Formation of highly porous aerosol particles by atmospheric freeze-drying in ice clouds

The cycling of atmospheric aerosols through clouds can change their chemical and physical properties and thus modify how aerosols affect cloud microphysics and, subsequently, precipitation and climate. Current knowledge about aerosol processing by clouds is rather limited to chemical reactions within water droplets in warm low-altitude clouds. However, in cold high-altitude cirrus clouds and anvils of high convective clouds in the tropics and midlatitudes, humidified aerosols freeze to form ice, which upon exposure to subsaturation conditions with respect to ice can sublimate, leaving behind residual modified aerosols. This freeze-drying process can occur in various types of clouds. Here we simulate an atmospheric freeze-drying cycle of aerosols in laboratory experiments using proxies for atmospheric aerosols. We find that aerosols that contain organic material that undergo such a process can form highly porous aerosol particles with a larger diameter and a lower density than the initial homogeneous aerosol. We attribute this morphology change to phase separation upon freezing followed by a glass transition of the organic material that can preserve a porous structure after ice sublimation. A porous structure may explain the previously observed enhancement in ice nucleation efficiency of glassy organic particles. We find that highly porous aerosol particles scatter solar light less efficiently than nonporous aerosol particles. Using a combination of satellite and radiosonde data, we show that highly porous aerosol formation can readily occur in highly convective clouds, which are widespread in the tropics and midlatitudes. These observations may have implications for subsequent cloud formation cycles and aerosol albedo near cloud edges.Aerosol particles are modified and age during their atmospheric lifetime, resulting in changes in their optical and hygroscopic properties that subsequently determine the aerosols’ radiative characteristics and their ability to affect cloud properties via nucleation of ice particles and water droplet formation. Outside of clouds, aerosol particles undergo changes in size, composition, volatility, and concentration following gas–particle partitioning, oxidative aging induced by photochemistry, and heterogeneous uptake of gas-phase reactive trace gases (1). Varying conditions of temperature and ambient relative humidity (RH) affect the aerosols’ hygroscopic growth and phase state, further changing their properties (2, 3).In haze, fog, and cloud droplets, aerosols can also be modified following aqueous-phase reactions (such as oxidation, nitration, and acid–base reactions) (1, 4). Studies of aerosol processing by clouds have predominantly focused on warm clouds and aqueous-phase reactions (4) that lead to the formation of secondary organic aerosol material (5) and modify the size and composition of the resulting particles (6).Despite the importance of tropospheric ice clouds to Earth’s radiative balance and climate (7, 8) and extensive efforts to understand many of the processes within cirrus clouds (9), little attention has been given to the processing of aerosols by the cloud ice phase. As clouds continually form and dissipate, it is likely that aerosol particles undergo cycles of freezing and drying during ice crystal formation and subsequent ice sublimation when exposed to dry, subsaturated conditions outside of the cloud. Such conditions occur in the lower parts of cirrus clouds (10), in the outflow of high convective clouds (11, 12), and when clouds are injected into the dry lower stratosphere (13). Here we show that the atmospheric conditions for such aerosol processing are likely to occur readily in deep convective clouds that are common in the tropics and midlatitudes and reach the upper troposphere and tropical tropopause layer. We hypothesize that aerosols may undergo significant modifications following such freeze-drying cycles. In contrast to chemical aging, the modifications to the aerosols in a freeze-drying cycle may be induced by changes in the thermodynamic state of the aerosol particles, which may lead to unexplored phenomena with regard to aerosol particle morphology and phase.Organic material and sulfates are prevalent in high-altitude interstitial aerosol and ice crystal residues of cirrus clouds throughout the tropics, subtropics, and midlatitudes (14, 15). Recent studies have proposed that atmospheric organic aerosol particles may be in an amorphous glassy state, depending on the temperature and liquid water content (16, 17). The glassy state affects ice nucleation capability (18–20) and water uptake and release (18, 19, 21) and slows down chemical aging of atmospheric particles due to slower diffusion of water and reactants into the particle (22). Previous studies suggested that glassy aerosols emanating from deep convective clouds act as potential ice nuclei in the upper troposphere and tropical tropopause layer (23, 24).The experiments in this study simulate the atmospheric freeze-drying cycle for organic and mixed organic/sulfate aerosol. We follow how this ice cloud process affects the phase, size, and morphology of the processed aerosol particles and assess experimentally how changes in these parameters affect their cloud condensation nucleation activity and optical properties.