The Regional Air Pollution Control Agency

Ozone

Addition Ozone Information

Nature of Pollutant

Ozone, more commonly known as smog, forms when a mixture of numerous chemicals, of which nonmethane hydrocarbons (NMHC) and nitrogen oxides (NOx) are the main constituents, collects in the atmosphere. When the sun shines on this mixture of ozone precursors in warm weather, a complex series of chemical reactions is set off resulting in the formation of a group of compounds known collectively as photochemical oxidants.

Approximately 90% of the end product of this chemical reaction is one type of oxidant - ozone. For convenience, ozone is the only photochemical oxidant measured. Measuring all the compounds in smog would be prohibitively complicated and expensive. From the above description of its formation, one can see that ozone must be a seasonal problem, occurring roughly from May through September, when warm sunny weather is abundant. The official ozone season for this region is April 1 through Oct. 31 The higher levels of ozone generally occur in the afternoon after the temperature has risen and the precursors have had time to react. Ground level ozone concentrations then fall off during the night.

Prevention and Avoidance

Because ozone is formed during hot, sunny weather, and relies on having precursors such as automobile exhaust and evaporated gasoline, it makes sense to reduce the emissions of the precursors, and or change the time they are emitted to miss the peak sunshine and heat.

The Regional Ozone Action Program, a coalition of which RAPCA is a member, attempts to reduce emissions through the voluntary efforts of citizens. RAPCA's part in this program is to call an Air Pollution Advisory when it appears the weather is conducive to causing an ozone exceedance. During Air Pollution Advisories, regional residents are urged to reduce their emissions of hydrocarbons through such strategies as reducing driving or limiting the use of other gasoline powered engines (e.g., lawnmowers) during the daylight hours. Furthermore, residents will be urged to curtail their exposure to ozone depending on their susceptability (age, health, etc.) The Air Quality Index (AQI) categories have specific warnings for the various susceptable groups, depending on the forecast ozone levels.

Precursors

Currently, there are no ambient air standards for the two major precursors to ozone: NOx and NMHC. Although there is a standard for the major constituent of NOx, nitrogen dioxide or NO2, RAPCA no longer monitors this pollutant because the past readings were very low in comparison with the standard and because our regional population base is too small (less than one million) for USEPA to support such monitoring. If air quality modeling of ozone is required or becomes important to us, then nitrogen oxide monitoring may be re-instituted.

Health and Environmental Effects

Photochemical oxidants such as ozone are powerful oxidizers and can irritate eyes, skin, and lungs by drying mucous and the protective moist membranes of the body. They can impair breathing, damage plants and reduce crop yields, cause major changes in ecosystems, and cause serious deterioration in organic materials such as rubber and dyes. Exposure to ozone damages biological tissue and cells. Exposure for 6 to 7 hours, even at relatively low concentrations, significantly reduces lung function in normal, healthy people during periods of moderate exercise. Ozone, which can seriously irritate eyes, mucous membranes, and the respiratory system, is not emitted directly into the atmosphere but is produced by a complex series of chemical reactions. These reactions are initiated when volatile organic compounds (VOCs) and nitrogen oxide emissions from automobiles and other sources are exposed to sunlight.

In a February 28, 2006 Federal Register Notice, EPA announced the release of EPA's latest ozone criteria document, "Air Quality Criteria for Ozone and Other Photochemical Oxidants." For more information about the effects of ozone, you can obtain the new Ozone Criteria Documents from this EPA web site.

Monitors and Sites

RAPCA monitors the presence of photochemical oxidants as indicated by the concentration of ozone (O3). Ozone concentrations are continuously monitored. The technique used is ultra-violet (UV) photometry and is based on the fact that ozone absorbs ultra-violet light. With the U-V photometry method, an air sample is diverted into a catalytic converter which changes any ozone present into oxygen. This sample is then passed through the absorption chamber to determine the amount of U-V light passed through it to serve as a reference. A second sample is then introduced into the absorption chamber and the amount of U-V light passed through it is also measured, (which will be a reduced amount because ozone absorbs U-V). The difference between the two values represents the amount of ozone present.