Ammonia emission factors for open-lot dairies: direct measurements and estimation by nitrogen intake.

Ammonia (NH3) emissions contribute to the formation of secondary particulate matter (PM) 10 microm 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.