Concentrations of Cloud Condensation Nuclei (CCN) and Ice Nucleating Particles (INP) around Antarctica during the 2016/17 ACE (Antarctic Circumnavigation Expedition) Cruise

It is a great challenge to assign exact numbers to the human influence on climate change. While we know the effect of carbon dioxide emissions quite well, there are anthropogenic emissions of other substances that effect climate through complex chains of interactions with atmospheric processes that are not yet well characterized [IPCC, 2013]. This lack of knowledge causes uncertainties in the quantification of how human activities influence weather and climate [Carslaw et al., 2013]. To isolate the anthropogenic contribution, we need to characterize the atmosphere as it was before industrialization - before substantial anthropogenic emissions of gases and particles to the air began. The effects of gases and particles in the pre-industrial atmosphere serve as a baseline against which anthropogenic climate effects are calculated. The region that qualifies best to observe pristine, pre-industrial like conditions is the Southern Ocean (SO) as discussed in the paper by Hamilton et al. [2014]. To tackle this challenge the “Study of Preindustrial-like-Aerosol Climate Effects” (SPACE) project was designed, which made use of the Antarctic Circumnavigation Expedition (ACE).

This expedition took place between December 2016 and March 2017 on board RV “Akademik Troyshnikov”, which went round the Antarctic continent from Cape Town to Hobart and Punta Arenas and back to Cape Town. Among a wide variety of experiments on board aiming to understand ocean chemical, microbiological and physical processes also atmospheric parameters where sampled, within the SPACE project. As partner in this project, TROPOS focused on aerosol particles involved in aerosol cloud interactions, specifically those that are able to act as cloud condensation nuclei (CCN), as well as particles able to nucleate ice (INP).

A CCN counter was operated in the aerosol measurement container of the Paul Scherrer Institute, which was positioned in the front of the upper deck. The CCN-100 instrument (DMT, Boulder, USA) was continuously operated during all three legs at five supersaturations (0.15%, 0.2%, 0.3%, 0.5%, 1%). It was connected to a common aerosol inlet (GAW type) and operated side by side with other aerosol characterization equipment measuring, e.g., total aerosol particle number concentration and size distribution.

Filter sampling for the off-line INP analysis was done by means of two DIGITEL instruments (Riemer Messtechnik, Germany) with automatic filter exchange. Both instruments were placed on the observation deck and fixed to the railing. The high-volume instrument DHA-80 was operated with a 24h sampling interval and 150mm quartz fiber filters as filter material with a PM10 inlet. The low volume instrument DPA-14 was run on an 8h sampling interval and the filter material was polycarbonate (Nuclepore) with 200nm pore size. The sampled filters were collected regularly from the instruments storage and stored at minus 20°C on board the vessel. The high-volume filters are shared between, chemical analysis and INP analysis, the low volume filters are used for INP analysis only.

Often the CCN concentration for all supersaturations reflects the total particle number with only some exceptions. Typical concentrations of CCN are between 5 cm⁻³ to 1300 cm⁻³. For 0.2% in supersaturation 15 to 260 cm⁻³ CCN were observed. This is generally higher than suggested by the GLOMAP model [Pringle et al., 2009], where concentrations between 0 and 25 cm⁻³ are predicted for the entire SO. The CCN number is surprisingly constant over the whole cruise, with an overall mean value at 0.2% of 110 cm⁻³. However, extreme values were found close to the Antarctic continent. At Mertz Glacier (67°30’S 144°45’E), very low CCN concentrations (around 15 cm⁻³) were observed, but also the highest pristine concentrations (up to 260 cm⁻³) were measured close to this region.

We find that INP concentrations are constantly low on the open ocean, but increase in the vicinity to the ports of Cape Town, Hobart and Punta Arenas. On the open ocean, concentrations of INP active at -10°C are often below 0.1m⁻³, the limit of detection. Our observed INP concentrations at -15°C are about a magnitude lower than Bigg’s measurements in 1969-1972 Bigg [1973], and closer to concentrations predicted by the global GLOMAP model (personal communication: Jesus Vergara-Temprado).

In our contribution, we will present and discuss preliminary results for both CCN and INP concentrations.

Acknowledgement: ACE was a scientific expedition carried out under the auspices of the Swiss Polar Institute, supported by funding from the ACE Foundation and Ferring Pharmaceuticals. The data analysis is funded by the German Research Foundation within the Priority Programme 1158 (Antarctic Research with Comparable Investigations in Arctic Sea Ice Areas, STR 453/12-1). EU FP7 project BACCHUS (project number 603445) is acknowledged for financial support.