Adsorption of Gas-Phase Nitric Acid ton-Hexane Soot:  Thermodynamics and Mechanism

Abstract

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 1 h had no effect on the adsorption. The atmospheric implications of this work are discussed.

Heterogeneous conversion of NO<sub>2</sub> and NO on HNO<sub>3</sub> treated soot surfaces: atmospheric implications

“…However, the influence of adsorbed HNO 3 on Reaction (1) is still unclear. The conclusion, that Reaction (1) cannot account for atmospheric HONO formation is in good agreement with a model study in which the deactivation of the soot surface was taken into consideration (Aumont et al, 1999). Only in the case that a fast reactivation of the soot surface was proposed (Aumont et al, 1999), yet not observed in the laboratory, or when the well documented deactivation was ignored (Kotamarthi et al, 2001;Lary et al, 1999) a significant influence on the atmosphere was calculated.…”

“…The formation of HONO in the reaction of HNO 3 on soot surfaces was also proposed in a model study by Lary et al (1999) and was observed in the reaction of soot with liquid HNO 3 solutions (Louisnard et al, 1995). However, in the more recent study of Aubin and Abbatt (2003) no formation of HONO was observed, although the authors could not determine the upper limit for the HONO yield, because of the missing calibration of their mass spectrometer. Although more recent studies point to the fact that the discrepancies of the modelled and measured NO x /HNO 3 ratio cannot be explained by Reaction (2) (e.g.…”

“…These calculations were based on the first study of Reaction (2) by Rogaski et al (1997), who investigated the reaction at high concentrations of HNO 3 and reported large uptake coefficients and the formation of NO and NO 2 . However, in more recent studies, performed at reactant concentrations closer to atmospheric conditions, only a reversible uptake of HNO 3 (Aubin and Abbatt, 2003;Choi and Leu, 1998;Longfellow et al, 2000) or the absence of any gas phase products (Saathoff et al, 2001;Seisel et al, 2000) was observed. The difference was explained by a surface reaction which takes place only at high HNO 3 concentrations for multilayer coverage of adsorbed HNO 3 (Choi and Leu, 1998).…”

“…Unfortunately, these model studies were based either on laboratory data determined under experimental conditions not prevailing in the atmosphere, i.e. very high reactant concentrations, or these studies did not take into consideration deactivation processes of the soot surface, which have been observed in laboratory studies, leading to unrealistic conclusions (Aumont et al, 1999).…”

Reaction Dynamics of Phenyl Radicals in Extreme Environments: A Crossed Molecular Beam Study

“…10 Those species are emitted to the atmosphere from natural and anthropogenic sources with an average global emission rate of anthropogenic carbon from fossil fuel combustion as high as 2.4 × 10 10 kg per year. 11 Once liberated into the ambient environment, PAHs and soot in respirable size of 10-100 nm can be transferred into the lungs by inhalation and are strongly implicated in the degradation of human health, 12 particularly due to their high carcinogenic risk potential. PAHs and carbonaceous nanoparticles are also serious water pollutants of marine ecosystems and bioaccumulate in the fatty tissue of living organisms.…”