Tropospheric Ozone Tutorial
What is “good” and “bad” ozone?
Ozone occurs in two layers of the atmosphere. The layer surrounding the
earth's surface is the troposphere. Here, ground-level or "bad" ozone is an air pollutant
that damages human health, vegetation, and many common materials. It is a key
ingredient of urban smog. The troposphere extends to a level about 10 miles up, where it
meets the second layer, the stratosphere. The stratospheric or "good" ozone layer extends
upward from about 10 to 30 miles and protects life on earth from the sun's harmful
ultraviolet rays (UV-b) Ozone has the same chemical structure whether it occurs high
above the earth or at ground level. Ground level, or “bad ozone”, is also called
tropospheric ozone while “good ozone” is called stratospheric ozone. Most of the ozone
in the atmosphere (80-90%) is of the “good” variety.
Why is it called “bad” ozone?
High levels, or concentrations of ozone at the earth’s causes damage to
people, plants, and animals. Ozone at the ground level occurs naturally but activities by
humans can cause concentrations to increase. In humans, when air with high
concentrations of ozone is inhaled it adversely affects the respiratory system. The most
sensitive people are children, the elderly, and those with asthma or other respiratory
illnesses. In vegetation, changes their color, injures the plant (sometimes visibly seen by
changes in leaf color) and decreases their yields.
Ozone effects human health mainly as an irritant to the lungs (Bates, 1994; Horvath and
McKee, 1994). Populations sensitive to increased ozone levels are people with preexisting conditions such as asthma, and those involved in outdoor activities. Symptoms
include decreased lung function, chest pains, and coughing. Additionally, studies
indicate that increased levels of ozone contribute to decreased crop yield and forest
decline (Office of Technology Assessment, 1989; Lefohn et al., 1987).
What is ozone?
Ozone is a gas that forms in the atmosphere when 3 atoms of oxygen are
combined (O3). At ground level it is formed by a chemical reaction between
oxides of nitrogen (NOx), and volatile organic compounds (VOC) in the presence
of sunlight. (EPA, Good up High, Bad Nearby)
What causes ozone to form?
Ozone is not directly emitted into the atmosphere but form when NOx and
VOCs are in the air and the sun is out. Motor vehicle exhaust and industrial
emissions, power plants, gasoline vapors, chemical solvents, and vegetation are
some of the major sources of NOx (pronounced as one word, Nox) and VOC
(pronounced as 3 separate letters, V-O-C), also known as ozone precursors.
Strong sunlight and hot weather cause ground-level ozone to form in harmful
concentrations in the air. Many urban areas tend to have high levels of "bad"
ozone, but other areas are also subject to high ozone levels as winds carry NOx
emissions and ozone hundreds of miles away from their original sources.(EPA,
Good up High, Bad Nearby)
Major sources of anthropogenic NOx include transportation, power plants, and
other combustion processes. Significant natural sources of NOx in the U.S. occur from
lightning, biomass burning, and soil. (Chameides and Cowling, 1995). Anthropogenic
sources of VOCs include transportation, chemical plants, and refineries. Natural VOC
emissions also contribute significantly through vegetation.
How does meteorology/weather influence ozone concentrations?
Tropospheric ozone is formed most efficiently on hot, sunny days under
stagnant and humid air conditions. High humidity and high temperatures enhance
the formation of ozone. High winds tend to dilute ozone concentrations locally
but they also transport ozone to other locations, often causing high ozone
concentrations in areas Low wind speeds lead to the buildup of high local
pollutant concentrations. Strong ventilation with high wind speeds prevents the
local build-up near the sources, but contributes to long-range transport and
regional ozone particularly during directionally persistent wind conditions.
Can ozone be transported from city to city?
Ozone and the compounds that help form it, Nox and VOC can be
transported significant distances from where their are emitted (originate) and can
cause ozone problems in other areas. Atmospheric conditions can exert a powerful
influence on the distribution of pollutant concentrations in space and time. The
distance of O3 transport between the precursor emissions and O3 removal is in
the range of 150 to 500 miles. The transport of ozone manifests itself differently
at the local, sub-regional and regional scales. In general, local (30-150 miles)
transport contributes most to the non attainment of the 120 ppb standard. Beyond
100 - 200 miles the ozone concentrations tend to decrease with increasing
transport distances. (OTAG)
How is ozone measured?
Tropospheric ozone can be measured using multiple methods. Since ozone is a
strong oxidizer, it chances the color of some chemical compounds and solutions. For
example, paper soaked in a mixture of starch and potassium iodide will change color
when exposed to ozone.
The reaction of ozone with various chemicals, gases, and even some lubricating
oils causes a faint luminescence that can be detected by a sensitive photomultiplier tube.
Detection systems such as this are known as chemiluminescence detectors.
Since ozone absorbs ultraviolet radiation so effectively, many kinds of ozone
detectors incorporate a UV lamp and a detector. Air is passed through a chamber, and any
attenuation is assumed to have been caused by ozone. A problem with this method is that
attenuation can also be caused by dust. Therefore, it is common practice to use two
chambers, one of which receives air form which any ozone has been scrubbed by a
catalytic converter. Alternatively, scrubbed and unscrubbed air can be passed through the
same chamber. Either way, the error caused by dust and other contaminants in the ozonefree sample can then be determined by subtraction. (F. Mims III)
The absorption of UV light by the ozone molecule provides a reasonably
straightforward and accurate means to measure ozone. Most instruments rely on the 254
nm emission line of mercury (which happens to coincide with an absorption maximum of
ozone) from a mercury discharge lamp as the UV light source. This tefchnique, which has
been incorporated into several high-quality commercially available instruments, is
reliable, and interference that occurs because of the absorption of the UV light by
molecules other than ozone can generally be ignored. Most high-quality, routine in situ
measurements of ozone have been made with this technique.
The chemiluminescence produced by the reaction of ozone with nitric oxide forms the
basis for a sensitie and specific ozone detection method. Although chemiluminescence
tends to be more complicated than the UV absorption method, it can make fast-response
ozone measurememtns because of its greater sensitivity. For this reason
chemiluminescence has been used to measure ozone fluxes that can be deduced from the
correlation of ozone variation with atmopshere turbulence.
Where are the highest ozone concentrations?
The highest ozone concentrations tend to occur in and around large urban areas
that generate the precursors necessary for ozone formation. However, rural areas can also
experience high ozone concentrations due to transport.
[any need to mention ozone scavenging in cities?]
When are ozone concentrations highest?
Ozone concentrations vary over different scales of time. In urban areas where ozone is
generated, ozone concentrations tend to peak in the middle of the day and dip to their
lowest concentrations during the middle of the night. This type of cycle is called diurnal.
Ozone concentrations vary from day to day depending on emissions and meteorology but
sometimes under certain meteorological conditions episodes occur. Episodes are periods
of a few days to a couple of weeks when ozone concentrations are unusually high.
For U.S. southern urban areas, episodes are caused by local stagnation, when wind speeds
are slow. This allows ozone concentrations to build up. In northern U.S. cities, episodes
are more heavily influenced by transport of ozone and precursors from other areas.
Episodes are also associated with high temperatures and high humidity.
Because ozone concentrations are dependent on high temperature and sunlight, the
highest concentrations during the year occur in the summer months. The lowest
concentrations, or the cleanest air, occurs during the winter months. This type of cycle is
called seasonal.