Obtaining AQI Data
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 there are two situations that complicate the Air Quality Index. First, the AQI regulation was not changed when the PM2.5 standard was updated, so the AQI 100 breakpoint for PM2.5 corresponds to the previous standard (40 ug/m3 and not 35 ug/m3). Second, 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. There is still no decision on how to properly implement this in our data systems and all SO2 AQI values are based on the 1-hour standard and are not defined above the "Unhealthy" category.
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.
Data for years 1999 - 2014 was last updated on April 13, 2015.