Temperature, humidity, and ionisation effect of iodine oxoacid nucleation

Iodine oxoacids are recognised for their significant contribution to the formation of new particles in marine and
polar atmospheres. Nevertheless, to incorporate the iodine oxoacid nucleation mechanism into global
simulations, it is essential to comprehend how this mechanism varies under various atmospheric conditions.
In this study, we combined measurements from the CLOUD (Cosmic Leaving OUtdoor Droplets) chamber at
CERN and simulations with a kinetic model to investigate the impact of temperature, ionisation, and humidity
on iodine oxoacid nucleation. Our findings reveal that ion-induced particle formation rates remain largely
unaffected by changes in temperature. However, neutral particle formation rates experience a significant
increase when the temperature drops from +10 °C to −10 °C. Running the kinetic model with varying
ionisation rates demonstrates that the particle formation rate only increases with a higher ionisation rate when
the iodic acid concentration exceeds 1.5 × 10$^7$ cm$^{−3}$
, a concentration rarely reached in pristine marine
atmospheres. Consequently, our simulations suggest that, despite higher ionisation rates, the charged cluster
... mehr
nucleation pathway of iodic acid is unlikely to be enhanced in the upper troposphere by higher ionisation
rates. Instead, the neutral nucleation channel is likely to be the dominant channel in that region. Notably, the
iodine oxoacid nucleation mechanism remains unaffected by changes in relative humidity from 2% to 80%.
However, under unrealistically dry conditions (below 0.008% RH at +10 °C), iodine oxides (I$_2$O$_4$ and I$_2$O$_5$)
significantly enhance formation rates. Therefore, we conclude that iodine oxoacid nucleation is the dominant
nucleation mechanism for iodine nucleation in the marine and polar boundary layer atmosphere.