T. Reddmann

Three-dimensional model simulations of SF6 with mesospheric chemistry

Multiannual integrations with the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA) have been performed using meteorological analyses of vorticity and divergence up to 10 hPa to analyze the influence of a simplified SF 6 mesospheric chemistry on estimation of mean age of air and to compare profiles of SF 6 mixing ratios observed in the stratosphere with model simulations. The chemical degradation scheme includes electron attachment of SF 6 and subsequent reactions of SF 6 - , such as photodetachment and charge transfer with ozone. Several combinations of reaction rate constants and electron profiles have been tested. Good agreement with observations is found for inert SF 6 transport. However, when mesospheric loss is inclnded in the model, significant deviations are found for polar winter observations above 25 km. Chemical loss by electron attachment without reactions yielding SF 6 again is not compatible with observations. The atmospheric lifetime of SF 6 spans 400 to 10,000 years, depending on the assumed loss mechanism and the value for the electron density in the stratosphere.

Recent Northern Hemisphere stratospheric HCl increase due to atmospheric circulation changes

The abundance of chlorine in the Earth’s atmosphere increased considerably during the 1970s to 1990s, following large emissions of anthropogenic long-lived chlorine-containing source gases, notably the chlorofluorocarbons. The chemical inertness of chlorofluorocarbons allows their transport and mixing throughout the troposphere on a global scale, before they reach the stratosphere where they release chlorine atoms that cause ozone depletion. The large ozone loss over Antarctica was the key observation that stimulated the definition and signing in 1987 of the Montreal Protocol, an international treaty establishing a schedule to reduce the production of the major chlorine- and bromine-containing halocarbons. Owing to its implementation, the near-surface total chlorine concentration showed a maximum in 1993, followed by a decrease of half a per cent to one per cent per year, in line with expectations. Remote-sensing data have revealed a peak in stratospheric chlorine after 1996, then a decrease of close to one per cent per year, in agreement with the surface observations of the chlorine source gases and model calculations. Here we present ground-based and satellite data that show a recent and significant increase, at the 2σ level, in hydrogen chloride (HCl), the main stratospheric chlorine reservoir, starting around 2007 in the lower stratosphere of the Northern Hemisphere, in contrast with the ongoing monotonic decrease of near-surface source gases. Using model simulations, we attribute this trend anomaly to a slowdown in the Northern Hemisphere atmospheric circulation, occurring over several consecutive years, transporting more aged air to the lower stratosphere, and characterized by a larger relative conversion of source gases to HCl. This short-term dynamical variability will also affect other stratospheric tracers and needs to be accounted for when studying the evolution of the stratospheric ozone layer.

NOy partitioning and budget and its correlation with N2O in the Arctic vortex and in summer midlatitudes in 1997

Vertical profiles of the most important species of nocturnal total reactive nitrogen (NO y = NO 2 + HNO 3 + CIONO 2 + 2 N 2 O 5 + HO 2 NO 2 ) together with its source gas N 2 O were retrieved from infrared limb emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding, Balloon-borne version (MIPAS-B) instrument inside the late winter arctic vortex from Kiruna (Sweden, 68°N) on 24 March 1997 and in summer midlatitudes from Gap (France, 44°N) on 2 July 1997. The measured data were compared to calculations performed with the three-dimensional chemistry transport model (CTM) Karlsruhe Simulation model of the Middle Atmosphere (KASIMA). The results show that in the late winter arctic vortex most of the available nitrogen and chlorine is in the form of HNO 3 and CIONO 2 , respectively. An anomalous N 2 O-NO y correlation observed in March 1997 appears to be caused to a large extent by quasi-horizontal mixing of air masses across the vortex edge. However, near 20 km some denitrification of ∼1.5 to 2 ppbv NO y could be observed. The N 2 O profile measured in July 1997 indicates remnants of polar vortex air and is not reproduced by the CTM at the same location. However, the profile shapes of the individual compounds of the NO y family as well as the NO x /NO y ratio are reproduced fairly well by the model.

Streamers observed by the CRISTA experiment and simulated in the KASIMA model

Data from the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) showed three narrow streamers of air with tropical mixing ratios of HNO3 and N2O pointing from the tropics toward middle latitudes in the middle stratosphere on November 6, 1994. By means of the mechanistic prognostic model, the diagnostic chemical transport model (CTM) and the combined nudged model, which are all versions of the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA), the hypothesis is checked of whether these streamers are due to adiabatic transport processes on a timescale of days. Whereas the prognostic model reproduces the northern hemisphere streamers only qualitatively in their position, the CTM and the nudged model show a good agreement between their simulated tracer structures and the observed streamers. Because of the clear streamer signal in the nudged model compared to the CTM, its data are used for the investigation of isentropic tracer deformations. They show that the northern hemisphere streamers are mainly built by adiabatic transport on a timescale of days. Rossby wave breaking plays a role in the dissolution of the streamers. In the southern hemisphere, the production of Ertels potential vorticity (EPV) and the net heating rate is large, and the observed streamers are therefore not reproduced in the EPV. Moreover, the isentropic deformations of the EPV due to the horizontal flow are that strong during a minor warming in the end of October that the reproduction of the southern hemisphere streamer by means of artificial tracers fails.

The influence of the OH + NO2+ M reaction on the NOy, partitioning in the late Arctic winter 1992/1993 as studied with KASIMA

The northern hemispheric winter 1992/1993 is simulated with the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA). The model is a combination of an off-line model using analyses from the European Centre for Medium Range Weather Forecasts up to a pressure altitude of 10 hPa and a mechanistic prognostic model on top with an entire altitude range from 10 up to 120 km. The chemistry scheme included in the model represents the gas phase chemistry as well as heterogeneous reactions on polar stratospheric clouds and on liquid sulfate aerosols. We compare model results with global measurements performed by the instruments of the Upper Atmosphere Research Satellite. The model is able to simulate the global distribution of source gases and long-lived species as well as reactive species in good agreement with the observations. The most significant discrepancies occur inside the vortex where the calculated ozone mixing ratio is overestimated. Below the pressure altitude of 30 hPa inside the vortex the calculated NO2 mixing ratio is underestimated compared to the measurement. A sensitivity study with a new recommendation of the OH + NO2 + M rate constant using the full form of the falloff function for the pressure dependence has been performed. The mixing ratios of the NOy species differ by only about 5% due to the new recommended rate constant inside the vortex with no significant effect on the denoxification.

NO y production, ozone loss and changes in net radiative heating due to energetic particle precipitation in 2002–2010

We analyze the impact of energetic particle precipitation on the stratospheric nitrogen budget, ozone abundances and net radiative heating using results from three global chemistry-climate models considering solar protons and geomagnetic forcing due to auroral or radiation belt electrons. Two of the models cover the atmosphere up to the lower thermosphere, the source region of auroral NO production. Geomagnetic forcing in these models is included by prescribed ionization rates. One model 5 reaches up to about 80 km, and geomagnetic forcing is included by applying an upper boundary condition of auroral NO mixing ratios parameterized as a function of geomagnetic activity. Despite the differences in the implementation of the particle effect, the resulting modeled NOy in the upper mesosphere agrees well between all three models, demonstrating that geomagnetic forcing is represented in a consistent way either by prescribing ionization rates or by prescribing NOy at the model top. Compared with observations of stratospheric and mesospheric NOy from the MIPAS instrument for the years 2002–2010, 10 the model simulations reproduce the spatial pattern and temporal evolution well. However, after strong sudden stratospheric warmings, particle induced NOy is underestimated by both high-top models, and after the solar proton event in October 2003, NOy is overestimated by all three models. Model results indicate that the large solar proton event in October 2003 contributed about 1–2 Gmol (10 mol) NOy per hemisphere to the stratospheric NOy budget, while downwelling of auroral NOx from the upper mesosphere and lower thermosphere contributes up to 4 Gmol NOy. Accumulation over time leads to a constant particle15 induced background of about 0.5–1 Gmol per hemisphere during solar minimum, and up to 2 Gmol per hemisphere during solar maximum. Related negative anomalies of ozone are predicted by the models nearly in every polar winter, ranging from 10– 50% during solar maximum to 2–10% during solar minimum. Ozone loss continues throughout polar summer after strong solar proton events in the Southern hemisphere and after large sudden stratospheric warmings in the Northern hemisphere. During mid-winter, the ozone loss causes a reduction of the infrared radiative cooling, i.e., a positive change of the net radiative heating 20 (effective warming), in agreement with analyses of geomagnetic forcing in stratospheric temperatures which show a warming in the late winter upper stratosphere. In late winter and spring, the sign of the net radiative heating change turns to negative (effective cooling). This spring-time cooling lasts well into summer and continues until the following autumn after large solar proton events in the Southern hemisphere, after sudden stratospheric warmings in the Northern hemisphere. 1 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-514 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 22 June 2017 c

The Influence of Energetic Particles on the Chemistry of the Middle Atmosphere

Energetic particle precipitation (EPP) during solar and geomagnetic active periods causes chemical disturbances in the lower thermosphere and in the middle atmosphere. Additional HOx (H, OH, HO2) and NOx (N, NO, NO2) are produced in the middle atmosphere, and enhancements of NOx produced in these events can be transported to the winter stratosphere. These trace species take part in ozone chemistry and, by chemical-radiative coupling, the dynamical state in the middle atmosphere can be altered. There is evidence both from observations and from chemistry-climate models that the EPP induced signal in the middle atmosphere may then propagate into the troposphere. Thus particle precipitation could connect to possible climate effects. The first step in this functional chain is the impact of EPP on the chemical composition in the middle atmosphere and lower thermosphere, and the downward transport in the polar winter middle atmosphere. The general objective of this project was to assess quantitatively the chemical composition change in the middle atmosphere by combining model simulations and observations. The study relays mainly on the observations of the MIPAS instrument on the ENVISAT satellite, whose data set has been expanded in the context of this project by a newly developed retrieval of the gas H2O2, a reservoir for the members of the HOx family. Simulations have been carried out with the two chemical transport models CLaMS and KASIMA, which cover chemistry and transport effects in the stratosphere up to the mesosphere/lower thermosphere region. The impact on the global NOy budget and (the resulting) total ozone change are assessed in these studies. In addition, the ion reaction mechanism for the conversion of N2O5 to HNO3 based on positive ion chemistry was refined. The detailed comparison of model results and observation for the SPE 2003 showed that models can simulate the impact of EPP on ozone chemistry but deficiencies exist for some minor species.

Atmospheric circulation changes identified thanks to ground-based FTIR monitoring of hydrogen chloride (HCl)

Monitoring the success of the Montreal Protocol on substances that deplete stratospheric ozone is one of the primary tasks of the NDACC network. Among the various techniques involved, giving access to numerous relevant parameters, the ground-based FTIR instruments contribute significantly by providing total and partial columns of key tropospheric and stratospheric constituents. Indeed, high-resolution solar infrared spectra contain the signatures of a suite of halogenated organic source gases. The current list includes CFC-11, CFC-12, HCFC-22, HCFC-142b, CCl4, CF4, SF6 (e.g., Krieg et al., 2005; Zander et al., 2008; Rinsland et al., 2012; Mahieu et al., 2013a; Mahieu et al., 2014a) and efforts are ongoing to expand this list. In addition, the respective evolutions of the inorganic chlorine and fluorine loadings in the stratosphere are also accessible to this technique through observations of the main reservoirs, i.e. hydrogen chloride (HCl) and chlorine nitrate (ClONO2), hydrogen fluoride (HF) and carbonyl fluoride (COF2). Time series and trends of all these species have been reported and analyzed in successive studies, notably allowing characterising from the ground the rapid increase of inorganic chlorine (Cly) in the Earth’s stratosphere (e.g., Zander et al., 1987; Rinsland et al., 1991, 1996; Reisinger et al., 1995), following large emissions of anthropogenic halogenated source gases during the 1970s to 1990s. Later, studies involving several NDACC ground-based FTIR stations provided evidence for a stabilisation of HCl and ClONO2 around the mid-1990s (Rinsland et al., 2003), and then a near-global characterisation of the Cly decrease at rates close to 1%/yr in both hemispheres at 17 sites between 80oN and 78°S (Kohlhepp et al., 2012).