Research_Climate-induced Air Quality Deterioration

As summers get increasingly hotter, expect more adverse effects on health. EPICenter grant recipient Pengfei Liu is exploring the connection between the two as a necessary step in climate adaptation. 

The trendlines point in one direction: The ten hottest years on record were all just in this nascent century.  

Summers, especially, are getting hotter. And that’s a problem for many reasons, including its adverse effect on human health. Research has found that “the risk of heat-related deaths during extreme summer heat has already increased rapidly over the past 20 years,” and will become the norm if nothing is done to address the issue. Pengfei Liu, Assistant Professor at the School of Earth & Atmospheric Sciences, is doing his part to address the issue.  

The mechanisms for increased mortality in hotter summers 

Through an EPICenter grant, Liu is researching the exact mechanisms through which higher summer temperatures increase mortality rates.  

The heat itself is one pathway: Even short-term heat waves can affect health adversely and increase mortality. Liu and his team propose an additional, indirect pathway: through air pollution. In this case, warmer temperatures worsen air pollution, which increases mortality. 

This indirect pathway forms the basis of research for Liu and his team “to better quantify the link between temperature and air quality and its implications on human health.”  

The impact on public health policy 

The EPICenter grant focuses the scope of Liu’s research to the southeastern U.S. which is not only home to Georgia Tech but also to a high population of seniors.  

Epidemiological evidence indicates that even a rise of 1°C in summer mean temperatures corresponds to an estimated increase in mortality of 1% to 2.5% among older adults in US populations. It’s why Liu’s research is important; parsing the link between mortality and hotter summers will be one of the key factors that could help shape public health policy.  

But research has to be placed in context, Liu cautions. “For policy, we have to consider air quality, climate change, and human health and energy in a bigger framework,” he says.  

But Liu’s research is especially suited to frame policy because it focuses on both spatial and temporal trends. “For example, we can examine whether people living in urban regions versus rural areas are more sensitive to air pollution,” Liu says. 

Complementary evaluation techniques 

Traditional methods of evaluating the relationship between high temperatures and air pollution use chemical transport models. Starting with emissions inventory from different places and times, the mechanism simulates chemical reactions in the air with the addition, transport, and removal of pollutants. Using such a method allows researchers to study how the air quality—ozone and particulate matter in particular—change over time and across different places. 

A newer alternative to chemical transport models is statistical modeling with machine learning. Using ground-level observations and satellite data, statistical modeling techniques yield better temporal and spatial resolution and are computationally more efficient. On the other hand, chemical transport models are based on physics and chemistry principles and are hardy enough to study historical patterns and to make future predictions.  

For increased clarity, Liu is relying on both kinds of models leveraging the strengths of each. “We’re trying to use a statistical model to evaluate air pollution temperature sensitivity and we want to use that information to improve the chemical transport model,” Liu says, explaining that results from one can help boost the robustness of the other.  

Liu’s research feeds on data from the last 20 years to develop benchmarks for the relationship between air pollution and temperature. The resultant robust model can then deliver accurate predictions in the future. 

The findings so far 

Liu’s research on heat and mortality has delivered some striking results so far. Zooming out beyond the southeastern US, Liu and team found a strong positive relationship between temperature and air pollution, which was expected. “If we don’t change anything else, then if we have a warmer summer, the air quality will get worse,” Liu says.  

What was unexpected was that the sensitivity values changed in different regions: In the southeastern United States and in the Western US, the air pollution is more sensitive to summer temperatures than the rest of the country.  

And the reasons for each region might be different. The southeastern US has dense vegetation, which can emit volatile organic compounds, which are precursors for ozone and air pollutants. In the western United States, hotter summers increased the incidence of wildfires, which also increased air pollution. While the net cause and effect is not that simple—human-emitted NOx and SO2 can complicate the picture—the findings are striking. “I’m not saying anthropogenic emission is not important at all, but I think the biogenic factor is a big part of explaining the sensitivity, particularly under a scenario of large reduction of anthropogenic emissions” Liu says.  

Another significant finding: In the eastern United States the sensitivity of air pollution to heat is decreasing and that might be because of the decrease in anthropogenic emissions.  

One of the many key takeaways from Liu’s research is that decreasing anthropogenic emissions is making a dent in addressing climate change as data from the Northeast shows. “If we can reduce emissions as much as we can, we can reduce the temperature sensitivity to air pollutants and then a warmer summer will not mean terrible air pollution,” Liu predicts.  

And that just might mean avoiding many heat-related deaths in increasingly hot summers all over the country—and the world.