To calculate PM2.5 AQI, governments use a machine called a beta attenuation monitor that estimates the number of micrograms (the mass or weight) per cubic meter of air.
This calculation is for the US AQI scale, which many other countries use. However, different countries follow different scales. For instance, China uses its own AQI scale and India uses a scale it calls National Air Quality Index (NAQI).
To put it simple, PM2.5 is a definite value and AQI is a index calculated based on various standards.
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