We got a very good and relevant question this week from Severine P., who asked about pm2.5 concentrations versus pm10 concentrations. What Severine asked in her mail was:

I don't understand why pm2.5 concentrations are often higher than pm10 concentrations.
Aren't PM smaller than 2.5 micro included in PM smaller than 10 micro?
Thank you for your answer

Severine's question is totally correct: PM2.5 are, by definition, particulates smaller than 2.5 micrometers, so indeed, they should also be included as smaller than 10 micrometers.

But the assumption about concentration is not correct: PM2.5 concentration can be smaller than PM10, but yet, the converted AQI value can be higher. For instance, an AQI of 50 for PM2.5 corresponds to 15.5 mg/m³, while it corresponds to 55 mg/m³ for PM10.

So, even if the full PM2.5 15.5 mg/m³ are added to the PM10 concentration, the PM10 AQI still remains much lower than the PM2.5 AQI, For instance, earlier this week, the Dong Cheng Dong Si (东城东四) PM10 concentration was 366, resulting in 216 AQI, while the Dong Cheng Dong Si (东城东四) PM2.5 was 348, resulting in 398 AQI.

In order to double confirm that our understanding is correct, we did contact the world renown Air Quality expert Dr Sarath Guttikunda from urbanemissions.info, who wrote us back:

You are right on these. The differences are due to the differences in the break points for PM2.5 and PM10 and how the epidemiological data is correlated to each of the fractions.

So, the next questions is about the relevance of the PM10, especially in China. It is totally right that most of the time (empirically confirmed), PM2.5 is the dominant value in the AQI. So, do we still need the PM10 measurement? Is it still relevant? And which conditions are reflected when PM10 AQI is higher than the PM2.5?

We asked the question again to Dr Sarath Guttikunda, who replied:

There are now new studies presenting evidence that PM2.5 is more harmful than PM10. Physically, this makes sense - smaller than particle, more probability that it will go deeper into the lungs and harm us. This is also one of the important reasons for WHO to push for all countries to have standards for PM2.5.

To the question, do we still need to have PM10 measurement, when we are doing PM2.5 - yes we still do. While the fraction of PM2.5 is higher in the PM10 fraction, for the most cities with pollution from transport and other combustion sources, an often neglected non-combustion source is dust re-suspension (from road dust and dust storms), which forms part of the coarser fraction (PM2.5 to PM10). If we suddenly stop measuring PM10, we will be neglecting this source.

Most cities still measure only PM10 - in China and in India. So, another reason we cannot remove it from the equation.

Many thanks, Dr Sarath, for the quick and professional answers.

For more information about the conversion from concentration to AQI, you can refer to the AQI calculator as well as to this AQI scale explanation.