There is, for the first time, an absence of exceedances to the annual limit value (LV) in large agglomerations, with a decrease in the levels measured in AML Norte, from 55 µg/m3 in 2019 to 40 µg/m3 in 2020, in Porto Litoral, from 48 µg/m3 in 2019 to 40 µg/m3 in 2020 and in Entre Douro and Minho, from 57 µg/m3 in 2019 to 32 µg/m3 in 2020.
The significant decrease in the average annual concentration in the agglomeration Entre Douro e Minho may be related to the fact that the station that has determined the exceedance of the LV was not used in the assessment, in 2020, due to breakdowns or malfunctions of the measuring equipment.
It is also possible to observe that in the remaining zones and agglomerations, there was a generalised decrease in the order of magnitude of the measured levels, noting however, in the Coimbra agglomeration a slight increase in the measured levels of NO2 compared to 2019 (of 15 µg/m3 to 22 µg/m3). This situation may result from the use, in the 2019 assessment, of the urban background station instead of the usual traffic station, which that year, due to equipment failure, was not operational.
The hourly LV of 200 µg/m3, not to be exceeded more than 18 times a year, was an air quality objective fully met in all zones and agglomerations in the national territory.