Co-occurrence of extremes in surface ozone, particulate matter, and temperature over eastern North America

Study key findings

• Examines 15 years (1999–2013) of surface observations of "extended summer" (April through September) surface ozone (O3), fine particulate matter (PM2.5), and maximum temperature (TX) over the eastern United States and Canada
• Constructs a climatology of the coincidence, overlap, and lag in space and time of each quantity's extremes
• Defines extremes as the 50 days with the highest values in three 5-year windows (at the 95th percentile)
• Finds that all three extremes occur primarily in large-scale, multiday, spatially connected episodes and coincide with large-scale meteorological features
• Results show that the largest, longest-lived episodes have the highest incidence of co-occurrence
• Results demonstrate the need to evaluate these extremes as synergistic costressors to accurately quantify their impacts on human health, as heat waves and air pollution episodes may worsen under future climate change

Author quotes

The weather factors that drive heat waves also contribute to intensified surface ozone and air pollution episodes. These extreme, multiday events tend to cluster and overlap, worsening the health impacts beyond the sum of their individual effects.

Michael J. Prather, study co-author and professor at the University of California, Irvine

Abstract

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 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 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 d with the highest values in three 5-y windows (∼95th percentile). Any two extremes occur on the same day in the same grid cell more than 50% of the time in the northeastern United States, but on a domain average, co-occurrence is approximately 30%. Although not exactly co-occurring, many of these 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, multiday, 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 their local 95th percentile threshold, by +7 ppb for O3, +6 µg m−3 for PM2.5, and +1.7 °C for TX. Our results demonstrate the need to evaluate these extremes as synergistic costressors to accurately quantify their impacts on human health.