Toward Understanding Secondary Aerosol Formation in Marine and Polar Atmospheres

Clouds in the marine and polar atmospheres play a crucial role in climate through cloud-radiation interactions. Secondary aerosols formed from gas precursors serve as potential cloud droplet nuclei, influencing cloud reflectivity and lifespan, and consequently impacting the climate indirectly. Understanding the processes involved in secondary aerosol and cloud formation in these regions is essential for predicting future changes, given the extensive coverage of marine and polar clouds and the observed rapid transformations in the Arctic. Despite their significance, marine and polar aerosol formation processes remain among the least investigated due to the challenges posed by experiments and logistics. In this presentation, compelling evidence derived from field observations and laboratory experiments will be showcased, highlighting the significance of iodine species as crucial aerosol sources in polar and marine atmospheres.

A significant limitation in the study of iodine aerosol formation has been the absence of instruments capable of accurately measuring gaseous iodine species with high sensitivity. To address this, we have developed a bromide chemical ionization mass spectrometric method that allows for near-comprehensive and highly sensitive detection of oxidized iodine species. Through our method, we have achieved a detection limit of below 10⁵molecules cm^-3, surpassing the sensitivity of most previous techniques.

To gain a deeper understanding of the aerosol formation mechanism in marine and polar atmospheres, we integrated this instrument with the Cosmics Leaving Outdoor Droplet (CLOUD) chamber at CERN. Through our investigations, we found two aerosol precursors that were previously underestimated: iodic acid (HIO₃) and iodous acid (HIO₂). Our measurements revealed a distinct sequence of HIO₃-HIO₂ clusters, with HIO₂ playing a crucial role in stabilizing HIO₃ clusters through halogen and hydrogen bonding. Notably, this mechanism exhibits exceptional efficiency at temperatures below 0 °C, making it particularly relevant for polar boundary layers and the global upper troposphere-lower stratosphere. Furthermore, we show that atmospheric negative ions can substitute for the function of HIO₂, thereby accelerating HIO₃ aerosol formation processes. This mechanism emerges as critical for temperatures above 0 °C, highlighting its significance for the marine boundary atmospheres.

The involvement of iodic acid and iodous acid in secondary aerosol formation processes has also been observed on a global scale. Traditionally, iodine aerosol formation has been predominantly associated with mid-latitudinal regions. However, our study has revealed the unequivocal presence of HIO₃ in a much broader range of environments, including polar regions, boreal forests, marine sites, and even polluted urban areas. To comprehensively evaluate the worldwide implications of iodine aerosol formation processes, a systematic analysis combining ambient observations, laboratory experiments, and global simulations is imperative.