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Revealing the hidden story behind air quality measurements of fine particulate matter (PM2.5)

Looking at the Breathe London map, you can see levels of two types of air pollutant: nitrogen dioxide (NO2), a gas, and fine particulate matter (PM2.5), small polluting particles. In this article, we look at PM 2.5 in particular, and ask the question: how much can our data tell us about where particulate pollution is coming from? Quite a bit, it turns out…

Comparing local vs non-local sources of pollution

Our team took a closer look at measurements of PM2.5 (tiny airborne particles that can harm our health) coming through our network of sensors. We monitored these fine particles at one-minute intervals across the whole Breathe London (BL) network, but in this example, we zoom in on just two locations: BL1, a 'roadside' site, and BL2, a 'background' site. BL1 is situated near a road junction where two A-roads meet. It is classed as a roadside site in the BL network. In contrast, BL2 is located in a green area away from major roads, and is classified as a background site within the same network.

We looked at these two sites over a two-week period from 5 to 19 June this year to see what they could tell us about where particulate matter in London is coming from – and this is what we found…

Usually, air quality is summarised as hourly averages, which is what we see from our sensors in Figure 1. This gives a useful overview of overall exposure to particulate pollution during the period, which can help us to make informed decisions about our health and how to tackle this problem. But is this the full picture? Not quite.

Figure 1. Time series of hourly PM2.5 from two BL locations (roadside and urban background) for an approximate two-week period (5 to 19 June 2025). This is derived from 1-minute data shown in Figure 2.

Figure 2. Time series of the 1-minute PM2.5 from the same two BL locations (roadside and urban background) for an approximate two-week period (5 to 19 June 2025). Note the y-axis range is 3 times the range shown in Figure 1 and 3.

By making use of the high time-resolution data that our network of sensors provides (BL one-minute readings, Figure 2), we can dig a lot deeper. Specifically, we can separate the sources of the particulate pollution we're seeing into two components:

• Local: pollution originating from nearby sources,
• Non-local: pollution transported in from farther away.

This process, known as scale separation, allows us to tease apart these different contributions and better understand where the pollution our sensors detect is coming from.

The technical bit: How can we tell the local from non-local through scale separation?

There are several ways to find the non-local pollution, but here it is achieved by using the 1 minute measurements to look for the periods in every hour which are not influenced by local pollution – these are the lowest values. We know these low values correspond to the non-local sources, because all the BL sites see the same features at any one time. The local components are then the amounts of pollution above the non-local part, and these of course can vary by site.

Figure 3. Time series of hourly non-local PM2.5 from two BL locations (roadside and urban background) for an approximate two-week period (5 to 19 June 2025).

Figure 4. Time series of hourly local PM2.5 from two BL locations (roadside and urban background) for an approximate two-week period (5 to 19 June 2025). Note the y-axis range is half the range in Figure 3.

In Figure 3, we show just the non-local PM2.5 component at both locations. Interestingly, the values are very similar across the two sites, confirming that they are both affected in similar ways by pollution from regional or distant sources (the average non-local PM2.5 over the period was approximately 6.4 µg/m³ at the two locations).

However, the story changes when we look at Figure 4, which focuses on the local PM2.5 contribution (this is the difference between the signals shown in Figure 1 and Figure 3 for each location). Here, we see a striking contrast:

• The background site shows low hourly values, generally under 4 µg/m³, with an average local value over the period of 1.1 µg/m³.
• The roadside site, on the other hand, sometimes records local spikes of up to 6-10 µg/m³ and the corresponding local average PM2.5 over the period shown is 1.6 µg/m³.

What do the findings tell us about London’s PM problem?

The above may sound complex, but the story it tells us is quite simple.

The differences between the ‘roadside’ and ‘background' sites, shown in Figure 4, highlight how much sources local to that sensor, such as road traffic, heating, or nearby industry, can influence what we breathe -even within relatively short distances.

However, the similarities in the non-local PM2.5 shown in Figure 3 indicates that sources of particulate matter from further afield are also contributing to London’s pollution problem. We know that PM can travel great distances, from other parts of the city, country, and can even travel over from Europe. Our data suggests that this non-local pollution is impacting the quality of the air we breathe in our city in a significant way – only 15-20% of PM2.5 at these sites comes from local sources!

What does this mean for tackling air pollution?

While traditional air quality reports give us valuable summaries, there’s a richer, more detailed picture hidden in the high-resolution data that our Breathe London network provides. With the right tools and analysis, we can uncover insights like this and better understand how to protect our health.

When it comes to particulate matter, this shows us that tackling the problem relies on the efforts of people at all levels. To address local particulate pollution, like we saw at the roadside site, it’s important to employ a range of strategies from reducing our own individual emissions, working together to implement measures to address air pollution in our community, and calling on our local leaders to act. But this also emphasises the need to take our efforts even further, holding national and even international leaders to account for cleaning the air we breathe and helping to reduce this non-local, transboundary pollution.

To protect the health of our communities, action is needed at home and further afield, and decision-makers at all levels must work together to make our air cleaner and safer to breathe.

Interested in doing more to help clean up London’s air? Explore our resource hub, where you can find materials designed to support you to take air quality action.