Abstract
AbstractThe integrated horizontal flux method is commonly used to estimate ammonia emission from field‐applied manure. The method involves measuring the wind speed and ammonia concentration at various heights on a post in the middle of a manured plot. Wind speed and concentration profiles are subsequently fitted to these measurements. The product of the profiles represents the amount of ammonia displaced by the wind, and the calculated ammonia emission is based on integrating the product of the profiles along the height. A crucial step is the functional form of the profiles, and linear relationships employing the logarithm of the height are generally used. In this study, 160 Dutch emission experiments on grassland were re‐analysed to evaluate alternative profiles for the concentration and wind speed. It is shown that an exponential concentration model usually provides a better fit than the commonly used profile and that the measurement error for the concentration should be modelled by means of a gamma distribution. Based on the re‐analysis, this new model reduces the calculated ammonia emission by around 10%. It is further shown that adding a displacement parameter to the wind speed model only has a minor effect on the calculated emission. Finally, a simulation study reveals that misspecification of the concentration profile may lead to a relative bias of up to 27%, that the precision of the estimated emission can be improved by increasing the number of concentration measurements near the ground and that wind speed measurements at three heights could suffice.
Highlights
Over the last decades, many studies have been carried out to estimate the ammonia (NH3) emission from field-a pplied manure (Hafner et al, 2018)
This paper proposes an alternative statistical model for the concentration profile which employs an exponential relationship between the ammonia concentration and height, includes the measured background concentrations to fit the model and uses a gamma distribution for the concentration measurement error
The modified Park test for heteroscedasticity resulted in an estimate of 1.87 for the power P in the variance function; detailed results can be found in Appendix S4. This indicates that the measurement errors are more or less gamma-distributed, with P = 2, and the exponential model was fitted employing the gamma distribution
Summary
Many studies have been carried out to estimate the ammonia (NH3) emission from field-a pplied manure (Hafner et al, 2018). The IHF method as applied by Ryden and McNeill (1984), here referred to as the R&M approach, involves application of manure on a medium-sized plot, measuring the concentration of ammonia at various heights both at a windward location and at a leeward location, and measuring the wind speed at the location These measurements are used to fit a wind speed and concentration profile by means of a linear regression on the logarithm of the measurement height, from which the resulting ammonia emission is calculated. The IHF wind speed regression model, as applied by R&M, does not include a displacement height parameter which takes into account that, depending on the vegetation or other obstacles on the soil, a zero average wind speed may be reached at some height above the surface (Oke, 1987) Omission of this parameter could exaggerate the horizontal transportation of ammonia close to the ground, resulting in an overestimation of the ammonia emission
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