Mode Locked-laser Cavity-enhanced near UV spectrometer for sensitive and quantitative local Measurement of Atmospheric Reactive molecules. – MoCaMAR

The atmospheric physicists community suffers a lack of fast, reliable, space resolved measurements for a wide set of molecules playing a crucial role in atmospheric chemistry despite their very low concentrations. Many oxidants act on the atmosphere composition, giving rise to a local chemistry and having sometimes a 'cleaning' effect. As an example, although it is well established that BrO is the key agent in ozone destruction and mercury deposition in the snowpack, its origin remains poorly defined because of the lack of time and space resolved measurements. A better knowledge on distribution and/or time variations of these oxidants would help to further refine and complete the existing models, an important step in understanding climate and predicting its trends. We have a long-standing experience in the development of laser based field instruments dedicated to in-situ real-time analysis of trace species in the atmosphere via their infrared rovibrational absorption spectrum. Through several collaborations we deployed our optical instruments in various contexts: In the laboratory (water isotopic ratio measurements, Groningen), in the vicinity of a volcano (fumaroles composition, Naples), on board of an aircraft (tropospheric methane on NASA DC8, California 2004, tropospheric CO on a Piper equipped by LSCE, 2007 Paris), or integrated in a unpressurized stratospheric airplane (water isotopic ratio on Geophysica, AMMA-SCOUT campaign 2006, Burkina-Faso). This latter campaign has clearly proven that our spectrometers can be engineered to work in harsh conditions of pressure, temperature and vibration (70 mbar, -50°C, turboreactor airplane). On the other hand, we also developed beginning in 2000 a high sensitivity technique exploiting modelocked lasers, especially suitable to access strong electronic absorption bands in the near UV. Recent technological progress makes now available these laser sources as compact and robust systems featuring excellent performances, in particular a wide spectral coverage. This will permit setting up a transportable instrument capable of direct, rapid and local measurements for a wide variety of trace species, a performance not available by other existing techniques. In the laboratory, we already demonstrated the potentialities of this novel technique, which consists in making the wide (5nm) laser emitted mode comb to coincide with the comb of transmission resonances of a high-finesse optical cavity, where a constant ambient air flux is injected. Using a diffraction grating to disperse the transmitted light onto a detectors array opens a spectral window as large as the laser source which, in addition, is widely tuneable. Using a 1m cell, this multiplex absorption scheme allows for the recording in a few seconds a spectrum equivalent to that we would obtain after a 10km pathlength. In the near U.V., we already reached a detection limit equivalent to a 10-9cm-1 absorption, giving a concentration detection limit around 1pptv for the majority of the molecules considered here and less than 1 pptv for some possessing stronger absorption bands (BrO, IO, OBrO, OClO). Our project first focuses on species (BrO, IO, H2CO) that are detectable in the blue (340- 460nm). At a second stage, other molecules will be made accessible by laser frequency tripling or quadrupling (OH, SO2, O3, DMS, and DMSO). A transportable femtosecond laser spectrometer will be finalized as LSP within one year of obtaining the laser source. Several months will be devoted to validation tests (side by side with the LGGE team) using calibrated samples. We will then install the system at Dumont d'Urville, a permanent station in coastal Antarctica where the LGGE team will supervise the measurement strategy by the device in the presence of a member of our team. The objective is here to document for the first time IO and BrO, two potentially important radicals for the atmospheric DMS cycle studied by LGGE in the framework of the ORE CESOA (Observatoires de Recherche en Environnement, CyclE du SOufre dans les zones Australes).