

  2026-04-29

  article

  High spatial resolution satellite instruments operated by GHGSat estimate methane (CH4) emissions from relatively large point and area sources. The objective of this study is to assess the performance of GHGSat’s measurements of CH4 plumes using the Weather Research and Forecasting-large eddy simulation (WRF-LES) at a landfill in Indianapolis, Indiana, a well-studied, urban source of 20-25 mol s-1 of CH4. Our WRF-LES configuration used to simulate dispersion of tracers in the turbulent atmospheric boundary layer with realistic mesoscale forcing includes six one-way nested domains with 87 vertical levels. The 37 m horizontal grid spacing of the innermost domain is comparable to the 30 m spatial resolution of GHGSat. With landfill emissions represented as a constant, continuous source, methane dispersion in the innermost domain is compared to GHGSat observed CH4 plumes. The analyses performed for a total of 15 different days show considerable day-to-day variation and can be grouped as: 1) good agreement in plume dispersion direction and shape; 2) large differences in the plume dispersion direction and shape; and 3) plumes not observed by GHGSat (three of 15 days; no clear reasons identified). The plume shapes and directions diverge when the 10 m wind speed is below 2 m s-1. Compared with the WRF-LES simulations, GHGSat measurements underestimated plume strength by 38-47%. No clear dependence of bias or absolute error on GHGSat reported uncertainty or environmental parameters is identified. Our findings suggest that GHGSat could be used for detecting and quantifying moderate magnitude CH4 sources even in complex urban environments, but systematic underestimation and occasional non-detections should be considered when interpreting derived plume strength.

  PublisherDOI

• Laboratory and field assessment of mid-infrared absorption (MIRA) instrument performance for methane and ethane dry mole fractions

  Atmospheric measurement techniques · 2026-02-10

  articleOpen access

  Abstract. Concurrent measurements of methane (CH4) and ethane (C2H6) can be used to identify and separate methane sources, as ethane is present in thermogenic sources (e.g., oil and natural gas) but not in biogenic sources (e.g., agriculture). In this study, we evaluated the performance of multiple Aeris MIRA Ultra instruments (Versions 1 and 2) through controlled laboratory tests and tower-based deployments under field conditions. The systems were modified with an external pump, flow control, a Nafion dryer, and a custom-built auxiliary box to automate the system and transmit near real-time data. We determined the best calibration approach for our application, given practical limitations, to be a full calibration cycle (with ambient and high calibration cylinders) about once per day and an ambient calibration cylinder sampled hourly. Measurement uncertainty was assessed, including the uncertainty due to instrument noise as a function of calibration frequency, uncertainty in the water vapor correction, and cylinder assignment uncertainty. Instrument noise was the dominant source of uncertainty for C2H6, while the water vapor correction dominated the CH4 uncertainty. For Version 2 systems with hourly calibrations and a Nafion dryer with counterflow, the mean total uncertainty, including both systematic errors and noise, of hourly averages was 0.8–3.0 ppb CH4 and 0.35–0.37 ppb C2H6. Laboratory intercomparisons showed network compatibility within 1.2 ppb CH4 and 0.23 ppb C2H6, and a collocated deployment with a NOAA Picarro system agreed within 1.8 ppb CH4. Instrument noise varied substantially amongst the instruments, with errors reaching up to 11 ppb CH4 and 2 ppb C2H6 for hourly means, with similar variability indicated in a 50 h cylinder test. With appropriate engineering and calibration, the Aeris MIRA Ultra has the potential to distinguish regional methane emission sources in many field settings.

  PublisherOA PDFDOI

• Evaluation of Aircraft-based Methane Emissions Estimates and Estimation Methodologies with Six Years of Data from Indianapolis

  2026-05-15

  articleOpen access

  Indianapolis CH 4 emissions are 49 16 mol s -1 , with the South Side Landfill contributing 27 10 mol s -1 Boundary layer entrainment must be corrected when using upwind flights to define CH 4 background concentrations A mesoscale CH 4 model can filter complex background days, reducing emission variance by 25% in the case of Indianapolis

  PublisherOA PDFDOI

• Scaling Urban Methane Emissions: Utility of Single-Site Measurements in Five Urban Domains

  Environmental Science & Technology · 2025-07-09 · 1 citations

  articleOpen access

  Urban methane (CH4) missions remain poorly understood due to limited observational constraints. Most estimates rely on bottom-up inventories based on assumed emission factors and activity data or downscaling methods, which often underestimate emissions, sometimes by a factor of 2 or more in United States and European cities. While satellite and mobile observations can improve understanding, they face limitations in spatial resolution, coverage, and frequency. In contrast, fixed in situ measurements calibrated to World Meteorological Organization standards offer high precision continuous data, although with limited spatial coverage due to logistical constraints. This study uses in situ observations from single tower sites in five northeastern United States cities to estimate total urban CH4 emissions using a Bayesian scaling factor framework. Despite limited spatial sampling, the approach yields robust emission estimates consistent with other studies. To explore drivers of variability, the analysis examines correlations between inferred emissions and urban characteristics including population, residential gas usage, and infrastructure. Results show that residential building volume outperforms population as a predictor in some regions, highlighting the importance of infrastructure-specific factors. By demonstrating a scalable observation-based approach using minimal sites, this work addresses key gaps in urban CH4 monitoring and emphasizes the value of robust measurements and tailored proxies for improving emission estimates in diverse urban settings.

  PublisherDOI

• Seasonality and declining intensity of methane emissions from the Permian and nearby US oil and gas basins

  2025-07-02 · 2 citations

  preprintOpen access

  We quantify weekly methane emissions and trends from oil and gas production in the US Permian Basin for 2019–2023, and in nearby basins for 2022–2023, by analytical inversion of Tropospheric Monitoring Instrument (TROPOMI) satellite observations with the Integrated Methane Inversion (IMI) at 25-km resolution. Permian oil and gas emissions averaged 4.0 ± 1.1 Tg a-1 over 2019–2023, with large seasonal variation but little interannual variability. Methane intensity fell from 5.2% to 3.2% as production surged. Intensity in the New Mexico Permian fell from 4.5% to 2.1%, approaching the state’s 2026 target of <2%. Emissions were on average 60% higher in the winter than summer, which we corroborate with Permian Basin Tower Network measurements, Insight M aircraft data, and GHGSat satellite observations. This seasonality may be driven in part by higher winter emissions from liquid storage tanks due to decreased separator efficiency in cold conditions. Similar but weaker seasonality along with decreasing emissions and intensities is found in weekly inversions for the Anadarko, Barnett, Eagle Ford, and Haynesville basins in 2022–2023. Our work suggests that better weatherization of oil and gas facilities could significantly reduce methane emissions.

  PublisherOA PDFDOI

• Examining Daily Temporal Characteristics of Oil and Gas Methane Emissions in the Delaware Basin Using Continuous Tower Observations

  Journal of Geophysical Research Atmospheres · 2025-03-19 · 2 citations

  articleOpen access1st authorCorresponding

  Abstract Top‐down studies have found consistent underestimations in the United States Environmental Protection Agency (EPA) methane emissions inventory from the oil and gas (O&G) sector. Many of these studies use observations that bias toward hours when worktime activity occurs. In this study, we analyze over 2 years of methane measurements from a tower network in the Delaware basin to analyze hourly temporal emission patterns. Inversion results suggest a range in emissions from 137 Mg/hr at night to 197 Mg/hr during the day, present during both weekdays and weekends. If these results are applicable to other basins, daytime‐influenced methodologies may overestimate daily emission rates by up to 27%. This bias does not reconcile the more than 200% difference between the EPA inventory and top‐down estimates in the Delaware basin. This study demonstrates how continuous measurement networks can be combined with detailed activity data to improve bottom‐up inventories.

  PublisherOA PDFDOI

• Laboratory and field assessment of mid-infrared absorption (MIRA) instrument performance for methane and ethane dry mole fractions

  2025-10-16

  articleOpen accessCorresponding

  Abstract. Concurrent measurements of methane (CH4) and ethane (C2H6) can be used to identify and separate methane sources, as ethane is present in thermogenic sources (e.g., oil and natural gas) but not in biogenic sources (e.g., agriculture). In this study, we evaluated the performance of multiple Aeris MIRA Ultra instruments (Versions 1 and 2) through controlled laboratory tests and tower-based deployments under field conditions. The systems were modified with an external pump, flow control, a Nafion dryer, and a custom-built auxiliary box to automate the system and transmit near real-time data. We determined the best calibration approach for our application, given practical limitations, to be a full calibration cycle (with ambient and high calibration cylinders) about once per day and an ambient calibration cylinder sampled hourly. Measurement uncertainty was assessed, including the uncertainty due to instrument noise as a function of calibration frequency, uncertainty in the water vapor correction, and cylinder assignment uncertainty. Instrument noise was the dominant source of uncertainty for C2H6, while the water vapor correction dominated the CH4 uncertainty. For Version 2 systems with hourly calibrations and a Nafion dryer with counterflow, the mean total uncertainty, including both systematic errors and noise, of hourly averages was 0.8–3.0 ppb CH4 and 0.35–0.37 ppb C2H6. Laboratory intercomparisons showed network compatibility within 1.2 ppb CH4 and 0.23 ppb C2H6, and a collocated deployment with a NOAA Picarro system agreed within 1.8 ppb CH4. Instrument noise varied substantially amongst the instruments, with errors reaching up to 11 ppb CH4 and 2 ppb C2H6 for hourly means, with similar variability indicated in a 50-h cylinder test. With appropriate engineering and calibration, the Aeris MIRAUltra shows the capability to measure ethane and methane with sufficient stability to distinguish regional methane emission sources in many field settings.

  PublisherOA PDFDOI

• Simulating Complex CO ₂ Background Conditions for Indianapolis, IN, With a Simple Ecosystem CO ₂ Flux Model

  Journal of Geophysical Research Biogeosciences · 2025-06-01

  articleOpen access

  Abstract We evaluated the ability of a simple ecosystem carbon dioxide (CO 2 ) flux model, the Vegetation Photosynthesis and Respiration Model (VPRM), to capture complex CO 2 background conditions observed in Indianapolis, IN. Using simulated biogenic CO 2 fluxes and mole fraction tower influence functions, we estimated biogenic CO 2 mole fractions at three background towers in the Indianapolis Flux Experiment (INFLUX) network from April 2017 to March 2020. The model captures afternoon average CO 2 enhancements, the difference between the background towers and a common reference tower, at a monthly time scale with no significant bias, with monthly mean residuals rarely differing significantly from zero. Although not central to our application, the model could not capture day‐to‐day variations of observed afternoon average CO 2 enhancements. Random errors, when averaged over monthly to yearly time scales, were an order of magnitude smaller than typical urban enhancements. VPRM captured site‐to‐site differences in the average observed daily cycle of CO 2 fluxes at agricultural eddy covariance flux sites well, indicating that the model is able to capture rural CO 2 fluxes in our domain in addition to capturing differential impacts of the fluxes on CO 2 mole fractions. VPRM can be effectively used in CO 2 inversions to represent complex seasonal variations in background conditions observed in Indianapolis. Indianapolis, a modest‐size city surrounded by strong ecosystem fluxes, represents a rigorous test for the VPRM system. Further, this study presents an evaluation system that can be applied to assess the performance of other ecosystem CO 2 flux models in cities with similar monitoring networks.

  PublisherOA PDFDOI

• Using Tower-based Observational Networks to Assess the Impact of COVID-19 Lockdowns on Greenhouse Gas Emissions in Six North American Cities

  2025-08-18

  preprintOpen access1st authorCorresponding

  The onset of the COVID-19 pandemic in North America and the lockdowns that followed in March 2020 brought forth a rapid change to societal functions disrupting many aspects of normal life, including the greenhouse gas emissions associated with them. In this work, we examine the capabilities of tower networks established in six North American cities in quantifying the change in these emissions. Influence functions, which relate tower-based observational sites to their upwind source regions, were created for sites in Los Angeles, the D.C/Baltimore urban corridor, Indianapolis, Salt Lake City, Boston, and Toronto for the months of February-April of 2017-2020, and model CO2 enhancements were generated by multiplying the influence functions by regional inventories. Scaling factors are assigned to the city emissions to minimize the difference between observed and modeled afternoon CO2 enhancements in 15 day intervals. Scaling factors from the 2020 period are then compared directly to those from the 2017-2019 timeframe to calculate a relative change in the emissions during the COVID-lockdown timeframe. Results across all six cities show a consistent message; by the end of March 2020, CO2 emissions decreased by an average of 34% relative to the same 2017-2019 timeframe. This decrease matches values observed from bottom-up inventories during the same period. A similar technique is performed for methane across 4 cities with more variable trends across cities. The results of this paper demonstrate the ability to utilize simple approaches to detect and quantify temporal changes in emissions using a tower network.

  PublisherOA PDFDOI

Frequent coauthors

• Thomas Lauvaux

  Université de Reims Champagne-Ardenne

  73 shared

• K. J. Davis

  Pennsylvania State University

  67 shared

• Colm Sweeney

  National Oceanic and Atmospheric Administration

  37 shared

• N. L. Miles

  Pennsylvania State University

  35 shared

• Sha Feng

  Pennsylvania State University

  32 shared

• Alan Fried

  Institute of Arctic and Alpine Research

  27 shared

• Bianca C. Baier

  25 shared

• Joshua P. DiGangi

  Langley Research Center

  23 shared