Abstract
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.
Highlights
Estimation of trace gas emission from confined source areas on a local scale (i.e., receptor distance to sources less than 500 m) using the combination of inverse dispersion modelling with either concentration or flux measurements is a widespread method, especially in the agricultural sector (e.g., [1,2,3,4,5,6]).Atmosphere 2018, 9, 146; doi:10.3390/atmos9040146 www.mdpi.com/journal/atmosphereIn particular, the combination of concentration measurements with backward Lagrangian stochastic modelling is a convenient way of emission estimation that has spurred its utilization in the past decade [7]
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
The combination of concentration measurements with backward Lagrangian stochastic modelling is a convenient way of emission estimation that has spurred its utilization in the past decade [7]
Summary
Estimation of trace gas emission from confined source areas on a local scale (i.e., receptor (measurement) distance to sources less than 500 m) using the combination of inverse dispersion modelling with either concentration or flux measurements is a widespread method, especially in the agricultural sector (e.g., [1,2,3,4,5,6]).Atmosphere 2018, 9, 146; doi:10.3390/atmos9040146 www.mdpi.com/journal/atmosphereIn particular, the combination of concentration measurements with backward Lagrangian stochastic (bLS) modelling is a convenient way of emission estimation that has spurred its utilization in the past decade [7]. Estimation of trace gas emission from confined source areas on a local scale (i.e., receptor (measurement) distance to sources less than 500 m) using the combination of inverse dispersion modelling with either concentration or flux measurements is a widespread method, especially in the agricultural sector (e.g., [1,2,3,4,5,6]). Cm , both in μg NH3 m−3 , Qsrc is the emission rate of the source in μg NH3 m−2 s−1 and Q is sim the modelled C/Q ratio in s m−1 calculated from Equation (1) if dry deposition was not included n o or Equation (19) if deposition was included in the model run.
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