The woodchips-based gas phase biofilter is capable of mitigating airborne ammonia efficiently. The ability to monitor real-time moisture content of biofilter media is preferred for maintaining the ammonia mitigation efficiency. The focus of this study is to enhance the understanding of impedance-based moisture content measurement and improve methodologies to monitor the moisture content of gas phase ammonia mitigation biofilters more accurately. A proposed sensor consisting of a sensing unit (three parallel plates) and a circuit generating DC voltage outputs was used in the study. The sensor readings changed with step-wise increase of moisture content as well as different particle size distribution and nitrogen (ammonia-nitrogen, nitrate-nitrogen) concentrations of biofilter media. The results showed the particle size distribution and nitrogen concentrations have significant impacts on impedance-based moisture sensing. A mathematical model was established to correlate the sensor reading of the impedance-based moisture sensor with moisture content, ammonia-nitrogen concentration, and nitrate-nitrogen concentration based on experimental results. Further studies are needed to improve the performance of the sensor considering other potential factors.