Formation Mechanisms of Iodine–Ammonia Clusters in Polluted Coastal Areas Unveiled by Thermodynamics and Kinetic Simulations

Abstract

It has been revealed that iodine species play important roles in atmospheric new particle formations (NPFs) in pristine coastal areas. However, it is unclear whether other atmospheric species, such as NH3, for which the levels in coastal areas of China are >2.5 × 1010 molecules·cm–3 are involved in the NPFs of iodine species, although NH3 has been proved to promote particle formation of H2SO4. Via high-level quantum chemical calculations and atmospheric cluster dynamic code simulations, this study unveiled new mechanisms of nucleation, in which NH3 mediates the formation of iodine particles by assisting hydrolysis of I2O5 or reacting with HIO3. The simulated formation rates of iodine–ammonia clusters via the new mechanisms are much higher than those simulated via sequential addition of HIO3 with subsequent release of H2O, under the condition that NH3 concentrations are higher than 1010 molecules·cm–3. The new mechanisms can well explain the observed cluster formation rates at a coastal site in Zhejiang of China. The findings not only expand the current understandings of the role of NH3 in NPFs but also highlight the importance of monitoring and evaluating NPFs via the iodine–ammonia cluster pathway in the coastal areas of China and other regions worldwide.

Insights into the Chemistry of Iodine New Particle Formation: The Role of Iodine Oxides and the Source of Iodic Acid

“…Our present nitrate CIMS experiments indicate that this competition likely occurs between I 2 O 5 –I x O y clustering and slow hydrolysis by the water dimer. 32 The concentration of I x O y in our flow tube is high (∼10 10 to 10 12 cm –3 ) compared to atmospheric conditions (10 7 to 10 9 cm –3 ). For low water concentrations, I 2 O 5 and HIO 3 are mainly removed by clustering with I x O y , and low concentrations of HIO 3 and I 2 O y ·HIO 3 clusters exist, which may be too low to be detectable by PIMS in experiments with the same time scale as in the present ones.…”

“…Hence, these observations suggest that I 2 O 5 reacts with water to generate HIO 3 . The reaction between I 2 O 5 and H 2 O is precluded by a large barrier in the PES, 30 but recent ab initio calculations at the CCSD(T)//M06-2X/aug-ccpVTZ-PP + ECP28 level 32 indicate that hydrolysis of I 2 O 5 by the water dimer is feasible …”

Iodous acid – a more efficient nucleation precursor than iodic acid

“…15,18 Extensive studies indicate that iodine oxoacids play important roles in NPF in marine regions. 17,[19][20][21][22] In Mace head, a coast in Ireland, NPF events were observed along with a high-concentration of iodic acid (HIO 3 ) and a low-concentration of iodous acid (HIO 2 ), and the nucleation was speculated to proceed by the sequential addition of HIO 3 , followed by intracluster restructuring to I 2 O 5 . 17 Recently, a theoretical study indicated that HIO 3 can self-nucleate in clean coastal regions and the selfnucleation of HIO 3 can also be influenced by sulfuric acid and ammonia in polluted coastal regions.…”

“…19 Additionally, a theoretical study has studied the formation mechanism of HIO 3 -ammonia clusters in polluted coastal areas and this new mechanism can well explain the observed cluster formation rates at a coastal site in Zhejiang of China. 20,21 Moreover, a recent experimental study indicates that HIO 3 is the major driver in both nucleation and growth processes of iodine oxoacid particles and remains as the dominant constituent of the freshly formed particles, and HIO 2 plays an important role in neutral nucleation by stabilizing HIO 3 clusters against evaporation. 22 Hence, the participation of HIO 2 in nucleation has caught our concern.…”

Molecular-level evidence for marine aerosol nucleation of iodic acid and methanesulfonic acid

“…In addition to the above sulfur precursors, recent experimental and theoretical studies (He et al, 2021;Martí n et al, 2020;Xia et al, 2020;Rong et al, 2020) have also recognized the critical role of iodine compounds in marine NPF process.…”