Outdoor nitrogen dioxide from natural and anthropogenic sources also influences indoor levels. The most important indoor sources include tobacco smoke and gas-, wood-, oil-, kerosene- and coal-burning appliances such as stoves, ovens, space and water heaters and fireplaces, particularly unflued or poorly maintained appliances. Road traffic is the principal outdoor source of nitrogen dioxide. In indoor air, however, this oxidation process is generally much slower ( 2). In ambient conditions, nitric oxide is rapidly oxidized in air to form nitrogen dioxide by available oxidants (such as oxygen, ozone and VOCs) and this rapid oxidation velocity is such that it is nitrogen dioxide that is usually considered as a primary pollutant. Indeed, 90–95% of the nitrogen oxides are usually emitted as nitric oxide and only 5–10% as nitrogen dioxide, although substantial variations from one source type to another have been observed. The higher the combustion temperature, the more nitric oxide is generated. In ambient air, the oxides of nitrogen are formed by various combinations of oxygen and nitrogen at high temperatures during the combustion process. It reacts with water and is soluble in sulfuric and nitric acids. Its molecular weight is 46.01 g/mol, melting point −11.2 ☌, boiling point 21.15 ☌ and density 1.59 (air = 1).
It is a strong oxidant, corrosive and poorly soluble in water ( 1).
In that form, nitrogen dioxide is volatile, reddish-brown in colour and heavier than air, and has a characteristic pungent odour perceptible from a concentration of 188 μg/m 3 (0.1 ppm). While the boiling point of nitrogen dioxide is 21.15 ☌, in normal ambient conditions its low partial pressure in the atmosphere (908 mmHg at 25 ☌) prevents condensation so that it exists in the air in its gaseous form. In its liquid form, nitrogen dioxide is colourless to brown. Nitric oxide is oxidized in air to form nitrogen dioxide. Nitrous acid (HONO) is a common pollutant in ambient and indoor environments, produced by the reaction of nitrogen dioxide with water.
Ambient concentrations of these two gases vary widely according to local sources and sinks, but can exceed a total concentration (NO + NO 2) of 500 μg/m 3 in dense urban areas. However, nitric oxide (NO) and nitrogen dioxide (NO 2) are the two principal nitrogen oxides associated with combustion sources. Nitrous oxide (N 2O) is a greenhouse gas with significant anthropogenic sources contributing to its worldwide abundance (∼ 0.3 ppm). There are seven oxides of nitrogen that may be found in the ambient air.
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