Increasing co-occurrence of fine particulate matter and ground-level ozone extremes in the western United States.

The maximum daily fraction of western US grid cells with simultaneous PM 2.5 /ozone co-occurrence during July to September has more than doubled (from 18.9 to 44.6%) over the past two decades, with an increasing trend of ~1.35% per year ( P = 0.02) ( Fig. 1D ).

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