Modelling the Antarctic ozone hole

Researchers performed model calculations of the ozone depletions taking place in the Antarctic lower stratosphere. Making the assumption that odd nitrogen is frozen out on stratospheric haze particles, an analysis is given of how much homogeneous reactions can contribute to ozone loss during September-October. Comparisons with observations indicate the potential importance of reactions with HCl in the polar stratospheric cloud particles

Acute effects of a large bolide impact simulated by a global atmospheric circulation model

The goal is to use a global three-dimensional atmospheric circulation model developed for studies of atmospheric effects of nuclear war to examine the time evolution of atmospheric effects from a large bolide impact. The model allows for dust and NOx injection, atmospheric transport by winds, removal by precipitation, radiative transfer effects, stratospheric ozone chemistry, and nitric acid formation and deposition on a simulated Earth having realistic geography. Researchers assume a modest 2 km-diameter impactor of the type that could have formed the 32 km-diameter impact structure found near Manson, Iowa and dated at roughly 66 Ma. Such an impact would have created on the order of 5 x 10 to the 10th power metric tons of atmospheric dust (about 0.01 g cm(-2) if spread globally) and 1 x 10 to the 37th power molecules of NO, or two orders of magnitude more stratospheric NO than might be produced in a large nuclear war. Researchers ignore potential injections of CO2 and wildfire smoke, and assume the direct heating of the atmosphere by impact ejecta on a regional scale is not large compared to absorption of solar energy by dust. Researchers assume an impact site at 45 N in the interior of present day North America

A discussion on the determination of atmospheric OH and its trends

The oxidation efficiency of the troposphere is largely determined by the hydroxyl radical and its global distribution. Its presence limits the lifetime of most trace gases. Because of the great importance of several of these gases for climate, ozone budget and OH itself, it is of fundamental importance to acquire knowledge about atmospheric OH and possible trends in its concentrations. In the past, average concentrations of OH and trends were largely derived using industrially produced CH₃CCl₃ as a chemical tracer. The analyses have given valuable, but also rather uncertain results. In this paper we describe an idealized computer aided tracer experiment which has as one of its goals to derive tracer concentration weighted, global average <<i>k</i>(OH)>, where the temporal and spatial OH distribution is prescribed and <i>k</i> is the reaction rate coefficient of OH with a hitherto never produced (Gedanken) tracer, which is injected at a number of surface sites in the atmosphere in well known amounts over a given time period. Using a three-dimensional (3-D) time-dependent chemistry transport model, <<i>k</i>(OH)> can be accurately determined from the calculated 3-D tracer distribution. It is next explored how well <<i>k</i>(OH)> can be retrieved solely from tracer measurements at a limited number of surface sites. The results from this analysis are encouraging enough to actually think about the feasibility to carry out a global dedicated tracer experiment to derive <<i>k</i>(OH)> and its temporal trends. However, before that, we propose to test the methods that are used to derive <<i>k</i>(OH)>, so far largely using CH₃CCl₃, with an idealized tracer experiment, in which a global chemistry transport model is used to calculate the ``Gedanken'' tracer distribution, representing the real 3-D world, from which <<i>k</i>(OH)> is derived, using only the tracer information from a limited set of surface sites. We propose here that research groups which are, or will be, involved in global average OH studies to participate in such an inter-comparison of methods, organized and over-seen by a committee appointed by the International Global Atmospheric Chemistry (IGAC) program

Model study of multiphase DMS oxidation with a focus on halogens

We studied the oxidation of dimethylsulfide (DMS) in the marine boundary layer (MBL) with a one-dimensional numerical model and focused on the influence of halogens. Our model runs show that there is still significant uncertainty about the end products of the DMS addition pathway, which is especially caused by uncertainty in the product yield of the reaction of the intermediate product methyl sulfinic acid (MSIA) with OH. BrO strongly increases the importance of the addition branch in the oxidation of DMS even when present at mixing ratios smaller than 0.5pmol&nbsp;mol^-1. The inclusion of halogen chemistry leads to higher DMS oxidation rates and smaller DMS to SO₂ conversion efficiencies. The DMS to SO₂ conversion efficiency is also drastically reduced under cloudy conditions. In cloud-free model runs between 5 and 15% of the oxidized DMS reacts further to particulate sulfur, in cloudy runs this fraction is almost 100%. Sulfate production by HOCl_aq and HOBr_aq is important in cloud droplets even for small Br^- deficits and related small gas phase halogen concentrations. In general, more particulate sulfur is formed when halogen chemistry is included. A possible enrichment of HCO₃^- in fresh sea salt aerosol would increase pH values enough to make the reaction of S(IV)^* (=SO_2,aq+HSO₃^-+SO₃^2-) with O₃ dominant for sulfate production. It leads to a shift from methyl sulfonic acid (MSA) to non-sea salt sulfate (nss-SO₄^2-) production but increases the total nss-SO₄^2- only somewhat because almost all available sulfur is already oxidized to particulate sulfur in the base scenario. We discuss how realistic this is for the MBL. We found the reaction MSA_aq+OH to contribute about 10% to the production of nss-SO₄^2- in clouds. It is unimportant for cloud-free model runs. Overall we find that the presence of halogens leads to processes that decrease the albedo of stratiform clouds in the MBL

Influence of a solar proton event on stratospheric ozone

Ozone depletion in the stratosphere associated with the solar proton event of August 4, 1972, was observed with the backscattered ultraviolet experiment on the Nimbus 4 satellite. An abrupt ozone decrease in the 75-80 deg N zone of about 0.002 atm-cm above 4 mb was observed to persist throughout the month of August. A decrease was noted in the 55-65 deg N zone on days 219 and 220, but recovery occurred on day 221. Thereafter, a more gradual decrease was observed. The equatorial zone also showed gradual decrease after day 218, but these were not uniquely distinguished from seasonal variations. The observed change in total ozone following the event was -0.003 atm-cm for the 75-80 deg N zone, corresponding to a 1.3 percent reduction in the 0.305 atm-cm zonal average, but within the 0.019 atm-cm standard deviation. Above the 10 mb surface in the 75-80 deg N zone however, a decrease of 0.004 atm-cm may be compared with a standard deviation of 0.001 atm-cm

Development of a tunable diode laser absorption spectrometer for measurement of the 13C/12C ratio in methane

A tunable diode laser absorption spectrometer (TDLAS) for measuring the ratio in methane has been developed. Using a triple path arrangement the spectra of the CH4 sample, a isotope standard and pure 13CH4 are recorded simultaneously and compared to evaluate the ratio of the sample, using a 13CH4---12CH4 absorption line pair near 3007 cm−1. Systematic effects due to variations in temperature, pressure, and optical density were measured for this rotational-vibrational transition pair. Optical interference effects are effectively suppressed by linearly polarizing the laser beam and using Brewster windows for gas cells and detectors. The overall δ13C accuracy vs. the PDB scale is about ± 1 ‰ for a CH4 concentration of 2.5 % (sample size: 5 μmoles = 0.11 STP cm3 CH4) using 36 cm long absorption cells. The future application of a multipass cell should allow measurement concentrations of CH4 down to about 50 ppm. The main advantages of the new method are the short measurement time of 10–15 min for one sample and the direct measurement on the CH4 molecule without the need to chemically convert it to CO2. With the present accuracy the new method should be useful for the measurement of CH4 sources, allowing a greater sample throughput compared to the conventional mass spectrometry technique

The impact of the spacecraft system SÄNGER on the composition of the middle atmosphere

A two-dimensional chemical model and physical considerations are used to estimate the impact of the spacecraft system SÄNGER on stratospheric and mesospheric ozone in relation to other spacecraft and other anthropogenic perturbations. Perturbations of middle atmospheric NOy H2O and H2 concentrations, and their impact on the radiative balance of the atmosphere, including contrail formation, are discussed. It is found, that in case of about 24 launches per year the perturbations due to SÄGER are about negligible on a global scale. However, if a SÄGER version would be used for a hypersonic fleet of commercial aircraft a serious ozone depletion is predicted. © 1992 by Wax Planck Society

Will climate change increase ozone depletion from low-energy-electron precipitation?

We investigate the effects of a strengthened stratospheric/mesospheric residual circulation on the transport of nitric oxide (NO) produced by energetic particle precipitation. During periods of high geomagnetic activity, energetic electron precipitation (EEP) is responsible for winter time ozone loss in the polar middle atmosphere between 1 and 6 hPa. However, as climate change is expected to increase the strength of the Brewer-Dobson circulation including extratropical downwelling, the enhancements of EEP NO&lt;sub&gt;x&lt;/sub&gt; concentrations are expected to be transported to lower altitudes in extratropical regions, becoming more significant in the ozone budget. Changes in the mesospheric residual circulation are also considered. We use simulations with the chemistry climate model system EMAC to compare present day effects of EEP NO&lt;sub&gt;x&lt;/sub&gt; with expected effects in a climate change scenario for the year 2100. In years of strong geomagnetic activity, similar to that observed in 2003, an additional polar ozone loss of up to 0.4 μmol/mol at 5 hPa is found in the Southern Hemisphere. However, this would be approximately compensated by an ozone enhancement originating from a stronger poleward transport of ozone from lower latitudes caused by a strengthened Brewer-Dobson circulation, as well as by slower photochemical ozone loss reactions in a stratosphere cooled by risen greenhouse gas concentrations. In the Northern Hemisphere the EEP NO&lt;sub&gt;x&lt;/sub&gt; effect appears to lose importance due to the different nature of the climate-change induced circulation changes

Methane production from mixed tropical savanna and forest vegetation in Venezuela

International audienceMeasurements of methane concentrations in the boundary layer in the northern part of the Guayana shield, Venezuela, during the wet season (October 1988), showed the presence of substantial methane surface emissions. The measuring site is within the savanna climate region, but is affected by emissions from savanna and forest vegetation. From day versus night concentration measurements, with higher concentrations during night, a methane source strength near the site of 3?7×1011 molecules/cm2/s can be estimated, which includes emissions from small tracts of flooded soils, termites and especially tropical vegetation. Extrapolated to the entire savanna, this may imply a methane source of ~30?60 Tg yr?1 similar to the one calculated for tropical vegetation on the basis of recently published in vitro plant emission experiments by Keppler et al. (2006), which indicate emissions of ~30 Tg yr?1 for tropical savannas and grasslands and ~78 Tg yr?1 for tropical forests