1.4 Conclusion
The filamentation regime, described in the present chapter, consists of the propa-gation of self-confined narrow light channels, that feature, within their core, op-tical intensities large enough to efficiently initiate photo-chemical reactions of air constituents. Although very thin, with a typical diameter of 100 µm, these light structures can reach up to several meters of length, thus providing an attractive tool to remotely irradiate an extended air volume with high power density.
This spectacular phenomenon results from the nonlinear interaction between the propagating light pulse and the air, and thus spontaneously arises as soon as the input average power exceeds a critical threshold, which, at 800 nm, is approxi-mately 4 GW. The choice of this near-IR wavelength is not arbitrary, but rather depends on the available technology for the amplification of ultrashort pulses, which is necessary to attain such high laser powers.
This spectral region is particularly favourable for long-range propagation, as the atmosphere is almost transparent and the molecular scattering is negligible for dis-tances shorter than 30 km. Furthermore, filaments exhibit a surprising robustness against atmospheric turbulence, and can propagate also in adverse meteorological conditions.
For this reasons filamentation offers a promising approach for remotely perform photo-nucleation experiments in the real atmosphere; therefore, this research field, pioneered by the GAP Biophotonics group, relies on this spectacular light propa-gation regime.
Chapter 2
Atmospheric condensation of water vapor: spontaneous and
photo-induced processes
As discussed in in Chapter 1, ultrashort light filaments feature intriguing proper-ties that support their potential use in atmospheric applications. The possibility to generate, at a desired location, an extended volume of air-light interaction at high-intensities yields fascinating perspectives on laser-based weather modula-tion. The multi-photon processes induced by the high electric field in the filament core, indeed, are expected to strongly alter the thermodynamic equilibrium of air constituents.
In this chapter, the main atmospheric processes involving aerosols formation and growth are described, in order to give an overview of the physico-chemical atmo-spheric scenario that will be locally modulated by the laser irradiation.
New particles in the atmosphere ubiquitously form at any location, especially dur-ing daytime, via the nucleation of new clusters out of the gas phase and their subsequent growth. Several mechanisms may lead to a particle formation event, that can be distinguished by the number and the nature of the species involved.
Most of these processes are reviewed in Sec. 2.3 and Sec. 2.4.
Furthermore, aerosols that grow up to 100 nm or more, may potentially provide a condensation substrate for water vapor and other soluble trace gases, leading to a virtually indefinite growth: the particle is thus said to be activated. The activation of aerosols is the first step in the formation of clouds, and depends upon the at-mospheric conditions as well as on the chemical composition of the aerosols and the presence of atmospheric trace gases. The latter can be strongly modified by the laser action. Therefore, the knowledge of cloud physics is extremely useful in order to investigate the effects of laser irradiation; the activation of aerosols and the cloud formation are presented in Sec. 2.5.
photo-induced processes It is beyond the scope of this work, however, to provide an extensive treatment of the thermodynamics of aerosols as well as of the nucleation and activation of atmospheric particles. What follows is rather an overview of these relevant atmo-spheric processes involving aerosols, a comprehensive description of which can be found in Seinfeld and Pandis [6] or Pruppacher and Klett [70].
2.1 Atmospheric aerosols
Aerosols are defined as small particles suspended in air, and have a strong in-fluence on local weather, atmospheric chemistry, and health. They scatter the solar radiation and cool the atmosphere and thus have a direct impact on climate forcing; furthermore, they control cloud formation, their albedo effect and the pre-cipitations on a global scale, which is often referred to as indirect climate forcing.
They can be directly injected into the atmosphere from primary sources, such as combustion reactions, emissions from biological processes, suspension of indus-trial dust etc, or they can result from the gas to particle conversion of vapour species. This processes is referred to asNew particle formation (NPF), and will be discussed in Sec. 2.2. The evidence of such phenomenon has been reported for the first time by Aitken [71] in 1897; the scientific community is currently very active on this research field even if the state-of-the-art of particle monitor-ing devices doesn’t provide access to sizes below∼3 nm, where the direct phase conversion is expected to occur [72]. Therefore, no direct observations of such transition have been achieved yet.
2.1.1 Aerosol modes
An aerosol population is commonly described by its size distribution. As many different processes are responsible for the production of aerosols, typical size dis-tributions of atmospheric particles feature distinct modes, as displayed in Fig. 2.1.
In most areas, the volume or mass distribution is double-peaked (bottom panel):
one corresponds to theaccumulation mode(0.1−2µm), which collects particles from primary emission as well as condensation of trace gases and organics, the other one represents thecoarse mode(2−50µm), which includes particles usu-ally produced by mechanical processes such as erosion.
The upper plot of Fig. 2.1 focuses on the number of aerosols, while the lower one displays their volume. It is interesting to note that, while particles larger than 0.1 µm basically constitute all the aerosol population mass, they are negligible in number as compared to smaller particles that yield, conversely, a minor contri-bution to the total mass. By plotting the size districontri-bution of the aerosol number
2.1 Atmospheric aerosols
Figure 2.1: Size and volume distribution modes of atmospheric aerosols. Reprint from [6].
concentration, two main modes emerge: the nucleation mode (clusters smaller than 10 nm) where particles directly created in situ out of the gas phase accumu-late, and theAitken mode(10−100 nm) which corresponds to the accumulation mode of the mass distribution.
It is common to refer to particles smaller than 100 nm, between 100 nm and 2.5µm and larger than 2.5µm as, respectively,ultrafine,fineandcoarseparticles.
photo-induced processes