A new instrument for kinetics and branching ratio studies of gas phase collisional processes at very low temperatures.

A new instrument dedicated to the kinetic study of low-temperature gas phase neutral-neutral reactions, including clustering processes, is presented. It combines a supersonic flow reactor with vacuum ultra-violet synchrotron photoionization time-of-flight mass spectrometry. A photoion-photoelectron coincidence detection scheme has been adopted to optimize the particle counting efficiency. The characteristics of the instrument are detailed along with its capabilities illustrated through a few results obtained at low temperatures (<100 K) including a photoionization spectrum of n-butane, the detection of formic acid dimer formation, and the observation of diacetylene molecules formed by the reaction between the C2H radical and C2H2.

How measurements of rate coefficients at low temperature increase the predictivity of photochemical models of Titan's atmosphere.

The predictivity of photochemical models of Titan's atmosphere depends strongly on the precision and accuracy of reaction rates. For many reactions, large uncertainty results from the extrapolation of rate laws to low temperatures. A few reactions have been measured directly at temperatures relevant to Titan's atmosphere. In the present study, we observed the consequences of the reduced uncertainty attributed to these reactions. The global predictivity of the model was improved, i.e., most species are predicted with lower uncertainty factors. Nevertheless, high uncertainty factors are still observed, and a new list of key reactions has been established.

Effects of atmospheric pollution on a Lichenophagous lepidopteran.

Terrestrial epiphytic algae were used to absorb atmospheric pollutants and this material was then fed to larvae of a bagmoth to assess the toxicological effects of the pollutants. Samples of algae were amended with pesticides to simulate agricultural spray drift and with vehicle exhaust gases to simulate road traffic emissions. Larval mortality, immobilization, and feeding were used as the toxicological endpoints. Feeding was the most sensitive of these, but clear dose-response relationships were observed with all three endpoints. The effects of atmospheric pollution on an impacted environment were then investigated using these organisms. Algal samples were collected along a transect that included an area of arable farming, a motorway, and a woodland. Feeding was reduced when the larvae were fed algae from the areas proximal to the arable land and to the motorway. It is suggested that pesticide spray drift and vehicle exhaust emissions may have been the causal agents for these effects.

Gas-phase reactions and energy transfer at very low temperatures.

Experimental studies of gas-phase chemical reactions and molecular energy transfer at very low temperatures and between electrically neutral species are reviewed. Although work of collisionally induced vibrational and rotational transfer is described, emphasis is placed on very recent results on the rates of free radical reactions obtained by applying the pulsed laser photolysis (PLP)-laser-induced fluorescence (LIF) technique in a CRESU (Cinétique de Réactions en Ecoulement Supersonique Uniforme) apparatus at temperatures as low as 13 K. These measurements demonstrate that quite a wide variety of reactions-including those between two radicals, those between radicals and unsaturated molecules, and even some of those between radicals and saturated molecules-remain rapid at very low temperatures. Theoretical efforts to explain some of these results are described, as is their impact on attempts to model the synthesis of molecules in interstellar clouds.