Selected Publications

2022
K. Alapaty, B. Cheng, J. Bash, J. W. Munger, J. T. Walker, and S. Arunachalam. 11/2022. “Dry Deposition Methods Based on Turbulence Kinetic Energy: 1. Evaluation of Various Resistances and Sensitivity Studies Using a Single‐Point Model.” Journal of Geophysical Research: Atmospheres, 127, Pp. e2022JD036631.
Aaron Teets, David J. P. Moore, M. Ross Alexander, Peter D. Blanken, Gil Bohrer, Sean P. Burns, Mariah S. Carbone, Mark J. Ducey, Shawn Fraver, Christopher M. Gough, David Y. Hollinger, George Koch, Thomas Kolb, J. William Munger, Kimberly A. Novick, Scott V. Ollinger, Andrew P. Ouimette, Neil Pederson, Daniel M. Ricciuto, Bijan Seyednasrollah, Christoph S. Vogel, and Andrew D. Richardson. 5/2022. “Coupling of Tree Growth and Photosynthetic Carbon Uptake Across six North American Forests.” Journal of Geophysical Research: Biogeosciences, 127, Pp. e2021JG006690. Publisher's VersionAbstract
Abstract Linking biometric measurements of stand-level biomass growth to tower-based measurements of carbon uptake – gross primary productivity (GPP) and net ecosystem productivity (NEP) – has been the focus of numerous ecosystem-level studies aimed to better understand the factors regulating carbon allocation to slow-turnover wood biomass pools. However, few of these studies have investigated the importance of previous year uptake to growth. We tested the relationship between wood biomass increment (WBI) and different temporal periods of carbon uptake from the current and previous year to investigate potential lagged allocation of fixed carbon to growth among six mature, temperate forests. We found WBI was strongly correlated to carbon uptake across space (i.e., long-term averages at the different sites) but on annual timescales, WBI was much less related to carbon uptake, suggesting a temporal mismatch between C fixation and allocation to biomass. We detected lags in allocation of previous year carbon uptake to WBI at three of six sites. Sites with higher annual WBI had overall stronger correlations to carbon uptake, with the strongest correlations to carbon uptake from the previous year. Only one site had WBI with strong positive relationships to current year uptake and not the previous year. Forests with low rates of WBI demonstrated weak correlations to carbon uptake from the previous year and stronger relationships to current year climate conditions. Our work shows an important, but not universal, role of lagged allocation of previous year carbon uptake to growth in temperate forests.
2021
E. J. Larson, L. D. Schiferl, R. Commane, J. W. Munger, A. T. Trugman, T. Ise, E. S. Euskirchen, S. Wofsy, and P. M. Moorcroft. 12/30/2021. “The changing carbon balance of tundra ecosystems: results from a vertically-resolved peatland biosphere model.” Environmental Research Letters, 17, 1, Pp. 014019. Publisher's VersionAbstract
An estimated 1700 Pg of carbon is frozen in the Arctic permafrost and the fate of this carbon is unclear because of the complex interaction of biophysical, ecological and biogeochemical processes that govern the Arctic carbon budget. Two key processes determining the region’s long-term carbon budget are: (a) carbon uptake through increased plant growth, and (b) carbon release through increased heterotrophic respiration (HR) due to warmer soils. Previous predictions for how these two opposing carbon fluxes may change in the future have varied greatly, indicating that improved understanding of these processes and their feedbacks is critical for advancing our predictive ability for the fate of Arctic peatlands. In this study, we implement and analyze a vertically-resolved model of peatland soil carbon into a cohort-based terrestrial biosphere model to improve our understanding of how on-going changes in climate are altering the Arctic carbon budget. A key feature of the formulation is that accumulation of peat within the soil column modifies its texture, hydraulic conductivity, and thermal conductivity, which, in turn influences resulting rates of HR within the soil column. Analysis of the model at three eddy covariance tower sites in the Alaskan tundra shows that the vertically-resolved soil column formulation accurately captures the zero-curtain phenomenon, in which the temperature of soil layers remain at or near 0 °C during fall freezeback due to the release of latent heat, is critical to capturing observed patterns of wintertime respiration. We find that significant declines in net ecosystem productivity (NEP) occur starting in 2013 and that these declines are driven by increased HR arising from increased precipitation and warming. Sensitivity analyses indicate that the cumulative NEP over the decade responds strongly to the estimated soil carbon stock and more weakly to vegetation abundance at the beginning of the simulation.
L. D’Orangeville, M. Itter, D. Kneeshaw, J. W. Munger, A. D. Richardson, J. M. Dyer, D. A. Orwig, Y. Pan, and N. Pederson. 7/26/2021. “Peak radial growth of diffuse-porous species occurs during periods of lower water availability than for ring-porous and coniferous trees.” Publisher, Volume, issueNumber, Pp. pageNBumber. Publisher's VersionAbstract
Climate models project warmer summer temperatures will increase the frequency and heat severity of droughts in temperate forests of Eastern North America. Hotter droughts are increasingly documented to affect tree growth and forest dynamics, with critical impacts on tree mortality, carbon sequestration and timber provision. The growing acknowledgement of the dominant role of drought timing on tree vulnerability to water deficit raises the issue of our limited understanding of radial growth phenology for most temperate tree species. Here, we use well-replicated dendrometer band data sampled frequently during the growing season to assess the growth phenology of 610 trees from 15 temperate species over 6 years. Patterns of diameter growth follow a typical logistic shape, with growth rates reaching a maximum in June, and then decreasing until process termination. On average, we find that diffuse-porous species take 16–18 days less than other wood-structure types to put on 50% of their annual diameter growth. However, their peak growth rate occurs almost a full month later than ring-porous and conifer species (ca. 24 ± 4 days; mean ± 95% credible interval). Unlike other species, the growth phenology of diffuse-porous species in our dataset is highly correlated with their spring foliar phenology. We also find that the later window of growth in diffuse-porous species, coinciding with peak evapotranspiration and lower water availability, exposes them to a higher water deficit of 88 ± 19 mm (mean ± SE) during their peak growth than ring-porous and coniferous species (15 ± 35 mm and 30 ± 30 mm, respectively). Given the high climatic sensitivity of wood formation, our findings highlight the importance of wood porosity as one predictor of species climatic sensitivity to the projected intensification of the drought regime in the coming decades.
Yenny Gonzalez, Róisín Commane, Ethan Manninen, Bruce C. Daube, Luke D. Schiferl, J. Barry McManus, Kathryn McKain, Eric J. Hintsa, James W. Elkins, Stephen A. Montzka, Colm Sweeney, Fred Moore, Jose L. Jimenez, Pedro Campuzano Jost, Thomas B. Ryerson, Ilann Bourgeois, Jeff Peischl, Chelsea R. Thompson, Eric Ray, Paul O. Wennberg, John Crounse, Michelle Kim, Hannah M. Allen, Paul A. Newman, Britton B. Stephens, Eric C. Apel, Rebecca S. Hornbrook, Benjamin A. Nault, Eric Morgan, and Steven C. Wofsy. 7/22/2021. “Impact of stratospheric air and surface emissions on tropospheric nitrous oxide during ATom.” Atmospheric Chemistry and Physics, 21, 14, Pp. 11113-11132. Publisher's VersionAbstract
We measured the global distribution of tropospheric N2O mixing ratios during the NASA airborne Atmospheric Tomography (ATom) mission. ATom measured concentrations of ∼ 300 gas species and aerosol properties in 647 vertical profiles spanning the Pacific, Atlantic, Arctic, and much of the Southern Ocean basins, nearly from pole to pole, over four seasons (2016–2018). We measured N2O concentrations at 1 Hz using a quantum cascade laser spectrometer (QCLS). We introduced a new spectral retrieval method to account for the pressure and temperature sensitivity of the instrument when deployed on aircraft. This retrieval strategy improved the precision of our ATom QCLS N2O measurements by a factor of three (based on the standard deviation of calibration measurements). Our measurements show that most of the variance of N2O mixing ratios in the troposphere is driven by the influence of N2O-depleted stratospheric air, especially at mid- and high latitudes. We observe the downward propagation of lower N2O mixing ratios (compared to surface stations) that tracks the influence of stratosphere–troposphere exchange through the tropospheric column down to the surface. The highest N2O mixing ratios occur close to the Equator, extending through the boundary layer and free troposphere. We observed influences from a complex and diverse mixture of N2O sources, with emission source types identified using the rich suite of chemical species measured on ATom and the geographical origin calculated using an atmospheric transport model. Although ATom flights were mostly over the oceans, the most prominent N2O enhancements were associated with anthropogenic emissions, including from industry (e.g., oil and gas), urban sources, and biomass burning, especially in the tropical Atlantic outflow from Africa. Enhanced N2O mixing ratios are mostly associated with pollution-related tracers arriving from the coastal area of Nigeria. Peaks of N2O are often associated with indicators of photochemical processing, suggesting possible unexpected source processes. In most cases, the results show how difficult it is to separate the mixture of different sources in the atmosphere, which may contribute to uncertainties in the N2O global budget. The extensive data set from ATom will help improve the understanding of N2O emission processes and their representation in global models.
Gilberto Pastorello, Carlo Trotta, Eleonora Canfora, Housen Chu, Danielle Christianson, You-Wei Cheah, Cristina Poindexter, Jiquan Chen, Abdelrahman Elbashandy, Marty Humphrey, Peter Isaac, Diego Polidori, Markus Reichstein, Alessio Ribeca, Catharine van Ingen, Nicolas Vuichard, Leiming Zhang, Brian Amiro, Christof Ammann, M. Altaf Arain, Jonas Ardö, Timothy Arkebauer, Stefan K. Arndt, Nicola Arriga, Marc Aubinet, Mika Aurela, Dennis Baldocchi, Alan Barr, Eric Beamesderfer, Luca Belelli Marchesini, Onil Bergeron, Jason Beringer, Christian Bernhofer, Daniel Berveiller, Dave Billesbach, Thomas Andrew Black, Peter D. Blanken, Gil Bohrer, Julia Boike, Paul V. Bolstad, Damien Bonal, Jean-Marc Bonnefond, David R. Bowling, Rosvel Bracho, Jason Brodeur, Christian Brümmer, Nina Buchmann, Benoit Burban, Sean P. Burns, Pauline Buysse, Peter Cale, Mauro Cavagna, Pierre Cellier, Shiping Chen, Isaac Chini, Torben R. Christensen, James Cleverly, Alessio Collalti, Claudia Consalvo, Bruce D. Cook, David Cook, Carole Coursolle, Edoardo Cremonese, Peter S. Curtis, Ettore D’Andrea, Humberto da Rocha, Xiaoqin Dai, Kenneth J. Davis, Bruno De Cinti, Agnes de Grandcourt, Anne De Ligne, Raimundo C. De Oliveira, Nicolas Delpierre, Ankur R. Desai, Carlos Marcelo Di Bella, Paul di Tommasi, Han Dolman, Francisco Domingo, Gang Dong, Sabina Dore, Pierpaolo Duce, Eric Dufrêne, Allison Dunn, Jiří Dušek, Derek Eamus, Uwe Eichelmann, Hatim Abdalla M. ElKhidir, Werner Eugster, Cacilia M. Ewenz, Brent Ewers, Daniela Famulari, Silvano Fares, Iris Feigenwinter, Andrew Feitz, Rasmus Fensholt, Gianluca Filippa, Marc Fischer, John Frank, Marta Galvagno, Mana Gharun, Damiano Gianelle, Bert Gielen, Beniamino Gioli, Anatoly Gitelson, Ignacio Goded, Mathias Goeckede, Allen H. Goldstein, Christopher M. Gough, Michael L. Goulden, Alexander Graf, Anne Griebel, Carsten Gruening, Thomas Grünwald, Albin Hammerle, Shijie Han, Xingguo Han, Birger Ulf Hansen, Chad Hanson, Juha Hatakka, Yongtao He, Markus Hehn, Bernard Heinesch, Nina Hinko-Najera, Lukas Hörtnagl, Lindsay Hutley, Andreas Ibrom, Hiroki Ikawa, Marcin Jackowicz-Korczynski, Dalibor Janouš, Wilma Jans, Rachhpal Jassal, Shicheng Jiang, Tomomichi Kato, Myroslava Khomik, Janina Klatt, Alexander Knohl, Sara Knox, Hideki Kobayashi, Georgia Koerber, Olaf Kolle, Yoshiko Kosugi, Ayumi Kotani, Andrew Kowalski, Bart Kruijt, Julia Kurbatova, Werner L. Kutsch, Hyojung Kwon, Samuli Launiainen, Tuomas Laurila, Bev Law, Ray Leuning, Yingnian Li, Michael Liddell, Jean-Marc Limousin, Marryanna Lion, Adam J. Liska, Annalea Lohila, Ana López-Ballesteros, Efrén López-Blanco, Benjamin Loubet, Denis Loustau, Antje Lucas-Moffat, Johannes Lüers, Siyan Ma, Craig Macfarlane, Vincenzo Magliulo, Regine Maier, Ivan Mammarella, Giovanni Manca, Barbara Marcolla, Hank A. Margolis, Serena Marras, William Massman, Mikhail Mastepanov, Roser Matamala, Jaclyn Hatala Matthes, Francesco Mazzenga, Harry McCaughey, Ian McHugh, Andrew M. S. McMillan, Lutz Merbold, Wayne Meyer, Tilden Meyers, Scott D. Miller, Stefano Minerbi, Uta Moderow, Russell K. Monson, Leonardo Montagnani, Caitlin E. Moore, Eddy Moors, Virginie Moreaux, Christine Moureaux, J. William Munger, Taro Nakai, Johan Neirynck, Zoran Nesic, Giacomo Nicolini, Asko Noormets, Matthew Northwood, Marcelo Nosetto, Yann Nouvellon, Kimberly Novick, Walter Oechel, Jørgen Eivind Olesen, Jean-Marc Ourcival, Shirley A. Papuga, Frans-Jan Parmentier, Eugenie Paul-Limoges, Marian Pavelka, Matthias Peichl, Elise Pendall, Richard P. Phillips, Kim Pilegaard, Norbert Pirk, Gabriela Posse, Thomas Powell, Heiko Prasse, Suzanne M. Prober, Serge Rambal, Üllar Rannik, Naama Raz-Yaseef, Corinna Rebmann, David Reed, Victor Resco de Dios, Natalia Restrepo-Coupe, Borja R. Reverter, Marilyn Roland, Simone Sabbatini, Torsten Sachs, Scott R. Saleska, Enrique P. Sánchez-Cañete, Zulia M. Sanchez-Mejia, Hans Peter Schmid, Marius Schmidt, Karl Schneider, Frederik Schrader, Ivan Schroder, Russell L. Scott, Pavel Sedlák, Penélope Serrano-Ortíz, Changliang Shao, Peili Shi, Ivan Shironya, Lukas Siebicke, Ladislav Šigut, Richard Silberstein, Costantino Sirca, Donatella Spano, Rainer Steinbrecher, Robert M. Stevens, Cove Sturtevant, Andy Suyker, Torbern Tagesson, Satoru Takanashi, Yanhong Tang, Nigel Tapper, Jonathan Thom, Michele Tomassucci, Juha-Pekka Tuovinen, Shawn Urbanski, Riccardo Valentini, Michiel van der Molen, Eva van Gorsel, Ko van Huissteden, Andrej Varlagin, Joseph Verfaillie, Timo Vesala, Caroline Vincke, Domenico Vitale, Natalia Vygodskaya, Jeffrey P. Walker, Elizabeth Walter-Shea, Huimin Wang, Robin Weber, Sebastian Westermann, Christian Wille, Steven Wofsy, Georg Wohlfahrt, Sebastian Wolf, William Woodgate, Yuelin Li, Roberto Zampedri, Junhui Zhang, Guoyi Zhou, Donatella Zona, Deb Agarwal, Sebastien Biraud, Margaret Torn, and Dario Papale. 7/9/2021. “The FLUXNET2015 dataset and the ONEFlux processing pipeline for eddy covariance data.” Scientific Data, 8, 1. Publisher's VersionAbstract
The FLUXNET2015 dataset provides ecosystem-scale data on CO2, water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such as gap-filled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the first time in this paper. In addition, 206 of these sites are for the first time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible.
Itziar Irakulis-Loitxate, Luis Guanter, Yin-Nian Liu, Daniel J. Varon, Joannes D. Maasakkers, Yuzhong Zhang, Apisada Chulakadabba, Steven C. Wofsy, Andrew K. Thorpe, Riley M. Duren, Christian Frankenberg, David R. Lyon, Benjamin Hmiel, Daniel H. Cusworth, Yongguang Zhang, Karl Segl, Javier Gorroño, Elena Sánchez-García, Melissa P. Sulprizio, Kaiqin Cao, Haijian Zhu, Jian Liang, Xun Li, Ilse Aben, and Daniel J. Jacob. 6/30/2021. “Satellite-based survey of extreme methane emissions in the Permian basin.” Science Advances, 7, 27. Publisher's VersionAbstract
Industrial emissions play a major role in the global methane budget. The Permian basin is thought to be responsible for almost half of the methane emissions from all U.S. oil- and gas-producing regions, but little is known about individual contributors, a prerequisite for mitigation. We use a new class of satellite measurements acquired during several days in 2019 and 2020 to perform the first regional-scale and high-resolution survey of methane sources in the Permian. We find an unexpectedly large number of extreme point sources (37 plumes with emission rates >500 kg hour−1), which account for a range between 31 and 53% of the estimated emissions in the sampled area. Our analysis reveals that new facilities are major emitters in the area, often due to inefficient flaring operations (20% of detections). These results put current practices into question and are relevant to guide emission reduction efforts.
E. B. Wiggins, A. Andrews, C. Sweeney, J. B. Miller, C. E. Miller, S. Veraverbeke, R. Commane, S. Wofsy, J. M. Henderson, and J. T. Randerson. 6/7/2021. “Boreal forest fire CO and CH4 emission factors derived from tower observations in Alaska during the extreme fire season of 2015.” Atmospheric Chemistry and Physics, 21, 11, Pp. 8557-8574. Publisher's VersionAbstract
Recent increases in boreal forest burned area, which have been linked with climate warming, highlight the need to better understand the composition of wildfire emissions and their atmospheric impacts. Here we quantified emission factors for CO and CH4 from a massive regional fire complex in interior Alaska during the summer of 2015 using continuous high-resolution trace gas observations from the Carbon in Arctic Reservoirs Vulnerability Experiment (CRV) tower in Fox, Alaska. Averaged over the 2015 fire season, the mean CO / CO2 emission ratio was 0.142 ± 0.051, and the mean CO emission factor was 127 ± 40 g kg−1 dry biomass burned. The CO / CO2 emission ratio was about 39 % higher than the mean of previous estimates derived from aircraft sampling of wildfires from boreal North America. The mean CH4 / CO2 emission ratio was 0.010 ± 0.004, and the CH4 emission factor was 5.3 ± 1.8 g kg−1 dry biomass burned, which are consistent with the mean of previous reports. CO and CH4 emission ratios varied in synchrony, with higher CH4 emission factors observed during periods with lower modified combustion efficiency (MCE). By coupling a fire emissions inventory with an atmospheric model, we identified at least 34 individual fires that contributed to trace gas variations measured at the CRV tower, representing a sample size that is nearly the same as the total number of boreal fires measured in all previous field campaigns. The model also indicated that typical mean transit times between trace gas emission within a fire perimeter and tower measurement were 1–3 d, indicating that the time series sampled combustion across day and night burning phases. The high CO emission ratio estimates reported here provide evidence for a prominent role of smoldering combustion and illustrate the importance of continuously sampling fires across time-varying environmental conditions that are representative of a fire season.
C. Staebell, K. Sun, J. Samra, J. Franklin, C. C. Miller, X. Liu, E. Conway, K. Chance, S. Milligan, and S. Wofsy. 5/25/2021. “Spectral calibration of the MethaneAIR instrument.” Atmospheric Measurement Techniques, 14, 5, Pp. 3737-3753. Publisher's VersionAbstract

MethaneAIR is the airborne simulator of MethaneSAT, an area-mapping satellite currently under development with the goal of locating and quantifying large anthropogenic CH4 point sources as well as diffuse emissions at the spatial scale of an oil and gas basin. Built to closely replicate the forthcoming satellite, MethaneAIR consists of two imaging spectrometers. One detects CH4 and CO2 absorption around 1.65 and 1.61 µm, respectively, while the other constrains the optical path in the atmosphere by detecting O2 absorption near 1.27 µm. The high spectral resolution and stringent retrieval accuracy requirements of greenhouse gas remote sensing in this spectral range necessitate a reliable spectral calibration. To this end, on-ground laboratory measurements were used to derive the spectral calibration of MethaneAIR, serving as a pathfinder for the future calibration of MethaneSAT. Stray light was characterized and corrected for through fast-Fourier-transform-based Van Cittert deconvolution. Wavelength registration was examined and found to be best described by a linear relationship for both bands with a precision of ∼ 0.02 spectral pixel. The instrument spectral spread function (ISSF), measured with fine wavelength steps of 0.005 nm near a series of central wavelengths across each band, was oversampled to construct the instrument spectral response function (ISRF) at each central wavelength and spatial pixel. The ISRFs were smoothed with a Savitzky–Golay filter for use in a lookup table in the retrieval algorithm. The MethaneAIR spectral calibration was evaluated through application to radiance spectra from an instrument flight over the Colorado Front Range.

Housen Chu, Xiangzhong Luo, Zutao Ouyang, W. Stephen Chan, Sigrid Dengel, Sébastien C. Biraud, Margaret S. Torn, Stefan Metzger, Jitendra Kumar, M. Altaf Arain, Tim J. Arkebauer, Dennis Baldocchi, Carl Bernacchi, Dave Billesbach, T. Andrew Black, Peter D. Blanken, Gil Bohrer, Rosvel Bracho, Shannon Brown, Nathaniel A. Brunsell, Jiquan Chen, Xingyuan Chen, Kenneth Clark, Ankur R. Desai, Tomer Duman, David Durden, Silvano Fares, Inke Forbrich, John A. Gamon, Christopher M. Gough, Timothy Griffis, Manuel Helbig, David Hollinger, Elyn Humphreys, Hiroki Ikawa, Hiroki Iwata, Yang Ju, John F. Knowles, Sara H. Knox, Hideki Kobayashi, Thomas Kolb, Beverly Law, Xuhui Lee, Marcy Litvak, Heping Liu, J. William Munger, Asko Noormets, Kim Novick, Steven F. Oberbauer, Walter Oechel, Patty Oikawa, Shirley A. Papuga, Elise Pendall, Prajaya Prajapati, John Prueger, William Quinton, Andrew D. Richardson, Eric S. Russell, Russell L. Scott, Gregory Starr, Ralf Staebler, Paul C. Stoy, Ellen Stuart-Haëntjens, Oliver Sonnentag, Ryan C. Sullivan, Andy Suyker, Masahito Ueyama, Rodrigo Vargas, Jeffrey D. Wood, and Donatella Zona. 5/15/2021. “Representativeness of Eddy-Covariance flux footprints for areas surrounding AmeriFlux sites.” Agricultural and Forest Meteorology, 301. Publisher's VersionAbstract
Large datasets of greenhouse gas and energy surface-atmosphere fluxes measured with the eddy-covariance technique (e.g., FLUXNET2015, AmeriFlux BASE) are widely used to benchmark models and remote-sensing products. This study addresses one of the major challenges facing model-data integration: To what spatial extent do flux measurements taken at individual eddy-covariance sites reflect model- or satellite-based grid cells? We evaluate flux footprints—the temporally dynamic source areas that contribute to measured fluxes—and the representativeness of these footprints for target areas (e.g., within 250–3000 m radii around flux towers) that are often used in flux-data synthesis and modeling studies. We examine the land-cover composition and vegetation characteristics, represented here by the Enhanced Vegetation Index (EVI), in the flux footprints and target areas across 214 AmeriFlux sites, and evaluate potential biases as a consequence of the footprint-to-target-area mismatch. Monthly 80% footprint climatologies vary across sites and through time ranging four orders of magnitude from 103 to 107 m2 due to the measurement heights, underlying vegetation- and ground-surface characteristics, wind directions, and turbulent state of the atmosphere. Few eddy-covariance sites are located in a truly homogeneous landscape. Thus, the common model-data integration approaches that use a fixed-extent target area across sites introduce biases on the order of 4%–20% for EVI and 6%–20% for the dominant land cover percentage. These biases are site-specific functions of measurement heights, target area extents, and land-surface characteristics. We advocate that flux datasets need to be used with footprint awareness, especially in research and applications that benchmark against models and data products with explicit spatial information. We propose a simple representativeness index based on our evaluations that can be used as a guide to identify site-periods suitable for specific applications and to provide general guidance for data use.
T. Foken, W. Babel, J. W. Munger, T. Gronholm, T. Vesala, and A. Knohl. 5/4/2021. “Selected breakpoints of net forest carbon uptake at four eddy-covariance sites.” Tellus Series B-Chemical and Physical Meteorology, 73, 1, Pp. 1-12. Publisher's VersionAbstract
Extensive studies are available that analyse time series of carbon dioxide and water flux measurements of FLUXNET sites over many years and link these results to climate change such as changes in atmospheric carbon dioxide concentration, air temperature and growing season length and other factors. Many of the sites show trends to a larger carbon uptake. Here we analyse time series of net ecosystem exchange, gross primary production, respiration, and evapotranspiration of four forest sites with particularly long measurement periods of about 20 years. The regular trends shown are interrupted by periods with higher or lower increases of carbon uptake. These breakpoints can be of very different origin and include forest decline, increased vegetation period, drought effects, heat waves, and changes in site heterogeneity. The influence of such breakpoints should be included in long-term studies of land-atmosphere exchange processes.
D. Obrist, E. M. Roy, J. L. Harrison, C. F. Kwong, J. W. Munger, H. Moosmueller, C. D. Romero, S. Sun, J. Zhou, and R. Commane. 3/21/2021. “Previously unaccounted atmospheric mercury deposition in a midlatitude deciduous forest.” Proceedings of the National Academy of Sciences of the United States of America, 118, 29. Publisher's VersionAbstract
Mercury is toxic to wildlife and humans, and forests are thought to be a globally important sink for gaseous elemental mercury (GEM) deposition from the atmosphere. Yet there are currently no annual GEM deposition measurements over rural forests. Here we present measurements of ecosystem–atmosphere GEM exchange using tower-based micrometeorological methods in a midlatitude hardwood forest. We measured an annual GEM deposition of 25.1 µg ⋅ m−2 (95% CI: 23.2 to 26.7 1 µg ⋅ m−2), which is five times larger than wet deposition of mercury from the atmosphere. Our observed annual GEM deposition accounts for 76% of total atmospheric mercury deposition and also is three times greater than litterfall mercury deposition, which has previously been used as a proxy measure for GEM deposition in forests. Plant GEM uptake is the dominant driver for ecosystem GEM deposition based on seasonal and diel dynamics that show the forest GEM sink to be largest during active vegetation growing periods and middays, analogous to photosynthetic carbon dioxide assimilation. Soils and litter on the forest floor are additional GEM sinks throughout the year. Our study suggests that mercury loading to this forest was underestimated by a factor of about two and that global forests may constitute a much larger global GEM sink than currently proposed. The larger than anticipated forest GEM sink may explain the high mercury loads observed in soils across rural forests, which impair water quality and aquatic biota via watershed Hg export.
C. Floerchinger, P. B. Shepson, K. Hajny, B. C. Daube, B. H. Stirm, C. Sweeney, and S. C. Wofsy. 2/18/2021. “Relative flux measurements of biogenic and natural gas-derived methane for seven US cities.” Elementa-Science of the Anthropocene, 9, 1, Pp. 000119. Publisher's VersionAbstract
Using the Purdue University Airborne Laboratory for Atmospheric Research, we measured concentrations of methane and ethane emanating from seven U.S. cities (New York, NY, Philadelphia, PA, Washington, D.C./Baltimore, MD, Boston, MA, Chicago, IL, Richmond, VA, and Indianapolis, IN), in order to determine (with a median 95% CI of roughly 7%) the fraction of methane emissions attributable to natural gas (Thermogenic Methane Emission Ratio [TMER]), for both summer and winter months. New methodology is introduced to compute inflow concentrations and to accurately define the spatial domain of the sampling region, using upwind measurements coupled with Lagrangian trajectory modeling. We show discrepancies in inventory-estimated TMER from cities when the sample domain is defined using political boundaries versus urban centers encircled by the flight track and highlight this as a potential source of error common to top-down studies. We found that methane emissions of natural gas were greater than winter biogenic emissions for all cities except Richmond, where multiple landfills dominate. Biogenic emissions increased in summer, but natural gas remained important or dominant (20%–80%). National inventories should be updated to reflect the dominance of natural gas emissions for urban environments and to account for seasonal increases in biogenic methane in summer.
Junjie Liu, Latha Baskaran, Kevin Bowman, David Schimel, A. Anthony Bloom, Nicholas C. Parazoo, Tomohiro Oda, Dustin Carroll, Dimitris Menemenlis, Joanna Joiner, Roisin Commane, Bruce Daube, Lucianna V. Gatti, Kathryn McKain10, John Miller, Britton B. Stephens, Colm Sweeney, and Steven Wofsy. 2/10/2021. “Carbon Monitoring System Flux Net Biosphere Exchange 2020 (CMS-Flux NBE 2020).” Earth System Science, 13, 2, Pp. 299–330. Publisher's VersionAbstract
Here we present a global and regionally resolved terrestrial net biosphere exchange (NBE) dataset with corresponding uncertainties between 2010–2018: Carbon Monitoring System Flux Net Biosphere Exchange 2020 (CMS-Flux NBE 2020). It is estimated using the NASA Carbon Monitoring System Flux (CMS-Flux) top-down flux inversion system that assimilates column CO2 observations from the Greenhouse Gases Observing Satellite (GOSAT) and NASA's Observing Carbon Observatory 2 (OCO-2). The regional monthly fluxes are readily accessible as tabular files, and the gridded fluxes are available in NetCDF format. The fluxes and their uncertainties are evaluated by extensively comparing the posterior CO2 mole fractions with CO2 observations from aircraft and the NOAA marine boundary layer reference sites. We describe the characteristics of the dataset as the global total, regional climatological mean, and regional annual fluxes and seasonal cycles. We find that the global total fluxes of the dataset agree with atmospheric CO2 growth observed by the surface-observation network within uncertainty. Averaged between 2010 and 2018, the tropical regions range from close to neutral in tropical South America to a net source in Africa; these contrast with the extra-tropics, which are a net sink of 2.5±0.3 Gt C/year. The regional satellite-constrained NBE estimates provide a unique perspective for understanding the terrestrial biosphere carbon dynamics and monitoring changes in regional contributions to the changes of atmospheric CO2 growth rate. The gridded and regional aggregated dataset can be accessed at https://doi.org/10.25966/4v02-c391 (Liu et al., 2020).
N. Bautista, B. D. V. Marino, and J. W. Munger. 2/6/2021. “Science to Commerce: A Commercial-Scale Protocol for Carbon Trading Applied to a 28-Year Record of Forest Carbon Monitoring at the Harvard Forest.” Land, 10, 2, Pp. 163. Publisher's VersionAbstract
Forest carbon sequestration offset protocols have been employed for more than 20 years with limited success in slowing deforestation and increasing forest carbon trading volume. Direct measurement of forest carbon flux improves quantification for trading but has not been applied to forest carbon research projects with more than 600 site installations worldwide. In this study, we apply carbon accounting methods, scaling hours to decades to 28-years of scientific CO2 eddy covariance data for the Harvard Forest (US-Ha1), located in central Massachusetts, USA and establishing commercial carbon trading protocols and applications for similar sites. We illustrate and explain transactions of high-frequency direct measurement for CO2 net ecosystem exchange (NEE, gC m−2 year−1) that track and monetize ecosystem carbon dynamics in contrast to approaches that rely on forest mensuration and growth models. NEE, based on eddy covariance methodology, quantifies loss of CO2 by ecosystem respiration accounted for as an unavoidable debit to net carbon sequestration. Retrospective analysis of the US-Ha1 NEE times series including carbon pricing, interval analysis, and ton-year exit accounting and revenue scenarios inform entrepreneur, investor, and landowner forest carbon commercialization strategies. CO2 efflux accounts for ~45% of the US-Ha1 NEE, an error of ~466% if excluded; however, the decades-old coupled human and natural system remains a financially viable net carbon sink. We introduce isoflux NEE for t13C16O2 and t12C18O16O to directly partition and quantify daytime ecosystem respiration and photosynthesis, creating new soil carbon commerce applications and derivative products in contrast to undifferentiated bulk soil carbon pool approaches. Eddy covariance NEE methods harmonize and standardize carbon commerce across diverse forest applications including, a New England, USA regional eddy covariance network, the Paris Agreement, and related climate mitigation platforms.
Kelvin H. Bates, Daniel J. Jacob, Siyuan Wang, Rebecca S. Hornbrook, Eric C. Apel, Michelle J. Kim, Dylan B. Millet, Kelley C. Wells, Xin Chen, Jared F. Brewer, Eric A. Ray, Róisín Commane, Glenn S. Diskin, and Steven C. Wofsy. 2/3/2021. “The Global Budget of Atmospheric Methanol: New Constraints on Secondary, Oceanic, and Terrestrial Sources.” Publisher, 126, 4, Pp. e2020JD033439. Publisher's VersionAbstract
Methanol is the second-most abundant organic gas in the remote atmosphere after methane, but its sources are poorly understood. Here, we report a global budget of methanol constrained by observations from the ATom aircraft campaign as implemented in the GEOS-Chem global atmospheric chemistry model. ATom observations under background marine conditions can be fit in the model with a surface ocean methanol concentration of 61 nM and a methanol yield of 13% from the newly implemented CH3O2 + OH reaction. While terrestrial biogenic emissions dominate the global atmospheric methanol budget, secondary production from CH3O2 + OH and CH3O2 + CH3O2 accounts for 29% of the total methanol source, and makes up the majority of methanol in the background marine atmosphere sampled by ATom. Net emission from the ocean is comparatively minor, particularly because of rapid deposition from the marine boundary layer. Aged anthropogenic and pyrogenic plumes sampled in ATom featured large methanol enhancements to constrain the corresponding sources. Methanol enhancements in pyrogenic plumes did not decay with age, implying in-plume secondary production. The atmospheric lifetime of methanol is only 5.3 days, reflecting losses of comparable magnitude from photooxidation and deposition. GEOS-Chem model results indicate that methanol photochemistry contributes 5%, 4%, and 1.5% of the tropospheric burdens of formaldehyde, CO, and ozone, respectively, with particularly pronounced effects in the tropical upper troposphere. The CH3O2 + OH reaction has substantial impacts on radical budgets throughout the troposphere and should be included in global atmospheric chemistry models.
Susan S. Kulawik, John R. Worden, Vivienne H. Payne, Dejian Fu, Steven C. Wofsy, Kathryn McKain, Colm Sweeney, Bruce C. Daube Jr., Alan Lipton, Igor Polonsky, Yuguang He, Karen E. Cady-Pereira, Edward J. Dlugokencky, Daniel J. Jacob, and Yi Yin. 1/15/2021. “Evaluation of single-footprint AIRS CH4 profile retrieval uncertainties using aircraft profile measurements.” Atmospheric Measurement Techniques, 14, 1, Pp. 335-354. Publisher's VersionAbstract
We evaluate the uncertainties of methane optimal estimation retrievals from single-footprint thermal infrared observations from the Atmospheric Infrared Sounder (AIRS). These retrievals are primarily sensitive to atmospheric methane in the mid-troposphere through the lower stratosphere (∼2 to ∼17 km). We compare them to in situ observations made from aircraft during the HIAPER Pole to Pole Observations (HIPPO) and Atmospheric Tomography Mission (ATom) campaigns, and from the NOAA GML aircraft network, between the surface and 5–13 km, across a range of years, latitudes between 60∘ S to 80∘ N, and over land and ocean. After a global, pressure-dependent bias correction, we find that the land and ocean have similar biases and that the reported observation error (combined measurement and interference errors) of ∼27 ppb is consistent with the SD between aircraft and individual AIRS observations. A single observation has measurement (noise related) uncertainty of ∼17 ppb, a ∼20 ppb uncertainty from radiative interferences (e.g., from water or temperature), and ∼30 ppb due to “smoothing error”, which is partially removed when making comparisons to in situ measurements or models in a way that accounts for this regularization. We estimate a 10 ppb validation uncertainty because the aircraft typically did not measure methane at altitudes where the AIRS measurements have some sensitivity, e.g., the stratosphere, and there is uncertainty in the truth that we validate against. Daily averaging only partly reduces the difference between aircraft and satellite observation, likely because of correlated errors introduced into the retrieval from temperature and water vapor. For example, averaging nine observations only reduces the aircraft–model difference to ∼17 ppb vs. the expected ∼10 ppb. Seasonal averages can reduce this ∼17 ppb uncertainty further to ∼10 ppb, as determined through comparison with NOAA aircraft, likely because uncertainties related to radiative effects of temperature and water vapor are reduced when averaged over a season.
R. A. Stern, N. Mahmoudi, C. O. Buckee, A. T. Schartup, P. Koutrakis, S. T. Ferguson, J. M. Wolfson, S. C. Wofsy, B. C. Daube, and E.M. Sunderland. 1/1/2021. “The Microbiome of Size-Fractionated Airborne Particles from the Sahara Region.” Environmental Science & Technology, 55, 3, Pp. 1487-1496. Publisher's VersionAbstract
Diverse airborne microbes affect human health and biodiversity, and the Sahara region of West Africa is a globally important source region for atmospheric dust. We collected size-fractionated (>10, 10–2.5, 2.5–1.0, 1.0–0.5, and <0.5 μm) atmospheric particles in Mali, West Africa and conducted the first cultivation-independent study of airborne microbes in this region using 16S rRNA gene sequencing. Abundant and diverse microbes were detected in all particle size fractions at levels higher than those previously hypothesized for desert regions. Average daily abundance was 1.94 × 105 16S rRNA copies/m3. Daily patterns in abundance for particles <0.5 μm differed significantly from other size fractions likely because they form mainly in the atmosphere and have limited surface resuspension. Particles >10 μm contained the greatest fraction of daily abundance (51–62%) and had significantly greater diversity than smaller particles. Greater bacterial abundance of particles >2.5 μm that are bigger than the average bacterium suggests that most airborne bacteria are present as aggregates or attached to particles rather than as free-floating cells. Particles >10 μm have very short atmospheric lifetimes and thus tend to have more localized origins. We confirmed the presence of several potential pathogens using polymerase chain reaction that are candidates for viability and strain testing in future studies. These species were detected on all particle sizes tested, including particles <2.5 μm that are expected to undergo long-range transport. Overall, our results suggest that the composition and sources of airborne microbes can be better discriminated by collecting size-fractionated samples.
2020
Benjamin Birner, Martyn P. Chipperfield, Eric J. Morgan, Britton B. Stephens, Marianna Linz, Wuhu Feng, Chris Wilson, Jonathan D. Bent, Steven C. Wofsy, Jeffrey Severinghaus, and Ralph F. Keeling. 10/30/2020. “Gravitational separation of Ar/N-2 and age of air in the lowermost stratosphere in airborne observations and a chemical transport model.” Atmospheric Chemistry and Physics, 20, 21, Pp. 12391–12408. Publisher's VersionAbstract
Accurate simulation of atmospheric circulation, particularly in the lower stratosphere, is challenging due to unresolved wave–mean flow interactions and limited high-resolution observations for validation. Gravity-induced pressure gradients lead to a small but measurable separation of heavy and light gases by molecular diffusion in the stratosphere. Because the relative abundance of Ar to N2 is exclusively controlled by physical transport, the argon-to-nitrogen ratio (Ar∕N2) provides an additional constraint on circulation and the age of air (AoA), i.e., the time elapsed since entry of an air parcel into the stratosphere. Here we use airborne measurements of N2O and Ar∕N2 from nine campaigns with global coverage spanning 2008–2018 to calculate AoA and to quantify gravitational separation in the lowermost stratosphere. To this end, we develop a new N2O–AoA relationship using a Markov chain Monte Carlo algorithm. We observe that gravitational separation increases systematically with increasing AoA for samples with AoA between 0 and 3 years. These observations are compared to a simulation of the TOMCAT/SLIMCAT 3-D chemical transport model, which has been updated to include gravitational fractionation of gases. We demonstrate that although AoA at old ages is slightly underestimated in the model, the relationship between Ar∕N2 and AoA is robust and agrees with the observations. This highlights the potential of Ar∕N2 to become a new AoA tracer that is subject only to physical transport phenomena and can supplement the suite of available AoA indicators.
B. D. V. Marino, T. Vinh, J. W. Munger, and R. Gyimah. 10/2020. “Direct measurement forest carbon protocol: a commercial system-of-systems to incentivize forest restoration and management.” PeerJ, 8, issueNumber, Pp. e8891. Publisher's VersionAbstract
Forest carbon sequestration offsets are methodologically uncertain, comprise a minor component of carbon markets and do not effectively slow deforestation. The objective of this study is to describe a commercial scale in situ measurement approach for determination of net forest carbon sequestration projects, the Direct Measurement Forest Carbon Protocol™, to address forest carbon market uncertainties. In contrast to protocols that rely on limited forest mensuration, growth simulation and exclusion of CO2 data, the Direct Measurement Forest Carbon Protocol™ is based on standardized methods for direct determination of net ecosystem exchange (NEE) of CO2 employing eddy covariance, a meteorological approach integrating forest carbon fluxes. NEE is used here as the basis for quantifying the first of its kind carbon financial products. The DMFCP differentiates physical, project and financial carbon within a System-of-Systems™ (SoS) network architecture. SoS sensor nodes, the Global Monitoring Platform™ (GMP), housing analyzers for CO2 isotopologues (e.g., 12CO2,13CO2, 14CO2) and greenhouse gases are deployed across the project landscape. The SoS standardizes and automates GMP measurement, uncertainty and reporting functions creating diverse forest carbon portfolios while reducing cost and investment risk in alignment with modern portfolio theory. To illustrate SoS field deployment and operation, published annual NEE data for a tropical (Ankasa Park, Ghana, Africa) and a deciduous forest (Harvard Forest, Petersham, MA, USA) are used to forecast carbon revenue. Carbon pricing scenarios are combined with historical in situ NEE annual time-series to extrapolate pre-tax revenue for each project applied to 100,000 acres (40,469 hectares) of surrounding land. Based on carbon pricing of $5 to $36 per ton CO2 equivalent (tCO2eq) and observed NEE sequestration rates of 0.48 to 15.60 tCO2eq acre−1 yr−1, pre-tax cash flows ranging from $230,000 to $16,380,000 across project time-series are calculated, up to 5× revenue for contemporary voluntary offsets, demonstrating new economic incentives to reverse deforestation. The SoS concept of operation and architecture, with engineering development, can be extended to diverse gas species across terrestrial, aquatic and oceanic ecosystems, harmonizing voluntary and compliance market products worldwide to assist in the management of global warming. The Direct Measurement Forest Carbon Protocol reduces risk of invalidation intrinsic to estimation-based protocols such as the Climate Action Reserve and the Clean Development Mechanism that do not observe molecular CO2 to calibrate financial products. Multinational policy applications such as the Paris Agreement and the United Nations Reducing Emissions from Deforestation and Degradation, constrained by Kyoto Protocol era processes, will benefit from NEE measurement avoiding unsupported claims of emission reduction, fraud, and forest conservation policy failure.

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