AQ Monitoring Results: Ozone at Fremont School in Fresno
This page last reviewed April 21, 2009
Ozone at Fremont School
Ozone (O3) is a reactive gas produced naturally in small amounts in the atmosphere. Most of the atmospheric ozone (about 90%) is found in the stratosphere, which is located approximately from 20 to 30 kilometers above the earth's surface. The stratospheric ozone is beneficial ozone since it absorbs hazardous ultraviolet radiation emitted by the sun. In the troposphere, which is normally within 10 kilometers above the earth's surface, there exists the ozone that is a component of smog. The tropospheric ozone can cause breathing difficulties and even lung damage. Ozone can also damage vegetation, buildings, rubber, and plastics. Unfortunately, the beneficial stratospheric ozone layer is thinning, and the tropospheric ozone has increased dramatically due to human activities. The tropospheric ozone is one of the major pollutants that are used to measure ambient air quality.
The tropospheric ozone is not directly emitted from human activities; rather, it is produced by a series of chemical reactions of nitrogen oxides, volatile organic compounds and oxygen with the energy of sunlight. High tropospheric ozone concentrations normally occur on hot summer days with strong sunlight.
Motor vehicles are the dominant contributor to both nitrogen oxides and volatile organic compounds in California. Other sources of nitrogen oxides include fossil fuel fired electric utilities, and other industrial, commercial and residential combustion sources. In nature, soil bacterial action also produces nitrogen oxides. Sources of volatile organic compounds include gasoline vapors, chemical solvents, and consumer products like paints. In nature, biogenic emissions from plants include volatile organic compounds.
In order to reduce tropospheric ozone, California has adopted aggressive emission controls on motor vehicles and other sources, and as a consequence, the ozone concentration has reduced greatly over the last several decades. But for most areas in California, the measured ozone levels still exceed both national and state standards, which are:
90 ppb* for 1-hour average
120 ppb for 1-hour average
80 ppb for 8-hour average
The standards establish the levels above which ozone is known to cause adverse health effects in humans.
Ambient Monitoring Results
Ambient levels of ozone are routinely monitored hourly at approximately 190 sites in the California ambient air quality measurement network. Based on the ozone standards, the maximum measured ozone concentration is used to determine the air quality regarding to ozone. Daily maximum of the 1-hour ozone measurements is a useful parameter to indicate the ozone levels. From 1998 through 2001, the statewide average concentration of daily maximum 1-hour ozone measurements was 50 ppb (parts per billion). Relative to the statewide average, the Fresno County region was 28% higher for the same time period, averaging 64 ppb.
The ambient monitoring results at Fremont School are provided here:
- A graph comparing the monthly summaries of ozone at the community with historical statewide and regional levels
- A table of summary statistics
- Raw data in Excel format