How is attainment of the PM2.5 National Ambient Air Quality Standard (NAAQS) determined using monitoring data?
The federal Clean Air Act (CAA) established timeframes and milestones for states to meet and maintain National Ambient Air Quality Standards (NAAQS) for criteria pollutants. US EPA sets the NAAQS levels to protect public health and the environment. Each state, including Massachusetts, is required to monitor the ambient air to determine whether it meets each standard. Each monitoring site with Federal Reference Method (FRM) monitors is required to achieve 75% or greater data capture for the year in order for the data from the monitoring site to be considered valid in the attainment demonstration. The annual standard is met when the annual average of the quarterly mean PM2.5 concentrations is less than or equal to 15 g/m3 (3-year average). If spatial averaging is used, the annual average from all monitors within the county may be averaged in the calculation of the 3-year mean. The 24-hour standard is met when the 98th percentile value is less than or equal to 35 g/m3 (3-year average). If
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