Observing HNO₃ release dependent upon metal complexes in malonic acid/nitrate droplets

Although the dicarboxylic acid has been reported to react with nitrate for aged internally mixed aerosols in atmosphere, the quantitative nitrate depletion dependent upon composition in particles is still not well constrained. The chemical composition evolutions for malonic acid/sodium nitrate (MA/SN), malonic acid/magnesium nitrate (MA/MN) and malonic acid/calcium nitrate (MA/CN) particles with the organic to inorganic molar ratio (OIR) of 1:1 are investigated by vacuum Fourier transform infrared spectroscopy (FTIR). Upon dehydration, the intensity of the asymmetric stretching mode of COO^– group (ν_as-COO⁻) increases, accompanying the decrease in OH feather band and COOH band and NO₃ ⁻ band. These band changes suggest malonate salts formation and HNO₃ release. The quantitative NO₃ ⁻ depletion data shows that the reactivity of MA-MN is most and that of MA-SN is least. Analysis of the stretching mode of COO^– indicates the different bond type between metal cation and carboxylate anion. In addition, water content in particles decreases at the constant RH, implying water loss with the chemical reaction. When the RH changes very quickly, water uptake delay during the humidification process reveals that water mass transport is limited below 37% RH.