NOx Chemistry

Nitrogen-containing compounds are key players in any discussion of tropospheric chemistry. Typical nitrogen containing compounds include nitrous oxide (N₂O), nitric oxide (NO), and nitrogen dioxide (NO₂). NO and NO₂ are collectively known as "NO_x", where the x represents a subscript.

The principle source of NO_x is the high-temperature combustion of fossil fuels. There are two primary sources of NO_x formation:

• fuel NO_x: occurs when nitrogen-containing compounds in fossil fuels are oxidized
• thermal NO_x: occurs when an oxygen-nitrogen mixture is raised to a high temperature.

In the thermal process, high temperature and high oxygen concentration favor NO formation. Rate constants for thermal NO_x production in the forward reactions are temperature dependent, while only the oxidation of NO is temperature dependent.

The mechanisms for fuel-based NO_x formation are not fully understood. It is known that the amout of NO formed during this process depends on the fuel-to-air equivalence ratio. The amount of fuel NO_x formed is not temperature dependent, but rather is a linear function of nitrogen content, usually measured in percentage (by weight). There appears to be little difference between fuels that are derived from petroleum, shale, or coal.

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Important NO_x are shown belowfor the three key nitrogen-containing compounds found in the troposphere: nitric oxide, nitrogen dioxide, and nitric acid (HNO₃). With NO, reacts with free radicals to form NO₂. It can also be oxidized in a reaction with atomic oxygen to form NO₂ and O₂. NO is also produced by the photolysis of NO₂ to form NO and a singlet oxygen.

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