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

Modelling the Antarctic lower stratosphere

Results form modeling studies of the Antarctic lower stratosphere which have attempted to simulate the large springtime ozone losses and corresponding changes in other trace constituents are given. These studies were carried out in a photochemical box model, a one-dimensional model without transport and in a two-dimensional photochemical-dynamical-radiation model. The photochemical studies have investigated inter alia the sensitivity of ozone to inclusion in the model of heterogeneous chemistry, and to the inclusion of the ClO dimer. When both of these are incorporated in the model, ozone depletions resembling whose found in Halley Bay in 1987 (J.C. Farman, Nature, 329, 1987) can be reproduced. The temporal variations (both diurnal and during the August to October period) of a number of important tracers including HCl, ClONO2, OClO and BrO are discussed. The two-dimensional study concentrated on the difficulty of establishing in the model the dynamical preconditioning of the lower polar stratosphere - low temperatures, low N2O, etc., high ClOx. Calculations are presented to show: (1) the depletion of ozone during the springtime season, (2) the effect of large ozone losses on lower latitudes, and (3) the longer term (multi-year) variations of ozone in Antarctica, assuming realistic increases in the atmospheric halogen burden

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

Mid-latitude ozone changes: studies with a 3-D CTM forced by ERA-40 analyses

International audienceWe have used an off-line three-dimensional (3-D) chemical transport model (CTM) to study long-term changes in stratospheric O3. The model was run from 1977?2004 and forced by ECMWF ERA-40 and operational analyses. Model runs were performed to examine the impact of increasing halogens and additional stratospheric bromine from short-lived source gases. The analyses capture much of the observed interannual variability in column ozone, but there are also unrealistic features. In particular the ERA-40 analyses cause a large positive anomaly in northern hemisphere (NH) column O3 in the late 1980s. Also, the change from ERA-40 to operational winds at the start of 2002 introduces abrupt changes in some model fields which affect analysis of trends. The model reproduces the observed column increase in NH mid-latitudes from the mid 1990s. Analysis of a run with fixed halogens shows that this increase is not due to a significant decrease in halogen-induced loss, i.e. is not an indication of recovery. The model predicts only a small decrease in halogen-induced loss after 1999. In the upper stratosphere, despite the modelled turnover of chlorine around 1999, O3 does not increase to the effects of increasing ECMWF temperatures, decreasing modelled CH4 at this altitude, and abrupt changes to the SH temperatures at the end of the ERA-40 period. The impact of an additional 5 pptv stratospheric bromine from short-lived species decreases mid-latitude column O3 by about 10 DU. However, the impact on the modelled relative O3 anomaly is generally small except during periods of large volcanic loading