Methods of Estimating Deposition Using Atmospheric Concentration Measurements: Using Synthetic Observations Downwind of a CAFO to Quantify Ammonia Deposition

Ammonia is an important chemical species in the atmosphere because of its role in aerosol formation and ability to act as a reactive nitrogen species in the global nitrogen cycle. However, because ammonia is challenging to measure with sufficient time resolution for eddy-covariance-type approaches, other strategies are needed to measure dry deposition in a cost-effective way. Livestock feeding operations are a major source of ammonia to the atmosphere and surrounding ecosystems; ammonia concentrations near these large feedlots can be many orders of magnitude higher than background. The impact that these feedlots can have on regional ecology and air quality can be difficult to determine, in large part due to the challenges of measuring and modelling ammonia dry deposition adjacent to these major sources. Feedlots housing ruminant livestock such as cattle are also sources of methane. Because methane does not undergo dry deposition and is chemically inert on relevant spatial scales, we can use it as a tracer for feedlot emissions.

Here, we demonstrate a new strategy to constrain ammonia deposition fluxes by measuring the ammonia:methane concentration ratio downwind of a feedlot, and using the change in this ratio to calculate the deposition rate. We use Large Eddy Simulation to simulate the dispersion of tracer species from an area source to represent ammonia and methane emissions from a feedlot. We then use the ammonia:methane concentration ratio to construct a mass balance, and calculate the depositional flux. To further test this using two realistic measurement platforms (small unmanned aerial vehicle and a truck), we sample our LES model output to produce pseudo-observations of the turbulent dispersion of our ammonia and methane tracers. We show that by measuring the ammonia:methane concentration ratios downwind of the source on an aerial platform, we can accurately estimate the depositional flux, while a surface-only measurement strategy is biased high by a factor of 2, due to the faster removal of ammonia near the surface.