A054-05
CCN over the Southern Ocean: Sources and Relevance for Cloud Formation

Tuesday, 8 December 2020: 19:16
Virtual
Julia Schmale1,2, Athanasios Nenes3, Christian Tatzelt4, Avichay Efraim5, Silvia Henning4, Andrea Baccarini6, Sebastian Landwehr7, Alireza Moallemi6, Robin L. Modini1, Frank Stratmann4, Daniel Rosenfeld8, Leighton Anunda Regayre9, Ken S Carslaw10, Josef Dommen6, Urs Baltensperger11 and Martin Gysel12, (1)Paul Scherrer Institute, Villingen, Switzerland, (2)École Polytechnique Fédérale de Lausanne, ENAC, Lausanne, Switzerland, (3)Swiss Federal Institute of Technology Lausanne, Laboratory of Atmospheric Processes and their Impacts, School of Architecture, Civil & Environmental Engineering, Lausanne, Switzerland, (4)Leibniz Institute for Tropospheric Research, Leipzig, Germany, (5)The Hebrew Univeristy of Jerusalem, Jerusalem, Israel, (6)Paul Scherrer Institute, Villigen, Switzerland, (7)National University of Ireland, Galway, Lausanne, Ireland, (8)Hebrew University, Jerusalem, Israel, (9)University of Leeds, Leeds, LS2, United Kingdom, (10)University of Leeds, Leeds, United Kingdom, (11)Paul Scherrer Inst, Villigen, Switzerland, (12)Paul Scherrer Institute, 5232 Villigen, Switzerland
Recorded Presentation
Abstract:
Aerosol-cloud interactions are the least understood anthropogenic influence on climate change. A major cause of this limited understanding is the poorly quantified state of aerosols in the pristine preindustrial atmosphere, which defines the baseline against which anthropogenic effects are calculated. In addition, models significantly underestimate cloud solar reflectance over the Southern Ocean. The poor representation of aerosol-cloud interactions is one of reason for the bias.

We present results from the Antarctic Circumnavigation Expedition project “Study of Preindustrial-like Aerosol Climate Effect” (ACE-SPACE) showing the varying influence of different natural sources to the aerosol and cloud condensation nuclei (CCN) concentrations and the sensitivity of cloud droplet formation to aerosol availability. ACE-SPACE covered the Indian, Pacific and Atlantic sectors of the Southern Ocean between 34 to 72 °S from December 2016 to March 2017. We operated amongst other a CCN counter, several ion and particle size distribution instruments covering the range from 2 nm to 19 µm and mass spectrometers for trace gas and particle composition.

Based on trace gas analysis, aerosol measured south of 60 °S can be classified as pristine and almost exclusively of natural source. Our analysis shows that sea spray contributes at maximum 30 % to CCN at a supersaturation of 0.2 % (CCN0.2), particularly when crossing the westerly wind belt. New particle formation is not a source of CCN in the boundary layer. Microbial emissions such as DMS, which is converted to methanesulfonic acid (MSA) and particulate sulfate, influence activation rates and concentrations of CCN0.2. Comparison of satellite retrieved Nd with in situ measured CCN shows that clouds are mostly fed from aerosol originating in the marine boundary layer as opposed to the free troposphere. Our Nd simulations with a state-of-the-art cloud droplet activation parameterization show that in some areas of the Southern Ocean cloud formation is limited by the availability of aerosol particles. Our results can improve model simulations of clouds over the Southern Ocean and we have shown that our data can significantly reduce the uncertainty of anthropogenic radiative forcing.

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