Confocal Raman imaging and atomic force microscopy of the surface reaction of NO 2 and NaCl(100) under humidity

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 NO 2 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 NO 2 generates surface conversion to NaNO 3 monolayer capping the NaCl(100) surface and releases NOCl. The subsequent exposure of this NaNO 3 layer to RH below ∼45% induces the formation of rare NaNO 3 tetrahedral crystals less than 0.5 µm in size. The crystallization occurs through two‐dimensional NO 3 − migration under the H 2 O monolayer regime. After another subsequent exposure to RH above 45% and below 75%, supermicrometric NaNO 3 rhombohedral plates were obtained under the H 2 O multilayer regime. On the other hand, the simultaneous exposure of NaCl(100) to NO 2 and H 2 O below ∼45% RH rapidly generates numerous submicrometric NaNO 3 tetrahedra on the NaCl(100) surface. The dramatic increase of NaNO 3 production in the presence of water vapour is explained by the formation of HNO 3 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 NO 2 /H 2 O mixtures under RH above 75% induces the coexistence of both solid‐state NaNO 3 and dissolved NO 3 − in droplets. Copyright © 2008 John Wiley & Sons, Ltd.