Molecular-scale evidence of aerosol particle formation via sequential addition of HIO3

Homogeneous nucleation and subsequent cluster growth leads to the formation of new aerosol particles in the atmosphere1. Nucleation of sulphuric acid and organic vapours is thought to be responsible for new particle formation over continents1,2 while iodine oxide vapours have been implicated in particle formation over coastal regions3–7. Molecular clustering pathways involved in atmospheric particle formation have been elucidated in controlled laboratory studies of chemically simple systems2,8–10. But no direct molecular-level observations of nucleation in atmospheric field conditions involving either sulphuric acid, organic or iodine oxide vapours have been reported to date11. Here we report field data from Mace Head, Ireland and supporting data from northern Greenland and Queen Maud Land, Antarctica that allow for the identification of the molecular steps involved in new particle formation in an iodine-rich, coastal atmospheric environment. We find that the formation and initial growth process is almost exclusively driven by iodine oxoacids and iodine oxide vapours with average resulting cluster O:I ratios of 2.4. Based on the high O:I ratio, together with observed high concentrations of iodic acid, HIO₃, we suggest that cluster formation primarily proceeds by sequential addition of iodic acid HIO₃, followed by intra-cluster restructuring to I₂O₅ and recycling of water in the atmosphere or upon drying. Overall, our study provides ambient atmospheric molecular-level observations of nucleation, supporting the previously suggested role of iodine containing species in new particle formation3–7, 12–18, and identifies the key nucleating compound.

Lidar-based Studies of Aerosol Optical Properties Over Coastal Areas

Aerosol size distribution and concentration strongly depend on wind speed, direction, and measuring point location in the marine boundary layer over coastal areas. The marine aerosol particles which are found over the sea waves in high wind conditions affect visible and near infrared propagation for paths that pass very close to the surface as well as the remote sensing measurements of the sea surface. These particles are produced by various air sea interactions. This paper presents the results of measurements taken at numerous coastal stations between 1992 and 2006 using an FLS-12 lidar system together with other supporting instrumentation. The investigations demonstrated that near-water layers in coastal areas differ significantly from those over open seas both in terms of structure and physical properties. Taking into consideration the above mentioned factors, aerosol concentrations and optical properties were determined in the marine boundary layer as a function of offshore distance and altitude at various coastal sites in two seasons. The lidar results show that the remote sensing algorithms used currently in coastal areas need verification and are not fully reliable.