The Ambient Monitoring Section began operating a network of PM-2.5 monitors in 1999. Designated monitors for which PM-2.5 concentrations exceed 15.0 µg/m³ (micrograms per cubic meter) annual arithmetic mean averaged over 3 consecutive years are likely to be deemed "not attaining" the "annual" standard. Those for which the annual 98th percentile concentrations exceed 35 µg/m³ averaged over 3 consecutive years are likely to be deemed "not attaining" the "daily" standard.

Maps of county weighted annual arithmetic means and annual 98th percentiles from this network have been drawn for successive 3-year periods. County statistics are displayed on each map and may also be found in the corresponding tables.

Certain data documented as having been affected by forest fires have been excluded from our averages. However, we anticipate that the US EPA will publish averages in which these data are included.

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