On the Co-occurrence of Air Quality Extremes and Heat Waves

Abstract:

We investigate the relationship between observed maximum extremes of ozone, PM_2.5, temperature over eastern North America during 15 extended summer seasons (April-September, 1999-2013). We use an objective mapping algorithm to calculate a 1° x 1° grid-cell averaged product of (1) the maximum daily 8-hour average of surface ozone abundance and (2) the daily average PM_2.5 abundance from surface monitoring networks in the US and Canada. In addition, we use ECMWF reanalysis data to generate a 1° x 1° grid-cell averaged product of (3) the maximum temperature at 2-meter height. The extreme maxima for these 3 data sets are defined at each grid cell as the 50 days with the highest value in three 5-year windows (~94.5 percentile of all Apr-Sep days). These extremes for ozone, particles and temperature are denoted OX, PX, and TX, respectively.

Extreme ozone and PM_2.5 most often occur together (35% of OX and PX events in a cell occur simultaneously), followed by PM_2.5 and temperature (29%), ozone and temperature (27%), and all three (15%). In all cases, the greatest co-occurrence is found in the northeast US (>50% for two co-occurring events). We find that the day after any extreme is also likely to be an extreme of any kind (p > 75%) and that the most likely follow-on extreme is of the same type (p = 20-40%). The northeast US is an exception where OX are more likely to be followed by PX.

Extreme episodes (defined as multi-day, spatially connected events) typically originate as an OX event, followed by PX and then TX. This ordering is also evident in the generalized spatial structure of episodes: OX occur at the center and the eastern leading edge, PX are found to the immediate northwest, and TX surround the OX and PX events. The largest OX and PX episodes are similar in size, but TX episodes are usually larger and longer lasting. In general, extremes are more likely to co-occur for larger episodes.

The intensity of OX, PX, and TX events is measured as the % above the 5-year mean. It is greatest for any of these 3 extremes when it co-occurs with another extreme, and greatest when all three extremes occur together.