Iodine Drives New Particle Formation in the Central Arctic Ocean

The Arctic is warming faster than Earth on average (Arctic amplification) and the extent of the sea ice coverage has decreased dramatically over the past decades, especially in summer. However, the underlying processes behind this amplification are not well understood because of many feedback mechanisms between the ocean, the atmosphere and the cryosphere. Clouds are an important player in the Arctic radiative budget, as they almost always have a net surface warming effect. However, models struggle to reproduce Arctic clouds partly because aerosol processes and aerosol sources in this area are poorly understood. This is particularly true in the central Arctic Ocean where aerosols can reach extremely low concentrations and clouds may even become limited by cloud condensation nuclei (CCN) availability. New particle formation (NPF) has been identified as a potential source of CCN in this region, and a modelling study showed that NPF is needed to explain the observed Aitken mode particle concentration. However, no conclusive results were reported concerning the nature of these NPF events and the chemical composition of the nucleating vapours. In this contribution we present the first comprehensive physicochemical characterization of several NPF events in the central Arctic Ocean during the Arctic Ocean 2018 expedition. We identified a marked increase of the iodic acid concentration towards the end of the summer, potentially associated with the onset of the freeze-up season and a concurrent increase of the ozone concentration. Moreover, our results indicate that iodic acid is a main driver for the formation of new particles and their early growth. The frequency of new particle formation events follows the same seasonal trend as the ultrafine particle concentration, which is one order of magnitude higher in fall compared to summer. In addition, we developed a simple parametrization that describes iodic acid variability, providing an estimate of the iodine precursor emission rates. Finally, we also provide direct evidences that Aitken mode particles activate as cloud condensation nuclei suggesting that iodine NPF can influence the formation of clouds and their properties in the central Arctic.

Physical and Chemical Properties of Cloud Droplet Residuals and Aerosol Particles During the Arctic Ocean 2018 Expedition

Andrea Baccarini, Lubna Dada, Julia Schmale

Detailed knowledge of the physical and chemical properties and sources of particles that form clouds is especially important in pristine areas like the Arctic, where particle concentrations are often low and observations are sparse. Here, we present in situ cloud and aerosol measurements from the central Arctic Ocean in August–September 2018 combined with air parcel source analysis. We provide direct experimental evidence that Aitken mode particles (particles with diameters ≲70 nm) significantly contribute to cloud condensation nuclei (CCN) or cloud droplet residuals, especially after the freeze-up of the sea ice in the transition toward fall. These Aitken mode particles were associated with air that spent more time over the pack ice, while size distributions dominated by accumulation mode particles (particles with diameters ≳70 nm) showed a stronger contribution of oceanic air and slightly different source regions. This was accompanied by changes in the average chemical composition of the accumulation mode aerosol with an increased relative contribution of organic material toward fall. Addition of aerosol mass due to aqueous-phase chemistry during in-cloud processing was probably small over the pack ice given the fact that we observed very similar particle size distributions in both the whole-air and cloud droplet residual data. These aerosol–cloud interaction observations provide valuable insight into the origin and physical and chemical properties of CCN over the pristine central Arctic Ocean.

2022

Frequent new particle formation over the high Arctic pack ice by enhanced iodine emissions

Andrea Baccarini, Julia Schmale

In the central Arctic Ocean the formation of clouds and their properties are sensitive to the availability of cloud condensation nuclei (CCN). The vapors responsible for new particle formation (NPF), potentially leading to CCN, have remained unidentified since the first aerosol measurements in 1991. Here, we report that all the observed NPF events from the Arctic Ocean 2018 expedition are driven by iodic acid with little contribution from sulfuric acid. Iodic acid largely explains the growth of ultrafine particles (UFP) in most events. The iodic acid concentration increases significantly from summer towards autumn, possibly linked to the ocean freeze-up and a seasonal rise in ozone. This leads to a one order of magnitude higher UFP concentration in autumn. Measurements of cloud residuals suggest that particles smaller than 30 nm in diameter can activate as CCN. Therefore, iodine NPF has the potential to influence cloud properties over the Arctic Ocean.

2020

Annual cycle observations of aerosols capable of ice formation in central Arctic clouds

Ivo Fabio Beck, Julia Schmale

The Arctic is warming faster than anywhere else on Earth, prompting glacial melt, permafrost thaw, and sea ice decline. These severe consequences induce feedbacks that contribute to amplified warming, affecting weather and climate globally. Aerosols and clouds play a critical role in regulating radiation reaching the Arctic surface. However, the magnitude of their effects is not adequately quantified, especially in the central Arctic where they impact the energy balance over the sea ice. Specifically, aerosols called ice nucleating particles (INPs) remain understudied yet are necessary for cloud ice production and subsequent changes in cloud lifetime, radiative effects, and precipitation. Here, we report observations of INPs in the central Arctic over a full year, spanning the entire sea ice growth and decline cycle. Further, these observations are size-resolved, affording valuable information on INP sources. Our results reveal a strong seasonality of INPs, with lower concentrations in the winter and spring controlled by transport from lower latitudes, to enhanced concentrations of INPs during the summer melt, likely from marine biological production in local open waters. This comprehensive characterization of INPs will ultimately help inform cloud parameterizations in models of all scales.

2022