Benedikt Steil

The impact of greenhouse gases and halogenated species on future solar UV radiation doses

The future development of stratospheric ozone layer depends on the concentration of chlorine and bromine containing species. The stratosphere is also expected to be affected by future enhanced concentrations of greenhouse gases. These result in a cooling of the winter polar stratosphere and to more stable polar vortices which leads to enhanced chemical depletion and reduced transport of ozone into high latitudes. One of the driving forces behind the interest in stratospheric ozone is the impact of ozone on solar UV-B radiation. In this study UV scenarios have been constructed based on ozone predictions from the chemistry-climate model runs carried out by GISS, UKMO and DLR. Since cloudiness, albedo and terrain height are also important factors, climatological values of these quantities are taken into account in the UV calculations. Relative to 1979–92 conditions, for the 2010–2020 time period the GISS model results indicate a springtime enhancement of erythemal UV doses of up to 90% in the 60–90 °N region and an enhancement of 100% in the 60–90 °S region. The corresponding maximum increases in the annual Northern Hemispheric UV doses are estimated to be 14% in 2010–20, and 2% in 2040–50. In the Southern Hemisphere 40% enhancement is expected during 2010–20 and 27% during 2040–50.

The summer circulation over the eastern Mediterranean and the Middle East: influence of the South Asian monsoon

The summer circulation over the eastern Mediterranean and the Middle East (EMME) is dominated by persistent northerly winds (Etesians) whose ventilating effect counteracts the adiabatic warming induced by large scale subsidence. The ERA40 dataset is used to study the vertical distribution of these circulation features, which both appear to be reconciled manifestations of the South Asian monsoon influence. As predicted by past idealized modeling studies, in late spring a westward expanding upper level warm structure and subsidence areas are associated with Rossby waves excited by the monsoon convection. Steep sloping isentropes that develop over the EMME facilitate further subsidence on the western and northern periphery of the warm structure, which is exposed to the midlatitude westerlies. The northerly flow and descent over the eastern Mediterranean have maxima in July that are strikingly synchronous to the monsoon convection over northern India, where the weaker easterly jet favors a stronger Rossby wave response and consequent impact on the EMME circulation. The pronounced EMME topography modifies the monsoon induced structure, firstly, by inducing orographically locked summer anticyclones. These enhance the mid and low level northwesterly flow at their eastern flanks, leading to distinct subsidence maxima over the eastern Mediterranean and Iran. Secondly, topography amplifies the subsidence and the northerly flow over the Aegean, Red Sea, the Iraq—Gulf region and to the east of the Caspian Sea.

Chemical effects in 11-year solar cycle simulations with the freie universitat Berlin climate middle atmosphere model with online chemistry (FUB-CMAM-CHEM)

The impact of 11-year solar cycle variations on stratospheric ozone (O3) is studied with the Freie Universitat Berlin Climate Middle Atmosphere Model with interactive chemistry (FUB-CMAM-CHEM). To consider the effect of variations in charged particle precipitation we included an idealized NO x source in the upper mesosphere representing relativistic electron precipitation (REP). Our results suggest that the NO x source by particles and its transport from the mesosphere to the stratosphere in the polar vortex are important for the solar signal in stratospheric O3. We find a positive dipole O3 signal in the annual mean, peaking at 40–45 km at high latitudes and a negative O3 signal in the tropical lower stratosphere. This is similar to observations, but enhanced due to the idealized NO x source and at a lower altitude compared to the observed minimum. Our results imply that this negative O3 signal arises partly via chemical effects.

Impact of aircraft NOx emissions on tropospheric and stratospheric ozone. Part II: 3-D model results

Abstract The global three-dimensional dynamic-chemical model ECHAM3/CHEM is employed to estimate the impact of present and future sub- and supersonic aircraft NOx emissions on ozone. Multinual simulations are performed to investigate changes of large-scale climatological features. An increase of tropospheric ozone of 3–4% is found in the Northern Hemisphere for a present day scenario, independently from season. No indications for significant ozone changes are indicated in the lower stratosphere. NOx emissions expected for the year 2015 by sub- and supersonic (500 civil aircraft, Mach 2.4, EINOx=15) air traffic yield much larger ozone changes in the model. An ozone increase of approximately 15% is predicted near the cruise altitude of future subsonic air traffic. This is accompanied by a clear ozone reduction of 3–4% in the lower stratosphere caused by supersonic aircraft. A stronger decrease of ozone is detected in polar winter due to the impact of heterogeneous reactions on polar stratospheric clouds.

Chemical reaction pathways affecting stratospheric and mesospheric ozone

Catalytic cycles and other chemical pathways affecting ozone are normally estimated empirically in atmospheric models. In this work we have automatically quantified such processes by applying a newly developed analysis package called the Pathway Analysis Program (PAP). It used modeled chemical rates and concentrations as input. These were supplied by the Module Efficiently Calculating the Chemistry of the Atmosphere MECCA box model, itself initialized by the Free University of Berlin Climate Middle Atmosphere Model with Chemistry. We analyzed equatorial, midlatitude and high-latitude locations over 24-hour periods during spring in both hemispheres. We present results for locations in the lower stratosphere, upper stratosphere and midmesosphere. Oxygen photolysis dominated (>99%) in situ ozone production in the equatorial lower stratosphere, in the upper stratosphere and in the mesosphere. In the lower stratosphere midlatitudes the ozone smog cycle (already established in the troposphere) rivaled oxygen photolysis as an in situ ozone source in both hemispheres. However, absolute ozone production rates in midlatitudes were rather slow compared with at the equator, typically 1650 ppt ozone/day. In the equatorial lower stratosphere, five catalytic sinks were important (each contributing at least 5% to chemical ozone loss): a HOx cycle, a HOBr cycle and its HOCl analog, a water-HOx cycle and a mixed chlorine-bromine cycle. Important in midlatitudes were the HOx cycle, a NOx cycle, the HOBr cycle and the mixed chlorine-bromine cycle. In lower-stratosphere high latitudes, the chlorine dimer cycle and the mixed chlorine-bromine cycle dominated in both hemispheres. A variant on the latter, involving BrCl formation, also featured. In the upper stratosphere high latitudes (where strong negative ozone trends are observed), a nitrogen cycle, a chlorine cycle, and a mixed chlorine-nitrogen cycle were found. In the mesosphere, three closely related HOx cycles dominated ozone loss.

Acetone and PAN in the upper troposphere: impact on ozone production from aircraft emissions

Abstract The role of acetone and PAN in the chemistry of the upper troposphere has been investigated with a time-dependent photochemical box model. The results show that acetone strongly enhances aircraft-related ozone production, as exemplified in this study over North Atlantic during summer. For typical flight corridor conditions, the inclusion of about 1 nmol mol −1 acetone more than doubles net ozone production by aircraft NO emissions and resolves discrepancies between observations and several global model predictions. The actual magnitude of the acetone-attributed enhancement of ozone production can, however, be reduced by PAN formation which acts as a sink for NOx (NO+NO2) and also lowers the OH/HO2 ratio.

Intercomparison of Stratospheric Chemistry Models under Polar Vortex Conditions

Several stratospheric chemistry modules from box, 2-D or 3-D models, have been intercompared. The intercomparison was focused on the ozone loss and associated reactive species under the conditions found in the cold, wintertime Arctic and Antarctic vortices. Comparisons of both gas phase and heterogeneous chemistry modules show excellent agreement between the models under constrained conditions for photolysis and the microphysics of polar stratospheric clouds. While the mean integral ozone loss ranges from 4–80% for different 30–50 days long air parcel trajectories, the mean scatter of model results around these values is only about ±1.5%. In a case study, where the models employed their standard photolysis and microphysical schemes, the variation around the mean percentage ozone loss increases to about ±7%. This increased scatter of model results is mainly due to the different treatment of the PSC microphysics and heterogeneous chemistry in the models, whereby the most unrealistic assumptions about PSC processes consequently lead to the least representative ozone chemistry. Furthermore, for this case study the model results for the ozone mixing ratios at different altitudes were compared with a measured ozone profile to investigate the extent to which models reproduce the stratospheric ozone losses. It was found that mainly in the height range of strong ozone depletion all models underestimate the ozone loss by about a factor of two. This finding corroborates earlier studies and implies a general deficiency in our understanding of the stratospheric ozone loss chemistry rather than a specific problem related to a particular model simulation.

Aerosol Chemistry Interactions After the Mt. Pinatubo Eruption

A coupled chemistry climate model is used to study the 1991 eruption of Mt. Pinatubo, specifically the highly nonlinear interactions with atmospheric chemistry, radiation and dynamics. Both volcanic aerosol and O 3 are treated as prognostic variables and are coupled with the radiation scheme. With a bulk approach and a simple parameterization for the aerosol size distribution our model is able to reproduce the broad features of the atmospheric effects after the Pinatubo eruption. The simulated aerosol mode radius is overestimated during the first months after the eruption leading to strong perturbations in the modeled heating rates and enhanced downward transport. The evolution of simulated aerosol surface area density is found to agree rather well with observations from the Northern Hemisphere midlatitudes. Discrepancies appear to be related to inaccuracies in the simulated long range transport and to numerical vertical diffusion in the model. The simulated changes in the atmospheric trace gas concentration are reasonable. The modifications in the chemical concentration due to volcanic aerosol are a combined effect of changes in the heterogeneous chemistry, in the photolysis rates and in the heating rates. Column ozone decreases in the tropics by about 2%-3% in autumn 1991 which agrees with the observations. The strong ozone decrease in polar winter is also reproduced by the model. For a more realistic determination of the atmospheric effects of the Pinatubo aerosol, microphysical processes of the formation and the development of stratospheric aerosol must be considered.