Frequently asked questions

Our year for sampling data now runs from 16 March to 15 March, the date on which we received our new ministerial authorisation. All the data for the current year should therefore be available towards the end of April of the following year.

The quarterly average for the first quarter is calculated from 16 March to 15 June 2024. Samples are collected over a 24-hour period, every other day. The values obtained are then added together and divided by the total number of samples collected over the quarter.

The daily values fluctuate according to : 

• meteorological conditions; if the wind blows in the direction of the analyser for a greater number of hours then a higher value is expected, which is why the measuring station was installed in the dominant wind corridor at the property boundary to measure the highest annual value.
• the completion of projects; projects implemented in recent years, particularly transitional projects, make it possible to reduce emissions at source, so even if the winds blow for the same amount of time in the direction of the analyser, a lower value will be measured compared with historical data.

The time taken to publish sampling results is explained by sample collection, transmission to the laboratory, sampling to be sent to the various laboratories for certain parameters, analysis of the samples, compilation of the results, validation by quality assurance and quality control, conversion into ng/m³, then posting on the database.

The sampling stations analyse the metals present in total dust and particulate matter, as well as SO₂.

SO₂ is measured using spectrophotometric analysers that provide a continuous, real-time reading of its concentration in the air. This data can then be broadcast in real time on the website.

For metallic dust, samplers capture the air for 24 hours through a filter. These data take longer to publish because of the time involved in collecting the sample, sending it to the laboratory, taking samples to send to different laboratories for certain parameters, analysing the samples, compiling the results, validating quality assurance and quality control, converting it into ng/m³ and then putting it online on the database.

The targets applicable in the ministerial authorisation are not quarterly, but annual. We have to wait until we have the results for the whole year (from 16 March to 15 March of the following year) to know whether the targets have been achieved.

It is important to note, however, that factors over which we have no control (weather factors) can influence the average and cause it to vary from one sampling period to the next, which ultimately has an impact on the annual average.