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Cloud condensation nuclei (CCN) can affect cloud properties and therefore the Earth’s radiative balance. New particle formation (NPF) from condensable vapours in the free troposphere has been suggested to contribute to CCN, especially in remote, pristine atmospheric regions, but direct evidence is sparse, and the magnitude of this contribution is uncertain. Here we use in-situ aircraft measurements of vertical profiles of aerosol size distributions to present a global-scale survey of NPF occurrence. We observed intense NPF occurring at high altitude in tropical convective regions over both the Pacific and Atlantic Oceans. Together with the results of chemical-transport models, our findings indicate that NPF persists at all longitudes as a global-scale band in the tropical upper troposphere, covering about 40% of the Earth’s surface. Furthermore, we find that this NPF in the tropical upper troposphere is a globally important source of CCN in the lower troposphere, where they can affect cloud properties. Our findings suggest that the production of CCN, as these new particles descend towards the surface, is currently not adequately captured in global models, because they tend to underestimate both the magnitude of tropical upper tropospheric NPF and the subsequent growth to CCN sizes. This has potential implications for cloud albedo and the global radiative balance.

Owing to the low-gravity conditions in space, space-borne laboratories enable experiments with extended free-fall times. Because Bose-Einstein condensates have an extremely low expansion energy, space-borne atom interferometers based on Bose-Einstein condensation have the potential to have much greater sensitivity to inertial forces than do similar ground-based interferometers. On 23 January 2017, as part of the sounding-rocket mission MAIUS-1, we created Bose-Einstein condensates in space and conducted 110 experiments central to matter-wave interferometry, including laser cooling and trapping of atoms in the presence of the large accelerations experienced during launch. Here we report on experiments conducted during the six minutes of in-space flight in which we studied the phase transition from a thermal ensemble to a Bose-Einstein condensate and the collective dynamics of the resulting condensate. Our results provide insights into conducting cold-atom experiments in space, such as precision interferometry, and pave the way to miniaturizing cold-atom and photon-based quantum information concepts for satellite-based implementation. In addition, space-borne Bose-Einstein condensation opens up the possibility of quantum gas experiments in low-gravity conditions .

Sea spray aerosols impact Earth's radiation balance by directly scattering solar radiation. They also act as cloud condensation nuclei, thereby altering cloud properties including reflectivity, lifetime and extent. The influence of sea spray aerosol on cloud properties is thought to be particularly strong over remote ocean regions devoid of continental particles. Yet the contribution of sea spray aerosol to the population of cloud condensation nuclei in the marine boundary layer remains poorly understood. Here, using a lognormal-mode-fitting procedure, we isolate sea spray aerosols from measurements of particle size and abundance over the Pacific, Southern, Arctic and Atlantic oceans to determine the contribution of sea spray aerosol to the population of cloud condensation nuclei in the marine boundary layer. On a global basis, with the exception of the high southern latitudes, sea spray aerosol makes a contribution of less than 30% to the cloud condensation nuclei population for air that is supersaturated at 0.1 to 1.0%--the supersaturation range typical of marine boundary layer clouds. Instead, the cloud condensation nuclei population between 70° S and 80° N is composed primarily of non-sea-salt sulfate aerosols, due to large-scale meteorological features that result in entrainment of particles from the free troposphere.

Surface-active compounds present in aerosols can increase their cloud condensation nuclei (CCN) activation efficiency by reducing the surface tension (σ) in the growing droplets. However, the importance of this effect is poorly constrained by measurements. Here we present estimates of droplet surface tension near the point of activation derived from direct measurement of droplet diameters using a continuous flow streamwise thermal gradient chamber (CFSTGC). The experiments used sea spray aerosol (SSA) mimics composed of NaCl coated by varying amounts of (i) oleic acid, palmitic acid or myristic acid, (ii) mixtures of palmitic acid and oleic acid, and (iii) oxidized oleic acid. Significant reductions in σ relative to that for pure water were observed for these mimics at relative humidity (RH) near activation (∼ 99.9 %) when the coating was sufficiently thick. The calculated surface pressure (π = σH2O − σobserved) values for a given organic compound or mixture collapse onto one curve when plotted as a function of molecular area for different NaCl seed sizes and measured RH. The observed critical molecular area (A0) for oleic acid determined from droplet growth was similar to that from experiments conducted using macroscopic solutions in a Langmuir trough. However, the observations presented here suggest that oleic acid in microscopic droplets may exhibit larger π values during monolayer compression. For myristic acid, the observed A0 compared well to macroscopic experiments on a fresh subphase, for which dissolution has an important impact. A significant kinetic limitation to water uptake was observed for NaCl particles coated with pure palmitic acid, likely as a result of palmitic acid (with coating thicknesses ranging from 67 to 132 nm) being able to form a solid film. However, for binary palmitic-acid–oleic-acid mixtures there was no evidence of a kinetic limitation to water uptake. Oxidation of oleic acid had a minor impact on the magnitude of the surface tension reductions observed, potentially leading to a slight reduction in the effect compared to pure oleic acid. A CCN counter was also used to assess the impact on critical supersaturations of the substantial σ reductions observed at very high RH. For the fatty-acid-coated NaCl particles, when the organic fraction (εorg) was > 0.90 small depressions in critical supersaturation were observed. However, when εorg < 0.90 the impact on critical supersaturation was negligible. Thus, for the fatty acids considered here, the substantial σ reductions observed at high RH values just below activation have limited impact on the ultimate critical supersaturation. A surface film model is used to establish the properties that surface-active organic molecules must have if they are to ultimately have a substantial impact on the activation efficiency of SSA. To influence activation, the average properties of surface-active marine-derived organic molecules must differ substantially from the long-chain fatty acids examined, having either smaller molecular volumes or larger molecular areas. The model results also indicate that organic-compound-driven surface tension depression can serve to buffer the critical supersaturation against changes to the organic-to-salt ratio in particles in which the organic fraction is sufficiently large.

Long-range transport of biogenic emissions from the coast of Antarctica, precipitation scavenging, and cloud processing are the main processes that influence the observed variability in Southern Ocean (SO) marine boundary layer (MBL) condensation nuclei (CN) and cloud condensation nuclei (CCN) concentrations during the austral summer. Airborne particle measurements on the HIAPER GV from north-south transects between Hobart, Tasmania, and 62.sup.\" S during the Southern Ocean Clouds, Radiation Aerosol Transport Experimental Study (SOCRATES) were separated into four regimes comprising combinations of high and low concentrations of CCN and CN. In 5 d HYSPLIT back trajectories, air parcels with elevated CCN concentrations were almost always shown to have crossed the Antarctic coast, a location with elevated phytoplankton emissions relative to the rest of the SO in the region south of Australia. The presence of high CCN concentrations was also consistent with high cloud fractions over their trajectory, suggesting there was substantial growth of biogenically formed particles through cloud processing. Cases with low cloud fraction, due to the presence of cumulus clouds, had high CN concentrations, consistent with previously reported new particle formation in cumulus outflow regions. Measurements associated with elevated precipitation during the previous 1.5 d of their trajectory had low CCN concentrations indicating CCN were effectively scavenged by precipitation. A coarse-mode fitting algorithm was used to determine the primary marine aerosol (PMA) contribution, which accounted for 20 % of CCN (at 0.3 % supersaturation) and cloud droplet number concentrations. Vertical profiles of CN and large particle concentrations (D.sub.p 0.07 µm) indicated that particle formation occurs more frequently above the MBL; however, the growth of recently formed particles typically occurs in the MBL, consistent with cloud processing and the condensation of volatile compound oxidation products. CCN measurements on the R/V Investigator as part of the second Clouds, Aerosols, Precipitation, Radiation and atmospheric Composition Over the southeRn Ocean (CAPRICORN-2) campaign were also conducted during the same period as the SOCRATES study. The R/V Investigator observed elevated CCN concentrations near Australia, likely due to continental and coastal biogenic emissions. The Antarctic coastal source of CCN from the south, CCN sources from the midlatitudes, and enhanced precipitation sink in the cyclonic circulation between the Ferrel and polar cells (around 60.sup.\" S) create opposing latitudinal gradients in the CCN concentration with an observed minimum in the SO between 55 and 60.sup.\" S. The SOCRATES airborne measurements are not influenced by Australian continental emissions but still show evidence of elevated CCN concentrations to the south of 60.sup.\" S, consistent with biogenic coastal emissions. In addition, a latitudinal gradient in the particle composition, south of the Australian and Tasmanian coasts, is apparent in aerosol hygroscopicity derived from CCN spectra and aerosol particle size distribution. The particles are more hygroscopic to the north, consistent with a greater fraction of sea salt from PMA, and less hygroscopic to the south as there is more sulfate and organic particles originating from biogenic sources in coastal Antarctica.

The spontaneous growth of cloud condensation nuclei (CCN) into cloud droplets under supersaturated water vapour conditions is described by classic Köhler theory. This spontaneous activation of CCN depends on the interplay between the Raoult effect, whereby activation potential increases with decreasing water activity or increasing solute concentration, and the Kelvin effect, whereby activation potential decreases with decreasing droplet size or increases with decreasing surface tension, which is sensitive to surfactants. Surface tension lowering caused by organic surfactants, which diminishes the Kelvin effect, is expected to be negated by a concomitant reduction in the Raoult effect, driven by the displacement of surfactant molecules from the droplet bulk to the droplet-vapour interface. Here we present observational and theoretical evidence illustrating that, in ambient air, surface tension lowering can prevail over the reduction in the Raoult effect, leading to substantial increases in cloud droplet concentrations. We suggest that consideration of liquid-liquid phase separation, leading to complete or partial engulfing of a hygroscopic particle core by a hydrophobic organic-rich phase, can explain the lack of concomitant reduction of the Raoult effect, while maintaining substantial lowering of surface tension, even for partial surface coverage. Apart from the importance of particle size and composition in droplet activation, we show by observation and modelling that incorporation of phase-separation effects into activation thermodynamics can lead to a CCN number concentration that is up to ten times what is predicted by climate models, changing the properties of clouds. An adequate representation of the CCN activation process is essential to the prediction of clouds in climate models, and given the effect of clouds on the Earth's energy balance, improved prediction of aerosol-cloud-climate interactions is likely to result in improved assessments of future climate change.

We present two novel matter-wave Sagnac interferometers based on ring-shaped time-averaged adiabatic potentials, where the atoms are put into a superposition of two different spin states and manipulated independently using elliptically polarized rf-fields. In the first interferometer the atoms are accelerated by spin-state-dependent forces and then travel around the ring in a matter-wave guide. In the second one the atoms are fully trapped during the entire interferometric sequence and are moved around the ring in two spin-state-dependent 'buckets'. Corrections to the ideal Sagnac phase are investigated for both cases. We experimentally demonstrate the key atom-optical elements of the interferometer such as the independent manipulation of two different spin states in the ring-shaped potentials under identical experimental conditions.

New particle formation is globally one of the major sources of aerosol particles and cloud condensation nuclei. As primary emissions are a minor contributor to particle concentrations, secondary new particle formation processes are probably key in determining Antarctic aerosol number concentrations. However, our knowledge of new particle formation and its mechanisms in Antarctica is very limited. Here we study summertime open ocean and coastal new particle formation in the Antarctic Peninsula region based on both ship and station measurements. The rates of particle formation relative to sulfuric acid concentrations, as well as the sulfuric acid dimer-to-monomer ratios, were similar to those seen for sulfuric acid–dimethylamine–water nucleation. Numerous sulfuric acid–amine peaks were identified during new particle formation events, providing evidence that alkylamines were the bases that facilitated sulfuric acid nucleation. Most new particle formation events occurred in air masses arriving from the ice-covered Weddell Sea and its marginal ice zone, which are an important source of volatile sulfur and alkylamines. This nucleation mechanism is more efficient than the ion-induced sulfuric acid–ammonia pathway previously observed in Antarctica, and one that can occur rapidly under neutral conditions. This hitherto overlooked pathway to biologically driven aerosol formation should be considered for estimating aerosol and cloud condensation nuclei numbers in ocean–sea ice–aerosols–climate feedback models. New particles can form rapidly in Antarctica through the reactions of sulfuric acid and amines, suggest ship and station measurements around the Antarctic Peninsula.

The presence of microplastics and nanoplastics (MnPs) in the atmosphere and their transport on a global scale has previously been demonstrated. However, little is known about their environmental impacts in the atmosphere. MnPs could act as cloud condensation nuclei (CCN) or ice-nucleating particles (INPs), affecting cloud formation processes. In sufficient quantities, they could change the cloud albedo, precipitation and lifetime, collectively impacting the Earth’s radiation balance and climate. In this Perspective, we evaluate the potential impact of MnPs on cloud formation by assessing their ability to act as CCN or INPs. Based on an analysis of their physicochemical properties, we propose that MnPs can act as INPs and potentially as CCN after environmental aging processes such as photochemical weathering and the sorption of macromolecules or trace soluble species onto the particle surface. The actual climate impact(s) of MnPs depend on their abundance relative to other aerosols. The concentration of MnPs in the atmosphere is currently low, so they are unlikely to make a substantial contribution to radiative forcing in regions exposed to other aerosols, either from natural sources or anthropogenic pollution. Nevertheless, MnPs will potentially cause non-negligible perturbations in unpolluted remote or marine clouds and generate local climate impacts, particularly in view of an increase in the release of MnPs to the environment in the future. Further measurements, coupled with better characterization of the physiochemical properties of MnPs, will enable a more accurate assessment of the climate impacts of MnPs acting as INPs and CCN.Microplastics and nanoplastics may affect cloud formation processes by acting as ice-nucleating particles and cloud condensation nuclei.

Sea spray aerosols (SSAs) make up a substantial proportion of aerosols in the global atmosphere and, especially when considering marine haze and cloud layers, can have a large impact on cloud formation and atmospheric radiative balance. Although SSA has the highest cloud condensation nuclei (CCN) activation potential, the majority of its population, residing in sub-micrometre sizes, are often obscured by non-sea-spray CCN. Quantification of SSA-derived CCN is fundamental in understanding the radiative budget. Recent approaches to estimate the sub-micrometre SSA employed a free-monomodal lognormal analysis that depicts the global oceanic CCN population comprising less than 30% SSA. Here we derive SSA distributions from a unique five-year dataset of aerosol microphysics and hygroscopicity (water uptake ability) over Atlantic waters. This approach utilizes the distinctive ultra-high hygroscopicity signature of inorganic sea salt and is able to identify the sub-micrometre sea spray down to 35 nm diameter with high time and size resolution. In stark contrast to previous studies, the hygroscopicity coupled multimodal fitting analysis yields SSA-derived CCN as much as 500% in excess of estimates produced using the free-monomodal approach. Our results suggest the contribution of SSA to global CCN, particularly Aitken mode SSA, has probably been overlooked. Very small aerosols from sea spray make up a larger proportion of cloud condensation nuclei than previously recognized, according to an analysis of five years of aerosol ground-based measurement data from over the Atlantic Ocean.