By Ji Liu
China is notorious for its heavily polluted air. Different statistics are named to represent the air quality to draw the public attention for their outdoor activities. One of the most famous one is AQI, air quality index, which translates the density of harmful air pollutant jargons into some readable ranges.
There are mainly two resources to acquire the data of AQI, aqicn.org as well as the China Mission AQI (a.k.a the Beijing, Shanghai … embassy index). So far, both of these two index are using instant index to show public.
There’s a new way to calculate the air quality, named as “nowcast”. The concept behind the nowcast is to compensate the “24 hours averaging“, which should be used when converting concentrations to AQI. The reason for this averaging is that the AQI scale specifies that each of the Levels of Health Concern (i.e. Good, Moderate,… Unhealthy…) is valid under a 24 hours exposure. For example, when seeing a 188 AQI (Unhealthy), one need to read it as “if I stay out for 24 hours, and the AQI is 188 during those 24 hours, then the health effect is Unhealthy“. This is quite different from saying that “if the AQI reported now is 188, then the health effect is Unhealthy“.
Does this mean to use average is a better idea? Certainly not.
- First, the dynamic of Air Pollution is such that wind than completely clean the air in less than 30 minutes! This phenomenon is frequently seen in Beijing with the strong north winds able to bring the PM2.5 AQI from more than 300 to less than 50 in less than one hour. When this happens, no one wants to wait for 24 hours before knowing that the Air Quality is good (and go out for a walk to enjoy the fresh air!)
- The second reason is when the Air Quality suddenly gets worse. One famous case is the Indonesian wildfire causing the Singapore Smog when the winds are heading to the north, under which circumstances the AQI can go from below 50 to more than 150 is just one hour. As a matter of fact, we have had many requests from asthmatic/sensitive people when Singapore was still only providing 24 hours average readings.
Rather than simply using average, the scientist brings in weight according to time sequence, which well avoid the problem of wrong statics usage. The NowCast is computed from the most recent 12 hours of PM monitoring data, but the NowCast weights the most recent hours of data more heavily than an ordinary 12-hour average when pollutant levels are changing. The NowCast is used in lieu of a 24-hour average PM concentration in the calculation of the AQI until an entire calendar day of hourly concentrations has been monitored.