Accounting for Field-Scale Dry Deposition in Backward Lagrangian Stochastic Dispersion Modelling of NH3 Emissions
A controlled ammonia (NH3) release experiment was performed at a grassland site. The aim was to quantify the effect of dry deposition between the source and the receptors (NH3 measurement locations) on emission rate estimates by means of inverse dispersion modelling. NH3 was released for three hours at a constant rate of Q = 6.29 mg s−1 from a grid of 36 orifices spread over an area of 250 m2. The increase in line-integrated NH3 concentration was measured with open-path optical miniDOAS devices at different locations downwind of the artificial source. Using a backward Lagrangian stochastic (bLS) dispersion model (bLSmodelR), the fraction of the modelled release rate to the emitted NH3 ( Q bLS / Q ) was calculated from the measurements of the individual instruments. Q bLS / Q was found to be systematically lower than 1, on average between 0.69 and 0.91, depending on the location of the receptor. We hypothesized that NH3 dry deposition to grass and soil surfaces was the main factor responsible for the observed depletion of NH3 between source and receptor. A dry deposition algorithm based on a deposition velocity approach was included in the bLS modelling. Model deposition velocities were evaluated from a ‘big-leaf’ canopy resistance analogy. Canopy resistances (generally termed R c ) that provided Q bLS / Q = 1 ranged from 75 to 290 s m−1, showing that surface removal of NH3 by dry deposition can plausibly explain the original underestimation of Q bLS / Q . The inclusion of a dry deposition process in dispersion modelling is crucial for emission estimates, which are based on concentration measurements of depositing tracers downwind of homogeneous area sources or heterogeneously-distributed hot spots, such as, e.g., urine patches on pastures in the case of NH3.
Soil greenhouse gas budget of two intensively managed grazing systems
Abstract Increasing the soil carbon stock by optimizing the grassland management is seen as a potential cost-effective mitigation strategy to counteract greenhouse gas (GHG) emissions from pasture fields associated with the application of fertilizer or cattle excreta. This study presents results of GHG flux measurements in a paired field experiment of two neighboring grazing systems over a full year. Each pasture was grazed by 12 dairy cows in a rotational grazing management. Different feeding strategies (system G: grazing only; system M: grazing with supplement of maize silage) resulted in different nitrogen excretion rates. The field scale emissions of CO2, methane (CH4) and nitrous oxide (N2O) were quantified using the eddy covariance (EC) technique on both parallel pasture fields excluding the direct emissions by the animals. Small-scale CH4 emissions from excreta patches and background areas were also measured using the ‘fast-box’ chamber technique. The fast-box measurements dominated by dung patch emissions were up-scaled to the field and compared to the EC measurements revealing some discrepancies between the two measurement approaches. N2O emissions resulted mainly from slurry and mineral fertilization (44 ± 17%) while both animal excreta and background emissions contributed each about 28% to the annual emissions. Total N2O emissions amounted to 3.8 ± 1.1 kg N2O-N ha−1 yr−1 and 3.7 ± 1.4 kg N2O-N ha−1 yr−1 for the pasture fields M and G, respectively. In order to determine the net ecosystem carbon budget (NECB) of the pasture fields, additional non-gaseous carbon fluxes were either measured (harvest, slurry application) or estimated based on an animal feeding model (grazing intake, excreta). For the investigated year, the NECB (negative sign indicating sequestration) resulted in –103 ± 74 g C m−2 yr−1 and –63 ± 67 g C m−2 yr−1 for pastures M and G, respectively, indicating the tendency of carbon storage especially in the pasture M soil. However, the emissions of CH4 and N2O almost balanced the NECB resulting in non-significant (near-neutral) net GHG budgets for the pasture soils (field M:–144 ± 277 g CO2-eq. m−2 yr−1, field G:–9 ± 256 g CO2-eq. m−2 yr−1). Due to the paired plot design of the experiment, the difference between the two pasture fields was significant with pasture M showing a reduction in net GHG emissions by 135 ± 89 g CO2-eq. m−2 yr−1 compared to pasture G.
Two instruments based on differential optical absorption spectroscopy (DOAS) to measure accurate ammonia concentrations in the atmosphere
Abstract. We present two Differential Optical Absorption Spectroscopy (DOAS) instruments built at RIVM: the RIVM DOAS and the miniDOAS. Both instruments provide virtually interference-free measurements of NH3 concentrations in the atmosphere, since they measure over an open path, without suffering from inlet problems or interference problems by ammonium aerosols dissociating on tubes or filters. They measure concentrations up to at least 200 μg m−3, have a fast response, low maintenance demands, and a high up-time. The RIVM DOAS has a high accuracy of typically 0.15 μg m−3 for ammonia for 5-min averages and over a total light path of 100 m. The miniDOAS has been developed for application in measurement networks such as the Dutch National Air Quality Monitoring Network (LML). Compared to the RIVM DOAS it has a similar accuracy, but is significantly reduced in size, costs, and handling complexity. The RIVM DOAS and miniDOAS results showed excellent agreement (R2 = 0.996) during a field measurement campaign in Vredepeel, the Netherlands. This measurement site is located in an agricultural area and is characterized by highly variable, but on average high ammonia concentrations in the air. The RIVM-DOAS and miniDOAS results were compared to the results of the AMOR instrument, a continuous-flow wet denuder system, which is currently used in the LML. Averaged over longer time spans of typically a day, the (mini)DOAS and AMOR results agree reasonably well, although an offset of the AMOR values compared to the (mini)DOAS results exists. On short time scales, the (mini)DOAS shows a faster response and does not show the memory effects due to inlet tubing and transport of absorption fluids encountered by the AMOR. Due to its high accuracy, high uptime, low maintenance and its open path, the (mini)DOAS shows a good potential for flux measurements by using two (or more) systems in a gradient set-up and applying the aerodynamic gradient technique.