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Frequency Distribution and Diurnal Pattern


Ground-level ozone is a natural occuring compound in the atmosphere, but ozone concentrations can increase when we release large quanities of air pollution (especially nitrogen oxides and volatile organic compounds) on hot, sunny days. Ozone is monitored at numerous locations in the United States and at each of these sites there are thousands of hourly average values of ozone concentrations collected each year the site is active. Examining the distribution of the data and examining the data for a diurnal pattern will allow you to determine if this monitoring site (shown in the three graphs below) is being influenced by air pollution emissions released from people's activities.

 

Frequency Distribution of Ozone Concentrations: The following two graphics allow you to examine the historical distribution of the hourly average ozone concentrations measured at the monitoring location.

 

The first graphic shows a histogram of the results and each hourly ozone concentration is placed into a predetermined ozone category (or bin). The ozone categories are arranged in 0.020 parts per million (ppm) bins. Typically, sites with a normal (bell-shaped) distribution indicate that anthropogenic emissions have a minor impact on ozone concentrations at the site. Major urban areas that generate large amounts of air pollution and ozone can have a high frequency of ozone concentrations above 0.060 ppm and sometimes nearby rual areas will also be impacted by the high concentrations as the ozone is trasported downwind of the urban area. In areas that have a lower population and/or less industry the frequency distribution may have an increase frequency on both the left and right sides of the distribution when compared to "clean" ambient monitoring sites.

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Graph Instructions

The X- and Y- axes can be adjusted. Click and Drag your mouse along the axes to scroll through the values. To adjust the scale of the axes, Click and Drag your mouse along the axes while holding the shift key and left mouse button.


The X-axis adjusts on each graph in unison with other graph, but each graph's Y-axes adjust independent of other graph.

 

The next graphic is a second way to examine the distribution of hourly ozone concentrations at the site. The box shows where 50% of the hourly average ozone concentrations fall between. If the median (blue line) is in the middle of the upper and lower lines of the box then the hourly ozone concentrations are normally distributed. If the median is closer to the lower end of the box then the hourly average ozone have a large frequency of high values; while if the median is closer to the upper end of the box then the concentrations have a high frequency of low hourly average concentations. The opens circles are the minimum and maximum values and if either are found outside of the vertical lines (called whiskers) then the values are considered to be outliers.

Diurnal Pattern: Some ambient ozone monitoring sites have a diurnal pattern where the concentrations increase during the day and then decrease at night. The presence of a diurnal pattern provides information about the characteristics of a specific ozone monitoring site (Lefohn et al., 1990). Seasonal ozone profiles can distinguish between sites that experience ozone scavenging and those that do not. Ozone is rapidly depleted near the surface below the nocturnal inversion layer (Berry, 1964). Mountainous sites, which are above the nocturnal inversion layer, do not necessarily experience this depletion (Stasiuk and Coffey, 1974). Ozone trapped below the inversion layer is depleted by dry deposition and chemical reactions (such as with nitrogen oxides) if other reactants are present in sufficient quantities (Kelly et al., 1984).

 

Above the nocturnal inversion layer, dry deposition generally does not occur and the concentration of ozone scavengers is generally lower so that ozone concentrations remains fairly constant (Wolff et al, 1987). For some low elevation sites, intra-day variability is most significant due to the pronounced daily amplitude in ozone concentration between the pre-dawn minimum and mid-afternoon-to-early-evening maximum (Taylor and Hanson, 1992). If the graph below has a flat diurnal pattern then this is usually interpreted as indicating a lack of efficient scavenging of ozone and/or a lack of photochemical precursors, whereas a varying diurnal pattern is taken to indicate the opposite.

 

The diurnal composite diagrams alone should not be used to quantify exposures of ozone because they represent long-term average concentrations. Long-term average concentrations tend to mask the occurrence of infrequent, but biologically important, peak concentrations. However, diurnal curves can provide information on scavenging, as well as when the maximum hourly average concentrations tend to occur during the day. If you review the hourly average concentration information over a 24-hour period then you will find that peak ozone concentrations can occur later in the day in rural areas than in urban areas, with the distances downwind from urban centers generally determining how much later the peaks occur (Lefohn, 1992).

Please note: If the graph above has an hour with a missing value or a value of zero this is because the daily automatic equipment calibrations occurs at this time each day. Consequently, there are not enough valid ozone measurements during the hour to compute an hourly average.

 

Please note: the results will not display properly in Internet Explorer version 8.0 or earlier.

Literature Cited

Berry, C. R. 1964. Differences in concentrations of surface oxidant between valley and mountaintop conditions in the southern Appalachians. J. Air Pollut. Control Assoc. 14:238-239.

 

Kelly, N.A.; Wolff, G.T.; Ferman, M.A. 1984. Sources and sinks of ozone in rural areas. Atmos. Environ. 18:1251-1266.

 

Lefohn, A.S.; Benkovitz, C.M.; Tanner, R.L.; Shadwick, D.S.; Smith, L.A. 1990. Air quality measurements and characterizations for terrestrial effects research. Washington, DC: National Acid Precipitation Assessment Program; State of Science and Technology Report no. 7.

 

Lefohn, A. S., 1992. Ozone Exposures Within or Near National Forests in North Carolina, South Carolina, and Tennessee. Report on file at: National Forests in North Carolina, Asheville, NC 28801.

 

Stasiuk, W.N.; Coffey, P.E. 1974. Rural and urban ozone relationships in New York State. J. Air Pollut. Control Assoc. 24:564-568.

 

Taylor, G.E., Jr.; Hanson, P.J. 1992. Forest trees and tropospheric ozone: role of canopy deposition and leaf uptake in developing exposure – response relationships. Agriculture, Ecosystems and Environment 42:255-273.