Nitrogen oxide monitors are used to collect continuous measurements of nitric oxide, nitrogen dioxide, and total oxides of nitrogen. The instrument actually detects only NO. NO monitor makes NO reacts in the sample with ozone. This reaction produces light, which can then be measured. To determine the amount of NO2 in the air, it converts the NO2 to NO and then measures that by again reacting with ozone. The second reading is actually total NO and the amount of N02 is assumed equal to the difference between the first and second readings.