John M. C. Plane

Extensive halogen-mediated ozone destruction over the tropical Atlantic Ocean

Increasing tropospheric ozone levels over the past 150 years have led to a significant climate perturbation; 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 layer 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 models, 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.

Photochemical formation of hydrogen peroxide in natural waters exposed to sunlight

Hydrogen peroxide is formed in most natural waters when they are exposed to sunlight. The rate at which H/sub 2/O/sub 2/ accumulates is related to the concentration of light-absorbing (>295 nm) organic substances in these waters. The photochemical accumulation rate of H/sub 2/O/sub 2/ in sunlight has been measured for several surface waters and ground waters and was found to be 2.7 /times/ 10/sup /minus/7/ to 48 /times/ 10/sup /minus/7/ mol L/sup /minus/1/ h/sup /minus/1/, in waters ranging from 0.53 to 18 mgL/sup -1/ dissolved organic carbon (DOC), respectively. These rates were determined in midday sunlight, 0.4 W m/sup /minus/2/ (295-385 nm), latitude 24.3/degrees/ N. Apparent quantum yields of H/sub 2/O/sub 2/ have been determined for natural waters at different wavelengths. These quantum yields decreased with increasing wavelength, from 10/sup /minus/3/ in the near-ultraviolet to 10/sup /minus/6/ in the visible spectral range. The quantum yields have been used in a photochemical model to calculate H/sub 2/O/sub 2/ accumulation rates of natural water samples. Model calculations agree with H/sub 2/O/sub 2/ accumulation rates obtained from exposing three different water samples to sunlight.

On the photochemical production of new particles in the coastal boundary layer

Concurrent measurements of ultra-fine (r<5 nm) particle (UFP) formation, OH and SO2 concentrations in the coastal environment are examined to further elucidate the processes leading to tidal-related homogeneous heteromolecular nucleation. During almost daily nucleation events, UFP concentration approached ≈300,000 cm−3 under conditions of solar radiation and low tide. Simultaneous measurements of OH illustrate that, as well as occurring during low tide, these events occur during conditions of peak OH concentration, suggesting that at least one of the nucleating species is photochemically produced. Derived H2SO4 production also exhibited remarkable coherence, although phase-lagged, with UFP formation, thus suggesting its involvement, although binary nucleation of H2SO4 and H2O can be ruled out as a plausible mechanism. Ternary nucleation involving NH3 seems most likely as a trigger mechanism, however, at least a fourth condensable species, X, is required for growth to detectable sizes. Since UFP are only observed during low tide events, it is thought that species X, or its parent, is emitted from the shore biota - without which, no nucleation is detected. Species X remains to be identified. Model simulations indicate that, in order to reproduce the observations, a nucleation rate of 107 cm−3 s−1, and a condensable vapour concentration of 5 × 107 cm−3, are required.

A modeling study of iodine chemistry in the marine boundary layer

An observationally constrained photochemical box model has been developed to investigate the atmospheric chemistry of iodine in the marine boundary layer, motivated by recent measurements of the iodine monoxide (IO) radical (Allan et al., this issue). Good agreement with the time series of IO measured at a midlatitude coastal station was achieved by using a reaction scheme that included recycling of iodine through marine aerosol. The strong diurnal variation in IO observed in the subtropical Atlantic was satisfactorily modeled by assuming a constant concentration of iodocarbons that photolyzed to produce roughly 1×104 iodine atoms cm−3 s−1 at midday. The significance of the occurrence of IO at concentrations of up to 4 parts per trillion in the marine boundary layer was then considered from three angles. First, the iodine-catalyzed destruction of ozone was shown to be of a magnitude similar to that caused by odd-hydrogen photochemistry, with up to 13% of the available ozone destroyed per day in a marine air mass. Second, the enrichment factor of iodine in marine aerosol compared with surface seawater was predicted to increase to values of several thousand, in sensible accord with observations. Most of the enrichment should be due to the accumulation of iodate, although other iodine species may also be present, depending on the rate of aerosol recycling. Third, the denoxification of the marine boundary layer was found to be significantly enhanced as a result of aerosol uptake of IONO2, formed from the recombination of IO with NO2.

The chemistry of meteoric metals in the Earth's upper atmosphere

Abstract The presence of thin layers of free metal atoms at around 90 km in the upper atmosphere has been known for about fifty years. Layers of the alkali metals Na, K and Li, as well as Ca and Fe, have been observed. This discovery has posed two important questions. First, what is the source of the metals: interplanetary or terrestrial? Secondly, what is the nature of the chemistry that causes reactive metals such as sodium to exist in their atomic form in the atmosphere? The first part of this review covers the techniques that have been developed to observe the metal layers, including ground-, rocket- and space-based photometers, and in particular metal lidars. The many curious phenomena that have been observed are then described, such as the small scale-heights of the layers, the quite different seasonal variations of the three alkali metals, the large depletions of Ca and Fe relative to Na, and the dramatic appearance of sporadic Na layers. The second part of the review describes the recent advances ...

Atmospheric Chemistry of Iodine

Atmospheric Chemistry of Iodine Alfonso Saiz-Lopez,* John M. C. Plane,* Alex R. Baker, Lucy J. Carpenter, Roland von Glasow, Juan C. G omez Martín, Gordon McFiggans, and Russell W. Saunders Laboratory for Atmospheric and Climate Science (CIAC), CSIC, Toledo, Spain School of Chemistry, University of Leeds, Leeds, LS2 9JT, United Kingdom School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom Department of Chemistry, University of York, Heslington, York YO10 5DD, United Kingdom School of Earth, Atmospheric & Environmental Sciences, University of Manchester, Manchester, M13 9PL, United Kingdom

Observations of iodine monoxide in the remote marine boundary layer

We report measurements of the iodine monoxide (IO) radical in the marine boundary layer at three remote sites: Mace Head (Ireland), Tenerife (Canary Islands), and Cape Grim (Tasmania). IO was observed by long-path differential optical absorption spectroscopy using the A(2)Pi(3/2)-X(2)Pi(3/2) electronic transition between 415 and 450 nm. The daytime IO concentration at these three locations was found to vary from below the detection limit (less than or equal to 0.2 parts per trillion (ppt)) to a maximum of 4 ppt, with an average of about 1 ppt, Of particular note is that the IO observed off the north coast of Tenerife, which is probably typical of the open ocean sub-tropical North Atlantic, exhibited a distinct diurnal cycle which correlated strongly with the solar actinic flux in the near UV. IO was also observed at Cape Grim to be present at much lower levels (approximate to 0.3 ppt) in westerly air from the Southern Ocean. As is shown in the companion paper (McFiggans et al., this issue), these measurements of IO are satisfactorily reproduced by a photochemical box model incorporating the recycling of iodine through marine aerosol. This model indicates that the direct iodine-catalyzed destruction of ozone in the boundary layer may well be similar to the losses caused by odd-hydrogen photochemistry and dry deposition. The significance of this work is that IO is probably present in much of the open ocean boundary layer, at levels where it may cause significant depletion of ozone.

An ion‐molecule mechanism for the formation of neutral sporadic Na layers

This paper describes a laboratory study into the chemical pathways by which Na + is converted to Na in the upper atmosphere. The termolecular clustering reactions of Na + with N 2 , O 2 , and CO 2 were studied in a low-temperature fast flow reactor coupled to a quadrupole mass spectrometer. This yielded κ(Na + + N 2 + He, 93-255 K) = (1.20 ± 0.13) x 10 -30 (T/200 K) -(2.20±0.09) , κ(Na + + O 2 + He, 93-130 K) = (5.20 ± 2.62) x 10-31 (T/200 K) -(2.64±0.74) κ(Na + + CO 2 + He, 158-300 K) = (9.05±1.38) x 10 -30 (T/200 K) -(2.84±0.48) , where the units are cm 6 molecule -2 s -1 and the stated errors are a combination of the 2σ standard errors in the kinetic data and the systematic errors in the temperature, pressure, and flow rates. It was then shown that atomic O will ligand switch with Na.N 2 + but not with Na.CO 2 + , and that the former reaction proceeds essentially at the Langevin collision frequency. The neutralization of Na + in the upper atmosphere is therefore rather complex. The first step is formation of the Na.N 2 + ion from the recombination of Na + with N 2 . This cluster ion can then either switch with CO 2 , which leads to a stable cluster ion that will undergo dissociative electron recombination to form Na; or switch with atomic O, which reforms Na + . The result of this is that the lifetime of Na + changes very rapidly from more than a day above 100 km to just a few minutes at 90 km. Furthermore, the rate of neutralization is largely independent of the electron concentration. A simple model describing the conversion of Na + to atomic Na in a descending sporadic E layer demonstrates that this ion-molecule mechanism appears to fulfil many of the major criteria for producing sporadic sodium layers.

Meteoric smoke fallout over the Holocene epoch revealed by iridium and platinum in Greenland ice

An iridium anomaly at the Cretaceous/Tertiary boundary layer has been attributed to an extraterrestrial body that struck the Earth some 65 million years ago. It has been suggested that, during this event, the carrier of iridium was probably a micrometre-sized silicate-enclosed aggregate or the nanophase material of the vaporized impactor. But the fate of platinum-group elements (such as iridium) that regularly enter the atmosphere via ablating meteoroids remains largely unknown. Here we report a record of iridium and platinum fluxes on a climatic-cycle timescale, back to 128,000 years ago, from a Greenland ice core. We find that unexpectedly constant fallout of extraterrestrial matter to Greenland occurred during the Holocene, whereas a greatly enhanced input of terrestrial iridium and platinum masked the cosmic flux in the dust-laden atmosphere of the last glacial age. We suggest that nanometre-sized meteoric smoke particles, formed from the recondensation of ablated meteoroids in the atmosphere at altitudes >70 kilometres, are transported into the winter polar vortices by the mesospheric meridional circulation and are preferentially deposited in the polar ice caps. This implies an average global fallout of 14 ± 5 kilotons per year of meteoric smoke during the Holocene.

The Mesosphere and Metals: Chemistry and Changes

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.