January 1999 Air Quality Report

• Weather patterns affecting air quality
• Highest monitored levels of air pollution
• Highest levels of particulate matter (PM10 & PM2.5), carbon monoxide (CO), and ozone (O3)

Pollutant dispersion varied from good to poor during January and included two periods of stagnant days, the 1st through 6th, and the 24th through 26th. Precipitation was above normal again in western Washington, with SeaTac airport receiving 127% of normal rainfall for January. The three month period, November through January, is the wettest on record at SeaTac airport. The airport average monthly temperature of 42.1 degrees F was 2.0 degrees above normal for January with a maximum temperature of 53° F on the 10th and a minimum temperature of 30° F on the 3rd and 24th.

• Because of elevated particulate levels and a stable weather pattern, a first stage burn ban was declared on Tuesday the 5th at 2 p.m. The ban was ended the next morning on Wednesday the 6th at 10 a.m. after a weather disturbance aloft effectively removed the temperature inversion.
• The highest PM2.5 value of 60 µg/m³ was in the "Moderate" category (PSI of 95) and was measured on Monday the 4th at the Alexander Avenue site in the Tacoma tideflats. The highest PM10 value of 88 µg/m³ was in the "Moderate" category (PSI of 67) and was measured on Tuesday the 5th at the East 11th Street site in the Tacoma tideflats.
• The highest 8-hour average for carbon monoxide was 6.5 ppm, in the "Moderate" category (PSI of 71), and occurred during the evening hours of Monday the 4th at the Pacific Avenue site in downtown Tacoma.
• The worst visual range of 8.1 miles occurred the evening of Monday the 4th from 11 pm - midnight PST at the Kent site.

Puget Sound Clean Air Agency