Ozone Season is Here

Here’s a chemistry question for you: Mix nitrogen oxides (NOx) with volatile organic compounds (VOCs), then add heat and sunlight.  What do you get? Ground-level ozone, a potent pollutant that can seriously impact air quality, human health, and even plants.

Warmer weather means a return to ozone season – the time of the year when ground-level ozone is most likely to form in the lower atmosphere.  Like the ozone that protects earth from the sun’s harmful ultraviolet radiation, ground-level ozone is a molecule composed of three oxygen atoms (O3).  But unlike the protective ozone layer, ground-level ozone occurs in the lower atmosphere, where humans live, work, and play.  A good way to remember the difference: Ozone is good up high, but bad nearby.

Ozone is unique among the criteria pollutants in that it is not directly emitted from a smokestack or source.  Instead, sunlight spurs a photochemical reaction between NOx and VOCs to create the ground-level pollutant.  Sources of NOx include emissions from motor vehicles, boilers, and power plants.  Sources of VOCs include motor vehicles, dry cleaning, paint solvents, and evaporation of gasoline from storage and fuel transfer facilities.

Ozone pollution affects everyone, but especially children and those with existing respiratory problems.  Exposure to low levels of ozone can trigger health problems including inflamed and damaged airways, coughing, throat irritation, and congestion.  People with asthma or other lung problems may see their symptoms worsen with exposure to ozone.  Long term exposure can result in permanent lung damage.

For these reasons and more, the U.S. EPA classifies ground-level ozone as a “criteria pollutant” and sets specific limits to how much ozone may be measured in a particular area.  The current health-based standard for ground-level ozone is 75 parts per billion (ppb), or 75 parts ozone to one billion parts of air by volume, measured as an eight-hour average.

The Division for Air Quality, the National Park Service at Mammoth Cave, and Louisville Metro Air Pollution Control District operate a network of 26 ozone monitors across Kentucky.  Ozone monitors operate continuously from March 1st until October 31st each year, when meteorological conditions favor the formation of ozone.

You might expect this colorless, odorless gas to be difficult to detect in the air – but it’s actually pretty straightforward.  It turns out that ozone absorbs ultraviolet light, so by passing a beam of ultraviolet light through an air sample, we can check to see if the ultraviolet light decreases in intensity – which would indicate the presence of ozone.

The intensity of the ultraviolet light beam is first measured with no ozone present to determine a reference value.  Next, an ambient air sample is introduced and the intensity of light passing through it is measured by an ultraviolet detector.  The amount of light absorbed by the sample indicates the level of ozone present.

If you are sensitive to air pollution, you’re probably already familiar with the Air Quality Index.  This color-coded index tells you at a glance how clean or polluted your air is.  The current AQI level is determined by whichever pollutant is the highest in concentration – usually ozone or particulate matter – relative to the respective health-based standard.  The AQI also tells you the health effects associated with the current reading, as well as suggestions for modifying outdoor activities during high-ozone days.

If you’re a real air quality geek, here’s another way to find the current ozone level in your area.  Visit EPA’s AirNow website and click on the “More Maps” option, then choose “Current Ozone”.  Click on the map for your location, and the map will zoom in for a close-up view.  AQI values will be displayed next to the map.  To convert these values into actual ozone levels, visit the AQI Calculator, where you can select “ozone” as your pollutant, enter an AQI number from the previous page, and calculate.  Pretty cool, huh?