Laser-induced breakdown spectroscopy and laser photofragmentation spectroscop y are investigated as potential approaches to the detection and quanti cation of the sulfuric acid aerosols resulting from the oxidation of dimethylsul de in air. The form er, direct technique, where aerosols are introduced in the plasma formed by focusing the fundamental freq uency of a Nd:YAG laser in air, and atomic sulfur emission is measured in the region 180 nm, gives a limit of detection of 165 ppbv for a 15 m in integration time, which is inadequate for our purposes. The second, indirect approach, based on the photofragmentation, with 193 nm photons, of the compound resulting from interaction between sodium chloride and sulfuric acid aerosols, gives a detection limit of 46.5 ppbv in 10 s measuring time. With this method, a complete ''titration curve'' for sulfuric acid aerosols can be obtained in ''quasi'' real-tim e.

A major interest of our Institute lies in the study of the atmospheric sulphur cycle and in the oxidation mechanism(s) of Dimethylsulfide (DMS) in air. DMS originates from the ocean biota and its crucial role in the formation of atmospheric aerosols is well-documented (1,2). When DMS is oxidized, it generates SO2 that can be further oxidized to H2SO4, inducing the production and growth of new aerosol particles that can act as cloud condensation nuclei and affect the Earth’s albedo (2). The possibility of detecting sulphuric acid aerosols with laser excitation has been discussed, to the best of our knowledge, only in one paper (3), where the aerosols generated by mixing SO3 and H2O were addressed with laser photons at 193 nm. The resulting broadband emission was attributed to sulphuric acid and was similar to the emission spectrum of gaseous SO2.

Several time-integrated and time-resolved experiments have been performed on different sodium-containing dry aerosols in air to investigate the behavior of atomic sodium emission at the yellow doublet after photofragmentation with 193 nm photons provided by an ArF excimer laser. It is shown that sodium chloride, sodium hydroxide, and especially sodium sulfate require different thresholds for the overall process of vaporization, dissociation, and excitation of atomic emission. This can be clearly seen by the experimental fact that (1) the onset of the time-integrated emission signal starts at different laser energies, and (2) the time-resolved shapes of the emission signal during the laser pulse (13 ns) are markedly different in the case of chloride and sulfate aerosols, the former rising faster than the latter. From an analytical point of view, the detection limit for sodium in airborne particles is 0.3 ng of sodium per gram of air (∼ 0.017 ppbv), which is about one order of magnitude better than the detection limit reported by early laser-induced plasma experiments.