Deliquescence and efflorescence of potassium salts relevant to biomass-burning aerosol particles

Particles containing potassium salts are common in biomass burning plumes and include compounds such as KCl, KNO3, and K2SO4. We determined the deliquescence and efflorescence relative humidities of single- and multi-component potassium salts using an environmental transmission electron microscope and an environmental scanning electron microscope. KCl particles deliquesced at a relative humidity (RH) of 85% and effloresced at 56%. K2SO4 particles deliquesced at 96% and effloresced at 60%. KNO3 particles grew continuously with increasing humidity. If KNO3 particles were heat-treated at 104C overnight, they deliquesced at an RH above 90%. Atomized solutions of multi-component potassium compounds (KCl KNO3 and KCl K2SO4) formed particles that contained distinct grains of each compound upon drying. Particles formed from the KCl KNO3 mixtures grew continuously with increasing humidity until deliquescence occurred at an RH of 82% and efflorescence occurred at 58%. Particles from the KCl K2SO4 mixture began to take up water at an RH of 84%. They deliquesced at 90% and effloresced at 52%. This work provides new data for mixed aqueous electrolytes that are important for atmospheric science. Efflorescence values for K2SO4, KNO3, and internally mixed particles of K2SO4 and KCl are reported here for the first time.