On August 27, a large landslide in the Maurienne region, in French territory, caused the closure of the Fréjus tunnel. Traffic was diverted to other road routes, including the Mont Blanc tunnel.
Despite ARPA Valle d’Aosta has not detected exceeding the limits established by lawthe increase in the number of heavy vehicles in transit has had a important impact on the concentrations of nitrogen dioxidewhile the influence on PM10 values is less evident.
How much has traffic increased?
The data provided by the EEIG-TMB indicate that starting from 28 August the daily number of heavy vehicles in transit at the Mont Blanc tunnel is doubled compared to the previous days, while light traffic, which had high values typical of the summer period, decreased, perhaps due to the coincidence with the end of the holidays.
Have the pollutant values increased?
The monitoring of the effects of cross-border traffic on air quality in Courmayeur Entrèves, along the access road to the tunnel, is carried out with a fixed station for the measurement of nitrogen oxides and atmospheric particulates.
The station is owned by the GEIE-TMB and i data are validated and processed by ARPA Valle d’Aosta.
Nitrogen dioxide
From August 28th values have increased, with a particularly high average daily concentration from 30 August to 8 September. These are the highest values measured since the beginning of 2023.
The limit value for the hourly average was not exceeded defined in Legislative Decree 155/2010 (200 µg/m3): the maximum hourly recorded was 129 µg/m3.
But how “anomalous” are these values?
Even in comparison with the historical series of the last 5 years, the measurements of this period are particularly high.
In general, in 2023 the concentrations were lower than those of previous years, with the exception of the last period in which the effect of the increase in the number of heavy vehicles in transit is evident.
And are they really due to traffic?
The concentrations of pollutants in the air depend not only on the quantity of substance released into the atmosphere, but also on meteorological conditions, which can favor their increase (situations of poor ventilation, transport phenomena) or decrease (intense ventilation, precipitation).
Using a Machine Learning method it is possible to eliminate the influence of meteorology from air quality data sets: the model is trained on the historical series of measured data (average daily concentrations of nitrogen dioxide and meteorological variables) in Courmayeur, to obtain the “typical” concentration value of a given meteorological situation.
After training, the model is applied to 2023 starting from real meteorological conditions to obtain the expected concentration values. If the measurements deviate greatly from the forecasts it means that some factor, other than meteorology, has significantly influenced the measured values.
Atmospheric particulate matter
Even the PM10 concentrations have increased since August 28but the values of the previous days were very low due to the rain.
In the second half of August the arrival of dust from the Sahara resulted in values of very high PM10 concentrationswith the regulatory limit being exceeded on August 22nd.
In comparison with the historical series of the last 5 years, the measurements of this period are slightly higher than average on some days.
In 2023 we can see several episodes with high values, which led to exceeding the regulatory limit, such as that of August 22nd due to the arrival of dust from the Sahara.
