Future warming exacerbated by aged-soot effect on cloud formation

Lohmann, Ulrike; Friebel, Franz; Kanji, Zamin A.; Mahrt, Fabian; Mensah, A. A.; Neubauer, David

doi:10.1038/s41561-020-0631-0

Cited by 71 publications

(69 citation statements)

References 74 publications

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“…Many or even most observations of pre-activation can be explained by pore ice that has nucleated in a preceding activation step/cloud cycle (Marcolli, 2017b), allowing ice crys-tal formation by pure depositional growth when the particles re-encounter ice supersaturation. Yet, some observations of pre-activation may also be explained by the contact of soot aggregates with ice or liquid water changing their (pore) morphology or modifying surface functionalization (Bhandari et al, 2019;China et al, 2015;Colbeck et al, 1990;Ma et al, 2013;Miljevic et al, 2012;Zuberi et al, 2005) and thus enhancing their ice nucleation activity in subsequent cloud cycles, even in the absence of ice pockets preserved in pores (Mahrt et al, 2020b). Here, we focus on pre-activation through ice preserved in pores.…”

Section: Capillary Condensation Within Soot Aggregatesmentioning

confidence: 99%

“…Since evaporating water or ice exerts a tension on the surrounding soot spherules (e.g. China et al, 2015;Ma et al, 2013), these spherules are attracted to each other and open loops can close, giving rise to ring pores and larger cavities.…”

Section: Atmospheric Implicationsmentioning

confidence: 99%

“…While accounting for soot aggregate size is indispensable to correctly estimating the number of primary particles and pores present that ultimately determine the ice nucleation activity, explicitly measuring the size-and relative humidity-dependent ice nucleation activity has only been the focus of a limited number of studies to date. Recently, Lohmann et al (2020) used a more realistic parameterization derived from the ice nucleation activity of 400 nm miniCAST black soot aggregates, measured at 233 and 218 K, with the AF being a function of RH i but still independent of aggregate size. Moreover, Zhou and Penner (2014) and Lohmann et al (2020) assumed that soot aggregates are rendered inactive over time upon acquiring a coating of three monolayers or one monolayer, respectively.…”

Section: Atmospheric Implicationsmentioning

confidence: 99%

“…The ice nucleation activity of soot particles has been shown to impact cirrus cloud coverage, emissivity, and lifetime, thereby affecting the radiative balance (e.g. Lohmann, 2002;Lohmann et al, 2020;McGraw et al, 2020). Knowledge of the ice nucleation ability of soot particles is essential for estimating the anthropogenic influence on cirrus clouds (Kärcher, 2017;Penner et al, 2018;Zhao et al, 2019).…”

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confidence: 99%

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Soot PCF: pore condensation and freezing framework for soot aggregates

2021

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Abstract. Atmospheric ice formation in cirrus clouds is often initiated by aerosol particles that act as ice-nucleating particles. The aerosol–cloud interactions of soot and associated feedbacks remain uncertain, in part because a coherent understanding of the ice nucleation mechanism and activity of soot has not yet emerged. Here, we provide a new framework that predicts ice formation on soot particles via pore condensation and freezing (PCF) that, unlike previous approaches, considers soot particle properties, capturing their vastly different pore properties compared to other aerosol species such as mineral dust. During PCF, water is taken up into pores of the soot aggregates by capillary condensation. At cirrus temperatures, the pore water can freeze homogeneously and subsequently grow into a macroscopic ice crystal. In the soot-PCF framework presented here, the relative humidity conditions required for these steps are derived for different pore types as a function of temperature. The pore types considered here encompass n-membered ring pores that form between n individual spheres within the same layer of primary particles as well as pores in the form of inner cavities that form between two layers of primary particles. We treat soot primary particles as perfect spheres and use the contact angle between soot and water (θsw), the primary particle diameter (Dpp), and the degree of primary particle overlap (overlap coefficient, Cov) to characterize pore properties. We find that three-membered and four-membered ring pores are of the right size for PCF, assuming primary particle sizes typical of atmospheric soot particles. For these pore types, we derive equations that describe the conditions for all three steps of soot PCF, namely capillary condensation, ice nucleation, and ice growth. Since at typical cirrus conditions homogeneous ice nucleation can be considered immediate as soon as the water volume within the pore is large enough to host a critical ice embryo, soot PCF becomes limited by either capillary condensation or ice crystal growth. We use the soot-PCF framework to derive a new equation to parameterize ice formation on soot particles via PCF, based on soot properties that are routinely measured, including the primary particle size, overlap, and the fractal dimension. These properties, along with the number of primary particles making up an aggregate and the contact angle between water and soot, constrain the parameterization. Applying the new parameterization to previously reported laboratory data of ice formation on soot particles provides direct evidence that ice nucleation on soot aggregates takes place via PCF. We conclude that this new framework clarifies the ice formation mechanism on soot particles in cirrus conditions and provides a new perspective to represent ice formation on soot in climate models.

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Section: Capillary Condensation Within Soot Aggregatesmentioning

confidence: 99%

Section: Atmospheric Implicationsmentioning

confidence: 99%

Section: Atmospheric Implicationsmentioning

confidence: 99%

mentioning

confidence: 99%

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Soot PCF: pore condensation and freezing framework for soot aggregates

2021

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“…The ice nucleation activity of soot particles has been shown to impact cirrus cloud coverage, emissivity and lifetime, thereby affecting the radiative balance (e.g. Lohmann, 2002;Lohmann et al, 2020;McGraw et al, 2020). Knowledge of the ice nucleation ability of soot particles is essential for estimating the anthropogenic influence on cirrus clouds (Kärcher, 2017;Penner et al, 2018;Zhao et al, 2019).…”

Section: Introductionmentioning

confidence: 99%

Soot-PCF: Pore condensation and freezing framework for soot aggregates

Marcolli¹,

Mahrt²,

Kärcher³

2020

Preprint

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Abstract. How ice crystals form in the troposphere strongly affects cirrus cloud properties. Atmospheric ice formation is often initiated by aerosol particles that act as ice nucleating particles. The aerosol-cloud interactions of soot and associated feedbacks remain uncertain, in part because a coherent understanding of the ice nucleation mechanism and activity of soot has not yet emerged. Here, we provide a new framework that predicts ice formation on soot particles via pore condensation and freezing (PCF) that, unlike previous approaches, considers soot particle properties capturing their vastly different pore properties compared to other aerosol species such as mineral dust. During PCF, water is taken up below water saturation into pores on soot aggregates by capillary condensation. At cirrus temperatures, pore water can freeze homogeneously and subsequently grow into a macroscopic ice crystal. In the soot-PCF framework presented here, the relative humidity conditions required for these steps are derived for different pore types as a function of temperature. The pore types considered here evolve from idealized stacking of equally sized primary particles, either in tetrahedral or cubic packing arrangements. Specifically, we encompass n-membered ring pores that form between n individual spheres within the same layer of primary particles as well as pores in the form of inner cavities that form between two layers of primary particles. We treat soot primary particles as perfect spheres and use the contact angle between soot and water (θsw), the primary particle diameter (Dpp) and the degree of primary particle overlap (overlap coefficient, Cov) to characterize soot pore properties. We find that n-membered ring pores are the dominant pore structures for soot-PCF, as they are common features of soot aggregates and have a suitable geometry for both, filling with water and growing ice below water saturation. We focus our analysis on three-membered and four-membered ring pores as they are of the right size for PCF assuming primary particle sizes typical for atmospheric soot particles. For these pore types, we derive equations that describe the conditions for all three steps of soot-PCF, namely capillary condensation, ice nucleation, and ice growth. Since at typical cirrus conditions homogeneous ice nucleation can be considered immediate as soon as the water volume within the pore is large enough to host a critical ice embryo, soot-PCF becomes either limited by capillary condensation or ice crystal growth. For instance, our results show that at typical cirrus temperatures of T = 220 K, three-membered ring pores formed between primary particles with θsw = 60°, Dpp = 20 nm, and Cov = 0.05 are ice growth limited, as the ice requires a relative humidity with respect to ice of RHi = 137 % to grow out of the pore, while a sufficient volume of pore water for ice nucleation has condensed already at RHi = 86 %. Conversely, four-membered ring pores with the same primary particle size and an overlap coefficient of Cov = 0.1 are capillary condensation limited as they require RHi = 129 % to gather enough water for ice nucleation, compared with only 124 % RHi, required for ice growth. We use the soot-PCF framework to derive a new equation to parameterize of ice formation on soot particles via PCF. This equation is based on soot properties that are routinely measured, including the primary particle size and overlap, and the fractal dimension. These properties, along with the number of primary particles making up an aggregate and the contact angle between water and soot, constrain the parameterization. Applying the new parameterization to previously reported laboratory data of ice formation on soot particles provides direct evidence that ice nucleation on soot aggregates takes place via PCF. We conclude that this new framework clarifies the ice formation mechanism on soot particles at cirrus conditions and provides a new perspective to represent ice formation on soot in climate models.

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