Air Pollution Goes Round and Round

by Bill Chameides | January 28th, 2010
posted by Erica Rowell (Editor)

Permalink | 2 comments

A new study suggests that T-shirts and electronics are not the only imports we get from Asia.

The Good, the Bad, and the Ugly

Dan Albritton, who ran a research lab at the National Oceanic and Atmospheric Administration for many decades, liked to refer to atmospheric ozone (O3) as “the good, the bad, and the ugly.”

When found in the stratosphere (some six to 31 miles above), O3 is “good” because it protects us from the sun’s harmful UV radiation. But when found in the troposphere (the atmosphere below about six miles), O3 is “bad” because it is a greenhouse gas (and when transported can contribute to the ugly form). O3 is “ugly” at ground-level because it is toxic to people, other animals, and plants. (Learn more here, here and here.)

Atmospheric O3 is produced from photochemical reactions; i.e., reactions triggered by solar radiation (see graphic). In the lower atmosphere, some O3 is produced as a component of photochemical smog generated from air pollutants from power plants, factories, and automobiles. For the chemically curious, the key air pollutants in this process are volatile organic compounds (VOC) and nitrogen oxides (NOx) (see graphic).

The Good, the Bad, and the Ugly. Ozone (O3) is most abundant in the stratosphere where it protects us from the sun’s harmful UV radiation. Less abundant, but worrisome, is O3 in the lower atmosphere or troposphere and at ground level where it is toxic and acts as a greenhouse gas. In both regions, O3 is produced from photochemical processes. In the lower atmosphere and at ground level these reactions involve volatile organic compounds (VOCs) and nitrogen oxides (NOx).

While concerns about stratospheric O3 focus on declining levels (like the ozone hole), concerns about tropospheric and ground-level ozone focus on increasing concentrations. In the 1950s with the exploding popularity of the automobile, photochemical smog and the O3 that accompanies it became ubiquitous to urban environments. Later in the century, with more sensitive instrumentation to monitor O3 concentrations, it became clear that ground-level O3 pollution was not limited to the urban environment but was a regional phenomenon extending over many hundreds if not thousands of miles.

But the atmosphere at the ground is connected to the atmosphere above and so scientists began to wonder if O3 pollution was not just a ground-level, regional problem but also a global phenomenon extending throughout the troposphere.

In 1988 scientists Dieter Kley and Andreas Volz published a startling paper in the journal Nature — they uncovered records of O3 measurements made by French monks living in the Alps way back in the 1880s. Analyses of these measurements indicate that O3 concentrations at that time were more than a factor of less than they are today. Since then, many other early 20th and late 19th century datasets have been uncovered that generally confirm the conclusions of Volz and Kley.

At least two important questions grew out of their work:

  1. If real, was this increase caused by air pollution?
  2. Are O3 concentrations increasing today?

The answers to these questions are quite important — not only because a global increase in ground-level O3 implies an increasing toxicity of the air we breathe but also, because O3 in the troposphere is a greenhouse gas, an increase implies human activity is creating yet another global warming source.

The answer to the first question is difficult to answer because we don’t have the detailed VOC and NOx emissions and concentration data from the early and mid-20th century, but model simulations suggest that qualitatively the increase is likely to be real and due in large part to air pollution.

The second question is also quite difficult to work out because tropospheric and ground-level O3 vary quite a lot in both space and time. To infer a trend requires teasing it out from the large natural variability.

Ever willing to jump into the breach, atmospheric chemists have been attempting to see if a trend exists by monitoring O3 concentrations in greater and greater detail. The results of these studies have been mixed: at some locations an increase, at others no trend, and at others still a downward trend [pdf].

Last week, a paper in Nature by Owen Cooper at the University of Colorado at Boulder and colleagues added a new piece to the puzzle. They used an extensive dataset of O3 measurements collected from the mid-troposphere two to five miles above the Earth’s surface from 1995-2008 along with an extensive dataset from 1984. These measurements were collected in the spring (when long-range global transport of O3 dominates local ground-level sources) by a variety of different platforms including balloons, remote sensing (lidar), and aircraft.

The authors estimate that springtime O3 concentrations above the western United States have been increasing at a rate of 0.63 ± 0.34 parts per billion per year. (One part per billion means a concentration of 1 molecule for each billion atmospheric molecules.)

By comparison, the springtime background concentration of O3 in that region is typically about 60 parts per billion which gives a trend of about one percent of the background concentration per year. Still, it appears to be statistically significant and a percent per year increase over 25 years adds up to a net increase of about 15 parts per billion or almost 30 percent since 1984.

If that isn’t enough, the Cooper et al. paper has an added kicker. Their analysis suggests that much of the increase they are seeing over the western United States is being driven by pollution from Asia.

If true, it suggests that Asian pollution could frustrate efforts here to mitigate ground-level O3 pollution. For example, as we progressively clamp down on local, ground-level O3 pollution by setting tighter and tighter air quality standards, background O3 from the troposphere becomes a greater and greater component of the gound-level O3.

By increasing the background O3 concentrations over the United States, Asian air pollution increases the m
inimum concentration we can achieve at the ground.

But before you get too indignant about Southeast Asians exporting their dirty air to us, keep in mind that because winds tend to blow from west to east, we send our pollution to Europe, and European pollution tends to flow to Asia. When it comes to global air pollution, what goes around comes around.

filed under: Asia, atmosphere, climate change, Europe, faculty, global warming, nature, science
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  1. Bill Chameides
    Jan 29, 2010

    Tom, the story of O3 regulation is full of missteps as we slowly came up the learning curve. As it happens, I published a paper in Science way back in the 1980s that argued that control of NOx emissions is essential to effectively mitigate O3 pollution.

  2. Tom Sharpless
    Jan 29, 2010

    It took awhile for air pollution experts to realize how extensive ozone is. EPA’s original decision not to regulate NO seems to have been based on smog chamber results showing NO delaying O3 formation each morning. Perhaps it was believed that by the time the rush hour was over, the O3 would be dispersed. Only later (than 1970) was it conclusively shown that there is O3 aloft that mixes in with ground level NO when the overnight inversion is eroded by the sun’s warmth, rapidly causing NO to become NO2 and thereby starting the rapid buildup of O3 at ground level.

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