Nitrogen Oxides

Introduction

Nitrogen is the most abundant component of the atmosphere. Its diverse forms are critical to plant health in the biosphere and energy equilibrium above it. Most atmospheric nitrogen occurs in the gaseous molecular form, (N₂), but lesser amounts occur as nitrous oxide (N₂O), nitrogen dioxide NO₂, nitric oxide (NO) and other oxides of nitrogen as well as ammonia (NH3). The lowest threshold of injury due to nitrogen dioxide appears to be around 2 ppm for a 4-hour exposure, which is rarely exceeded in nature. Average exposure, throughout the growing season would have to exceed 0.25 ppm in order to expect any growth suppression (Ashenden, 1979a). harmful effects of oxides of nitrogen, especially nitrogen dioxide, have been recognized for many years. Alone, but more particularly in pollutant interaction, they can adversely affect plant growth when concentrations exceed natural background levels.

Sources

Nitrogen oxides occur naturally and also are produced by man's activities. In nature, they are a result of bacterial processes, biological growth and decay, lightning, and forest and grassland fires. The primary source of man-made nitrogen oxides is from the burning of fossil fuels. Of the nitrogen oxides emitted, most is nitric oxide, some is nitrous oxide and less than 10 per cent is nitrogen dioxide. The amount of nitrogen dioxide emitted varies with the temperature of combustion; as temperature increases so does the level of nitrogen dioxide. Agriculture also plays a role in nitrogen oxide emissions with the use of fertilizers contributing nitrous oxide to the atmosphere. Alberta contributes about 23 per cent of the 1887-kilo tones of nitrogen oxides emitted annually in Canada. In Alberta, the oil and gas industry is responsible for about 41 per cent of nitrogen oxide emissions, transportation (including planes, trains and automobiles) 25 per cent, electrical utilities 17 per cent, and other industrial, commercial and residential users the remaining 17 per cent.

Emission of Nox from different Sources :

Category	Percent
Other Transportation	40%
Road Vehicles	28%
Other Industrial Processes	11%
Utilities	8%
Cement and Concrete	5%
Miscellaneous	6%
Smelters/Primary Metals	2%

Effects

• At high concentration levels, nitrogen dioxide is potentially toxic to plants, can injure leaves and reduce growth and yield.
• In combination with either ozone (O₃) or sulphur dioxide (SO₂), nitrogen dioxide may cause injury at even lower concentration levels.
• As one of the components of smog, nitrogen dioxide is known to irritate the lungs and increase susceptibility to respiratory infections.
• Nitrous oxide is a greenhouse gas. As well it contributes to ozone depletion in the stratosphere, as discussed in the fact sheets Greenhouse Effect and Ozone - Stratospheric and Ground-Level.
• Nitric oxide by itself is non-toxic, but it is readily converted in the air to nitrogen dioxide.

Indicator

Some crop species such as bean, leak and pea are regarded as very sensitive to nitrogen oxides. Kovacs (1992b) lists sensitive and accumulating plant indicators of nitrous gases.

Coniferous tree

Chlorosis of young needles is a common symptom in response to NOx. Tip burn of older needles is often observed in coniferous species. Pine trees display bleaching followed by sharply defined red/brown bands between necrotic and healthy tissue in older needles. Immediate abscission of older needles occurs in spruce.

Deciduous trees and shrubs

Herringbone necrosis in the older leaves of beech, hazel and apple trees have been observed. Ivory necrosis, red/brown necrosis and black necrosis have been recorded in certain species.

Herbs/grasses/crops

Many species show a water soaked appearance on the leaves followed by necrosis in response to acute NOx exposure. Leaf glazing has been observed in annual poa, cabbage and spinach. Nectrotic streaking and interveinal necrosis has been recorded in many narrow-leaved & broad-leaved species. Legumes among many other species show ivory necrosis while some species display yellow, orange or brown necrosis. Tip necrosis has been observed on other plant parts such as awns, bracts & sepals.

Factor Effecting Sensitivity

Young leaves and needles are more sensitive to NOx than older ones. High relative humidity and N deficiency increase plant sensitivity to Nox, whereas N excess and drought conditions decrease sensitivity. NOx-sensitive conditions are associated with high VOC/NOx ratios and VOC-sensitive conditions are associated with low VOC/NOx ratios. A more reactive VOC mix is associated with NOx-sensitive conditions. NOx-sensitive versus VOC-sensitive conditions are correlated with the ratio of reactivity-weighted VOC to NOx, rather than the ratio of the simple sum of VOC to NOx. Naturally occurring VOC (especially isoprene, emitted primarily by oaks and other deciduous trees) represents a significant fraction of total ambient VOC, especially in suburban and rural settings and when VOC is weighted by reactivity. Biogenic VOC is especially important because they are usually highly reactive. Biogenic NOx is far smaller relative to the anthropogenic source. A high rate of biogenic VOC increases the ratio of reactivity-weighted VOC to NOx and makes NOx-sensitive conditions more likely. (Chameides et al., 1988, 1992; Pierce et al., 1998).