Figure 5.12.8). The term NOx refers to all nitrogen oxides; however, in air pollution work, NOx refers only to NO (about 95-98%) and NO2. Because NO is essentially transparent in the visible and UV light region, it must be converted to NO2 before it can be measured. NO can then react with ozone to form NO2 with chemiluminescence. The light emitted can be measured photometrically as an indication of the reaction's extent. With excess ozone, the emission is in proportion to the amount of NO. Including NO2 (which reacts slowly with ozone) involves bringing the sample gas in contact with a hot molybdenum catalyst. This technique converts all NO2 to NO before it reacts with ozone.

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