By Arizona Department of Environmental Quality

The wave of low pressure that brought breezy winds to the Valley on Friday and Saturday departed to the east on Sunday. As this system slides on out, high pressure will filter in behind it. This high pressure has our attention since it will result in a more stagnant atmosphere over Arizona on Monday. Furthermore, light winds and plenty of sunlight will be favorable conditions for the formation and accumulation of ozone near the ground. Therefore, an Ozone Health Watch was in place for Sunday AND an Ozone High Pollution Advisory will be issued for Monday.

And just as a reminder: an Ozone Health Watch means that ozone is expected to approach the federal health standard (90-100 AQI); an Ozone High Pollution Advisory means that ozone is expected to exceed the health standard (>100 AQI).

The good news, however, is that ozone levels are expected to decrease during the latter half of the week as another large pattern of low pressure begins to affect the U.S. West. This system will result in stronger winds out of the southwest, particularly Wednesday and Thursday. For Tuesday though, there’s a little more uncertainty regarding the strength of the winds just above the ground. Ozone is currently forecast in the Health Watch AQI (Air Quality Index) range for Tuesday as light winds near the ground should favor ozone accumulation. Then by Wednesday, ozone levels are forecast to decrease and fall below the Health Watch range.

Regarding PM10 (dust), no issues are expected, save for the possibility of elevated PM10 levels southwest of the Phoenix metro area on weekday mornings. PM10 is forecast in the Moderate AQI category on weekdays to account for this activity. Elsewhere around the Valley, PM10 is expected to remain in the Good AQI category.

Lastly, PM2.5 (smoke, soot) levels are expected to remain in the Good Air Quality Index (AQI) category each day.