International Air Quality Advisory Board
SPECIAL REPORT ON
2. NITROGEN OXIDES - THE PIVOTAL POLLUTANT?
In considering the recent history of air quality assessment and air pollution management, the Board suggests that nitrogen oxides are currently the most significant of the common pollutants.
Nitrogen oxides are a collection of gaseous nitrogen species that play a major role in atmospheric chemistry and that can significantly affect ecosystems and human health. The predominant source of NOX is the combustion of fossil fuel to produce thermal, electrical, and mechanical energy. NOX is most often emitted as NO, sometimes accompanied by smaller amounts of NO2. It contributes to the formation of photochemical oxidants (i.e. ozone) that cause adverse effects in humans, plants and materials. Nitrogen species are also implicated in global warming and stratospheric ozone depletion (i.e. formation of the "ozone hole").
In air, NOX is converted to nitric acid and particulate nitrate, which then contribute to precipitation acidity and PM2.5, and negatively affect visibility and human health. Acid precipitation and deposition of nitrogen species have resulted in episodic acidification of fresh waters, fish kills in eastern North America, eutrophication of estuaries, soil-water acidification in forests, and changes in soil microbes.
2.1 Health Effects
It has proven difficult to establish with certainty the existence of adverse human health effects solely attributable to ambient NO2. Animal studies give evidence that NO2 interferes with lung defenses, which may explain the considerable amount of data indicating that, at current high levels, NO2 exposure (both indoors and outdoors) increases the incidence of respiratory infections in children. One episode in London, England, in December 1991, when NO2 reached 400 parts per billion (ppb) for eight consecutive hours on two consecutive days, did cause increases in the values of some health indicators, but there was no explosive increase in asthma attacks. It was not possible to clearly distinguish the effects of the rise in NO2 level from the concomitant rise in particulate matter in this case.
Studies on human subjects have shown that pre-exposure to low levels of NO2 increases the response to subsequent exposure to low levels of ozone -- a phenomenon that may be of significance in real-life situations. However, a great deal is not yet known about the patterns of human exposure to NO2: it is difficult to make inferences from differing annual exposure levels, and the close correlation between NO2 and PM10 in many environments also complicates epidemiological studies attempting to isolate the effects of NO2.
2.2 The Importance of Monitoring
Since NOX is a collection of compounds that can be transformed into such secondary species as nitric acid, ammonium nitrate, and organic nitrogen, it is important to understand the speciation distribution of nitrogen compounds in both emissions and secondary air pollutants. Such an understanding, which is essential for determining how NOX affects the environment and human health, requires routine and continuous chemical monitoring of emissions, ambient air, and deposition. Without this, the role of NOX in the atmosphere and ecosystem cannot be completely and quantitatively understood. Currently, our knowledge of NOX releases from pollution sources is based on a vehicle emission inventory, which is believed to be comprehensive, and on emission inventories for stationary, area, and fugitive sources, which are comparatively incomplete.
2.3 Emission and Deposition Trends
Recent trends in nitrogen speciation and deposition can be derived from wet deposition data of the U.S. National Atmospheric Deposition Program (NADP). Trends in emissions in the United States indicate that NOX emissions appear to have remained level over the past decade, while sulfur oxide (SOX) emissions have been substantially reduced.
In some areas, however, data indicate that wet nitrate deposition has increased, while sulfate deposition has decreased. For example, NADP precipitation chemistry data, which track the fate and transport of pollutants from local and upwind sources, show transfers of air contaminants to sensitive Vermont aquatic and terrestrial ecosystems. Figure 2-1 shows nitrate and sulfate concentrations in rain and snow in Bennington, Vermont, from 1981 to 1995. Nitrate concentrations appear to have increased, while sulfate concentrations have decreased. This decline in sulfate deposition is believed to be a consequence of SOX reductions from power plants in the eastern United States, where further reductions are anticipated as Title IV of the U.S. CAA continues to be implemented. The increase in nitrate deposition is believed to be due to the lack of concurrent efforts to reduce NOX emissions from vehicles and power plants.
2.4 Anticipated Future NOX Reductions
NOX emission reductions can be expected in the next few years as the U.S. Clean Air Act Amendments of 1990 continue to be implemented and as the recent OTAG recommendations take hold in the eastern United States. This will provide a significant opportunity, through ambient air monitoring and atmospheric research before, during, and after the reductions, to understand and quantify the role of NOX in the atmosphere and in ecosystems. Changes in emissions, chemical transformations, deposition, and environmental and human health effects can be studied with an eye toward clarifying the role of NOX in the formation of ozone, acid rain, and particulate matter. Such a clarification is essential to identify and understand the probable impacts of further NOX reductions. As significant effects are associated with the long-range (including transboundary) transport of this pollutant, it is important that the United States and Canada collaborate as soon as possible to design and execute such studies.
2.5 Technology: Emission Controls and Energy Conservation
NOX emissions can be reduced by retrofitting sources with control technology and by adopting energy conservation measures. Proven NOX control technology, although expensive, is now available and in use. Energy conservation measures have played a major role in restraining the growth rate of NOX emissions, but emissions have nonetheless continued to increase in many areas. Because of the link between energy use and air pollution, it is clear that additional energy conservation steps -- with a particular emphasis on vehicular efficiency -- are needed. Over the long term, these steps will need to include widespread use of low- and zero-emission vehicles (LEVs and ZEVs), development of alternative fuels and energy sources, and further advances in energy conservation in the industrial and residential sectors. These steps -- which may be taken as part of efforts to reduce carbon dioxide (CO2) releases in response to concerns about anticipated global climate change -- will provide a significant additional benefit in the form of reduced NOX emissions.
"End of pipe" emission control policies, while imperative in the short term, will be inadequate to achieve widespread reductions of NOX and other emissions in the long term. As the twenty-first century begins, technologies that are highly energy efficient and designed around the philosophy of "zero discharge" must be developed. While this philosophy should be applied to all sectors and activities, particular emphasis should be placed on decreasing the intensity of energy use and increasing the energy efficiency of both fossil fuel-fired, thermal-electric generating plants and the transportation sector.
While many complex factors influence their production, it is likely that NOX emissions will change substantially in eastern North America. Given this, the Board recommends the following: