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In convective clouds, satellite-observed deepening or increased amount of clouds with increasing aerosol concentration has been reported and is sometimes interpreted as aerosol-induced invigoration of the clouds. However, such correlations can be affected by meteorological factors that affect both aerosol and clouds, as well as observational issues. In this study, we examine the behaviour in a 660x660 km2 region of the South Pacific during June 2007, previously found by Koren et al., [2014] to show strong correlation between cloud fraction, cloud top pressure and aerosols, using a cloud-resolving model with meteorological boundary conditions specified from a reanalysis. The model assumes constant aerosol loading, yet reproduces vigorous clouds at times of high real-world aerosol concentrations. Days with high and low aerosol loading exhibit deep-convective and shallow clouds respectively, in both observations and the simulation. Synoptic analysis shows that vigorous clouds occur at times of strong surface troughs, which are associated with high winds and advection of boundary-layer air from the Southern Ocean where sea salt aerosol is abundant, thus accounting for the high correlation. Our model results show that aerosol-cloud relationships can be explained by coexisting but independent wind-aerosol and wind-cloud relationships and that no cloud condensation nuclei (CCN) effect is required.

Prior observations have documented the process of cloud cleansing, through which cloudy, polluted air from a continent is slowly transformed into cloudy, clean air typical of a maritime environment. During that process, cloud albedo changes gradually, followed by a sudden reduction in cloud fraction and albedo as drizzle forms and convection changes from closed to open cellular. Experiments in a cloud chamber that generates a turbulent environment show a similar cloud cleansing process followed by rapid cloud collapse. Observations of 1) cloud droplet size distribution, 2) interstitial aerosol size distribution, 3) cloud droplet residual size distribution, and 4) water vapor supersaturation are all consistent with the hypothesis that turbulent fluctuations of supersaturation accelerate the cloud cleansing process and eventual cloud collapse. Decay of the interstitial aerosol concentration occurs slowly at first then more rapidly. The accelerated cleansing occurs when the cloud phase relaxation time exceeds the turbulence correlation time.

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 (Cly) and bromine (Bry). The effects of additional sea salt on atmospheric chemistry have not been explored. We simulate sea salt aerosol injections for MCB under two scenarios (212–569 Tg/a) in the GEOS‐Chem global chemical transport model, only considering their impacts as a halogen source. Globally, tropospheric Cly and Bry increase (20–40%), leading to decreased ozone (−3 to −6%). Consequently, OH decreases (−3 to −5%), which increases the methane lifetime (3–6%). Our results suggest that the chemistry of the additional sea salt leads to minor total radiative forcing compared to that of the sea salt aerosol itself (~2%) but may have potential implications for surface ozone pollution in tropical coastal regions., Key Points Sea salt aerosol emissions for Marine Cloud Brightening geoengineering are implemented in a global chemical transport modelThis leads to changes in global tropospheric Bry and Cly (+20 to 40%), ozone (−3 to −6%), OH (−2 to −4%), and methane lifetime (+3 to 6%)Chemistry of the added sea salt leads to minor total radiative forcing (−20 to −50 mW/m2) but may have implications for ozone pollution

After many field studies, the establishment of connections between marine microbiological processes, sea spray aerosol (SSA) composition, and cloud condensation nuclei (CCN) has remained an elusive challenge. In this study, we induced algae blooms to probe how complex changes in seawater composition impact the ability of nascent SSA to act as CCN, quantified using the apparent hygroscopicity parameter (κapp). Throughout all blooms, κapp ranged between 0.7 – 1.4 (average 0.95 ± 0.15), consistent with laboratory investigations using algae-produced organic matter, but differing from climate model parameterizations and in situ SSA generation studies. The size distribution of nascent SSA dictates that changes in κapp associated with biological processing induce less than 3% change in expected CCN concentrations for typical marine cloud supersaturations. The insignificant effect of hygroscopicity on CCN concentrations suggests that the SSA production flux and/or secondary aerosol chemistry may be more important factors linking ocean biogeochemistry and marine clouds.

[1] Primary-produced sea spray aerosol, typically comprising sea-salt, but also enriched with organic matter (OM) in biologically active oceanic regions, impacts the global radiative budget through contributions to aerosol optical depth. We show that sea spray light-scattering enhancement, f(RH), as a function of relative humidity (RH) is suppressed when enriched with OM. A new hygroscopic growth factor parameterization reveals a dual hygroscopicity state, flipping from high hygroscopicity and high f(RH) to low hygroscopicity and low f(RH) as the OM mixing volume percentage exceeds ~ 55% in sea spray. Under elevated wind speeds, this affects the Top-of-atmosphere direct radiative forcing (ΔF) by reducing the cooling contribution of sea spray by ~ 5.5 times compared to pure sea-salt spray. These results suggest a positive feedback coupling between the marine biosphere, sea spray aerosol, and the direct radiative budget.

Natural aerosol plays a significant role in the Earth's system due to its ability to alter the radiative balance of the Earth. Here we use a global aerosol microphysics model together with a radiative transfer model to estimate radiative effects for five natural aerosol sources in the present-day atmosphere: dimethyl sulfide (DMS), sea-salt, volcanoes, monoterpenes, and wildfires. We calculate large annual global mean aerosol direct and cloud albedo effects especially for DMS-derived sulfate (-0.23 Wm and -0.76 Wm, respectively), volcanic sulfate (-0.21 Wm and -0.61 Wm) and sea-salt (-0.44 Wm and -0.04 Wm ). The cloud albedo effect responds nonlinearly to changes in emission source strengths. The natural sources have both markedly different radiative efficiencies and indirect/direct radiative effect ratios. Aerosol sources that contribute a large number of small particles (DMS-derived and volcanic sulfate) are highly effective at influencing cloud albedo per unit of aerosol mass burden. Key Points New, consistent estimates of natural aerosol radiative effects are given We find substantial variability in natural aerosol radiative efficiency Non-linear sensitivity of cloud albedo effects to emission source strength.

Marine cloud brightening (MCB) is a proposed technique to limit global warming through injections of sea spray into the marine boundary layer. Using the Norwegian Earth System Model, the sensitivity of MCB to sea salt amount and particle size was studied by running a set of simulations in which Aitken (re = 0.04 µm), accumulation (re = 0.22 µm), or coarse (re = 2.46 µm) mode sea salt emissions were increased uniformly by 10−11 to 10−8 kg m−2 s−1. As desired, accumulation mode particles had a negative radiative effect of down to −3.3 W m−2. Conversely, for Aitken mode particles, injections of 10−10 kg m−2 s−1 or greater led to a positive forcing of up to 8.4 W m−2, caused by a strong competition effect combined with the high critical supersaturation of Aitken mode sea salt. The coarse mode particles gave a positive forcing of up to 1.2 W m−2 because of a decrease in activation of background aerosols. Sensitivity experiments show that the competition effect dominated our results. MCB may have a cooling effect, but if the wrong size or injection amount is used, our simulations show a warming effect on the climate system.

Frost flowers collected from the surface of new sea ice near the Brunt Ice Shelf, Antarctica, show depletion in sulphate and sodium relative to other sea water ions. This is consistent with loss of mirabilite (Na2SO4) during formation of the brine from which the frost flowers grow. Aerosol generated from frost flowers would have higher sodium:sulphate ratios than aerosol generated from sea water. This would explain low values of non-sea-salt sulphate encountered in winter aerosol, and winter layers in ice cores, at coastal Antarctic sites. Calculations confirm that the frost flower source should be significant compared to an open water source for coastal regions.

Atmospheric particles scatter incoming solar radiation and cause regional cooling for areas in which they increase the net backscattering. Stratospheric aerosols have been widely observed to have this cooling effect on the Earth's surface following major volcanic eruptions. A smaller effect is expected to result from the regular entrainment of aerosols in the troposphere from natural and anthropogenic surface sources. Over long time scales, even a small cooling by tropospheric aerosols could be significant in offsetting warming caused by greenhouse gases. We looked for indications of this effect at the ocean's surface, when the albedo is raised in the presence of tropospheric aerosols. To test whether a correlation exists between tropospheric aerosol outbreaks and a cooler ocean surface, we compared satellite-derived aerosol optical depth (AOD) measurements to in situ sea surface temperatures (SSTs) over a four year period. We selected a subtropical North Atlantic region where the upwind mobilization of desert dust results in an aerosol signal large enough to outweigh other factors which cause cooling of the sea surface. In this location, the differing seasonal variations of aerosol amount, mixed layer depth, and cloudiness permit a glimpse of the causal relationship between aerosols and temperature changes at the sea surface. Although we were limited by in situ SST coverage, we found temperature perturbations correlated to large aerosol outbreaks from the Sahara Desert.

The pH of deliquesced sea-salt aerosol was derived from the Cl phase partitioning measured in marine air, corresponding meteorological conditions, and the thermodynamic properties of HCl. Under moderately polluted conditions at Bermuda during spring 1996, calculated sea-salt aerosol pH ranged from the mid-2s to the mid-3 s. Cl− and NO3−concentrations in the lower half of the sea-salt size distribution were near thermodynamic equilibrium with respect to HClg and HNO3g; shorter aerosol lifetimes and slower reaction kinetics involving larger aerosol sustained moderate phase disequilibria. The active multiphase cycling of HCl buffered sea-salt aerosol pH to approximately the same value across the particle size spectrum.

[1] Aerosol indirect effects, i.e., the interactions of aerosols with clouds by serving as cloud condensation nuclei or ice nuclei constitute the largest uncertainty in climate forcing and projection. Previous IPCC reported negative aerosol indirect forcing, which does not account for aerosol-convective cloud interactions because the complex processes involved are poorly understood and represented in climate models. Here we elucidated how aerosols change convective intensity, diabatic heating, and regional circulation under different environmental conditions. We found that aerosol indirect effect on deep convective cloud systems could lead to enhanced regional convergence and a strong top-of-atmosphere warming. Aerosol invigoration effect occurs mainly in warmed-based convection with weak shear. This could result in a strong radiative warming in the atmosphere (up to +5.6 W m−2), a lofted latent heating, and a reduced diurnal temperature difference, all of which could potentially impact regional circulation and modify weather systems. The positive aerosol radiative forcing on deep clouds could offset the negative aerosol radiative forcing on low clouds to an unknown extent.

[1] In contrast to fine anthropogenic aerosols (radii ∼

[1] Effects of natural and anthropogenic aerosol particles on the radiation budget in cloudy atmospheres are still a major research topic. For example, can an increase or decrease in aerosol particle number, originating from changed dimethylsulfide (DMS) and isoprene emissions by marine phytoplankton, impact the earth radiation budget via increasing or decreasing planetary albedo and lifetime of clouds? And if so, is a shifted cloud droplet spectrum accompanied by a regional change in precipitation? Here, we show by a synergistic analysis of satellite observations (MODIS, SeaWiFS, AIRS, SSM/I and CERES) that the phytoplankton related emission of the mentioned gases into the atmosphere strongly influences cloud properties within a broad latitude belt in the Southern Hemisphere during the austral summer. For this season we detected indirect aerosol effects over the Southern Ocean from 45°S to 65°S, especially in regions with plankton blooms, indicated by high chlorophyll-a concentration in seawater. The strong increase in cloud condensation nuclei column content from 2.0 × 108 to more than 5.0 × 108 CCN/cm2 for a chlorophyll increase from 0.3 to about 0.5 mg/m3 in these regions decreases cloud droplet effective radius and increases cloud optical thickness for water clouds. Consequently, the upward short-wave radiative flux at the top of the atmosphere increases. Our analysis also reveals reduced precipitation over the Antarctic Polar Frontal Zone during strong plankton blooms. We suggest that due to fine particles formed in the atmosphere originating from gaseous DMS and possibly isoprene emissions the reduction of precipitation is caused by delayed homogeneous freezing in water clouds.

The ice-core record of the carbon-13 content of atmospheric methane (δ13CH4) has largely been used to constrain past changes in methane sources. The aim of this paper is to explore, for the first time, the contribution that changes in the strength of a minor methane sink―oxidation by atomic chlorine in the marine boundary layer (ClMBL)―could make to changes in δ13CH4 on glacial-interglacial timescales. Combining wind and temperature data from a variety of general circulation models with a simple formulation for the concentration of ClMBL, we find that changes in the strength of this sink, driven solely by changes in the atmospheric circulation, could have been responsible for changes in δ13CH4 of the order of 10% of the glacial-interglacial difference observed. We thus highlight the need to quantify past changes in the strength of this sink, including those relating to changes in the sea-ice source of sea salt aerosol.

[1] The number of cloud droplets formed from a population of aerosols depends on the aerosol number concentration, NA, the size distribution, and the chemical composition. The cloud albedo effect occurs when increasing NA causes increases to the droplet concentration, ND. We examined the effects of changing aerosol size, composition, and number on ND within the United States. We found that changing the water-soluble organic carbon (WSOC) fraction from 50% to 0.05% in the fine mode aerosol and from 50% to 95% in the coarse mode aerosol decreased ND by an average of 34%. Our results show that the changes to the aerosol composition cause over a 20% change to ND, a magnitude previously estimated to cause a 1 W m−2 change in radiative forcing. Given the realistic range of aerosol compositions used here, it is not possible for global models to correctly calculate the cloud albedo effect if composition is ignored.

[1] Trends from April 1998 to March 2008 in surface global irradiance, aerosol surface direct radiative forcing, and aerosol optical properties (optical thickness, single scattering albedo, and asymmetry factor) were investigated based on measurements of the surface solar radiation under clear skies at Tsukuba, Japan. The optical properties were estimated by optimizing the external mixture of four assumed aerosols (water soluble, soot, sea salt, and dust) to the surface direct and diffuse irradiances in visible and near infrared regions. The surface global irradiance was increased by an increase of single scattering albedo, not by a decrease of optical thickness. Furthermore, the increasing trend of single scattering albedo may be related to the reduced emission of black carbon in East Asia.

[1] We have used a global three-dimensional chemical transport model coupled to a detailed size-resolved aerosol microphysics module to study the impact of BrO on dimethylsulfide (DMS) in the remote marine boundary layer. Our model results suggest BrO contributes 16% of the global annual DMS oxidation sink. This effect is most profound over the SH oceans where low NOx concentrations and a high sea salt aerosol source, coupled with high DMS concentrations, drives a large contribution of BrO to DMS oxidation (>20%). Bromine chemistry also results in an 18% reduction in the global DMS burden and lifetime. In addition, when we use an alternative DMS source parameterization resulting in a factor 2 increase in DMS flux the release of bromine from sea salt aerosol increases by 50–60% in the southern hemisphere summer because of additional aerosol acidity. This suggests a possible DMS-SO2-sea salt-BrO marine aerosol feedback mechanism that acts to reduce the sensitivity of the DMS lifetime to increases in DMS emission.

It is illustrated how climate forcing by stratospheric aerosols depends on aerosol properties. The climate forcing is a function of aerosols size distribution, but the size dependence can be described well by a single parameter: the area-weighted mean radius, r(eff). If r(eff) is greater than about 2 microns, the global average greenhouse effect of the aerosols exceeds the albedo effect, causing a surface heating. The aerosol climate forcing is less sensitive to other characteristics of the size distribution, the aerosol composition, and the altitude of the aerosols. Thus stratospheric aerosol forcing can be defined accurately from measurements of aerosol extinction over a broad wavelength range.

[1] Cloud responses to changes in aerosol remain a dominant uncertainty in the radiative forcing of climate. Two main constructs related to aerosol effects on clouds have been postulated: (i) the “albedo effect” whereby anthropogenic aerosol results in increased droplet concentrations that generate increases in cloud albedo, all else (particularly cloud water) being equal; (ii) the “lifetime effect” whereby anthropogenic aerosol suppresses precipitation and results in clouds with more liquid water, higher fractional cloudiness, and longer lifetimes. Based on new observations presented here, and supported by previous fine-scale modeling studies, we suggest that the balance of evidence shows that non-precipitating cumulus clouds can experience an evaporation-entrainment feedback, and respond to aerosol perturbations in a manner inconsistent with the traditional “lifetime effect.” Because most cumulus clouds evaporate without producing significant precipitation, this is particularly relevant to estimates of aerosol indirect effects on climate.

[1] Snow lying on sea ice could be a potentially important source of sea salt aerosol, as small snow particles, rich in salts, can be easily lifted into the air though blowing-snow events. Using a measured distribution of snow salinity on Antarctic sea ice and a blowing snow sublimation parameterization, we derive a method for estimating sea salt aerosol production, and bromine release, during blowing-snow events. Compared with sea salt aerosol production rates from the open ocean, we find that the aerosol production rate from snow can be more than an order of magnitude larger per unit area under typical weather conditions. The large sea ice cover may thus enhance the supply of sea salt to the polar lower atmosphere. This is consistent with observations of sea salt aerosol seasonality and with the ice-core record. This large emission of sea salt from snow also implies an additional tropospheric bromine source in these regions.

[1] A new approach to sea salt parameterization is proposed which incorporates wind-wave characteristics into the sea salt emission function and can be employed globally and under swell-influenced conditions. The new source function was applied into Navy Aerosol Analysis and Prediction System model together with predictions from the global wave model Wave Watch III. The squared surface wind velocity U10 and the wave's orbital velocity Vorb=πHs/TP are shown to be the key parameters in the proposed parameterization. Results of the model simulations are validated against multi-campaign shipboard measurements of the sea salt aerosol. The validations indicate a good correlation between Vorb and the measured surface concentrations. The model simulations with the new parameterization exhibit an improved agreement with the observations when compared to a wind-speed-only approach. The proposed emission parameterization has the potential to improve the simulations of sea salt emission in aerosol transport models.

[1] Results from a numerical one-dimensional model show that continental pollution that is advected over the ocean leads to substantial acid displacement from newly emitted sea salt aerosol. Oxidation of HCl by OH and cycling between the gas and aerosol phase lead to a build-up of nighttime Cl2 mixing ratios of up to 90 pmol mol−1 in our base scenario and 125 pmol mol−1 in a sensitivity study with high wind speeds. Therefore the recirculation of continental airmasses over the ocean and back to the coast might explain previous measurements of high nighttime Cl2 in onshore winds. Furthermore, the model runs suggest a substantial contribution of daytime Cl-radical chemistry to the oxidation of volatile organic compounds under these conditions.

[1] One important role of anthropogenic aerosol particles is their influence on climate by acting as cloud condensation nuclei. However, these particles are diverse in composition and mixing state, and our knowledge of which particle types act as cloud condensation nuclei is incomplete. Here we present direct measurements of individual organic particles that nucleated cloud droplets in the atmosphere. These results indicate that nitrogenated organic aerosol particles can act as cloud condensation nuclei without being mixed with inorganic material, and thus influence climate through cloud formation.

[1] Scattering of incoming solar radiation by sea salt aerosol is strongly dependent on relative humidity (RH) since hygroscopic particles take up water at high RH. Organic compounds may constitute up to 50% of marine aerosol mass in internal mixtures. We used a detailed thermodynamic and optical model to calculate hygroscopic growth and extinction of sea salt aerosol internally mixed with a soluble organic compound. Increasing organic content from 10 to 50% suppresses growth at high RH compared to a pure NaCl particle by 4 to 20%. For a mildly absorbing organic, the scattering increase with RH is reduced by up to 32% for these mixtures, consistent with observations. Internal mixtures of 90% NaCl and 10% non-absorbing organics cause 3% less cooling than 100% NaCl particles in the visible spectrum over the clear-sky oceans. For a mildly absorbing organic compound, 10% organic content reduces radiative cooling substantially compared to 100% NaCl aerosol.

[1] High concentrations of small atmospheric aerosols are known to reduce the size of cloud droplets, increase cloud albedo and suppress precipitation formation. In contrast, cloud simulations suggest that even low concentrations of large soluble aerosols should promote droplets' growth and rainfall. Until now, though, no observational evidence of such microphysical effects in natural circumstance over land has been presented. By using NOAA-AVHRR retrievals on cases where salt-dust from the Aral Sea interacts with clouds we show that large salt-containing dust particles increase cloud drops to sizes that promote precipitation. These findings are in line with the findings of the microphysical models and recent results from hygroscopic cloud seeding experiments for rain enhancement.

Sea spray aerosol particles are an integral part of the Earth's radiation budget. To date, the inorganic composition of nascent sea spray aerosol particles has widely been assumed to be equivalent ...