The Bent Creek data shows a typical pattern (called a diurnal pattern) of ozone concentrations throughout the day.  Ozone concentrations begin to rise in the morning and then decrease after the sun sets in the evening.  Remember the recipe to form ozone is on warm sunny days, nitrogen oxides react with the VOCs.  One pattern the Bent Creek data shows (above) is the ozone concentrations increase as the solar radiation and temperatures increase during the day.  The Bent Creek data also reflects people’s daily activities.  Typically, electrical generation (a major source of nitrogen oxides) increases in the morning as people get ready for work, and remains high on hot days in order to provide electricity to cool people’s homes and businesses.  Also, when people drive to work each day they release nitrogen oxides from the tailpipes of their automobiles.  The large amount of nitrogen oxides released early in the day contributes to recipe that forms ozone.   The combination of a favorable environment and high nitrogen oxide emissions makes high ozone concentrations during the day.

Conversely, later in the day, many people drive home from work and electrical demand remains high on the hot days – thus there are still large amounts of nitrogen oxides released into the atmosphere.  Solar radiation declines until sunset and the temperature also decreases. As nightfall approaches there is a lower likelihood that ozone will form because there is not enough sunlight (ultraviolet radiation) to cause the reactions necessary to form ozone.  The nitrogen oxide emissions then serve an interesting role due to their abundance.  Instead of contributing to ozone formation, the nitrogen oxides react with the ozone present in the atmosphere and cause a reduction of ozone concentrations during the nighttime.  This occurs because nitrogen oxide molecules, in the absence of heat and strong sunlight, remove the third oxygen atom from the unstable ozone (O₃) molecule.

In mountain valleys, such as occur near Asheville, ozone-forming pollution comes from both local and out-of-state sources.  Winds can carry ozone formed in urban areas long distances to surrounding rural areas.  Much of the ozone pollution at high elevations in the mountains of Western North Carolina is transported by winds from other states.  The results from the high elevation ozone monitoring site near Shining Rock Wilderness (figure above) show ozone concentrations due not change throughout the day and that average concentrations are greater then at the Bent Creek site.  Consequently, people and vegetation at higher elevations are exposed to more ozone then people and vegetation at low elevations.

Effect on Plants:  Ozone effects on plants are most pronounced when soil moisture and nutrients are adequate and ozone concentrations are high. Under good soil moisture and nutrient conditions the ozone will enter through openings into the leaf and damage the cells that produce the food for the plants.  Once the ozone is absorbed into the leaf, some plants spend energy to produce bio-chemicals that can neutralize a toxic effect from the ozone.  Other plants will suffer from a toxic effect, and growth loss and/or visible symptoms may occur.  The presence of ozone in an area can be detected when consistent and known symptoms are observed on the upper-leaf surface of a sensitive plant species.  For example, some air specialists use blackberry plants as a “bio-indicator” of ground level ozone.  The photograph below shows the severe reddening of the blackberry foliage near Shining Rock Wilderness in western North Carolina when both adequate soil moisture and high ozone concentrations were present.

Exposure Indices: There are two important statistics used to estimate the growth loss to vegetation when summarizing data from an ozone monitor. The N100 statistic is the number of hours when the measured ozone concentration is greater than or equal to 0.100 parts per million (ppm).  Experimental trials with a frequent number of peaks (hourly averages greater than or equal to 0.100 ppm) have been demonstrated to cause greater growth loss to vegetation than trials with no peaks in the exposure regime (Hogsett et al., 1985; Mussleman et al., 1983; and Mussleman et al., 1986).  For this reason, the W126 (Lefohn and Runeckles, 1987) was developed as a biologically meaningful way to summarize hourly average ozone data. The W126 places a greater weight on the measured values as the concentrations increase.  Thus, it is possible for a high W126 value to occur with few to no hours above 0.100 ppm. Therefore, it is also necessary to determine the number of hours the ozone concentrations are greater than or equal to 0.100 ppm.  It should also be noted the lack of N100 values does not mean ozone symptoms will not be present when field surveys are conducted.

Vegetation's sensitivity to ozone varies  -- not only between species, but also within a species.  For example, there may be two black cherry trees growing next to one another, and one will have severe ozone symptoms while the adjacent black cherry has no visible symptoms.   An example of the variation between species can be seen when an analysis is conducted with Ozone Calculator.  In 1998, Shining Rock Wilderness had a W126 value of 104.3227 and had 43 hours when the ozone concentration was greater than or equal to 0.100 ppm.  Using these values as inputs, the estimated growth loss for one black cherry study was 26.1 percent, for red maple the growth loss was 5.6 percent, and for sugar maple the growth loss was estimated to be 0.1 percent.  Assuming there were adequate nutrients and soil moisture, the predicted growth losses indicated that black cherry is more sensitive to ozone than red maple, which is more sensitive than sugar maple.

1998 Shining Rock

W126 = 104.3227, N100 = 43 hours