Why do the values shown on the IRCELINE website differ from those on aqicn.org?
The air quality measurements that are shown on the IRCEL - CELINE website for the different pollutants (PM10, PM2.5, NO2, ozone, "black carbon", etc.) are concentrations expressed as micrograms per cubic meter (µg/m³), or as mass per unit of volume. Microgram per cubic meter is a conventional and the most frequently used unit in the field of the scientific analysis of air pollution.
The European target and threshold values as given in the European directives, are also expressed in µg/m³. In the same way, the WHO recommendations are expressed as µg/m³. As an example, the daily limit for PM10 as described in directive 2008/50/EC is 50µg/m³, not to be exceeded on more than 35 days per year, while the WHO recommendation on the daily PM10 limit concentration is 50µg/m³, not to be exceeded on more than 3 days per year.
On the IRCEL - CELINE website, the air quality data measured by the telemetric networks of the three Regions is published in real time and updated on an hourly basis. These measurements are, after validation by the authorised regional environmental agencies, also used for the mandatory annual reporting on Belgian air quality to the European Commission.
The air quality data on the IRCEL - CELINE website are publicly accessible. Through so-called 'web services', they can be used by external users, for instance to develop smartphone apps. As such, aqicn.org also uses these services. The air quality data shown on http://www.aqicn.org are derived from the IRCEL - CELINE website. However, the values published by AQICN are not concentrations, but "index numbers". The data from IRCEL - CELINE are thus first converted by AQICN to dimensionless numbers, to condense the concentrations of multiple pollutants into a single number. Indexes serve the purpose of allowing a simple yet qualitative evaluation of air quality, such as the BelATMO index in Belgium. Worldwide, various air quality indexes are used.
Aqicn.org uses the index of the American Environmental Protection Agency (US EPA). However, to compute this index for particulate matter (PM2.5), AQICN erroneously uses hourly measured particulate matter concentrations, while the USEPA index for particulate matter is based on 24-hourly mean concentrations. The latter is legitimate, since 24-hourly mean concentrations are used in epidemiological studies which investigate the short term health effects caused by particulate matter. The index value on aqicn.org is only correct if the concentrations do not change in the following 23 hours.
The correct conversion between concentrations and the EPA index can be calculated with the following online application: https://www.airnow.gov/index.cfm?action=airnow.calculator