Some new laboratory approaches to studying tropospheric heterogeneous reactions

Assessing the potential for global climate change requires a detailed understanding of the fundamental chemical and physical processes controlling the concentrations of key gases as well as particles in the atmosphere. Laboratory studies are used to obtain the basic kinetic and mechanistic data needed for inputs to models as well as for interpreting field observations. While gas-phase reactions are reasonably well-understood, “heterogeneous” processes involving gases and solids are not. We briefly describe applications of three approaches to laboratory studies of heterogeneous atmospheric reactions which have not been widely used for this purpose in the past: diffuse reflectance infrared Fourier transform spectrometry (DRIFTS), transmission electron microscopy with energy dispersive X-ray spectroscopy (TEM-EDS), and X-ray photoelectron spectroscopy (XPS). The application of these techniques to studying the reactions of the oxides of nitrogen with NaCl and NaBr found in sea-salt particles is described and used to illustrate their utility in obtaining both kinetic and mechanistic data. The reaction of NO2 with NaBr is found to be approximately second order in NO2, suggesting that the dimer N2O4 may be the reacting species. If this is the case, a preliminary value for the reaction probability for the N2O4---NaBr reaction at 298 K is 2 × 10−4 with an uncertainty of a factor of three. That for the HNO3---NaCl reaction was found using XPS to be (4 ± 2) × 10−4. The kinetic data from these studies indicate that the NO2 reaction is too slow to be competitive with the N2O5 and HNO3 reactions. Mechanistically, both the DRIFTS and TEM-EDS studies show that water vapor even at relative humidities well below the deliquescence point causes a selective recrystallization of surface nitrate into microcrystallites of NaNO3, regenerating a fresh salt surface. This may explain field observations of some marine particles which are essentially totally devoid of chloride ions.