Co-occurrence of Extremes in Surface Ozone, Particulate Matter, and Temperature Over Eastern North America

Observations indicated strong correlations between heat extremes and multiple health-stressors.

The Science

In this paper, using 15 years of surface observations over the eastern United States and Canada, the authors show that the extremes cluster together in often overlapping large-scale episodes, and that the largest episodes have the hottest temperatures and highest levels of pollution.

The Impact

Exposure to extreme temperatures and high levels of the pollutants ozone and particulate matter poses a major threat to human health. Heat waves and pollution episodes share common underlying meteorological drivers and thus often coincide, which can synergistically worsen their health impacts beyond the sum of their individual effects. Furthermore, there is evidence that pollution episodes and heat waves will worsen under warmer conditions, making it imperative to understand the nature of their co-occurrence.


Heat waves and air pollution episodes pose a serious threat to human health and may worsen under future climate change. In this paper, we use 15 years (1999-2013) of commensurately 1°x1°- gridded surface observations of extended summer (April-September) surface ozone (O3), fine particulate matter (PM2.5), and maximum temperature (TX) over the eastern United States and Canada to construct a climatology of the coincidence, overlap, and lag in space and time of their extremes.  Extremes of each quantity are defined climatologically at each grid cell as the 50 days with the highest values in three 5-yr windows (˜(95th %ile). Any two extremes (O3X, PMX, TXX) occur on the same day in the same grid cell more than 50% of the time in the northeastern United States.  Many extremes show connectedness with consistent offsets in space and in time, which often defy traditional mechanistic explanations.  All three extremes occur primarily in large-scale, multi-day, spatially connected episodes with scales of >1,000 km and clearly coincide with large-scale meteorological features.  The largest, longest-lived episodes have the highest incidence of co-occurrence and contain extreme values well above even their local threshold (95th%), by +7 ppb for O3, +6 µg m-3 for PM2.5, and +1.7 °C for TX.  The results demonstrate the need to evaluate these extremes as synergistic co-stressors to accurately quantify their impacts on human health.

Principal Investigator(s)

Michael Prather


The U.S. Department of Energy Office of Science, Biological and Environmental Research, Earth System Modeling.


Schnell, JL; & Prather, MJ. (2017). Co-occurrence of extremes in surface ozone, particulate matter, and temperature over eastern North America.. Proceedings of the National Academy of Sciences of the United States of America, 114(11), 2854 – 2859. doi: 10.1073/pnas.1614453114. UC Irvine: 1817348.