Heterogeneous Chemistry of Individual Mineral Dust Particles with Nitric Acid. Laboratory and Field Observations.

This study presents laboratory and field evidence of complete, irreversible processing of solid calcium carbonate (calcite)-containing particles and quantitative formation of liquid calcium nitrate particles apparently as result of heterogeneous reaction of calcite with nitric acid. The heterogeneous chemistry of individual dust particles from four authentic dust samples with gas-phase nitric acid is presented. Morphology and compositional changes of individual particles as they react with nitric acid were observed using conventional scanning electron microscopy with energy dispersive analysis of X-rays (SEM/EDX) and computer controlled SEM/EDX. Environmental Scanning Electron Microscopy (ESEM) was utilized to investigate the hygroscopic behavior of mineral dust particles reacted with nitric acid. Differences in the reactivity of mineral dust particles from these four different dust source regions with nitric acid were observed. Mineral dust from source regions containing high levels of calcium, namely China loess dust and Saudi coastal dust, were found to react to the greatest extent. Formation of nitrates from individual calcite particles was also followed as a function of time in field aerosol samples collected at Shoresh, Israel. We show that calcium carbonate rich dust aerosols may exhibit continuous, extensive reactivity with nitric acid resulting in formation of highly hygroscopic calcium nitrate particles. Transformation of hygroscopically inert dry mineral dust particles into hygroscopic wet aerosol may have substantial impacts on the light scattering properties, the ability to modify clouds and heterogeneous chemistry.