The N statistic is the number of hours when the measured ozone concentration is greater than or equal to 0. Experimental trials with a frequent number of peaks hourly averages greater than or equal to 0.
For this reason, the W Lefohn and Runeckles, was developed as a biologically meaningful way to summarize hourly average ozone data. The W places a greater weight on the measured values as the concentrations increase. Thus, it is possible for a high W value to occur with few to no hours above 0. Therefore, it is also necessary to determine the number of hours the ozone concentrations are greater than or equal to 0. It should also be noted the lack of N values does not mean ozone symptoms will not be present when field surveys are conducted.
The W exposure index does not utilize a threshold value, but weights differentially all hourly average concentrations. Significant weighting greater than 0 occurs at all hourly average concentrations above 0.
The use of a 0 weighting for hourly average concentrations less than 0. The W has weighting of approximately 1 for all hourly average concentrations equal to and above 0. The weighting of 1 at these concentrations was based on informal discussions with vegetation researchers in California whose vegetation experienced repeated occurrences of hourly values equal to and above 0. This was a subjective decision and it is recognized that every species will have a different weighting function.
Taken from A. 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 , Shining Rock Wilderness had a W value of Using these values as inputs, the estimated growth loss for one black cherry study was 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.
Berry, C. Differences in concentrations of surface oxident between valley and mountain conditions in the Southern Appalachians. Thursday, Nov. State offices will reopen Friday, Nov. Reporting will resume on Friday, Nov. Ozone is a molecule made up of three oxygen atoms, often referenced as O 3. In a corona discharge ozone reactor cell, the dielectric facilitates the formation of a broad and continuous corona across its surface to maximize the effective area of the corona in the reactor cell.
As shown in the figure below, the dielectric material is bonded to one of the conductive electrode surfaces. The space between the dielectric surface and the other conductive electrode surface forms an air gap called the dielectric gap.
It is in this gap that the corona discharge forms when sufficient voltage is applied across the two electrodes. The characteristics of the dielectric materials and the size of the dielectric gap largely determine the behavior of the reactor cell. The greater the effective dielectric gap, the greater the power that must be applied to form and sustain the corona.
Power is applied to a given reactor cell at a specific frequency. This frequency is typically optimized based on the characteristics of the reactor cell and is set by the manufacturer of the ozone generator. The ozone concentration and total ozone output from an ozone reactor cell or generator is a function of three user-controllable parameters: the power applied to the reactor cell, the oxygen concentration and the flow rate of the feed gas through the reactor cell.
Ozone concentration and output grams per hour or pounds per day may be optimized by adjusting these parameters. Ozone concentration and output from the reactor cell increase as more power is applied to the reactor cell. As with the frequency of the power, the range of power that can be applied to the ozone reactor cell and the duration of each pulse is typically optimized and set by the manufacturer of the generator.
Unfortunately, ozone becomes less stable at higher temperatures. NOx and VOC combine chemically with oxygen to form ozone during sunny, high-temperature conditions of late spring, summer and early fall. High levels of ozone are usually formed in the heat of the afternoon and early evening, dissipating during the cooler nights. Photo courtesy NASA Worldwide seasonal changes in tropospheric ozone: Tropospheric ozone increases during summers in the northern and southern hemispheres when the climate is hot.
The most tropospheric ozone is observed during summer in the northern hemisphere. Although ozone pollution is formed mainly in urban and suburban areas, it ends up in rural areas as well, carried by prevailing winds or resulting from cars and trucks that travel into rural areas.
Significant levels of ozone pollution can be detected in rural areas as far as miles kilometers downwind from urban industrial zones. When you inhale ozone, it travels throughout your respiratory tract.
Because ozone is very corrosive, it damages the bronchioles and alveoli in your lungs , air sacs that are important for gas exchange see How Your Lungs Work for details.
Repeated exposure to ozone can inflame lung tissues and cause respiratory infections. Ozone exposure can aggravate existing respiratory conditions such as asthma, reduce your lung function and capacity for exercise and cause chest pains and coughing.
Young children, adults who are active outdoors and people with respiratory diseases are most susceptible to the high levels of ozone encountered during the summer. In addition to effects on humans, the corrosive nature of ozone can damage plants and trees. High levels of ozone can destroy agricultural crops and forest vegetation. EPA Ozone-damaged plant left and normal plant right. Avoiding Ozone Exposure To protect yourself from ozone exposure, you should be aware of the Air Quality Index AQI in your area everyday -- you can usually find it in the newspaper or on a morning weather forecast on TV or radio.
You should also be familiar with the U. What do the numbers in the AQI mean? The AQI measures concentrations of five air pollutants: ozone, sulfur dioxide, particulate matter, carbon monoxide and nitrogen dioxide. The EPA has chosen these pollutants as criteria pollutants, but these are not all of the pollutants in the air.
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