John T Walker

John T Walker
United States Environmental Protection Agency | US EPA · Office of Research and Development

Ph.D. Soil Science

About

121
Publications
14,732
Reads
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4,186
Citations
Introduction
My research interests include: measurement and modeling of atmosphere-biosphere exchange of mass and energy, measurement and modeling of trace gas emissions from natural and agricultural sources, measurement and modeling of trace gas and particle air-surface exchange in agricultural and forest landscapes, biogeochemical cycling of nitrogen, processes of secondary aerosol formation in the atmosphere, and micrometeorology.
Additional affiliations
May 2012 - present
North Carolina State University
Position
  • Professor (Associate)
July 1998 - present
United States Environmental Protection Agency
Education
August 2000 - May 2005
North Carolina State University
Field of study
  • Soil Science

Publications

Publications (121)
Article
Full-text available
Waters impounded behind dams (i.e., reservoirs) are important sources of greenhouses gases (GHGs), especially methane (CH4), but emission estimates are not well constrained due to high spatial and temporal variability, limitations in monitoring methods to characterize hot spot and hot moment emissions, and the limited number of studies that investi...
Article
Full-text available
The dry deposition process refers to flux loss of an atmospheric pollutant due to uptake of the pollutant by the Earth's surfaces, including vegetation, underlying soil, and any other surface types. In chemistry transport models (CTMs), the dry deposition flux of a chemical species is typically calculated as the product of its surface layer concent...
Preprint
Full-text available
With increasing complexity of air quality models, additional chemical species have been included in model simulations for which dry deposition processes need to be parameterized. For this purpose, the gaseous dry deposition scheme of Zhang et al. (2003) is extended to include 12 oxidized volatile organic compounds (oVOCs) and hydrogen cyanide (HCN)...
Preprint
Full-text available
Waters impounded behind dams (i.e. reservoirs) are important sources of greenhouses gases, especially methane (CH4), but their contribution is not well constrained due to high spatial and temporal variability, limitations in monitoring methods to characterize hot spot and hot moment emissions, and the limited number of studies that investigate diur...
Article
Full-text available
Acidity, defined as pH, is a central component of aqueous chemistry. In the atmosphere, the acidity of condensed phases (aerosol particles, cloud water, and fog droplets) governs the phase partitioning of semivolatile gases such as HNO3, NH3, HCl, and organic acids and bases as well as chemical reaction rates. It has implications for the atmospheri...
Preprint
Full-text available
Abstract. Acidity, defined as pH, is a central component of aqueous chemistry. In the atmosphere, the acidity of condensed phases (aerosol particles, cloud water, and fog droplets) governs the phase partitioning of semi-volatile gases such as HNO<sub>3</sub>, NH<sub>3</sub>, and HCl, as well as chemical reaction rates. It has implications for the a...
Article
This review summarizes the state of the science of measurements of dry deposition of reactive nitrogen (Nr) compounds in North America, beginning with current understanding of the importance of dry deposition at the U.S. continental scale followed by a review of micrometeorological flux measurement methods. Measurements of Nr air-surface exchange i...
Article
Frameworks for limiting ecosystem exposure to excess nutrients and acidity require accurate and complete deposition budgets of reactive nitrogen (Nr). While much progress has been made in developing total Nr deposition budgets for the U.S., current budgets remain limited by key data and knowledge gaps. Analysis of National Atmospheric Deposition Pr...
Article
Determination of the amount of reactive nitrogen (Nr) deposition in excess of the ecosystem critical load (CL) requires an estimate of total deposition. Because the CL exceedance is used to inform policy decisions, uncertainty in both the CL and the exceedance itself must be understood. In this paper we review the state of the science with respect...
Technical Report
Full-text available
Critical load frameworks for limiting ecosystem exposure to nutrients and acidity require accurate and complete deposition budgets of reactive nitrogen (Nr). Determination of the amount of deposition in excess of the ecosystem critical load (i.e. “exceedance”) requires an estimate of total deposition, which is typically derived from gridded chemica...
Poster
This study is aimed at characterizing seasonal emissions of three reactive nitrogen (Nr) species, i.e., ammonia (NH3), nitric oxide (NO), and nitrous oxide (N2O), from turfgrass over the course of a year. These species are produced in the soil via nitrification and denitrification and are known for their roles in atmospheric processes including the...
Poster
Full-text available
Reservoirs are a globally important source of methane (CH4) to the atmosphere, but measuring CH4 emission rates from reservoirs is difficult due to the spatial and temporal variability in emissions via the emission pathways of ebullition (bubbling) and diffusion. The dominant source of CH4 in reservoirs is production by methanogens in the reservoir...
Article
Full-text available
Excess nitrogen and phosphorus (i.e., nutrients) environmental loadings continue to affect ecosystem function and human health across the U.S. Our ability to connect atmospheric inputs of nutrients to aquatic endpoints remains limited due to uncoupled air and water quality monitoring. Where connections exist, the information provides insights about...
Article
Full-text available
To quantify differences between dry deposition algorithms commonly used in North America, five models were selected to calculate dry deposition velocity (Vd) for O3 and SO2 over a temperate mixed forest in southern Ontario, Canada, where a 5-year flux database had previously been developed. The models performed better in summer than in winter with...
Article
Full-text available
Reservoirs are a globally significant source of methane (CH4) to the atmosphere. However, emission rate estimates may be biased low due to inadequate monitoring during brief periods of elevated emission rates (that is, hot moments). Here we investigate CH4 bubbling (that is, ebullition) during periods of falling water levels in a eutrophic reservoi...
Article
Full-text available
This study investigates the composition of organic particulate matter in PM2.5 in a remote montane forest in the southeastern US, focusing on the role of organic nitrogen (N) in sulfur-containing secondary organic aerosol (nitrooxy-organosulfates) and aerosols associated with biomass burning (nitro-aromatics). Bulk water-soluble organic N (WSON) re...
Conference Paper
Full-text available
Reservoirs are a globally important source of methane (CH4) to the atmosphere, but measuring CH4 emission rates from reservoirs is difficult due to the spatial and temporal variability of the various emission pathways, including ebullition and diffusion. We used the eddy covariance method to measure fluxes of CH4 over a mid-sized (2.4 km2), eutroph...
Article
Nitrogen loss from fertilized turf has been a concern for decades, with most research focused on inorganic (NO3-) leaching. The present work examined both inorganic and organic N species in leachate and soil N2O emissions from intact soil cores of a bermudagrass chronosequence (1, 15, 20, and 109 years old) collected in both winter and summer. Meas...
Article
Full-text available
This study investigates the composition of organic particulate matter in a remote montane forest in the southeastern U.S., focusing on the role of organic nitrogen (N) in sulfur-containing secondary organic aerosol (nitrooxy-organosulfates) and aerosols associated with biomass burning (nitro-aromatics). Bulk water soluble organic N (WSON) represent...
Article
Full-text available
Evaluation of the semi-continuous Monitor for AeRosols and GAses in ambient air (MARGA, Metrohm Applikon B.V.) was conducted with an emphasis on examination of accuracy and precision associated with processing of chromatograms. Using laboratory standards and atmospheric measurements, analytical accuracy, precision and method detection limits derive...
Article
Land application of compost has been a promising remediation strategy for soil health and environmental quality, but substantial emissions of greenhouse gases, especially N2O, need to be controlled during making and using compost of high N-load wastes, such as chicken manure. Biochar as a bulking agent for composting has been proposed as a novel ap...
Article
Full-text available
Evaluation of the semi-continuous Monitor for AeRosols and GAses in ambient air (MARGA, Metrohm Applikon B.V.) was conducted with an emphasis on examination of accuracy and precision associated with processing of chromatograms. Using laboratory standards and atmospheric measurements, analytical accuracy, precision, and method detection limits deriv...
Article
A coupled photosynthesis-stomatal conductance model with single layer sunlit and shaded leaf canopy scaling is implemented and evaluated in a diagnostic box model with the Pleim-Xiu land surface model (PX LSM) and ozone deposition model components taken directly from the meteorology and air quality modeling system - WRF/CMAQ (Weather Research and F...
Article
Full-text available
New particle formation (NPF) can potentially alter regional climate by increasing aerosol particle (hereafter particle) number concentrations and ultimately cloud condensation nuclei. The large scales on which NPF is manifest indicate potential to use satellite-based (inherently spatially averaged) measurements of atmospheric conditions to diagnose...
Article
Isotopocule analysis by quantum cascade laser spectrometry (QCL) is a promising approach for in situ, real-time tracking of the biological sources of soil N2O emissions. However, background atmospheric N2O is an important source of variability in the measurement of isotopocule ratios (i.e., 14N15N16O/14N14N16O and 15N14N16O/14N14N16O) of gas sample...
Article
In PNAS, we (1) highlight changes in the balance of oxidized and reduced nitrogen contributions to United States reactive nitrogen deposition, pointing out that decreases in nitrogen oxides (NOx) emissions, combined with modest growth in ammonia emissions, have altered the inorganic nitrogen deposition budget from majority deposition by oxidized sp...
Article
Full-text available
The dry component of total nitrogen and sulfur atmospheric deposition remains uncertain. The lack of measurements of sufficient chemical speciation and temporal extent make it difficult to develop accurate mass budgets and sufficient process level detail is not available to improve current air–surface exchange models. Over the past decade, signific...
Article
Full-text available
Rapid development of agriculture and fossil fuel combustion greatly increased US reactive nitrogen emissions to the atmosphere in the second half of the 20th century, resulting in excess nitrogen deposition to natural ecosystems. Recent efforts to lower nitrogen oxides emissions have substantially decreased nitrate wet deposition. Levels of wet amm...
Article
Full-text available
The dry component of total nitrogen and sulfur atmospheric deposition remains uncertain. The lack of measurements of sufficient chemical speciation and temporal extent make it difficult to develop accurate mass budgets and sufficient process level detail is not available to improve current air-surface exchange models. Over the past decade, signific...
Conference Paper
While few dispute the significance of within-canopy sources or sinks of reactive gaseous and particulate compounds, their estimation continues to be the subject of active research and debate. Reactive species undergo turbulent dispersion within an inhomogeneous flow field, and may be subjected to chemical, biological and/or physical deposition, emi...
Article
Full-text available
Recent laboratory chamber studies indicate a significant role for highly oxidized low-volatility organics in new particle formation (NPF), but the actual role of these highly oxidized low-volatility organics in atmospheric NPF remains uncertain. Here, particle size distributions (PSDs) measured in nine forest areas in North America are used to char...
Article
Full-text available
This article focuses on modeling of the nitrogen cascade and briefly reviews measurement networks and emerging measurement techniques in North America, with an emphasis on reduced nitrogen.
Article
Full-text available
Recent laboratory chamber studies indicate a significant role for highly oxidized low volatility organics in new particle formation (NPF) but the actual role of these highly oxidized low volatility organics in atmospheric NPF remains uncertain. Here, particle size distributions (PSDs) measured in nine forest areas in North America are used to chara...
Chapter
The background for this discussion was the background document in this book entitled: “Surface/atmosphere exchange of atmospheric acids and aerosols, including the effect and model treatment of chemical interactions”.
Chapter
Chemical processes occurring on leaf surface and in air inside vegetation canopies play significant and sometimes dominant roles on pollutant dry deposition budgets. Yet, these processes are seldom explicitly treated in dry deposition parameterizations/models. This report briefly summarizes the current knowledge, outstanding issues, and recommendat...
Article
Atmospheric deposition of reactive nitrogen (Nr) can contribute to acidification and nutrient enrichment in aquatic and terrestrial ecosystems.1-3 This can lead to reduced fish and insect populations in aquatic systems4 and their near depletion in highly acidified water bodies.5 Nutrient enrichment can disrupt the nutrient balance, leading to reduc...
Article
Ammonia (NH3) and carbon monoxide (CO) are primary pollutants emitted to the Earth's atmosphere from common as well as distinct sources associated with anthropogenic and natural activities. The seasonal and global distributions and correlations of NH3 and CO from the Tropospheric Emission Spectrometer (TES) satellite observations and GEOS-Chem mode...
Article
Lawn management practices that produce aesthetically appealing landscapes may also create environmental conditions that stimulate soil nitrous oxide (N2O) fluxes. The purpose of this exploratory study is to investigate the effects of lawn management practices on N2O fluxes from three non-replicated distinctly different residential landscapes: a hig...
Article
Full-text available
We developed and validated an automated, inexpensive, and continuous multiple-species gas-flux monitoring system that can provide data for a variety of relevant atmospheric pollutants, including O-3, CO2, and NOx. Validation consisted of conducting concurrent gas-phase dry-deposition experiments, using both dynamic flux chambers and an eddy-covaria...
Book
The background for this discussion was the background document in this book entitled: “Surface/atmosphere exchange of atmospheric acids and aerosols, including the effect and model treatment of chemical interactions”
Book
Chemical processes occurring on leaf surface and in air inside vegetation canopies play significant and sometimes dominant roles on pollutant dry deposition budgets. Yet, these processes are seldom explicitly treated in dry deposition parameterizations/models. This report briefly summarizes the current knowledge, outstanding issues, and recommendat...
Article
Full-text available
We developed and validated an automated, inexpensive, and continuous multiple-species gas-flux monitoring system that can provide data for a variety of relevant atmospheric pollutants, including O3, CO2, and NOx. Validation consisted of conducting concurrent gas-phase dry-deposition experiments, using both dynamic flux chambers and an eddy-covarian...
Article
A semi-empirical bi-directional flux modeling approach is used to estimate NH3 air concentrations and dry deposition fluxes to a portion of the Pocosin Lakes National Wildlife Refuge (PLNWR) downwind of a large poultry facility. Meteorological patterns at PLNWR are such that some portion of the refuge is downwind of the poultry facility 52%, 66%, 5...
Article
A new class of enclosed path gas analyzers suitable for eddy covariance applications combines the advantages of traditional closed-path systems (small density corrections, good performance in poor weather) and open-path systems (good spectral response, low power requirements), and permits estimates of instantaneous gas mixing ratio. Here, the exten...
Article
Full-text available
Ambient air monitoring as part of the US Environmental Protection Agency's (US EPA's) Clean Air Status and Trends Network (CASTNet) currently uses filter packs to measure weekly integrated concentrations. The US EPA is interested in supplementing CASTNet with semi-continuous monitoring systems at select sites to characterize atmospheric chemistry a...
Article
Wet deposition and transport analysis has been performed for ammonium (NH4+) in North Carolina, USA. Multiple regression analysis is employed to model the temporal trend and seasonality in monthly volume-weighted mean NH4+ concentrations in precipitation from 1983 to 1996 at six National Atmospheric Deposition Program/National Trends Network (NADP/...
Article
Full-text available
Reactive nitrogen (Nr) is removed by surface fluxes (air-surface exchange) and wet deposition. The chemistry and physics of the atmosphere result in a complicated system in which competing chemical sources and sinks exist and impact that removal. Therefore, uncertainties are best examined with complete regional chemical transport models that simula...
Article
Full-text available
This study examines the characteristics of new particle formation at a forest site in southeastern US. Particle size distributions above a Loblolly pine plantation were measured between November 2005 and September 2007 and analyzed by event type and frequency, as well as in relation to meteorological and atmospheric chemical conditions. Nucleation...
Article
Full-text available
Existing descriptions of bi-directional ammonia (NH3) land-atmosphere exchange incorporate temperature and moisture controls, and are beginning to be used in regional chemical transport models. However, such models have typically applied simpler emission factors to upscale the main NH3 emission terms. While this approach has successfully simulated...
Article
Atmospheric particle size distribution measurements (10 <= aerodynamic diameter, Dp <= 250 nm), which took place above a loblolly pine plantation in the Southeast U.S. from November 2005 to September 2007, were made using Scanning Mobility Particle Sizer (SMPS). The size distributions were investigated to identify new particle formation and to clas...
Article
Full-text available
The impacts of meteorology and air quality on the concentrations and relative distributions of free and combined amino acids (FAA; CAA) are evaluated during a month-long sampling campaign at a semi-urban site in the southeastern U.S. The average FAA concentration in fine aerosols (PM2.5) was 11�6 ng m–3, while CAA was found to be several times high...
Article
Full-text available
Animal feeding operations (AFOs) satisfy the demand for meat, dairy, and eggs; however, they may negatively impact air quality. In this study, the concentrations of PM2.5 and PM10 were simultaneously monitored at four ambient locations in the vicinity of a commercial egg production farm for over two years. Overall, concentrations and temporal patte...
Article
Full-text available
Atmospheric ammonia (NH3) is the primary atmospheric base and an important precursor for inorganic particulate matter and when deposited NH3 contributes to surface water eutrophication, soil acidification and decline in species biodiversity. Flux measurements indicate that the air-surface exchange of NH3 is bi-directional. However, the effects of b...
Article
Passive ambient air sampling for nitrogen dioxide (NO2), coarse particulate matter (PMc), and gaseous ammonia (NH3) was conducted at 22 monitoring sites, a compliance site, and a background site in the Cleveland, Ohio, USA area during summer 2009 and winter 2010. This air monitoring network was established to assess intra–urban gradients of air pol...
Article
Full-text available
Recent incorporation of coupled soil biogeochemical and bi-directional NH 3 air-surface exchange algorithms into regional air quality models holds promise for further reducing uncertainty in estimates of NH 3 emissions from fertilized soils. While this represents a significant advancement over previous approaches, the evaluation and improvement