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The 7 November 2016 at 11h00

Wide Interest Seminar: Michael Prather


How do pollution episodes and heatwaves interact? The co-occurrence of extremes in ozone, particulate matter, and temperature

By Michael Prather and Jordan Schnell*, University of California, Irvine

* now Program in Atmospheric and Oceanic Sciences, Princeton University


Exposure to extreme temperatures and high levels of the pollutants ozone and particulate matter pose 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 future climate change making it imperative to understand the nature of their co-occurrence.

In this work, we use fifteen years of commensurately gridded (1° x 1°) surface observations of extended summer (April–September) surface ozone (O3), fine particulate matter (PM2.5), and maximum temperature (TX) over the Eastern US and Canada to construct a climatology of the coincidence, overlap, and lag in space and time of the extremes.  Extremes of each quantity are defined climatologically at each grid cell as the 50 days with the highest values in three 5-year windows (~95th percentile). We find that any two extremes co-occur on average between 27–35% of the time and >50% of the time in the Northeast US. The extremes also show consistent offsets in space in time, which contradict simple mechanistic explanations in some locations. The extremes most often occur in large-scale, multi-day, spatially connected episodes on scales of >1,000 km episodes and are clearly driven by meteorology, namely stagnation. The largest, longest-lived, clusters have the highest incidence of co-occurrence and contain extremes well above the 95th percentile threshold, i.e., 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.


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