## 22 October 2015

### A Walk In The Haze

Monday (19 October) was an evening to remember.

My friends and I attended a function in the western part of Singapore. When we entered the air-conditioned room, the latest (i.e., 7 pm) 24-hour PSI (west) was 125, the 3-hour PSI was 77 and the 1-hour PM2.5 (west) was 48 micrograms per cubic metre (equivalent to a 1-hour PSI of 91).

When the function ended two hours later, we found the air outdoors thick with haze.

We whipped out our cell phones to check the pollution numbers.

One person read with disbelief that the latest (i.e., 9 pm) 24-hour PSI (west) was 122, lower than what it was two hours ago.

Another person remarked that the pollution app on his phone showed that the PSI at 9:50 pm was only 96. He thought that the PSI reading was real time and location specific. Instead, what he was getting was the national 3-hour PSI reading published by National Environment Agency ("NEA") presented in a different format.

A third person noted that the 1-hour PM2.5 concentration (west) was 164 micrograms per cubic metre. However, she sighed that it was just a number to her and wondered what it was telling her about the intensity of the haze (it was equivalent to a 1-hour PSI of 214).

As we waited for our transport home, we noted that the 24-hour PSI (west) had risen to 136 at 10 pm, the 3-hour PSI to 152 and the 1-hour PM2.5 concentration (west) to 442 micrograms per cubic metre (equivalent to a 1-hour PSI of 461).

24-Hour PSI
The 24-hour PSI in each of the five regions is derived from averaging the pollution concentration over the preceding 24 hours. NEA updates the 24-hour PSI reading for each region every hour.

When the PM2.5 concentration is changing rapidly, it is clear that the 24-hour PSI is of no use.

3-Hour PSI
The 3-hour PSI is derived from averaging the pollution concentration (typically the PM2.5 concentration during the current hazy conditions) over the preceding 3 hours and the five regions. There is only one 3-hour PSI reading for the whole country at any one time, which NEA updates every hour.

When the PM2.5 concentration is changing rapidly, especially when it changes at different speeds in the five regions, it is clear the 3-hour PSI is of limited usefulness.

This is illustrated by the situation at 9 pm on 19 October when 1-hour PM2.5 concentration in the west was 164 micrograms per cubic metre (equivalent to a 1-hour PSI of 214) while the 5-region 3-hour average PM2.5 concentration was lagging far behind at 52 micrograms per cubic metre (equivalent to a 3-hour PSI of 96), as shown in the following table:

At 10 pm, the 1-hour PM2.5 concentration in the west was 442 micrograms per cubic metre (equivalent to a 1-hour PSI of 461) while the 5-region 3-hour average PM2.5 concentration was still lagging far behind at 105 micrograms per cubic metre (equivalent to a 3-hour PSI of 152), as shown in the following table:

At 11 pm, the 1-hour PM2.5 concentration in the west was 471 micrograms per cubic metre (equivalent to a 1-hour PSI of 480) while the 5-region, 3-hour average PM2.5 concentration was still lagging far behind at 159 micrograms per cubic metre (equivalent to a 3-hour PSI of 209), as shown in the following table:

Given their limited usefulness, why then does NEA publish the 3-hour PSI readings?

Presumably, the 3-hour PSI, being an average of 15 data points, is always lower than the highest data point, and is therefore not as alarming as a region-specific 1-hour PSI, which can be extremely high, as it was at 11 pm on 19 October.

1-Hour PSI
I explained n my recent articles NEA's Stubborn Refusal to Publish the 1-Hour PSI and The Value Of 1-Hour PM2.5 that inasmuch as NEA already provides the 3-hour PSI and the 1-hour PM2.5 concentration, neither of which comes with Ministry of Health's advisory, NEA should take the logical next step and provide the 1-hour PSI based on the 1-hour PM2.5 concentration so that people can more easily get a grasp of the intensity of the pollution.

Without the help of a 1-hour PSI (or an index by any other name, derived from PM2.5 concentration), many people struggle with making sense of PM2.5. To make matters worse, the relationship between 1-hour PM2.5 and 1-hour PSI is not continuously linear throughout the range from 0 to 500, as can be seen from the following table:
1-Hour PM2.5
μgrams per cubic metre
1-Hour PSI
12
50
55
100
150
200
250
300
350
400
500
500