Enhancement of marine cloud albedo via controlled sea spray injections: a global model study of the influence of emission rates, microphysics and transport

Abstract

Abstract. Modification of cloud albedo by controlled emission of sea spray particles into the atmosphere has been suggested as a possible geoengineering option to slow global warming. Previous global studies have imposed changes in cloud drop concentration in low level clouds to explore the radiative and climatic effects. Here, we use a global aerosol transport model to quantify how an imposed flux of sea spray particles affects the natural aerosol processes, the particle size distribution, and concentrations of cloud drops. We assume that the proposed fleet of vessels emits sea spray particles with a wind speed-dependent flux into four regions of persistent stratocumulus cloud off the western coasts of continents. The model results show that fractional changes in cloud drop number concentration (CDNC) vary substantially between the four regions because of differences in wind speed (which affects the spray efficiency of the vessels), transport and particle deposition rates, and because of variations in aerosols from natural and anthropogenic sources. Using spray emission rates comparable to those implied by previous studies we find that the predicted CDNC changes are very small (maximum 20%) and in one of the four regions even negative. The weak or negative effect is because the added particles suppress the in-cloud supersaturation and prevent existing aerosol particles from forming cloud drops. A scenario with five times higher emissions (considerably higher than previously assumed) increases CDNC on average by 45-163%, but median concentrations are still below the 375 cm −3 assumed in previous studies. An inadvertent effect of the spray emissions is that sulphur dioxide concenCorrespondence to: H. Korhonen (hannele.korhonen@fmi.fi) trations are suppressed by 1-2% in the seeded regions and sulphuric acid vapour by 64-68% due to chemical reactions on the additional salt particles. The impact of this suppression on existing aerosol is negligible in the model, but should be investigated further in the real environment so that inadvertent impacts can be excluded.

Marine cloud brightening – as effective without clouds

“…The size of the injected particles in this study is in the same size range as most previous ESM studies on sea spray climate engineering that simulate the aerosol injection (e.g. Alterskjaer et al, 2012Jones and Haywood, 2012;Korhonen et al, 2010;Muri et al, 2015;and Wang et al, 2011). It should also be mentioned that extensive measurements show that organics contribute substantially to the composition of sea spray aerosol, and in many areas is even the dominant constituent (e.g.…”

“…One of the advantages of simulating sea spray climate engineering in ESMs through sea salt aerosol emissions, compared to just increasing the CDNC, is that the cloud droplet activation process is taken into account. Previous studies have shown that injection of sea spray particles in some circumstances may actually reduce the CDNC due to increased competition for water vapour and reduced activation of background aerosol particles (Korhonen et al, 2010;Alterskjaer et al, 2012). showed in a single-model study that while the injection of accumulation-mode particles increased the CDNC, the injections of Aitken-or coarse-mode particles could have the opposite effect with a reduction in CDNC.…”

Modelling artificial sea salt emission in large eddy simulations

“…perpendicular to the wind direction as suggested by Salter et al [7]. The vessel emits particles of 300 nm dry diameter [14,15], 25 m above the sea surface [7], at a rate of 15 kg s −1 (half the rate suggested by Salter et al [7]) for about 40 min and travels across the domain (4.8 km) once. From this emission rate, assuming the salinity of seawater to be 35 g kg −1 and NaCl being the sole solute, we derived a particle number flux of 1.72 × 10 16 s −1 .…”

Ocean–Atmosphere Interactions of Particles

“…The recognition of the importance of sea salt nuclei in marine cloud processes has led to the interesting suggestion that global warming could be ameliorated somewhat through controlled enhancement of albedo and lifetime in low level marine clouds , in other words, geoengineering, whereby artificial floating sea spray generators are deployed in regions of persistent stratocumulus clouds. While intriguing as a geo-engineering solution to global warming, Korhonen et al (2010b), using a global model, and simulating emissions from a fleet of spray-emitting vessels in four regions of persistent stratocumulus fields found increases in cloud droplet concentration of maximum 20 %, and even a reduction was predicted in one region.…”