Real time emission measurements were conducted at two commercial dairy freestall barns located in Midwestern (WI) U.S. during a 24 month period from 2007 to 2009. The two cross-flow mechanically ventilated buildings (275 and 375 cow capacities) were part of the National Air Emissions Monitoring Study (NAEMS), which collected emissions of several gases including ammonia (NH 3) and hydrogen sulfide (H2S) as well as particulate matter (PM) from various U.S. dairy, pig, and poultry buildings. Additional funds were obtained to install methane (CH4) and nitrous oxide (N2O) analyzers in the on-farm instrument shelter (OFIS) located adjacent to the freestall dairy barns. The OFIS contained gas analyzers and PM measurement instruments, calibration gas cylinders for the gas analyzers, a gas sampling system (GSS), and a computer for sampling control and data collection. Barn ventilation airflow rates were measured by recording run times of the exhaust fans (total of 125 fans). All exhaust fans were the same model (belt driven, 127 cm blade diameter with 0.746 kW motor). Airflow measurements of a representative number of similar exhaust fans in each building were conducted in situ at the start and halfway through the two year monitoring project with the Fan Assessment Numeration System (FANS). The fan airflow measurements were used to develop an airflow model for the fan model used on this farm, and to assess the uncertainty of the fan airflow model for estimating the total barn ventilation rate (VR). For the average barn VR, the measurement uncertainty was 4%. Comparison of this direct measurement was made with an indirect measurement (CO2 balance) which yielded close agreement.