Abstract
Abstract. Ammonia (NH3) fluxes were estimated from a field being grazed by dairy cattle during spring by applying a backward Lagrangian stochastic model (bLS) model combined with horizontal concentration gradients measured across the field. Continuous concentration measurements at field boundaries were made by open-path miniDOAS (differential optical absorption spectroscopy) instruments while the cattle were present and for 6 subsequent days. The deposition of emitted NH3 to clean patches on the field was also simulated, allowing both net and gross emission estimates, where the dry deposition velocity (vd) was predicted by a canopy resistance (Rc) model developed from local NH3 flux and meteorological measurements. Estimated emissions peaked during grazing and decreased after the cattle had left the field, while control on emissions was observed from covariance with temperature, wind speed and humidity and wetness measurements made on the field, revealing a diurnal emission profile. Large concentration differences were observed between downwind receptors, due to spatially heterogeneous emission patterns. This was likely caused by uneven cattle distribution and a low grazing density, where hotspots of emissions would arise as the cattle grouped in certain areas, such as around the water trough. The spatial complexity was accounted for by separating the model source area into sub-sections and optimising individual source area coefficients to measured concentrations. The background concentration was the greatest source of uncertainty, and based on a sensitivity/uncertainty analysis the overall uncertainty associated with derived emission factors from this study is at least 30–40 %.Emission factors can be expressed as 6 ± 2 g NH3 cow−1 day−1, or 9 ± 3 % of excreted urine-N emitted as NH3, when deposition is not simulated and 7 ± 2 g NH3 cow−1 day−1, or 10 ± 3 % of excreted urine-N emitted as NH3, when deposition is included in the gross emission model. The results suggest that around 14 ± 4 % of emitted NH3 was deposited to patches within the field that were not affected by urine or dung.
Highlights
Over 90 % of anthropogenic ammonia (NH3) emissions in Europe have agricultural sources (Erisman et al, 2008; Reidy et al, 2008; Hertel et al, 2011), of which 70–90 % have been estimated to be produced by livestock (Pain et al, 1998; Hutchings et al, 2001)
The configuration of the three miniDOAS sensors and the grazed field during Period 2 led to certain wind directions being unsuitable for emission estimates, while additional miniDOAS sensors placed at field boundaries would have been beneficial
The miniDOAS systems were well suited to the task, providing continuous high time resolution concentration measurements at field boundaries across the field
Summary
Over 90 % of anthropogenic ammonia (NH3) emissions in Europe have agricultural sources (Erisman et al, 2008; Reidy et al, 2008; Hertel et al, 2011), of which 70–90 % have been estimated to be produced by livestock (Pain et al, 1998; Hutchings et al, 2001). In addition to decreasing nitrogen efficiency for farming systems, the volatilisation of NH3 from agricultural areas is a principal factor in the formation of fine-fraction secondary aerosols due to its reactions with nitric and sulfuric acids in the atmosphere and upon deposition is linked to acidification and eutrophication of natural. Following the application of urine and dung to the soil surface by grazing livestock, urea is microbially converted to NH3 which is volatilised at rates which vary extensively depending on soil and canopy layer properties, weather and culture conditions (Laubach et al, 2013a). NH3 emissions have been measured from cattle urine patches at the ratio of 7–25.7 % of excreted urine nitrogen (N) for grazed pastures (Jarvis et al, 1989; Ryden et al, 1987; Laubach et al, 2012, 2013a), and measurements from sheep urine patches in summer–winter experiments have suggested emissions which represent 12.2–22.2 % of excreted urine-N (Sherlock and Goh, 1984)
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