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: PM_2.5 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: PM_2.5 concentration can be smaller than PM₁₀, but yet, the converted AQI value can be higher. For instance, an AQI of 50 for PM_2.5 corresponds to 15.5 mg/m³, while it corresponds to 55 mg/m³ for PM₁₀.
So, even if the full PM_2.5 15.5 mg/m³ are added to the PM₁₀ concentration, the PM₁₀ AQI still remains much lower than the PM_2.5 AQI, For instance, earlier this week, the 东城东四 PM₁₀ concentration was 366, resulting in 216 AQI, while the 东城东四 PM_2.5 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 PM_2.5 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), PM_2.5 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 PM_2.5? We asked the question again to Dr Sarath Guttikunda, who replied:

There are now new studies presenting evidence that PM_2.5 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 PM_2.5.

To the question, do we still need to have PM₁₀ measurement, when we are doing PM_2.5 - yes we still do. While the fraction of PM_2.5 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 (PM_2.5 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.

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