scholarly journals Tight coupling of particle size, number and composition in atmospheric cloud droplet activation

2012 ◽  
Vol 12 (7) ◽  
pp. 3253-3260 ◽  
Author(s):  
D. O. Topping ◽  
G. McFiggans
Keyword(s):  
Water Vapour ◽  
Organic Compounds ◽  
Radiative Forcing ◽  
Aerosol Particles ◽  
Cloud Droplet ◽  
Compositional Dependence ◽  
Ambient Conditions ◽  
Tight Coupling ◽  
Aerosol Composition ◽  
Droplet Activation

Abstract. The substantial uncertainty in the indirect effect of aerosol particles on radiative forcing in large part arises from the influences of atmospheric aerosol particles on (i) the brightness of clouds, exerting significant shortwave cooling with no appreciable compensation in the long wave, and on (ii) their ability to precipitate, with implications for cloud cover and lifetime. Predicting the ambient conditions at which aerosol particles may become cloud droplets is largely reliant on an equilibrium relationship derived by Köhler (1936). However, the theoretical basis of the relationship restricts its application to particles solely comprising involatile compounds and water, whereas a substantial fraction of particles in the real atmosphere will contain potentially thousands of semi-volatile organic compounds in addition to containing semi-volatile inorganic components such as ammonium nitrate. We show that equilibration of atmospherically reasonable concentrations of organic compounds with a growing particle as the ambient humidity increases has potentially larger implications on cloud droplet formation than any other equilibrium compositional dependence, owing to inextricable linkage between the aerosol composition, a particles size and concentration under ambient conditions. Whilst previous attempts to account for co-condensation of gases other than water vapour have been restricted to one inorganic condensate, our method demonstrates that accounting for the co-condensation of any number of organic compounds substantially decreases the saturation ratio of water vapour required for droplet activation. This effect is far greater than any other compositional dependence; more so even than the unphysical effect of surface tension reduction in aqueous organic mixtures, ignoring differences in bulk and surface surfactant concentrations.

scholarly journals Tight coupling of particle size and composition in atmospheric cloud droplet activation

2011 ◽  
Vol 11 (9) ◽  
pp. 25155-25171 ◽  
Author(s):  
D. Topping ◽  
G. McFiggans
Keyword(s):  
Water Vapour ◽  
Organic Compounds ◽  
Radiative Forcing ◽  
Aerosol Particles ◽  
Cloud Droplet ◽  
Compositional Dependence ◽  
Ambient Conditions ◽  
Tight Coupling ◽  
Aerosol Composition ◽  
Droplet Activation

Abstract. The substantial uncertainty in the indirect effect on radiative forcing in large part arises from the influences of atmospheric aerosol particles on (i) the brightness of clouds, exerting significant shortwave cooling with no appreciable compensation in the longwave, and on (ii) their ability to precipitate, with implications for cloud cover and lifetime. Predicting the ambient conditions at which aerosol particles may become cloud droplets is largely reliant on an equilibrium relationship derived in 1936. However, the theoretical basis of the relationship restricts its application to particles solely comprising involatile compounds and water, whereas a substantial fraction of particles in the real atmosphere will contain potentially thousands of semi-volatile organic compounds in addition to containing semi-volatile inorganic components such as ammonium nitrate. We show that equilibration of atmospherically reasonable concentrations of organic compounds with a growing particle as the ambient humidity increases has larger implications on cloud droplet formation than any other equilibrium compositional dependence, owing to inextricable linkage between the aerosol composition and a particles size under ambient conditions. Whilst previous attempts to account for co-condensation of gases other than water vapour have been restricted to one inorganic condensate, our method demonstrates that accounting for the co-condensation of any number of organic compounds substantially decreases the saturation ratio of water vapour required for droplet activation. This effect is far greater than any other compositional dependence; moreso even than the unphysical effect of surface tension reduction in aqueous organic mixtures, ignoring differences in bulk and surface surfactant concentrations.

scholarly journals Importance of aerosol composition and mixing state for cloud droplet activation over the Arctic pack ice in summer

2015 ◽  
Vol 15 (5) ◽  
pp. 2545-2568 ◽  
Author(s):  
C. Leck ◽  
E. Svensson
Keyword(s):  
Water Vapour ◽  
Cloud Droplet ◽  
Polymer Gels ◽  
Water Soluble ◽  
The Arctic ◽  
Local Source ◽  
Aerosol Composition ◽  
Pack Ice ◽  
Kohler Theory ◽  
Droplet Activation

Abstract. Concentrations of cloud condensation nuclei (CCN) were measured throughout an expedition by icebreaker around the central Arctic Ocean, including a 3 week ice drift operation at 87° N, from 3 August to 9 September 2008. In agreement with previous observations in the area and season, median daily CCN concentrations at 0.2% water vapour supersaturation (SS) were typically in the range of 15 to 30 cm−3, but concentrations varied by 2 to 3 orders of magnitude over the expedition and were occasionally below 1 cm−3. The CCN concentrations were highest near the ice edge and fell by a factor of 3 in the first 48 h of transport from the open sea into the pack ice region. For longer transport times they increased again, indicating a local source over the pack ice, suggested to be polymer gels, via drops injected into the air by bubbles bursting on open leads. We inferred the properties of the unexplained non-water soluble aerosol fraction that was necessary for reproducing the observed concentrations of CCN. This was made possible by assuming Köhler theory and simulating the cloud nucleation process using a Lagrangian adiabatic air parcel model that solves the kinetic formulation for condensation of water on size resolved aerosol particles. We propose that the portion of the internally/externally mixed water insoluble particles was larger in the corresponding smaller aerosol size ranges. These particles were physically and chemically behaving as polymer gels: the interaction of the hydrophilic and hydrophobic entities on the structures of polymer gels during cloud droplet activation would at first only show a partial wetting character and only weak hygroscopic growth. Given time, a high CCN activation efficiency is achieved, which is promoted by the hydrophilicity or surface-active properties of the gels. Thus the result in this study argues that the behaviour of the high Arctic aerosol in CCN-counters operating at water vapour SSs > 0.4% (high relative humidities) may not be properly explained by conventional Köhler theory.

scholarly journals Cloud droplet activation of secondary organic aerosol is mainly controlled by molecular weight, not water solubility

2018 ◽  
Author(s):  
Jian Wang ◽  
John E. Shilling ◽  
Jiumeng Liu ◽  
Alla Zelenyuk ◽  
David M. Bell ◽  
...  
Keyword(s):  
Molecular Weight ◽  
Water Solubility ◽  
Global Climate ◽  
Cloud Condensation Nuclei ◽  
Aerosol Particles ◽  
Average Molecular Weight ◽  
Cloud Droplet ◽  
Oxidation Level ◽  
Organic Species ◽  
Droplet Activation

Abstract. Aerosol particles strongly influence global climate by modifying the properties of clouds. An accurate assessment of the aerosol impact on climate requires knowledge of the concentration of cloud condensation nuclei (CCN), a subset of aerosol particles that can activate and form cloud droplets in the atmosphere. Atmospheric particles typically consist of a myriad of organic species, which frequently dominate the particle composition. As a result, CCN concentration is often a strong function of the hygroscopicity of organics in the particles. Earlier studies showed organic hygroscopicity increases nearly linearly with oxidation level. Such increase of hygroscopicity is conventionally attributed to higher water solubility for more oxidized organics. By systematically varying the water content of activating droplets, we show that for the majority of secondary organic aerosols (SOA), essentially all organics are dissolved at the point of droplet activation. Therefore, the organic hygroscopicity is not limited by solubility, but is dictated mainly by the molecular weight of organic species. Instead of increased water solubility as previously thought, the increase of the organic hygroscopicity with oxidation level is largely because (1) SOA formed from smaller precursor molecules tend to be more oxidized and have lower average molecular weight and (2) during oxidation, fragmentation reactions reduce average organic molecule weight, leading to increased hygroscopicity. A simple model of organic hygroscopicity based on molecular weight, oxidation level, and volatility is developed, and it successfully reproduces the variation of SOA hygroscopicity with oxidation level observed in the laboratory and field studies.

scholarly journals An investigation into the performance of four cloud droplet activation parameterisations

2014 ◽  
Vol 7 (4) ◽  
pp. 1535-1542 ◽  
Author(s):  
E. Simpson ◽  
P. Connolly ◽  
G. McFiggans
Keyword(s):  
Aerosol Particle ◽  
Large Scale ◽  
Aerosol Particles ◽  
Cloud Droplet ◽  
Particle Growth ◽  
Systematic Evaluation ◽  
Climate Modelling ◽  
Median Diameter ◽  
Droplet Activation ◽  
Simulation Time

Abstract. Cloud droplet number concentration prediction is central to large-scale weather and climate modelling. The benchmark cloud parcel model calculation of aerosol particle growth and activation, by diffusion of vapour to aerosol particles in a rising parcel of air experiencing adiabatic expansion, is too computationally expensive for use in large-scale global models. Therefore the process of activation of aerosol particles into cloud droplets is parameterised with an aim to strike the optimum balance between numerical expense and accuracy. We present a detailed systematic evaluation of three cloud droplet activation parameterisations that are widely used in large-scale models and one recent update. In all cases, it is found that there is a tendency to overestimate the fraction of activated aerosol particles when the aerosol particle "median diameter" is large (between 250 and 2000 nm) in a single lognormal mode simulation. This is due to an infinite "effective simulation time" of the parameterisations compared to a prescribed simulation time in the parcel model. This problem arises in the parameterisations because it is assumed that a parcel of air rises to the altitude where maximum supersaturation occurs, regardless of whether this altitude is above the cloud top. Such behaviour is problematic because, in some cases, large aerosol can completely suppress the activation of drops. In some cases when the "median diameter" is small (between 5 and 250 nm) in a single lognormal mode the fraction of activated drops is underestimated by the parameterisations. Secondly, it is found that in dual-mode cases there is a systematic tendency towards underestimation of the fraction of activated drops, which is due to the methods used by the parameterisations to approximate the sink of water vapour.

Cloud Droplet Activation of Amino Acid Aerosol Particles

2010 ◽  
Vol 114 (1) ◽  
pp. 379-386 ◽  
Author(s):  
Adam Kristensson ◽  
Thomas Rosenørn ◽  
Merete Bilde
Keyword(s):  
Amino Acid ◽  
Aerosol Particles ◽  
Cloud Droplet ◽  
Droplet Activation

scholarly journals The role of turbulent fluctuations in aerosol activation and cloud formation

2020 ◽  
Vol 117 (29) ◽  
pp. 16831-16838
Author(s):  
Prasanth Prabhakaran ◽  
Abu Sayeed Md Shawon ◽  
Gregory Kinney ◽  
Subin Thomas ◽  
Will Cantrell ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Aerosol Particles ◽  
Cloud Droplet ◽  
Radiative Properties ◽  
Cloud Formation ◽  
Saturation Ratio ◽  
Ambient Relative Humidity ◽  
Cloud Properties ◽  
The Mean ◽  
Droplet Activation

Aerosol indirect effects are one of the leading contributors to cloud radiative properties relevant to climate. Aerosol particles become cloud droplets when the ambient relative humidity (saturation ratio) exceeds a critical value, which depends on the particle size and chemical composition. In the traditional formulation of this problem, only average, uniform saturation ratios are considered. Using experiments and theory, we examine the effects of fluctuations, produced by turbulence. Our measurements, from a multiphase, turbulent cloud chamber, show a clear transition from a regime in which the mean saturation ratio dominates to one in which the fluctuations determine cloud properties. The laboratory measurements demonstrate cloud formation in mean-subsaturated conditions (i.e., relative humidity <100%) in the fluctuation-dominant activation regime. The theoretical framework developed to interpret these measurements predicts a transition from a mean- to a fluctuation-dominated regime, based on the relative values of the mean and standard deviation of the environmental saturation ratio and the critical saturation ratio at which aerosol particles activate or become droplets. The theory is similar to the concept of stochastic condensation and can be used in the context of the atmosphere to explore the conditions under which droplet activation is driven by fluctuations as opposed to mean supersaturation. It provides a basis for future development of cloud droplet activation parameterizations that go beyond the internally homogeneous parcel calculations that have been used in the past.

Measurements of cloud droplet activation of aerosol particles at a clean subarctic background site

2005 ◽  
Vol 110 (D6) ◽  
pp. n/a-n/a ◽  
Author(s):  
Mika Komppula ◽  
Heikki Lihavainen ◽  
Veli-Matti Kerminen ◽  
Markku Kulmala ◽  
Yrjö Viisanen
Keyword(s):  
Aerosol Particles ◽  
Cloud Droplet ◽  
Background Site ◽  
Droplet Activation

scholarly journals Reduced effective radiative forcing from cloud–aerosol interactions (ERF<sub>aci</sub>) with improved treatment of early aerosol growth in an Earth system model

2021 ◽  
Vol 21 (23) ◽  
pp. 17243-17265
Author(s):  
Sara Marie Blichner ◽  
Moa Kristina Sporre ◽  
Terje Koren Berntsen
Keyword(s):  
Radiative Forcing ◽  
Cloud Droplet ◽  
Early Growth ◽  
Earth System Model ◽  
Aerosol Concentration ◽  
System Model ◽  
Earth System ◽  
Secondary Aerosols ◽  
Effective Radiative Forcing ◽  
Droplet Activation

Abstract. Historically, aerosols of anthropogenic origin have offset some of the warming from increased atmospheric greenhouse gas concentrations. The strength of this negative aerosol forcing, however, is highly uncertain – especially the part originating from cloud–aerosol interactions. An important part of this uncertainty originates from our lack of knowledge about pre-industrial aerosols and how many of these would have acted as cloud condensation nuclei (CCN). In order to simulate CCN concentrations in models, we must adequately model secondary aerosols, including new particle formation (NPF) and early growth, which contributes a large part of atmospheric CCN. In this study, we investigate the effective radiative forcing (ERF) from cloud–aerosol interactions (ERFaci) with an improved treatment of early particle growth, as presented in Blichner et al. (2021). We compare the improved scheme to the default scheme, OsloAero, which are both embedded in the atmospheric component of the Norwegian Earth System Model v2 (NorESM2). The improved scheme, OsloAeroSec, includes a sectional scheme that treats the growth of particles from 5–39.6 nm in diameter, which thereafter inputs the particles to the smallest mode in the pre-existing modal aerosol scheme. The default scheme parameterizes the growth of particles from nucleation up to the smallest mode, a process that can take several hours. The explicit treatment of early growth in OsloAeroSec, on the other hand, captures the changes in atmospheric conditions during this growth time in terms of air mass mixing, transport, and condensation and coagulation. We find that the ERFaci with the sectional scheme is −1.16 W m−2, which is 0.13 W m−2 weaker compared to the default scheme. This reduction originates from OsloAeroSec producing more particles than the default scheme in pristine, low-aerosol-concentration areas and fewer NPF particles in high-aerosol areas. We find, perhaps surprisingly, that NPF inhibits cloud droplet activation in polluted and/or high-aerosol-concentration regions because the NPF particles increase the condensation sink and reduce the growth of the larger particles which may otherwise activate. This means that in these high-aerosol regions, the model with the lowest NPF – OsloAeroSec – will have the highest cloud droplet activation and thus more reflective clouds. In pristine and/or low-aerosol regions, however, NPF enhances cloud droplet activation because the NPF particles themselves tend to activate. Lastly, we find that sulfate emissions in the present-day simulations increase the hygroscopicity of secondary aerosols compared to pre-industrial simulations. This makes NPF particles more relevant for cloud droplet activation in the present day than the pre-industrial atmosphere because increased hygroscopicity means they can activate at smaller sizes.

scholarly journals Importance of aerosol composition and mixing state for cloud droplet activation in the high Arctic

2014 ◽  
Vol 14 (15) ◽  
pp. 21223-21283 ◽  
Author(s):  
C. Leck ◽  
E. Svensson
Keyword(s):  
Water Vapor ◽  
Cloud Droplet ◽  
Polymer Gels ◽  
High Arctic ◽  
Water Soluble ◽  
Local Source ◽  
Aerosol Composition ◽  
Pack Ice ◽  
Kohler Theory ◽  
Droplet Activation

Abstract. Concentrations of cloud condensation nuclei (CCN) were measured throughout an expedition by icebreaker around the central Arctic Ocean, including a 3 week ice drift operation at 87° N, from 3 August to 9 September 2008. In agreement with previous observations in the area and season median daily CCN concentrations at 0.2% water vapor supersaturation were typically in the range of 15 to 30 cm−3, but concentrations varied by two to three orders of magnitude over the expedition and were occasionally below 1 cm−3. The CCN concentrations were highest near the ice edge and fell by a factor of three in the first 48 h of transport from the open sea into the pack ice region. For longer transport times they increased again indicating a local source over the pack ice, suggested to be polymer gels, via drops injected into the air by bubbles bursting on open leads. By assuming Köhler theory and simulating the cloud nucleation process using a Lagrangian adiabatic air parcel model that solves the kinetic formulation for condensation of water on size resolved aerosol particles we inferred the properties of the unexplained non-water soluble aerosol fraction that is necessary for reproducing the observed concentrations of CCN. We propose that the portion of the internally/externally mixed water insoluble particles was larger in the corresponding smaller aerosol sizes ranges. These particles were physically and chemically behaving as polymer gels: the interaction of the hydrophilic and hydrophobic entities on the structures of polymer gels during cloud droplet activation would at first only show a partial wetting character and only weak hygroscopic growth. Given time, a high CCN activation efficiency is achieved, which is promoted by the hydrophilicity or surface-active properties of the gels. Thus the result in this study argues for that the behavior of the high Arctic aerosol in CCN-counters operating at water vapor supersaturations > 0.4% (high relative humidities) may not be properly explained by conventional Köhler theory.

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