The ablation of cosmic dust particles entering the Earth’s upper atmosphere produces a layer of Ca atoms around 90 km. Here, we present a set of kinetic experiments designed to understand the nature of the Ca molecular reservoirs on the underside of the layer. CaOH was produced by laser ablation of a Ca target in the fast flow tube and detected by non-resonant laser-induced fluorescence, probing the D(2Σ+) ← X(2Σ1) transition at 346.9 nm. The following rate constants were measured (at 298 K): k(CaOH + H → Ca + H2O) = (1.04 ± 0.24) × 10–10 cm3 molecule–1 s–1, k(CaOH + O → CaO + OH) < 1 × 10–11 cm3 molecule–1 s–1, and k(CaOH + O2 → O2CaOH, 1 Torr) = (5.9 ± 1.8) × 10–11 cm3 molecule–1 s–1 (uncertainty at the 2σ level of confidence). The recycling of CaOH from reaction between O2CaOH and O proceeds with an effective rate constant of keff(O2CaOH + O → CaOH + products, 298 K) = 2.8–1.2+2.0 × 10–10 cm3 molecule–1 s–1. Master equation modeling of the CaOH + O2 kinetics is used to extrapolate to mesospheric temperatures and pressures. The results suggest that the formation of O2CaOH slows the conversion of CaOH to atomic Ca via reaction with atomic H, and O2CaOH is likely to be a long-lived reservoir species on the underside of the Ca layer and a building block of meteoric smoke particles.Keywords: calcium hydroxide; gas-phase metal chemistry; laser spectroscopy; mesosphere−lower thermosphere; meteoric calcium layer;

•First laboratory study of CO oxidation by O2 on meteoric smoke analogues.•CO oxidation rate increases significantly with a higher Fe content in the dust.•Cosmic dust input to Venus is about 1.3 tonnes per hour, of which 34% ablates.•Distribution of resulting meteoric smoke particles modelled in Venus’ atmosphere.•Substantial depletion of O2 predicted in the hot troposphere below 40 km.The heterogeneous oxidation of CO by O2 on olivine, Fe sulfate and Fe oxide particles was studied using a flow tube apparatus between 300 and 680 K. These particles were chosen as possible analogues of unablated cosmic dust and meteoric smoke in Venus’ atmosphere. On olivine and Fe oxides, the rate of CO oxidation to CO2 only becomes significant above 450 K. For iron sulfates, CO2 production was not observed until these dust analogues had decomposed into iron oxides at ∼ 540 K. The CO oxidation rate increases significantly with a higher Fe content in the dust, implying that oxidation occurs through Fe active sites (no reaction was observed on Mg2SiO4). The oxidation kinetics can be explained by CO reacting with chemi-sorbed O2 through an Eley–Rideal mechanism, which is supported by electronic structure calculations. Uptake coefficients were measured from 450 to 680 K, yielding: log10(γ (CO on MgFeSiO4)) = (2.9 ± 0.1) × 10-3 T(K) – (8.2 ± 0.1); log10(γ (CO on Fe2SiO4)) = (2.3 ± 0.3) × 10-3 T(K)  – (7.7 ± 0.2); log10(γ (CO on FeOOH/Fe2O3)) = (5.6 ± 0.8) × 10-3 T(K) – (9.3 ± 0.4). A 1-D atmospheric model of Venus was then constructed to explore the role of heterogeneous oxidation. The cosmic dust input to Venus, mostly originating from Jupiter Family Comets, is around 32 tonnes per Earth day. A chemical ablation model was used to show that ∼34% of this incoming mass ablates, forming meteoric smoke particles which, together with unablated dust particles, provide a significant surface for the heterogeneous oxidation of CO to CO2 in Venus’ troposphere. This process should cause almost complete removal of O2 below 40 km, but have a relatively small impact on the CO mixing ratio (since CO is in large excess over O2). Theoretical quantum calculations indicate that the gas-phase oxidation of CO by SO2 in the lower troposphere is not competitive with the heterogeneous oxidation of CO. Finally, the substantial number density of meteoric smoke particles predicted to occur above the cloud tops may facilitate the low temperature heterogeneous chemistry of other species.

•Mg and K species deposited on ice do not co-desorb when the ice sublimes.•Mg is unreactive on an ice surface, while K reacts to form KOH.•An effective and distinct MSP coagulation mechanism occurring in PMCs is proposed.The uptake and potential reactivity of metal atoms on water ice can be an important process in planetary atmospheres and on icy bodies in the interplanetary and interstellar medium. For instance, metal atom uptake affects the gas-phase chemistry of the Earth's mesosphere, and has been proposed to influence the agglomeration of matter into planets in protoplanetary disks. In this study the fate of Mg and K atoms incorporated into water-ice films, prepared under ultra-high vacuum conditions at temperatures of 110–140 K, was investigated. Temperature-programmed desorption experiments reveal that Mg- and K-containing species do not co-desorb when the ice sublimates, demonstrating that uptake on ice particles causes irreversible removal of the metals from the gas phase. This implies that uptake on ice particles in terrestrial polar mesospheric clouds accelerates the formation of large meteoric smoke particles (≥1 nm radius above 80 km) following sublimation of the ice. Energetic sputtering of metal-dosed ice layers by 500 eV Ar+ and Kr+ ions shows that whereas K reacts on (or within) the ice surface to form KOH, adsorbed Mg atoms are chemically inert. These experimental results are consistent with electronic structure calculations of the metals bound to an ice surface, where theoretical adsorption energies on ice are calculated to be −68 kJ mol−1 for K, −91 kJ mol−1 for Mg, and −306 kJ mol−1 for Fe. K can also insert into a surface H2O to produce KOH and a dangling H atom, in a reaction that is slightly exothermic.

•Ground meteorites and some terrestrial minerals can be useful cosmic dust analogues.•Ordinary chondrites can be appropriate substitutes for carbonaceous chondrites.•Sol-gel methods produce analogues for likely meteoric smoke particle compositions.Analogues have been developed and characterised for both interplanetary dust and meteoric smoke particles. These include amorphous materials with elemental compositions similar to the olivine mineral solid solution series, a variety of iron oxides, undifferentiated meteorites (chondrites) and minerals which can be considered good terrestrial proxies to some phases present in meteorites. The products have been subjected to a suite of analytical techniques to demonstrate their suitability as analogues for the target materials.

•Substantial depletion of the Na and Fe mesospheric layers is observed during a major SSW event.•A whole atmosphere model shows that this is caused by SSW-driven cooling in the MLT, and the impact extends to the tropics.•The response of the K layer is predicted to be much smaller than Na, Fe and Mg.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.

The gas-phase reactions of a selection of sodium-containing species with atmospheric constituents, relevant to the chemistry of meteor-ablated Na in the upper atmosphere, were studied in a fast flow tube using multiphoton ionization time-of-flight mass spectrometry. For the first time, unambiguous observations of NaO and NaOH in the gas phase under atmospheric conditions have been achieved. This enabled the direct measurement of the rate constants for the reactions of NaO with H2, H2O, and CO, and of NaOH with CO2, which at 300–310 K were found to be (at 2σ confidence level): k(NaO + H2O) = (2.4 ± 0.6) × 10–10 cm3 molecule –1 s–1, k(NaO + H2) = (4.9 ± 1.2) × 10–12 cm3 molecule –1 s–1, k(NaO + CO) = (9 ± 4) × 10–11 cm3 molecule –1 s–1, and k(NaOH + CO2 + M) = (7.6 ± 1.6) × 10–29 cm6 molecule –2 s–1 (P = 1–4 Torr). The NaO + H2 reaction was found to make NaOH with a branching ratio ≥99%. A combination of quantum chemistry and statistical rate theory calculations are used to interpret the reaction kinetics and extrapolate the atmospherically relevant experimental results to mesospheric temperatures and pressures. The NaO + H2O and NaOH + CO2 reactions act sequentially to provide the major atmospheric sink of meteoric Na and therefore have a significant impact on the underside of the Na layer in the terrestrial mesosphere: the newly determined rate constants shift the modeled peak to about 93 km, i.e., 2 km higher than observed by ground-based lidars. This highlights further uncertainties in the Na chemistry cycle such as the unknown rate constant of the NaOH + H reaction. The fast Na-recycling reaction between NaO and CO and a re-evaluated rate constant of the NaO + CO2 sink should be now considered in chemical models of the Martian Na layer.

The dissociative recombination (DR) of FeO+ ions with electrons has been studied in a flowing afterglow reactor. FeO+ was generated by the pulsed laser ablation of a solid Fe target, and then entrained in an Ar+ ion/electron plasma where the absolute electron density was measured using a Langmuir probe. A kinetic model describing gas-phase chemistry and diffusion to the reactor walls was fitted to the experimental data, yielding a DR rate coefficient at 298 K of k(FeO+ + e–) = (5.5 ± 1.0) × 10–7 cm3 molecule–1 s–1, where the quoted uncertainty is at the 2σ level. Fe+ ions in the lower thermosphere are oxidized by O3 to FeO+, and this DR reaction is shown to provide a more important route for neutralizing Fe+ below 110 km than the radiative/dielectronic recombination of Fe+ with electrons. The experimental system was first validated by measuring two other DR reaction rate coefficients: k(O2+ + e–) = (2.0 ± 0.4) × 10–7 and k(N2O+ + e–) = (3.3 ± 0.8) × 10–7 cm3 molecule–1 s–1, which are in good agreement with the recent literature.

The fluorinated gases SF6 and C2F5Cl (CFC-115) are chemically inert with atmospheric lifetimes of many centuries which, combined with their strong absorption of IR radiation, results in unusually high global warming potentials. Very long lifetimes imply that mesospheric sinks could make important contributions to their atmospheric removal. In order to investigate this, the photolysis cross sections at the prominent solar Lyman-α emission line (121.6 nm), and the reaction kinetics of SF6 and CFC-115 with the neutral meteoric metal atoms Na, K, Mg, and Fe over large temperature ranges, were measured experimentally. The Na and K reactions exhibit significant non-Arrhenius behavior; quantum chemistry calculations of the potential energy surfaces for the SF6 reactions indicate that the Na and K reactions with SF6 are probably activated by vibrational excitation of the F-SF5 (v3) stretching mode. A limited set of kinetic measurements on Na + SF5CF3 are also presented. The atmospheric removal of these long-lived gases by a variety of processes is then evaluated. For SF6, the removal processes in decreasing order of importance are electron attachment, VUV photolysis, and reaction with K, Na, and H. For CFC-115, the removal processes in decreasing order of importance are reaction with O(1D), VUV photolysis, and reaction with Na, K, and H.

NF3 is a potent anthropogenic greenhouse gas with increasing industrial usage. It is characterized by a large global warming potential due in part to its large atmospheric lifetime. The estimated lifetime of about 550 years means that potential mesospheric destruction processes of NF3 should also be considered. The reactions of NF3 with the neutral metal atoms Na, K, Mg and Fe, which are produced by meteoric ablation in the upper mesosphere, were therefore studied. The observed non-Arrhenius temperature dependences of the reactions between about 190 and 800 K are interpreted using quantum chemistry calculations of the relevant potential energy surfaces. The NF3 absorption cross section at the prominent Lyman-α solar emission line (121.6 nm) was determined to be (1.59 ± 0.10) × 10–18 cm2 molecule–1 (at 300 K). In the mesosphere above 60 km, Lyman-α photolysis is the dominant removal process of NF3; the reactions with K and Na are 1–2 orders of magnitude slower. However, the atmospheric lifetime of NF3 is largely controlled by reaction with O(1D) and photolysis at wavelengths shorter than 190 nm; these processes dominate below 60 km.

This review discusses the magnitude of the cosmic dust input into the earth's atmosphere, and the resulting impacts from around 100 km to the earth's surface. Zodiacal cloud observations and measurements made with a spaceborne dust detector indicate a daily mass input of interplanetary dust particles ranging from 100 to 300 tonnes, which is in agreement with the accumulation rates of cosmic-enriched elements (Ir, Pt, Os and super-paramagnetic Fe) in polar ice cores and deep-sea sediments. In contrast, measurements in the middle atmosphere – by radar, lidar, high-flying aircraft and satellite remote sensing – indicate that the input is between 5 and 50 tonnes per day. There are two reasons why this huge discrepancy matters. First, if the upper range of estimates is correct, then vertical transport in the middle atmosphere must be considerably faster than generally believed; whereas if the lower range is correct, then our understanding of dust evolution in the solar system, and transport from the middle atmosphere to the surface, will need substantial revision. Second, cosmic dust particles enter the atmosphere at high speeds and undergo significant ablation. The resulting metals injected into the atmosphere are involved in a diverse range of phenomena, including: the formation of layers of metal atoms and ions; the nucleation of noctilucent clouds, which are a sensitive marker of climate change; impacts on stratospheric aerosols and O3 chemistry, which need to be considered against the background of a cooling stratosphere and geo-engineering plans to increase sulphate aerosol; and fertilization of the ocean with bio-available Fe, which has potential climate feedbacks.

Marine aerosol is highly enriched in iodine, mostly in the form of iodate (IO3–) ions, compared to its relative abundance in seawater. This paper describes a laboratory study of the photochemical reduction of IO3– in the presence of humic acid. Spectroscopic analysis showed that ∼20% of IO3– was converted to “free” iodide (I–) ions and this fraction remained constant as a function of time. Direct detection of an organically fixed fraction (i.e., ∼ 80%) was not possible, but a number of test reactions with surrogate organic compounds containing functional groups identified in humic acid structures indicate that efficient substitution of iodine occurs at aromatic 1,2 diol sites. These iodinated humic acids are stable with respect to photolysis at near-UV/visible wavelengths and are likely to account for a significant proportion of the soluble iodine-containing organic material occurring within aerosols. In the lower atmosphere, oxidation of I– to I2 in marine aerosol occurs mostly through the uptake of O3, with H2O2 playing a very minor role. A model of iodine chemistry in the open ocean tropical boundary layer, which incorporates these experimental results, is able to account for the observed enrichment of iodine in marine aerosol.

This paper describes the kinetic study of a number of gas-phase reactions involving neutral Mg-containing species, which are important for the chemistry of meteor-ablated magnesium in the upper mesosphere/lower thermosphere region. The study is motivated by the very recent observation of the global atomic Mg layer around 90 km, using satellite-born UV–visible spectroscopy. In the laboratory, Mg atoms were produced thermally in the upstream section of a fast flow tube and then converted to the molecular species MgO, MgO2, OMgO2, and MgCO3 by the addition of appropriate reagents. Atomic O was added further downstream, and Mg was detected at the downstream end of the flow tube by laser-induced fluorescence. The following rate coefficients were determined at 300 K: k(MgO + O → Mg + O2) = (6.2 ± 1.1) × 10–10; k(MgO2 + O → MgO + O2) = (8.4 ± 2.8) × 10–11; k(MgCO3 + O → MgO2 + CO2) ≥ 4.9 × 10–12; and k(MgO + CO → Mg + CO2) = (1.1 ± 0.3) × 10–11 cm3 molecule–1 s–1. Electronic structure calculations of the relevant potential energy surfaces combined with RRKM theory were performed to interpret the experimental results and also to explore the likely reaction pathways that convert MgCO3 and OMgO2 into long-lived reservoir species such as Mg(OH)2. Although no reaction was observed in the laboratory between OMgO2 and O, this is most likely due to the rapid recombination of O2 with the product MgO2 to form the relatively stable O2MgO2. Indeed, one significant finding is the role of O2 in the mesosphere, where it initiates holding cycles by recombining with radical species such as MgO2 and MgOH. A new atmospheric model was then constructed which combines these results together with recent work on magnesium ion–molecule chemistry. The model is able to reproduce satisfactorily some of the key features of the Mg and Mg+ layers, including the heights of the layers, the seasonal variations of their column abundances, and the unusually large Mg+/Mg ratio.

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 O3, measured using a flow tube with a laser ablation source and detection of ions by quadrupole mass spectrometry. The results obtained are (2σ uncertainty): k(Si+ + O3, 298 K) = (6.5 ± 2.1) × 10−10 cm3 molecule−1 s−1, with three product channels (branching ratios): SiO+ + O2 (0.52 ± 0.24), SiO + O2+ (0.48 ± 0.24), and SiO2+ + O (<0.1); k(SiO+ + O3, 298 K) = (6 ± 4) × 10−10 cm3 molecule−1 s−1, where the major products (branching ratio ≥ 0.95) are SiO2 + O2+. 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 O2(1Δg), 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+ + O2 + He reaction was measured to be k(298 K) = (9.0±1.3) × 10−30 cm6 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 O2, with a rate constant of k(298 K) = (1.5 ± 1.0) × 10−13 cm3 molecule−1 s−1.

This paper describes the kinetic study of a number of gas-phase reactions involving neutral Ca-containing species, many of which are important for describing the chemistry of meteor-ablated calcium in the Earth's upper atmosphere. Ca atoms were produced thermally in the upstream section of a fast flow tube, and then converted to the molecular species CaO, CaO2, CaO3, CaCO3 or Ca(OH)2 by the addition of appropriate reagents. Atomic O or H was added further downstream, and both Ca and CaO were detected at the downstream end of the flow tube by laser-induced fluorescence. The following rate coefficients were determined: k(CaO + O → Ca + O2) = (3.1+2.0−1.5) × 10−10 at 300 K and (1.3+3.4−0.6) × 10−10 at 203 K; k(CaO2 + O → CaO + O2) = (2.2+7.0−1.4) × 10−11 at 300 K and (1.6+2.9−0.7) × 10−11 at 203 K; k(CaO2 + H → products, 298 K) = (1.2 ± 0.6) × 10−11; k(CaCO3 + O → CaO2 + CO2, 300 K) ≤ 1.0 × 10−12; k(CaCO3 + H→ CaOH + CO2, 298 K) ≥ 2.8 × 10−12 and ≤3.6 × 10−11; k(CaO3 + H→ CaOH + O2, 298 K) ≥ 1.7 × 10−11; k(Ca(OH)2 + H → CaOH + H2O, 298 K) ≥ 1.1 × 10−11; k(CaOH + H → Ca + H2O, 298 K) ≥ 1.1× 10−11 cm3 molecule−1 s−1. The kinetics of the reactions of Ca and CaO with NO2 and N2O were also studied, yielding k(Ca + NO2 → CaO + NO) = (2.6 ± 0.3) × 10−10 at 300 K and (2.0 ± 0.3) × 10−10 at 203 K; k(CaO + NO2 → CaO2 + NO) = (8.1 ± 2.0) × 10−10 at 300 K and (2.9 ± 1.0) × 10−10 at 202 K; k(CaO + N2O → CaO2 + N2) = (4.2 ± 1.7) × 10−11 at 300 K and (2.2 ± 1.2) × 10−12 at 206 K; k(CaO + H2 → Ca + H2O, 300 K) = (3.4 ± 1.3) × 10−10 cm3 molecule−1 s−1. Electronic structure calculations of the relevant potential energy surfaces were performed to interpret the experimental results, and the atmospheric implications of these measurements are then discussed.

A laser flash photolysis technique and quasi-classical trajectory (QCT) calculations have been used to determine the rate coefficients for the title process. The experimental high-pressure-limiting rate coefficient is 7.0 × 10−11 cm3 s−1 at T = 300 K, which compares with the computed QCT value for the Mg+ + H2O capture rate of 2.75 ± 0.08 × 10−9 cm3 s−1 at the same temperature. The 39-fold difference between the experimental and simulation results is explained by further QCT calculations for the He + Mg+·H2O* collision process. In particular, our simulation results indicate that collision-induced dissociation (CID) of the Mg+·H2O* excited adduct is very likely compared with collisional stabilization (CS), which is an order of magnitude less likely. Including the relative rates of CID and CS in the calculation and assuming that those Mg+·H2O* complexes that perform only one inner turning point in the dissociation coordinate are unlikely to be stabilized by CS, the computed rate coefficient compares well with the high-pressure experimental value.

Silicon monoxide (SiO) is injected directly into the Earth’s upper atmosphere by ablating meteoroids. SiO is also produced by the reaction of atomic Si (another ablation product) with O2 and O3. The reactions of SiO with several atmospherically relevant oxidants have been studied by the pulsed laser photolysis of a Si atom precursor in the presence of O2, followed by time-resolved non-resonant laser-induced fluorescence of SiO at 282 nm. This yielded: k(SiO + O3, 190–293 K) = (4.4 ± 0.6) × 10−13 cm3 molecule−1 s−1; k(SiO + O2 + He, 293 K) ≤ 3 × 10−32 cm6 molecule−2 s−1, k(SiO + O + He, 293 K) ≤ 1 × 10−30 cm6 molecule−2 s−1, k(SiO + H2O, 293 K, 4–20 Torr) ≤ 4 ×10−14 cm3 molecule−1 s−1, and k(SiO + OH, 293 K, 4–20 Torr) = (5.7 ± 2.0) × 10−12 cm3 molecule−1 s−1. These results are explained by combining ab initio quantum chemistry calculations with transition state theory and RRKM theory. An upper limit of 5 × 10−13 cm3 molecule−1 s−1 for the reaction SiO2 + O → SiO + O2 was determined, but calculations indicate the existence of a high barrier (104.7 kJ mol−1) which will make this reaction very slow at mesospheric temperatures.

Atomic silicon is generated by meteoric ablation in the Earth’s upper atmosphere (70–110 km). The reactions of Si(3PJ) atoms with several atmospherically relevant species were studied by the pulsed laser photolysis of a Si atom precursor (typically PheSiH3), followed by time-resolved laser induced fluorescence at 251.43 nm (Si(3p23P0→ 4s 3P1)). This yielded: k(Si + O2, 190–500 K) = 9.49 × 10−11 + 1.80 × 10−10× exp(−T/115 K) cm3 molecule−1 s−1 (uncertainty ≤±15%), in good accord with recent high-level theoretical calculations but in marked disagreement with previous experimental work; k(Si + O3, 190–293 K) = (4.0 ± 0.5) × 10−10 cm3 molecule−1 s−1; k(Si + CO2, 293 K) ≤ 1.2 × 10−14 cm3 molecule−1 s−1; and k(Si + H2O, 293 K) ≤ 2.6 × 10−13 cm3 molecule−1 s−1. These results are explained using a combination of quantum chemistry calculations and long-range capture theory. The quenching rate coefficients k(Si(1D2) + N2, 293 K) = (4.0 ± 0.7) × 10−11 cm3 molecule−1 s−1 and k(Si(1D2) + H2O, 293 K) = (2.3 ± 0.3) × 10−10 cm3 molecule−1 s−1 were also determined.

A series of gas-phase reactions involving molecular Ca-containing ions was studied by the pulsed laser ablation of a calcite target to produce Ca+ in a fast flow of He, followed by the addition of reagents downstream and detection of ions by quadrupole mass spectrometry. Most of the reactions that were studied are important for describing the chemistry of meteor-ablated calcium in the earth’s upper atmosphere. The following rate coefficients were measured: k(CaO+ + O → Ca+ + O2) = (4.2 ± 2.8) × 10−11 at 197 K and (6.3 ± 3.0) × 10−11 at 294 K; k(CaO+ + CO → Ca+ + CO2, 294 K) = (2.8 ± 1.5) × 10−10; k(Ca+·CO2 + O2 → CaO2+ + CO2, 294 K) = (1.2 ± 0.5) ×10−10; k(Ca+·CO2 + H2O → Ca+·H2O + CO2) = (13.0 ± 4.0) × 10−10; and k(Ca+·H2O + O2 → CaO2+ + H2O, 294 K) = (4.0 ± 2.5) × 10−10 cm3 molecule−1 s−1. The quoted uncertainties are a combination of the 1σ standard errors in the kinetic data and the systematic errors in the models used to extract the rate coefficients. Rate coefficients were also obtained for the following recombination (also termed association) reactions in He bath gas: k(Ca+·CO2 + CO2 → Ca+·(CO2)2, 294 K) = (2.6 ± 1.0) × 10−29; k(Ca+·H2O + H2O → Ca+·(H2O)2) = (1.6 ± 1.1) × 10−27; and k(CaO2+ + O2 → CaO2+·O2) < 1 × 10−31 cm6 molecule−2 s−1. These recombination rate coefficients, as well as those for the ligand-switching reactions listed above, were then interpreted using a combination of high level quantum chemistry calculations and RRKM theory using an inverse Laplace transform solution of the master equation. The surprisingly slow reaction between CaO+ and O was explained using quantum chemistry calculations on the lowest 2A′, 2A″ and 4A″ potential energy surfaces. These calculations indicate that reaction mostly occurs on the 2A′ surface, leading to production of Ca+(2S) + O2(1Δg). The importance of this reaction for controlling the lifetime of Ca+ in the upper mesosphere and lower thermosphere is then discussed.

Increasing tropospheric ozone levels over the past 150 years have led to a significant climate perturbation1; the prediction of future trends in tropospheric ozone will require a full understanding of both its precursor emissions and its destruction processes. A large proportion of tropospheric ozone loss occurs in the tropical marine boundary layer2, 3 and is thought to be driven primarily by high ozone photolysis rates in the presence of high concentrations of water vapour. A further reduction in the tropospheric ozone burden through bromine and iodine emitted from open-ocean marine sources has been postulated by numerical models4, 5, 6, 7, but thus far has not been verified by observations. Here we report eight months of spectroscopic measurements at the Cape Verde Observatory indicative of the ubiquitous daytime presence of bromine monoxide and iodine monoxide in the tropical marine boundary layer. A year-round data set of co-located in situ surface trace gas measurements made in conjunction with low-level aircraft observations shows that the mean daily observed ozone loss is ~50 per cent greater than that simulated by a global chemistry model using a classical photochemistry scheme that excludes halogen chemistry. We perform box model calculations that indicate that the observed halogen concentrations induce the extra ozone loss required for the models to match observations. Our results show that halogen chemistry has a significant and extensive influence on photochemical ozone loss in the tropical Atlantic Ocean boundary layer. The omission of halogen sources and their chemistry in atmospheric models may lead to significant errors in calculations of global ozone budgets, tropospheric oxidizing capacity and methane oxidation rates, both historically and in the future.

Abstract

Halogens influence the oxidizing capacity of Earth's troposphere, and iodine oxides form ultrafine aerosols, which may have an impact on climate. We report year-round measurements of boundary layer iodine oxide and bromine oxide at the near-coastal site of Halley Station, Antarctica. Surprisingly, both species are present throughout the sunlit period and exhibit similar seasonal cycles and concentrations. The springtime peak of iodine oxide (20 parts per trillion) is the highest concentration recorded anywhere in the atmosphere. These levels of halogens cause substantial ozone depletion, as well as the rapid oxidation of dimethyl sulfide and mercury in the Antarctic boundary layer.

The reactions between Ca+(42S1/2) and O3, O2, N2, CO2 and H2O were studied using two techniques: the pulsed laser photo-dissociation at 193 nm of an organo-calcium vapour, followed by time-resolved laser-induced fluorescence spectroscopy of Ca+ at 393.37 nm (Ca+(42P3/2–42S1/2)); and the pulsed laser ablation at 532 nm of a calcite target in a fast flow tube, followed by mass spectrometric detection of Ca+. The rate coefficient for the reaction with O3 is essentially independent of temperature, k(189–312 K) = (3.9 ± 1.2) × 10−10 cm3 molecule−1 s−1, and is about 35% of the Langevin capture frequency. One reason for this is that there is a lack of correlation between the reactant and product potential energy surfaces for near coplanar collisions. The recombination reactions of Ca+ with O2, CO2 and H2O were found to be in the fall-off region over the experimental pressure range (1–80 Torr). The data were fitted by RRKM theory combined with quantum calculations on CaO2+, Ca+·CO2 and Ca+·H2O, yielding the following results with He as third body when extrapolated from 10−3–103 Torr and a temperature range of 100–1500 K. For Ca+ + O2: log10(krec,0/cm6 molecule−2 s−1) = −26.16 − 1.113log10T − 0.056log102T, krec,∞ = 1.4 × 10−10 cm3 molecule−1 s−1, Fc = 0.56. For Ca+ + CO2: log10(krec,0/ cm6 molecule−2 s−1) = −27.94 + 2.204log10T − 1.124log102T, krec,∞ = 3.5 × 10−11 cm3 molecule−1 s−1, Fc = 0.60. For Ca+ + H2O: log10(krec,0/ cm6 molecule−2 s−1) = −23.88 − 1.823log10T − 0.063log102T, krec,∞ = 7.3 × 10−11exp(830 J mol−1/RT) cm3 molecule−1 s−1, Fc = 0.50 (Fc is the broadening factor). A classical trajectory analysis of the Ca+ + CO2 reaction is then used to investigate the small high pressure limiting rate coefficient, which is significantly below the Langevin capture frequency. Finally, the implications of these results for calcium chemistry in the mesosphere are discussed.

The absolute absorption cross section of IONO2 was measured by the pulsed photolysis at 193 nm of a NO2/CF3I mixture, followed by time-resolved Fourier transform spectroscopy in the near-UV. The resulting cross section at a temperature of 296 K over the wavelength range from 240 to 370 nm is given by log10(σ(IONO2)/cm2 molecule−1) = 170.4 − 3.773 λ + 2.965 × 10−2λ2 − 1.139 × 10−4λ3 + 2.144 × 10−7λ4 − 1.587 × 10−10λ5, where λ is in nm; the cross section, with 2σ uncertainty, ranges from (6.5 ± 1.9) × 10−18 cm2 at 240 nm to (5 ± 3) × 10−19 cm2 at 350 nm, and is significantly lower than a previous measurement [J. C. Mössinger, D. M. Rowley and R. A. Cox, Atmos. Chem. Phys., 2002, 2, 227]. The photolysis quantum yields for IO and NO3 production at 248 nm were measured using laser induced fluorescence of IO at 445 nm, and cavity ring-down spectroscopy of NO3 at 662 nm, yielding ϕ(IO) ≤ 0.02 and ϕ(NO3) = 0.21 ± 0.09. It is likely that photolysis to I + NO3 is the only significant channel, as shown by accompanying quantum chemistry calculations. The low ϕ(NO3) is explained by the production of hot NO3, most of which dissociates to NO2 + O. In terms of atmospheric relevance, the noon photolysis frequency of J(IONO2) = (3.0 ± 2.1) × 10−3 s−1 (40° N, July) is fast enough to limit the effectiveness of IONO2 as a daytime reservoir of iodine oxides, but the formation and subsequent photolysis of IONO2 is very inefficient as an ozone-depleting cycle.

•Laser ablation in a fast flow tube is used to generate fully thermalized AlO.•The rate constant of the Al + O2 reaction is well stablished.•Evidence of the AlO + AlO self-reaction is observed.•First determination of the AlO absorption cross section at the B(0) ← X(0) bandhead.•LIF scans of the B(0) ← X(0) and B(1) ← X(0) bands agree with calculated spectra.•The possibility of lidar detection of meteoric AlO in the atmosphere is discussed.The rate coefficient of the Al + O2 reaction has been measured in a laser ablation-fast flow tube apparatus by monitoring atomic Al resonance absorption and AlO laser induced fluorescence (LIF). The rate constant has been found to be k(298 K) = (1.68 ± 0.24) × 10−10 cm3 molecule−1 s−1. Under conditions of near-stoichiometric conversion of Al into AlO, the absorption cross section of AlO at the bandhead of the B2Σ+(v' = 0) ← X2Σ+(v'' = 0) transition has been determined to be σ(298 K, 1 hPa) = (6.7 ± 1.6) × 10−15 cm2 molecule−1 (0.003 nm resolution), in very good agreement with theoretical predictions.Figure optionsDownload full-size imageDownload high-quality image (98 K)Download as PowerPoint slide

•When ice containing Fe atoms evaporates, Fe does not co-desorb.•Auroral sputtering of Fe from ice particles is ineffective.•Sputtering from the rings of Saturn is a possible source of magnetospheric Fe+.Icy particles containing a variety of Fe compounds are present in the upper atmospheres of planets such as the Earth and Saturn. In order to explore the role of ice sublimation and energetic ion bombardment in releasing Fe species into the gas phase, Fe-dosed ice films were prepared under UHV conditions in the laboratory. Temperature-programmed desorption studies of Fe/H2O films revealed that no Fe atoms or Fe-containing species co-desorbed along with the H2O molecules. This implies that when noctilucent ice cloud particles sublimate in the terrestrial mesosphere, the metallic species embedded in them will coalesce to form residual particles. Sputtering of the Fe-ice films by energetic Ar+ ions was shown to be an efficient mechanism for releasing Fe into the gas phase, with a yield of 0.08 (Ar+ energy=600 eV). Extrapolating with a semi-empirical sputtering model to the conditions of a proton aurora indicates that sputtering by energetic protons (>100 keV) should also be efficient. However, the proton flux in even an intense aurora will be too low for the resulting injection of Fe species into the gas phase to compete with that from meteoric ablation. In contrast, sputtering of the icy particles in the main rings of Saturn by energetic O+ ions may be the source of recently observed Fe+ in the Saturnian magnetosphere. Electron sputtering (9.5 keV) produced no detectable Fe atoms or Fe-containing species. Finally, it was observed that Fe(OH)2 was produced when Fe was dosed onto an ice film at 140 K (but not at 95 K). Electronic structure theory shows that the reaction which forms this hydroxide from adsorbed Fe has a large barrier of about 0.7 eV, from which we conclude that the reaction requires both translationally hot Fe atoms and mobile H2O molecules on the ice surface.

•Uptake of nitric acid on synthetic meteoric smoke is relatively efficient.•Olivine and haematite/goethite particles synthesised by sol–gel method.•Potentially significant removal of HNO3 in the polar vortex by uptake on smoke.The uptake of HNO3, H2O, NO2 and NO was studied on meteoric smoke particle analogues using a low-pressure Knudsen cell operating at 295 K. The analogues used were olivine (MgFeSiO4) and a haematite/goethite (Fe2O3/FeO(OH)) mixture synthesised by the sol–gel process. For uptake on MgFeSiO4, the following uptake coefficients were obtained: γ(HNO3)=(1.8±0.3)×10−3, γ(H2O)=(4.0±1.3)×10−3, γ(NO2)=(5.7±0.2)×10−4 and γ(NO)<3×10−4. γ(HNO3) did not show a dependence on the mass of MgFeSiO4 in the Knudsen cell (when varied by a factor of 6) implying that, because of relatively efficient uptake, HNO3 is removed only by near-surface particles. This was corroborated by application of a surface uptake model. Saturating the MgFeSiO4 particles with water vapour before exposing them to NO2 increased γ(NO2) to (2.1±0.7)×10−3, but had a very small effect on γ(HNO3). For uptake on Fe2O3/FeO(OH), γ(HNO3)=(1.5±0.2)×10−3. These results were then included in a whole atmosphere chemistry–climate model, which shows that the heterogeneous removal on meteoric smoke particles in the winter polar vortex between 30 and 60 km appears to provide an important sink for HNO3.

•Experimental determination of minimum H2O flux to trap CO2 in ice.•Mesospheric temperatures need to be below 100 K to trap CO2.•Polar mesospheric particles are therefore unlikely to contain CO2.Polar mesospheric clouds form in the summer high latitude mesopause region and are primarily comprised of H2O ice, forming at temperatures below 150 K. Average summertime temperatures in the polar mesosphere (78°N) are approximately 125 K and can be driven lower than 100 K by gravity waves. Under these extreme temperature conditions and given the relative mesospheric concentrations of CO2 and H2O (~360 ppmv and ~10 ppmv, respectively) it has been hypothesised that CO2 molecules could become trapped within amorphous mesospheric ice particles, possibly making a significant contribution to the total condensed volume. Studies of CO2 trapping in co-deposited gas mixtures of increasing CO2:H2O ratio (deposited at 98 K) were analysed via temperature programmed desorption. CO2 trapping was found to be negligible when the H2O flux to the surface was reduced to 4.8×1013 molecules cm−2 s−1. This corresponds to an average of 0.4 H2O molecules depositing on an adsorbed CO2 molecule and thereby trapping it in amorphous ice. Extrapolating the experimental data to mesospheric conditions shows that a mesospheric temperature of 100 K would be required (at a maximum mesospheric H2O concentration of 10 ppmv) in order to trap CO2 in the ice particles. Given the rarity of this temperature being reached in the mesosphere, this process would be an unlikely occurrence.

Particle size distributions from a series of experiments involving the photo-oxidation of iron pentacarbonyl [Fe(CO)5] in ozone at atmospheric pressure and 295 K are reported for a range of initial reactant concentrations, varying photolysis rates and particle growth times. These data sets were used to test a model which describes the formation of FeO3 in the gas phase, followed by clustering to produce primary Fe2O3 particles. These subsequently coagulate to form fractal-like structures as a result of magnetic dipole coupling of the primary particles.For the smallest size, spherical particles, Smoluchowski theory was used to determine a coagulation constant (kS) of 7.0×10−10 cm3 s−1, indicating a primary particle diameter of 6.6 nm, in very good agreement with the optimised value used in the particle growth model for this system.Finally, these findings are used in discussion of the formation and growth of Fe2O3 ‘meteoric smoke’ particles in the upper atmosphere.

The reactions between Ca+(42S1/2) and O3, O2, N2, CO2 and H2O were studied using two techniques: the pulsed laser photo-dissociation at 193 nm of an organo-calcium vapour, followed by time-resolved laser-induced fluorescence spectroscopy of Ca+ at 393.37 nm (Ca+(42P3/2–42S1/2)); and the pulsed laser ablation at 532 nm of a calcite target in a fast flow tube, followed by mass spectrometric detection of Ca+. The rate coefficient for the reaction with O3 is essentially independent of temperature, k(189–312 K) = (3.9 ± 1.2) × 10−10 cm3 molecule−1 s−1, and is about 35% of the Langevin capture frequency. One reason for this is that there is a lack of correlation between the reactant and product potential energy surfaces for near coplanar collisions. The recombination reactions of Ca+ with O2, CO2 and H2O were found to be in the fall-off region over the experimental pressure range (1–80 Torr). The data were fitted by RRKM theory combined with quantum calculations on CaO2+, Ca+·CO2 and Ca+·H2O, yielding the following results with He as third body when extrapolated from 10−3–103 Torr and a temperature range of 100–1500 K. For Ca+ + O2: log10(krec,0/cm6 molecule−2 s−1) = −26.16 − 1.113log10T − 0.056log102T, krec,∞ = 1.4 × 10−10 cm3 molecule−1 s−1, Fc = 0.56. For Ca+ + CO2: log10(krec,0/ cm6 molecule−2 s−1) = −27.94 + 2.204log10T − 1.124log102T, krec,∞ = 3.5 × 10−11 cm3 molecule−1 s−1, Fc = 0.60. For Ca+ + H2O: log10(krec,0/ cm6 molecule−2 s−1) = −23.88 − 1.823log10T − 0.063log102T, krec,∞ = 7.3 × 10−11exp(830 J mol−1/RT) cm3 molecule−1 s−1, Fc = 0.50 (Fc is the broadening factor). A classical trajectory analysis of the Ca+ + CO2 reaction is then used to investigate the small high pressure limiting rate coefficient, which is significantly below the Langevin capture frequency. Finally, the implications of these results for calcium chemistry in the mesosphere are discussed.

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 O3, measured using a flow tube with a laser ablation source and detection of ions by quadrupole mass spectrometry. The results obtained are (2σ uncertainty): k(Si+ + O3, 298 K) = (6.5 ± 2.1) × 10−10 cm3 molecule−1 s−1, with three product channels (branching ratios): SiO+ + O2 (0.52 ± 0.24), SiO + O2+ (0.48 ± 0.24), and SiO2+ + O (<0.1); k(SiO+ + O3, 298 K) = (6 ± 4) × 10−10 cm3 molecule−1 s−1, where the major products (branching ratio ≥ 0.95) are SiO2 + O2+. 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 O2(1Δg), 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+ + O2 + He reaction was measured to be k(298 K) = (9.0±1.3) × 10−30 cm6 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 O2, with a rate constant of k(298 K) = (1.5 ± 1.0) × 10−13 cm3 molecule−1 s−1.

Atomic silicon is generated by meteoric ablation in the Earth’s upper atmosphere (70–110 km). The reactions of Si(3PJ) atoms with several atmospherically relevant species were studied by the pulsed laser photolysis of a Si atom precursor (typically PheSiH3), followed by time-resolved laser induced fluorescence at 251.43 nm (Si(3p23P0→ 4s 3P1)). This yielded: k(Si + O2, 190–500 K) = 9.49 × 10−11 + 1.80 × 10−10× exp(−T/115 K) cm3 molecule−1 s−1 (uncertainty ≤±15%), in good accord with recent high-level theoretical calculations but in marked disagreement with previous experimental work; k(Si + O3, 190–293 K) = (4.0 ± 0.5) × 10−10 cm3 molecule−1 s−1; k(Si + CO2, 293 K) ≤ 1.2 × 10−14 cm3 molecule−1 s−1; and k(Si + H2O, 293 K) ≤ 2.6 × 10−13 cm3 molecule−1 s−1. These results are explained using a combination of quantum chemistry calculations and long-range capture theory. The quenching rate coefficients k(Si(1D2) + N2, 293 K) = (4.0 ± 0.7) × 10−11 cm3 molecule−1 s−1 and k(Si(1D2) + H2O, 293 K) = (2.3 ± 0.3) × 10−10 cm3 molecule−1 s−1 were also determined.

The absolute absorption cross section of IONO2 was measured by the pulsed photolysis at 193 nm of a NO2/CF3I mixture, followed by time-resolved Fourier transform spectroscopy in the near-UV. The resulting cross section at a temperature of 296 K over the wavelength range from 240 to 370 nm is given by log10(σ(IONO2)/cm2 molecule−1) = 170.4 − 3.773 λ + 2.965 × 10−2λ2 − 1.139 × 10−4λ3 + 2.144 × 10−7λ4 − 1.587 × 10−10λ5, where λ is in nm; the cross section, with 2σ uncertainty, ranges from (6.5 ± 1.9) × 10−18 cm2 at 240 nm to (5 ± 3) × 10−19 cm2 at 350 nm, and is significantly lower than a previous measurement [J. C. Mössinger, D. M. Rowley and R. A. Cox, Atmos. Chem. Phys., 2002, 2, 227]. The photolysis quantum yields for IO and NO3 production at 248 nm were measured using laser induced fluorescence of IO at 445 nm, and cavity ring-down spectroscopy of NO3 at 662 nm, yielding ϕ(IO) ≤ 0.02 and ϕ(NO3) = 0.21 ± 0.09. It is likely that photolysis to I + NO3 is the only significant channel, as shown by accompanying quantum chemistry calculations. The low ϕ(NO3) is explained by the production of hot NO3, most of which dissociates to NO2 + O. In terms of atmospheric relevance, the noon photolysis frequency of J(IONO2) = (3.0 ± 2.1) × 10−3 s−1 (40° N, July) is fast enough to limit the effectiveness of IONO2 as a daytime reservoir of iodine oxides, but the formation and subsequent photolysis of IONO2 is very inefficient as an ozone-depleting cycle.

A series of gas-phase reactions involving molecular Ca-containing ions was studied by the pulsed laser ablation of a calcite target to produce Ca+ in a fast flow of He, followed by the addition of reagents downstream and detection of ions by quadrupole mass spectrometry. Most of the reactions that were studied are important for describing the chemistry of meteor-ablated calcium in the earth’s upper atmosphere. The following rate coefficients were measured: k(CaO+ + O → Ca+ + O2) = (4.2 ± 2.8) × 10−11 at 197 K and (6.3 ± 3.0) × 10−11 at 294 K; k(CaO+ + CO → Ca+ + CO2, 294 K) = (2.8 ± 1.5) × 10−10; k(Ca+·CO2 + O2 → CaO2+ + CO2, 294 K) = (1.2 ± 0.5) ×10−10; k(Ca+·CO2 + H2O → Ca+·H2O + CO2) = (13.0 ± 4.0) × 10−10; and k(Ca+·H2O + O2 → CaO2+ + H2O, 294 K) = (4.0 ± 2.5) × 10−10 cm3 molecule−1 s−1. The quoted uncertainties are a combination of the 1σ standard errors in the kinetic data and the systematic errors in the models used to extract the rate coefficients. Rate coefficients were also obtained for the following recombination (also termed association) reactions in He bath gas: k(Ca+·CO2 + CO2 → Ca+·(CO2)2, 294 K) = (2.6 ± 1.0) × 10−29; k(Ca+·H2O + H2O → Ca+·(H2O)2) = (1.6 ± 1.1) × 10−27; and k(CaO2+ + O2 → CaO2+·O2) < 1 × 10−31 cm6 molecule−2 s−1. These recombination rate coefficients, as well as those for the ligand-switching reactions listed above, were then interpreted using a combination of high level quantum chemistry calculations and RRKM theory using an inverse Laplace transform solution of the master equation. The surprisingly slow reaction between CaO+ and O was explained using quantum chemistry calculations on the lowest 2A′, 2A″ and 4A″ potential energy surfaces. These calculations indicate that reaction mostly occurs on the 2A′ surface, leading to production of Ca+(2S) + O2(1Δg). The importance of this reaction for controlling the lifetime of Ca+ in the upper mesosphere and lower thermosphere is then discussed.

Silicon monoxide (SiO) is a structurally complex compound exhibiting differentiated oxide-rich and silicon-rich nano-phases at length scales covering nanoclusters to the bulk. Although nano-sized and nano-segregated SiO has great technological potential (e.g. nano-silicon for optical applications) and is of enormous astronomical interest (e.g. formation of silicate cosmic dust) an accurate general description of SiO nucleation is lacking. Avoiding the deficiencies of a bulk-averaged approach typified by classical nucleation theory (CNT) we employ a bottom-up kinetic model which fully takes into account the atomistic details involved in segregation. Specifically, we derive a new low energy benchmark set of segregated (SiO)N cluster ground state candidates for N ≤ 20 and use the accurately calculated properties of these isomers to calculate SiO nucleation rates. We thus provide a state-of-the art evaluation of the range of pressure and temperature conditions for which formation of SiO will or will not proceed. Our results, which match with available experiment, reveal significant deficiencies with CNT approaches. We employ our model to shed light on controversial issue of circumstellar silicate dust formation showing that, at variance with the predictions from CNT-based calculations, pure SiO nucleation under such conditions is not viable.

This review discusses the magnitude of the cosmic dust input into the earth's atmosphere, and the resulting impacts from around 100 km to the earth's surface. Zodiacal cloud observations and measurements made with a spaceborne dust detector indicate a daily mass input of interplanetary dust particles ranging from 100 to 300 tonnes, which is in agreement with the accumulation rates of cosmic-enriched elements (Ir, Pt, Os and super-paramagnetic Fe) in polar ice cores and deep-sea sediments. In contrast, measurements in the middle atmosphere – by radar, lidar, high-flying aircraft and satellite remote sensing – indicate that the input is between 5 and 50 tonnes per day. There are two reasons why this huge discrepancy matters. First, if the upper range of estimates is correct, then vertical transport in the middle atmosphere must be considerably faster than generally believed; whereas if the lower range is correct, then our understanding of dust evolution in the solar system, and transport from the middle atmosphere to the surface, will need substantial revision. Second, cosmic dust particles enter the atmosphere at high speeds and undergo significant ablation. The resulting metals injected into the atmosphere are involved in a diverse range of phenomena, including: the formation of layers of metal atoms and ions; the nucleation of noctilucent clouds, which are a sensitive marker of climate change; impacts on stratospheric aerosols and O3 chemistry, which need to be considered against the background of a cooling stratosphere and geo-engineering plans to increase sulphate aerosol; and fertilization of the ocean with bio-available Fe, which has potential climate feedbacks.

Silicon monoxide (SiO) is injected directly into the Earth’s upper atmosphere by ablating meteoroids. SiO is also produced by the reaction of atomic Si (another ablation product) with O2 and O3. The reactions of SiO with several atmospherically relevant oxidants have been studied by the pulsed laser photolysis of a Si atom precursor in the presence of O2, followed by time-resolved non-resonant laser-induced fluorescence of SiO at 282 nm. This yielded: k(SiO + O3, 190–293 K) = (4.4 ± 0.6) × 10−13 cm3 molecule−1 s−1; k(SiO + O2 + He, 293 K) ≤ 3 × 10−32 cm6 molecule−2 s−1, k(SiO + O + He, 293 K) ≤ 1 × 10−30 cm6 molecule−2 s−1, k(SiO + H2O, 293 K, 4–20 Torr) ≤ 4 ×10−14 cm3 molecule−1 s−1, and k(SiO + OH, 293 K, 4–20 Torr) = (5.7 ± 2.0) × 10−12 cm3 molecule−1 s−1. These results are explained by combining ab initio quantum chemistry calculations with transition state theory and RRKM theory. An upper limit of 5 × 10−13 cm3 molecule−1 s−1 for the reaction SiO2 + O → SiO + O2 was determined, but calculations indicate the existence of a high barrier (104.7 kJ mol−1) which will make this reaction very slow at mesospheric temperatures.

This paper describes the kinetic study of a number of gas-phase reactions involving neutral Ca-containing species, many of which are important for describing the chemistry of meteor-ablated calcium in the Earth's upper atmosphere. Ca atoms were produced thermally in the upstream section of a fast flow tube, and then converted to the molecular species CaO, CaO2, CaO3, CaCO3 or Ca(OH)2 by the addition of appropriate reagents. Atomic O or H was added further downstream, and both Ca and CaO were detected at the downstream end of the flow tube by laser-induced fluorescence. The following rate coefficients were determined: k(CaO + O → Ca + O2) = (3.1+2.0−1.5) × 10−10 at 300 K and (1.3+3.4−0.6) × 10−10 at 203 K; k(CaO2 + O → CaO + O2) = (2.2+7.0−1.4) × 10−11 at 300 K and (1.6+2.9−0.7) × 10−11 at 203 K; k(CaO2 + H → products, 298 K) = (1.2 ± 0.6) × 10−11; k(CaCO3 + O → CaO2 + CO2, 300 K) ≤ 1.0 × 10−12; k(CaCO3 + H→ CaOH + CO2, 298 K) ≥ 2.8 × 10−12 and ≤3.6 × 10−11; k(CaO3 + H→ CaOH + O2, 298 K) ≥ 1.7 × 10−11; k(Ca(OH)2 + H → CaOH + H2O, 298 K) ≥ 1.1 × 10−11; k(CaOH + H → Ca + H2O, 298 K) ≥ 1.1× 10−11 cm3 molecule−1 s−1. The kinetics of the reactions of Ca and CaO with NO2 and N2O were also studied, yielding k(Ca + NO2 → CaO + NO) = (2.6 ± 0.3) × 10−10 at 300 K and (2.0 ± 0.3) × 10−10 at 203 K; k(CaO + NO2 → CaO2 + NO) = (8.1 ± 2.0) × 10−10 at 300 K and (2.9 ± 1.0) × 10−10 at 202 K; k(CaO + N2O → CaO2 + N2) = (4.2 ± 1.7) × 10−11 at 300 K and (2.2 ± 1.2) × 10−12 at 206 K; k(CaO + H2 → Ca + H2O, 300 K) = (3.4 ± 1.3) × 10−10 cm3 molecule−1 s−1. Electronic structure calculations of the relevant potential energy surfaces were performed to interpret the experimental results, and the atmospheric implications of these measurements are then discussed.