Ice Nucleation Enhancement in Ambient Aerosols Under High‐Salinity Conditions: Insights From Cloud Chamber Experiments

Ice‐nucleating particles (INPs) play a role in mixed‐phase clouds, yet the processes controlling their properties remain poorly understood. Water‐soluble salts such as sodium chloride (NaCl) depress the freezing temperature of cloud droplets. However, using cloud‐chamber‐based optical particle measurements after NaCl injection, we show that high NaCl concentrations can enhance, rather than suppress, the ice‐nucleating ability of ambient aerosols, resulting in ice forming at temperatures where untreated ambient aerosols were inactive. Experiments using only inorganic or mineral aerosols did not reproduce this behavior. Importantly, the enhancement observed with ambient aerosols occurred several minutes after NaCl addition, suggesting that hypersaline exposure may modify unidentified salt‐sensitive constituents in ambient aerosols. Such conditions are atmospherically plausible during sea‐spray evaporation and saline aerosol processing. Our findings provide the first experimental evidence that NaCl addition can enhance the ice‐nucleating ability of ambient aerosols, suggesting that hypersaline exposure may be a previously unrecognized pathway influencing mixed‐phase clouds.