Oxides of Nitrogen (NO2, NO and NOx)
Oxides of nitrogen (NOx) are the total of nitrogen dioxide
(NO2) and nitric oxide (NO). During high temperature combustion
as in the burning of natural gas, coal, oil and gasoline atmospheric
nitrogen may combine with molecular oxygen to form NO. NO is colourless
and odourless. Most NO in the ambient air will react with O3
to form NO2. NO2 is a reddish-brown gas with a
pungent odour and is partially responsible for the "brown haze" observed
near large cities.
Transportation (automobiles, locomotives and aircraft) is the major
source of NOx in Alberta. Other major sources include industrial
sources (oil and gas industries) and power plants. Smaller sources of
NOx include natural gas combustion, heating fuel combustion,
and forest fires. The largest urban source of NOx is emissions
from motor vehicles.
Monitoring Method
NOx are measured continuously by the principle of chemiluminescence.
In this method, the air sample is split into two pathways; one to measure
NO, and the other to measure total NOx. In the first pathway, the sample goes
directly to the analysis chamber, is mixed with O3, and light is produced. The
amount of light detected is proportional to the NO concentration and is a measurement of
NO in the sample air. In the second pathway, a catalytic converter is used to first change
all the NO in the sample air to NO2, and then the sample goes on to
the analysis chamber. This measurement is the sum of NO2 and NO, expressed as
NOx. The difference of the readings between the two pathways is determined
electronically and is the NO2 concentration.
Objectives
Air quality Objectives for NO2 are based on prevention of
human health effects. The Objectives are:
- 0.212 ppm as a 1-hour average concentration;
- 0.106 ppm as a 24-hour average concentration; and
- 0.032 ppm as an annual average concentration.
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