How can cities detect, and avoid, peaks in particulate-matter air pollution?

Pollution over Lyon. Image: Getty.

In January 2017, France and a large part of Europe were struck by episodes of particulate matter pollution. These microscopic particles are known as PM2.5 and PM10 when they measure less than 2.5 or 10 micrometers (µm) in diameter respectively. The Conversation

They are proven to be harmful to human health because they enter our respiratory system, and the smallest can even enter our blood flow. According to the European Environment Agency, air pollution is the cause of 467,000 premature deaths annually in Europe.

These particles can come from natural sources (sea salt, volcanic eruptions, forest fires, etc.) or human activities (transport, heating, industry, etc.)

What is a pollution peak?

Pollution peaks occur when regulatory warning thresholds, as defined in 2008 by the European Union and transposed to French law in late 2010, are exceeded. In virtue of these regulations, the first level of severity (known as the “public information and warning threshold”) is reached for PM10 particles when there are ≥50 µg per cubic meter of air (m³) in the atmosphere; the warning level is reached at ≥80 µg/m³.

There is no trigger limit for PM2.5, but just a set maximum amount of 25 µg/m³ on average per year.

However, these regulations have serious limitations. The “mass” concentration thresholds which indicate the total mass of particles in the air and which are used to assess the danger of particulate matter pollution are higher than the levels recommended by the WHO; the latter have been set for PM10 at 20 µg/m³ on average per year and 50 µg/m³ on average per day, in order to take account of chronic and short-term exposure.

In addition, the only parameter taken into account in European and French regulations concerns mass concentration. The concentration in terms of number (i.e. the number of particles per m³ of air), and the chemical composition are not taken into account for the triggering of warnings.

Lastly, there are no regulations for very small particulate matter (less than 1 µm), which is mainly produced by human activity, even though it is potentially the most harmful.

Comparison of the size of microscopic particles with a hair and grain of sand. Image: EPA/creative commons.

How are they detected?

In France, the Ministry for the Environment has delegated the task of monitoring air quality and regulated pollutants across the country to certified associations united under Fédération Atmo France. They are supported in this task by the Central Laboratory for the Monitoring of Air Quality.

These associations put in place automatic measurements for the concentration of pollutants, as well as other monitoring measures to allow a better understanding of the phenomena observed, such as the chemical composition of particles, or weather conditions.

These measurements can be combined with approaches for modeling particle concentration, thanks in particular to Prevair, the French forecast platform. Calculating the history of air mass can also be used to reveal the origin of particles, and it is therefore now possible to describe the phenomena at the origin of the increase in concentrations in relative detail.

Explanation of a real case

The graph below, produced from observations by our research department and measurements by Atmo Hauts-de-France, illustrates an example of pollution peaks that affected the local area in January 2017.

During this period, anticyclonic weather conditions contributed to the stagnation of air masses above pollutant-emitting areas. In addition, cooler temperatures led to an increase in emissions (notably linked to domestic wood heating) and the formation of “secondary” particles which formed after chemical reactions in the atmosphere.

Image: Data V. Riffault/SAGE (Cappa and Climibio projects)/creative commons.

The graphs show changes in mass concentrations of PM10 and PM2.5 over a period of several days at the Lille Fives monitoring station, as well as changes in several chemical species measured in PM1 4 km away on the University of Lille campus.

We can see that almost all the particles fell within the PM2.5 proportion, something which rules out natural phenomena such as a dust being blown in from deserts, since such particles mainly fall within the range of 2.5 to 10 µm. Furthermore, the particles in question are generally smaller in size than 1 µm.

The pollution episode began on the evening of 21 January  and continued throughout the weekend, in spite of a lower level of road traffic. This can be explained by an increase in wood burning (as suggested by the m/z 60 tracer, which is a fragment of levoglucosan, a molecule emitted by pyrolysis of cellulose found in wood).

Wood burning and other forms of combustion (such as traffic or certain industries) also emit nitrogen dioxide (NO2) as a gas, which can turn into nitric acid (HNO3) through a reaction with hydroxyl radicals (•OH) in the atmosphere.

At sufficiently low temperatures, HNO3 combines with ammonia (NH3) produced by farming activity to form ammonium nitrate (NH4NO3) solid. These are known as “secondary particles”.

A slight decrease in concentrations of particulate matter was observed at the end of the weekend, with more favorable weather conditions for the dispersion and elimination of pollutants.

In this episode, the very low concentrations of sulfates rule out an impact from coal power stations in Germany and Eastern Europe. It is therefore definitely a question of local and regional pollution linked to human activity and which accumulated as a result of unfavorable weather conditions.

How can this be avoided?

Since we cannot control the weather conditions, levers of action are primarily based on reducing pollutant emissions.

For example, reducing the formation of secondary particles will entail limiting NO2emissions linked to road traffic through road space rationing measures; for NH3 emissions, action must be taken regarding farming practices (spreading and rearing methods).

Concerning emissions from wood heating, replacing older devices with cleaner ones will enable better burning and fewer particulate matter emissions; this could be accompanied by an investment in housing insulation.

But these measures should not make us forget populations’ chronic exposure to concentrations of particulate matter which exceed the recommended WHO thresholds. This type of pollution is insidious and is damaging to health in the medium and long term, notably with the development of cardio-vascular and respiratory diseases and lung cancer.

Véronique Riffault is professor of atmospheric science at IMT Lille Douai – Institut Mines-Télécom.

This article was originally published on The Conversation. Read the original article.


Why doesn’t London build an RER network, like Paris did?

A commuter walking by a map of the RER B line at the Chatelet-Les Halles station in Paris. Image: Getty.

I’ve heard many people make many different complaints about the Parisian transport system. That it does a bad job of linking a rich, white city with its poorer, more diverse suburbs. That, even as subway systems go, it’s a hostile environment for women. That the whole thing smells distractingly of urine.

I’m familiar with all of these complaints – I’ve often smelt the urine. And I’m aware that, in many ways, London’s is the superior transport network.

And yet I can’t help be jealous of Paris – In large part, because of the RER.

Central Paris. The Metro lines are thinner, and in pastel shades; the RER lines are thicker, and in brighter colours. Image: RATP.

Paris, you see, has not one but two underground railway systems. The more famous one is the original Paris Metro, opened in 1900: that’s the one with those fancy green portals with the word “metropolitain” written above them in a vaguely kooky font.

The Metro, though, mostly serves Paris Intra-muros: the official city, inside the Boulevard Périphérique ring road, site of the city’s last set of walls. As a result, it’s of very little use in most of the city’s suburbs. Its stations are very close together, which places a limit on how fast its trains can cross town. It was also, by the mid 20th century, becoming annoyingly overcrowded.

So starting in the 1960s, the city transport authorities began planning a second underground railway network. The Réseau Express Régional – Regional Express Network – would link suburban lines on either side of Paris, through new heavy rail tunnels beneath the city. Its stations would be much further apart than those of the metro – roughly one every 3km, rather than every 600m – so its trains can run faster.

And fifty years and five lines later, it means that 224 stations in the suburbs of Paris are served by trains which, rather than terminating on the edge of the city, now continue directly through tunnels to its centre.

The RER network today. Image: RATP.

London is, belatedly, doing something similar. The Elizabeth Line, due to open in stages from later this year, will offer express-tube style services linking the suburban lines which run west from Paddington to those which run east from Liverpool Street. And Thameslink has offered cross-town services for 30 years now (albeit not at tube-level frequencies). That, too, is going to add more routes to its network over the next few years, meaning direct trains from the southern suburbs to north London and vice versa.

Yet the vast majority of suburban National Rail services in London still terminate at big mainline stations, most of which are on the edge of the centre. For many journeys, especially from the south of the city, you still need to change to the London Underground.

So, could London ape Paris – and make Thameslink and Crossrail the first element of its own RER network?

In a limited way, of course, it’s doing just that. The next big project after Crossrail is likely to be (original name, this) Crossrail 2. If that gets funding, it’ll be a new south-west to north-east route, connecting some of the suburban lines into Waterloo to those in the Lea Valley.

The proposed route of Crossrail 2. Click to expand.

But it’s not immediately obvious where you could go next – what Crossails 3, 4 or 5 should cover.

That’s because there’s an imbalance in the distribution of the remaining mainline rail services in London. Anyone who’s even remotely familiar with the geography of the city will know that there are far more tube lines to its north. But the corollary of that is that there are far more mainlines to the south.

To usefully absorb some of those, Crossrail 3 would probably need to run south to south in some way. There is actually an obvious way of doing this: build a new tunnel from roughly Battersea to roughly Bermondsey, and take over the Richmond lines in the west and North Kent lines in the east, as a sort of London equivalent of RER C:

Our suggestion for Crossrail 3. Image: Google Maps/CityMetric.

But that still leaves a whole load of lines in south and south east London with nowhere to send them beyond their current terminal stations.

In fact, there are reasons for thinking that the whole RER concept doesn’t really fit the British capital. It was designed, remember, for a city in which the Metro only served the centre (roughly equivalent of London’s zones 1 & 2).

But London Underground wasn’t like that. From very early in its history, it served outer London too: it was not just a way of getting people around the centre, but for getting them there from their suburban homes too.

This is turn is at least in part a function of the economic geography of the two cities. Rich Parisians have generally wanted to live in the centre, pushing poorer people out to the banlieues. In London, though, the suburbs were where the good life was to be found.

To that end, the original operators of some lines weren’t just railway companies, but housing developers, too. The Metropolitan Railway effectively built large chunks of north west London (“Metroland”), partly to guarantee the market for its trains, but partly too because, well, housing is profitable.

In other parts of town, existing main line railways were simply added to the new underground lines. The Central line swallowed routes originally built by the Great Western Railway and London & North Eastern Railway. The District line absorbed part of the London, Tilbury & Southend Railway.

At any rate: the Tube was playing the same role as the RER as early as the 1930s. London could still benefit from some RER-type services, so hopefully the Elizbaeth Line won’t be the last. But it doesn’t need an entire second metro network in the way 1960s Paris did.

There is another idea we could more profitably steal from Paris. Those suburban railways which aren’t connected to the RER are still run by the national rail operator, SNCF. But it uses the Transilien brand name, to mark them out as a part of the Parisian transport network, and – as with the RER – each route has its own letter and its own colour.

The Transilien & RER networks in Paris. Image: Maximilian Dörrbecker/Wikimedia Commons.

This would not have the transformative effect on London that building another half a dozen Crossrails would. But it would make the network much easier to navigate, and would be almost infinitely cheaper. Perhaps we should be starting there.

Jonn Elledge is the editor of CityMetric. He is on Twitter as @jonnelledge and on Facebook as JonnElledgeWrites

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