Marine Cloud Condensation Nuclei Research Articles

Multi-campaign ship and aircraft observations of marine cloud condensation nuclei and droplet concentrations

In-situ marine cloud droplet number concentrations (CDNCs), cloud condensation nuclei (CCN), and CCN proxies, based on particle sizes and optical properties, are accumulated from seven field campaigns: ACTIVATE; NAAMES; CAMP2EX; ORACLES; SOCRATES; MARCUS; and CAPRICORN2. Each campaign involves aircraft measurements, ship-based measurements, or both. Measurements collected over the North and Central Atlantic, Indo-Pacific, and Southern Oceans, represent a range of clean to polluted conditions in various climate regimes. With the extensive range of environmental conditions sampled, this data collection is ideal for testing satellite remote detection methods of CDNC and CCN in marine environments. Remote measurement methods are vital to expanding the available data in these difficult-to-reach regions of the Earth and improving our understanding of aerosol-cloud interactions. The data collection includes particle composition and continental tracers to identify potential contributing CCN sources. Several of these campaigns include High Spectral Resolution Lidar (HSRL) and polarimetric imaging measurements and retrievals that will be the basis for the next generation of space-based remote sensors and, thus, can be utilized as satellite surrogates.

Rapid cloud removal of dimethyl sulfide oxidation products limits SO2 and cloud condensation nuclei production in the marine atmosphere

Oceans emit large quantities of dimethyl sulfide (DMS) to the marine atmosphere. The oxidation of DMS leads to the formation and growth of cloud condensation nuclei (CCN) with consequent effects on Earth's radiation balance and climate. The quantitative assessment of the impact of DMS emissions on CCN concentrations necessitates a detailed description of the oxidation of DMS in the presence of existing aerosol particles and clouds. In the unpolluted marine atmosphere, DMS is efficiently oxidized to hydroperoxymethyl thioformate (HPMTF), a stable intermediate in the chemical trajectory toward sulfur dioxide (SO2) and ultimately sulfate aerosol. Using direct airborne flux measurements, we demonstrate that the irreversible loss of HPMTF to clouds in the marine boundary layer determines the HPMTF lifetime (τHPMTF < 2 h) and terminates DMS oxidation to SO2 When accounting for HPMTF cloud loss in a global chemical transport model, we show that SO2 production from DMS is reduced by 35% globally and near-surface (0 to 3 km) SO2 concentrations over the ocean are lowered by 24%. This large, previously unconsidered loss process for volatile sulfur accelerates the timescale for the conversion of DMS to sulfate while limiting new particle formation in the marine atmosphere and changing the dynamics of aerosol growth. This loss process potentially reduces the spatial scale over which DMS emissions contribute to aerosol production and growth and weakens the link between DMS emission and marine CCN production with subsequent implications for cloud formation, radiative forcing, and climate.

CAICE Studies: Insights from a Decade of Ocean-Atmosphere Experiments in the Laboratory.

Ocean-atmosphere interactions control the composition of the atmosphere, hydrological cycle, and temperature of our planet and affect human and ecosystem health. Our understanding of the impact of ocean emissions on atmospheric chemistry and climate is limited relative to terrestrial systems, despite the fact that oceans cover the majority (71%) of the Earth. As a result, the impact of marine aerosols on clouds represents one of the largest uncertainties in our understanding of climate, which is limiting our ability to accurately predict the future temperatures of our planet. The emission of gases and particles from the ocean surface constitutes an important chemical link between the ocean and atmosphere and is mediated by marine biological, physical, and chemical processes. It is challenging to isolate the role of biological ocean processes on atmospheric chemistry in the real world, which contains a mixture of terrestrial and anthropogenic emissions. One decade ago, the NSF Center for Aerosol Impacts on Chemistry of the Environment (CAICE) took a unique ocean-in-the-laboratory approach to study the factors controlling the chemical composition of marine aerosols and their effects on clouds and climate. CAICE studies have demonstrated that the complex interplay of phytoplankton, bacteria, and viruses exerts significant control over sea spray aerosol composition and the production of volatile organic compounds. In addition, CAICE experiments have explored the physical production mechanisms and their impact on the properties of marine cloud condensation nuclei and ice nucleating particles, thus shedding light on connections between the oceans and cloud formation. As these ocean-in-the-laboratory experiments become more sophisticated, they allow for further exploration of the complexity of the processes that control atmospheric emissions from the ocean, as well as incorporating the effects of atmospheric aging and secondary oxidation processes. In the face of unprecedented global climate change, these results provide key insights into how our oceans and atmosphere are responding to human-induced changes to our planet.This Account presents results from a decade of research by chemists in the NSF Center for Aerosol Impacts on Chemistry of the Environment. The mission of CAICE involves taking a multidisciplinary approach to transform the ability to accurately predict the impact of marine aerosols on our environment by bringing the full real-world chemical complexity of the ocean and atmosphere into the laboratory. Toward this end, CAICE has successfully advanced the study of the ocean-atmosphere system under controlled laboratory settings through the stepwise simulation of physical production mechanisms and incorporation of marine microorganisms, building to systems that replicate real-world chemical complexity. This powerful approach has already made substantial progress in advancing our understanding of how ocean biology and physical processes affect the composition of nascent sea spray aerosol (SSA), as well as yielded insights that help explain longstanding discrepancies in field observations in the marine environment. CAICE research is now using laboratory studies to assess how real-world complexity, such as warming temperatures, ocean acidification, wind speed, biology, and anthropogenic perturbations, impacts the evolution of sea spray aerosol properties, as well as shapes the composition of the marine atmosphere.

Substantial Seasonal Contribution of Observed Biogenic Sulfate Particles to Cloud Condensation Nuclei

Biogenic sources contribute to cloud condensation nuclei (CCN) in the clean marine atmosphere, but few measurements exist to constrain climate model simulations of their importance. The chemical composition of individual atmospheric aerosol particles showed two types of sulfate-containing particles in clean marine air masses in addition to mass-based Estimated Salt particles. Both types of sulfate particles lack combustion tracers and correlate, for some conditions, to atmospheric or seawater dimethyl sulfide (DMS) concentrations, which means their source was largely biogenic. The first type is identified as New Sulfate because their large sulfate mass fraction (63% sulfate) and association with entrainment conditions means they could have formed by nucleation in the free troposphere. The second type is Added Sulfate particles (38% sulfate), because they are preexisting particles onto which additional sulfate condensed. New Sulfate particles accounted for 31% (7 cm−3) and 33% (36 cm−3) CCN at 0.1% supersaturation in late-autumn and late-spring, respectively, whereas sea spray provided 55% (13 cm−3) in late-autumn but only 4% (4 cm−3) in late-spring. Our results show a clear seasonal difference in the marine CCN budget, which illustrates how important phytoplankton-produced DMS emissions are for CCN in the North Atlantic.

Small fraction of marine cloud condensation nuclei made up of sea spray aerosol

Sea spray aerosols are thought to alter cloud properties in remote ocean regions. Aerosol analyses over four ocean regions reveal that these aerosols represent less than 30% of cloud condensation nuclei in typical marine boundary layer clouds.

Combined Effect of Glycine and Sea Salt on Aerosol Cloud Droplet Activation Predicted by Molecular Dynamics Simulations

The present study illustrates the combined effect of organic and inorganic compounds on cloud droplet nucleation and activation processes representative for the marine environment. Amino acids and sea salt are common marine cloud condensation nuclei (CCN) which act as a prerequisite for growth of cloud droplets. The chemical and physical properties of these CCN play a key role for interfacial properties such as surface tension, which is important for the optical properties of clouds and for heterogeneous reactions. However, there is a lack of detailed information and in situ measurements of surface tension of such nanosized droplets. Here we present a study of the combined effect of zwitterionic glycine (ZGLY) and sea salt in nanosized water droplets using molecular dynamics simulations, where particular emphasis is placed on the surface tension for the nanosized droplets. The critical supersaturation is estimated by the Köhler equation. It is found that dissolved sea salt interacts with ZGLY through a water bridge and weakens the hydrogen bonds among ZGLYs, which has a significant effect on both surface tension and water vapor supersaturation. Clusters of glycine mixed with sea salt deliquesce more efficiently and have higher growth factors.

Aircraft Measurements of Atmospheric Aerosol over North China in Autumn

Using an aircraft equipped with Passive Cavity Aerosol Spectrum Probe (PCASP-100X) and streamwise thermal gradient cloud condensation nuclei (CCN) counter, six flight measurements of atmospheric aerosol and CCN were conducted over North China in autumn. Five flight observations over Shijiazhuang urban area show that there is great difference of aerosol microphysical character between clear day and haze day. Vertical distributions show that aerosols mainly concentrated on the levels from near surface to the low level of troposphere and the concentration generally decreased with height increasing. The max aerosol concentration was 8110 cm-3on haze day and 1614 cm-3on clear day. There was more than 4 times difference in concentration of aerosol in boundary layer between haze day and clear day. CCN has the similar vertical distributions as aerosol. Most CCN concentrated on the boundary layer suggesting that the main source of CCN is from surface. Observation over Bohai Bay shows that the marine aerosol and CCN were greatly influenced by anthropogenic emissions.

Intercomparison of modal and sectional aerosol microphysics representations within the same 3-D global chemical transport model

Abstract. In the most advanced aerosol-climate models it is common to represent the aerosol particle size distribution in terms of several log-normal modes. This approach, motivated by computational efficiency, makes assumptions about the shape of the particle distribution that may not always capture the properties of global aerosol. Here, a global modal aerosol microphysics module (GLOMAP-mode) is evaluated and improved by comparing against a sectional version (GLOMAP-bin) and observations in the same 3-D global offline chemistry transport model. With both schemes, the model captures the main features of the global particle size distribution, with sub-micron aerosol approximately unimodal in continental regions and bi-modal in marine regions. Initial bin-mode comparisons showed that the current values for two size distribution parameter settings in the modal scheme (mode widths and inter-modal separation sizes) resulted in clear biases compared to the sectional scheme. By adjusting these parameters in the modal scheme, much better agreement is achieved against the bin scheme and observations. Annual mean surface-level mass of sulphate, sea-salt, black carbon (BC) and organic carbon (OC) are within 25% in the two schemes in nearly all regions. Surface level concentrations of condensation nuclei (CN), cloud condensation nuclei (CCN), surface area density and condensation sink also compare within 25% in most regions. However, marine CCN concentrations between 30° N and 30° S are systematically 25–60% higher in the modal model, which we attribute to differences in size-resolved particle growth or cloud-processing. Larger differences also exist in regions or seasons dominated by biomass burning and in free-troposphere and high-latitude regions. Indeed, in the free-troposphere, GLOMAP-mode BC is a factor 2–4 higher than GLOMAP-bin, likely due to differences in size-resolved scavenging. Nevertheless, in most parts of the atmosphere, we conclude that bin-mode differences are much less than model-observation differences, although some processes are missing in these runs which may pose a bigger challenge to modal schemes (e.g., boundary layer nucleation and ultra-fine sea-spray). The findings here underline the need for a spectrum of complexity in global models, with size-resolved aerosol properties predicted by modal schemes needing to be continually benchmarked and improved against freely evolving sectional schemes and observations.

Low sensitivity of cloud condensation nuclei to changes in the sea-air flux of dimethyl-sulphide

Abstract. The emission of dimethyl-sulphide (DMS) gas by phytoplankton and the subsequent formation of aerosol has long been suggested as an important climate regulation mechanism. The key aerosol quantity is the number concentration of cloud condensation nuclei (CCN), but until recently global models did not include the necessary aerosol physics to quantify CCN. Here we use a global aerosol microphysics model to calculate the sensitivity of CCN to changes in DMS emission using multiple present-day and future sea-surface DMS climatologies. Calculated annual fluxes of DMS to the atmosphere for the five model-derived and one observations based present day climatologies are in the range 15.1 to 32.3 Tg a−1 sulphur. The impact of DMS climatology on surface level CCN concentrations was calculated in terms of summer and winter hemispheric mean values of ΔCCN/ΔFluxDMS, which varied between −43 and +166 cm−3/(mg m−2 day−1 sulphur), with a mean of 63 cm−3/(mg m−2 day−1 sulphur). The range is due to CCN production in the atmosphere being strongly dependent on the spatial distribution of the emitted DMS. The relative sensitivity of CCN to DMS (i.e. fractional change in CCN divided by fractional change in DMS flux) depends on the abundance of non-DMS derived aerosol in each hemisphere. The relative sensitivity averaged over the five present day DMS climatologies is estimated to be 0.02 in the northern hemisphere (i.e. a 0.02% change in CCN for a 1% change in DMS) and 0.07 in the southern hemisphere where aerosol abundance is lower. In a globally warmed scenario in which the DMS flux increases by ~1% relative to present day we estimate a ~0.1% increase in global mean CCN at the surface. The largest CCN response occurs in the Southern Ocean, contributing to a Southern Hemisphere mean annual increase of less than 0.2%. We show that the changes in DMS flux and CCN concentration between the present day and global warming scenario are similar to interannual differences due to variability in windspeed. In summary, although DMS makes a significant contribution to global marine CCN concentrations, the sensitivity of CCN to potential future changes in DMS flux is very low. This finding, together with the predicted small changes in future seawater DMS concentrations, suggests that the role of DMS in climate regulation is very weak.

Contributions from DMS and ship emissions to CCN observed over the summertime North Pacific

Abstract. Measurements of cloud condensation nuclei (CCN) made over the North Pacific Ocean in July 2002 are analysed with concurrent measurements of aerosol number, mass and composition. Overall the CCN are controlled by the sulphate, including one case that suggests particle nucleation and growth resulting from dimethyl sulphide oxidation that enhanced CCN concentrations. Hourly CCN concentrations are correlated with concentrations of sulphate plus methanesulphonic acid (MSA) over the entire study period (r2=0.43 and 0.52 for supersaturations of 0.34% and 0.19%, respectively), and are not well correlated with other organics (r2&lt;0.2). One case study reveals elevated mass and number concentrations of ultrafine and fine organic particles due to regional ship emissions, identified through quadrupole aerosol mass spectrometer (Q-AMS) measurements, during which organic mass concentrations are correlated with CCN values (r2=0.39 and 0.46 for supersaturations of 0.19% and 0.34%, respectively). The evolution of the time series and mass distributions of organics, sulphate and MSA over this timeframe indicate that the regional distribution of small, diffuse ship-sourced organic particles act as condensation sites for sulphur species, resulting in a subsequent increase in number concentrations of CCN. We conclude that, where present, direct emissions of anthropogenic organic particles may exert a strong control on marine CCN concentrations once diffused into the marine atmosphere, by acting as condensation sites for biogenic and anthropogenic sulphur species.

Phytoplankton and Cloudiness in the Southern Ocean

The effect of ocean biological productivity on marine clouds is explored over a large phytoplankton bloom in the Southern Ocean with the use of remotely sensed data. Cloud droplet number concentration over the bloom was twice what it was away from the bloom, and cloud effective radius was reduced by 30%. The resulting change in the short-wave radiative flux at the top of the atmosphere was -15 watts per square meter, comparable to the aerosol indirect effect over highly polluted regions. This observed impact of phytoplankton on clouds is attributed to changes in the size distribution and chemical composition of cloud condensation nuclei. We propose that secondary organic aerosol, formed from the oxidation of phytoplankton-produced isoprene, can affect chemical composition of marine cloud condensation nuclei and influence cloud droplet number. Model simulations support this hypothesis, indicating that 100% of the observed changes in cloud properties can be attributed to the isoprene secondary organic aerosol.

Cloud condensation nuclei measurements at Mace Head, Ireland, over the period 1994–2002

Analyses of cloud condensation nuclei (CCN) number concentrations (cm − 3 ) measured at the Mace Head Atmospheric Research Station, near Carna, County Galway, Ireland, using a DH Associates Model M1 static thermal diffusion cloud chamber over the period from March 1994 to September 2002 are presented in this work. Air masses are defined as being ‘marine’ if they originate from a wind direction of 180–300° and ‘continental’ air masses are defined as originating from a wind direction of 45–135°. Air masses without such filtering were classified as ‘undefined’ air masses. Air masses were found to be dominated by marine sector air, re-affirming Mace Head as a baseline atmospheric research station. CCN levels for specific air masses at Mace Head were found to be comparable with earlier studies both at Mace Head and elsewhere. Monthly averaged clean marine (wind direction of 180–300° and black carbon absorption coefficient < 1.425 Mm − 1 ) CCN and marine CCN varied between 15–247 cm − 3 and 54–670 cm − 3 , respectively. As expected, significant increases in number concentration were found in continentally sourced CCN over that of marine CCN and were found to follow a log-normal distribution significantly tighter than that of clean marine air masses. No significant trend was found for CCN over the 9-year period. While polluted continental air masses showed a slight increase in CCN concentrations over the winter months, most likely due to increased fuel usage and a lower mixed boundary layer, the dominance of marine sector air arriving at Mace Head, which generally consists of background CCN concentrations, reduced seasonal differences for polluted air. Marine air showed a distinct seasonal pattern, with elevated values occurring over the spring and summer seasons. This is thought to be due to enhanced biogenic aerosol production as a result of phytoplankton bloom activity in the North Atlantic.

An ultrafine sea‐salt flux from breaking waves: Implications for cloud condensation nuclei in the remote marine atmosphere

Sea‐salt aerosol (SSA) is an important constituent of natural marine aerosol to which anthropogenic aerosols must be compared when assessing their climatic influence. Size distributions of particles, produced by bubbles from coastal oceanic breaking waves, were found to have sizes as small as 0.01 μm, with 60% smaller than 0.1 μm diameter. The thermal stability of these particles and their growth factor measured under increasing humidity indicate that most are sea salt. These SSA size distributions were used in conjunction with the measured number flux for bubbles from coastal breaking waves to develop a new sea‐salt source function. This source function and the associated SSA flux compare favorably with previously published estimates available for sizes larger than 0.5 μm but extend the source function down to much smaller sizes. When this SSA flux is applied to oceanic whitecaps that have a highly nonlinear dependence on wind speed, it implies strong regional and temporal differences in the open ocean surface number flux. In marine regions with little continental impact we estimate that this SSA flux can contribute ∼5–90% of the marine cloud condensation nuclei (CCN), with the rest accounted for by the flux of aerosol entrained from the free troposphere. These two fluxes are large enough to account for commonly observed aerosol and CCN concentrations in the clean MBL without requiring a nucleation source in the MBL. These observations have significant implications for modeling marine aerosol concentrations and evolution.

Growth of freshly nucleated particles in the troposphere: Roles of NH3, H2SO4, HNO3, and HCl

Growth of airborn, nanometer‐size nuclei was examined in a H2SO4‐HNO3‐HCl‐NH3‐H2O system using a single‐particle condensation model. Under conditions typical of the lower troposphere, growth is driven initially by the flux of sulfuric acid, followed by thermodynamic equilibration by the more abundant water and ammonia vapors. Pure sulfuric acid‐water‐ammonia condensation was shown to be rather slow and unable to produce cloud condensation nuclei (CCN) in the atmosphere. Under favorable conditions and after reaching a certain size, nuclei may start to absorb large quantities of nitric or hydrochloric acid together with ammonia, which produces CCN‐size particles over a timescale of a few minutes. The resulting CCN are susceptible to evaporation as the ambient conditions change but may become stable via cloud processing. The fast, HNO3‐HCl‐NH3‐driven condensation was estimated to occur frequently in continental aerosol systems. Marine CCN production cannot be explained by this process; it requires either the participation of other condensable vapors, such as organics, or a different growth mechanism.

Particle formation due to SO2 oxidation and high relative humidity in the remote marine boundary layer

Nucleation in the marine boundary layer affects the distribution of natural sulfur across the aerosol size spectrum and thus may contribute to marine cloud condensation nuclei (CCN) concentrations and climate forcing. We investigated how nucleation probability in remote marine areas develops under different conditions. Our study suggests that conditions favorable for particle formation correspond to transitions from moderate to high relative humidities. The highest nucleation probabilities were associated with transitions that (1) are relatively rapid, (2) occur near the local solar noon, and (3) are a result of the cooling of the air. The probability of these nucleation events occurring is greater under cleaner conditions and is probably more important than those associated with precipitation scavenging in remote marine air.

Atmospheric sulfur chemistry and cloud condensation nuclei (CCN) concentrations over the northeastern Pacific Coast

Correlated measurements of dimethylsulfide (DMS), gas phase dimethylsulfoxide (DMSO), methane‐sulfonic acid (MSA)(g), sulfuric acid (H2SO4), and cloud condensation nuclei (CCN) were conducted in April 1991 at a Pacific coastal site in northern Washington. Measurements of SO2, aerosol methanesulfonate (MSA)(p), and non‐sea‐salt sulfate (nss‐SO4) concentrations were also included. Maximum DMS concentrations between 100 and 240 pptv were observed when the measurement site (480 m above sea level) was embedded in clouds and air from the marine boundary layer was flowing upslope to the site. DMS levels measured in continental air and/or above the mixed layer were typically less than 20 pptv. The sulfur gases DMSO, H2SO4, and MSA(g) were measured in real time on a continuous basis (once every 60–150 s) using selected ion chemical ionization mass spectrometry. Corresponding concentrations ranged between &lt;0.5–3.2 pptv, 0.001–1.19 pptv, and 0.002–0.19 pptv, respectively. All three sulfur gases showed significant diel variations mostly in phase with each other. Their corresponding lifetimes in the marine atmosphere are estimated to be of the order of a few hours. The results in connection with recent laboratory studies and model calculations suggest that dimethylsulfone (DMSO2) was the dominant end product of DMS oxidation under the present conditions. CCN concentrations measured in marine air ranged roughly between 10–200 cm−3 and 200–400 cm−3 at 0.3% and 0.9% supersaturation, respectively. A statistical analysis using only data obtained in predominantly marine air and during non‐fog/non‐precipitation periods showed significant correlations between individual sulfur species and between CCN (0.3% ss) and H2SO4, and CCN (0.3% ss) and nss‐SO4. The results indirectly support a relationship between DMS and CCN concentrations. However, other observations also suggest that at higher supersaturations (0.9%), compounds less soluble than sulfate may become important in marine CCN formation.

Bistability of CCN concentrations and thermodynamics in the cloud-topped boundary layer

RECENT discussions of the effect of cloud condensation nuclei (CCN) in controlling the albedo of marine stratus cloud and hence global climate1–3have raised questions concerning the factors that control CCN populations and how (or whether) such factors should be included in climate models4. The observed near-constancy5 of CCN concentrations in remote marine air is remarkable in view of the wide variety of possible conditions, the range over which they might be expected to vary, and the considerable difference between marine and continental CCN concentrations. Here we explain this relative lack of variability, thought to be due to a balance of source and sink processes6,7, in a simple model of the marine cloud-topped boundary layer. Solutions to this model give two stable CCN concentration regimes—one corresponding to the low concentration observed over the ocean, the other corresponding to the higher concentration observed over land—each dominated by a different sink mechanism. The two regimes have very different optical properties, and thus may be of climatic significance.

Model test of CCN‐cloud albedo climate forcing

Cloud condensation nuclei (CCN) influence cloud albedo through their effect on the cloud droplet size distribution. A number of studies have evaluated the climatic impact of the CCN‐cloud albedo feedback, but all have assumed that cloud distributions, cloud thicknesses, and cloud liquid water contents would remain constant as the climate adjusted. We have tested this assumption using the Livermore version of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM1). Our results indicate that there are no significant compensating changes in cloud properties that would counteract the 1.7% global albedo increase resulting from a fourfold increase in marine CCN concentration. Furthermore, when ocean surface temperatures are decreased 4° C in a manner broadly consistent with the enhanced cloud albedos, we predict an increase in cloud fraction of 3.5% and a reduction in cloud altitude, leading to a positive feedback from clouds that would imply a climate impact roughly double that calculated from cloud droplet size distribution change alone.