Atmospheric chemistry of meteoric metals.

The subject of this review is the atmospheric chemistry of the metals which ablate from meteoroids in the Earth’s upper atmosphere. The major meteoric species are Fe, Mg, Si, and Na, against which two minor species, Ca and K, offer surprising contrasts. These metals exist as layers of atoms between about 80 and 105 km and atomic ions at higher altitudes. Below 85 km they form compounds—oxides, hydroxides, and carbonates—which polymerize into nanometer-sized meteoric smoke particles (MSPs). These particles probably act as condensation nuclei for clouds in the mesosphere and stratosphere and eventually after about 4 years are deposited at the Earth’s surface. The subject of meteoric metal chemistry was reviewed in 19911 and 2003,2 and there were also more focused reviews on laboratory studies of metal reactions in 19943 and 20024 and the atmospheric modeling of metals in 2002.5

The present review will therefore concentrate on the many developments that have taken place in the past decade. On the observational side, these developments include the near-global measurement of the Na, K, Mg, and Mg+ layers from satellite-borne spectrometers and lidar observations of Na and Fe from several Antarctic observatories, the discovery that metal atoms are removed in the vicinity of noctilucent (or polar mesospheric) clouds, the surprising observation of metal atoms up to around 180 km in the thermosphere, the unexpected finding that the ratio of the Na d lines in the terrestrial nightglow is variable, the first observations of the molecular bands of FeO and NiO in the nightglow, the first measurements of the vertical flux of Na atoms in the upper mesosphere, the measurement of MSPs from rockets, incoherent scatter radars, satellites, and aircraft, and measurements of the depositional flux of meteoric smoke in polar ice cores.

Laboratory measurements (including the application of quantum chemistry calculations) have addressed several issues: the ion and neutral gas-phase chemistries of compounds containing Fe, Ca, Mg, Si, and K, leading to the first chemically closed reaction schemes for these metals, the uptake of metal atoms on low-temperature ice surfaces and the resulting photoelectric emission, understanding the variable Na d line ratio observations, and the formation of a variety of iron oxide and Fe–Mg–silicate nanoparticles as analogues of meteoric smoke.

There have also been significant developments in modeling: a chemical ablation model to predict the evaporation rates of individual elements from a meteoroid, coupling this ablation model with an astronomical model of dust input to generate the meteoric input function (MIF), the inclusion of the MIF together with metal chemistry in a whole atmosphere chemistry climate model to create the first global models of the Na, Fe, Mg, and K layers, an explanation for the 50 year old puzzle of why the Na and K layers exhibit such different seasonal behavior, modeling the growth and transport of MSPs through the mesosphere and stratosphere, the paleoclimate implications of an enhanced cosmic dust input, and the climate implications of the deposition of meteoric Fe into the Southern Ocean.

The present review is divided into five sections following this Introduction. Section 2 is a general review of the mesosphere and lower thermosphere from the perspective of understanding the metal layers and the sensitivity of this atmospheric region to solar activity and longer term anthropogenic changes. Section 3 describes the atmospheric chemistry of the meteoric metals and then reviews observations of the metal layers and MSPs. Section 4 deals with laboratory and theoretical studies of gas-phase metal reactions and particle formation under mesospheric conditions. Section 5 is concerned with the development of global models of metal chemistry which describe the input and ablation of cosmic dust, the gas-phase chemistry of metallic species, the formation of MSPs, and transport to the Earth’s surface. Section 6 is then a summary with a discussion of future directions for the field.

The mesosphere and metals: chemistry and changes.

Modeling the microphysics of mesospheric ice particles: Assessment of current capabilities and basic sensitivities

In the summers of 1993 and 1994 a series of meteorological rockets and sounding rockets were launched from the Andoya Rocket Range (69°N) during the SCALE and ECHO campaigns in order to investigate the state of the mesosphere and lower thermosphere (SCALE = “SCAttering Layer Experiment”; ECHO indicates that radar and lidar echoes are investigated). At the same location a Rayleigh lidar was operational during these campaigns and searched for enhanced backscatter signals from the upper mesosphere indicative of the presence of noctilucent clouds (NLC). In five cases the lidar detected a NLC and the atmospheric temperature profile was obtained simultaneously by in situ techniques. A literature survey shows that there are only three more cases of unambiguous and simultaneous observations of NLC temperature and altitude. The temperature profiles obtained during SCALE and ECHO are as expected for the high-latitude summer season: The mean mesopause temperature is 135 K at an altitude of 87.5 km. The variability of the temperatures is smaller above ∼84 km altitude than below. The mean temperature below the mesopause shows a remarkable repeatability ever since the first measurements 30 years ago; at the lower edge of the NLC heights (82 km) it is again and again observed to be in the range 150 ± 2 K and the variability within each data set is only a few Kelvins. Such an “equithermal submesopause” in summer puts a serious constraint on any model prediction of secular changes of temperatures in the upper mesosphere. The mean altitude of the NLCs as determined from our lidar measurements is 83.1 km which is surprisingly close to the very first height determinations more than 100 years ago. It is conceivable that this repeatability in altitude reflects a similar repeatability of the thermal structure. There is no apparent correlation between the conditions at the mesopause and the occurrence of NLCs at lower altitudes. The physical reason behind this is presumably related to the fact that the wind direction changes at ∼87 km height, which implies that the air masses observed above the rocket site near mesopause altitudes have been advected from different locations than those at NLC heights. The NLCs are always located below the mesopause, and the temperature in the NLC layer is observed to be lower than 154 K. Our results are compatible with the idea that NLCs consist of ice particles which start to nucleate around the mesopause, sediment to lower altitudes while growing, become observable by the lidar and/or by the naked eye, and finally evaporate once they approach the higher temperatures around 82 km.

Noctilucent clouds and the thermal structure near the Arctic mesopause in summer

We investigate the dust particle charging process in the Earth’s upper atmosphere. Calculating the spectra of solar radiation, we study the influence of the photoelectric effect on the charging process. We show that both positively and negatively charged dust particles are present in the upper atmosphere. We consider the mechanisms which can be responsible for the formation of dust structures like noctilucent clouds and polar mesosphere summer echoes.

Dust particle charging and formation of dust structures in the upper atmosphere

With a Mattauch-Herzog type of mass spectrometer the positive ion composition was measured in the mesosphere between 80 and 98.4 km over Kiruna, Sweden, in summer 1993. During the rocket flight an intense noctilucent cloud (NLC) was present and detected by a particle impact sensor at 83.0–83.6 km and 82.5–85.6 km on ascent and descent, respectively. For the first time a significant depletion of the main positive proton hydrates was observed on ascent and a complete disappearance of these ions on descent in the entire NLC-layer. The observation is associated with an atypical low ion production of 0.01–0.1 cm−3s−1, which favors the ion loss by particle attachment and/or vertical transport. In order to explain the loss of mobile charge a second population of at least 105 cm−3 1–2 nm ice particles is required in the NLC.

Positive ion depletion in a noctilucent cloud

A novel mass spectrometer designed to measure simultaneously positive ion composition in the mesosphere, was successfully launched during the NLC-91 project Instruments supporting the mass spectrometer were a probed to measure both electrons and positive ions as well as a wave propagation experiment The location of the Noctilucent Clouds (NLC) was determined by a particle impact sensor to detect secondary electrons and ions from the impact of NLC particle The density of proton hydrates and of the related total ions is depleted in the NLC region at 83 km An improved detection limit of 5 x 10(exp 4)/cu m for positive ions and improved height resolution revealed for the first time large gradients in the O2(+), H(+)(H2O)2 and H(+)(H2O)6 densities within a small height range of the order of 50 m Such gradients at the altitude of NLC and Polar Mesospheric Summer Echoes (PMSE) are associated with strong variability of mesospheric water vapor, temperature and neutral air density

Small‐scale structure of O2+ and proton hydrates in a noctilucent cloud and polar mesospheric summer echo of August 9/10 1991 above Kiruna

During a recent rocket campaign to study the phenomenology of noctilucent clouds (NLC-93), mass spectrometer measurements showed the presence of ions at m/e=45 and 61. These ions are attributed to SiOH+ and SiO2H+; the former is formed by reaction of Si+ with H2O and the latter is formed by reaction of SiOH+ with O3. The data provide the first observation of a chemical sequence of reactions beginning with the ablation of meteoric elements and a sequential reaction scheme.

Silicon molecular ions in the D‐region

First Positive Ion Depletion in a Noctilucent Cloud and its Implication on the Particle Size Distribution

In its first part, this paper shows why a recently made attempt to reduce the special theory of relativity to Newtonian kinematics is bound to fail. In the second part, we propose a differentiated notion of incommensurability which enables us to amend the contention that the special theory of relatively and Newtonian kinematics are “incommensurable”.

Sind die klassische Mechanik und die spezielle Relativitätstheorie kommensurabel

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