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The Belgian air quality index, the BelAQi

An air quality index makes it possible to globally assess the quality of the air in the short term over a geographical area determined from the results of several pollutants in the ambient air.

The BelAQi index (Belgian Airquality Index) is based on the concept developed within the framework of the European JOAQUIN project, which is used in the Netherlands (www.luchtmeetnet.nl). This is an index based on the effects of air pollution on health. It results from a combination of sub-indices relating to the concentrations of 4 pollutants in the ambient air: PM10 (fine particles whose (aerodynamic) diameter is less than 10 microns), PM2.5 (less than 2.5 microns) , ozone (O3), and nitrogen dioxide (NO2).

This index aims to translate the results of air quality measurements into a single, understandable qualitative assessment.

The BelAQI is calculated using hourly concentrations measured in real time at stations in the Walloon network. The index is calculated on a daily or hourly basis, and is expressed on a graduated scale from 1 to 10; the higher the value of the index, the poorer the air quality. The highest sub-index value is attributed to the BelAQi index.

Two calculation methods are carried out:

- the daily index (from the day before) is based on the daily average concentrations (D-1) of PM10 and PM2.5, and the hourly maximum concentrations of the day (D-1) for O3 and NO2 ;

- the current index is based on the moving average concentrations over 24 hours for PM10  and PM2.5, and on hourly concentrations for O3 and NO2.

Index

 Classification

PM10 - Daily average
(µg/m³)

PM2.5 - Daily average
(µg/m³)

O3 - Max 1h/d
(µg/m³)

NO2 - Max 1h/d
(µg/m³)

1 excellent 0 - 10 0 - 5 0 - 25  0 - 10
2 very good 11 - 20 6 - 10 26 - 50 21 - 50
3 good 21 - 30 11 - 15 51 - 70 51 - 70
4 quite good 31 - 40 16 - 25 71 - 120 71 - 120
5 average 41 - 50 26 - 35 121 - 160 121 - 150
6 mediocre 51 - 60 36 - 40 161 - 180 151 - 180
7 very mediocre 61 - 70 41 - 50 181 - 240 181 - 200
8 poor 71 - 80 51 - 60 241 - 280 201 - 250
9 very poor 81 - 100 61 - 70 281 - 320 251 - 300
10 appalling > 100 > 70 > 320 > 300
 

Table 1 : Scale of the BelAQI index (*moving average concentrations over 24 hours for PM10 and PM2.5, and hourly concentrations for O3 and NO2 in the case of the calculation of the current index)

Limits and constraints of the index

The index is a qualitative assessment of the ambient air quality which has only limited scientific value. It responds to a need to provide information adapted to a requesting public. This information system is simple but nevertheless reflects the complex situation of air quality. It is not representative of particular situations or pollution peaks that may be encountered in the immediate vicinity of sources of pollution, such as roads, industrial zones, etc. It should also be noted that the method of constructing the index does not allow rigorous comparisons to be made between the different sites. Indeed, the value of the index is set on that of the highest sub-index and does not include the levels reached by the other pollutants, nor the cumulative pollution due to the four pollutants. The index therefore has only limited scientific value and does not allow the identification of local pollution phenomena. It should also be remembered that the index is calculated on the data obtained in real time from the analysers, which have not yet undergone all the validation tests.

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