SOME NEW LABORATORY APPROACHES TO STUDYING TROPOSPHERIC HETEROGENEOUSREACTIONS

Assessing the potential for global climate change requires a detailed understanding of the fundamental chemical and physical processes contr olling the concentrations of key gases as well as particles in the atm osphere. Laboratory studies are used to obtain the basic kinetic and m echanistic data needed for inputs to models as well as for interpretin g field observations. While gas-phase reactions are reasonably well-un derstood, ''heterogeneous'' processes involving gases and solids are n ot. We briefly describe applications of three approaches to laboratory studies of heterogeneous atmospheric reactions which have not been wi dely used for this purpose in the past: diffuse reflectance infrared F ourier transform spectrometry (DRIFTS), transmission electron microsco py with energy dispersive X-ray spectroscopy (TEM-EDS), and X-ray phot oelectron spectroscopy (XPS). The application of these techniques to s tudying the reactions of the oxides of nitrogen with NaCl and NaBr fou nd in sea-salt particles is described and used to illustrate their uti lity in obtaining both kinetic and mechanistic data. The reaction of N O2 with NaBr is found to be approximately second order in NO3, suggest ing that the dimer N2O4 may be the reacting species. If this is the ca se, a preliminary value for the reaction probability for the N2O4-NaBr reaction at 298 K is 2 x 10(-4) with an uncertainty of a factor of th ree. That for the HNO3-NaCl reaction was found using XPS to be (4 +/- 2) x 10(-4). The kinetic data from these studies indicate that the NO2 , reaction is too slow to be competitive with the N2O5 and HNO3 reacti ons. Mechanistically, both the DRIFTS and TEM-EDS studies show that wa ter vapor even at relative humidities well below the deliquescence poi nt causes a selective recrystallization of surface nitrate into microc rystallites of NaNO3, regenerating a fresh salt surface. This may expl ain held observations of some marine particles which are essentially t otally devoid of chloride ions.