A Chemical-dynamical Model of Wave-driven Sodium Fluctuations

A comprehensive chemical-dynamical model is used to investigate the interaction of gravity waves with twenty minor species involved in the atomic sodium chemistry in the mesopause region. We find that chemistry becomes important on the underside of the sodium layer, primarily below 85 km altitude, where the relative importance of chemistry in wave-driven sodium fluctuations increases with increasing wave period and increasing horizontal wavelength. We also find that for altitudes below 80 km an adequate determination of the effects of chemistry in these fluctuations requires the inclusion of several reactions related to ozone chemistry. However, the atomic Na density is too low this region to be routinely observed by current sodium lidars. Importantly, we find that above 85 km altitude sodium can be treated as a passive tracer of gravity wave motions

The Meteoric Ni Layer in the Upper Atmosphere

The first global atmospheric model of Ni (WACCM-Ni) has been developed to understand recent observations of the mesospheric Ni layer by ground-based resonance lidars. The three components of the model are: the Whole Atmospheric Community Climate Model (WACCM6); a meteoric input function derived by coupling an astronomical model of dust sources in the solar system with a chemical meteoric ablation model; and a comprehensive set of neutral, ion-molecule, and photochemical reactions pertinent to the chemistry of Ni in the upper atmosphere. In order to achieve closure on the chemistry, the reaction kinetics of three important reactions were first studied using a fast flow tube with pulsed laser ablation of a Ni target, yielding k(NiO + O) = (4.6 ± 1.4) × 10−11, k(NiO + CO) = (3.0 ± 0.5) × 10−11, and k(NiO2 + O) = (2.5 ± 1.2) × 10−11 cm3 molecule−1 s−1 at 294 K. The photodissociation rate of NiOH was computed to be J(NiOH) = 0.02 s−1. WACCM-Ni simulates satisfactorily the observed neutral Ni layer peak height and width, and Ni+ measurements from rocket-borne mass spectrometry. The Ni layer is predicted to have a similar seasonal and latitudinal variation as the Fe layer, and its unusually broad bottom-side compared with Fe is caused by the relatively fast NiO + CO reaction. The quantum yield for photon emission from the Ni + O3 reaction, observed in the nightglow, is estimated to be between 6% and 40%. ©2020. The Authors

Impacts of a sudden stratospheric warming on the mesospheric metal layers

We report measurements of atomic sodium, iron and temperature in the mesosphere and lower thermosphere (MLT) made by ground-based lidars at the ALOMAR observatory (69°N, 16°E) during a major sudden stratospheric warming (SSW) event that occurred in January 2009. The high resolution temporal observations allow the responses of the Na and Fe layers to the SSW at high northern latitudes to be investigated. A significant cooling with temperatures as low as 136 K around 90 km was observed on 22 − 23 January 2009, along with substantial depletions of the Na and Fe layers (an ~80% decrease in the column abundance with respect to the mean over the observation period). The Whole Atmosphere Community Climate Model (WACCM) incorporating the chemistry of Na, Fe, Mg and K, and nudged with reanalysis data below 60 km, captures well the timing of the SSW, although the extent of the cooling and consequently the depletion in the Na and Fe layers is slightly underestimated. The model also predicts that the perturbations to the metal layers would have been observable even at equatorial latitudes. The modelled Mg layer responds in a very similar way to Na and Fe, whereas the K layer is barely affected by the SSW because of the enhanced conversion of K+ ions to K atoms at the very low temperatures

A combined rocket-borne and ground-based study of the sodium layer and charged dust in the upper mesosphere

The Hotel Payload 2 rocket was launched on January 31st 2008 at 20.14 LT from the Andøya Rocket Range in northern Norway (69.31° N, 16.01° E). Measurements in the 75–105 km region of atomic O, negatively-charged dust, positive ions and electrons with a suite of instruments on the payload were complemented by lidar measurements of atomic Na and temperature from the nearby ALOMAR observatory. The payload passed within 2.58 km of the lidar at an altitude of 90 km. A series of coupled models is used to explore the observations, leading to two significant conclusions. First, the atomic Na layer and the vertical profiles of negatively-charged dust (assumed to be meteoric smoke particles), electrons and positive ions, can be modelled using a self-consistent meteoric input flux. Second, electronic structure calculations and Rice–Ramsperger–Kassel–Markus theory are used to show that even small Fe–Mg–silicates are able to attach electrons rapidly and form stable negatively-charged particles, compared with electron attachment to O2 and O3. This explains the substantial electron depletion between 80 and 90 km, where the presence of atomic O at concentrations in excess of 1010 cm−3 prevents the formation of stable negative ions

Cosmic dust fluxes in the atmospheres of Earth, Mars and Venus

The ablation of cosmic dust injects a range of metals into planetary upper atmospheres. In addition, dust particles which survive atmospheric entry can be an important source of organic material at a planetary surface. In this study the contribution of metals and organics from three cosmic dust sources – Jupiter-Family comets (JFCs), the Asteroid belt (AST), and Halley-Type comets (HTCs) – to the atmospheres of Earth, Mars and Venus is estimated by combining a Chemical Ablation Model (CABMOD) with a Zodiacal Cloud Model (ZoDy). ZoDy provides the mass, velocity, and radiant distributions for JFC, AST, and HTC particles. JFCs are shown to be the main mass contributor in all three atmospheres (68% for Venus, 70% Earth, and 52% for Mars), providing a total input mass for Venus, Earth and Mars of 31 ± 18 t d⁻¹, 28 ± 16 t d⁻¹ and 2 ± 1 t d⁻¹, respectively. The mass contribution of AST particles increases with heliocentric distance (6% for Venus, 9% for Earth, and 14% for Mars). A novel multiphase treatment in CABMOD, tested experimentally in a Meteoric Ablation Simulator, is implemented to quantify atmospheric ablation from both the silicate melt and Fe-Ni metal domains. The ratio of Fe:Ni ablation fluxes at Earth, Mars and Venus are predicted to be close to their CI chondritic ratio of 18, in agreement with mass spectrometric measurements of Fe+:Ni+ = 20.0–₈.₀+¹³·⁰ in the terrestrial ionosphere. In contrast, lidar measurements of the neutral atoms at Earth indicate Fe:Ni = 38 ± 11, and observations by the Neutral Gas and Ion Mass Spectrometer on the MAVEN spacecraft at Mars indicate Fe+:Ni+ = 43–₁₀+¹³. Given the slower average entry velocity of cosmic dust particles at Mars, the accretion rate of unmelted particles in Mars represents 60% of the total input mass, of which a significant fraction of the total unmelted mass (22%) does not reach an organic pyrolysis temperature (~900 K), leading to a flux of intact carbon of 14 kg d⁻¹. This is significantly smaller than previous estimates

Seasonal variations of the Na and Fe layers at the South Pole and their implications for the chemistry and general circulation of the polar mesosphere

Lidar observations, conducted at the South Pole by University of Illinois researchers, are used to characterize the seasonal variations of mesospheric Na and Fe above the site. The annual mean layer abundances are virtually identical to midlatitude values, and the mean centroid height is just 100 m higher for Na and 450 m higher for Fe compared with 40 N. The most striking feature of the metal profiles is the almost complete absence of Na and Fe below 90 km during midsummer. This leads to summertime layers with significantly higher peaks, narrower widths, and smaller abundances than are observed at lower latitudes. The measurements are compared with detailed chemical models of these species that were developed at the University of East Anglia. The models accurately reproduce most features of these observations and demonstrate the importance of rapid uptake of the metallic species on the surfaces of polar mesospheric clouds and meteoric smoke particles. The models show that vertical downwelling in winter, associated with the meridional circulation system, must be less than about 1 cm s 1 in the upper mesosphere in order to avoid displacing the minor constituents O, H, and the metal layers too far below 85 km. They also show that an additional source of gas-phase metallic species, that is comparable to the meteoric input, is required during winter to correctly model the Na and Fe abundances. This source appears to arise from the wintertime convergence of the meridional flow over the South Pole.Ope

Kinetic studies of atmospherically relevant silicon chemistry. Part III: Reactions of Si+ and SiO+ with O3, and Si + with O2

Silicon ions are generated in the Earth's upper atmosphere by hyperthermal collisions of material ablated from incoming meteoroids with atmospheric molecules, and from charge transfer of silicon-bearing neutral species with major atmospheric ions. Reported Si+ number density vs. height profiles show a sharp decrease below 95 km, which has been commonly attributed to the fast reaction with H2O. Here we report rate coefficients and branching ratios of the reactions of Si+ and SiO+ with O-3, measured using a flow tube with a laser ablation source and detection of ions by quadrupole mass spectrometry. The results obtained are (2 sigma uncertainty): k(Si+ + O-3, 298 K) = (6.5 +/- 2.1) x 10(-10) cm(3) molecule(-1) s(-1), with three product channels (branching ratios): SiO+ + O-2 (0.52 +/- 0.24), SiO + O-2 + (0.48 +/- 0.24), and SiO2+ + O (= 0.95) are SiO2 + O-2(+). Reactions (1) and (2) therefore have the unusual ability to neutralise silicon directly, as well as forming molecular ions which can undergo dissociative recombination with electrons. These reactions, along with the recently reported reaction between Si (+) and O-2((1)Delta(g)), largely explain the disappearance of Si (+) below 95 km in the atmosphere, relative to other major meteoric ions such as Fe+ and Mg+. The rate coefficient of the Si+ + O-2 + He reaction was measured to be k(298 K) = (9.0 +/- 1.3) x 10(-30) cm(6) molecule(-2) s(-1), in agreement with previous measurements. The SiO2+ species produced from this reaction, which could be vibrationally excited, is observed to charge transfer at a relatively slow rate with O-2, with a rate constant of k(298 K) = (1.5 +/- 1.0) x 10(-13) cm(3) molecule(-1) s(-1).This work was supported by the Natural Environment Research Council (grant NE/E005659/1).Peer reviewe

Determination of the O-IO bond dissociation energy by photofragment excitation spectroscopy

Pulsed laser photolysis combined with laser induced fluorescence of IO has been used to study the photofragment excitation spectrum of OIO. Analysis of the O-IO dissociation threshold observed around 480 nm provides the first experimental determinations of the OIO bond dissociation energy and heat of formation at 298 K, which are given by D0{ring operator} (O s(-) IO) = (248.8 ± 1.0) kJ mol- 1 and Δf H298{ring operator} (OIO) = (118.5 ± 2.0) kJ mol- 1, respectively. © 2009 Elsevier B.V. All rights reserved.This work was supported by the Natural Environment Research Council (Grant NE/E005659/1).Peer reviewe

A Chemical-dynamical Model of Wave-driven Sodium Fluctuations

A comprehensive chemical-dynamical model is used to investigate the interaction of gravity waves with twenty minor species involved in the atomic sodium chemistry in the mesopause region. We find that chemistry becomes important on the underside of the sodium layer, primarily below 85 km altitude, where the relative importance of chemistry in wave-driven sodium fluctuations increases with increasing wave period and increasing horizontal wavelength. We also find that for altitudes below 80 km an adequate determination of the effects of chemistry in these fluctuations requires the inclusion of several reactions related to ozone chemistry. However, the atomic Na density is too low this region to be routinely observed by current sodium lidars. Importantly, we find that above 85 km altitude sodium can be treated as a passive tracer of gravity wave motions