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.
Abstract. The crystal formation of nitric acid trihydrate (NAT) 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 reprecipitate 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 MSPs and meteoric fragments may play in NAT nucleation in PSCs. We compared meteoric fragments that had recently been 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 it is unlikely for there to be sufficient fragments to contribute to the nucleation of crystalline NAT particles. We then considered silica formed from sulfuric acid-processed MSPs. Our previous work showed that nanoparticulate 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 that size is critical. Here, we account for surface curvature of primary grains using the 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 tens of nanometres, is also effective enough to contribute to NAT crystals in early season PSCs where there is an absence of ice. This study demonstrates that the modelling of crystal nucleation in PSCs and resulting ozone depletion relies on an 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.
<p>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. However, nucleation of these crystals is not understood.</p><p>It has been suggested previously that either fragmented meteoroids or meteoric smoke particles (MSPs), or possibly both, are important as heterogeneous nuclei. The role of H<sub>2</sub>SO<sub>4</sub>, which is present in liquid PSCs, in these nucleation processes has not been investigated. It is known that metal-containing Meteoric Smoke Particles (MSPs) are processed, partially dissolving whilst some components re-precipitate within H<sub>2</sub>SO<sub>4</sub> droplets, producing silica and alumina particles which differ in size from the original MSPs. We recently found that analogues for nanoparticulate MSPs have a low ability to nucleate NAT relative to larger particles of similar material, suggesting that the size of particles may be a critical parameter for the nucleating ability of silica particles. &#160;We previously showed experimentally that nano-particulate fumed silica is a poor promoter of nucleation, whilst micron scale fused quartz was found to be effective. Both materials have similar chemical and structural (crystallographically amorphous) properties.</p><p>In this study we developed a model using Classical Nucleation Theory (CNT) where we account for surface curvature of primary grains. This model is able to account for the discrepancy in nucleation effectiveness of fumed silica and fused quartz, by treating them as having the same nucleating ability (contact angle) but differing particle size (or equivalently surface curvature), assuming interfacial energies which are physically reasonable given literature measurements. We also performed new experiments which allowed us to refine our understanding of the H<sub>2</sub>SO<sub>4</sub> sensitivity of NAT nucleation by meteoric fragments. Combining sedimentation modelling with our results and recent experiments on fragmentation of incoming meteoroids suggests that fragments are unlikely to be important as heterogeneous nuclei. However, the CNT model developed here provides evidence that nucleation of NAT on (10s nm) MSP analogues is effective enough to explain observed NAT crystal number concentrations in PSCs (without ice).</p>
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