Cumulus Support

I have also posted this on the wxforum, but lets see if anyone here has the answer...

My question is specifically about the UK and their COMEAP Air Quality Index (AQI) scale, but it probably applies more widely - though the answers may be different.

AQI values for the UK are a 24 hour measurement, and for any moment in time we can quote a rolling 24 hour number for the AQI.

But when the AQI is either forecast for future days, or recorded for past days what is the value that is quoted?

The highest rolling 24 hour AQI value of the day?
The mean of the rolling AQI values for the day?
The AQI for the 24 hours of that day - i.e. the value at 23:59?
The AQI at midday?
or...?

Should be...

mcrossley wrote: ↑Tue 08 Dec 2020 4:44 pm The AQI for the 24 hours of that day - i.e. the value at 23:59?

It is made very clear in
https://assets.publishing.service.gov.u ... _index.pdf

(page 17) that 24 hour mean at end of day is what should be reported in UK for past days. I have not seen any mention of the time at which a day ends, so it is reasonable to assume that past days are reported by calendar days. Again, the same reference does not say explicitly what is used for future forecasts, but it is reasonable to assume these are also calendar days.

In the late 1970s, when I worked in air pollution, we used filters for measuring particulate pollution, and the colour of the filter gave the reading (against a standard scale). Some sites, if my memory is correct, had mechanisms for controlling which set of filters was exposed, so the figures probably were by calendar day. Some relied on manual action to change the filter, so these were actually left alone overnight and the change time might be 9am or 10am, so the figures were not by calendar day. Obviously back then we could not produce real-time numbers!

[In case it is of interest to anybody, back then, for gaseous pollutants we reported the 98th percentile (selected to eliminate the maximum just in case it was an outlier, i.e. false) of all reported measurements arranged not by time of day but in ascending order in calendar day. This was new at the time, having previously reported the median, which also required sorting by magnitude.

I did not actually work in the team reporting daily values, my team reported monthly and annual values (except for Pb where in those days before lead-free petrol we reported weekly values); our computers back then were not powerful enough to sort all reported measurements for monthly or annual periods, so for these we had a finite number of ranges that measurements were assigned to, so the 98th percentile for these longer periods was an approximation]

Excellent, thank you.

mcrossley wrote: ↑Tue 08 Dec 2020 4:44 pm AQI values for the UK are a 24 hour measurement, and for any moment in time we can quote a rolling 24 hour number for the AQI.

But when the AQI is either forecast for future days, or recorded for past days what is the value that is quoted?

I am not sure it would be useful to use the 24hr average as a prediction.
Most predictions nowadays come from satellite and models and there is a huge science around it. So making even the suggestion that a 24hr av value or a derivative from there would be a prediction I think is not a correct signal.

I will give some links below for a view on the problem and techniques used

• https://www.hindawi.com/journals/amete/2019/1279565/
• https://www.sciencedirect.com/science/a ... 9719334813
• https://www.sciencedirect.com/science/a ... 0920307249
• https://aqicn.org/forecast/world/

And in addition to this I made a post On Air Quality which points to the sites where you can contribute your sensor data and which tries to show what can be done with the 'cheap sensors' in the context of prediction and analysis.

Anyway, single station values have no predictive value is my point. In that sense the 24 hr value is locally of little interest it is the 1 or 3 hr value which dampen the actual values a bit which help to evaluate when you got to run or stay and close windows and doors. I assume that answers your question a bit?

Well, yes, but the UK AQI scale like many of the others is based on a 24 hour average, so a 3 hour figure whilst of interest is of no value to compare against official figures. The premise being that exposure to short term pollution is less detrimental to health than the exposure over the longer 24 hour period. The paper referenced above recognises that using a 24 hour value has some short comings, like their example of the evening of 5th November, but that is what has been chosen as the standard so it is what I will use for my daily figures.

I do also graph the 3 hour value for what it is worth on my web site alongside the raw readings, so I can see both short term and 24 hour values.

mcrossley wrote: ↑Tue 08 Dec 2020 11:45 pm Well, yes, but the UK AQI scale like many of the others is based on a 24 hour average, so a 3 hour figure whilst of interest is of no value to compare against official figures. The premise being that exposure to short term pollution is less detrimental to health than the exposure over the longer 24 hour period. The paper referenced above recognises that using a 24 hour value has some short comings, like their example of the evening of 5th November, but that is what has been chosen as the standard so it is what I will use for my daily figures.

I do also graph the 3 hour value for what it is worth on my web site alongside the raw readings, so I can see both short term and 24 hour values.

True, but I was more on the predictive value of the 24 hr av.
No matter what the standard of the country is, the 24 hr av. does not contain predictive value.

Agreed. But I do not think I implied that did I? I just asked how they arrive at the AQI value for a day - and the predicted value for a day presumably must be the same calculation. How they arrive at predicted values I have no idea but obvious things like the expected weather conditions will be the major factor in high values (no wind, high pressure system, cold etc)

Well you started with

But when the AQI is either forecast for future days, ...

that looks pretty predictive to me.
But again, non-native English speaker etc...

Something I've pondered before, regards air quality graphs is the inclusion of some sort of wind information.

Here I know two basic sources of particle matter.
One to the West @ 5km, the other around North @ under 1km.

The sort of thing that would be useful would be a wind indicator on the top border of the graph....
Bit like this, which is also colour coded for speed.
Now that would make another great @Beteljuice Play Thing......

Cheers.

Just found the Legends for the above.