Molecular understanding of sulphuric acid-amine particle nucleation in the atmosphere

4 pages 359-363 in the print version, additional 7 pages online.Peer reviewe

Effect of radiation defects on the early stages of nanoindentation tests in bcc Fe and Fe-Cr alloys

Classical molecular dynamics (MD) simulations of nanoindentation tests are performed in defect-free at 300, 600 and 1000 K and defected crystals at 300 and 600 K of pure Fe and Fe-9Cr alloy. The aim of the work is to evaluate the effect of the presence of various defects specific to neutron/heavy ion irradiation (i.e., voids, dislocation loops and Cr precipitates) on the material response during the early stages of nanoindentation (i.e., at indentation depths up to 20 angstrom). We establish that even a relatively large density of the radiation defects (similar to 10(24) m(-3)) does not induce any significant change in the material response, i.e., it is not unambiguously detectable at the force-depth curves neither in pure Fe nor in Fe-9Cr alloys. The macroscopic parameters, which can be derived from these curves, such as hardness and reduced modulus, calculated in the crystals with and without radiation defects also cannot clearly reveal the contribution from the presence of the radiation defects given the resulting uncertainty of their extraction. However, several distinct features typical for nanoindentation tests observed in crystals with radiation defects only were identified, such as a) obstruction of emission of dislocation loops under the indenter during loading for crystals with precipitates and dislocation loops, and b) special residual imprint pattern for crystals with dislocation loops. The results of this work provide useful data for the parameterization and validation of the higher-scale methods, such as dislocation dynamics and (crystal plasticity-) finite-element method. The details of the transferability of MD results to these methods are also discussed.Peer reviewe

Oxidation Products of Biogenic Emissions Contribute to Nucleation of Atmospheric Particles

Atmospheric new-particle formation affects climate and is one of the least understood atmospheric aerosol processes. The complexity and variability of the atmosphere has hindered elucidation of the fundamental mechanism of new-particle formation from gaseous precursors. We show, in experiments performed with the CLOUD (Cosmics Leaving Outdoor Droplets) chamber at CERN, that sulfuric acid and oxidized organic vapors at atmospheric concentrations reproduce particle nucleation rates observed in the lower atmosphere. The experiments reveal a nucleation mechanism involving the formation of clusters containing sulfuric acid and oxidized organic molecules from the very first step. Inclusion of this mechanism in a global aerosol model yields a photochemically and biologically driven seasonal cycle of particle concentrations in the continental boundary layer, in good agreement with observations

Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol nucleation

Atmospheric aerosols exert an important influence on climate1 through their effects on stratiform cloud albedo and lifetime2 and the invigoration of convective storms3. Model calculations suggest that almost half of the global cloud condensation nuclei in the atmospheric boundary layer may originate from the nucleation of aerosols from trace condensable vapours4, although the sensitivity of the number of cloud condensation nuclei to changes of nucleation rate may be small5, 6. Despite extensive research, fundamental questions remain about the nucleation rate of sulphuric acid particles and the mechanisms responsible, including the roles of galactic cosmic rays and other chemical species such as ammonia7. Here we present the first results from the CLOUD experiment at CERN. We find that atmospherically relevant ammonia mixing ratios of 100 parts per trillion by volume, or less, increase the nucleation rate of sulphuric acid particles more than 100–1,000-fold. Time-resolved molecular measurements reveal that nucleation proceeds by a base-stabilization mechanism involving the stepwise accretion of ammonia molecules. Ions increase the nucleation rate by an additional factor of between two and more than ten at ground-level galactic-cosmic-ray intensities, provided that the nucleation rate lies below the limiting ion-pair production rate. We find that ion-induced binary nucleation of H2SO4–H2O can occur in the mid-troposphere but is negligible in the boundary layer. However, even with the large enhancements in rate due to ammonia and ions, atmospheric concentrations of ammonia and sulphuric acid are insufficient to account for observed boundary-layer nucleation