Influence of stearic acid coating of the NaCl surface on the reactivity with NO2 under humidity

Abstract

In the atmosphere, sea salt aerosols, containing mainly NaCl, can accumulate fatty acids and undergo heterogeneous chemistry with atmospheric nitrogen oxides. The effect of stearic acid (SA) coating on the reactivity of the NaCl(100) surface with NO₂ under humidity was studied by atomic force microscopy (AFM), Raman mapping and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to highlight processes occurring on NaCl surfaces. The vapor-deposition of SA on the NaCl surface generates heterogeneous coating with discontinuous monolayer islands. The SA molecules with all-trans conformation stick to the NaCl surface through –CO₂H groups and are organized in parallel between them and nearly perpendicularly to the surface. The SA coating does not prevent the NaNO₃ particle formation when the sample is exposed to NO₂ under low humidity conditions. The initial abilities of the NaCl surface coated with SA to pick up NO₂ from the gas phase are correlated with the fraction of bare NaCl area evidencing the spatially heterogeneous reactivity of the surface. The role of H₂O in the NO₂ uptake and the catalytic conversion of NaCl to NaNO₃ is shown. Under humidity (RH = 50%), the H₂O uptake by NaNO₃ particles on the coated-NaCl surface is significantly more important than that adsorbed under analogous conditions without the presence of NaNO₃ particles. This unusual water absorption initiates transitions (i) from solid NaNO₃ particles to NaNO₃ aqueous solution and (ii) from the SA monolayer with well-ordered all trans alkyl chains to the SA gel with completely disordered conformation. This mixed SA/NaNO₃ layer on the particle surface may have significant consequences on the hygroscopic properties and reactivity of the sea salt aerosols in the atmosphere.