Confocal Raman imaging and atomic force microscopy of the surface reaction of NO2 and NaCl(100) under humidity

✍ Scribed by S. Scolaro; S. Sobanska; J. Barbillat; J. Laureyns; F. Louis; D. Petitprez; C. Brémard

• Publisher: John Wiley and Sons
• Year: 2009
• Tongue: English
• Weight: 294 KB
• Volume: 40
• Category: Article
• ISSN: 0377-0486
• DOI: 10.1002/jrs.2098

✦ Synopsis

Abstract

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.