Confocal Raman imaging and atomic force microscopy of the surface reaction of NO2 and NaCl(100) under humidity
Résumé
Polarized confocal Raman imaging combined with non-contact atomic force microscopy (AFM) was used to study the three-dimensional evolution of the NaCl(100) surface during its reaction with NO2 at low pressure as a function of relative humidity (RH) from 0% to nearly 80%. Sea salt particles containing NaCl as the main constituent are believed to be the major source of reactive tropospheric chlorine and nitrate fallouts. At an RH of 0%, the reaction of dry NO2 generates surface conversion to NaNO3 monolayer capping the NaCl(100) surface and releases NOCl. The subsequent exposure of this NaNO3 layer to RH below 45% induces the formation of rare NaNO3 tetrahedral crystals less than 0.5 µm in size. The crystallization occurs through two-dimensional NO3- migration under the H2O monolayer regime. After another subsequent exposure to RH above 45% and below 75%, supermicrometric NaNO3 rhombohedral plates were obtained under the H2O multilayer regime. On the other hand, the simultaneous exposure of NaCl(100) to NO2 and H2O below 45% RH rapidly generates numerous submicrometric NaNO3 tetrahedra on the NaCl(100) surface. The dramatic increase of NaNO3 production in the presence of water vapour is explained by the formation of HNO3 and its easy reaction with the NaCl(100) surface. For RH above 45% and below 75%, the tetrahedra evolve to rhombohedral plates of supermicrometric size. The exposure of NaCl(100) to NO2/H2O mixtures under RH above 75% induces the coexistence of both solid-state NaNO3 and dissolved NO3- in droplets.