The contribution of anthropogenic bromine emissions to past stratospheric ozone trends: a modelling study

International audienceBromine compounds play an important role in the depletion of stratospheric ozone. We have calculated the changes in stratospheric ozone in response to changes in the halogen loading over the past decades, using a two-dimensional (latitude/height) model constrained by source gas mixing ratios at the surface. Model calculations of the decrease of total column ozone since 1980 agree reasonably well with observed ozone trends, in particular when the contribution from very short-lived bromine compounds is included. Model calculations with bromine source gas mixing ratios fixed at 1959 levels, corresponding approximately to a situation with no anthropogenic bromine emissions, show an ozone column reduction between 1980 and 2005 at northern hemisphere mid-latitudes of only ?55% compared to a model run including all halogen source gases. In this sense anthropogenic bromine emissions are responsible for ?45% of the model estimated column ozone loss at northern hemisphere mid-latitudes. The chemical efficiency of bromine relative to chlorine for global total ozone depletion from our model calculations, expressed by the so called ?-factor, is about 73 on an annual average. This value is much higher than previously published results. Updates in reaction rate constants can explain only part of the differences in ?. The inclusion of bromine from very short-lived source gases has only a minor effect on the global mean ?-factor

The contribution of anthropogenic bromine emissions to past stratospheric ozone trends: a modelling study

Bromine compounds play an important role in the depletion of stratospheric ozone. We have calculated the changes in stratospheric ozone in response to changes in the halogen loading over the past decades, using a two-dimensional (latitude/height) model constrained by source gas mixing ratios at the surface. Model calculations of the decrease of total column ozone since 1980 agree reasonably well with observed ozone trends, in particular when the contribution from very short-lived bromine compounds is included. Model calculations with bromine source gas mixing ratios fixed at 1959 levels, corresponding approximately to a situation with no anthropogenic bromine emissions, show an ozone column reduction between 1980 and 2005 at Northern Hemisphere mid-latitudes of only ≈55% compared to a model run including all halogen source gases. In this sense anthropogenic bromine emissions are responsible for ≈45% of the model estimated column ozone loss at Northern Hemisphere mid-latitudes. However, since a large fraction of the bromine induced ozone loss is due to the combined BrO/ClO catalytic cycle, the effect of bromine would have been smaller in the absence of anthropogenic chlorine emissions. The chemical efficiency of bromine relative to chlorine for global total ozone depletion from our model calculations, expressed by the so called α-factor, is 64 on an annual average. This value is much higher than previously published results. Updates in reaction rate constants can explain only part of the differences in α. The inclusion of bromine from very short-lived source gases has only a minor effect on the global mean α-factor

Estimating the contribution of bromoform to stratospheric bromine and its relation to dehydration in the tropical tropopause layer

International audienceThe contribution of bromoform to the stratospheric bromine loading is estimated using the one-dimensional tropical mean model of Folkins and Martin (2005), which is constrained by observed mean profiles of temperature and humidity. In order to reach the stratosphere, bromoform needs to be lifted by deep convection into the tropical tropopause layer (TTL), above the level of zero radiative heating. The contribution of bromoform to stratospheric bromine then depends critically on the rate of removal of the degradation products of bromoform (collectively called Bry here) from the TTL, which is believed to be due to scavenging by falling ice. This relates the transport of short-lived bromine species into the stratosphere to processes of dehydration in the TTL. In the extreme case of dehydration occurring only through overshooting deep convection, the loss of Bry from the TTL may be negligible and consequently bromoform will fully contribute with its boundary layer mixing ratio to the stratospheric bromine loading, i.e. with 3 pptv for an assumed 1 pptv of bromoform in the boundary layer. For the other extreme that Bry is removed from the TTL almost instantaneously, the model calculations predict a contribution of about 0.5 pptv for the assumed 1 pptv of boundary layer bromoform. While this gives some constraints on the contribution of bromoform to stratospheric bromine, a key uncertainty in estimating the contribution of short-lived bromine source gases to the stratospheric bromine loading is the mechanism and rate of removal of Bry within the TTL

Rapid meridional transport of tropical airmasses to the Arctic during the major stratospheric warming in January 2003

International audienceWe present observations of unusually high values of ozone and N2O in the middle stratosphere that were observed by the airborne submillimeter radiometer ASUR in the Arctic. The observations took place in the meteorological situation of a major stratospheric warming that occurred in mid-January 2003 and was dominated by a wave 2 event. On 23 January 2003 the observed N2O and O3 mixing ratios around 69° N in the middle stratosphere reached maximum values of ~190 ppb and ~10 ppm, respectively. The similarities of these N2O profiles in a potential temperature range between 800 and 1200 K with N2O observations around 20° N on 1 March 2003 by the same instrument suggest that the observed Arctic airmasses were transported from the tropics quasi-isentropically. This is confirmed by 5-day back trajectory calculations which indicate that the airmasses between about 800 and 1000 K had been located around 20° N 3?5 days prior to the measurement in the Arctic. Calculations with a linearized ozone chemistry model along calculated as well as idealized trajectories, initialized with the low-latitude ASUR ozone measurements, give reasonable agreement with the Arctic ozone measurement by ASUR. PV distributions suggest that these airmasses did not stay confined in the Arctic region which makes it unlikely that this dynamical situation lead to the formation of dynamically caused pockets of low ozone

Impact of deep convection and dehydration on bromine loading in the upper troposphere and lower stratosphere

Stratospheric bromine loading due to very short-lived substances is investigated with a three-dimensional chemical transport model over a period of 21 years using meteorological input data from the European Centre for Medium-Range Weather Forecasts ERA-Interim reanalysis from 1989 to the end of 2009. Within this framework we analyze the impact of dehydration and deep convection on the amount of stratospheric bromine using an idealized and a detailed full chemistry approach. We model the two most important brominated short-lived substances, bromoform (CHBr<sub>3</sub>) and dibromomethane (CH<sub>2</sub>Br<sub>2</sub>), assuming a uniform convective detrainment mixing ratio of 1 part per trillion by volume (pptv) for both species. The contribution of very short-lived substances to stratospheric bromine varies drastically with the applied dehydration mechanism and the associated scavenging of soluble species ranging from 3.4 pptv in the idealized setup up to 5 pptv using the full chemistry scheme. In the latter case virtually the entire amount of bromine originating from very short-lived source gases is able to reach the stratosphere thus rendering the impact of dehydration and scavenging on inorganic bromine in the tropopause insignificant. Furthermore, our long-term calculations show that the mixing ratios of very short-lived substances are strongly correlated to convective activity, i.e. intensified convection leads to higher amounts of very short-lived substances in the upper troposphere/lower stratosphere especially under extreme conditions like El Niño seasons. However, this does not apply to the inorganic brominated product gases whose concentrations are anti-correlated to convective activity mainly due to convective dilution and possible scavenging, depending on the applied approach

Towards a climatology of stratospheric bromine monoxide from SCIAMACHY limb observations

International audienceRetrievals of stratospheric bromine monoxide (BrO) profiles from two years of limb measurements from the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instrument onboard ENVISAT are analysed and a global climatology of stratospheric BrO is prepared. A comparison of the SCIAMACHY BrO retrievals with a set of four balloon-borne BrO profiles shows mean relative differences in the altitude range from 18 to 30 km between ?42%. The SCIAMACHY BrO observations provide for the first time a picture of the seasonal variation of stratospheric BrO on a global scale. At mid-latitudes of both hemispheres BrO shows a strong seasonal cycle with a maximum in winter and a minimum in summer. The seasonal variation of BrO is closely correlated with changes in nitrogen dioxide (NO2), confirming our present understanding of gas phase bromine chemistry. Using the SCIAMACHY BrO observations together with the calculated bromine partitioning from a photochemical model constrained by the SCIAMACHY NO2 observations, the total stratospheric bromine loading is estimated to be 18.5±4 pptv. This indicates a contribution of about 3.5±4 pptv from short lived bromine species in addition to methyl bromide and the halons

Contribution of very short-lived substances to stratospheric bromine loading: uncertainties and constraints

Very short-lived substances (VSLS) still represent a major factor of uncertainty in the quantification of stratospheric bromine loading. One of the major obstacles for short-lived source gases in contributing to the stratosphere is generally thought to be loss of inorganic bromine (Bry) in the tropical tropopause layer (TTL) due to dehydration. We use sensitivity calculations with a three-dimensional chemistry transport model comprising a consistent parametrization of convective transport and a comprehensive chemistry scheme to investigate the associated processes. The model considers the two most important bromine VSLS, bromoform (CHBr3) and dibromomethane (CH2Br2). The organic bromine source gases as well as the resulting profile of inorganic bromine in the model are consistent with available observations. In contrast to its organic precursors, Bry is assumed to have a significant sorption capacity regarding sedimenting liquid or frozen particles thus the fraction of intact source gases during their ascent through the TTL is a critical factor. We find that source gas injection is the dominant pathway into the stratosphere, about 50% of CHBr3 and 94% of CH2Br2 is able to overcome the cold point tropopause at approximately 17 km altitude, modulated by the interannual variability of the vertical transport efficiency. In fact, our sensitivity calculations indicate that the extent of source gas injection of CHBr3 is highly sensitive to the strength of convection and large-scale ascent; in contrast, modifying the photolysis or the destruction via OH yields a significantly smaller response. In principle, the same applies as well to CH2Br2, though it is considerably less responsive due to its longer lifetime. The next important aspect we identified is that the partitioning of available Bry from short-lived sources is clearly shifted away from HBr, according to our current state of knowledge the only member of the Bry family which is efficiently adsorbed on ice particles. This effect is caused by very efficient heterogeneous reactions on ice surfaces which reduce the HBr/Bry fraction below 15% at the tropical tropopause. Under these circumstances there is no significant loss of Bry due to dehydration in the model, VSLS contribute fully to stratospheric bromine. In addition, we conduct several sensitivity calculations to test the robustness of this result. If heterogeneous chemistry is ignored, the HBr/Bry fraction exceeds 50% and about 10% of bromine from VSLS is scavenged. Dehydration plays a minor role for Bry removal under the assumption that HOBr is efficiently adsorbed on ice as well since the heterogeneous reactions alter the partitioning equilibrium of Bry in favor of HOBr. In this case, up to 12% of bromine from VSLS is removed. Even in the extreme and unrealistic case that adsorbed species on ice particles are instantaneously removed the maximum loss of bromine does not exceed 25 %. Assuming 6 parts per trillion by volume (pptv) of bromine short-lived source gases in convective updrafts, a value that is supported by observational data, we find a most likely contribution of VSLS to stratospheric bromine in the range of 4.5–6 pptv

Local impact of solar variation on NO2 in the lower mesosphere and upper stratosphere from 2007 to 2012

MIPAS/ENVISAT data of nighttime NO2 volume mixing ratios (VMR) from 2007 until 2012 between 40 km and 62 km altitude are compared with the geomagnetic Ap index and solar Lyman-α radiation. The local impact of variations in geomagnetic activity and solar radiation on the VMR of NO 2 in the lower mesosphere and upper stratosphere in the Northern Hemisphere is investigated by means of superposed epoch analysis. Observations in the Northern Hemisphere show a clear 27-day period of the NO2 VMR. This is positively correlated with the geomagnetic Ap index at 60-70° N geomagnetic latitude but also partially correlated with the solar Lyman-α radiation. However, the dependency of NO2 VMR on geomagnetic activity can be distinguished from the impact of solar radiation. This indicates a direct response of NOx (NO + NO2) to geomagnetic activity, probably due to precipitating particles. The response is detected in the range between 46 km and 52 km altitude. The NO2 VMR epoch maxima due to geomagnetic activity is altitude-dependent and can reach up to 0.4 ppb, leading to mean production rates of 0.029 ppb (Ap d)-1. Observations in the Southern Hemisphere do not have the same significance due to a worse sampling of geomagnetic storm occurances. Variabilities due to solar variation occur at the same altitudes at 60-70° S geomagnetic latitude but cannot be analyzed as in the Northern Hemisphere. This is the first study showing the direct impact of electron precipitation on NOx at those altitudes in the spring/summer/autumn hemisphere. © 2014 Author(s).F. Friederich and M. Sinnhuber gratefully acknowledge funding by the Helmholtz Association of German Research Centres (HGF), grant VH-NG-624Peer Reviewe

Retrieval of nitric oxide in the mesosphere and lower thermosphere from SCIAMACHY limb spectra

We use the ultra-violet (UV) spectra in the range 230–300 nm from the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) to retrieve the nitric oxide (NO) number densities from atmospheric emissions in the gamma-bands in the mesosphere and lower thermosphere. Using 3-D ray tracing, a 2-D retrieval grid, and regularisation with respect to altitude and latitude, we retrieve a whole semi-orbit simultaneously for the altitude range from 60 to 160 km. We present details of the retrieval algorithm, first results, and initial comparisons to data from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). Our results agree on average well with MIPAS data and are in line with previously published measurements from other instruments. For the time of available measurements in 2008–2011, we achieve a vertical resolution of 5–10 km in the altitude range 70–140 km and a horizontal resolution of about 9° from 60° S–60° N. With this we have independent measurements of the NO densities in the mesosphere and lower thermosphere with approximately global coverage. This data can be further used to validate climate models or as input for them