Ground level ozone is formed by chemical reactions between volatile organic compounds (VOCs), also known as hydrocarbons and oxides of nitrogen (NO_x) in the presence of sunlight. NO_x and VOCs are known as the chief "precursors" of ozone. These compounds react in the presence of sunlight to produce ozone. These gaseous compounds mix like a thin soup in the atmosphere, and when they are exposed to sunlight, ozone is formed. Due to the nature of these reactions, ozone concentrations can reach unhealthful levels when the weather is hot and sunny with little or no wind. As a result, ground level ozone pollution, or smog, is mainly a daytime problem during summer months.

Large industrial areas and cities with heavy summer traffic are the main contributors to ozone formation. When temperatures are high and the movement of air is limited, ozone can accumulate to unhealthy levels.

What is considered to be an unhealthy level? The EPA has established a general index to guide citizens in planning driving and outdoor activities during the ozone season. The Air Quality Index, or AQI, is a scale used to report ground level ozone and other common pollutants in the air. The higher the AQI value, the greater the health concern. A specific color has been assigned to each AQI category. The color scheme can help to quickly determine if air pollutants are reaching unhealthy levels.

EPA Air Quality Guide for Ozone

• Good – Air Quality Index 0-50 (Green)
  • Partly sunny to cloudy skies or rain
  • Temperatures in mid-70s to low 80s
  • Windy
  • No health impacts are expected when air quality is in this range.
     
• Moderate – Air Quality Index 51-100 (Yellow)
  • Partly cloudy to sunny skies
  • Temperatures in upper 70s to mid-80s
  • Light to moderate winds
  • Unusually sensitive people should consider limiting prolonged outdoor exertion.
     
• Unhealthy-Sensitive Groups – Air Quality Index 101 – 150 (Orange)
  • Sunny skies
  • Temperatures in high 80s to 90s
  • Light winds
  • Active children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion.
     
• Unhealthy (more people affected) – Air Quality Index 151-200 (Red)
  • Sunny skies
  • Increasing humidity
  • Temperatures in high 80s to 90s
  • Little to no wind
  • Active children and adults, and people with respiratory disease, such as asthma, should avoid prolonged outdoor exertion; everyone else, especially children, should limit prolonged outdoor exertion.
     
• Very Unhealthy (Alert) - Air Quality Index 201-300 (Purple)
  • Hazy, hot and humid
  • Temperatures in 90s and above
  • Little or no wind
  • Active children and adults, and people with respiratory disease, such as asthma, should avoid all outdoor exertion; everyone else, especially children, should limit outdoor exertion.
     
• Hazardous  (Alert) - Air Quality Index > 300 (Brown)
  • Health warnings of emergency conditions.
  • The entire population is more likely to be affected.

In areas where ground level ozone has the potential to reach unhealthy levels, Ozone Action Day programs have been established. An Ozone Action Day program is a voluntary initiative by government, environmental groups, and business leaders working with the general public to take extra action to prevent air pollution when high ozone levels are predicted. Because ground level ozone forms under certain weather conditions, a regional team of meteorologists can predict days when ground level ozone concentrations may exceed health standards. These are generally hot, sunny days with little or no wind.

Ozone Action Days can be declared by the state environmental protection agencies if exceptionally high concentrations of ground level ozone are predicted. Awareness of such high levels of ground level ozone allows citizens to take measures to help reduce the amount of ozone or smog.

In this lesson, the students will analyze the same type of data that the meteorologists use to determine the formation of ground level ozone.
 

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