Source Of Cloud Condensation Nuclei Research Articles

The impact of the 1783–1784 AD Laki eruption on global aerosol formation processes and cloud condensation nuclei

Abstract. The 1783–1784 AD Laki flood lava eruption commenced on 8 June 1783 and released 122 Tg of sulphur dioxide gas over the course of 8 months into the upper troposphere and lower stratosphere above Iceland. Previous studies have examined the impact of the Laki eruption on sulphate aerosol and climate using general circulation models. Here, we study the impact on aerosol microphysical processes, including the nucleation of new particles and their growth to cloud condensation nuclei (CCN) using a comprehensive Global Model of Aerosol Processes (GLOMAP). Total particle concentrations in the free troposphere increase by a factor ~16 over large parts of the Northern Hemisphere in the 3 months following the onset of the eruption. Particle concentrations in the boundary layer increase by a factor 2 to 5 in regions as far away as North America, the Middle East and Asia due to long-range transport of nucleated particles. CCN concentrations (at 0.22% supersaturation) increase by a factor 65 in the upper troposphere with maximum changes in 3-month zonal mean concentrations of ~1400 cm−3 at high northern latitudes. 3-month zonal mean CCN concentrations in the boundary layer at the latitude of the eruption increase by up to a factor 26, and averaged over the Northern Hemisphere, the eruption caused a factor 4 increase in CCN concentrations at low-level cloud altitude. The simulations show that the Laki eruption would have completely dominated as a source of CCN in the pre-industrial atmosphere. The model also suggests an impact of the eruption in the Southern Hemisphere, where CCN concentrations are increased by up to a factor 1.4 at 20° S. Our model simulations suggest that the impact of an equivalent wintertime eruption on upper tropospheric CCN concentrations is only about one-third of that of a summertime eruption. The simulations show that the microphysical processes leading to the growth of particles to CCN sizes are fundamentally different after an eruption when compared to the unperturbed atmosphere, underlining the importance of using a fully coupled microphysics model when studying long-lasting, high-latitude eruptions.

Growth and Shrinkage of New Particles in the Atmosphere in Hong Kong

Grown nucleated particles > 50 nm in diameter are an important source of cloud condensation nuclei (CCN) and when the size is > 100 nm, they can also have direct influence on the climate. In this study, the nucleation and growth of new particles in the atmosphere in Hong Kong were investigated during dry season (monthly averaged RH < 75%). The maximum size of grown nucleated particles was generally less than 40 nm during new particle burst and growth events. The exception, accounting for ∼ 20% of all burst and growth events, was those induced by strong photochemical reactions, in which subsequent particle shrinkage occurred. Temporal particle and gas concentration variability and meteorological conditions support the occurrence of particle shrinkage. The shrinkage rate calculated (∼ 8 nm h–1) was close to the growth rate. The observation of particle shrinkage sheds new light on particle transformation dynamics and it would add to the understanding of particle behavior in the atmosphere.

Cloud condensation nuclei from biomass burning during the Amazonian dry-to-wet transition season

Aircraft measurements of cloud condensation nuclei (CCN) during the Large-Scale Biosphere–Atmosphere Experiment in Amazonia (LBA) were conducted over the Southwestern Amazon region in September–October 2002, to emphasize the dry-to-wet transition season. The CCN concentrations were measured for values within the range 0.1–1.0% of supersaturation. The CCN concentration inside the boundary layer revealed a general decreasing trend during the transition from the end of the dry season to the onset of the wet season. Clean and polluted areas showed large differences. The differences were not so strong at high levels in the troposphere and there was evidence supporting the semi-direct aerosol effect in suppressing convection through the evaporation of clouds by aerosol absorption. The measurements also showed a diurnal cycle following biomass burning activity. Although biomass burning was the most important source of CCN, it was seen as a source of relatively efficient CCN, since the increase was significant only at high supersaturations.

Evidence of the cloud lifetime effect from wildfire‐induced thunderstorms

A case study is presented of pyro‐cumulonimbi (pyroCbs) forming over Canadian forest fires. Cloud‐top ice effective radius values of these pyroCbs are significantly smaller than are those within surrounding convection. The smoke provides a massive source of cloud condensation nuclei (CCN), resulting in smaller cloud droplets which freeze homogeneously at temperatures around −40°C and produce very small ice crystals. It is also shown that the pyroCb anvils persist 6–12 hours longer than convectively‐generated cirrus anvils from nearby convection. This provides evidence for the so‐called cloud lifetime effect, an aerosol indirect effect identified by the most recent Intergovernmental Panel on Climate Change (IPCC) report.

Simulation of Daily Variation of Suspended Particulate Matter over Delhi: Relative Roles of Vehicular Emission, Dust, and Domestic Appliances

Abstract The massive growth in the size and the population of cities over the past few decades has led to serious deterioration in the quality of air. One of the important constituents of airborne pollutants, which is a major health hazard, is suspended particulate matter (SPM). SPM is also an important source of cloud condensation nuclei; accurate simulations of SPM with sufficiently long lead thus have many applications, from issuing health advisories to forecasting fog. One of the biggest challenges in modeling air pollution in general and SPM in particular is to identify and mathematically represent the (location specific) sources and sinks. In this study the authors present a dynamical model for daily values of SPM over Delhi, India. The meteorological parameters are taken from the daily values from NCEP reanalysis. The validation is carried out against observations generated by the Central Pollution Control Board (CPCB) in India for the period 2000–05. Error statistics show that the model can capture a significant part of the observed variability of SPM. An evaluation of the relative contributions of various sources show that while vehicular pollution accounts for a large fraction of the SPM throughout the year, steep increases in the winter and the premonsoon periods are accounted for by fossil fuel burning and wind blown dust, respectively. Simulation with a doubling scenario for traffic congestion shows the effect to have strong seasonality. Such a model can be also interfaced with a seasonal forecast model or a climate model for enhanced scope of seasonal forecasts or for an investigation of the impact of SPM on regional climate change.

Influence of oceanic dimethyl sulfide emissions on cloud condensation nuclei concentrations and seasonality over the remote Southern Hemisphere oceans: A global model study

We use a global chemical transport model with size‐resolved aerosol microphysics to investigate the sources of cloud condensation nuclei (CCN) in the Southern Hemisphere remote marine boundary layer (MBL). Long‐term observations of CCN number at Cape Grim (40°41′S, 144°41′E) show a clear seasonal cycle with a 2–3 times higher concentration in summer than in winter, which has been attributed to seasonal changes in the dimethyl sulfide (DMS) ocean‐to‐atmosphere flux. We find that this cycle at Cape Grim and also throughout the 30°–45°S latitude band is caused mostly by changes in the regional‐scale DMS ocean water concentration. In this latitude band, DMS emissions increase the simulated CCN concentrations from November to April, with a maximum effect of 46% in January (calculated at 0.23% supersaturation). Farther south, the impact of DMS on CCN is apparent only from December to February and increases the CCN concentration at most by 18% at 45°–60°S and by 40% at 60°–75°S. These model‐derived contributions of DMS to Southern Ocean summertime CCN are smaller than the 80% derived from correlations between satellite‐observed chlorophyll and column CCN, which we explain in terms of nonlinear behavior of CCN from the free troposphere (FT). We show that the main microphysical pathway of DMS influence on CCN number is nucleation of DMS‐derived H2SO4 in the FT and subsequent growth of formed particles by condensation and coagulation during entrainment into the MBL. Our simulations suggest that &gt;90% of the increase in MBL CCN when DMS is added to the model is formed in this way. The growth of ultrafine sea spray particles to CCN sizes due to condensation of DMS‐derived H2SO4 in the MBL affects the simulated CCN concentrations by less than 6%. Overall, entrainment of nucleated sulfate aerosol into the MBL from the FT accounts for 43–65% of the summer zonal mean CCN concentrations but only 7–20% of the winter CCN over the Southern Hemisphere oceans.

Sensitivity Studies of the Role of Aerosols in Warm-Phase Orographic Precipitation in Different Dynamical Flow Regimes

Abstract Aerosols serve as a source of cloud condensation nuclei (CCN) and influence the microphysical properties of clouds. In the case of orographic clouds, it is suspected that aerosol–cloud interactions reduce the amount of precipitation on the upslope side of the mountain and enhance the precipitation on the downslope side when the number of aerosols is increased. The net effect may lead to a shift of the precipitation distribution toward the leeward side of mountain ranges, which affects the hydrological cycle on the local scale. In this study aerosol–cloud interactions in warm-phase clouds and the possible impact on the orographic precipitation distribution are investigated. Herein, simulations of moist orographic flow over topography are conducted and the influence of anthropogenic aerosols on the orographic precipitation formation is analyzed. The degree of aerosol pollution is prescribed by different aerosol spectra that are representative for central Switzerland. The simulations are performed with the Consortium for Small-Scale Modeling’s mesoscale nonhydrostatic limited-area weather prediction model (COSMO) with a horizontal grid spacing of 2 km and a fully coupled aerosol–cloud parameterization. It is found that an increase in the aerosol load leads to a downstream shift of the orographic precipitation distribution and to an increase in the spillover factor. A reduction of warm-phase orographic precipitation is observed at the upslope side of the mountain. The downslope precipitation enhancement depends critically on the width of the mountain and on the flow dynamics. In the case of orographic precipitation induced by stably stratified unblocked flow, the loss in upslope precipitation is not compensated by leeward precipitation enhancement. In contrast, flow blocking may lead to leeward precipitation enhancement and eventually to a compensation of the upslope precipitation loss. The simulations also indicate that latent heat effects induced by aerosol–cloud–precipitation interactions may considerably affect the orographic flow dynamics and consequently feed back on the orographic precipitation development.

CCN activity and droplet growth kinetics of fresh and aged monoterpene secondary organic aerosol

Abstract. The ability of secondary organic aerosol (SOA) produced from the ozonolysis of α-pinene and monoterpene mixtures (α-pinene, β-pinene, limonene and 3-carene) to become cloud droplets was investigated. A static CCN counter and a Scanning Mobility CCN Analyser (a Scanning Mobility Particle Sizer coupled with a Continuous Flow counter) were used for the CCN measurements. Consistent with previous studies monoterpene SOA is quite active and would likely be a good source of cloud condensation nuclei (CCN) in the atmosphere. A decrease in CCN activation diameter for α-pinene SOA of approximately 3 nm hr−1 was observed as the aerosol continued to react with oxidants. Hydroxyl radicals further oxidize the SOA particles thereby enhancing the particle CCN activity with time. The initial concentrations of ozone and monoterpene precursor (for concentrations lower than 40 ppb) do not appear to affect the activity of the resulting SOA. Köhler Theory Analysis (KTA) is used to infer the molar mass of the SOA sampled online and offline from atomized filter samples. The estimated average molar mass of online SOA was determined to be 180±55 g mol−1 (consistent with existing SOA speciation studies) assuming complete solubility. KTA suggests that the aged aerosol (both from α-pinene and the mixed monoterpene oxidation) is primarily water-soluble (around 65%). CCN activity measurements of the SOA mixed with (NH4)2SO4 suggest that the organic can depress surface tension by as much as 10 N m−1 (with respect to pure water). The droplet growth kinetics of SOA samples are similar to (NH4)2SO4, except at low supersaturation, where SOA tends to grow more slowly. The CCN activation diameter of α-pinene and mixed monoterpene SOA can be modelled to within 10–15% of experiments by a simple implementation of Köhler theory, assuming complete dissolution of the particles, no dissociation into ions, a molecular weight of 180 g mol−1, a density of 1.5 g cm−3, and the surface tension of water.

Aerosol–cloud–precipitation interactions. Part 1. The nature and sources of cloud-active aerosols

Atmospheric aerosol particles serve as condensation nuclei for the formation of both, cloud droplets and atmospheric ice particles. As a result, they exert a substantial influence on the microphysical properties of water and ice clouds, which in turn affect the processes that lead to the formation of rain, snow, hail, and other forms of precipitation. In recent years, considerable progress has been made in understanding the chemical composition of aerosols, their microphysical properties, and the factors that enable them to act as cloud condensation nuclei (CCN) and ice nuclei (IN). The first part of this review article will focus on the nature and sources of CCN and IN. We discuss the fundamentals of the cloud droplet and ice nucleation processes, and the role that the chemical composition and particle size play in this process. We show that, in many instances, the influence of chemical composition can be represented by a simple parameterization, which leaves particle size as the main variable controlling CCN efficiency. Aerosol particles are produced either directly by anthropogenic and natural sources (dust, sea salt, soot, biological particles, etc.), or they are formed in the atmosphere by condensation of low-volatility compounds (e.g., sulfuric acid or oxidized organic compounds). We discuss the magnitude of these sources, and the CCN and IN characteristics of the particles they produce. In contrast to previous assessments, which focused on the aerosol mass, we are emphasizing the number of particles being produced, as this is the key variable in cloud microphysics. Large uncertainties still exist for many aerosol sources, e.g., the submicron part of the seaspray aerosol, the particles produced by the biosphere, and the secondary organic aerosol. We conclude with a discussion on what particle concentrations may have been in the pristine atmosphere, before the onset on anthropogenic pollution. Model calculations and observations in remote continental regions consistently suggest that CCN concentrations over the pristine continents were similar to those now prevailing over the remote oceans, suggesting that human activities have modified cloud microphysics more than what is reflected in conventional wisdom. The second part of this review will address the effects of changing CCN and IN abundances on precipitation processes, the water cycle, and climate.

Contribution of particle formation to global cloud condensation nuclei concentrations

We use a global aerosol microphysics model to predict the contribution of boundary layer (BL) particle formation to regional and global distributions of cloud condensation nuclei (CCN). Including an observationally derived particle formation scheme, where the formation rate of molecular clusters is proportional to gas‐phase sulfuric acid to the power one, improves modeled particle size distribution and total particle number concentration at three continental sites in Europe. Particle formation increases springtime BL global mean CCN (0.2% supersaturation) concentrations by 3–20% and CCN (1%) by 5–50%. Uncertainties in particle formation and growth rates must be reduced before the accuracy of these predictions can be improved. These results demonstrate the potential importance of BL particle formation as a global source of CCN.

Doppler lidar observations of Russian forest fire plumes over Helsinki

In August 2006, forest fires in north-western Russia led to elevated concentrations of fine particles reducing air quality in southern Finland over a number of weeks. Predominately easterly and south-easterly winds resulted in smoke plumes extending over the Gulf of Finland and affecting eastern and southern Finland during 7–14 August 2006 and 21–23 August 2006. This article describes observations using Doppler lidar of two episodes where smoke plumes from the Russian forest fires were evident over the Helsinki area on 7 and 9 August 2006. The observations were made during a convective field campaign, part of the Helsinki Testbed, an international mesoscale meteorology research project running from January 2005 until September 2007 (Dabberdt et al., 2005). During the 2006 fire season, a total of 23 046 fires were recorded, affecting 999 080 hectares of forested lands and 579 234 hectares of non-forested lands with most fires reported in the Kareliya, Leningradskaya and Murmanskaya regions of north-western and western Russia (Global Fire Monitoring Center, 2006). Emissions from boreal fires are important because of their location at climatically sensitive northern latitudes. The main pollutants released during vegetation fires are aerosols, carbon monoxide and dioxide, nitrogen oxides and non-methane hydrocarbons. These emissions play an important role in the atmospheric chemistry and radiative properties of the atmosphere (Damoah et al., 2004). Smoke and aerosol particles scatter sunlight and reflect it directly back to space thereby having a cooling effect on the atmosphere by increasing cloud reflectance (Kaufman and Fraser, 1997). Smoke particles are also a major source of cloud condensation nuclei (CCN) and increases in CCN affect the microphysics of clouds, thereby altering cloud albedo, affecting the way clouds absorb and reflect sunlight. Emissions from forest fires also have a warming effect on the atmosphere, as greenhouse gases are emitted contributing to the greenhouse effect. Biomass burning is thought to account for as much as 25% of the total global emission of greenhouse gases (Rebelo, 2006). Changes in the concentration of carbon monoxide and aerosols affect ozone, which plays an important role in the global climate system (Daniel and Solomon, 1998). In 1998, global boreal forest regions experienced some 17.9 million hectares of fire and it has been estimated from models that global fire emissions contribute 8.9% of total carbon emissions, 13.8% of carbon monoxide emissions and 12.4% of methane emissions (Kasischke and Bruhwiler, 2003). Occurrence of smoke plumes over southern Finland

Cloud condensation nucleus production from nucleation events at a highly polluted region

Atmospheric nucleation events have received increasing attention as a potentially important source of aerosol particles affecting climate and human health. Here, we report 2.5 years of continuous nucleation observations from San Pietro Capofiume, Italy (44°39′N, 11°37′W). We show that nucleation events occur frequently in the Po Valley region, even though the region is rather polluted with high pre‐existing particle concentrations. The nucleation events are often very intensive, and the newly formed particles can grow to sizes as large as 100–200 nm in diameter within a few hours. Our calculations of the enhancement of particle number concentrations in different size ranges and comparison with steady‐state and regional emission estimates indicate that the nucleation events produce a significant fraction of particles larger than 100 nm, and thus constitute an important source of cloud condensation nuclei (CCN).

Formation Pathways and Composition of Iodine Oxide Ultra-Fine Particles

Environmental Context.Bursts of ultra-fine particles (diameter &lt; 10 nm) in the daytime coastal marine boundary layer at low tide coincide with the observation of iodine oxide radicals. The detection of iodine in the particles suggests a direct link between the biogenic emission of iodine-containing vapours and subsequent particle nucleation and growth. These coastal aerosols are therefore most likely iodine oxide polymers. However, the reaction pathways leading to the homogeneous nucleation of these particles are currently an area of uncertainty, as is their final composition. These ultra-fine particles are potentially important as a source of cloud condensation nuclei, and as a major pathway for enriching iodine in marine aerosol. Abstract.Iodine oxide nanoparticles were generated photochemically from I2 in the presence of O3, and their morphology and composition analyzed by transmission electron microscopy (TEM). The particles exhibit fractal morphologies consistent with agglomerative coagulation, and have an O/I ratio of 2.45 ± 0.08, indicating that they are composed of I2O5. Quantum calculations show that gas-phase I2O5 could be formed by a series of exothermic reactions involving the oxidation of I2O2, I2O3 and I2O4 by O3. In order to form pure I2O5 particles, modelling calculations indicate that the rate coefficients for these reactions probably need to be faster than 6 × 10−13 cm3 molecule−1 s−1 at 295 K. Applying this model to the atmosphere shows that ultra-fine iodine oxide particles formed in the coastal marine boundary layer would then consist of I2O5.

Reply to comment by N. D. Marsh and H. Svensmark on “Solar influences on cosmic rays and cloud formation: A reassessment”

[1] Svensmark and Friis-Christensen [1997] (hereinafter referred to as SFC97) proposed a ‘‘galactic cosmic ray (GCR), clouds, and climate’’ hypothesis that cosmic ray flux, modulated by solar activity, may modify global total cloud cover (TCC) and thus global surface temperature by changing the number of ions in the atmosphere and thus the cloud droplet formation. This GCR-TCC hypothesis has been questioned by many authors who examined correlations of GCR with various satellite cloud properties and over different time intervals. We note that the SFC97 hypothesis differs from the one proposed by Marsh and Svensmark [2000] (hereinafter referred to as MS00), who postulated that the GCR influence on cloudiness is restricted to low cloud cover (LCC). [2] All the articles in this debate were based on satellite data and limited to the ocean. By contrast, Sun and Bradley [2002] (hereinafter SB02) combined short-period satellite data (from the International Satellite Cloud Climatology Project, ISCCP) with long-term visual total cloud observations to reassess the GCR-TCC relationship over the ocean and the land as well. The SB02 study was initialized in early 2000, focusing on the GCR-TCC hypothesis. In the early spring of 2001 we added to our revision several comments regarding the GCR-LCC hypothesis. We concluded in our analysis that there is no solid evidence for the existence of a GCR-cloud correlation (either total cloud or low cloud cover). [3] Marsh and Svensmark [2004] (hereinafter referred to as MS04) raised five comments about our analysis and finally claimed that ‘‘On the basis of satellite observations, there continue to be strong indications of a globally distributed correlation between cosmic rays and low-cloud cover.’’ We will demonstrate in this reply that their comments are either unfounded or illogical. All of their comments are dealt with in section 2, but issues relevant to the GCR-LCC hypothesis will be separately addressed in section 3. New evidence will be present in section 3 to indicate that the quality of the ISCCP infrared (IR) LCC data set, which MS00 and Marsh and Svensmark [2003] (hereinafter referred to as MS03) used to create the GCR-LCC hypothesis, is highly questionable. This reply confirms the view we expressed on the work of SB02 that there is no solid GCR-cloud relationship.

The sensitivity of dimethyl sulfide production to simulated climate change in the Eastern Antarctic Southern Ocean

Dimethyl sulfide (DMS) is a radiatively active trace gas produced by enzymatic cleavage of its precursor compound, dimethyl sulfoniopropionate (DMSP), which is released by marine phytoplankton in the upper ocean. Once ventilated to the atmosphere, DMS is oxidised to form non-sea-salt sulfate and methane sulfonate (MSA) aerosols, which are a major source of cloud condensation nuclei (CCN) in remote marine air and may thus play a role in climate regulation. Here we simulate the change in DMS flux in the Eastern Antarctic ocean from 1960–2086, corresponding to equivalent CO 2 tripling relative to pre-industrial levels. Calibration to contemporary climate conditions was carried out using a genetic algorithm to fit the model to surface chlorophyll from the 4-yr SeaWiFs satellite archive and surface DMS from an existing global database. Following the methodology used previously in the Subantarctic Southern Ocean, we then simulated DMS emissions under enhanced greenhouse conditions by forcing the DMS model with output from a coupled atmospheric–ocean general circulation model (GCM). The GCM was run in transient mode under the IPCC/IS92a radiative forcing scenario. By 2086, the change simulated in annual integrated DMS flux is around 20% in ice-free waters, with a greater increase of 45% in the seasonal ice zone (SIZ). Interestingly, the large increase in flux in the SIZ is not due to higher in situ production but mainly because of a loss of ice cover during summer–autumn and an increase in sea-to-air ventilation of DMS. These proportional changes in areal mean flux (25%) are much higher than previously estimated for the Subantarctic Southern Ocean (5%), and point to the possibility of a significant DMS–climate feedback at high Southern latitudes. Due to the nexus between ice cover and food-web structure, the potential for ecological community shifts under enhanced greenhouse conditions is high, and the implications for DMS production are discussed. DOI: 10.1034/j.1600-0889.2003.00077.x

Spatial and seasonal variations in CCN distribution and the aerosol‐CCN relationship over southern Africa

The sources and transport of cloud condensation nuclei (CCN) over southern Africa have been investigated using in situ measurements collected during the Aerosol Recirculation and Rainfall Experiment (ARREX) wet season projects and during the Southern African Regional Science Initiative (SAFARI‐2000) intensive dry season campaign. CCN concentrations over the subcontinent are generally higher in the dry season than in the wet season and exceed 2000 cm−3in highly polluted air masses. In the late dry season, CCN concentrations are highest in the northern regions of the subcontinent due to the burning of savanna biomass. Emissions from industries and power plants on the South African Highveld are a prolific year‐round source of CCN and are sufficient to account for CCN levels south of 20°S throughout the year. Most CCN are contained within the mixing layer, which extends to an altitude of 3000–4000 m over the plateau and is capped by a temperature inversion. Multiple inversions and absolutely stable layers control the stratification of aerosols and CCN. Biomass burning particles are efficient CCN, and the median diameter of the accumulation mode is large (up to 0.19 μm). Recently emitted industrial aerosols are less soluble and have a smaller median diameter (0.11 μm). Twice as many aerosols act as CCN in the dry season (68%) than in the wet season (34%). The fraction is highest in the dry season over the tropical regions (&gt;80%), where smoke aerosol predominates. Elevated aerosol and CCN concentrations are expected to have implications for direct radiative forcing, especially in the dry season, and for indirect forcing in the wet season.

Potential impact of climate change on marine dimethyl sulfide emissions

Dimethyl sulfide (DMS) is a biogenic compound produced in sea-surface water and outgased to the atmosphere. Once in the atmosphere, DMS is a significant source of cloud condensation nuclei in the unpolluted marine atmosphere. It has been postulated that climate may be partly modulated by variations in DMS production through a DMS-cloud condensation nuclei-albedo feedback. We present here a modelled estimation of the response of DMS sea-water concentrations and DMS fluxes to climate change, following previous work on marine DMS modeling (Aumont et al., 2002) and on the global warming impact on marine biology (Bopp et al., 2001). An atmosphere—ocean general circulation model (GCM) was coupled to a marine biogeochemical scheme and used without flux correction to simulate climate response to increased greenhouse gases (a 1% increase per year in atmospheric CO2 until it has doubled). The predicted global distribution of DMS at 1 × CO2 compares reasonably well with observations; however, in the high latitudes, very elevated concentrations of DMS due to spring and summer blooms of Phaeocystis can not be reproduced. At 2 × CO2, the model estimates a small increase of global DMS flux to the atmosphere (+2%) but with large spatial heterogeneities (from −15% to +30% for the zonal mean). Mechanisms affecting DMS fluxes are changes in (1) marine biological productivity, (2) relative abundance of phytoplankton species and (3) wind intensity. The mean DMS flux perturbation we simulate represents a small negative feedback on global warming; however, the large regional changes may significantly impact regional temperature and precipitation patterns.

Cloud activation of single‐component organic aerosol particles

The organic fraction of ambient aerosols is a complex mixture of hundreds of organic compounds varying in chemical structure and physical properties. The cloud condensation nuclei (CCN) activities of single‐component organic particles were experimentally investigated. Activation diameters were determined using a Tandem Differential Mobility Analyzer and a thermal diffusion Cloud Condensation Nucleus Counter. Studies were performed at supersaturations of 0.3 and 1% with dry particle diameters ranging between 0.02 and 0.2 μm. The focus was on both hygroscopic secondary organics and hydrophobic primary organics. Laboratory experiments were performed with sodium chloride, ammonium sulfate, glutaric acid, adipic acid, pinonic acid, glutamic acid, leucine, cholesterol, pinic acid, norpinic acid, hexadecane, hexadecanol, myristic acid, palmitic acid, and stearic acid. The results were compared with the classical Köhler theory and a theory that accounts for the limited solubility of many organics. It was discovered that organic species with bulk solubilities in water less than 0.01 g cm−3 can be a good source of CCN in the atmosphere if their contact angles with water are zero. Experiments confirmed that Köhler theory works well in predicting activation of soluble inorganic species and organics that are wettable by water, while an extension of the theory appears necessary to account for the low‐solubility organic species that are not wettable by water.

Chemical composition and sources of the high Arctic aerosol relevant for cloud formation

The chemical composition, sources, and methods of production of cloud condensation nuclei (CCN) were studied over the pack ice covered Arctic Ocean in summer. After ∼2 days' travel of air over the pack ice, two local CCN sources were identified: particles derived from the bursting bubbles on open water between ice floes (film and jet drops) and particles reaching CCN size by acquisition of oxidation products of dimethyl sulfide. The bubble‐derived particle modes were found to have an exponential dependence on prevailing wind speed. The film drop generation depended less on wind speed and jet drop production depended more on wind speed than particles over open water did. However, the enhanced presence of film drop particles at wind speeds &lt;5 m s−1 suggested a source of bubbles, other than wind, that is most active during clear sky days and ice melting. The film drop particles were mainly organic in nature, but their sea‐salt component increased at high wind speeds &gt;12 m s−1 to dominate both mass and number. The jet drop mode was composed largely of sea salt, making a significant mass contribution already for wind speeds &gt;5 m s−1. However, the smaller‐sized jet drops were not pure sodium chloride but contained a varied and appreciable organic component. The sulfur‐containing particles dominated the CCN population and mass for the most common wind speeds of 5–12 m s−1. Measured CCN concentrations were on average less than would have been expected from either a sulfate or a sea‐salt composition and the observed particle‐size distribution. It is concluded that other components, probably organic, depressed the nucleating ability of the particles. However, on clear sky days, there were a majority of occasions on which CCN concentrations were more than predicted from a sulfate composition and the measured size distribution.

Dimethylsulphide production in the subantarctic southern ocean under enhanced greenhouse conditions

Dimethylsulphide (DMS) is an important sulphur-containing trace gas produced by enzymatic cleavage of its precursor compound, dimethylsulphoniopropionate (DMSP), which is released by marine phytoplankton in the upper ocean. After ventilation to the atmosphere, DMS is oxidised to form sulphate aerosols which in the unpolluted marine atmosphere are a major source of cloud condensation nuclei (CCN). Because the micro-physical properties of clouds relevant to climate change are sensitive to CCN concentration in air, it has been postulated that marine sulphur emissions may play a rôle in climate regulation. The Subantarctic Southern Ocean (41–53°S) is relatively free of anthropogenic sulphur emissions, thus sulphate aerosols will be mainly derived from the biogenic source of DMS, making it an ideal region in which to evaluate the DMS-climate regulation hypothesis. We have extended a previous modelling analysis of the DMS cycle in this region by employing a coupled general circulation model (CGCM) which has been run in transient mode to provide a more realistic climate scenario. The CGCM output provided meteorological data under the IPCC/IS92a radiative forcing scenario. A DMS production model has been forced with the CGCM climate data to simulate the trend in the sea-to-air DMS flux for the period 1960 to 2080, corresponding to equivalent CO₂ tripling relative to pre-industrial levels. The results confirm a minor but non-negligible increase in DMS flux in this region, in the range +1% to +6% predicted over the period simulated. Uncertainty analysis of the DMS model predictions have confirmed the positive sign for the change in DMS flux, that is a negative DMS feedback on warming.