Detecting Long-Term Changes in Air Pollution from Space

Developing Tools for High-Resolution Air Quality Data

To keep our air clean, we need to implement effective regulations. To understand the effect of regulations, we need access to high-quality data that can show long-term trends.

Amir SouriAmir Souri, Ph.D. student in atmospheric sciences, is developing high-resolution methods to detect changes in air pollution.Amir Souri, a Ph.D. student who is advised by Yunsoo Choi, assistant professor in the Department of Earth and Atmospheric Sciences, is working on high-resolution methods to detect changes in air pollution.

Recently, Souri was the lead author on a paper, published in the Journal of Geophysical Research: Atmospheres, in which he used these methods to detect changes in emission levels in East Asia. This method, which can be used for detecting multiple types of air pollution, focused on ozone, which is also a problem here in the United States.

Ozone at Ground Level Harms Human Health

“In the stratosphere, ozone is useful because it blocks UV radiation,” said Souri, who arrived at the University of Houston in 2015.

However, on ground level, ozone poses a human health risk, leading to complications such as asthma and reduced lung function.

Ozone pollution is especially hard to predict, given that it is formed due to the photochemical reaction of different air pollutants. Souri’s research has focused on detecting the two primary reactants, which are NO2 and a class of compounds known as volatile organic compounds (VOCs). The main source of NO2 and VOC emissions are from vehicles, power plants and industry in East Asia.

Under the right circumstances, one that is dependent on sunlight and the right proportions of reactants, these can combine to form ozone.

Ozone Pollution Capable of Traveling Long Distances

“Air pollution doesn’t remain confined to one country,” Souri said. “Ozone has a long lifetime, lasting from a few days to several weeks. So even though we have regulations, ozone is still transported to the western United States from China.”

Using data from NASA’s Ozone Monitoring Instrument, which orbits the Earth recording daily air quality measurements, Souri was able to construct high-resolution maps dating back to 2004. These maps are of both NO2 and VOC levels.

High-Resolution Methods Show Hidden Trends

“We devised a new method called trend classification, which gave us a high resolution method to determine how NO2 levels changed during a long period of time,” Souri said.

These maps, which required the use of a high-performance computing cluster to construct, showed changes in pollution levels in East Asia. What Souri found was that NO2 levels are decreasing in many regions, especially in areas such as southern China.

“This map offers a much more detailed assessment,” Souri said. “Based on this map, we can get a lot of hidden information.”

One observation was the fact there are a few places, such as in parts of Korea and Japan, which have seen an increase in recent years. Some of this can be attributed to relaxed regulations in Korea, as well as increased use of coal-burning power plants in Japan following the Fukushima Daiichi nuclear disaster.

Defining Priorities for Regulations

The formation of ozone is dependent not only on the presence of NO2 and VOCs but also their relative proportions. In terms of regulating pollution levels, this creates an extra complication, as the relative proportions dictate what strategy to use. In some areas, reducing VOCs will have the greater impact, while in other areas, targeting NO2 will be more effective.

“For some regions, we are finding ozone production is becoming more sensitive to VOC levels. Meanwhile, other regions, ozone production is becoming more sensitive to NO2 levels,” said Souri. “These regions will require different priorities for regulations.”

- Rachel Fairbank, College of Natural Sciences and Mathematics