Summer time Fe depletion in the Antarctic mesopause region

We report common volume measurements of Fe densities, temperatures and ice particle occurrence in the mesopause region at Davis Station, Antarctica (69°S) in the years 2011–2012. Our observations show a strong correlation of the Fe-layer summer time depletion with temperature, but no clear causal relation with the onset or occurrence of ice particles measured as noctilucent clouds (NLC) or polar mesosphere summer echoes (PMSE). The combination of these measurements indicates that the strong summer depletion can be explained by gas-phase chemistry alone and does not require heterogeneous removal of Fe and its compounds on ice particles

Nighttime atmospheric chemistry of iodine

Little attention has so far been paid to the nighttime atmospheric chemistry of iodine species. Current atmospheric models predict a buildup of HOI and I₂ during the night that leads to a spike of IO at sunrise, which is not observed by measurements. In this work, electronic structure calculations are used to survey possible reactions that HOI and I₂ could undergo at night in the lower troposphere, and hence reduce their nighttime accumulation. The new reaction NO₃+ HOI  →  IO + HNO₃ is proposed, with a rate coefficient calculated from statistical rate theory over the temperature range 260–300 K and at a pressure of 1000 hPa to be k(T)  =  2.7  ×  10¯¹² (300 K/T)²·⁶⁶ cm³ molecule¯¹ s¯¹. This reaction is included in two atmospheric models, along with the known reaction between I₂ and NO₃, to explore a new nocturnal iodine radical activation mechanism. The results show that this iodine scheme leads to a considerable reduction of nighttime HOI and I₂, which results in the enhancement of more than 25 % of nighttime ocean emissions of HOI +I₂ and the removal of the anomalous spike of IO at sunrise. We suggest that active nighttime iodine can also have a considerable, so far unrecognized, impact on the reduction of the NO₃ radical levels in the marine boundary layer (MBL) and hence upon the nocturnal oxidizing capacity of the marine atmosphere. The effect of this is exemplified by the indirect effect on dimethyl sulfide (DMS) oxidation

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

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

A global atmospheric model of meteoric iron

The first global model of meteoric iron in the atmosphere (WACCM-Fe) has been developed by combining three components: the Whole Atmosphere Community Climate Model (WACCM), a description of the neutral and ion-molecule chemistry of iron in the mesosphere and lower thermosphere (MLT), and a treatment of the injection of meteoric constituents into the atmosphere. The iron chemistry treats seven neutral and four ionized iron containing species with 30 neutral and ion-molecule reactions. The meteoric input function (MIF), which describes the injection of Fe as a function of height, latitude, and day, is precalculated from an astronomical model coupled to a chemical meteoric ablation model (CABMOD). This newly developed WACCM-Fe model has been evaluated against a number of available ground-based lidar observations and performs well in simulating the mesospheric atomic Fe layer. The model reproduces the strong positive correlation of temperature and Fe density around the Fe layer peak and the large anticorrelation around 100 km. The diurnal tide has a significant effect in the middle of the layer, and the model also captures well the observed seasonal variations. However, the model overestimates the peak Fe+concentration compared with the limited rocket-borne mass spectrometer data available, although good agreement on the ion layer underside can be obtained by adjusting the rate coefficients for dissociative recombination of Fe-molecular ions with electrons. Sensitivity experiments with the same chemistry in a 1-D model are used to highlight significant remaining uncertainties in reaction rate coefficients, and to explore the dependence of the total Fe abundance on the MIF and rate of vertical transport

Iodine chemistry after dark

Little attention has so far been paid to the nighttime atmospheric chemistry of iodine species. Current atmospheric models predict a buildup of HOI and I₂ during the night that leads to a spike of IO at sunrise, which is not observed by measurements. In this work, electronic structure calculations are used to survey possible reactions that HOI and I₂ could undergo at night in the lower troposphere, and hence reduce their nighttime accumulation. The new reaction NO₃ + HOI → IO + HNO₃ is proposed, with a rate coefficient calculated from statistical rate theory over the temperature range 260 - 300 K and at a pressure of 1000 hPa to be k(T) = 2.7 x 10-¹² (300 K / T ) ².⁶⁶ cm³ molecule-¹ s-¹ . This reaction is included in two atmospheric models, along with the known reaction between I₂ and NO₃, to explore a new nocturnal iodine radical activation mechanism. The results show that this iodine scheme leads to a considerable reduction of nighttime HOI and I₂, which results in the enhancement of more than 25% of nighttime ocean emissions of HOI + I₂ and the removal of the anomalous spike of IO at sunrise. We suggest that active nighttime iodine can also have a considerable, so far unrecognized, impact on the reduction of the NO₃ radical levels in the MBL and hence upon the nocturnal oxidizing capacity of the marine atmosphere. The effect of this is exemplified by the indirect effect on dimethyl sulfide (DMS) oxidation

Kinetics of isothermal and non-isothermal precipitation in an Al-6at%Si alloy

A novel theory which describes the progress of a thermally activated reaction under isothermal and linear heating conditions is presented. It incorporates nucleation, growth and impingement and takes account of temperaturedependent solubility. The model generally fits very well to isothermal calorimetry and differential scanning calorimetry data on precipitation in an Al-6 at.% Si alloy. Analysis of the data shows that two processes occur in this precipitation reaction: growth of large Si particles and growth of pre-existing small nuclei. Determination of the sizes of Si precipitates by transmission electron microscopy indicates that interfacial energy contributions are small and have a negligible influence on solubilit

The intensification of metallic layered phenomena above thunderstorms through the modulation of atmospheric tides

We present a multi-instrument experiment to study the effects of tropospheric thunderstorms on the mesopause region and the lower ionosphere. Sodium (Na) lidar and ionospheric observations by two digital ionospheric sounders are used to study the variation in the neutral metal atoms and metallic ions above thunderstorms. An enhanced ionospheric sporadic E layer with a downward tidal phase is observed followed by a subsequent intensification of neutral Na number density with an increase of 600 cm−3 in the mesosphere. In addition, the Na neutral chemistry and ion-molecule chemistry are considered in a Na chemistry model to simulate the dynamical and chemical coupling processes in the mesosphere and ionosphere above thunderstorms. The enhanced Na layer in the simulation obtained by using the ionospheric observation as input is in agreement with the Na lidar observation. We find that the intensification of metallic layered phenomena above thunderstorms is associated with the atmospheric tides, as a result of the troposphere-mesosphere-ionosphere coupling