Please note that this article was orginally written in June 2013.
In April 2014, the Singapore NEA has updated the PSI calculation to also include PM_2.5. http://www.nea.gov.sg/anti-pollution-radiation-protection/air-pollution-control/psi

With the recent Southeast Asia Smog from Indonesia affecting Singapore and now Malaysia, we got many questions about why there is a difference between the data which can be read from Singapore NEA website (nea.gov.sg) and the World Air Quality Index project Singapore webpage.

For instance, here is what can be read today in NEA website:

For some historical reason, Singapore is using the PSI (Pollutant Standard Index) to evaluate the Air Quality. In the image above, number 1 corresponds to the PM10 value that is used for the PSI evalation. The value 67(/59) can be read as 67 μg/m3, corresponding to a PSI of 59. The PSI is evaluated as the maximum of the individual PSI for each of the pollutants: PM₁₀, SO₂, NO₂, O₃ (Ozone) and CO₂.

PSI(Singapore-North) = max( PSIPM10-based, ... PSIO3-based )= max (59, ..., 77) = 77

This said, the interesting thing is that, in this same table, the PM2.5 data is also provided in the last column (see 3). This data is provided in μg/m3 only (see 2), and there is no conversion to something like the PSI (i.e. a conversion from the PM2.5 mass to a Pollution or Quality Index). This conversion however exists, and is defined by the US Environmental Protection Agency. The easiest way to do the conversion is to use the online calculator, available at airnow.gov:

If you select the PM2.5 (1), then enter the mass concentration of 48 (2), can click on Calculate, you will obtain the AQI of 132 (3). So, based on the PM2.5 AQI conversion, the PSI that is used for Singapore could be extended (let's call it PSI++) to also take into account the PM2.5 information. In which case, the PSI++ would be the maximum of the regular PSI (based on PM10 only) and the PM2.5 AQI:

PSI++ = max( PSI, AQIPM.25 ) = max( 77, 132 ) = 132
This PSI++, that is commonly referred as AQI (or Air Quality Index), is what is being used on the the World Air Quality Index project, for all the cities (provided PM2.5 is available for the city). And this explains why the values are different between the NEA website and the World Air Quality Index project.

Note: With the PSI update from April 2014, the only remaining difference is that the World Air Quality Index project is using a 1-hour PM_2.5 reading (following the instantcast) while Singpore PSI is based on the 3 hours average.

Moreover, when doing the convertion, make sure you use the 1-hour reading for the PM2.5 concentration rather than the 24-hours averaged value, as shown on the below image:


http://www.haze.gov.sg/haze-updates/pollutant-concentrations/type/PM25-1Hr

If you want to know more about PM10 vs PM2.5, and especially why PM10 is still used, please check the faq entry about why is PM2.5 often higher than PM10? Is PM10 still a relevant measure?


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