Arizona Air Quality Information

Unusually Low Ozone for March


This March is on track for the history books. Are you aware of how many days ozone has remained in the Good Air Quality Index (AQI) category this month so far? Or to put it another way, how many days ozone has NOT reached the Moderate AQI category?

Over the past 15 years, on average, the number of days ozone reaches the Moderate AQI category in March is 17 days. This year, if current ozone forecasts hold, Phoenix could only see 6 days of Moderate ozone. It should be noted that the greatest number of Moderate ozone days in March was 27 days in 1999; the least was 1 day in 1984. Furthermore, this March could finish in the top 10 Marches for the least number of Moderate days in the past 40 years! Needless to say, Phoenix has enjoyed below normal ozone levels for almost all of this March so far. But why is this?

When comparing ozone levels (or any air pollutant) from year to year, differences are highly dependent upon the weather. This year, the jet stream has been persistently active over the Southwest through much of March. As a result, low pressure waves have been able to move over Arizona more frequently. These waves have then brought winds, clouds, and rain to Phoenix (photo: rain in downtown Phoenix on the morning of March 12). Three rain-producing systems affected Phoenix in March, resulting in Sky Harbor seeing its 10th wettest March on record so far, with a monthly rainfall total of 1.94". Since ozone needs sunlight to form and light winds to accumulate, these active weather conditions have helped to prevent ozone levels from rising into the Moderate AQI category.

But once weather conditions calm down--when winds become light and skies are sunny--ozone levels will begin to increase. We've already seen evidence of this as ozone levels finally reached the Moderate AQI category over this past weekend (March 21 and 22). Moreover, ozone is forecast in the Moderate AQI category today and Wednesday because of expected calm conditions. We'll have to wait and see how long this trend of active weather lasts.

View the Phoenix Air Quality forecast here.