Marine aerosol formation contributes significantly to the global radiative budget given the high susceptibility of marine stratiform cloud radiative properties to changes in cloud condensation nuclei (CCN) availability. Atmospheric new-particle-formation is thought to involve nucleation of sulfuric acid with water, ammonia or amines followed by condensation/growth in the presence of organic vapors. Unique in the marine boundary layer (MBL), new particle formation involves clustering of iodine oxides (I_xO_y) or sequential addition of HIO₃. A very recent study in Science even shows that the efficacy of iodine to form new particles exceeds that of the sulfuric acid-ammonia system at the same acid concentrations.

However, the growth of nucleated nanoparticles cannot be accounted for by condensation of typical species driving the initial nucleation. Condensation of organic vapors has been suggested as the most probable mechanism, but condensation growth requires condensing organic molecules of low effective volatilities. The growth of nucleated iodine-oxide particles to CCN sizes in open ocean, therefore, has been a mystery for about two decades.

An international team led by Prof. Dr. HUANG Ru-Jin of the Institute of Earth Environment, Chinese Academy of Sciences investigated the mechanism of iodine-derived marine new particle formation. They show, through a combination of laboratory experiments, ambient field measurements and model studies, that nucleated iodine oxide clusters provide unique sites for the accelerated growth of organic vapors to overcome the coagulation sink. Heterogeneous reactions of iodine-oxide particle with intermediate oxidized organics from oxidation of C₁-C₅ volatile organic compounds released from phytoplankton produce low-volatility organic acids and alkylaminium salts in the particle phase, while further oligomerization of small α-dicarbonyls (e.g., glyoxal) drives the particle growth.

This newly-identified heterogeneous mechanism explains the occurrence of particle production events at organic vapor concentrations almost an order of magnitude lower than those required for growth via condensation alone. Moreover, a notable fraction of iodine associated with these growing particles is recycled back into the gas phase, suggesting an effective transport mechanism for iodine to remote regions, acting as a ‘catalyst’ for nucleation and subsequent new particle production in marine air.

Fig. 1 New insights into iodine-organic multiphase reactions fast-tracking marine new particle formation. Iodine released from the ocean surface is photolyzed and oxidized to form iodine oxides or iodine oxoacids, which can nucleate to form iodine-oxide particles (IOPs) or contribute to the early cluster growth by chemical activation of the pre-nucleation cluster. Within the recently formed ultrafine particles heterogeneous reactions between the higher iodine oxides and condensing alcohols or carbonyls from oxidation of marine VOCs lead to the formation of low volatility oxidized organics where the produced organic acids can further react with basic molecules (e.g., amines) to form highly hygroscopic salts, accelerating the early particle growth into Aitken mode and ultimately CCN. During this process, the higher iodine oxides are recycled, re-starting the reaction sequence.(Image by HUANG, et al)

This study, entitled “Heterogeneous iodine-organic chemistry fast-tracks marine new particle formation”, is published in the Proceedings of the National Academy of Sciences (PNAS) on 2 August 2022. Prof. HUANG Ru-Jin is the first- and corresponding-author, the international team includes scientists from the University of Mainz, National University of Ireland Galway, Finnish Meteorological Institute, Texas A&M University, and California Institute of Technology. The work was supported by the National Natural Science Foundation of China, Chinese Academy of Sciences, Science Foundation Ireland, and the EU.

Contact: Prof. HUANG Ru-Jin, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China. Email: rujin.huang@ieecas.cn