Uptake of nitric acid on NaCl single crystals measured by backscattering spectrometry

Abstract The uptake of nitric acid in sea salt aerosols via the reaction HNO 3 (g) + NaCl (c) → NaNO 3 (c) + HCl (g) is a main contributor to the chlorine balance in the troposphere, especially in polluted, coastal regions. Backscattering spectrometry was used to measure oxygen concentration profiles on the surface of NaCl single crystals (1 0 0) after exposure to a nitric acid vapor pressure of 10 −3  torr and different relative humidities at room temperature. Comparison with the chlorine signal leads to the conclusion that the replacement of chlorine by nitrate is the only significant chemical reaction that occurs. The relative humidity is revealed to be a crucial parameter, because it determines the kinetics of the uptake. At 65% RH, the nitrate concentration increases almost linearly with time and shows no saturation at all. At 20% RH, it shows square root dependence of time implying diffusion control.