September 1998 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 ( G33C0003F110 REPLACEMENT LAPTOP LCD SCREEN O3)

Good pollutant dispersion generally prevailed during September. Precipitation was well below normal with Sea-Tac airport receiving 38% of normal rainfall for September. The airport average monthly temperature of 63.1 degrees F was 2.5 degrees above normal for September with a maximum temperature of 86° F on the 1st and a minimum temperature of 46° F on the 26th.

• In last month's report we informed you of a violation of the federal standard (NAAQS) for lead during the period April through June 1998 at the Harbor Island monitoring site. Our investigation is progressing and we hope to complete that part of the enforcement process soon. This report includes updated lead averages through mid-August. Please note that lead concentrations have returned to their normally low levels.
• Elevated ozone levels occurred during a 1-day period on September 1st. This period was characterized by stable weather with a nighttime low-level temperature inversion followed by warm afternoon temperatures in the lower 80's. Preliminary data shows that one measured ozone value elevated to just over the 8-hour federal standard (0.086 ppm) or reached the "Unhealthy for Sensitive Groups" category on the national Pollutant Standards Index (PSI). The Puget Sound region avoided exceeding the 8-hour federal standard for the years 1996-1998. For the years 1996-1998, the 3-year average of the 4th highest concentration for Enumclaw is 0.082 ppm, Pack Forest is 0.077 ppm and Lake Sammamish is 0.065 ppm.

1-Hour Standard: The highest 1-hour average for ozone (0.097 ppm) was in the "Moderate" category (PSI of 81) and occurred on Tuesday the 1st at the Pack Forest site in La Grande.

8-Hour Standard: The highest 8-hour average for ozone (0.086 ppm) was in the "Unhealthy for Sensitive Groups" category (proposed PSI of 104) and also occurred on Tuesday the 1st at the Pack Forest site.

• The highest PM10 value of 59 µg/m³ (PSI of 55) was measured on Tuesday the 8th at the 54th Ave. NE site in Northeast Tacoma. The highest PM2.5 value of 20 µg/m³ (proposed PSI of 54) was measured on Tuesday the 1st at the Kent and Issaquah sites and on the 21st and 23rd at the Duwamish site.
• The highest 8-hour average for carbon monoxide was 4.1 ppm (PSI of 46) and occurred the afternoon commute hours of Monday the 21st and 28th at the Everett site.
• The worst visual range of 12.3 miles occurred the afternoon of Thursday the 3rd during the hour from 2:00-3:00 pm PDT at the Kent site.

Puget Sound Air Pollution Control Agency