Advancing the understanding of ground-level ozone pollution

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Advancing the understanding
of ground-level ozone pollution

by Tom Ryerson, Aeronomy Laboratory

Research led by scientists at the NOAA Aeronomy Laboratory has illustrated two major factors that could be used to help guide future programs designed to clean up the air downwind of the Nation's fossil-fuel burning electric power plants: the size of the power plant, and the location of the power plant.

These power plants emit about one-quarter of the total U.S. human-made contribution of nitrogen oxides, or NOx, to the atmosphere.

Fossil-fueled electric power plant in Thomas Hill, MO

NOAA research on ozone formation downwind of fossil-fueled electric power plants such as this one in Thomas Hill, MO, has better defined two important factors that shape air quality.

NOx emissions from these plants can combine with volatile organic compounds (VOCs) in the atmosphere and sunlight to form ozone pollution. This ozone is produced near the Earth's surface, where it is harmful to human health and damaging to forests and crops. NOAA research has shown that the amount of ozone “smog” produced in power plant plumes is strongly dependent on their size and geographic location.

NOAA WP-3D aircraft was turned into a flying chemical laboratory for studies of ozone smog formation in fossil-fueled power plant plumes.

Often used for hurricane and weather research, the NOAA WP-3D aircraft shown here was turned into a flying chemical laboratory for studies of ozone smog formation in fossil-fueled power plant plumes.

NOAA aircraft used to study ozone formation

The team of researchers used a highly instrumented WP-3D research aircraft to sample the NOx-rich plumes from power plants. They measured a host of chemical species inside and outside the plumes at distances of 2 to 200 km downwind of the stacks.

Careful comparison of data from various power plant plumes in the eastern and southeastern U.S. permitted researchers to separate the effects of the plume's NOx concentration from the atmospheric reactive VOC composition and concentration.

Dependence on power plant location
Low levels of reactive VOCs were shown to slow the rate of ozone formation and minimize the ozone production yield from a given power plant NOx source. Because power plants do not emit substantial quantities of reactive VOCs, these must typically be mixed from the surrounding atmosphere into the plume as the plume is carried downwind after emission. As a consequence, the amount of VOCs mixed into the plume is very dependent on whether the power plant in located in a source region of reactive VOCs.

Map os U.S. showing that the largest 50 power plants are generally located in or near isoprene source regions.

The principal source of reactive VOCs in the U.S. is the natural emission of isoprene from hardwood forests. The Nation's largest 50 power plants (open circles) are generally located in or near isoprene source regions. The region enclosed by the green box is shown in more detail in the following figure. (After Figure 1b, Science, 292. 719-723 (2001)). Click on map for larger image.

Heavily forested areas in the eastern U.S. are characterized by high concentrations of reactive VOCs (because certain tree species are strong emitters of reactive VOCs), so the potential to form ozone from a given NOx emission is much higher there compared to, for example, the agricultural midwest. As it happens, many of the Nation's power plants (including most of the strongest NOx sources) are located in the eastern U.S., in or near forested regions where the levels of natural VOCs are high.

Dependence on power plant size
NOx can catalyze ozone formation in the presence of reactive VOCs and sunlight. However, theoretical models predict that very high concentrations of NOx can actually suppress the catalytic formation of ozone. This research used airborne measurements to confirm that such suppression occurs in the extremely concentrated plumes of the largest power plants.

Example of flight track which sampled plumes from power plants of different NOx emission rates.

(left) Example of flight track (blue line) which sampled plumes from power plants of different NOx emission rates. Measurements (black and red lines) superimposed on the track show where the plumes were encountered.

(right) Ozone is formed more slowly and in lower yield when NOx concentrations are very high or when reactive VOC concentrations are very low, as shown here by contrasting data from three different power plant plumes.

Chart showing that ozone is formed more slowly and in lower yield when NOx concentrations are very high or when reactive VOC concentrations are very low

Differences of a factor of 2 or greater in ozone formation rates, and in the amount of excess plume ozone formed per amount of NOx emitted, were consistently observed between plumes from the various plants. NOAA's research confirmed that the differences could be attributed to differences in power plant size and location. Plume ozone formation rates and yields are also dependent on many other factors, such as ambient temperature, sunlight, time of day of emission, and atmospheric dispersion of plume pollutants during transport. These factors can be difficult to control. However, the large dependence on power plant size and location can be usefully exploited in ozone control strategies based on deliberate reductions in power plant NOx emissions.

Implications for air quality in the U.S.

The U.S. is considering a policy to reduce ozone pollution that would require reductions in NOx emissions from power plants, whereby the overall amount of NOx emitted is decreased, but individual power plants could freely trade emission credits to minimize costs. This NOAA research suggests that emission trading strategies that value all NOx emissions equally might not be optimal with respect to air quality. There could be "good-for-air-quality" trades that result in less ozone pollution, such as trades that move NOx emissions away from high-VOC forested regions, or trades that shift emissions from smaller power plants to larger ones. However, there could also be "bad-for-air-quality" trades that actually result in more ozone pollution being produced, despite a reduction in the overall amount of NOx emitted. In addition, this new NOAA information is also important input into considerations of the location and size of planned new fossil-fuel electric power plants.

The Aeronomy Laboratory research findings provide a sound scientific basis for decisions made in industry and government related to ozone layer protection, air quality improvement, and climate change understanding.

The lab conducts scientific research aimed at understanding the fundamental chemical and physical processes of the Earth's atmosphere. This research concentrates on the lower two atmospheric layers known as the troposphere and stratosphere. In this century, it has become increasingly clear that humans are influencing the chemical composition of the troposphere and the stratosphere in ways that can impact conditions at the Earth's surface.

[7/16/01]

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