Air and Noise

Nitrogen Dioxide Air Pollution

The "Nitrogen Dioxide Air Pollution" fact sheet analyses the progress towards the objectives imposed for this pollutant across the country, in order to safeguard the protection of human health.
Description: 

Nitrogen dioxide (NO2) is one of the main air pollutants, a highly toxic gas that results from the burning of fossil fuels at high temperatures, originating mainly in car traffic and the industrial sector. Exposure to high concentrations can result in serious damage to human health, such as impaired lung function and increased risk of respiratory diseases. When oxidized in the atmosphere, it can even produce nitric acid, one of the components that increases the acidity of rain, and causes various damages to nature and materials, as it is corrosive. It is, however, an occurrence with little significance in our country.

Legislation stipulates air quality objectives for the protection of human health. The indicator used here is the annual average of NO2, which is an aggregated value, based on the hourly concentrations measured at each station, for comparison with the respective limit value (LV). The analysis of the air quality of the zones and agglomerations is carried out considering the worst value obtained in the stations belonging to each of these territorial units.

The type of measuring station, with regard to the environment and the influence of its location (urban, suburban or rural environment and traffic, industrial or background), identifies the nature of the emission sources and the order of magnitude of the levels measured. The trend analysis carried out, based on the aggregation of hourly mean values, depending on the type of station, allows a better clarification on the orientation and direction of the measures to be adopted to achieve the objectives of compliance with the limit values throughout the national territory. For this calculation, all stations with measurement efficiency above 75% are used, and in the case of stations covered by the indicative measurement evaluation strategy, measurements with efficiency above 14% are also used.

This fact sheet concerns mainland Portugal, the Autonomous Regions of Madeira and the Azores and will be updated annually.

 

Objectives and targets: 
  • Ensure compliance with the EU-wide targets for ambient air quality, which aim to avoid, prevent or limit the harmful effects of air pollutants on human health and the environment;
  • Measure ambient air quality across the country, with a special focus on urban centres;
  • Preserve air quality where it is good and improve it in other cases;
  • As targets, the limit values set forth in the legislation are not to be exceeded  (Decree-Law no. 102/2010):
    • Limit value for the NO2 hourly mean concentration of 200 μg/m3, not to be exceeded more than 18 times a calendar year;
    • Limit value for the NO2 annual mean concentration of 40 μg/m3.
Progress analysis:

The analysis of the concentrations of nitrogen dioxide (NO2), measured at the air quality stations, in the year 2020, shows an atypical year, due to the impact of the COVID-19 pandemic on air quality, resulting from several periods of lockdown which significantly influenced all economic activity and circulation in the national territory. In particular, the measures imposed during the state of alert and the state of emergency resulted in a strong reduction in emissions of air pollutants and led to a significant improvement in air quality, especially in areas with more intense traffic in large urban agglomerations. The reduction in concentrations was mainly felt in the concentrations of NO2, a pollutant whose main source is road traffic and which is responsible for most situations of air pollution in recent years in the cities of Lisbon, Porto and Braga (non-compliance with the annual limit value of 40 µg/m3).

Exceedances to the annual threshold limit value of NO2 in zones and agglomerations monitoring them (background, traffic and industrial stations in 2019 and 2020)

Notas: (1) - Zone with measurement efficiency <75% in 2019 (2) - Zone with measurement efficiency <75% in 2020 (3) - Zone assessed using air quality models in 2019 (a) - Agglomeration
Source: APA, CCDR Norte, CCDR Centro, CCDR Lisboa e Vale do Tejo, CCDR Alentejo, CCDR Algarve, DRA Madeira, DRA Açores, 2021

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.

Evolution of the average annual concentration of NO2 by type of station

Source: APA, CCDR Norte, CCDR Centro, CCDR Lisboa e Vale do Tejo, CCDR Alentejo, CCDR Algarve, DRA Madeira, DRA Açores, 2021

The trend analysis of average annual NO2 values by station typology allows the identification of a more accentuated downward trend from 2018 onwards in traffic stations where the most significant exposure of the population to this pollutant occurs, and a less significant decrease, in background stations (urban and suburban), from 2017.

For the remaining types, the order of magnitude of the measured concentrations has been maintained in recent years, with a break in this trend in 2020, with a reduction in NO2 levels, as a result of the restrictions imposed by the various lockdowns, resulting from the impact of the COVID-19 pandemic on the normal functioning of the economy and, consequently, on the improvement of air quality.

In the QualAr information system, historical air quality data are available online, as well as near real-time information on measurements carried out at all stations of the network and for all pollutants. The fact that the data is provisional means that it should be considered for information purposes only, and the validation process will be concluded by September of the following year.

 

Last update: 
Wednesday, 25 August, 2021
More information at: 

Portuguese Environment Agency - https://apambiente.pt/ar-e-ruido/
QualAr: National information system on air quality - http://qualar.apambiente.pt