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This page contains county level Air Quality Index (AQI) data that can be downloaded. Each file is column delimited and contains data for one calendar year. The first record contains the file format giving the field name, the start column and the field length. There is one record in the file for each day data exists for each county. The AQI value and defining pollutant are given as well as information about other non-defining measurements taken in the county for that day. To calculate the AQI value for a county, the AQI value for each monitor collecting sufficient data in the county for that day is examined and the highest reported AQI value becomes the "defining" AQI value for the county.
It should be noted that the SO2 AQI is a hybrid of two durations where the AQI values up through the "Unhealthy" category are based on the 1-hour values and the values above that are based on the 24-hour values.
Also note that PM2.5 data and AQI did not exist before 1998, so it could not factor into the defining AQI value for prior years.
Finally, the new ozone standard (from October 2015) has not been implemented in AQS yet, so the AQI reflects the 2008 standard.
Data was last updated on October 22, 2015.