A new method of 2D and 3D Air Quality monitoring using a lidar

<>A new method of 2D and 3D Air Quality monitoring using a lidar in French cities

L.Sauvage (1)*, B. Guinot (1), S. Loaëc (1), S. Lolli (1), M.Fofana(1) (1) LEOSPHERE, Bât. 503, Centre Scientifique d'Orsay, Plateau du Moulon, 91400 ORSAY (lsauvage@leosphere.fr / Phone : +33 169358821)

ABSTRACT

In order to characterize urban and industrial pollution, several measurements campaigns have been realized from December 2008 to July 2009 in association with ADEME and two French air quality agencies, ATMOPACA and COPARLY.

These measurements have been realized with a lidar from Leosphere. This instrument, equipped with a scanning device, allows realizing a mapping of particles.

These measurements have been conducted in industrials sites for plumes detection, urban site to show pollution from traffic and also in a tunnel with big circulation. These results are preliminary results.

Keywords: Lidar, Air quality, Extinction measurement, Mass concentration, Plumes

1.  INTRODUCTION

The pollution of particles becomes a real problem in our society. That's why it becomes necessary to evaluate the mass concentration, the sources and their spatialisation. In this way, Ademe and Leosphere have started some studies in order to characterize plumes in industrial site, and particles concentration in town.

These measurements have been realized in different French towns. The instrument used was a Lidar. For making mapping, the head of the lidar was fixed to a scanning device.

The main objective of these campaigns was to demonstrate the possibilities of the lidar in various environments. Another one is to find a way to convert the optical information from the lidar into a mass concentration.

2.  INSTRUMENTAL SET-UP

During this campaign we have used the EZLidar^TM ALS300, commercialized by Leosphere. The EZLidar is transportable, works at 355 nm and have a spatial resolution of 1.5m. It is equipped with a cross- polarised channel which discriminate non-spherical particles from the others.

For these measurements the lidar was placed at a horizontal position. The lidar signal is inverted using the so-called “slope method”. From this calculation we retrieve the backscatter profile and then we calculate an extinction value along the optical path, and detect the plumes very accurately.

3.  RESULTS AND DISCUSSION

<>1.      Plumes detection

In December 2008 at Nice, we have made measurements near a cement industry. The protocol was to make horizontal scanning n order to show the differences of particles concentration.

Fig 1: Plumes detection

In the Figure 1, we can observe the plumes and the different of concentration between two scan separated from 25minutes.

Lidar measurements permit to follow plumes and located accumulation of pollution. With volume measurements, we can have information about the size of the plume.

2.      Urban pollution

We have made measurements of urban pollution at Grenoble in April 2009, at Nice in May 2009 and at Lyon in July 2009.

In Grenoble, we have made fixed horizontal measurements in order to measure the pollution of a main axe of automobile circulation.

In the figure 2, we can see the extinction coefficient for the 3 days of measurements (a week-end: 11 & 12/04/09 and a Thursday: 14/04/09)

Figure 2: Evolution of extinction coefficient

We can observe a difference between the days-off and a day of work: the pollution is more important the Thursday.

We can see that on Thursday, we have a huge peak of pollution between 6 and 9 AM and a second one between 18 and 19PM, corresponding with the higher traffic time.

In Nice in May 2009, we have made horizontal scanning from the top of a building.

 

Figure 3: Horizontal scanning

In the figure 3, we can see several sources of aerosol. In the two pictures, on the left, we have particles from the railway station. The other sources of pollution can be attributing to the traffic.

 

Figure 4

Figure 4: Horizontal scanning

In the Figure 4, we have horizontal scanning from Lyon near a tunnel. We can observe a huge concentration of particles at the intersection of many roads.

At this same place, vertical scanning has been made. A first evaluation shows evidence of bursts from the tunnel and the road raisin up to 40m.

3.      Tunnel measurements

At Lyon, we have made measurements inside a tunnel and calculate the extinction coefficient.

Figure 5: Evolution of extinction coefficient (km^-1)

In the figure 5, we can see that the extinction coefficient is very high during the traffic period. The measurements have been made the 06/07/2009, this day the traffic has been stop at 21PM. One hour after, the extinction coefficient is approximately at 0.4 km^-1. The decrease is very fast, due to the circulation of air and the system of air extraction.

With the extinction coefficient and knowledge about the physic properties, we will be able to convert the optical measurement into mass concentration. For this conversion, we‘ll used the following expression:

α extinction coefficient (m^-1)

σ cross section (m²)

d density (µg.m^-3)

r³ mean cubic rayon (m³)

Thanks to this expression, we hope to convert the optical information into a mass concentration with an estimated error of 30% compared to the 20% error from the TEOM. We have already made this conversion during a campaign in an underground railway station.

4.  CONCLUSION AND PERSPECTIVES

After this campaign in different French town, we have proved the utility of a lidar.

This instrument is transportable and can be use in many configurations. We can have information about the spatilisation of the particles. This measures can indicates if a region is more exposed to pollution, it can be a strong help for agency of air quality in order to find the better place to put their instruments.

The other aim is to convert the optical signal into a mass concentration, after that, the lidar will be able to be used in order to respond to the limitations in mass concentrations.

5.  REFERENCES

[1] Sauvage L et al.- A new method of 2D Air Quality Monitoring using a lidar inside an underground railways station – ILRC24

[2] Raut J-C et al., Lidar measurements of aerosols in an underground station, ILRC24