K. Weber

Field measurements suggest the mechanism of laser-assisted water condensation

Because of the potential impact on agriculture and other key human activities, efforts have been dedicated to the local control of precipitation. The most common approach consists of dispersing small particles of dry ice, silver iodide, or other salts in the atmosphere. Here we show, using field experiments conducted under various atmospheric conditions, that laser filaments can induce water condensation and fast droplet growth up to several μm in diameter in the atmosphere as soon as the relative humidity exceeds 70%. We propose that this effect relies mainly on photochemical formation of p.p.m.-range concentrations of hygroscopic HNO3, allowing efficient binary HNO3–H2O condensation in the laser filaments. Thermodynamic, as well as kinetic, numerical modelling based on this scenario semiquantitatively reproduces the experimental results, suggesting that particle stabilization by HNO3 has a substantial role in the laser-induced condensation.

Laser-induced condensation by ultrashort laser pulses at 248 nm

We compare laser-induced condensation by UV laser pulses of femtosecond, sub-picosecond, and nanosecond duration between each other, as well as with respect to near-infrared (NIR) (800 nm) ultrashort laser pulses. Particle nucleation by UV pulses is so efficient that their growth beyond several hundreds of nm is limited by the local concentration of water vapour molecules. Furthermore, we evidence a dual mechanism: While condensation induced by ultrashort UV pulses rely on nitrogen photo-oxidative chemistry like in the NIR, nanosecond laser-induced condensation occurs without NO2 production, evidencing the domination of a mechanism distinct from that previously identified in the femtosecond regime.

Multijoule scaling of laser-induced condensation in air

Using 100 TW laser pulses, we demonstrate that laser-induced nanometric particle generation in air increases much faster than the beam-averaged incident intensity. This increase is due to a contribution from the photon bath, which adds up with the previously identified one from the filaments and becomes dominant above 550 GW/cm2. It appears related to ozone formation via multiphotondissociation of the oxygen molecules and demonstrates the critical need for further increasing the laser energy in view of macroscopic effects in laser-induced condensation.

Application of FTIR spectroscopy to open-path measurements at industrial sites in Germany

Emissions of air pollutants from industrial sources are of major public concern in Germany. Remarkable efforts have been made to control and reduce there emissions. A strong reduction of these so-called classical air pollutants like e.g. SO2 out of channeled industrial sources could be recorded throughout the last years. However, at some industrial sites there are still several measurement problems that cannot always be solved appropriately by conventional measurement systems. For example, screening of fugitive emissions or sudden releases out of leakages is difficult to monitor correctly by conventional point sensors. In these cases the open-path FTIR method can show up with several advantages because of its measurement principle. In this paper, different open-path measurements at industrial sites are presented, including measurements of sudden releases of ammonia and measurements at and above an olefin production plant.

Ground-based remote sensing of gas emissions from Teide volcano (Tenerife, Canary Islands, Spain): first results

Teide volcano, the Spanish highest peak (3716 m), is located in the central part of Tenerife, Canary Islands. An anomalous seismic activity was detected in and around Tenerife Island since middle of 2001 reaching a peak of seismicity in may 2004 by the National Seismic Network (IGN). Volcanic gases can provide a valuable information about this volcanic unrest at Tenerife; therefore, measurements of major and some minor volcanic gas components emitted by the fumarolic activity at the summit crater of Teide volcano were performed by means of optical remote sensing on September 2005. During this measurement campaign an UV-DOAS system and three op-TDL systems were used to monitor major and minor volcanic gas components in the open atmosphere inside the summit crater. The UV-DOAS system was used for monitoring SO2 and CS2, while three TDL-systems were used for monitoring CO2, H2S and HF. It could be demonstrated successfully, that the optical remote sensing instruments used (UV-DOAS and op-TDLs) were very appropriate to monitor continuously these volcanic gas components. In addition CO2 flux measurements were performed at the summit crater of Teide volcano by means of well established methods. By multiplying CO2 flux data by (gas)i/CO2 ratios provided by optical remote sensing measurements, it is possible to estimate flux values for several detected volcanic gas components during this measurement campaign.

Open-path FTIR measurements at an urban area in Germany

The open-path FTIR (op-FTIR) measurement method has several characteristics, which make the method attractive for monitoring air pollutants in urban areas. In this paper recent op-FTIR measurements at an urban site in Germany are presented. These measurements include the first direct intercomparisons between op-FTIR measurements and an official measurement system of the Environmental State Agency in North- Rhine-Westphalia. These intercomparisons revealed very good results.