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The Environmental Protection Agency has a saying about ozone: “good up high, bad nearby.” Ozone pollution near Earth’s surface is one of the main ingredients of summertime smog. But it is not directly measurable from space, because the abundance of ozone higher in the atmosphere masks the surface. Researchers in Arlene Fiore’s Atmospheric Chemistry Group at Lamont have used satellite measurements of two different precursor gases that contribute to ozone formation to track changes in this chemistry over the past decade, during which emission controls were put in place to lower surface ozone. The group’s observational analysis may help air quality managers assess the past success of their programs and identify the most effective approaches to reduce emissions and improve air quality in the future.

Unlike at high altitude, where ozone acts as Earth’s sunscreen to harmful ultraviolet radiation, ozone at low altitudes is a health hazard contributing to respiratory problems such as asthma and bronchitis. It is formed through complex chemical reactions initiated by sunlight and involving two types of gases: volatile organic compounds (VOCs) and nitrogen oxides (NOx). The group’s study included a major gas of each type, the VOC formaldehyde and nitrogen dioxide (NO2), which are both measurable from space.

“We are using satellite data to analyze the chemistry of ozone from space,” said group member and study lead author Xiaomeng Jin. Through a combination of computer models and space-based observations, Jin and colleagues used the concentrations of the precursor molecules to infer whether ozone production at a given location increased more in the presence of NOx, VOCs, or a mix of the two. Their study regions focused on North America, Europe, and East Asia during the northern summer, when abundant sunlight triggers the highest rates of ozone formation. To understand their impact on ozone formation, the team investigated whether VOC or NOx was the ingredient that most limited ozone formation. If emissions of that molecule can be reduced, then ozone formation will be reduced – critical information for air-quality managers.

“We are asking: ‘If I could reduce either VOCs or NOx, which one is going to get me the biggest bang for my buck in terms of the amount of ozone that we can prevent from being formed in the lower atmosphere?,’” said Fiore, who is also a member of NASA’s Health and Air Quality Applied Sciences Team.

“We are asking: ‘If I could reduce either VOCs or NOx, which one is going to get me the biggest bang for my buck…”

Fiore says the approach described in their study has important implications for ongoing work. Understanding trends and variability in atmospheric composition of chemical species such as ozone (both a greenhouse gas and an air pollutant) is a major driver of her group’s research. Many processes cause atmospheric ozone to vary, including climate and meteorology, natural and anthropogenic sources, as well as sinks through chemical reactions or uptake by the terrestrial biosphere and other surface processes. This application is an example of how researchers can use ever-growing satellite records to learn about one piece of this puzzle, in this case the changing emissions of ozone precursors.

As the major components of air pollution (ozone and particulate matter) also affect the climate system (through their interactions with the planet’s radiation budget), understanding changes in ozone formation chemistry connects to broader interests in the interactions between climate and air quality. “In our group, we’re keenly interested in understanding the impacts of changes in these air pollutants on the climate system,” said Fiore. “This research has not fundamentally changed our understanding of ozone formation chemistry,” said Fiore.

What is new is that, for the first time, researchers are able to observe how this chemistry varies in space and time. Newer work by Jin looks at even finer temporal and spatial scales, with the aim of understanding how ozone formation chemistry changes on the most polluted days compared with average conditions.

Among the most pressing questions for Fiore and air-quality research is how to clear the most unhealthy conditions as quickly as possible. “Billions of people on the Asian continent are breathing very unhealthy air most days of the year,” noted Fiore. “Which lessons learned and success stories from other regions are best suited to rapidly clearing the air and lowering the associated health burden in these areas?”

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