Comment on acp-2022-598

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.