Summertime Primary and Secondary Contributions to Southern Ocean Cloud Condensation Nuclei

Rafel Simó,Colin O’Dowd,Salvatore Marullo,Dantong Liu,Darius Ceburnis,Manuel Dall’Osto,Michael Flynn,Marco Bellacicco,Jurgita Ovadnevaite,Kirsten N Fossum,Andreas Zuend

doi:10.1038/s41598-018-32047-4

Abstract

Atmospheric aerosols in clean remote oceanic regions contribute significantly to the global albedo through the formation of haze and cloud layers; however, the relative importance of ‘primary’ wind-produced sea-spray over secondary (gas-to-particle conversion) sulphate in forming marine clouds remains unclear. Here we report on marine aerosols (PM1) over the Southern Ocean around Antarctica, in terms of their physical, chemical, and cloud droplet activation properties. Two predominant pristine air masses and aerosol populations were encountered: modified continental Antarctic (cAA) comprising predominantly sulphate with minimal sea-salt contribution and maritime Polar (mP) comprising sulphate plus sea-salt. We estimate that in cAA air, 75% of the CCN are activated into cloud droplets while in mP air, 37% are activated into droplets, for corresponding peak supersaturation ranges of 0.37–0.45% and 0.19–0.31%, respectively. When realistic marine boundary layer cloud supersaturations are considered (e.g. ~0.2–0.3%), sea-salt CCN contributed 2–13% of the activated nuclei in the cAA air and 8–51% for the marine air for surface-level wind speed < 16 m s−1. At higher wind speeds, primary marine aerosol can even contribute up to 100% of the activated CCN, for corresponding peak supersaturations as high as 0.32%.

Highlights

Atmospheric aerosols in clean remote oceanic regions contribute significantly to the global albedo through the formation of haze and cloud layers; the relative importance of ‘primary’ windproduced sea-spray over secondary sulphate in forming marine clouds remains unclear
The two predominant air masses were continental Antarctic and maritime polar, while the less frequent air mass was maritime tropical which was generally associated with polluted incursions from South American outflow
In contrast to other oceanic waters, the continental air outflowing from the polar region after subsiding from the free troposphere, and even becoming modified with marine sources as it advects over broken pack-ice, has brought with it a quite low-complexity aerosol population, seemingly comprising almost exclusively of biogenic sulphate products and close to insignificant values of organic aerosol

Summary

Introduction

Atmospheric aerosols in clean remote oceanic regions contribute significantly to the global albedo through the formation of haze and cloud layers; the relative importance of ‘primary’ windproduced sea-spray over secondary (gas-to-particle conversion) sulphate in forming marine clouds remains unclear. The Earth’s radiative budget encapsulates the difference between incoming solar shortwave radiation, less its reflected and back-scattered component plus a longwave (absorbed and re-emitted) radiation component, effectively determining the global climate It is strongly influenced by the marine boundary layer (MBL) as ocean surfaces are darker and more absorbing on average than land masses, and with 70% share of the Earth’s surface. The proposed aerosol response is typically based on an increase in CCN in a warmer climate, which has been observed[9,10], leading to an increase in cloud droplet concentration and related reflectivity of low-level clouds (a net radiative cooling effect) It is still unknown whether environmental changes from increased cooling (or from other environmental factors such as increasing atmospheric CO2 concentrations) will incur positive or negative responses in DMS production, due to the complexity of its generation in nature[11]

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Conclusion