Why the Air Quality Index would indicate good air quality even during during a smog advisory?
Ozone pollution builds up over the course of a hot sunny day and reaches it's peak in the late afternoon and early evening. The AQI reflects an average of the previous 8 hours of ozone pollution. For instance at 4 p.m. the AQI is an average of the 8 previous hourly readings (8 a.m. to 4 p.m.). That includes the period of time when ozone levels are at their lowest (in the morning).
The eight highest hours usually occur between 1 p.m. and 9 p.m. when temperatures are highest. The formation of ground-level ozone (smog) takes time. Ozone is formed when pollutants from cars, industry, other gas engines (lawnmowers, boats) and products we use every day (personal care items, spray cans, paint) react with oxygen in the presence of sunlight, particularly when it's hot.
Ground level ozone
Ozone, commonly called smog, is a colorless, odorless gas that can be harmful
to human health and plant life. All areas of Oregon currently meet the federal air quality
standard for ozone.
Because the Portland-Vancouver area at one time did not meet the standard,
DEQ developed an ozone maintenance plan that also includes the Salem-Keizer
area. Implementing these plans has ensured that ozone levels in the
Portland-Vancouver and Salem areas are in compliance with federal health