Synergistic effect in the humidifying process of atmospheric relevant calcium nitrate, calcite and oxalic acid mixtures

Hygroscopicity is a critical property for evaluating aerosol’s environmental and climate impacts. Traditional view always considered hygroscopic behavior of aerosol as a water adsorption/absorption–desorption cycle in which the size, water content and morphology etc. are well characterized. However, the chemical reactions between coexisting components in mixed particles during humidifying-dehumidifying process are almost neglected. In this study, we found that there exists synergistic effect among Ca(NO3)2, CaCO3 and H2C2O4 mixtures during humidifying process. Substitution of strong acid (HNO3) by medium acid (H2C2O4) take place during vapor absorption on Ca(NO3)2/H2C2O4 mixture. Moreover, the presence of nitrate exhibits a promotive effect to the reaction between H2C2O4 and CaCO3 under ambient condition. These results provoke us to rethink the hygroscopic behavior of mixed aerosol in which chemical reaction can greatly change the chemical composition, mixing state and consequently environmental and climate impacts.