The air pollution problem in China has bothered Chinese residents for decades. More and more people realize that air pollution affects their health seriously. A good air pollution monitoring system can help people reduce or avoid harm from pollutants. Chinese officials apply Air Pollution Index (API) as their monitoring system. In the U.S., EPA uses Air Quality Index (AQI). What’s the difference between API and AQI? What factors make great gaps when API value is relatively low(the lower the better for both API and AQI) but AQI value is high in the same area?( U.S. readings challenge China’s smog claims,Josh Levs, CNN, 2011)
Calculation. Both API and AQI share the same formula. So it is not the factor that makes difference between API and AQI.
AQI=(AQI,h-AQI,l)*(Concentration-BP,l)/(BP,h-BP,l)+AQI,l
BP,h and BP,l are concentration breakpoints, equal to AQI,h and AQI,l values.;
Concentration is the monitoring reading of a pollutant.
Breakpoints. There is slightly different between API and AQI when choosing breakpoints. For example, for category Unhealthy for Sensitive Groups, the AQI breakpoints of ozone ( 1-hour) are 0.125 and 0.164 ppm, which equal to AQI value 101 and 150 separately. But for API, the breakpoints are 0.102 and 0.164 ppm.Chart 1 shows the slight difference.
Chart 1
Chart 2 shows the tiny difference of PM10 (24-hour) between API and AQI.
Chart 2
Therefore, breakpoints choosing is not the factor that makes API and AQI value different dramatically.
Monitoring Method. For some pollutants, China and U.S. apply different monitoring methods. For example, U.S. monitors running 8 hour ozone reading from its hourly concentration. China only read daily average from hourly ozone concentration. Chart 3 indicates the great difference.
Chart 3
From this chart, AQI value(O3, 8-hour) rises dramatically. When ozone concentration is 0.1 ppm, API value (O3, 1-hour) stays under 100 which indicates moderate category, whereas AQI value (O3, 8-hour) goes above 150 which means unhealthy. Therefore, monitoring method makes great gap between API and AQI values.
PM2.5. Particle matter 2.5(PM2.5), which means particulates less than 2.5 micrometers in diameter, leads to respiratory diseases. It is a very important pollutant monitored by EPA in the U.S. Trace concentration of PM2.5 contributes to high AQI value. In China, PM2.5 concentration in some cities are relatively high. Unfortunately, China excludes the item out of the daily API report.
Daily API or AQI value is the maximum reading of all pollutants’ API or AQI values. For instance, if AQI(NO2)= 101, AQI(PM2.5)=120 and AQI(O3)=85, AQI daily is reported as 120.
Therefore, API daily value in China is lower than AQI daily reported by U.S. embassy also because of the absence of PM2.5.
Reference:
http://www.cnn.com/2011/11/01/world/asia/china-air/index.html?hpt=wo_bn4
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