Nitrogen oxides are a group of gases produced by the combustion of fossil fuels, especially when combustion takes place at high temperatures and pressures. Direct emissions are mainly in the form of nitric oxide (NO) with smaller amounts of nitrogen dioxide (NO2) and nitrous oxide (N2O). Nitric oxide and nitrogen dioxide are often lumped together as "NOx".

The main source of NO and NO2 in Saint John is the combustion of petroleum products by motor vehicles and electricity generating stations. Other NOx sources (not all of which are present within the Saint John metropolitan region) include kraft and paper mills, wood burning, waste incineration, oil refining and gas production, cement processing, steel and iron industries, and ethanol production.

Nitrogen oxide emissions can be controlled by a variety of means, including the use of catalytic converters, lowering combustion temperatures, careful air/fuel control during combustion, reducing the nitrogen content of fuels, and injection of either water or steam into the combustion chamber of boilers.

Nitric oxide persists only for a short time before reacting with ozone to form nitrogen dioxide. Nitric oxide is colourless, odourless and only slightly soluble in water. There are no air quality objectives or standards for nitric oxide.

Nitrogen dioxide (NO2) in high concentrations is a reddish gas with a sharp, pungent smell. In the environment it is never present in such large amounts, although if abundant it can lend a brownish colour to urban pollution. It is corrosive and a strong oxidiser - it assists fire. NO2 may combine with water in the air to produce acid rain, and, when combined with hydrocarbons in the presence of sunlight, contribute to the formation of ground-level ozone pollution. It may also contribute to the formation of visibility-reducing haze.

NO2 can irritate the lungs and lower resistance to respiratory infection, has an adverse effect on materials (for example, corrosion of metals, fading of fabric dyes, degradation of rubber) and can damage vegetation.

N2O (nitrous oxide) is not considered to be a directly harmful air pollutant in ambient air and there are no existing standards. However, it may cause indirect harm as it is a powerful greenhouse gas.

Nitric oxide

Nitric oxide is measured at two stations in Saint John (Forest Hills and Customs building).

Concentrations at the Customs building tend to be higher than those at Forest Hills, which reflects the greater impact of traffic emissions closer to the city centre. This can be seen in charts for both 1994 and 1995. There is some tendency for higher values in the summer at the Customs Building site. High peak values can occur at any time during weather conditions which restrict dispersion. There are no standards in effect for nitric oxide.

Since 1991, concentrations of NO have increased at both monitored locations, which may be a result of more vehicular traffic in the region.

Nitrogen dioxide is monitored at the same sites as nitric oxide. The maximum allowable 1 hour average for nitrogen dioxide is 210 parts per billion. Levels of NO2 in Saint John are moderate to low, with values seldom reaching 50% of the one hour average standard. The annual desirable and acceptable standards have not been exceeded in the period of record. Average monthly concentrations at Forest Hills were similar throughout 1994 and 1995. Concentrations tended to be higher at the Customs building site, and monthly means increased slightly at this site in 1995. Peak values did not show the same tendency and were similar in both years at Customs building. Monthly means showed a tendency to be lower in the winter months.

Considering the longer term trend since 1980, NO2 levels are apparently on the rise again after a general decreasing tendency since the early 1980's. The recent increase in NO2 is consistent with rising NO, as NO2 is produced in the air when NO combines with an oxidising agent such as ozone. It remains to be seen whether this recent upward trend will continue.

Since 1991, the relative concentrations at Forest Hills and Customs Building have reversed. Prior to 1991, Forest Hills had higher concentrations, often about double those recorded at the downtown location. Since 1991, concentrations have increased more steeply at the Customs location and in the last few years, values at that site have been considerably higher than those at Forest Hills. This change is probably the result of a site change at the Customs Building location in 1990-91. At that time, the monitoring location was moved approximately 75 m and this may have resulted in an increased exposure of the instrument to NO2. Whether or not this site move explains the switch to higher values at Customs Building relative to Forest Hills, it is clear that concentrations have been rising at both sites since 1991. Increasing motor vehicle traffic in the city centre may partially explain these changes.

It is difficult to obtain reliable indicators of vehicle activity which apply to regions, as opposed to individual roads. However, there is little doubt that the number of vehicle miles travelled in the Saint John metropolitan region has increased over the past 10-15 years. This trend seems to be continuing, although perhaps at a lesser rate than during the late 1980's. The number of vehicle trips made across the harbour bridge increased by approximately 45% between 1984 and 1995 to over nine million trips per year. Another index, calculated by taking the total number of vehicles crossing the top ten busiest intersections in Saint John, has increased a more modest 7% since 1984.

Attempting to relate ambient air pollutant concentrations to traffic activity in this way is simplistic, because tailpipe emissions from vehicles depend on many factors besides the number of vehicles and miles driven. Some important factors include vehicle age, state of maintenance, weather conditions, driver behaviour, what percentage of vehicles were fitted with emission control devices, average trip length and speed, road layout, and degree of traffic congestion.

Although measured values are in compliance with existing standards, NOx emissions are of concern as vehicle numbers and activity continue to increase, as does the consumption of electricity. All of these activities produce NOx. The production of NOx from industrial sources is regularly reviewed as industrial operating approvals are renewed.