Summertime Primary and Secondary Contributions to Southern Ocean Cloud Condensation Nuclei

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%.

Figure 1

Map of Southern Ocean region, ship course marked in red. Movement of air masses from three principal source regions shown in blue arrows; continental Antarctic (cAA), maritime Polar (mP) from the West, and maritime tropical (mT) or modified-mT, both from north and northwest of −60° latitude. Approximate summer regions of pack ice (green striped) and lasting sheet ice (black striped) in the Weddell Sea are shown.

Figure 2

Physical and chemical properties of cAA (Weddell Sea influenced) air masses (a) and mP air masses (b). All data from each steady-state case are lumped together into an air mass average. Bottom left: Median number-size distribution (black) and volumetric size distribution (blue), with D_p the SMPS-derived dry particle diameter. Shaded grey area represents 25^th–75^th percentile range with the total particle number and corresponding volume concentrations noted at the top. Bottom right: On top, the ratio of CCN to all particles greater than 20 nm, on bottom, the total number of CCN for varying supersaturation. Shaded range of supersaturations represent typical values for marine stratocumulus clouds. Top left: CCN activation efficiency as a function of critical supersaturation and particle diameter, on top, the Inter-modal minima point indicated in blue, with 10% SMPS sizing error (the blue shaded region corresponds to the spread of S_c values considering SMPS sizing errors) and the total number concentration of particles at sizes greater than the inter-modal minima. In black, fitted CCN activation curve obtained from the measurements with corresponding error bars. The red line represents partially neutralized sulphate according to the DON indicated (using mixture of H₂SO₄ + (NH₄)₂SO₄), the olive brown line represents NaCl, and the purple dashed line represents ammonia-neutralised MSA (i.e. NH₄-MSA salt) where all lines are based on predictions by the AIOMFAC model. Top right: Pie chart of chemical mass fractions, with the total mass concentration, average black carbon concentration, and DON_mol noted on top. The smaller pie chart is the breakdown of the non-sea-salt aerosol species.