The role of turbulent fluctuations in aerosol activation and cloud formation

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.

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

10.5194/acp-2018-715 ◽

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.

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Year-long observations of chemical properties of organic aerosols and cloud residuals at the Zeppelin Observatory, Svalbard

10.5194/egusphere-egu21-13266 ◽

2021 ◽

Author(s):

Yvette Gramlich ◽

Sophie Haslett ◽

Karolina Siegel ◽

Gabriel Freitas ◽

Radovan Krejci ◽

...

Keyword(s):

Cloud Condensation Nuclei ◽

Aerosol Particles ◽

Trace Gases ◽

Chemical Properties ◽

Radiative Properties ◽

Cloud Formation ◽

The Arctic ◽

Arctic Clouds ◽

Clear Sky ◽

Air Inlet

The number of cloud seeds, e.g. cloud condensation nuclei (CCN) and ice nucleation particles (INP), in the pristine Arctic shows a large range throughout the year, thereby influencing the radiative properties of Arctic clouds. However, little is known about the chemical properties of CCN and INP in this region. This study aims to investigate the chemical properties of aerosol particles and trace gases that are of importance for cloud formation in the Arctic environment, with focus on the organic fraction.Over the course of one full year (fall 2019 until fall 2020), we deployed a filter-inlet for gases and aerosols coupled to a chemical ionization high-resolution time-of-flight mass spectrometer (FIGAERO-CIMS) using iodide as reagent ion at the Zeppelin Observatory in Svalbard (480 m a.s.l.), as part of the Ny-&#197;lesund Aerosol Cloud Experiment (NASCENT). The FIGAERO-CIMS is able to measure organic trace gases and aerosol particles semi-simultaneously. The instrument was connected to an inlet switching between a counterflow virtual impactor (CVI) inlet and a total air inlet. This setup allows to study the differences in chemical composition of organic aerosol particles and trace gases at molecular level that are involved in Arctic cloud formation compared to ambient non-activated aerosol.We observed organic signal above background in both gas and particle phase all year round. A comparison between the gas phase mass spectra of cloud-free and cloudy conditions shows lower signal for some organics inside the cloud, indicating that some trace gases are scavenged by cloud hydrometeors whilst others are not. In this presentation we will discuss the chemical characteristics of the gases exhibiting different behavior during clear sky and cloudy conditions, and the implications for partitioning of organic compounds between the gas, aerosol particle and cloud hydrometeor (droplet/ice) phase.

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Coexistence of three liquid phases in atmospheric aerosol particles

10.5194/egusphere-egu21-3694 ◽

2021 ◽

Author(s):

Fabian Mahrt ◽

Yuanzhou Huang ◽

Shaun Xu ◽

Manabu Shiraiwa ◽

Andreas Zuend ◽

...

Keyword(s):

Air Quality ◽

Kinetic Modelling ◽

Aerosol Particles ◽

Nile Red ◽

Cloud Droplet ◽

Organic Aerosol ◽

Reaction Rates ◽

Phase Behaviour ◽

Radiative Properties ◽

Liquid Phases

Aerosol particles are ubiquitous in the atmosphere and play an important role for air quality and Earth&#8217;s climate. Primary organic aerosol (POA), secondary organic aerosol (SOA), and secondary inorganic aerosol (SIA) constitute a significant mass fraction of these particles. POA, SOA, and SIA can become internally mixed within the same particle though different processes such as coagulation, gas&#8211;particle partitioning. To predict the role of these internally mixed particles in climate and air quality information on their phase behaviour is needed, i.e. information on the number and type of phases present within these particles. As an example, a particle with a single homogeneous liquid phase can have different radiative properties, reaction rates, uptake kinetics, and potential to change cloud microphysical properties by activating into a cloud droplet, compared to a particle with multiple liquid or solid phases.In the current study we used Nile red, a solvatochromic dye, and fluorescence microscopy in order to determine the phase behaviour of POA+SOA+SIA particles. Squalane was used as a proxy of POA, ammonium sulfate was used as SIA and 1 of 23 different oxidized organic molecules were used as proxies of SOA. We demonstrate that three liquid phases often coexist within individual particles. We find that the phase behaviour strongly depends on the oxygen-to-carbon ratio of the SOA proxies. Experiments with SOA generated by dark ozonolysis of &#945;-pinene in an environmental chamber are consistent with these observations. We also used thermodynamic and kinetic modelling to investigate the atmospheric implications of our experimental results.

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An investigation into the performance of four cloud droplet activation parameterisations

Geoscientific Model Development ◽

10.5194/gmd-7-1535-2014 ◽

2014 ◽

Vol 7 (4) ◽

pp. 1535-1542 ◽

Cited By ~ 18

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.

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Aerosol-cirrus interactions: A number based phenomenon at all?

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-3-3625-2003 ◽

2003 ◽

Vol 3 (4) ◽

pp. 3625-3657

Author(s):

M. Seifert ◽

J. Ström ◽

R. Krejci ◽

A. Minikin ◽

A. Petzold ◽

...

Keyword(s):

Aerosol Particles ◽

Cloud Formation ◽

Data Sets ◽

Number Density ◽

Cloud Droplets ◽

Data Set ◽

Local Maxima ◽

The Mean ◽

Positive Correlations ◽

Aerosol Cloud Interactions

Abstract. In situ measurements of the partitioning of aerosol particles within cirrus clouds were used to investigate aerosol-cloud interactions in ice clouds. The number density of interstitial aerosol particles (non-activated particles in between the cirrus crystals) was compared to the number density of cirrus crystal residuals. The data was obtained during the two INCA (Interhemispheric Differences in Cirrus Properties form Anthropogenic Emissions) campaigns, performed in the Southern Hemisphere (SH) and Northern Hemisphere (NH) midlatitudes. Different aerosol-cirrus interactions can be linked to the different stages of the cirrus lifecycle. Cloud formation is linked to positive correlations between the number density of interstitial aerosol (Nint) and crystal residuals (Ncvi), whereas the correlations are smaller or even negative in a dissolving cloud. Unlike warm clouds, where the number density of cloud droplets is positively related to the aerosol number density, we observed a rather complex relationship when expressing Ncvi as a function of Nint for forming clouds. The data sets are similar in that they both show local maxima in the Nint range 100 to 200 cm−3, where the SH-maximum is shifted towards the higher value. For lower number densities Nint and Ncvi are positively related. The slopes emerging from the data suggest that a tenfold increase in the aerosol number density corresponds to a 3 to 4 times increase in the crystal number density. As Nint increases beyond the ca. 100 to 200 cm−3, the mean crystal number density decreases at about the same rate for both data sets. For much higher aerosol number densities, only present in the NH data set, the mean Ncvi remains low. The situation for dissolving clouds presents two alternative interactions between aerosols and cirrus. Either evaporating clouds are associated with a source of aerosol particles, or air pollution (high aerosol number density) retards evaporation rates.

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A light-driven burst of hydroxyl radicals dominates oxidation chemistry in newly activated cloud droplets

Science Advances ◽

10.1126/sciadv.aav7689 ◽

2019 ◽

Vol 5 (5) ◽

pp. eaav7689 ◽

Cited By ~ 9

Author(s):

Suzanne E. Paulson ◽

Peter J. Gallimore ◽

Xiaobi M. Kuang ◽

Jie Rou Chen ◽

Markus Kalberer ◽

...

Keyword(s):

Hydroxyl Radicals ◽

Uv Light ◽

Aerosol Particles ◽

Cloud Droplet ◽

Radiative Properties ◽

Size Distributions ◽

Cloud Droplets ◽

New Process ◽

Oxidation Chemistry ◽

The Impact

Aerosol particles and their interactions with clouds are one of the most uncertain aspects of the climate system. Aerosol processing by clouds contributes to this uncertainty, altering size distributions, chemical composition, and radiative properties. Many changes are limited by the availability of hydroxyl radicals in the droplets. We suggest an unrecognized potentially substantial source of OH formation in cloud droplets. During the first few minutes following cloud droplet formation, the material in aerosols produces a near-UV light–dependent burst of hydroxyl radicals, resulting in concentrations of 0.1 to 3.5 micromolar aqueous OH ([OH]aq). The source of this burst is previously unrecognized chemistry between iron(II) and peracids. The contribution of the “OH burst” to total OH in droplets varies widely, but it ranges up to a factor of 5 larger than previously known sources. Thus, this new process will substantially enhance the impact of clouds on aerosol properties.

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The influence of nitric acid on the cloud processing of aerosol particles

Atmospheric Chemistry and Physics ◽

10.5194/acp-6-1627-2006 ◽

2006 ◽

Vol 6 (6) ◽

pp. 1627-1634 ◽

Cited By ~ 6

Author(s):

S. Romakkaniemi ◽

H. Kokkola ◽

K. E. J. Lehtinen ◽

A. Laaksonen

Keyword(s):

Sulfuric Acid ◽

Nitric Acid ◽

Aerosol Particles ◽

Cloud Droplet ◽

Number Concentration ◽

Radiative Properties ◽

Aerosol Size Distribution ◽

Cloud Droplets ◽

Cloud Processing ◽

Cloud Droplet Number Concentration

Abstract. In this paper we present simulations of the effect of nitric acid (HNO3) on cloud processing of aerosol particles. Sulfuric acid (H2SO4) production and incloud coagulation are both affected by condensed nitric acid as nitric acid increases the number of cloud droplets, which will lead to smaller mean size and higher total surface area of droplets. As a result of increased cloud droplet number concentration (CDNC), the incloud coagulation rate is enhanced by a factor of 1–1.3, so that the number of interstitial particles reduces faster. In addition, sulfuric acid production occurs in smaller particles and so the cloud processed aerosol size distribution is dependent on the HNO3 concentration. This affects both radiative properties of aerosol particles and the formation of cloud droplets during a sequence of cloud formation-evaporation events. It is shown that although the condensation of HNO3 increases the number of cloud droplets during the single updraft, it is possible that presence of HNO3 can actually decrease the cloud droplet number concentration after several cloud cycles when also H2SO4 production is taken into account.

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