Development alters wind patterns, leading to pollution build up

When it comes to the pollution in our air, emissions aren't only to blame.

When it comes to the pollution in our air, emissions aren't only to blame. New research suggests that urban development actually alters wind patterns, leading to a build up of pollutants during warm summer months.

The study, published this month in the Journal of Geophysical Research - Atmospheres, was funded by the U.S. Air Force Weather Agency, the U.S. Defense Threat Reduction Agency and the National Science Foundation. I spoke recently with Fei Chen, the study's lead author and a scientist at the National Center for Atmospheric Research. Below are excerpts from our interview.

How does development alter wind patterns and lead to pollutant build up?

This is specific to the Houston area, which is close to the bay. Our study indicated that the urbanization of the Houston area affects wind patterns in a way that makes summertime pollution worse. An urban area and a large body of water don't cool down very much [at night]. The lack of a sharp temperature gradient has the effect of reducing wind. When you're walking across a parking lot in the summer, you can feel how much hotter it is than a nearby park. The paved area in cities has a major influence on local weather. During the day, the city heats up and it draws in air from offshore. However, this incoming air mass is offset by prevailing wind patterns. Buildings and other structures break up local winds far more than does the relatively smooth surface of croplands or a natural surface like grasslands. This tends to further reduce breezes.

What happens when those pollutants build up?

Pollution is a major health issue in Houston, even though they have worked for years to improve the situation. It can be very difficult to lower air pollution. The study shows that not only do emissions matter, but the city itself, the development, matters for public health.

How did you conduct this research?

I'm interested in how land use changes, how that impacts local weather and how that impacts air pollution. In terms of meteorologic conditions, [Houston] is quite an interesting place. We decided to study this because of the high visibility of the Houston pollution problem.

There have been studies showing how cities can affect temperature, precipitation and wind. We took it one step further to find out that the buildings and the pavement can contribute to an unhealthy level of air pollution by reducing wind that would otherwise push pollution out to sea. We used a complex computer model to do the simulations.

What should be done about this?

More research is needed to determine whether buildings and paved surfaces in other areas also contribute to air pollution. If this is the case, then city planners may be able to use my information to alter development patterns and reduce air pollution. Mitigating the air pollution problems in some cities may involve building more green space and lakes, for instance. The important point from this study is that reducing emissions is just one way of improving air quality.

Do you plan to continue research?

Yes. [We want] to expand this study to other cities. It's important for us to understand whether this will get worse in the future. We'll take a climate projection and use a similar approach to understand how our children will suffer from air pollution.

Photo: Fei Chen / By Carlye Calvin, University Corporation for Atmospheric Research

Image: Paved surfaces in the Houston area keep the city warmer than more natural surfaces. As a result, overnight temperatures are often similar between the city and nearby offshore areas, which weakens summertime breezes and enables air pollution to build up. The stagnant conditions also persist during the day because of larger-scale wind patterns. / Illustration by Lex Ivy, UCAR

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