| Measurement | Region | Annual Average, 1989-1991 | Annual Average, 2014-2016 | Percent Change | Number of Sites | Statistical Significance |
|---|---|---|---|---|---|---|
| Ambient particulate sulfate concentration (µg/m³) | Mid-Atlantic | 6.3 | 1.6 | -75 | 12 | *** |
| Midwest | 5.8 | 1.7 | -71 | 9 | *** | |
| Northeast | 3.4 | 0.9 | -74 | 4 | ||
| Southeast | 5.5 | 1.5 | -73 | 8 | *** | |
| Ambient sulfur dioxide concentration (µg/m³) | Mid-Atlantic | 13.0 | 2.0 | -85 | 12 | *** |
| Midwest | 11.0 | 1.5 | -86 | 9 | *** | |
| Northeast | 5.2 | 0.7 | -87 | 4 | ||
| Southeast | 5.1 | 0.6 | -88 | 8 | *** | |
| Ambient total nitrate concentration (µg/m³) | Mid-Atlantic | 3.3 | 1.5 | -55 | 12 | *** |
| Midwest | 4.6 | 2.4 | -48 | 9 | *** | |
| Northeast | 1.7 | 0.7 | -59 | 4 | ||
| Southeast | 2.2 | 1.1 | -50 | 8 | *** |