Heterogeneous chemistry of individual mineral dust particles with nitric acid: A combined CCSEM/EDX, ESEM, and ICP-MS study

The heterogeneous chemistry of individual dust particles from four authentic dust samples with gas phase nitric acid was investigated in this combined computer‐controlled scanning electron microscopy with energy‐dispersive X‐ray (CCSEM/EDX) analysis, environmental scanning electron microscopy (ESEM), and inductively coupled plasma mass spectrometry (ICP‐MS) study. Morphology and compositional changes of individual particles as they react with nitric acid were observed using conventional scanning electron microscopy with energy‐dispersive X‐ray analysis and CCSEM/EDX. 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. In particular, 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 dry mineral dust into an aqueous liquid aerosol has a tremendous impact on its light scattering properties, ability to modify clouds, heterogeneous chemistry, and gas to particle partitioning in the atmosphere. ICP‐MS was used to estimate maximal extent of possible CaCO3‐to‐Ca(NO3)2 conversion in the mineral dust samples. We show that in China loess dust and Saudi coastal dust aerosols, formation of calcium nitrate particles can take place at the total extent as high as ∼20–40 wt %.