High number concentrations of transparent exopolymer particles ( TEP ) in 1 ambient aerosol particles and cloud water – A case study at the tropical 2 Atlantic Ocean

High number concentrations of transparent exopolymer particles (TEP) in 1 ambient aerosol particles and cloud water – A case study at the tropical 2 Atlantic Ocean 3 4 Manuela van Pinxteren1, Tiera-Brandy Robinson, Sebastian Zeppenfeld1, Xianda Gong, 5 Enno Bahlmann4, Khanneh Wadinga Fomba1, Nadja Triesch1, Frank Stratmann, Oliver Wurl, 6 Anja Engel5, Heike Wex, Hartmut Herrmann1* 7 8 *Corresponding author: Hartmut Herrmann (herrmann@tropos.de) 9 10 1 Atmospheric Chemistry Department (ACD), Leibniz-Institute for Tropospheric Research 11 (TROPOS), 04318 Leipzig, Germany 12 2 Institute for Chemistry and Biology of the Marine Environment, Carl-von-Ossietzky 13 University Oldenburg, 26382 Wilhelmshaven, Germany 14 3 Dept. of Experimental Cloud and Microphysics, Leibniz-Institute for Tropospheric Research 15 (TROPOS), 04318 Leipzig, Germany 16 + now at: Center for Aerosol Science and Engineering, Department of Energy, Environmental 17 and Chemical Engineering, Washington University in St. Louis, 63130, MO, USA 18 4 Leibniz Centre for Tropical Marine Research (ZMT), 28359 Bremen, Germany 19 5 GEOMAR Helmholtz Centre for Ocean Research, Kiel 24105, Germany 20 21

and are therefore below 3% of PM10 and are subtracted from PM10 sodium concentration.
2) TEP volume concentration were calculated from the number concentrations in a size range between 5 and 10 µm (Tab. S5) assuming spherical particles.
3) Average sodium concentration in seawater.
4) Error discussion of the EFatm.: Even though the same TEP size ranges (diameters between 5 and 10 µm) were compared for oceanic and atmospheric measurements, the size distribution of TEP in the respective compartments might be affected by the different temperature, pressure, ion strength and pH in seawater and in the atmosphere. Such effects cannot be accounted for in the present study. Applying sodium concentrations for the EFaer. and EFcloud calculations also represents a source of error. TEP can form networks with inorganic cations and that might affect sodium concentrations in the seawater and in the atmosphere. However, it was reported that the TEP formation was essentially determined by the calcium concentration, while monovalent cations, such as sodium, seem not to be significantly involved in TEP formation (Meng and Liu, 2016). Hence, this error should be neglible. Finally, the TEP number concentrations in the ocean surface water were obtained from the biologically productive Mauritanian Upwelling region in the year 2012, hence at another time and season. Compared to other oceanic regions, the values from the Mauritanian Upwelling region were at the higher end (Engel et al., 2020). The region around the CVAO is rather oligotrophic and Chlorophyll-a values during the MarParCloud campaign were relatively low with 0.1 up to 0.6 µg L -1 (van Pinxteren et al., 2020). As TEP is often connected to phytoplankton activity, the TEP concentration at the CVAO might be lower compared to more productive regions (Robinson et al., 2019). Lower TEP concentrations would result in higher EFatm. (following equation 1 in the main manuscript). Hence the here reported EFatm. represent a lower limit. Nevertheless, even though absolute numbers can vary, the strong differences between the EFaer. ambient and EFaer. tank (Tab. S3) are evident, as they result from the same type of seawater. 56 1) For the tank-generated aerosol particles, solely the total number concentrations were available. TEP volume concentration were calculated from the number concentrations in a size range between 5 and 10 µm assuming spherical particles after the equation from Fig. S1: (y = 181.66x) assuming a similar size distribution of the tank-generated TEP to the ambient TEP.
2) Sodium was measured on TSP, however it was shown that particles greater than 10 µm radius are quickly removed in the atmosphere (Madry et al., 2011) and in sea spray model systems (Hoffman and Duce, 1976). Hence the sodium data on TSP is expected to be comparable to PM10 and therefore to the TEP volume concentrations between 5 and 10 µm.
2) TEP volume concentration was calculated from the number concentration in a size range between 5 and 10 µm assuming spherical particles. 84 85 86 Figure S1: Correlation between TEP number concentration and TEP volume concentration achieved 87 from the ambient aerosol samples (TEP diameter: 5-10 µm). The function was applied for the tank-88 generated aerosol particles and for the cloud water samples to calculate the volume concentration 89 from the measured number concentration. A similar size distribution of the TP in the size range 90 between 5 and 10 µm was shown for the cloud water and assumed for the tank -generated aerosol 91 particles