Mr Bob Joynt, Environmental Consultant and Mr Stephen Wu, Combustion Engineering Consultant
Environment Australia, February 2000
NOx refers to the gaseous nitrogen oxide compounds. In the discussions of nitrogen oxide combustion products and their impact to environment, the major nitrogen oxide species of concern are nitric oxide (NO) and nitrogen dioxide (NO2).
Under high temperature combustion conditions, the formation of NO is favoured and consequently, less than 10% of the NOx in typical exhaust is in the form of NO2 (Pereira and Amiridis, 1995). However, a higher percentage of NOx in the form of NO2 has been experienced in domestic applications. NO when it cools down in the atmosphere combines with oxygen in air to form NO2 (Eqn. 1).
In warm, sunny days the NO2 breaks down into NO and a nascent oxygen atom (Eqn. 2) which can combine with a molecule of oxygen to form ozone (Eqn. 3). The ozone reacts with NO to yield back NO2 almost as fast as it is formed.
When volatile organic compounds (VOC) exist in the air, they combine with the NO in the present of sunlight to change it back to NO2. Less NO is then available to remove the nascent oxygen, and hence ozone accumulates, resulting in photochemical smog.
The major effects of NOx on the ambient environment are:
- Health effects of NO2
Health effects are essentially the same indoors and outdoors if they are caused by the same concentrations and exposures under the same conditions.
- Health and visibility effects of photochemical smog
NOx and reactive organic compounds are the major contributors to the formation of ozone and other chemicals resulting from photochemical reactions in the atmosphere.
- Ecological and material damage from acidic precipitation
Extensive NOx emissions can contribute to acid precipitation but this is not significant in Australia.
Ambient air standards and goals were introduced in the National Environment Protection Measure for Ambient Air Quality (NEPC, 1998a). The one-hour average standard for NO2 is 0.12 ppm, with a goal to exceed this level on no more than one day a year to be achieved within ten years. There is also a one-year average standard of 0.03 ppm NO2 with a goal for no exceedances within ten years.
The health effects of NO2 have been summarised in many places, e.g. by EPAV (1993) and NEPC (1998b), where there is additional discussion of other effects of NOx and the costs and benefits of options for regulation of ambient NOx in Australia.
Indoor air quality studies indicate that substantially higher levels of NO2 and other pollutants can occur inside domestic buildings with gas appliances than outside. Recent Australian studies of indoor NO2 include Nitschke et al (1998), Brown (1998), Keywood et al (1998), Immig et al (1997), Garrett et al (1996) and Steer et al (1991). NOx emitted indoors would be partially removed by sinks before leaving the building.