<abstract> <b><sc>Abstract.</sc></b> Livestock barn emissions can affect public health, the environment, and quality of life. While these emissions can be mitigated using several methods, exhaust air treatment may be required in some situations. Biofiltration is one of the most cost-effective exhaust air treatment methods. In a biofilter, polluted air passes through a moist medium (e.g., compost) where the water-soluble gases are dissolved and then degraded by microorganisms into harmless or less harmful compounds. In this study, a downflow biofilter using a compost and wood chip medium was evaluated over summer, fall, and winter (August 2010 to January 2011) for its ability to mitigate emissions of ammonia (NH₃) and three greenhouse gases (GHGs): methane (CH₄), nitrous dioxide (N₂O), and carbon dioxide (CO₂). Biofilter medium properties were analyzed at the beginning and twice during the study. Changes in medium properties and CO₂ data indicated greater heterotrophic microbial activity during summer through fall and greater autotrophic activity during fall through winter. Regardless of empty bed residence time (EBRT) (5.3 to 26 s), NH₃ removal efficiency (RE) was about 90% with inlet concentrations of ≤1.1 mg m^-3. With higher NH₃ loading rates, the RE may differ from this study. In fall, CH₄ RE was 49% (EBRT = 26 s) but only 13% in summer (EBRT = 13 s). Nitrous oxide RE varied in a narrow range of 14% to 18% over the study. In summer, CO₂ removal was negligible but was 15% in fall and 34% in winter. While a compost based medium may be more effective for CH₄ and N₂O mitigation, a wood chip based medium would be more economical. Care should be taken when using a photoacoustic sensor for high-frequency and low-concentration NH₃ measurements.