Interactions of gaseous nitric acid with surfaces of environmental interest

Gaseous nitric acid removal by surfaces in experimental systems and in the atmospheric boundary layer is rapid. However, neither the form of HNO3 on surfaces nor its impact on the properties of the thin surface film are known. We report here studies of surfaces that have been exposed at room temperature (295 2 K) to gaseous mixtures of water vapor with HNO3 at concentrations from 46 ppb to 4 x 10(3) ppm. The surfaces were probed using a combination of Fourier transform infrared spectrometry (FTIR), non-contact atomic force microscopy (AFM), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and X-ray photoelectron spectroscopy (XPS). Exposure of borosilicate glass, quartz, and thin Teflon films to mixtures of gaseous HNO3 and water vapor leads to the subsequent uptake of much larger amounts of water than occurs on the corresponding unexposed surfaces. Infrared spectra show evidence for the formation of nitric acid-water complexes on the surface that leads to this enhanced water uptake. On borosilicate glass, exposure to the nitric acid-water vapor mixture results in surface segregation of the trace metal oxides and their nitrates formed from reaction with HNO3. The majority of these oxides can be removed by rinsing with water; however, smaller, segregated regions of ZnO remain on the surface. The implications for heterogeneous reactions in thin films on surfaces in laboratory systems and in the atmosphere are discussed.