Airborne and ground-level characterization of the ammonia plume from a beef cattle feedlot

In Canada, approximately 45% of NH3 emissions come from the cattle industry (based on data from 1995). As part of an effort to establish national emission inventories and to improve air quality forecasting capabilities, a study was recently conducted in Southern Alberta to investigate the emissions of a mid-size feedlot (20,000 head). A Cessna 207 equipped with fast NH3 and NOy instrumentation as well as an aerosol mass spectrometer (AMS) was flown in a dense grid pattern (spacing between tracks ~ 200m) covering an area of 5 by 10 km containing the 800 by 800 m feedlot, at several altitudes ranging from 30 to 300 m above ground. On the ground, this was supported by a more extensive suite of instruments on a mobile lab, as well as stationary measurements of long-path integrated NH3 concentrations and stability parameters downwind of the feedlot. Three flights were conducted in late September 2005 under varying meteorological conditions, ranging from very calm (unstable stratification) to very windy (nearly neutral). NH3 mixing ratios of 50 ppb were encountered up to 300 m above ground on the calm day. PM1.0 mass derived from the AMS during this flight was composed primarily of nitrate (40%) and organics (40%) with sulphate and ammonium contributing ~10% each. On average, the inorganic fraction of the particles was neutralized by NH4+ with particle formation appearing to be limited by inorganic acid levels.

The data collected provide an unprecedented opportunity to evaluate a Lagrangian dispersion model (WindTrax) which is used to predict emission rates from the concentration and turbulence measurements downwind of the source. The emission rate thus estimated for the feedlot (4 – 16 g/m2/day) was then fed into the model to predict the physical (3-D) plume characteristics, given the meteorological conditions. Comparison of the predicted plume characteristics with those observed on the aircraft provides information on factors not considered by the model, such as deposition or chemical transformation, and may shed light on other limitations and potential improvements. The end goal of the project is two-fold: to provide better estimates of the local net emission rate, and to improve the characterization of the sub-grid-scale dispersion and transformation of NH3 to account for these nonlinear processes in the national air quality models. Aircraft, mobile and stationary ground results will be presented and contrasted with model results.