For the last 5 days, we had experienced in Singapore what seems like a Hollywood movie script. “Crisis” loomed, leaders (perceived to be) giving bad decisions, people panic into revealing or rise to their true-selves.
Confusion, anger and desperation, partly because information was in bits and pieces in National Environment Agency (NEA) site. NEA website has finally re-organised it’s information with better clarity, although I think laymen would probably not understand. I thought I would share some of my takeaways after researching on the topic of air quality within and outside of NEA website. Some facts may well be my wrong interpretation, but all these are what I have concluded after going through NEA site, EPA/Airnow site, various countries environment agency sites such as Hong Kong, United States, Australia, Germany and UK.
- PM is no Prime Minister. PM10 refers to particles of size less than 10 micron, which means it includes those that are smaller than 2.5 micron but bigger than 100 nanometer. Particulate matter is one of the pollutants used to measure air quality, and usually measured in terms of its concentration (microgram/cubic metre)
- PM10 versus PM2.5. Because PM10 include particles smaller than 2.5 micron, you would naturally expect concentration of PM10 to be higher than that of PM2.5. E.g. a PM10 concentration of 54 ug/m3 has the same air quality effect as a PM2.5 concentration of 12 μg/m3.
- PM10/PM2.5 concentration can be measured as 24 hour average, 3 hours average (in case of NEA for their 3 hours PSI computation), or hourly (used in Australia and some states in US).
- (NEA) PSI calculation method is similar to other air quality index used in other countries. Air quality index (or pollution standard index) looks at 5 pollutants (particulate matter, sulphur dioxide, carbon monoxide, ozone and nitrogen dioxide). The so called better AQI used in some countries, is because they look at PM2.5 concentration whereaas NEA looks at PM10. Each pollutant measure is sub-indexed based on a table of scale that corresponds to the concentration. For e.g. sub-index 50 is equivalent to 24 hour particulate matter concentration of 50 ug/m3, or 8 hour Ozone of 118 ug/m3. The PSI (or AQI) is then determined by taking the maximum sub-index of the 5 pollutants. So if PM10 index is 50, but Carbon monoxide index is 200, the air quality is indexed as 200.
- PSI is AQI, no? While NEA uses PM10 instead of PM2.5 to calculate the PSI (or AQI), it does report 24 hours PM2.5 separately, used to be every 4 hours but recently reported hourly due to the escalating haze. The PSI/AQI is therefore inaccurate, if every pollutant has low concentration, but the high concentration of particulate matter is mostly due to 2.5 micron or smaller particles. In recent haze incidents, this is not true though, mainly because the concentration of PM2.5 is exceptionally high, so even if PM2.5 pollutant is not used to measure air quality, it’s concentration is already reflected in PM10 measurement.
- PSI is not accurate? When 3-hour PSI is low, but PM2.5 concentration shows exceptionally high, that’s most probably because the latter is measured past 24 hour (in the case in Singapore). Only PM10 is measured on 3 hourly basis. Whether it’s 24 hour, 3 hour or 1 hour average, it shows AVERAGE measurement, and it does not correspond to what you see/smell NOW. Obviously, an 1 hourly average measurement will be closer to what you see, while a 24 hourly average will prevent unnecessary panic alarm (or false sense of comfort) due to a particular spike / drop. Therefore it is important to know the duration in which the pollutant or air quality is being measured, and used it accordingly.
- Hourly or 24 hourly average is the way going forward? When the air quality is already at hazardous level, a forecast measurement would be more helpful than an hourly or 3 hour reading, because one should already be taking the necessary precautionary measure against the haze. (This is just my take)
- Hourly or Real time AQI? Not all countries are using true hourly air quality index measurement. In Beijing and Hong Kong, the hourly report pollutants based on the same definition as Singapore. e.g. it uses 24 hour PM10 concentration at 1500 hrs for a 1500 hr AQI calculation. In US, not all states have 1 hour average PM2.5 / PM10 concentration reading (e.g. Colorado state), let alone 1 hour AQI. In Germany and UK, there’s no reports of 1 hour average measurement. In Australia, it does look like all states have 1 hour moving average readings (at least 2 out of 2 states I checked have 1 hour average readings). Last but not least, NO countries provide real time measurement of air quality index, or the associated pollutants.
I do stand corrected for the above interpretation, but the purpose of the writing is because I read too many postings and accusation that’s based on seriously wrong assumption or understanding. I hope this doesn’t create more confusion than it is already in the cyberspace. If I have more time, I will reference some of the sites to the above points so that you can read for yourself if I have interpreted wrongly or not.
London AQI definition http://uk-air.defra.gov.uk/latest/currentlevels