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A down-flow biofilter was designed to treat exhaust air from a swine barn pit ventilation fan in Raleigh, NC. Computational Fluid Dynamics was used to model airflow to ensure spatial uniformity of treatment. The biofilter medium consisted of ~70% compost and 30% woodchips by volume. The biofilter was evaluated during August 2010 through April 2011 under different summer, fall, and winter conditions. The medium depth was 0.3 m, empty bed residence time (EBRT) was 7.6 s, residence time was 2.7 s, and the biofilter had a unit airflow rate (U) of 0.04 m3/m2-s. A photoacoustic multi-gas field monitor (Innova) was used to measure concentrations of ammonia, carbon dioxide, methane, and nitrous oxide. The Innova was evaluated with regard to its response time for ammonia, nitrous oxide, and methane. Boric acid scrubbers were also used to measure time averaged ammonia concentrations. Air samples were collected and analyzed in a gas chromatograph (GC) for methane and VOCs. Operating conditions such as temperature, medium moisture content, and system pressure drop were measured during biofilter operation. Pressure drop across the fan averaged 125 Pa. The biofilters removal efficiencies (RE) for ammonia ranged from 89 to 92%. Greenhouse gases methane and nitrous oxide REs ranged from 13 to 50% and 14 to 17% respectively, while carbon dioxide REs ranged from -6 to 37%. Results show that the biofilter can be effective at removing gases such as ammonia, but also, methane and nitrous oxide. The cost of the system was $1,225 per 0.50 m3/s.