How is the number of percent of population and person-days calculated?
The air quality measures takes into account both the number of days that the maximum daily PM2.5 concentrations exceed the NAAQS and the population within the county where monitors are located. The maximum daily and annual PM2.5 concentrations are calculated based on the methodology presented above. The number of persons in each county is derived from the Census Bureau population estimates. The Census Bureau typically updates the county-level population estimates for the years between censuses on an annual basis. The census county-level population is adjusted annually by considering births, deaths, immigration and other information sources. Documentation on the methodology used by the Census Bureau in estimating county-level can be found at http://www.census.gov/popest/topics/methodology/2009-st-co-meth.pdf.
