Impact of ambient relative humidity and acidity on chemical composition evolution for malonic acid/calcium nitrate mixed particles

The chemical compositions in atmospheric aerosols, which often evolve with environmental factors, have significant impact on climate and human health, while our fundamental understanding of chemical process is limited owing to their sensitive to atmospheric conditions. pH and RH are critical chemical factors of aerosols, impacting reaction pathways and kinetics that ultimately govern final components in particles. Herein, we monitored the chemical composition in internally mixed malonic acid/calcium nitrate with the mole ratio of 1:1 as a function of pH and relative humidity (RH). At 30% RH, lower than efflorescence relative humidity (ERH) of pure malonic acid aerosols, malonic acid still exhibits solution feature reflected by IR spectra, which was observed to transform to malonate, along with water loss and nitrate depletion. At another RH of 54% and 80%, the similar chemical process happened with less reaction rate. The response of chemical reaction between malonic acid and calcium nitrate to pH was studied by manipulating the starting pH of the bulk solution through dropping aqueous sodium hydroxide. Due to lower H⁺ concentration at higher pH, the formation and liberation of HNO₃ slow down, as well as water loss. After a down-up RH cycle, the water loss was obvious and grew with the decrease in pH. These measurements are improving our understanding of chemical composition evolution dependent upon pH and RH from a fundamental physical chemistry perspective and are critical for connecting chemistry and climate.