Effect of PM2.5 on AQI in Taiwan

US EPA has included sub-indices of 8-hour average O3 concentration and 24-hour average PM2.5 level into a newly-modified Air Quality Index (AQI). Calculations at air-quality monitoring stations in Taiwan show that the updated AQI system is associated with two to three times higher occurring frequency of days with AQI larger than 100 than that of the widely used Pollution Standard Index (PSI) larger than 100. On these unhealthy days, more than 50% are dominated by PM2.5 sub-index, followed by 8-hour average O3 sub-indices. By analyzing the PM2.5 data measured at four different stations, we note that PM2.5 tends to occupy more than 50% of the PM10 level. These results indicate that the importance of fine particles in urban smog has been magnified in the AQI system. With a new category between 101–150, AQI does pose as a better descriptor than PSI to warn of an unhealthy environment for the sensitive group.