Coupling sea‐salt and sulphate interactions and its impact on cloud droplet concentration predictions
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A parameterisation of internal mixing between sulphate and sea‐salt aerosol is developed to determine the available externally mixed sulphate cloud condensation nuclei (CCN) population. This parameterisation is then combined with a multi‐component aerosol‐cloud parameterisation to predict cloud droplet concentration incorporating the physical competition between sea‐salt and sulphate nuclei in the cloud nucleation processes. The results of the combined parameterisation indicate a significantly reduced role, compared to previous estimates, for sulphate in cloud droplet nucleation, and consequently, in indirect radiative forcing. However, the results also imply that cloud droplet concentration, and consequently, cloud albedo, has a greater susceptibility to change resulting from further anthropogenic SO 2 emissions.Keywords:
Cloud condensation nuclei
Sea salt aerosol
Cloud albedo
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Sea salt aerosol
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In this paper impacts of nss-sulfate, sea-salt and organic particles on microphysical properties of marine cloud are investigated, using a mutil-component size-resolving aerosol model. Numerical results show that the number and type of cloud condensation nuclei (CCN) depend on panicle physical and chemical properties (size distribution, chemical composition) of particles, and on environmental conditions (updrafts or supersaturation). Sea-salt particles play a critical role in cloud microphysical processes. Due to its large radius, sea-salt particles are activated into cloud drops in the initial cloud development. Sea-salt activation decreases supersaturation by consuming water vapor and suppresses nss-sulfate activation. Nss-sulfate indirect forcing may be overestimated in some conditions (such as updraft is low), because of the presence of sea-salt particles. Soluble organic components decrease maximum supersaturation, and lead to a decrease of cloud drops activated at the case of a high nss-sulfate and a high updraft velocity. Nss-sulfate CCN account for most variations of the cloud optical depth (COD). Sea-salt increases COD in the case of low nss-sulfate, but decreases COD when nss-sulfate concentration is high. The organic component enhances this influence of sea-salt on COD.
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Abstract The Arctic warms nearly four times faster than the global average, and aerosols play an increasingly important role in Arctic climate change. In the Arctic, sea salt is a major aerosol component in terms of mass concentration during winter and spring. However, the mechanisms of sea salt aerosol production remain unclear. Sea salt aerosols are typically thought to be relatively large in size but low in number concentration, implying that their influence on cloud condensation nuclei population and cloud properties is generally minor. Here we present observational evidence of abundant sea salt aerosol production from blowing snow in the central Arctic. Blowing snow was observed more than 20% of the time from November to April. The sublimation of blowing snow generates high concentrations of fine-mode sea salt aerosol (diameter below 300 nm), enhancing cloud condensation nuclei concentrations up to tenfold above background levels. Using a global chemical transport model, we estimate that from November to April north of 70° N, sea salt aerosol produced from blowing snow accounts for about 27.6% of the total particle number, and the sea salt aerosol increases the longwave emissivity of clouds, leading to a calculated surface warming of +2.30 W m −2 under cloudy sky conditions.
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Marine cloud brightening (MCB) is proposed to offset global warming by emitting sea salt aerosols to the tropical marine boundary layer, which increases aerosol and cloud albedo. Sea salt aerosol is the main source of tropospheric reactive chlorine (Cl
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Abstract The effect of sub‐cloud aerosol on cloud droplet concentration was explored over the north Atlantic and east Pacific under a variety of low and high wind speed conditions. A relationship of the form of D = 197{1 ‐ exp(‐6.13 × 10 3 * A )} was found to fit best the relationship between cloud droplet concentration ( D ; cm ‐3 ) and sub‐cloud aerosol concentration ( A ; cm ‐3 ) under low to moderate wind conditions. A few noticeable deviations from this relationship were observed which occurred under moderate to high wind speed condition. Under these high wind conditions, sea‐salt aerosol provided the primary source of cloud nuclei due to their higher nucleation activity and larger sizes, even under sulphate‐rich conditions. Simple model simulations reveal that the activation of sea‐salt nuclei suppresses the peak supersaturation reached in cloud, and thus inhibits the activation of smaller sulphate nuclei into cloud droplets. A multi‐component aerosol‐droplet parametrization for use in general circulation models is developed to allow prediction of cloud droplet concentration as a function of sea‐salt and non‐sea‐salt‐(nss) sulphate nuclei. The effects of enhancing an existing nss‐sulphate cloud condensation nuclei (CCN) population with sea‐salt nuclei are to reduce the number of cloud droplets activated under high (polluted) sulphate conditions and to increase the cloud droplet concentration under low (clean) sulphate conditions. The presence of sea‐salt CCN reduces the influence of nss‐sulphate CCN on cloud droplet concentrations, and thus is likely to reduce the predicted effect of nss‐sulphate indirect radiative forcing.
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The characteristics of the cloud drop size distribution near cloud base are initially determined by the aerosol particles that serve as cloud condensation nuclei (CCN) and the updraft velocity. Changes in CCN concentrations can change cloud drop number thereby affecting cloud optical properties and the global radiation budget. The CCN concentration depends on the composition and size distribution of the aerosol particles. Chemical reactions of the emitted gaseous sulfur compounds due to human activities will alter, through gas-to-particle conversion, the aerosol size distribution, total number, and its chemical composition. It is important to assess these changes in order to estimate the effect of anthropogenic sulfur emissions on cloud drop number concentrations. Here, we assume that the aerosol size distribution is modified, with total number unchanged, by two processes: condensation of sulfuric acid vapor (H2SO4) on a prescribed pre-existing particle size distribution and aqueous-phase oxidation of SO2 followed by evaporation of the drops. We examine the relationship between the resulting anthropogenic sulfate-containing aerosol size distribution and cloud drop number concentrations. The results are used to estimate the possible change in cloud optical thickness and cloud albedo due to an increase of anthropogenic sulfate mass concentration. This work is aimed at improving the assessment of the effects of anthropogenic sulfate on cloud optical properties and the global radiation budget.
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Abstract. We use a cloud-system-resolving model to study marine-cloud brightening. We examine how injected aerosol particles that act as cloud condensation nuclei (CCN) are transported within the marine boundary layer and how the additional particles in clouds impact cloud microphysical processes, and feedback on dynamics. Results show that the effectiveness of cloud brightening depends strongly on meteorological and background aerosol conditions. Cloud albedo enhancement is very effective in a weakly precipitating boundary layer and in CCN-limited conditions preceded by heavy and/or persistent precipitation. The additional CCN help sustain cloud water by weakening the precipitation substantially in the former case and preventing the boundary layer from collapse in the latter. For a given amount of injected CCN, the injection method (i.e., number and distribution of sprayers) is critical to the spatial distribution of these CCN. Both the areal coverage and the number concentration of injected particles are key players but neither one always emerges as more important than the other. The same amount of injected material is much less effective in either strongly precipitating clouds or polluted clouds, and it is ineffective in a relatively dry boundary layer that supports clouds of low liquid water path. In the polluted case and "dry" case, the CCN injection increases drop number concentration but lowers supersaturation and liquid water path. As a result, the cloud experiences very weak albedo enhancement, regardless of the injection method.
Cloud condensation nuclei
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Liquid water content
Marine stratocumulus
Supersaturation
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Sea salt aerosol
Cloud base
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Abstract. We use a cloud-system-resolving model to study marine-cloud brightening. We examine how injected aerosol particles that act as cloud condensation nuclei (CCN) are transported within the marine boundary layer and how the additional particles in clouds impact cloud microphysical processes, and feedback on dynamics. Results show that the effectiveness of cloud brightening depends strongly on meteorological and background aerosol conditions. Cloud albedo enhancement is very effective in a weakly precipitating boundary layer and in CCN-limited conditions preceded by heavy and/or persistent precipitation. The additional CCN help sustain cloud water by weakening the precipitation substantially in the former case and preventing the boundary layer from collapse in the latter. For a given amount of injected CCN, the injection method (i.e., number and distribution of sprayers) is critical to the spatial distribution of these CCN. Both the areal coverage and the number concentration of injected particles are key players but neither one always emerges as more important than the other. The same amount of injected material is much less effective in either strongly precipitating clouds or polluted clouds, and it is ineffective in a relatively dry boundary layer that supports clouds of low liquid water path. In the polluted case and "dry" case, the CCN injection increases drop number concentration but lowers supersaturation and liquid water path. As a result, the cloud experiences very weak albedo enhancement, regardless of the injection method.
Cloud condensation nuclei
Cloud albedo
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Marine stratocumulus
Supersaturation
Liquid water path
Sea salt aerosol
Albedo (alchemy)
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