Adsorption of gas-phase nitric acid to n-hexane soot:: Thermodynamics and mechanism

The adsorption of gas-phase nitric acid on n-hexane soot was measured as a function of temperature, relative humidity (RH), and nitric acid partial pressure in a coated-wall flow tube coupled to an electron-impact mass spectrometer. The specific surface area (SSA) of the soot, determined from the BET isotherm of Kr at 77 K for each sample, was 88-372 times larger than the geometric surface area. The SSA increased linearly with soot mass for thin samples but saturated at high mass. For the most part, the nitric acid adsorption was reversible in the submonolayer regime, and no indication of HONO formation was observed. The uptake increased with decreasing temperature and, for surface coverages between 10(12) and 10(13) molecules cm(-2), the average enthalpy of adsorption was -55.8 +/- 7.7 kJ mol(-1). A Langmuir-Freundlich model with the heterogeneity parameter equal to 0.5 closely describes the uptake data, implying either that the surface sites are energetically heterogeneous or that nitric acid is dissociating on the surface. The nitric acid adsorption isotherms were independent of relative humidity up to 80% RH at 243 K. Exposure to ppm levels of ozone for about I h had no effect on the adsorption. The atmospheric implications of this work are discussed.