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Highlights • Modeling the bi-directional NH 3 exchange at the field scale using SURFATM-NH 3. • Integrating an operational parameterization of emission potentials in the model. • Parameterizing the effect of urease inhibitor on NH 3 volatilization. • Satisfactory simulations of NH 3 fluxes by integrating the effect of urease inhibitor. • Good predictions of dynamic and order of magnitude of emission potentials by the model. Abstract Agriculture is the main source of ammonia (NH 3) emissions in the atmosphere. NH 3 is precursor to secondary fine particulate matter, which is of concern for its impacts on health and visibility. There are a limited number of field measurements of NH 3 emissions from fertilizer application in the US, and this limits our understanding of the importance of individual NH 3 source and sink processes in controlling timing and magnitude of NH 3 emissions. In this study, a new parameterization of the effect of urease inhibitor on NH 3 emissions from urea-based fertilizer was developed on the basis of experimental results found in the literature. This parameterization was combined with an existing operational parameterization of soil and stomatal emission potentials (Γ g , Γ s) and was implemented in a surface-atmosphere transfer model for NH 3 (SURFATM-NH 3) in order to evaluate the bi-directional fluxes of NH 3 at the field scale. The model was evaluated with field measurements obtained by the flux-gradient (FG) and relaxed eddy accumulation (REA) methods in a fertilized corn field in central Illinois. By integrating the effect of urease inhibitor, the timing of the highest NH 3 emission peak was successfully predicted and its magnitude was close to that measured (predicted 2106 ng m−2 s−1, measured by FG 2312 ± 582 ng m−2 s−1). Based on the model results, urease inhibitor has a considerable effect on the dynamics and order of magnitude of NH 3 fluxes. Furthermore, the model simulated the inhibiting action of N- (n -butyl) thiophosphoric (nBTPT) and suggests that it can reduce NH 3 volatilization by 32%. The model also successfully predicted environmental parameters, such as soil temperature. Finally, this new version of SURFATM-NH 3 is a valuable tool to estimate the NH 3 bi-directional fluxes at the field scale, which describes dynamic modeling of Γ s and Γ g by taking into account the effect of urease inhibitor which is commonly used in the US to improve the efficiency of urea fertilizers. [ABSTRACT FROM AUTHOR]

Highlights • Inter-comparison of relaxed eddy accumulation and flux gradient measurements. • Strong correlation between the two NH 3 flux measurement methods. • Peak NH 3 flux measured with both systems six days after fertilizer application. • Two elevated emission periods influenced by urease inhibitor and environmental conditions. Abstract Studies of NH 3 flux over agricultural ecosystems in the USA are limited by low temporal resolution (typically hours or days) and sparse spatial coverage, with no studies over corn in the Midwest USA. We report on NH 3 flux measurements over a corn canopy in Central Illinois, USA, using the relaxed eddy accumulation (REA) and flux gradient (FG) methods, providing measurements at 4 h and 0.5 h intervals, respectively. The REA and FG systems were operated for the duration of the 2014 corn-growing season. Flux-footprint analysis was used to select data from both systems, resulting in 82 concurrent measurements. Mean NH 3 flux of concurrent measurements was 205 ± 300 ng m−2 s−1 from REA and 110 ± 256 ng m−2 s−1 from FG for all concurrent samples. Results from both methods were not significantly different at a 95% confidence level for all concurrent measurements. The FG system resolved NH 3 emission peaks at 0.5 h averaging time that were otherwise un-observed with 4 h REA averaging. Two early-season peak emission periods were identified (DOY 130-132 and 140-143), where the timing and intensity of such emissions were attributed to a combination of urease inhibitor, applied as a field-management decision, and localized soil temperature and precipitation. Given the dependence of NH 3 fluxes on multiple parameters, this study further highlights the need for increased spatial coverage and high temporal resolution (e.g., <1 h) of measurements to better understand the impact of agricultural NH 3 emissions on air quality and the global nitrogen cycle. Such measurements are also needed for evaluation of models describing surface-atmosphere exchange of NH 3. [ABSTRACT FROM AUTHOR]

The objective of this research is to quantify NH 3 flux above an intensively managed cornfield in the Midwestern United States to improve understanding of NH 3 emissions and evaluations of new and existing emission models. A relaxed eddy accumulation (REA) system was deployed above a corn canopy in central Illinois, USA (40°3′46.209″N, 88° 11′46.0212″W) from May through September 2014 (day of year 115–273) to measure NH 3 fluxes due to chemical fertilizer application. NH 3 flux was measured in four-hour periods during mornings and afternoons. Mean atmospheric NH 3 concentration during the complete measurement period was 2.6 ± 2.0 μg m −3 . Larger upward fluxes of gaseous NH 3 were measured during the first 30 days after fertilization, with variations observed throughout the field campaign. Measured NH 3 fluxes ranged from −246.0 ng m −2 s −1 during wintertime background measurements to 799.6 ng m −2 s −1 within two weeks of fertilization (where negative flux indicates deposition). Mean positive flux was 233.3 ± 203.0 ng m −2 s −1 in the morning and 260.0 ± 253.3 ng m −2 s −1 in the afternoon while mean negative flux was −45.3 ± 38.6 ng m −2 s −1 in the morning and −78.35 ± 74.9 ng m −2 s −1 in the afternoon. NH 3 volatilization during the first 21 days after fertilization accounted for 79% of total nitrogen loss during the growing season. Such measurements are critical to improve understanding of agricultural NH 3 emissions in managed agricultural ecosystems dominated by rotations of highly fertilized corn and moderately to lightly fertilized soybeans, such as the plot studied herein. These measurements are also important to improve understanding of how managed agricultural ecosystems impact air quality, and contribute to the global nitrogen cycle, and to evaluate current NH 3 emission models. [ABSTRACT FROM AUTHOR]

Abstract: The seasonal and interannual variability in surface energy exchange and evapotranspiration (E) of two temperate semi-arid grasslands in southeastern Arizona, USA, were investigated using multi-year (2004–2007) eddy covariance measurements. The study sites, a post-fire site (AG) and an unburned site (KG), received 43–87% of the annual precipitation (P) during the North American Monsoon season (July–September), with the lowest values in the drought years of 2004 and 2005. Irrespective of the differences in temperature, surface albedo, vegetation cover and soil characteristics both sites responded similarly to changes in environmental conditions. The seasonal and interannual variations in the partitioning of net radiation to turbulent fluxes were mainly controlled by P through the changes in soil water content (θ) and vegetation growth. Drastic changes in albedo, vegetation growth and energy fluxes occurred following the onset of the monsoon season in July. During the dry or cool periods of autumn, winter and spring, the sensible heat flux was the largest component of the energy balance, whereas latent heat flux dominated during the warm and wet periods of summer. The dry-foliage Priestley–Taylor coefficient (α) declined when θ in the 0–15cm soil layer dropped below 0.08m3 m−3 at AG, and 0.09m3 m−3 at KG, respectively. The July–September average of dry-foliage surface conductance, α and E, reached their lowest values in 2004 at AG and in 2005 at KG. During July–September, monthly E was linearly correlated to the monthly mean θ and the broadband normalized difference vegetation index (NDVI), whereas during May–June the relationship between NDVI and E was not significant. Annual E varied from 264 to 322mm at AG and from 196 to 284mm at KG with the lowest value during the severe drought year at the site. July–September E had positive correlation with total P, the mean NDVI and the number of growing season days during that period. Annual P explained more than 80% of the variance in annual E. [Copyright &y& Elsevier]