The photolysis of FeOH and its effect on the bottomside of the mesospheric Fe layer

Metal layers in the upper mesosphere and lower thermosphere are created through meteoric ablation. They are important for understanding the temperature structure, dynamics and chemistry of this atmospheric region. Recent lidar observations have shown a regular downward extension of the Fe layer bottomside which correlates with solar radiation. In this study we combine lidar observations, quantum chemical calculations and model simulations to show that this bottomside extension is primarily caused by photolysis of FeOH. We determine the photolysis rate to be J(FeOH)=(6±3)×10−3s−1. We also show that the reaction FeOH+H→FeO+H2 is slower at mesospheric temperatures than previous estimates. With these updated rate coefficients, we are able to significantly improve the modelling of the Fe layer bottomside. The calculations further show the nearly complete depletion of FeOH during sunlit periods. This may have implications for cloud nuclei in the middle atmosphere

CO₂ ice structure and density under Martian atmospheric conditions

Clouds composed of CO2 ice form throughout the Martian atmosphere. In the mesosphere, CO2 ice clouds are thought to form via heterogeneous ice nucleation on nanoparticles of meteoric origin at temperatures often below 100 K. Lower altitude CO2 ice clouds in the wintertime polar regions form up to around 145 K and lead to the build-up of the polar ice caps. However, the crystal structure and related fundamental properties of CO2 ice under Martian conditions are poorly characterised. Here we present X-ray diffraction (XRD) measurements of CO2 ice, grown via deposition from the vapour phase under temperature and pressure conditions analogous to the Martian mesosphere. A crystalline cubic structure was determined, consistent with the low-pressure polymorph (CO2-I, space group Pa-3 (No. 205)). CO2 deposited at temperatures of 80 - 130 K and pressures of 0.01 – 1 mbar was consistent with dry ice and previous literature measurements, thus removing the possibility of a more complicated phase diagram for CO2 in this region. At 80 K, a lattice parameter of 5.578 ± 0.002 Å, cell volume of 173.554 ± 0.19 Å3 and density of 1.684 ± 0.002 g cm−3 was determined. Using these measurements, we determined the thermal expansion of CO2 across 80 – 130 K that allowed for a fit of CO2 ice density measurements across a larger temperature range (80 – 195 K) when combined with literature data (CO2 density = 1.72391 - 2.53×10−4 T - 2.87×10−6 T2). Temperature-dependent CO2 density values are used to estimate sedimentation velocities and heterogeneous ice nucleation rates, showing an increase in nucleation rate of up to a factor of 1000 when compared to commonly used literature values. This temperature-dependent equation of state is therefore suggested for use in future studies of Martian mesospheric CO2 clouds. Finally, we discuss the possible shapes of crystals of CO2 ice in the Martian atmosphere and show that a range of shapes including cubes and octahedra as well as a combination of the two in the form of cubo-octahedra are likely

Meteor trail characteristics observed by high time resolution lidar

We report and analyse the characteristics of 1382 meteor trails based on a sodium data set of ∼680 h. The observations were made at Yanqing (115.97° E, 40.47° N), China by a ground-based Na fluorescence lidar. The temporal resolution of the raw profiles is 1.5 s and the altitude resolution is 96 m. We discover some characteristics of meteor trails different from those presented in previous reports. The occurrence heights of the trails follow a double-peak distribution with the peaks at ∼83.5 km and at ∼95.5 km, away from the peak height of the regular Na layer. 4.7% of the trails occur below 80 km, and 3.25% above 100 km. 75% of the trails are observed in only one 1.5 s profile, suggesting that the dwell time in the laser beam is not greater than 1.5 s. The peak density of the trails as a function of height is similar to that of the background sodium layer. The raw occurrence height distribution is corrected taking account of three factors which affect the relative lifetime of a trail as a function of height: the meteoroid velocity (which controls the ratio of Na / Na+ ablated); diffusional spreading of the trail; and chemical removal of Na. As a result, the bi-modal distribution is more pronounced. Modelling results show that the higher peak corresponds to a meteoroid population with speeds between 20 and 30 km s?1, whereas the lower peak should arise from much slower particles in a near-prograde orbit. It is inferred that most meteoroids in this data set have masses of ∼1 mg, in order for ablation to produce sufficient Na atoms to be detected by lidar. Finally, the evolution of longer-duration meteor trails is investigated. Signals at each altitude channel consist of density enhancement bursts with the growth process usually faster than the decay process, and there exists a progressive phase shift among these altitude channels

Decay times of transitionally dense specularly reflecting meteor trails and potential chemical impact on trail lifetimes

Studies of transitionally dense meteor trails using radars which employ specularly reflecting interferometric techniques are used to show that measurable high-temperature chemistry exists at timescales of a few tenths of a second after the formation of these trails. This is a process which is distinct from the ambient-temperature chemistry that is already known to exist at timescales of tens of seconds and longer in long-lived trails. As a consequence, these transitionally dense trails have smaller lifetimes than might be expected if diffusion were the only mechanism for reducing the mean trail electron density. The process has been studied with four SKiYMET radars at latitudes varying from 10 to 75° N, over a period of more than 10 years, 24 h per day. In this paper we present the best parameters to use to represent this behaviour and demonstrate the characteristics of the temporal and latitudinal variability in these parameters. The seasonal, day-night and latitudinal variations correlate reasonably closely with the corresponding variations of ozone in the upper mesosphere. Possible reasons for these effects are discussed, but further investigations of any causative relation are still the subject of ongoing studies

Observations of the Nickel Layer in the Mesopause Region at Mid-Latitudes

Observations of the mesospheric Ni layer have been performed by lidar in January-March 2018 at Kuehlungsborn/Germany (54°N, 12°E). These soundings provide only the second Ni data set after initial observations by Collins et al. at Chatanika/Alaska (65°N, 147°W)[1]. We utilized for the first time a transition from the low-lying excited Ni(3D) state at 341 nm. For all soundings, nightly mean peak densities varied between ~280 cm−3 and 450 cm3, which is a factor of ~40 less than previously reported for Chatanika [1]. The observed Ni abundance is especially important if compared with the abundance of other metals like Fe, and with their respective abundances in evaporating meteoroids, which form the source of the metal layer in the upper mesosphere. Here, we present exemplarily a sounding from January 8, 2018. Beside the Ni raw data and density profiles we show a temperature profile as measured simultaneously be the co-located RMR lidar and the temperature variation due to gravity waves and tides

Nucleation of nitric acid hydrates in polar stratospheric clouds by meteoric material

Heterogeneous nucleation of crystalline nitric acid hydrates in polar stratospheric clouds (PSCs) enhances ozone depletion. However, the identity and mode of action of the particles responsible for nucleation remains unknown. It has been suggested that meteoric material may trigger nucleation of nitric acid trihydrate (NAT, or other nitric acid phases), but this has never been quantitatively demonstrated in the laboratory. Meteoric material is present in two forms in the stratosphere: smoke that results from the ablation and re-condensation of vapours, and fragments that result from the break-up of meteoroids entering the atmosphere. Here we show that analogues of both materials have a capacity to nucleate nitric acid hydrates. In combination with estimates from a global model of the amount of meteoric smoke and fragments in the polar stratosphere we show that meteoric material probably accounts for NAT observations in early season polar stratospheric clouds in the absence of water ice

Observations and Modeling of Potassium Emission in the Terrestrial Nightglow

The ablation of cosmic dust entering the atmosphere causes the formation of an atomic potassium (K) layer in the mesopause region. It can be studied via resonance fluorescence from the K(D1) line at 769.9 nm, stimulated by sunlight or a laser. In addition, the faint emission from a chemiluminescent cycle involving ozone and oxygen atoms has been observed with a nocturnal mean intensity of about 1 Rayleigh. In this study, the K nightglow is investigated in much greater detail, using 2,299 high‐resolution spectra taken with the astronomical echelle spectrograph Ultraviolet and Visual Echelle Spectrograph at Cerro Paranal in Chile (24.6°S) between 2000 and 2014. The seasonal variation is dominated by a maximum in June. During the night, the highest intensities are found close to sunrise. Moreover, there is a clear negative correlation with solar activity. These variations are very different from those of the well‐studied sodium (Na) nightglow. The K nightglow at Cerro Paranal was also simulated with the Whole Atmosphere Community Climate Model including K chemistry. The observed and modeled climatologies do not match well, largely because of unreliable Whole Atmosphere Community Climate Model ozone densities. Satellite‐based profile retrievals for ozone and temperature from Sounding of the Atmosphere using Broadband Emission Radiometry and K from Optical Spectrograph and Infrared Imaging System were then used to simulate the K nightglow and to derive the quantum yield of the K(D) emission with respect to the reaction of K with ozone. Considering that the obscured K(D2) line is expected on theoretical grounds to be 1.67 times brighter than K(D1), we find about 30% for this quantum yield, which is much higher than for Na(D) emission