Skip Maine state header navigation

Agencies | Online Services | Help

Skip First Level Navigation | Skip All Navigation

Air Home > Meteorology > Air Quality Trends

Particle Pollution Historical Trend

 

The design value is an important number because this is what EPA and DEP use to determine whether an area is attaining (meeting) the National Ambient Air Quality Standard (NAAQS) for Particle Pollution. Particle pollution has both a short-term and an annual standard. (A detailed technical description of design value can be found below the chart.) On these pages the design value is named for the final year in the 3 year averaging period.

Historically, Maine has not exceeded either Particle Pollution NAAQS. The monitors used to determine whether or not the NAAQS has been met are Federal Reference Method (FRM) monitors which operate on either a one in three day or a one in six day cycle.

There are two design values for particle pollution.

  • The (short-term) 24-hr design value is the three year average of the 98th percentile 24-hr concentration from each year. The yearly 98th percentile 24-hr concentration is determined by taking all of the monitored 24-hr concentrations for the year, ordering them from greatest to least, determining the 98th percentile position (the position equal to or greater than 98% of the data) and using the particle pollution 24-hr concentration at that position for that year. For example: if 50-99 valid 24-hr concentrations exist during the calendar year the 2nd highest concentration is the 98th percentile concentration and if 100-149 valid 24-hr concentrations exist during the calendar year the 3rd highest value is the 98th percentile 24-hr concentration.
  • The annual design value is the 3 year average of the annual average. The annual average is determined by first calculating the average 24-hr concentration for each quarter of the year and then calculating the average of the 4 quarters.

The design value for a site can be determined (is valid) if the monitor meets data completeness requirements. The site must have valid data for at least 75% of the scheduled sampling days for each quarter unless the quarterly average or the 98th percentile 24-hr concentration exceeds the NAAQS.

In the past we've displayed the maximum design value for the state for both the short-term and the annual standards. A key monitoring site in Portland was shutdown January 1, 2007 with the new monitoring site not approved and operational until February 2008. Therefore, in order to ensure that the data is interpreted correctly we now present the data by region for both the short-term and annual design values.