Nitrogen dioxide (NO2) must first be transformed into NO before it can
be measured using the chemiluminescent reaction. NO2 is converted to
NO by a molybdenum NO2-to-NO converter heated to about 325 °C (the
optional stainless steel converter is heated to 625 °C).
The ambient air sample is drawn into the Model 42i through the sample
bulkhead, as shown in Figure 1–1. The sample flows through a capillary,
and then to the mode solenoid valve. The solenoid valve routes the sample
either straight to the reaction chamber (NO mode) or through the NO2-toNO
converter and then to the reaction chamber (NOx mode). A flow
sensor to the reaction chamber measures the sample flow.
Dry air enters the Model 42i through the dry air bulkhead, passes through
a flow switch, and then through a silent discharge ozonator. The ozonator
generates the ozone needed for the chemiluminescent reaction. At the
reaction chamber, the ozone reacts with the NO in the sample to produce
excited NO2 molecules. A photomultiplier tube (PMT) housed in a
thermoelectric cooler detects the luminescence generated during this
reaction. From the reaction chamber, the exhaust travels through the ozone
(O3) converter to the pump, and is released through the vent.
The NO and NOx concentrations calculated in the NO and NOx modes
are stored in memory. The difference between the concentrations is used to
calculate the NO2 concentration. The Model 42i outputs NO, NO2, and
NOx concentrations to the front panel display, the analog outputs, and also
makes the data available over the serial or Ethernet connection.