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Lasers & Sources

Laser-induced water condensation in the atmosphere

Self-guided high-intensity filaments generated by ultrashort laser pulses can initiate water condensation and aerosol particle growth, even at less than 100% relative humidity.
16 January 2012, SPIE Newsroom. DOI: 10.1117/2.1201201.003996

Among atmospheric applications of lasers, laser-induced water condensation (LIC) is a rapidly emerging field driven by potential incentives for precipitation control. It was recently proposed1 as an alternative to cloud seeding with small particles of carbonic ice, silver iodide (AgI), or salts.2, 3 LIC relies on self-guided filaments4,5 generated by ultrashort laser pulses. Laser filaments result from a dynamic balance between intensity-dependent positive (self-focusing) and negative (defocusing) nonlinear contributions to the refractive index.4–7 The filaments have a typical intensity8 of 5×1013W/cm2 at kilometer-range distances,9 leaving strings of weakly ionized plasma (1015–1016cm−3) behind them. Furthermore, they can propagate through adverse conditions such as turbulent air10 or fog.11 These properties indicate possible atmospheric applications of filaments, which motivated the construction of the first mobile femtosecond terawatt laser, Teramobile.12

The ionization of air by the filaments suggests that they can trigger the nucleation of water droplets, by analogy with the Wilson chamber,13 or cloud chamber, in which ionizing particles are visualized by the trail of water droplets they leave behind them while propagating through an atmosphere saturated with humidity. Condensation was indeed observed when a Wilson chamber saturated with humidity was illuminated.1, 14 This effect was even visible with naked eye, as shown in Figure 1. More surprisingly, filaments can facilitate the formation of aerosol particles having sizes up to 10μm or larger, even well below saturation, down to 70% relative humidity (RH), as shown in Figure 2.1,15

Figure 1. Femtosecond-laser-induced condensation of water droplets in a Wilson chamber. The haze generated by the droplets scatters light from a green illumination laser.15

At such low RH, the charge-assisted condensation process active in the Wilson chamber is inefficient and cannot explain the observed effect. To gain more insight into the mechanism active in atmospheric conditions, we investigated LIC in the real atmosphere during an 8-month experimental campaign in Geneva using the Teramobile laser. We observed massive generation of both nanometer-scale particles (up to 2×107cm−3 for each laser shot within the volume swept by the laser filaments) and micrometer-scale particles (up to 10μm, 105cm−3) and characterized them over a broad range of temperature (2–36°C) and RH values (70–100%).15 In addition, we monitored trace gas concentrations, confirming previous indications of the production of ozone and NO2 concentrations in the parts per million range in the filaments.16 This data shows that the main LIC process in the atmosphere is photochemical and relies on the generation of highly hygroscopic species such as HNO3, which condense together with the water and stabilize the resulting binary particles.

This model can explain the observed particle density, its dependence on temperature and humidity, the particle diameters, and their very rapid growth within a few seconds.17 Other processes may also contribute, including the production of H2SO4 by laser oxidation of the background atmospheric SO2,18 charge-accelerated coagulation of particles less than 100nm in size, photochemistry involving excited radicals and ions, or even the formation of water–oxygen clusters, as proposed by Byers Brown.19 The identification and quantification of these alternative processes constitutes the main challenge ahead for the understanding of LIC.

Figure 2. Laser-filament-induced condensation at 75% relative humidity and 13°C. The measurements were performed by alternating 2min with the laser and 2min under reference conditions. (a) Nanoparticles (25–300nm). (b) Aerodynamic particles larger than 10μm (PM 10).15

A second challenge is scaling up these results to macroscopic volumes in the atmosphere. We launched 3J, 100TW pulses from the DRACO laser at Forschungszentrum Dresden-Rossendorf into a cloud chamber and measuring the yield in laser-generated particles. Surprisingly, we found that this yield scaled up faster than linearly with the incident laser intensity, with an exponent lying between 5 and 8. The fifth- and eighth-order processes may be identified as multiphoton dissociation of oxygen (the root cause of ozone production) and its ionization, respectively.19, 20 This nonlinear scaling could offer a method of producing laser-induced condensation over macroscopic scales and demonstrates the potential of high-energy lasers for that purpose.

The applicability of these results to the modulation of precipitation, however, is still highly questionable and requires further investigation. One may speculate that the laser-induced particles may serve as condensation nuclei and further condense water if the laser-activated air mass encounters appropriate atmospheric conditions, in particular, sufficient humidity. Conversely, excess condensation nuclei may compete for the available water molecules, preventing any of the resulting particles from reaching sufficient size to precipitate. On the other hand, LIC may provide a tool to remotely measure atmospheric conditions, especially the condensability of the atmosphere, in a pump-probe setup. The size distribution and density of the laser-generated aerosols depend on the local temperature and humidity. Atmospheric characterization would therefore be obtained by sounding these aerosols using a subsequent probe pulse, in a light detection and ranging configuration.

Laser-induced condensation therefore presents many challenges in both fundamental and applied science. The rapid growth of the community working on this subject21 illustrates the wide interest attracted by these stimulating challenges.

Jérôme Kasparian, Jean-Pierre Wolf
Group of Applied Physics–Biophotonics
University of Geneva
Geneva, Switzerland

Jérôme Kasparian is a research scientist. He is responsible for studies on the filamentation of ultrashort laser pulses and its atmospheric applications such as lightning control and laser-induced condensation, in particular within the Teramobile project, which he leads.

Jean-Pierre Wolf is a professor of physics and head of the Group of Applied Physics–Biophotonics. He is a specialist in ultrafast physics, including coherent control, laser filamentation, and atmospheric remote sensing and control.

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