About

Paul Shepson is a professor at Stony Brook University in the School of Marine and Atmospheric Sciences (SoMAS), where he conducts research on atmospheric chemistry and composition, interactions between the atmosphere and the surface, and the impacts of climate change on the physics, biology, and chemistry at the surface and in the atmosphere. His research group focuses on developing methods for quantifying greenhouse gas emission rates, particularly CO2 and CH4, from urban environments using aircraft-based measurements, tower observations, emissions models, and chemical transport models. This work connects to efforts at local, national, and international levels to mitigate climate change, and involves collaboration with organizations such as the National Institute for Standards and Technology (NIST). Shepson's research also includes studies of halogen chemistry in the Arctic Ocean environment, examining ocean-sea ice-snowpack-aerosol interactions that are changing due to climate change. His group has been involved in long-term Arctic research, including field projects in Alaska and campaigns like CHACHA, aimed at understanding atmospheric chemistry in the rapidly changing Arctic atmosphere. With a PhD from Pennsylvania State University obtained in 1982, Shepson has contributed extensively to the field through research on atmospheric chemistry, climate change, and related topics, with numerous publications and active involvement in grants and projects.

Research topics

• Statistics
• Atmospheric sciences
• Environmental chemistry
• Meteorology
• Organic chemistry
• Engineering
• Mathematics
• Geography
• Climatology
• Chemistry
• Environmental science
• Chromatography
• Ecology

Selected publications

• Synthesis of the Southeast Atmosphere Studies: Investigating Fundamental Atmospheric Chemistry Questions

  UNC Libraries · 2026-02-12

  articleOpen access

  The Southeast Atmosphere Studies (SAS), which included the Southern Oxidant and Aerosol Study (SOAS); the Southeast Nexus (SENEX) study; and the Nitrogen, Oxidants, Mercury and Aerosols: Distributions, Sources and Sinks (NOMADSS) study, was deployed in the field from 1 June to 15 July 2013 in the central and eastern United States, and it overlapped with and was complemented by the Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) campaign. SAS investigated atmospheric chemistry and the associated air quality and climate-relevant particle properties. Coordinated measurements from six ground sites, four aircraft, tall towers, balloon-borne sondes, existing surface networks, and satellites provide in situ and remotely sensed data on trace-gas composition, aerosol physicochemical properties, and local and synoptic meteorology. Selected SAS findings indicate 1) dramatically reduced NOx concentrations have altered ozone production regimes; 2) indicators of “biogenic” secondary organic aerosol (SOA), once considered part of the natural background, were positively correlated with one or more indicators of anthropogenic pollution; and 3) liquid water dramatically impacted particle scattering while biogenic SOA did not. SAS findings suggest that atmosphere–biosphere interactions modulate ambient pollutant concentrations through complex mechanisms and feedbacks not yet adequately captured in atmospheric models. The SAS dataset, now publicly available, is a powerful constraint to develop predictive capability that enhances model representation of the response and subsequent impacts of changes in atmospheric composition to changes in emissions, chemistry, and meteorology.

  PublisherDOI

• Integrated Analysis of Airborne In-situ Cloud and Aerosol Microphysics Data during the 2022 Chemistry in the Arctic: Clouds, Halogens, and Aerosols (CHACHA) Field Campaign

  2025-03-14

  preprintOpen access

  The Chemistry in the Arctic: Clouds, Halogens, and Aerosols (CHACHA) field project featured a wide collaboration from six universities to enhance the scientific understanding of multiphase halogen chemistry in the Arctic that took place in Utqiaġvik, Alaska during February-April 2022. This project was spurred by the pursuit of strengthening our understanding of how Arctic Sea ice loss and fossil fuel extraction affects atmospheric halogen chemistry.In this study, cloud flights from the University of Wyoming King Air are evaluated closely to assess the ambient conditions relevant to the Arctic boundary layer during flights targeting clouds emanating from open leads in the Arctic sea ice. During these flights, the Particle into Liquid Sampler (PILS) was utilized using a Roger’s inlet and Counterflow Virtual Impactor (CVI) with low volume (1.5 mL) samples being collected. This study aims to introduce a methodological basis for prioritizing samples and identifying samples that can be safely grouped together to maximize the chemical analysis possible. Instruments are used for this method include Aerosol microphysics data from instruments including Condensation Particle Counters (CPC), Portable Optical Particle Spectrometer (POPS), and Passive Cavity Aerosol Spectrometer Probe (PCASP) and cloud microphysics data from a Cloud Droplet Probe (CDP) and Two-Dimensional Stereo (2D-S). Ultimately, this work is a key step in chemical analysis of cloud flights that will be used to better understand multiphase Arctic halogen chemistry by constraining a Lagrangian chemical box model and cloud parcel modeling.

  PublisherDOI

• Modelling Arctic Lower Tropospheric Ozone: processes controlling seasonal variations

  2025-01-21 · 1 citations

  preprintOpen access

  Abstract. Previous assessments on modelling Arctic tropospheric ozone (O3) have shown that most atmospheric models continue to experience difficulties in simulating tropospheric O3 in the Arctic, particularly in capturing the seasonal variations at coastal sites, primarily attributed to the lack of representation of surface bromine chemistry in the Arctic. In this study, two independent chemical transport models (CTMs), DEHM (Danish Eulerian Hemispheric Model) and GEM-MACH (Global Environmental Multi-scale – Modelling Air quality and Chemistry), were used to simulate Arctic lower tropospheric O3 for the year 2015 at considerably higher horizontal resolutions (25-km and 15-km, respectively) than the large-scale models in the previous assessments. Both models include bromine chemistry and a representation of snow-sourced bromine mechanism: a blowing-snow bromine source mechanism in DEHM and a snowpack bromine source mechanism in GEM-MACH. Model results were compared with a suite of observations in the Arctic, including hourly observations from surface sites and mobile platforms (buoys and ship) and ozonesonde profiles, to evaluate models’ ability to simulate Arctic lower tropospheric O3, particularly in capturing the seasonal variations and the key processes controlling these variations. The study found that both models behave quite similarly outside the spring period and are able to capture the observed overall surface O3 seasonal cycle and synoptic scale variabilities, as well as the O3 vertical profiles in the Arctic. GEM-MACH (with the snowpack bromine source mechanism) was able to simulate most of the observed springtime Ozone Depletion Events (ODEs) at the coastal and buoy sites well, while DEHM (with the blowing-snow bromine source mechanism) simulated much fewer ODEs. The study showed that the springtime O3 depletion process plays a central role in driving the surface O3 seasonal cycle in Central Arctic, and that the bromine-mediated ODEs, while occurring most notably within the lowest few hundred metres of air above the Arctic Ocean, can induce a 5–7 % of loss in the total pan-Arctic tropospheric O3 burden during springtime. The model simulations also showed an overall enhancement in the pan-Arctic O3 concentration due to northern boreal wildfire emissions in summer 2015; the enhancement is more significant at higher altitudes. Higher O3 excess ratios (ΔO3/ΔCO) found aloft compared to near the surface indicate greater photochemical O3 production efficiency at higher altitudes in fire-impacted air masses. The model simulations further indicated an enhancement in NOy in the Arctic due to wildfires; a large portion of NOy produced from the wildfire emissions is found in the form of PAN that is transported to the Arctic, particularly at higher altitudes, potentially contributing to O3 production there.

  PublisherDOI

• Observational ozone datasets over the global oceans and polar regions (version 2024)

  Earth system science data · 2025-09-26 · 2 citations

  articleOpen access

  Abstract. Studying tropospheric ozone over the remote areas of the planet, such as the open oceans and the polar regions, is crucial to understand the role of ozone as a global climate forcer and regulator of atmospheric oxidative capacity. A focus on the pristine oceanic and polar regions complements the available land-based datasets and provides insights into key photochemical and depositional loss processes that control the concentrations and spatiotemporal variability in ozone as well as the physicochemical mechanisms driving these patterns. However, an assessment of the role of ozone over the oceanic and polar regions has been hampered by a lack of comprehensive observational datasets. Here, we present the first comprehensive collection of ozone data over the oceans and the polar regions. The overall dataset consists of 77 ship cruises/buoy-based observations and 48 aircraft-based campaigns. The dataset, consisting of more than 630 000 independent ozone measurement data points covering the period from 1977 to 2022 and an altitude range from the surface to 5000 m (with a focus on the lowest 2000 m), allows systematic analyses of the spatiotemporal distribution and long-term trends over the 11 defined ocean/polar regions. The datasets from ships, buoys, and aircraft are complemented by ozonesonde data from 29 launch sites or field campaigns and by 21 non-polar and 17 polar ground-based station datasets. The datasets contain information on how long the observed air masses were isolated from land, as estimated by backward trajectories from the individual observation points. To extract observations representative of oceanic conditions, we recommend using a subset of the data with an isolation time of 72 h or longer, from the analysis with coincident radon observations. These filtered oceanic and polar data showed typically flat diurnal cycles at high latitudes, whereas daytime decreases in ozone (11 %–16 %) were observed at lower latitudes. The ship/buoy- and aircraft-based datasets presented here will supplement the land-based ones in the TOAR-II (Tropospheric Ozone Assessment Report Phase II) database to provide a fully global assessment of tropospheric ozone. The described dataset is available at https://doi.org/10.17596/0004044 (Kanaya et al., 2025).

  PublisherOA PDFDOI

• Supplementary material to "Modelling Arctic Lower Tropospheric Ozone: processes controlling seasonal variations"

  2025-01-21 · 2 citations

  preprintOpen access
  PublisherDOI

• Modelling Arctic lower-tropospheric ozone: processes controlling seasonal variations

  Atmospheric chemistry and physics · 2025-08-01 · 1 citations

  articleOpen accessCorresponding

  Abstract. Previous assessments on modelling Arctic tropospheric ozone (O3) have shown that most atmospheric models continue to experience difficulties in simulating tropospheric O3 in the Arctic, particularly in capturing the seasonal variations at coastal sites, primarily attributed to the lack of representation of surface bromine chemistry in the Arctic. In this study, two independent chemical transport models (CTMs), DEHM (Danish Eulerian Hemispheric Model) and GEM-MACH (Global Environmental Multi-scale – Modelling Air quality and Chemistry), were used to simulate Arctic lower-tropospheric O3 for the year 2015 at considerably higher horizontal resolutions (25 and 15 km, respectively) than the large-scale models in the previous assessments. Both models include bromine chemistry but with different mechanistic representations of bromine sources from snow- and ice-covered polar regions: a blowing-snow bromine source mechanism in DEHM and a snowpack bromine source mechanism in GEM-MACH. Model results were compared with a suite of observations in the Arctic, including hourly observations from surface sites and mobile platforms (buoys and ships) and ozonesonde profiles, to evaluate models' ability to simulate Arctic lower-tropospheric O3, particularly in capturing the seasonal variations and the key processes controlling these variations. Both models are found to behave quite similarly outside the spring period and are able to capture the observed overall surface O3 seasonal cycle and synoptic-scale variabilities, as well as the O3 vertical profiles in the Arctic. GEM-MACH (with the snowpack bromine source mechanism) was able to simulate most of the observed springtime ozone depletion events (ODEs) at the coastal and buoy sites well, while DEHM (with the blowing-snow bromine source mechanism) simulated much fewer ODEs. The present study demonstrates that the springtime O3 depletion process plays a central role in driving the surface O3 seasonal cycle in central Arctic, and that the bromine-mediated ODEs, while occurring most notably within the lowest few hundred metres of air above the Arctic Ocean, can induce a 5 %–7 % of loss in the total pan-Arctic tropospheric O3 burden during springtime. The model simulations also showed an overall enhancement in the pan-Arctic O3 concentration due to northern boreal wildfire emissions in summer 2015; the enhancement is more significant at higher altitudes. Higher O3 excess ratios (ΔO3/ΔCO) found aloft compared to near the surface indicate greater photochemical O3 production efficiency at higher altitudes in fire-impacted air masses. The model simulations further indicated an enhancement in NOy in the Arctic due to wildfires; a large portion of NOy produced from the wildfire emissions is found in the form of PAN that is transported to the Arctic, particularly at higher altitudes, potentially contributing to O3 production there.

  PublisherOA PDFDOI

• Supplementary material to "Observational ozone data over the global oceans and polar regions: The TOAR-II Oceans data set version 2024"

  2025-02-13 · 2 citations

  preprintOpen access
  PublisherDOI

• Measurements of the Emission Rates of Nitrogen Oxides and Greenhouse Gases From the Prudhoe Bay Oil Field

  Journal of Geophysical Research Atmospheres · 2025-08-06 · 1 citations

  articleSenior author

  Abstract Oil and gas production regions are significant sources of greenhouse gases and reactive pollutants such as nitrogen oxides (NO x ) and volatile organic compounds. Research has also shown that methane (CH 4 ) emissions reported to the Environmental Protection Agency's (EPA) Greenhouse Gas Reporting Program (GHGRP) are generally underestimated. The Arctic accounted for 5.5% of global oil and gas production in 2022 but is estimated to contain significant undiscovered resources. The emitted NO x and volatile organic compounds can impact the composition and chemistry of the Arctic atmosphere. The Prudhoe Bay Oil Field in Alaska is one of the 10 largest oil fields in the US and has been approved for significant development expansion. However, only one recent study has reported measurements of its greenhouse gas emissions. We estimate the emission rates for carbon dioxide (CO 2 ), CH 4 , and NO x from the Prudhoe Bay Oil Field during the spring of 2022 using airborne mass balance methods and emission ratios. We also discuss emissions per energy produced and show an increase over time, with values higher than the national average for oil and gas producing regions, though within uncertainties. Our estimates are lower than the NO x emission estimate reported in the National Emissions Inventory (NEI), as seen in other oil and gas studies, but fall within the uncertainty range of the greenhouse gases reported in the GHGRP. This work provides a valuable snapshot of emissions before further expansion of extraction activities.

  PublisherDOI

• Observational ozone data over the global oceans and polar regions: The TOAR-II Oceans data set version 2024

  2025-02-13 · 2 citations

  preprintOpen access

  Abstract. Studying tropospheric ozone over the remote areas of the planet, such as the open oceans and the polar regions, is crucial to understand the role of ozone as a global climate forcer and regulator of atmospheric oxidative capacity. A focus on the pristine oceanic and polar regions complements the available land-based data sets and provides insights into key photochemical and depositional loss processes that control the concentrations, spatio-temporal variability of ozone, and the physico-chemical mechanisms driving these patterns. However, an assessment of the role of ozone over the oceanic and polar regions has been hampered by a lack of comprehensive observational data sets. Here, we present the first comprehensive collection of ozone data over the oceans and the polar regions. The overall data set consists of 77 ship cruises/buoy-based observations and 48 aircraft-based campaigns. The data set, consisting of more than 630,000 independent ozone measurement data points covering the period from 1977 to 2022 and an altitude range from the surface to 5000 m (with a focus on the lowest 2000 m), allows systematic analyses of the spatio-temporal distribution and long-term trends over the defined 11 ocean/polar regions. The data sets from ships, buoys, and aircrafts are complemented with an ozonesonde data set from 29 launch sites or field campaigns, and by 21 non-polar and 17 polar ground-based stations data sets. The data were filtered by using backward trajectories calculated with the HYSPLIT model from the individual observation points to extract essentially oceanic observations, defined as air masses that have travelled over oceans for 72 hours or more, which were further tested with the coincident Radon observations. The oceanic and polar data thus selected showed typically flat diurnal patterns at high latitudes and daytime decreases (11–16 %) at low latitudes, indicating the adequacy of the data collection and processing procedures, as well as the potential for further studies of processes with statistical robustness and coverage. The ship/buoy- and aircraft-based data sets presented here will supplement the land-based ones in the TOAR-II database to provide a fully global assessment of tropospheric ozone.

  PublisherDOI

• Greenhouse gas and short-lived pollutants in the Baltimore, MD and Washington, DC area: Coordinated measurements and models

  2024-03-08

  preprintOpen access

  The cities of Baltimore, MD and Washington, DC generate substantial amounts of air pollutants with adverse effects on health and climate, but the magnitude and origins of these contaminants remain uncertain.  The State of Maryland has committed to reducing statewide greenhouse gas emissions by 60% (relative to 2006 levels) by the year 2031. A team of scientists from the Maryland Department of the Environment (MDE), the National Institute of Standards and Technology (NIST), the National Oceanic and Atmospheric Administration (NOAA), the University of Maryland, and Stony Brook University have established a coordinated program of measurements and models to quantify and allocate emissions.  These include observations from aircraft, a mobile laboratory, a tower array, and surface monitors as well as Lagrangian and Eulerian models.  Results thus far indicate that methane emissions substantially exceed initial, traditional, bottom-up, inventory data and that leakage from the natural gas delivery system and landfills are major sources.  Urban methane emissions show a strong seasonality, consistent with natural gas usage – the flux in winter was 44% greater than in summer.  Model inversions suggest urban methane emissions in Washington and Baltimore decreased by 4-5%/yr between 2018 and 2021.  Mobile laboratory measurements of GHGs and air pollutants such as black carbon with high temporal and spatial resolution reveal a variety of sources in densely populated urban residential areas related to traffic and industry and with implications for environmental justice.  Analysis of long-term monitoring data with clustering of trajectories identified dominant transport pathways and sources in upwind states that likely contribute in a major way to ambient methane concentrations in the Baltimore/Washington area – these include the Marcellus oil and gas plays in Pennsylvania and West Virginia as well as swine production in North Carolina.  Ongoing and future work includes developing a landfill as a testbed for emissions quantification and control and use of carbon and hydrogen isotopes to partition fossil and biogenic emissions and biogenic losses.  The combination of State, federal, and university resources makes for a powerful tool to tackle air quality and climate problems.  

  PublisherDOI

Recent grants

• A Multiphase Study of the Nature, Sources, and Fate of Atmospheric Organic Nitrogen

  NSF · $751k · 2006–2013

• The Collaborative O-Buoy Project: Deployment of a Network of Arctic Ocean Chemical Sensors for the IPY and Beyond

  NSF · $508k · 2007–2011

• Studies of the Production of Molecular Halogens in Arctic Snowpacks and on Sea Ice Surfaces

  NSF · $527k · 2011–2014

• Development of a Proton Transfer Reaction Linear Ion Trap for Fast Atmospheric Pollutant Detection

  NSF · $535k · 2003–2008

• Collaborative Research: Program for Research on Oxidants: Photochemistry Emissions and Transport (PROPHET) 2009--Community Atmosphere-Biosphere INteractions EXperiment (CABINEX)

  NSF · $117k · 2009–2012

Frequent coauthors

• Brian H. Stirm

  Purdue University West Lafayette

  124 shared

• A. Karion

  National Institute of Standards and Technology

  104 shared

• Kerri A. Pratt

  98 shared

• Colm Sweeney

  National Oceanic and Atmospheric Administration

  91 shared

• J. W. Bottenheim

  Environment and Climate Change Canada

  77 shared

• J. C. Turnbull

  University of Colorado Boulder

  73 shared

• K. R. Gurney

  Northern Arizona University

  68 shared

• W. R. Simpson

  University of Alaska Fairbanks

  66 shared

Education

• Ph.D., Atmospheric Chemistry

  University of California, Los Angeles

  1981

• M.S., Atmospheric Chemistry

  University of California, Los Angeles

  1978

• B.S., Chemistry

  University of California, Los Angeles

  1976