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About half of present-day cloud condensation nuclei originate from atmospheric nucleation, frequently appearing as a burst of new particles near midday. Atmospheric observations show that the growth rate of new particles often accelerates when the diameter of the particles is between one and ten nanometres. In this critical size range, new particles are most likely to be lost by coagulation with pre-existing particles, thereby failing to form new cloud condensation nuclei that are typically 50 to 100 nanometres across. Sulfuric acid vapour is often involved in nucleation but is too scarce to explain most subsequent growth, leaving organic vapours as the most plausible alternative, at least in the planetary boundary layer. Although recent studies predict that low-volatility organic vapours contribute during initial growth, direct evidence has been lacking. The accelerating growth may result from increased photolytic production of condensable organic species in the afternoon, and the presence of a possible Kelvin (curvature) effect, which inhibits organic vapour condensation on the smallest particles (the nano-Köhler theory), has so far remained ambiguous. Here we present experiments performed in a large chamber under atmospheric conditions that investigate the role of organic vapours in the initial growth of nucleated organic particles in the absence of inorganic acids and bases such as sulfuric acid or ammonia and amines, respectively. Using data from the same set of experiments, it has been shown that organic vapours alone can drive nucleation. We focus on the growth of nucleated particles and find that the organic vapours that drive initial growth have extremely low volatilities (saturation concentration less than 10(-4.5) micrograms per cubic metre). As the particles increase in size and the Kelvin barrier falls, subsequent growth is primarily due to more abundant organic vapours of slightly higher volatility (saturation concentrations of 10(-4.5) to 10(-0.5) micrograms per cubic metre). We present a particle growth model that quantitatively reproduces our measurements. Furthermore, we implement a parameterization of the first steps of growth in a global aerosol model and find that concentrations of atmospheric cloud concentration nuclei can change substantially in response, that is, by up to 50 per cent in comparison with previously assumed growth rate parameterizations.

The effect of an increase in atmospheric aerosol concentrations on the distribution and radiative properties of Earth's clouds is the most uncertain component of the overall global radiative forcing from preindustrial time. General circulation models (GCMs) are the tool for predicting future climate, but the treatment of aerosols, clouds, and aerosol-cloud radiative effects carries large uncertainties that directly affect GCM predictions, such as climate sensitivity. Predictions are hampered by the large range of scales of interaction between various components that need to be captured. Observation systems (remote sensing, in situ) are increasingly being used to constrain predictions, but significant challenges exist, to some extent because of the large range of scales and the fact that the various measuring systems tend to address different scales. Fine-scale models represent clouds, aerosols, and aerosol-cloud interactions with high fidelity but do not include interactions with the larger scale and are therefore limited from a climatic point of view. We suggest strategies for improving estimates of aerosol-cloud relationships in climate models, for new remote sensing and in situ measurements, and for quantifying and reducing model uncertainty.

The growth of freshly formed aerosol particles can be the bottleneck in their survival to cloud condensation nuclei. It is therefore crucial to understand how particles grow in the atmosphere. Insufficient experimental data has impeded a profound understanding of nano-particle growth under atmospheric conditions. Here we study nano-particle growth in the CLOUD (Cosmics Leaving OUtdoors Droplets) chamber, starting from the formation of molecular clusters. We present measured growth rates at sub-3 nm sizes with different atmospherically relevant concentrations of sulphuric acid, water, ammonia and dimethylamine. We find that atmospheric ions and small acid-base clusters, which are not generally accounted for in the measurement of sulphuric acid vapour, can participate in the growth process, leading to enhanced growth rates. The availability of compounds capable of stabilizing sulphuric acid clusters governs the magnitude of these effects and thus the exact growth mechanism. We bring these observations into a coherent framework and discuss their significance in the atmosphere.

The effect of an increase in atmospheric aerosol concentrations on the distribution and radiative properties of Earth's clouds is the most uncertain component of the overall global radiative forcing from preindustrial time. General circulation models (GCMs) are the tool for predicting future climate, but the treatment of aerosols, clouds, and aerosol-cloud radiative effects carries large uncertainties that directly affect GCM predictions, such as climate sensitivity. Predictions are hampered by the large range of scales of interaction between various components that need to be captured. Observation systems (remote sensing, in situ) are increasingly being used to constrain predictions, but significant challenges exist, to some extent because of the large range of scales and the fact that the various measuring systems tend to address different scales. Fine-scale models represent clouds, aerosols, and aerosol-cloud interactions with high fidelity but do not include interactions with the larger scale and are therefore limited from a climatic point of view. We suggest strategies for improving estimates of aerosol-cloud relationships in climate models, for new remote sensing and in situ measurements, and for quantifying and reducing model uncertainty.

The global variation of ice-nucleating particle (INP) concentrations is an important modulator of the cloud-phase feedback, where the albedo of mixed-phase clouds increases in a warming climate. Shallow clouds such as those observed in cold-air outbreaks (CAOs) are particularly important for cloud-phase feedbacks and highly sensitive to INPs. To investigate the sources and concentrations of INPs in CAOs, we made airborne measurements over the Norwegian and Barents seas as part of the March 2022 Arctic Cold-Air Outbreak (ACAO) field campaign. Aerosol samples were collected on filters at locations above, below and upwind of CAO cloud decks. Throughout the campaign, INP concentrations were comparable to the highest previously observed in the Arctic. Scanning electron microscopy analysis of samples taken upwind of cloud decks showed that super-micron aerosol was dominated by mineral dusts. Analysis of aerosol particle size measurements to obtain an INP active site density suggested sea spray was unlikely to be the dominant INP type. These site densities were also too great for mineral components alone to be the dominant INP type above -20 °C. Accordingly, it is likely that the dominant INP type was mineral dust mixed with other ice nucleating materials, possibly of biogenic origin. Back-trajectory analysis and meteorological conditions suggested a lack of local INP sources. We therefore hypothesise that the high INP concentration is most likely to be associated with aged aerosol in Arctic haze that has undergone long-range transport from lower latitude regions. [ABSTRACT FROM AUTHOR]

The regulation introduced in 2020 that limits the sulfur content in shipping fuel has reduced sulfur emissions over global open oceans by about 80 %. This is expected to have reduced aerosols that both reflect solar radiation directly and affect cloud properties, with the latter also changing the solar radiation balance. Here we investigate the impacts of this regulation on aerosols and climate in the HadGEM3-GC3.1 climate model. The global aerosol effective radiative forcing caused by reduced shipping emissions is estimated to be 0.13 W m-2, which is equivalent to about 50 % of the positive forcing caused by the global reduction in all anthropogenic aerosols since late 20th century. Ensembles of global coupled simulations from 2020–2049 predict a global mean warming of 0.04 K averaged over this period. Our simulations are not clear on whether the global impact is yet to emerge or has already emerged because the present-day impact is masked by variability. Nevertheless, the impact of shipping emission reductions will have either already committed us to warming above the 1.5 K Paris target or will represent an important contribution that may help explain part of the rapid jump in global temperatures over the last 12 months. Consistent with previous aerosol perturbation simulations, the warming is greatest in the Arctic, reaching a mean of 0.15 K Arctic-wide and 0.3 K in the Atlantic sector of the Arctic (which represents greater than 10 % increase in the total anthropogenic warming since pre-industrial times). [ABSTRACT FROM AUTHOR]

Aerosol radiative forcing and cloud–climate feedbacks each have a large effect on climate, mainly through modification of solar short-wave radiative fluxes. Here we determine what causes the long-term trends in the upwelling short-wave (SW) top-of-the-atmosphere (TOA) fluxes (FSW↑) over the North Atlantic region. Coupled atmosphere–ocean simulations from the UK Earth System Model (UKESM1) and the Hadley Centre General Environment Model (HadGEM3-GC3.1) show a positive FSW↑ trend between 1850 and 1970 (increasing SW reflection) and a negative trend between 1970 and 2014. We find that the 1850–1970 positive FSW↑ trend is mainly driven by an increase in cloud droplet number concentration due to increases in aerosol, while the 1970–2014 trend is mainly driven by a decrease in cloud fraction, which we attribute mainly to cloud feedbacks caused by greenhouse gas-induced warming. In the 1850–1970 period, aerosol-induced cooling and greenhouse gas warming roughly counteract each other, so the temperature-driven cloud feedback effect on the FSW↑ trend is weak (contributing to only 23 % of the ΔFSW↑), and aerosol forcing is the dominant effect (77 % of ΔFSW↑). However, in the 1970–2014 period the warming from greenhouse gases intensifies, and the cooling from aerosol radiative forcing reduces, resulting in a large overall warming and a reduction in FSW↑ that is mainly driven by cloud feedbacks (87 % of ΔFSW↑). The results suggest that it is difficult to use satellite observations in the post-1970 period to evaluate and constrain the magnitude of the aerosol–cloud interaction forcing but that cloud feedbacks might be evaluated. Comparisons with observations between 1985 and 2014 show that the simulated reduction in FSW↑ and the increase in temperature are too strong. However, the temperature discrepancy can account for only part of the FSW↑ discrepancy given the estimated model feedback strength (λ=∂FSW∂T). The remaining discrepancy suggests a model bias in either λ or in the strength of the aerosol forcing (aerosols are reducing during this time period) to explain the too-strong decrease in FSW↑ , with a λ bias being the most likely. Both of these biases would also tend to cause too-large an increase in temperature over the 1985–2014 period, which would be consistent with the sign of the model temperature bias reported here. Either of these model biases would have important implications for future climate projections using these models. [ABSTRACT FROM AUTHOR]

The presence of ice in clouds can influence cloud lifetime, precipitation and radiative properties; here, organic material at the sea-air interface, possibly associated with phytoplankton cell exudates, is shown to nucleate ice under conditions relevant for ice cloud formation in the atmospheric environment. Ice-forming 'biological' particles in the marine atmosphere The presence of ice in clouds can have an influence on cloud lifetime and radiative properties. It remains uncertain to what extent sea spray may act as ice nucleating particles that facilitate the formation of ice in clouds. This study finds that organic material at the sea-air interface, probably derived from phytoplankton cell exudates, nucleates ice under conditions relevant for ice cloud formation in the atmospheric environment. Model simulations suggest that marine organic material may be an important source of ice nucleating particles in remote marine environments such as the Southern Ocean. The amount of ice present in clouds can affect cloud lifetime, precipitation and radiative properties.sup.1,2. The formation of ice in clouds is facilitated by the presence of airborne ice-nucleating particles.sup.1,2. Sea spray is one of the major global sources of atmospheric particles, but it is unclear to what extent these particles are capable of nucleating ice.sup.3,4,5,6,7,8,9,10,11. Sea-spray aerosol contains large amounts of organic material that is ejected into the atmosphere during bubble bursting at the organically enriched sea-air interface or sea surface microlayer.sup.12,13,14,15,16,17,18,19. Here we show that organic material in the sea surface microlayer nucleates ice under conditions relevant for mixed-phase cloud and high-altitude ice cloud formation. The ice-nucleating material is probably biogenic and less than approximately 0.2 micrometres in size. We find that exudates separated from cells of the marine diatom Thalassiosira pseudonana nucleate ice, and propose that organic material associated with phytoplankton cell exudates is a likely candidate for the observed ice-nucleating ability of the microlayer samples. Global model simulations of marine organic aerosol, in combination with our measurements, suggest that marine organic material may be an important source of ice-nucleating particles in remote marine environments such as the Southern Ocean, North Pacific Ocean and North Atlantic Ocean., Author(s): Theodore W. Wilson [sup.1] , Luis A. Ladino [sup.2] , Peter A. Alpert [sup.3] , Mark N. Breckels [sup.4] , Ian M. Brooks [sup.1] , Jo Browse [sup.1] , [...]

The amount of ice present in mixed-phase clouds, which contain both supercooled liquid water droplets and ice particles, affects cloud extent, lifetime, particle size and radiative properties (1,2). The freezing of cloud droplets can be catalysed by the presence of aerosol particles known as ice nuclei (2). One of the most important ice nuclei is thought to be mineral dust aerosol from arid regions (2,3). It is generally assumed that clay minerals, which contribute approximately two-thirds of the dust mass, dominate ice nucleation by mineral dust, and many experimental studies have therefore focused on these materials (1,2,4-6). Here we use an established droplet-freezing technique (4,7) to show that feldspar minerals dominate ice nucleation by mineral dusts under mixed-phase cloud conditions, despite feldspar being a minor component of dust emitted from arid regions. We also find that clay minerals are relatively unimportant ice nuclei. Our results from a global aerosol model study suggest that feldspar ice nuclei are globally distributed and that feldspar particles may account for a large proportion of the ice nuclei in Earth's atmosphere that contribute to freezing at temperatures below about -15 °C., Pure cloud droplets remain liquid until cooled to the homogeneous freezing threshold at around 237 K (ref. 2). At warmer temperatures, freezing can be catalysed by heterogeneous ice nuclei. Heterogeneous [...]

The nature and origin of organic aerosol in the atmosphere remain unclear. The gas-particle partitioning of semi-volatile organic compounds (SVOC) that constitute primary organic aerosols (POA) and the multigenerational chemical aging of SVOCs are particularly poorly understood. The volatility basis set (VBS) approach, implemented in air quality models such as the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), can be a useful tool to describe POA production and aging. However, the main disadvantage is its complexity, making the evaluation of model uncertainty and the optimal model parameterisation expensive to probe using only WRF-Chem simulations. Gaussian process emulators, trained on simulations from relatively few WRF-Chem simulations, are capable of reproducing model results and estimating the sources of model uncertainty within a defined range of model parameters. In this study, a WRF-Chem VBS parameterisation is proposed; we then generate a perturbed parameter ensemble of 111 model runs, perturbing ten parameters of the WRF-Chem model relating to organic aerosol emissions and the VBS oxidation reactions. This allowed us to cover the model's uncertainty space and compare output from each run to aerosol mass spectrometer observations of organic aerosol concentrations and O:C ratios measured in New Delhi, India. The simulations spanned the organic aerosol concentrations measured with the aerosol mass spectrometer (AMS). However, they also highlighted potential structural errors in the model that may be related to unsuitable diurnal cycles in the emissions and/or failure to adequately represent the dynamics of the planetary boundary layer. While the structural errors prevented us from clearly identifying an optimised VBS approach in WRF-Chem, we were able to apply the emulator in two periods: the full period (1st-29th May) and the period 14:00- 16:00 hrs local time, 1st-29th May. The combination of emulator analysis and model evaluation metrics allowed us to identify plausible parameter combinations for the analysed periods. We demonstrate that the methodology presented in this study can be used to determine the model uncertainty and identify the appropriate parameter combination for the VBS approach, and hence provide valuable information to improve our understanding on SOA production. [ABSTRACT FROM AUTHOR]