Ammonia Emission Factors for Open-Lot Dairies: Direct Measurements and Estimation by Nitrogen Intake

Abstract

Ammonia (NH₃) emissions contribute to the formation of secondary particulate matter (PM) 10 μm and under (PM₁₀). Dairies are significant sources of NH₃ in the San Joaquin Valley (SJV) of California, where the National Ambient Air Quality Standard for PM₁₀ is frequently exceeded. Detailed descriptions of diets, animal demographics, and production levels were obtained for two commercial open-lot dairies in the SJV and used to compute nitrogen intake for each feeding group (g N day⁻¹). Models derived from nutrition trials with cows, heifers, and calves were used to estimate urea-N excretion from N intake. Air NH₃ concentrations were also measured at the same dairies over 1-week periods in February 1999. NH₃ fluxes calculated from vertical profiles of concentrations at two or three locations downwind of the dairies were augmented with estimates of flux based on single-height concentrations measured at five or six additional downwind locations to compute NH₃ emission rates. NH₃ emission potentials, estimated from urea-N excretion, exceeded NH₃ emission rates measured by the micrometeorological methods by 1.5- and 3-fold on the two dairies. A diurnal pattern in NH₃ emission factors based on measurements showed peak emission occurring between 1:00 p.m. and 6:00 p.m. at both dairies. NH₃ emission potentials and measured NH₃ emission rates were higher for Dairy 2, which reported feeding heifers dietary crude protein in excess of National Research Council recommendations.