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Nicholas Cowan

Bio: Nicholas Cowan is an academic researcher from University of Edinburgh. The author has contributed to research in topics: Environmental science & Eddy covariance. The author has an hindex of 9, co-authored 26 publications receiving 220 citations.

Papers
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Journal ArticleDOI
TL;DR: In this paper, a Bayesian approach was used to simplify the calculation of emission factors and quantify the uncertainty with a lognormal model to fit the parameters of a LOG model.
Abstract: Nitrous oxide (N2O) is a greenhouse gas produced mainly by the microbial breakdown of agricultural fertilizer. ‘Emission factors’ (EFs, the fraction of nitrogen added that is released as N2O) are based on flux chamber measurements following the application of fertilizer. These measurements are very variable in space and time so that EFs are often uncertain, but this is rarely quantified. We developed a method that simplifies the calculation of EFs, incorporates prior knowledge and quantifies the uncertainty with a Bayesian approach to fit the parameters of a lognormal model. We compared this with the standard method for interpolating, extrapolating and integrating fluxes of N2O (trapezoidal integration). We verified both methods against process-based model output where the true integral was known and against eddy covariance data where the integral was estimated more accurately because of the greater spatial and temporal coverage. We used the process-based model to simulate flux chamber data and added a lognormal spatial distribution to the model output. The lognormal model performed better than the standard method, in terms of estimating the true underlying cumulative flux more accurately. Estimates based on chamber and eddy covariance data were sometimes substantially different, but with no clear systematic bias. The Bayesian approach with the lognormal model enabled us to combine both chamber and eddy covariance data to constrain cumulative fluxes. The standard trapezoidal method typically underestimates emission factors to some extent if fluxes are lognormally distributed in space. The Bayesian approach with the lognormal model is a robust method for quantifying the uncertainty in cumulative fluxes of N2O. Highlights Emission factors for N2O are based on sparse and variable measurements, and so are uncertain. We use a Bayesian approach to simplify the calculation and quantify the uncertainty. No observed systematic difference between eddy covariance and chamber measurement methods. The standard trapezoidal method will typically underestimate emission factors.

42 citations

Journal ArticleDOI
TL;DR: In this paper, a dynamic chamber method was developed to measure fluxes of N2O from soils with greater accuracy than previously possible, through the use of a quantum cascade laser (QCL).
Abstract: A dynamic chamber method was developed to measure fluxes of N2O from soils with greater accuracy than previously possible, through the use of a quantum cascade laser (QCL). The dynamic method was compared with the conventional static chamber method, where samples are analysed subsequently on a gas chromatograph. Results suggest that the dynamic method is capable of measuring soil N2O fluxes with an uncertainty of typically less than 1–2 µg N2O-N m−2 hour−1 (0.24–0.48 g N2O-N ha−1 day−1), much less than the conventional static chamber method, because of the greater precision and temporal resolution of the QCL. The continuous record of N2O and CO2 concentration at 1 Hz during chamber closure provides an insight into the effects that enclosure time and the use of different regression methods may introduce when employed with static chamber systems similar in design. Results suggest that long enclosure times can contribute significantly to uncertainty in chamber flux measurements. Non-linear models are less influenced by effects of long enclosure time, but even these do not always adequately describe the observed concentrations when enclosure time exceeds 10 minutes, especially with large fluxes.

34 citations

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TL;DR: In this paper, a combination of eddy covariance, high-resolution dynamic chamber and static chamber methods was used to compare N2O fluxes from two adjacent intensively managed grazed grasslands in Scotland, one of which was tilled.
Abstract: . Intensively managed grass production in high-rainfall temperate climate zones is a globally important source of N2O. Many of these grasslands are occasionally tilled to rejuvenate the sward, and this can lead to increased N2O emissions. This was investigated by comparing N2O fluxes from two adjacent intensively managed grazed grasslands in Scotland, one of which was tilled. A combination of eddy covariance, high-resolution dynamic chamber and static chamber methods was used. N2O emissions from the tilled field increased significantly for several days immediately after ploughing and remained elevated for approximately 2 months after the tillage event contributing to an estimated increase in N2O fluxes of 0.85 ± 0.11 kg N2O-N ha−1. However, any influence on N2O emissions after this period appears to be minimal. The cumulative N2O emissions associated with the tillage event and a fertiliser application of 70 kg N ammonia nitrate from one field were not significantly different from the adjacent untilled field, in which two fertiliser applications of 70 kg N ammonia nitrate occurred during the same period. Total cumulative fluxes calculated for the tilled and untilled fields over the entire 175-day measurement period were 2.14 ± 0.18 and 1.65 ± 1.02 kg N2O-N ha−1, respectively.

27 citations

01 Apr 2017
Abstract: Abstract. Uptake (or negative flux) of nitrous oxide (N2O) in agricultural soils is a controversial issue which has proved difficult to investigate in the past due to constraints such as instrumental precision and methodological uncertainties. Using a recently developed high-precision quantum cascade laser gas analyser combined with a closed dynamic chamber, a well-defined detection limit of 4 μg N2O-N m−2 h−1 could be achieved for individual soil flux measurements. 1220 measurements of N2O flux were made from a variety of UK soils using this method, of which 115 indicated uptake by the soil (i.e. a negative flux in the micrometeorological sign convention). Only four of these apparently negative fluxes were greater than the detection limit of the method, which suggests that the vast majority of reported negative fluxes from such measurements are actually due to instrument noise. As such, we suggest that the bulk of negative N2O fluxes reported for agricultural fields are most likely due to limits in detection of a particular flux measurement methodology and not a result of microbiological activity consuming atmospheric N2O.

26 citations

Journal ArticleDOI
TL;DR: Four distinctive types of vertical profiles of both NH3 and fine particle light extinction coefficient (bext) were observed and they were associated with well-mixed atmosphere, fast accumulation of local emissions, regional transport aloft, and the formation of low urban boundary layer, respectively.

24 citations


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01 Dec 2016
TL;DR: Analysis of δ(15)N values of aerosol NH4(+) to be a promising new tool for partitioning atmospheric NH3 sources, providing policy makers with insights into NH3 emissions and secondary aerosols for regulation in urban environments is demonstrated.
Abstract: The reduction of ammonia (NH3) emissions is urgently needed due to its role in aerosol nucleation and growth causing haze formation during its conversion into ammonium (NH4(+)). However, the relative contributions of individual NH3 sources are unclear, and debate remains over whether agricultural emissions dominate atmospheric NH3 in urban areas. Based on the chemical and isotopic measurements of size-resolved aerosols in urban Beijing, China, we find that the natural abundance of (15)N (expressed using δ(15)N values) of NH4(+) in fine particles varies with the development of haze episodes, ranging from -37.1‰ to -21.7‰ during clean/dusty days (relative humidity: ∼ 40%), to -13.1‰ to +5.8‰ during hazy days (relative humidity: 70-90%). After accounting for the isotope exchange between NH3 gas and aerosol NH4(+), the δ(15)N value of the initial NH3 during hazy days is found to be -14.5‰ to -1.6‰, which indicates fossil fuel-based emissions. These emissions contribute 90% of the total NH3 during hazy days in urban Beijing. This work demonstrates the analysis of δ(15)N values of aerosol NH4(+) to be a promising new tool for partitioning atmospheric NH3 sources, providing policy makers with insights into NH3 emissions and secondary aerosols for regulation in urban environments.

143 citations

Journal ArticleDOI
TL;DR: An international model comparison and benchmarking exercise, showing the potential of multi-model ensembles to predict productivity and nitrous oxide (N2 O) emissions for wheat, maize, rice and temperate grasslands.
Abstract: Simulation models are extensively used to predict agricultural productivity and greenhouse gas emissions. However, the uncertainties of (reduced) model ensemble simulations have not been assessed systematically for variables affecting food security and climate change mitigation, within multi-species agricultural contexts. We report an international model comparison and benchmarking exercise, showing the potential of multi-model ensembles to predict productivity and nitrous oxide (N2O) emissions for wheat, maize, rice and temperate grasslands. Using a multi-stage modelling protocol, from blind simulations (stage 1) to partial (stages 2 –4) and full calibration (stage 5), 24 process-based biogeochemical models were assessed individually or as an ensemble against long-term experimental data from four temperate grassland and five arable crop rotation sites spanning four continents.Comparisons were performed by reference to the experimental uncertainties of observed yields and N2O emissions. Results showed that across sites and crop/grassland types, 23%–40% of the uncalibrated individual models were within two standard deviations (SD) of observed yields, while 42 (rice) to 96% (grasslands) of the models were within 1 SD of observed N2O emissions. At stage 1, ensembles formed by the three lowest prediction model errors predicted both yields and N2O emissions within experimental uncertainties for 44% and 33% of the crop and grass-land growth cycles, respectively. Partial model calibration (stages 2–4) markedly reduced prediction errors of the full model ensemble E-median for crop grain yields (from 36% at stage 1 down to 4% on average) and grassland productivity (from 44%to 27%) and to a lesser and more variable extent for N2O emissions. Yield-scaled N2O emissions (N2O emissions divided by crop yields) were ranked accurately by three-model ensembles across crop species and field sites. The potential of using process-based model ensembles to predict jointly productivity and N2O emissions at field scale is discussed.

100 citations

Journal ArticleDOI
TL;DR: In this article, the authors reviewed the national emissions, particle size distributions and chemical composition of each of these sources and concluded that the introduction of battery electric vehicles had only a small effect on overall road traffic particle emissions.

80 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present a methodological protocol for eddy covariance measurements of CH4 and N2O fluxes as agreed for the ecosystem station network of the pan-European Research Infrastructure Integrated Carbon Observation System and provide a first international standard that is suggested to be adopted more widely.
Abstract: Commercially available fast-response analysers for methane (CH4) and nitrous oxide (N2O) have recently become more sensitive, more robust and easier to operate. This has made their application for long-Term flux measurements with the eddy-covariance method more feasible. Unlike for carbon dioxide (CO2) and water vapour (H2O), there have so far been no guidelines on how to optimise and standardise the measurements. This paper reviews the state-of-The-Art of the various steps of the measurements and discusses aspects such as instrument selection, setup and maintenance, data processing as well as the additional measurements needed to aid interpretation and gap-filling. It presents the methodological protocol for eddy covariance measurements of CH4 and N2O fluxes as agreed for the ecosystem station network of the pan-European Research Infrastructure Integrated Carbon Observation System and provides a first international standard that is suggested to be adopted more widely. Fluxes can be episodic and the processes controlling the fluxes are complex, preventing simple mechanistic gap-filling strategies. Fluxes are often near or below the detection limit, requiring additional care during data processing. The protocol sets out the best practice for these conditions to avoid biasing the results and long-Term budgets. It summarises the current approach to gap-filling. (Less)

65 citations

Journal ArticleDOI
TL;DR: Rather than providing universally prescriptive management for N2O emission reduction, the evidence supports mitigation strategies based upon tailored nutrient management approaches that keep N balances within safe limits, so as to minimize N 2O emissions while still achieving high crop yields.
Abstract: Improved nitrogen (N) use is key to future food security and environmental sustainability While many regions still experience N shortages, agriculture is the leading global emitter of N2 O due to losses exacerbated by N surpluses in other regions In order to sustainably maintain or increase food production, farmers and their advisors need a comprehensive and actionable understanding of how nutrient management affects both yield and N2 O emissions, particularly in tropical and subtropical agroecosystems We performed a meta-analysis to determine the effect of N management and other factors on N2 O emissions, plant N uptake, and yield Our analysis demonstrates that performance indicators-partial N balance and partial factor productivity-predicted N2 O emissions as well as or better than N rate While we observed consistent production and environmental benefits with enhanced-efficiency fertilizers, we noted potential trade-offs between yield and N2 O emissions for fertilizer placement Furthermore, we observed confounding effects due to management dynamics that co-vary with nutrient application practices, thus challenging the interpretation of the effect of specific practices such as fertilization frequency Therefore, rather than providing universally prescriptive management for N2 O emission reduction, our evidence supports mitigation strategies based upon tailored nutrient management approaches that keep N balances within safe limits, so as to minimize N2 O emissions while still achieving high crop yields The limited evidence available suggests that these relationships hold for temperate, tropical, and subtropical regions, but given the potential for expansion of N use in crop production, further N2 O data collection should be prioritized in under-represented regions such as Sub-Saharan Africa

61 citations