Ozone Forecasting

Ozone Forecasting Provides Valley an Early Warning System for Unhealthy Air Quality

What is ozone?

Ozone (O3) is one of six national "criteria" air pollutants and is a major component of "smog." Although naturally occurring, stratospheric O3 helps shield the earth from excessive solar radiation, excess ground-level O3—primarily from human-caused sources—can be detrimental to both human health and the environment.

How is ozone made?

Ground-level O3 is formed via a complex series of chemical reactions between reactive volatile organic compounds (VOCs) and oxides of nitrogen (NOX) in the presence of sunlight. Given the need for sunlight and the concentration of VOC and NOX sources in and around urban areas, peak ground-level O3 concentrations are usually found just downwind of large cities during hot and sunny summer afternoons. While peak short-term O3 concentrations are often found near cities, the highest O3 exposures are often found in rural areas. High-elevation rural areas are a particular concern because they can experience both elevated short-term concentrations and high long-term O3 exposures.

How does ozone affect health?

Exposure to excessive O3 levels can irritate the respiratory tract, particularly during physical exertion. Symptoms may include breathing difficulties, coughing, and throat irritation. Ozone exposure is also thought to increase susceptibility to respiratory infections, allergens, and effects of other air pollutants. Recovery from O3 damage to the respiratory system may take several days.

Who is sensitive to ozone?

Groups that may be especially sensitive to O3 include the very young, the very old, and those with respiratory or heart disease. However, even healthy people may experience some symptoms during high-O3 days. A recent California study suggests that physically active children living in areas with high levels of O3, such as Los Angeles, are more likely to develop asthma than similarly physically active children in areas with lower O3 levels.

What is ozone forecasting?

Ozone forecasting is the art and science of estimating tomorrow's maximum O3 levels with today's information. Our collaborative O3 forecasting approach is based upon a set of statistical models-one for each geographic area-which combine today's maximum midday 1-hour O3 measurements with tomorrow's weather forecasts. Since weather conditions and O3 levels often vary widely, our O3 forecasts consider city-specific monitoring and weather information.

While statistical models provide the basis for these forecasts, the Valley O3 forecast team can modify forecasts based on additional information, including weather pattern recognition and monitoring trends (Figure 2). The final forecasts are converted to EPA's color coded categories which depict relative levels of health concern (Table 1).

The primary objective of our O3 forecasting effort is to notify the public about the potential for unhealthy O3 levels so sensitive individuals may adjust their activities to minimize exposure. A secondary objective is to use this information to initiate voluntary O3 control programs.

What can we do to protect ourselves on unhealthy days?

According to EPA, sensitive groups, including active children and adults, and people with respiratory disease, such as asthma or emphysema, should limit prolonged outdoor exertion at forecast levels of orange or above (Table 1). For forecast levels of red (unhealthy) and above, even healthy people should try to limit their outdoor exertion.

So just like you might carry an umbrella when the weather report suggests a chance of showers, you might want to adjust your outdoor activities when unhealthy O3 levels are forecast. Since O3 levels are usually highest from mid- to late- afternoon, it's a good idea to avoid outdoor physical exertion during this time. You can effectively minimize O3 exposure by staying indoors, particularly in air conditioned spaces.

In addition to avoiding O3, you can also do things to help lower O3 on unhealthy forecast days. You can adjust your activities to minimize your contribution to excess O3 production. Nationally, many communities—such as the Tri-Cities here in the Tennessee Valley—announce "ozone action days" when unhealthy O3 levels are forecast.

What is an ozone action day?

Where these programs exist, ozone action days are initiated by state or local environmental or public health agencies when unhealthy O3 levels are forecast. These programs not only encourage people to avoid O3 exposure but also encourage people and businesses to take voluntary actions to lower the emission of O3-causing chemicals.

Action day tips to help improve air quality:

Conserve energy-at home, at work, everywhere. Ease off on the air conditioning by setting thermostats a bit higher.
Refuel vehicles during the cooler parts of the day to minimize vapor loss and always follow refueling instructions for vapor recovery. Don't spill fuel and tighten gas caps securely.
Keep engines cleaner and greener by maintaining and operating according to manufacturers' specifications.
Be sure tires are properly inflated for best mileage.
Drive less on action days, and when you do drive, ease off the gas pedal. Plan trips when traffic is light and allow some extra time.
Choose cleaner transportation options—combine trips, share a ride to work, or use public transportation.
Use environmentally safe paints and cleaning products whenever possible.
Use gasoline-powered lawn and garden equipment during cooler times of day.

When did ozone forecasting begin?

While it's been going on in some parts of the country for more than a decade, Tennessee Valley O3 forecasting began on May 1, 2001, when TVA, the Tennessee Department of Environment and Conservation (TDEC) and the Nashville/Davidson County Health Department issued next-day O3 forecasts for Nashville/Middle Tennessee. On June 1, 2001, with the support and assistance of the Ozone Action Partnership (OAP) of East Tennessee and Southwest Virginia, forecasts began for the Tri-Cities area (Johnson City TN, Kingsport TN, and Bristol VA).

TDEC posted these forecasts on the EPA AIRNow website throughout the summer of 2001. The Nashville forecasts were also posted on the Nashville/Davidson County Health Department website and are also available through a call-in line. The Tri-Cities forecasts were posted on the OAP website. On high-O3 days, special "ozone action day" faxes were sent to local industries to urge voluntary reductions of VOC and NOX emissions.

For 2002, our collaborative O3 forecasting effort, which includes our participation in AIRNow, commenced on May 1st and has grown to include areas surrounding Memphis, Chattanooga, Knoxville, and the Great Smoky Mountains National Park, in addition to Nashville and the Tri-Cities.

How are we doing?

Our overall performance goal is to forecast correctly the O3 color category in Table 1 at least two-thirds of the time and correctly forecast unhealthy days (orange category and above) at least half of the time. In 2001, we correctly forecast color category 65% of the time. Our ability to forecast unhealthy O3 days in 2001, however, was not very good—29% for Nashville and 13% for Tri-Cities.

In 2002, we added a set of "high" models to improve our ability to predict unhealthy days. On those days when our base model predicts next-day maximum 8-hour O3 values of 75 ppb or above, we switch to a "high" model, which is based on days with observed O3 concentrations of 75 ppb or greater.

In 2002, we are doing a much better job of forecasting unhealthy O3 levels. Through August, our correct color category forecasts range between 60% and 70% and, more importantly, our ability to forecast unhealthy days is above 50%.

What's next?

Following the 2002 O3 season, the forecast team will evaluate the results of this season's forecast effort, update the forecast models and process, and plan for the 2003 O3 forecasting season. Additionally, using the experience of this effort, the team will develop and test preliminary fine particle forecast models.

Information Contacts

William J. Parkhurst, (256) 386-2793, wjparkhurst@tva.gov
Frances P. Weatherford, (256) 386-2344,m fpweatherford@tva.gov
Jennifer M. Call, (256) 386-3435, jmcall@tva.gov

Last updated on 9-20-2002.
Inquiries and comments should be sent to wjparkhurst@tva.gov.