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Ozone 101

Austin Skyline - Comparison clean vs. polluted 

Ozone is the primary air pollutant of concern in the Austin-Round Rock Metropolitan Statistical Area (MSA), which includes Bastrop, Caldwell, Hays, Travis, and Williamson counties. Air quality readings taken from monitors within the MSA indicate that ozone levels have exceeded federal standards on numerous occasions. High ozone levels can negatively affect asthma and other upper respiratory illnesses. At-risk groups include children, the elderly, and those who work or exercise outdoors.

Information on ground-level ozone sources and how it is formed, the current status of ozone levels in the region, and initiatives being taken in Central Texas to maintain good air quality can be found at the following links:

What is ozone?

Ozone is a form of oxygen with three atoms, instead of the usual two atoms. It is a photochemical oxidant and, at ground level, is the main component of smog. Ozone is not emitted directly into the air but is formed through chemical reactions between natural and man-made emissions of volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of sunlight. These gaseous compounds mix like a thin soup in the ambient, or outdoor, air, and when they interact with sunlight, ozone is formed.
Sources of these pollutants include automobiles, gas-powered motors, refineries, chemical manufacturing plants, solvents used in dry cleaners and paint shops, and wherever natural gas, gasoline, diesel fuel, kerosene, and oil are combusted.

Ozone pollution is the periodic increase in the concentration of ozone in the ambient air, the natural air that surrounds us. It is mainly a daytime problem during the summer months because warm temperatures play a role in its formation. When temperatures are high, sunshine is strong, and winds are weak, ozone can accumulate to unhealthful levels.

Ozone levels or concentrations are typically expressed in parts per billion by volume (ppbV, or ppb), which represent the fraction of air molecules represented by ozone.

Ground-level Ozone

Ground-level (or tropospheric) ozone is the most prevalent air pollutant in Texas and the nation. Ozone is often one of several pollutants that make up "smog," which you may recognize as the reddish-brown haze that forms when air quality is poor. But because ozone itself is colorless, the air can look clear even when high ozone concentrations are present. 

One way ozone forms at ground level is when certain substances emitted by trees and other vegetation, soil microorganisms, and lightning react together to form low, background concentrations of ozone. If ground-level ozone were produced only from natural sources of emissions, it would be of no concern. Both animal and plant life tolerate natural background concentrations of ozone.

But many contemporary human activities result in emissions of additional chemical compounds — called precursor emissions — that also react in the air to form ozone or "bad ozone" and other harmful gases. These anthropogenic, or man-made emissions are the result of activities that include transportation, energy production and some industrial and commercial operations.

 Stratospheric Ozone 

Stratospheric ozone forms high in the atmosphere, 6-30 miles above the earth's surface, when intense sunlight causes oxygen molecules (O2) to break up and re-form as ozone molecules (O3). These ozone molecules form the ozone layer and are commonly referred to as "good ozone." At concentration levels as high as 12,000 ppb in the stratosphere, this ozone protects and shields people, trees, crops, property, and microorganisms from the harmful effects of the sun's ultraviolet light.

How does ozone occur?

Ozone pollution is a key component of smog. It is mainly a daytime problem during the summer months. Strong sunlight and hot weather cause ground-level ozone to form in harmful concentrations in the air.

Ozone is not emitted directly into the air. Instead it is formed in sunlight, which initiates a series of complex atmospheric chemical reactions. These reactions primarily involve nitrogen oxide (NOx) and volatile organic compound (VOC) emissions, called precursors. At ground level, ozone can harm plants and other materials through a process called oxidation. For these reasons, ozone is called a photochemical oxidant.

Precursor Emissions

NOx is produced almost entirely as a by-product of high-temperature combustion. 
Common sources of NOx include:
  • automobiles, trucks, and marine vessels
  • construction equipment
  • power generation
  • industrial processes
  • natural gas furnaces
VOCs include many organic chemicals that vaporize easily, such as those found in gasoline and solvents. They are emitted from many sources, including:
  • gasoline stations
  • motor vehicles, airplanes, trains, boats
  • petroleum storage tanks
  • oil refineries

In addition, biogenic, or natural emissions from trees and plants, are a major source of VOCs.

The concentration of ozone in the air is determined not only by the amounts of ozone precursor chemicals, but also by weather and climate factors. Intense sunlight, warm temperatures, stagnant high-pressure weather systems, and low wind speeds cause ozone to accumulate in harmful amounts.

How does ground-level ozone affect your health?

Ozone pollution near the ground is the most wide-spread air quality problem in the United States. The public in nearly 100 major cities is periodically exposed to harmful concentrations of ozone.The biggest concern with high ozone concentrations is the damage it causes to human health and vegetation.
High concentrations of ozone can cause:

  • shortness of breath
  • coughing or wheezing
  • headaches
  • nausea
  • throat and lung irritation

Who Can Suffer the Effects?

  1. Children often play outside for long periods during the summer. Their lungs are still developing, and they breathe more rapidly and inhale more air pollution per pound of body weight than adults. On days when ozone levels are high, these factors put children at increased risk for respiratory problems.
  2. People with lung diseases
  3. People who suffer from lung diseases like bronchitis, pneumonia, emphysema, asthma, and colds have even more trouble breathing when the air is polluted. The effects can be worse for anyone who spends significant periods of time exercising or working outdoors.
  4. Active adults. During exercise or strenuous work we breathe more often and draw air more deeply into the lungs. When we exercise heavily, we may increase our intake of air by as much as 10 times our level at rest. The interaction between air pollution and exercise is so strong that health scientists typically use exercising volunteers in their research.

National Ambient Air Quality Standards (NAAQS)

The Federal Clean Air Act

The U.S. Environmental Protection Agency's (EPA) Clean Air Act establishes NAAQS for six "criteria" air pollutants:

  1. Carbon Monoxide (CO)
  2. Lead (Pb)
  3. Nitrogen Oxides (NOx)
  4. Ozone (O3)
  5. Particulate Matter (PM)
  6. Sulfur Dioxide (SO2)

Units of measure for the standards are parts per billion (ppb) by volume, milligrams per cubic meter of air (mg/m3), and micrograms per cubic meter of air (µg/m3). These standards are designed to protect public health and the environment.  All counties in the United States are classified by the United States EPA based on whether or not they meet the NAAQS for a particular pollutant. Counties where pollutant concentrations regularly exceed the NAAQS are in violation of the standards and are classified as "nonattainment." States are required to develop and implement plans that will bring nonattainment areas into compliance with the NAAQS by a specified date. The Clean Air Act also contains additional specific requirements for emission reduction and planning activities related to emission sources (point sources, on-road mobile sources, non-road mobile sources, and non-point sources) and for nonattainment areas that fall into certain categories.

The Clean Air Act established two types of national air quality standards

  • Primary standards set limits to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly.
  • Secondary standards set limits to protect public welfare, including protection against visibility impairment, damage to animals, crops, vegetation, and buildings.

> Go to the EPA's website about NAAQS.
> Discover more about the Clean Air Act. 
> Learn more about the recent history of NAAQS.

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