29 Aug 2022
29 Aug 2022
Status: this preprint is currently under review for the journal ACP.

The importance of acid processed meteoric smoke relative to meteoric fragments for crystal nucleation in polar stratospheric clouds

Alexander D. James1, Finn Pace1, Sebastien N. F. Sikora2, Graham W. Mann2, John M. C. Plane1, and Benjamin J. Murray2 Alexander D. James et al.
  • 1School of Chemistry, University of Leeds, Leeds, LS9 2JT, UK
  • 2School of Earth and Environment, University of Leeds, LS9 2JT, UK

Abstract. Nitric Acid Trihydrate (NAT) crystal formation in the absence of water ice is important for a subset of Polar Stratospheric Clouds (PSCs) and thereby ozone depletion. It has been suggested that either fragmented meteoroids or meteoric smoke particles (MSPs), or possibly both, are important as heterogeneous nuclei of these crystals. Previous work has focused on the nucleating ability of meteoric material in nitric acid in the absence of sulfuric acid. However, it is known that when immersed in stratospheric sulfuric acid droplets, metal-containing meteoric material particles partially dissolve and components can re-precipitate as silica and alumina that have different morphologies to the original meteoric material. Hence, in this study we experimentally and theoretically explore the relative role that sulfuric acid-processed meteoric smoke and meteoric fragments may play in NAT nucleation in PSCs.

We compared meteoric fragments that had been recently prepared (by milling a meteorite sample) to a sample annealed under conditions designed to simulate heating during entry into the Earth’s atmosphere. Whilst the addition of sulfuric acid decreased the nucleating ability of the recently milled meteoric material relative to nucleation in binary nitric acid-water solutions (at similar NAT saturation ratio), the annealed meteoric fragments nucleated NAT with a similar effectiveness in both solutions. However, combining our results with measured fluxes of meteoric material to the Earth, sedimentation modelling and recent experiments on fragmentation of incoming meteoroids, suggests that there are unlikely to be sufficient fragments to contribute to the nucleation of crystalline NAT particles.

We then considered silica formed from sulfuric acid processed meteoric smoke particles. Our previous work showed that nano-particulate silica (radius ~6 nm) is a relatively poor promoter of nucleation compared with micron scaled silica particles, which were more effective. Both materials have similar chemical and structural (crystallographically amorphous) properties, indicating size is critical. Here we account for surface curvature of primary grains using Classical Nucleation Theory (CNT) to explore this size dependence. This model is able to explain the discrepancy in nucleation effectiveness of fumed silica and fused quartz, by treating their nucleating activity (contact angle) as equal but with differing particle size (or surface curvature), assuming interfacial energies that are physically reasonable. Here we use this CNT model to present evidence that nucleation of NAT on acid processed MSPs, where the primary grain size is 10s nm, is also effective enough to contribute to NAT crystals in early season PSCs where there is an absence of ice.

This study demonstrates that modelling of crystal nucleation in PSCs and resulting ozone depletion relies on accurate understanding of the transport and chemical processing of MSPs. This will affect estimated sensitivity of stratospheric chemistry to rare events such as large volcanic eruptions and long-term forecasting of ozone recovery in a changing climate.

Alexander D. James et al.

Status: open (until 10 Oct 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-598', Michel J. Rossi, 22 Sep 2022 reply
  • RC2: 'Comment on acp-2022-598', Anonymous Referee #2, 03 Oct 2022 reply

Alexander D. James et al.

Alexander D. James et al.


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Short summary
Here we examine whether several materials of meteoric origin can nucleate crystallisation in stratospheric cloud droplets, which would affect ozone depletion. We find that material which could fragment on atmospheric entry without melting is unlikely to be present in high enough concentration in the stratosphere to contribute to observed crystalline cloud. Material which ablates completely then forms a new solid can provide enough nucleation to explain observed cloud.