How is the maximum daily and annual average PM2.5 concentration calculated?
The maximum PM2.5 daily concentration is determined by retaining the maximum concentration reported for each monitoring site for each monitored day (i.e. if valid 24-hour averages are available for at least 75% of possible hours in a day). For the annual PM2.5 concentration, a complete (valid) quarterly average must have at least 11 observations (11 valid 24-hour values). For each valid quarter, a quarterly average is calculated. The annual average for each monitor is the average of four valid quarterly averages. The PM2.5 annual average measure is the maximum annual average among monitors with complete (4 valid quarters) data in that county. The values that exceed the NAAQS (i.e., 15.5 g/m3 and higher for annual or 35.5 g/m3 and higher for daily) are considered above the NAAQS.
