The ARM ground-based NO/NO2/NOy (Nitrogen Oxides) Monitor is based on detecting chemiluminescence resulting from the highly specific reaction of ambient NO reacting with reagent ozone. NO2 is converted to NO using a high-power LED photolysis cell which has a 75-80% conversion efficiency. The NO/NO2 channel alternately measures NO and NO + NO2 measured with a modified commercial NO instrument. A second dedicated NO instrument measures total NOy following conversion to NO in a heated molybdenum converter with ~99% conversion efficiency. A thermally controlled inlet box is external to the AOS at a sampling height of 10 m and contains mass flow controllers and converters to eliminate losses in the tubing. The instrument system includes automatic calibration capabilities at the sampling inlet for NO and gas-phase titration of NO to NO2 to measure basic sensitivity and conversion efficiency. The molybdenum converter is also tested against HNO3 in the laboratory.
The instrument system includes an internal computer that controls multiple switching cycles, real-time display, and sends a complete set of raw data to the site data system.