Environmental measurements
From raw sensor data to decoding and interpretation
January 2024
Changes in climatic conditions
This section presents a detailed analysis and discussion of the various measurements available and the processing carried out to get the best out of the citizen sensors used. Such processing is carried out every month on all the operational sensors, enabling us to deduce the average values presented on the home page for the past month or in the monthly reports tab for previous months; these detailed results for each month are not systematically presented on the site. This page is therefore aimed at enthusiast people who want to understand the behaviour and performance of their sensor.
Climatic conditions are important in analysing sensor data through temperature and humidity. Periods of fog disrupt the measurements of the citizen sensors, which are also sensitive to low temperatures; these phenomena are partially corrected as presented in the ‘Technique’ tab.
Changes in temperature and humidity were recorded as follows:
Temperature (°C): Some citizen sensors are equipped with operational and reliable temperature measurements. The corresponding readings show faithful reproducibility; the points with high temperatures correspond to sensors exposed to the sun during the day. There were two cold spells below the seasonal norms for the Toulouse region, and numerous occasions when the high seasonal norms were exceeded, reflecting the ambient warming of the climate. The average temperature for the month of January 2024 was 7.7°C, well above the usual norm (6.9°C).
Humidity rate (%): This measurement is indicative on the citizens’ sensors and does not reflect the periods of intense fog that occurred in January 2024, mainly outside urban areas. Periods of fog are indicated in greyish blue according to their intensity on the graph opposite and identified through the reference sensor (Reference curve). An average humidity level of 80% was recorded for this month. As humidity is a function of temperature, it decreases as the temperature rises, hence the stronger correlation with the temperature curves of certain sensors.
Measurements on the Toulouse ring road
These measurements are taken only by official Atmo Occitanie sensors, whose data are publicly available on their website.
It is therefore interesting to have the translation of these measurements to get an initial reference picture of the environment in the conurbation. The Atmo Occitanie website also presents these results, but more globally and over a longer period.
Concentration of fine particles PM10
Four sensors placed on or very close to the edge of the ring road provide measurements reflecting the impact of road traffic. There were periods of high concentration from 9 to 14 January, corresponding to a low-temperature phase, and from 23 to 28 January, a period of milder temperatures. A long period of lost data means that it is not possible to link the periods of high concentration. In general, the measurements are well above the annual values recommended by the WHO, confirming the critical nature of the ring road in relation to its environment. The most critical point is the southern part of the ring road at Rangueil (FR50048).
Concentration of fine particles PM2.5
Not all Atmo sensors provide PM2.5 measurements simultaneously with PM10 measurements. Only one is providing available measurements with numerous interruptions this month, at the level of the Route d’Albi (FR50054). These measurements show significant exceedances of the recommended thresholds.
The Toulouse ring road has always been identified as a critical point for air quality in the Toulouse conurbation. These results therefore come as no surprise, and are recognised and reflected in the simulations carried out by Atmo (see their website). However, these graphs give us a benchmark for the measurement profile of other sensors in the region.
Measurements in the centre of Toulouse
Two Atmo Occitanie sensors located in the centre of Toulouse monitor the city’s clean environment. Ten to twelve citizen sensors in the surrounding area will complete the picture; in particular, they could help to identify differences in behaviour from one neighbourhood to another.
Concentration of fine particles PM10
Atmo measurements: For two official sensors, one (FR50021) is located a few hectometres from the Canal du Midi in the Minimes district (blue curve) and the other (FR50030) near the Garonne (red curve). Their behaviour is similar, suggesting that the environment is uniform in the centre of Toulouse. The two sensors show a transmission anomaly from 17 to 23 January 2024.
Citizen sensors : Between the two Atmo sensors are two citizen sensors located in similar geographical environments (ID15655 and ID21138); their measurement curves are compared with the two Atmo sensors (dotted curves). These two types of sensor have a similar dynamic signature. During the low-temperature period from 7 to 14 January, a gain correction was applied to the citizen sensors to compensate for a drift observed on these low-temperature sensors (see technique tab). Outside cold periods, concentration peaks occur on both types of sensor, with potential differences in level that may be linked to their specific location (urban road traffic).
Citizen sensors : All the citizen sensors in the Toulouse conurbation give similar trends and results. Some sensors located in areas subject to fog during critical periods give peak values that are not representative of real PM10 pollution.
Outside periods of fog, differences in levels with potentially specific peaks can be observed, linked to the specific characteristics of their neighbourhood; a detailed analysis of their environment would help to explain the cause.
Generally speaking, the environment in terms of fine particles PM10 is acceptable on average in the centre of Toulouse. Critical periods with concentration peaks above the annual WHO standard are to be noted for the Atmo sensors and citizens, mainly during periods of more or less intense fog.
Concentration of fine particles PM2.5
Atmo sensors: The two sensors identified above also provide PM2.5 readings. The values exceed the recommended WHO standard mainly during periods of low temperatures. Both sensors show numerous interruptions in data supply during the month.
Citizen sensors : By comparing the two citizen sensors in the environment close to the Atmo sensors (dotted curves) we find the occurrence of peak values consistent with the Atmo sensors. As in the case of PM10, the values presented during cold periods (T<10°C) are lower due to a gain correction to compensate for the effects noted on the response of the sensors’ electronics (see technicaque tab). Apart from this correction, the responses of the Atmo and citizens sensors show similarities.
Citizen sensors : The response of the various sensors throughout the Toulouse conurbation remains homogeneous and consistent with the Atmo sensors at the dynamic level. Some measurements are also affected (peak values) by the presence of fog in the periods and zones affected.
The concentration of fine particles PM2.5 in the centre of Toulouse is more critical than that of PM10 in terms of WHO standards. This trend is shown by both official Atmo sensors and citizen sensors. There are a variety of reasons for this situation, including road traffic and heating …. interactions with weather conditions…..
Measurements in the western suburbs of Toulouse
Atmo Occitanie sensors, of which there are fewer in this zone, have targeted objectives:
– 2 sensors near the airport are used to monitor the very active airport area, and close to urban areas,
– 1 sensor on the motorway exit towards the Pyrenees and Spain, to monitor dense traffic on a strategic route; it is also located in a high-risk industrial zone.
– 2 other stations, located a few dozen kilometres from Toulouse (North and South-East) monitor the secondary effects of pollution in Toulouse (in particular NO2 and O3), but do not measure concentrations of fine particles.
Nine citizen sensors (although not numerous enough and strategically dispersed) provide a complementary view of the west of Toulouse.
Concentration of fine particles PM10 and PM2.5 in the vicinity of the airport
Atmo PM10 measurements: For the two Atmo sensors near the airport, levels were similar to the Atmo sensor located in the city centre (Minimes); this result is surprising given that the airport is a very active area bordering busy ring roads.
Between the airport and city centre sensors there is a sensor on the ring road giving higher results but with a comparable trend profile; we cannot therefore deduce that the airport and city centre environments are linked by wind effects.
PM2.5 Atmo measurements: The same effect was observed for PM2.5 measurements.
Further measurements would be interesting to understand the ambiguity of this finding.
There are no citizen sensors close to the airport that would allow a reliable correlation to be made. Only a trend with sensors in the western suburbs of Toulouse can be used to sketch out a comparison.
Concentration of fine particles PM10 and PM2.5 in the western suburbs of Toulouse
PM10 measurements: In suburbs far from the city, only citizen sensors provide a perception of the environment. Located in very different areas (urban and rural), their dynamic response is nevertheless similar, with noticeable differences in levels. These sensors show excessive PM10 levels during foggy periods (15, 25, 26 and 27 January); the measurements are also corrected during cold periods.
However, in undisturbed periods (temperature and fog), the levels are higher than those recorded in Toulouse, particularly in relation to the Atmo sensor at the airport. The reason for these high values remains to be defined.
PM2.5 measurements: The values recorded for PM2.5 content are close to logical expectations and in particular slightly lower than the values observed in the centre of Toulouse and also the measurements near the airport.
The fog effect is less marked for PM2.5 than for PM10 and results in only slight excesses during the corresponding periods.
Special case of the south-west route out of Toulouse
This area is special in that it includes the motorway link to Spain, the Thibaud industrial zone and the Toulouse urban waste recovery centre (incinerator).
Along with the ring road, it is one of Toulouse’s most critical areas in terms of air quality.
PM10 measurements: An official Atmo sensor (dotted line), located between the motorway to Spain and the ‘Arc-en-ciel’ highway (outer ring) gives measurements consistent with those seen on the ring road. Four sensors are located close to this road, including one (ID26046) near the waste recovery centre. Measurements at the latter are high and need to be monitored, while remaining compatible with WHO standards. The high peaks during periods of fog (15, 25, 26, 27 and 31 January) should be put into perspective, given the sensitivity of the sensors to this phenomenon.
PM2.5 measurements: The Atmo sensor in the area does not measure PM2.5 particles. Knowledge of this environment is based on citizens’ sensors: The concentration of fine PM2.5 particles appears high despite the minimisation corrections applied.
Overall, in the western suburbs of Toulouse, the concentration of PM10 particles over the current month is compatible with the recommended standards.
The situation regarding PM2.5 concentrations is more critical, as they exceed WHO recommendations.
Measures in the south-east suburbs of Toulouse
Focus on Castanet-Tolosan
Located around seven kilometres from the Toulouse conurbation, Castanet-Tolosan is not subject to the same environmental constraints as Toulouse, but remains under the influence of the large conurbation and the critical zone of the south-west axis, discussed above, located at the same distance but dominated by westerly winds.
A community of concerned citizens has therefore installed around ten sensors (there are no Atmo sensors in the area) to carry out self-monitoring, which is the basis of this initiative.
Concentration of fine particles PM10 and PM2.5 in Castanet-Tolosan
PM10 measurements: The sensors located in Castanet-Tolosan are presented here. All the sensors have been calibrated in relation to each other in order to give the same response in the same environment. It can be seen that the concentration of PM10 particles varies significantly depending on the position of the sensors, while still giving an acceptable average value. A detailed analysis shows disparities between the sensors depending on their location along the busy D813 and D79 roads. Once again, foggy periods produced excessive peaks (15, 25, 26, 27 and 31 January). During these periods, the concentrations measured were slightly higher than the WHO standards.
PM2.5 measurements: The same trends and developments are noticeable for PM2.5 concentrations. The average of the measurements over the whole period is slightly above the WHO recommendation. Monitoring over several months is needed to determine whether the situation is critical.
Influence of the busiest roads
Two busy roads cross the commune: the D813 (Toulouse – Carcassonne) and the D79 (Escalquens – Les Côteaux). In view of the measurements, the situation appears to be just as critical as in the centre of Toulouse.
PM10 measurements: Four sensors located on these roads (2 on the D813 and 2 on the D79, including 1 at the intersection with the D813 – green curve) often give values above the WHO recommended average value, even outside the critical periods of fog, which are less intense in the town centre.
PM2.5 measurements: PM2.5 measurements are also slightly above the average WHO standard.
Status in a residential area
A residential area stretches out in the lower part of Castanet between the town centre and the banks of the Canal du Midi, part of which runs alongside the A61 motorway (Toulouse – Carcassonne). Two citizen sensors have been installed in the middle of this area (ID78002 and ID81908); a new sensor recently installed (ID84351) on the edge of the motorway can be used to gauge the impact of road traffic.
PM10 measurements: Although located away from busy roads, the two sensors in residential areas show numerous peaks, even outside periods of fog in this area. Comparison with the measurements from the sensor on the edge of the motorway (dotted green curve) does not make it possible to link these peaks with motorway traffic. The peaks observed are therefore more likely to be due to heating emissions (wood fire in particular); many of these peaks also occur at the end of the day.
PM2.5 measurements: The frequency of PM2.5 peaks is confirmed. In view of their higher levels than those related to road traffic (see above), we can strongly suspect that these measurements are linked to heating residues.
Although the Castanet-Tolosan conurbation is not perceived as an area of concern in terms of air quality, which is deemed acceptable according to conventional criteria, the measurements nevertheless show a sensitivity on the town’s busy roads and a deterioration in residential areas linked to heating methods. This finding is based on a limited number of sensors, and certain sensitive areas have not yet been monitored, such as avenue Mandés France, which is heavily used by motorists and lined with schools and sports facilities. A more comprehensive monitoring plan is therefore recommended to better understand the risks and potential effects of values exceeding WHO standards.
What is the situation to the north and east of Toulouse?
Fine particle concentrations in eastern Toulouse
There are few sensors in this zone. Only one official Atmo Occitanie sensor is located in Bessières (FR 50047, about 20 km from Toulouse). Only three citizen sensors are listed in this zone: one to the north, which is not working properly (and therefore unusable), one to the east in the commune of Mons (giving back to the same location) and one to the south-east in Escalquens, near Castanet-Tolosan.
PM10 measurements: A detailed PM10 analysis of the two citizens’ sensors enabled us to identify periods of fog that had an impact on particle concentration measurements.
The periods of 14, 15 and 23 to 27 January, as well as 31 January, correspond to the appearance of fog of varying intensity in the two zones. More intense fog in the more rural Mons area led to more pronounced measurement impacts.
Outside these periods, PM10 concentration levels were relatively moderate, indicating a relatively undisturbed environment in these two zones. The signatures of the citizens‘ sensors and the Atmo sensor are close; the difference observed between 9 and 13 January results from the low-temperature gain correction made on the citizens’ sensors.
PM2.5 measurements: The same observations can be made as for PM10 measurements. Levels remain low and acceptable.
The sensors located in this part of the Toulouse suburbs do not allow us to draw any conclusive statements, apart from noting that they do not show any pronounced concentration of particulate matter. In the northern part, around the motorway to Bordeaux, a highly urbanised and industrialised area necessarily has an impact on air quality. In the eastern part, the exits towards Albi and Castres via major traffic routes (roads and motorway) cannot be without effect. The fact that Mons is located in a still rural environment is a particular feature that preserves air quality. In the south-east, residential areas, combined with small industrial zones, ensure that air quality remains acceptable.
However, monitoring, even using low-cost sensors, would be reassuring for the public in this somewhat neglected area.