Particulate matter in the atmosphere: it is not transport that makes the worst contribution

Particulate matter in the atmosphere: it is not transport that makes the worst contribution

Particulate matter in the atmosphere: it is not transport that makes the worst contribution, as demonstrated by the lockdown

Several national and international studies have analyzed the response of air pollution to the lockdown caused by anti-covid limitations. One thing is clear: overruns of particulate concentrations are increasingly frequent despite limitations to mobility.

According to the data collected in recent months by the Regional Agencies for the Protection of the Environment (ARPA), the levels of fine particles that are hazardous to health (PM2.5 and PM10) throughout the Po Valley, which is one of the areas most affected by this type pollution in Europe, are high although vehicular traffic is low. Numerous studies have been carried out to investigate the causes of this phenomenon having a good amount of data available in conditions that are certainly anomalous.

Did pollution decrease during the lockdown?

Perhaps the only positive effect expected from the prolonged lockdown imposed by the Covid-19 pandemic was a major drop in pollution. 

Unfortunately, the decrease was not as large as expected. 

This is demonstrated by a study conducted by the University of Birmingham, which analyzed the air quality in 11 cities around the world during the period of the lockdownThere was a decrease in nitrogen dioxide (NO2), a gas produced mostly by vehicle enginesWhile other secondary pollutantssuch as ozone (O3) and atmospheric particulate matter (PM10 and PM2.5), have maintained high levels in many cases. 

The cities considered in the study are: Beijing, Wuhan, Milan, Rome, Madrid, London, Paris, Berlin, New York, Los Angeles and Delhi. The analysis considers the period between the second and fourth week of lockdown. 

The results show that the decrease in traffic has produced an immediate decrease in nitrogen dioxide NO2which is concentrated mostly at ground level and is generated in the atmosphere by the oxidation of nitrogen monoxide (NO), produced directly by processes of combustion from vehiclesBut according to the researchers, the effects of the lockdown are not so markedif we exclude the effect of the weather, in fact, the closures have led to a reduction in NO2 of about 30%. 

What are the causes of the persistence of pollution in the Po Valley?

A study conducted by the Institute of Atmospheric Sciences and Climate of the National Research Council (CNR-Isac), coordinated by the Center for Studies for Air Quality and Climate Change of the Foundation for Research and Technology Hellas (C-STACC), shows and quantifies the effect of biomass combustion emissions on the chemical transformations responsible for the formation of secondary particulate matter. The researchers affirm that a large part of the PM in the Po Valley is of secondary origin, therefore originating from gaseous precursors that react in the atmosphere and that the mechanisms by which these pollutants are transformed into particulate matter are still under study.

One of the themes analyzed is the formation of secondary particulates even in the winter season when solar radiation, the engine of chemical transformation, is at its lowest levels. The study shows how the secondary particulate is easily formed even in the absence of radiation (through the dark aging process) and that this process is favored by the presence of liquid particles in the atmosphere, such as fog.

Until now, the forecasts of PM in the atmosphere were mainly based on sources from traffic and agriculture, considered the main causes, but the study shows that emissions from wood combustion for domestic heating also contribute to the total concentrations of PM more substantially than as assumed so far.

Also the 2020 report of the National Environmental Protection System confirms that, as in many Italian regions, also in the Po Valley the total quantities of PM10 emitted up to the first half of April have remained substantially unchanged compared to those of the same period of the previous years.

The reduction in the contribution from industry and transport was in fact offset by the increase in emissions from domestic heating; only starting from the second half of April, since there was no contribution from heating, there was a reduction in emissions mainly linked to the decrease in road traffic.

The main part of the emissions that make the Po Valley one of the most polluted areas in Europe come, not only from transport, but also from the chimneys of buildings. Furthermore, the characteristic conformation of the area hinders the circulation of air and favors the stagnation of pollutants. All these factors make the Po basin one of the most polluted areas in Europe, as underlined by the surveys of the World Health Organization (WHO).

Valeria Menichini

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