Probing heterogeneous chemistry of individual atmospheric particles using scanning electron microscopy and energy-dispersive X-ray analysis

In this paper, we demonstrate the utility of single-particle analysis to investigate the chemistry of isolated, individual particles of atmospheric relevance such as NaCl, sea salt, CaCO3, and SiO2. A variety of state-of-the-art scanning electron microscopy techniques, including environmental scanning electron microscopy and computer-controlled scanning electron microscopy/energy-dispersive X-ray analysis, were utilized for monitoring and quantifying phase transitions of individual particles, morphology, and compositional changes of individual particles as they react with nitric acid. Clear differences in reaction mechanisms were observed between SiO2, CaCO3, NaCl, and sea salt particles. SiO2 particles exposed to HNO3 showed no change, indicating that the reaction of SiO2 particles is limited to the particle surface and would not involve bulk atoms in its reactivity. Calcium carbonate was seen to convert to aqueous calcium nitrate droplets while sodium chloride formed microcrystallites of sodium nitrate on top of the particle. Sea salt particles showed morphology changes that could be described as a combination of these as both spherical droplets and microcrystallites were observed. This is consistent with the multicomponent composition of sea salt. Further differences were found in the reaction rates for sea salt and sodium chloride with nitric acid. Sea salt yielded a significant increase in reactivity when compared to the NaCl particles under similar conditions. The reaction of nitric acid with calcium carbonate was found to be strongly enhanced at higher relative humidity.