Ozone is an odorless, colorless gas composed of three oxygen atoms. It is not usually emitted directly into the air, but at ground level it is created by a chemical reaction between oxides of nitrogen (NO_x) and volatile organic compounds (VOCs) in the presence of heat and sunlight. Motor vehicle exhaust and industrial emissions, gasoline vapors, and chemical solvents are some of the major sources of NO_x and VOCs that help to form ozone. Other sources of VOCs include household products such as paints, paint strippers, and other solvents; wood preservatives; aerosol sprays; cleansers and disinfectants; moth repellents and air fresheners; stored fuels and automotive products; hobby supplies; dry-cleaned clothing.

VOCs + NO_x + Sunlight = Ozone

Sunlight and hot weather can cause ground level ozone to form in harmful concentrations in the air. As a result, it is known as a summertime air pollutant. Many urban areas tend to have high levels of ground level ozone, but even rural areas are subject to increased ozone levels because the wind can carry, or transports, ozone and pollutants that form it hundreds of miles away from their original sources.

Measuring ozone can be tricky and yield unexpected results. For instance, urban areas often have lower levels of ozone than adjoining suburban areas because of "ozone scavenging". Ozone scavenging occurs in locations where higher levels of NO_x are found, e.g. urban areas or busy highways, due to emissions. In these areas, emissions containing NO_x (Nitrogen oxide), react very quickly with O₃ (ozone) to form NO₂ (Nitrogen dioxide) and O₂ (oxygen).

NO_x + O₃ -> NO₂ + O₂

This creates overall lower levels of ozone in the urban areas.

The NO_x that does not "scavenge" ozone will continue to drift downwind (about 10 - 40 miles), eventually combine with VOCs and react in the sun to produce ground level ozone creating overall high levels of ground level ozone in the downwind location.

In summary, concentrations of ground level ozone tend to be reduced in urban areas due to ozone scavenging, but during ozone episodes, ozone concentrations can be high in areas 10-40 miles away due to wind transport. All of this may cause the ozone levels you measure to be lower or higher than you might expect.

Further, the EPA created air quality standards that limit the levels of ground level ozone in the air to protect public health. There are two standards; a one hour standard (120 ppb) and an eight hour standard (80 ppb). Every area of the country is eventually expected to meet or be below the standards. If an area does not meet, or "attain" the standard, it is considered a "non-attainment" area, and is subject to many more regulations to help make it an "attainment" area in the future.

Measurements are taken by the EPA to ensure the air quality standards are being met. The measurements you will be doing may not be taken at the same time intervals, or the level of precision as the EPA (or state) measurements. Therefore your measurements cannot be directly compared to the AirNow data, nor will you be able to calculate possible exceedences of the one hour or eight hour standard.