The Arctic is experiencing rapid warming, at least twice the global average, particularly in autumn and winter. This Arctic amplification poses challenges for accurate climate modeling, especially regarding clouds and their radiative properties. Cloud formation in the central Arctic Ocean can be limited by the availability of CCN, making even small increases in CCN concentration significantly impact surface warming. Previous research identified various CCN sources, including marine organic particles, sea salt, organic aerosols, and long-range transported continental emissions. However, the understanding of new particle formation (NPF) and its contribution to the Aitken mode particles and the overall CCN budget remains highly uncertain. Nucleation mode particles have been observed since 1991 but their source remained unidentified. Studies in the Arctic marginal ice zone (MIZ) linked NPF to sulfuric acid, ammonia, marine organics, and occasionally iodine, but these findings are not directly applicable to the central Arctic Ocean, which has much lower gas precursor concentrations. This study aimed to identify the source of frequent NPF events over the central Arctic Ocean during the Microbiology-Ocean-Cloud-Coupling in the High Arctic (MOCCHA) campaign in 2018, using advanced in-situ instrumentation.

Prior research highlighted the importance of understanding Arctic aerosol sources and their evolution to accurately model CCN populations and cloud formation. Various studies have explored CCN sources in the Arctic, including marine and continental influences. However, the role of new particle formation (NPF) and the specific chemical composition driving NPF events remained unclear, particularly in the central Arctic Ocean. While some studies in the marginal ice zone attributed NPF to various compounds, including iodine in a few cases, the dominant mechanism in the central Arctic, characterized by a low concentration of gas precursors, remained elusive. Existing models lacked information on the source regions of newly formed particles and did not incorporate iodine nucleation, highlighting the need for further investigation.

The MOCCHA campaign, part of the Arctic Ocean 2018 expedition, employed a suite of instruments aboard the icebreaker I/B Oden. Measurements were conducted over four weeks at latitudes exceeding 88°N. The core instrumentation included a nitrate chemical ionization mass spectrometer (CIMS) to measure iodic acid (HIO3), sulfuric acid, and methanesulfonic acid (MSA) concentrations. A negative atmospheric pressure interface time-of-flight (API-TOF) mass spectrometer was used to determine the chemical composition of naturally charged negative ions. A neutral cluster and air ion spectrometer (NAIS) and various particle counters and sizers were employed to determine the size distribution of particles, both neutral and charged. Cloud residuals were sampled using a counterflow virtual impactor (CVI) inlet, with the size distribution of these cloud residuals measured with a CPC and DMPS. Ozone measurements were performed using a model 205 ozone monitor from 2BTechnologies. Air mass back trajectories were calculated using the Lagrangian analysis tool LAGRANTO. Data cleaning was performed using an algorithm to remove ship pollution. Surface mixed-layer height, dry deposition velocity, and condensation sink were calculated based on separate measurements. A simple model was developed to quantify the influence of emission rates, dry deposition, boundary-layer height, and condensation sink on HIO3 concentration, and kinetic condensation model were used to estimate the contribution of sulfuric acid and HIO3 to observed particle growth rates. Critical diameters were calculated to assess CCN activation.

The study's key findings are as follows: 1. **Iodine drives NPF:** All eleven major NPF events observed over the pack ice were driven by HIO3, with negligible contribution from sulfuric acid. The negatively charged clusters consisted primarily of iodine oxides, with a maximum of eight iodine atoms per cluster, and much higher concentrations of HIO3 than sulfuric acid. 2. **Seasonal variation in HIO3:** The HIO3 concentration showed a clear increase from summer to autumn, leading to a more than tenfold increase in UFP concentration. This increase correlated strongly with the onset of sea ice formation and a rise in ozone concentration. 3. **Meteorological influence:** The HIO3 concentration and NPF events were closely linked to local meteorology. Fog acted as a significant sink for HIO3, reducing its concentration and inhibiting NPF. A simple model successfully explained a large portion of the HIO3 variability by considering emission rates, dry deposition velocity, boundary layer height, and condensation sink. The estimated median emission factor was 5.0 × 10^6 iodine atoms cm⁻²s⁻¹. 4. **Ultrafine particle growth:** UFP exhibited continuous growth, reaching 15-20 nm in size despite low growth rates. In most cases, over 50% of the growth was attributable to HIO3 alone. 5. **CCN activation:** Direct evidence demonstrated that particles as small as 20-40 nm can activate as CCN in Arctic fog, particularly when the concentration of larger aerosols is low, suggesting that iodine NPF is a relevant CCN source. 6. **Comparison with Alert data:** The observed seasonal trend in HIO3 concentration aligns remarkably well with historical iodine aerosol concentration data from Alert, Canada, further supporting the significance of iodine NPF in the Arctic.

This study provides the first direct molecular-scale evidence that iodine, specifically in the form of iodic acid (HIO3), is the primary driver of new particle formation (NPF) in the central Arctic Ocean during late summer and early autumn. The strong correlation between HIO3 concentration and the frequency and intensity of NPF events, coupled with the observed seasonal variations, highlight the importance of iodine emissions in the Arctic aerosol budget. The findings emphasize the connection between sea ice formation, ozone concentration, and enhanced iodine emissions, suggesting a previously unrecognized mechanism for aerosol production in the region. The observation that particles much smaller than previously thought can activate as CCN in the Arctic under CCN-limited conditions suggests that iodine-driven NPF could have a significant impact on the radiative properties of Arctic clouds and, consequently, on the regional climate system.

This research identifies iodic acid as the primary driver of frequent NPF events in the central Arctic Ocean, marking a significant advancement in our understanding of Arctic aerosol formation. The findings highlight a previously unknown link between sea ice formation, ozone levels, and iodine emissions. The observed activation of smaller particles as CCN suggests iodine NPF could significantly influence Arctic cloud properties and climate. Future research should focus on the detailed mechanisms of iodine release during sea ice formation and on refining the model to better understand iodine NPF dynamics. Improved incorporation of iodine NPF into climate models is crucial for more accurate climate projections.

The study was conducted during a single expedition in 2018, limiting the generalization of findings to other years. The model used to explain HIO3 variability is relatively simple and may not capture all relevant processes. While the study showed that particles smaller than 30 nm can activate as CCN, it could not completely quantify this process in a detailed manner, highlighting the need for further research into activation and cloud condensation.