Air quality monitoring is a major challenge for this decade. One of the most important and difficult issues concerns the monitoring of fine particles (1 µm <), and in particular ultrafine particles (100 nm <). In mid-2018, the French Agency for Food, Environmental and Occupational Health & Safety (ANSES), published a report recommending strengthening the monitoring of ultrafine particles, classified as priority pollutants in the air [1]. Furthermore, access to the number concentration of fine and ultrafine particles of a given elemental composition is crucial for the following reasons:
-    Ultrafine particles typically represent less than 1 % of the mass of aerosols, but more than 80 % of the total number of particles in the atmosphere.
-    Unlike micrometric particles, ultrafine particles remain suspended in the air for a very long time.
-    The health risks associated with inhalation of ultrafine particles are high. They are linked not only to their chemical composition, but also to their very small size, which enables them to penetrate deep into the respiratory system and can pass into the bloodstream.
The aim of the project proposed here is to obtain the number and mass concentrations, in real time and in situ, of particles of a given elemental composition, without limiting the type of particle. The size range considered is from a few tens of nm to 1000 nm, covering as far as possible the field of fine and ultrafine particles [2, 3].
To achieve these objectives, a new process based on Laser-Induced Breakdown Spectroscopy (LIBS) applied to aerosols is being developed. It differs from previous works [4] in that the interaction takes place under vacuum (and not at atmospheric pressure) between a focused pulsed laser with a high repetition rate and a jet of particles produced by an Aerodynamic Lens System (ALS) [5]. Interaction with the laser is therefore solely with the particles, without interaction with the gas, and is quasi-continuous. The ALS enables a very fine, dense particle jet to be produced under vacuum, while avoiding contamination of the optical elements by particle deposition. This deposition is a major limitation of on-line aerosol analysis using optical techniques.
The advantages of the presented technique over conventional ambient pressure techniques are:
-    No background noise from the surrounding plasma gas.
-    Highly enhanced sensitivity thanks to the concentration effect of ALS.
-    Detection of individual particles, enabling counting, linked to the number of particles in the air.
-    More efficient detection of elements emitting in the far UV range, even down to VUV.

References
[1] ANSES report, 2018, n°2015_SA_0216. https://www.anses.fr/fr/system/files/AIR2015SA0216Ra.pdf
[2] Picard J. et al., MRS Advances, 2017, 2, 1487. https://doi.org/10.1557/adv.2016.633
[3] O. Sublemontier et al.,” Dispositif de caractérisation de particules dans un jet de particules sous vide”, Eur. Pat. EP2889602, 2013. https://patentscope.wipo.int/search/fr/detail.jsf?docId=EP137965452
[4] Amodeo T. et al., Spectrochim. Acta B, 2008, 63, 1183. http://dx.doi.org/10.1016/j.sab.2008.09.005
[5] Liu P., et al., Aerosol Science and Technology, 1995, 22, 314. http://dx.doi.org/10.1080/02786829408959749