Contribution of carbonaceous aerosol to cloud condensation nuclei: processes and uncertainties evaluated with a global aerosol microphysics model
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Abstract. This paper explores the impacts of carbonaceous aerosol on cloud condensation nuclei (CCN) concentrations in a global climate model with size-resolved aerosol microphysics. Organic matter (OM) and elemental carbon (EC) from two emissions inventories were incorporated into a preexisting model with sulfate and sea-salt aerosol. The addition of carbonaceous aerosol increased CCN(0.2%) concentrations by 65–90% in the globally averaged surface layer depending on the carbonaceous emissions inventory used. Sensitivity studies were performed to determine the relative importance of the organic "solute effect", in which CCN concentrations increase because of the added soluble carbonaceous material, versus the "seeding effect", in which CCN concentrations increase because of increased particle number concentrations. In a sensitivity study where carbonaceous aerosol was assumed to be completely insoluble, concentrations of CCN(0.2%) still increased by 40–50% globally over the no carbonaceous simulation because primary carbonaceous emissions were able to become CCN via condensation of sulfuric acid. This shows that approximately half of the contribution of carbonaceous particles to CCN comes from the "seeding effect" and half from the "solute effect". The solute effect tends to dominate more in areas where there is less inorganic aerosol than organic aerosol and the seeding effect tends to dominate in areas where is more inorganic aerosol than organic aerosol. It was found that an accurate simulation of the number size distribution is necessary to predict the CCN concentration but assuming an average chemical composition will generally give a CCN concentration within a factor of 2. If a "typical" size distribution is assumed for each species when calculating CCN, such as is done in bulk aerosol models, the mean error relative to a simulation with size resolved microphysics is on the order of 35%. Predicted values of carbonaceous aerosol mass and aerosol number were compared to observations and the model showed average errors of a factor of 3 for carbonaceous mass and a factor of 4 for total aerosol number. These errors may be reduced by improving the emission size distributions of both primary sulfate and primary carbonaceous aerosol.Keywords:
Cloud condensation nuclei
Sea salt aerosol
Particle (ecology)
To understand how NO2 reacts with sea salt particles in the atmosphere of Mega-cities in coastal zones,the heterogeneous reaction of NO2 on the surface of wet sea salt was investigated with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and ion chromatography (IC).Kinetic measurements indicated that nitrate formation on sea salt was second order in NO 2 concentration and reactive uptake coefficients were (5.51 ± 0.19) × 10-7 and 1.26 × 10-6 respectively under 0% and 20% relative humidity (RH) at NO 2 molecular concentration of 1.96 × 1015 mol/cm3.The results showed that liquid water was formed at the site of MgCl2·6H2O,CaCl2·2H2O on the surface of sea salt and made the reaction more sustainable by releasing hydrated water and absorbing water from air even under a low RH (30%).Therefore,pure NaCl particles should not be used to represent sea salt in studies of the heterogeneous reaction with NO2.
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Ion chromatography
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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.
Cloud condensation nuclei
Sea salt aerosol
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Oceanic whitecaps are the major source of sea-salt aerosols. Sea-salt aerosols are the dominant natural aerosols in remote marine air and contribute to radiative and chemical processes affecting climate. The inclusion of the effects of sea-salt aerosols improves the predictions of climate models. The generation of sea-salt aerosols is the first of many processes that must be modeled. Modeling the production of sea-salt aerosols usually needs whitecap coverage, W, and its dependence on wind speed, U10. Various measurements of sea-salt production, however, do not constrain well the predictions of the sea-salt generation function using the relation W(U10) alone. Other variables, beside wind, affect the formation of whitecaps and production of sea-salt aerosols. In addition, the sea-salt generation function needs extension of its applicability toward smaller sea-salt aerosol sizes.
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Abstract Sulfate aerosols are typically the dominant source of cloud condensation nuclei (CCN) over remote oceans and their abundance is thought to be the dominating factor in determining oceanic cloud brightness. Their activation into cloud droplets depends on dynamics (i.e. vertical updrafts) and competition with other potential CCN sources for the condensing water. We present new experimental results from the remote Southern Ocean illustrating that, for a given updraft, the peak supersaturation reached in cloud, and consequently the number of droplets activated on sulfate nuclei, is strongly but inversely proportional to the concentration of sea-salt activated despite a 10-fold lower abundance. Greater sea-spray nuclei availability mostly suppresses sulfate aerosol activation leading to an overall decrease in cloud droplet concentrations; however, for high vertical updrafts and low sulfate aerosol availability, increased sea-spray can augment cloud droplet concentrations. This newly identified effect where sea-salt nuclei indirectly controls sulfate nuclei activation into cloud droplets could potentially lead to changes in the albedo of marine boundary layer clouds by as much as 30%.
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Liquid water content
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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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Sea salt particles are constantly produced from ocean surfaces by wave‐wind interactions and removed by deposition and precipitation scavenging. These particles constitute the background aerosol for light scattering in the marine boundary layer. In this work, the thermodynamic and optical properties of sea salt aerosol particles generated from seawater samples are measured at 25°C as a function of relative humidity, using a single‐particle levitation technique. Water activities, densities, and refractive indices of aqueous solution droplets containing a single salt NaCl, Na 2 SO 4 , MgCl 2 , or MgSO 4 are also reported as a function of concentration. The light‐scattering properties of the sea salt aerosol are modeled by the external mixture of these four salt systems selected to approximate the sea salt composition. Good agreements are obtained. It follows that in either visibility reduction or radiative forcing calculations, both freshly produced and aged sea salt aerosols may be modeled by external mixtures of the appropriate inorganic salts, whose solution properties are now available in the literature.
Sea salt aerosol
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Scavenging
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