Fig. 1. Schematic diagram of sea salt aerosol spectral detection system based on femtosecond laser filamentation. (a) The components include L, laser; M1, plane mirror; CL, concave lens (f = −150 mm); M2, dichroic mirror; F, fiber-optic probe; CM, concave mirror (f = 200 cm); O, oscilloscope; Mo, monochromator; P, PMT (photomultiplier tube); Le, convex lens (f = 5 cm); UR, ultrasound receiver; Mic, microphone; S, spectrometer; C, Istar-sCMOS camera; AG, aerosol generator. (b) Characterization of the filament by ultrasound signal. The red line represents 3 times the standard deviation of the blank sample (3σ). (c) Fluorescence signal at 337 nm, emitted after nitrogen is excited by the filament, is used to characterize the filament. (d) The pulse duration measured using a FROG from Femtoeasy (FWHM: 496.9 fs).

Fig. 2. Optimizing the interaction length between sea salt aerosol and the filament. (a) A diagram illustrating the variation of the interaction length; (b) emission spectrum (pseudo-color plot, see lines 171–174 of the Supplementary Material for detailed spectra) of the Na element under different interaction lengths of aerosol and the filament; (c) normalized integrated intensity values of Na element emission spectrum. The blue spheres represent the experimental measurements, while the red curve displays the data calculated using a nonlinear equation and Mie scattering theory.

Fig. 3. Detection limit of the Na element in sea salt aerosol at a 30 m detection distance. (a) Fluorescence spectrum (pseudo-color plot; see lines 175–178 of the Supplementary Material for detailed spectra) of the Na element under various concentrations of sea salt aerosols; (b) plot of the integrated fluorescence intensity of the Na element as a function of sea salt aerosol concentration. The orange sphere represents the limit of detection. (To highlight, the error bars are magnified by 5 times).