Bärbel Langmann

Volcanic sulfur emissions: Estimates of source strength and its contribution to the global sulfate distribution

Anthropogenic emission of SO 2 and conversion into SO 4 2- is argued to be the most important factor damping and modulating the global greenhouse effect. Recent estimates of the relative strength of the three important sources of volatile sulfur (SO 2 from fossil fuel combustion ∼78 Tg S/yr, from biomass burning ∼2 Tg S/yr, and from natural sources ∼25 Tg S/yr) suggest an overwhelming effect of the anthropogenic emissions for climate forcing. However, the radiatively relevant product SO 4 2- may have different patterns due to the distribution of the sources (some very dense areas near the surface for anthropogenic SO 2 , formation of SO 2 from dimethylsulfide in the marine boundary layer, and emission of volcanic SO 2 mostly in the free atmosphere in rural areas). In this paper we study the relative contribution of volcanic SO 2 emissions to the atmospheric sulfur budget applying an atmospheric general circulation model including a full sulfur cycle and prescribed source distributions. An off-line analysis tool is applied to determine the radiative forcing of sulfate aerosols. The results show that natural S sources are at least as important as the anthropogenic ones, even though their source strength is much smaller. The reasons are different lifetimes due to different production and emission processes. Therefore, we should improve our knowledge about the volcanic volatile sources and their time-space variability.

Numerical modelling of regional scale transport and photochemistry directly together with meteorological processes.

Abstract A three-dimensional regional scale atmosphere-chemistry model has been developed to contribute to an improved understanding of atmospheric photochemical processes. This on-line model determines meteorological processes directly together with tracer transport and photochemistry. Usually, off-line chemistry-transport models are applied which use archived data from a meteorological model as input information. However, a number of disadvantages result from the seperation of meteorological and photochemical modelling: only a part of the whole meteorological information is available in the sampled data sets and only in distinct time intervals. The availability of the whole meteorological information including subgrid scale dynamical motions like turbulence and moist convection at every model time step is the main advantage of the on-line procedure. In addition, on-line modelling allows to consider chemical–dynamical feedbacks which is not possible in the off-line mode. For validation and evaluation studies a 10-days simulation of a summersmog episode over Europe in July 1994 has been carried out with the first version of the on-line model. The results are compared with observations and with an off-line model simulation. The on-line model is able to reproduce measured near-surface concentrations in much better agreement than the off-line model does. The reason is an improved representation of tracer transport in convective clouds in the on-line model. It is responsible for the upward mixing of short-living precursor substances of photooxidants from the planetary boundary layer into the free troposphere, with a significant impact on photooxidants concentrations in the planetary boundary layer as well as in the free troposphere.

Simulation of stable water isotopes in precipitation over South america: Comparing regional to global circulation models

Abstract A simulation of the stable water isotope cycle over South America by the regional circulation model REMOiso is discussed. The performance of the regional model, with a resolution of 0.5° (∼55 km), is compared to simulations by the global circulation model ECHAMiso at two coarser resolutions and evaluated against observations of precipitation and δ18O. Here REMOiso is demonstrated to reproduce reasonably well climatic and isotopic features across South America. This paper explores further insights of δ18O as a climate proxy, based on REMOiso’s improvements as compared to ECHAMiso. In particular, the authors focus on the seasonal variation of the amount effect (δ18O decrease with precipitation amounts) and the anomalous δ18O continental gradient across the Amazon basin, as inferred from the REMOiso, ECHAMiso, and GNIP datasets. The finer resolution of topography in REMOiso enables a detailed analysis of the altitude effect: not only the first, but also the second derivative of δ18O with altitude is...

The contribution of Earth degassing to the atmospheric sulfur budget

Abstract Volcanic gases have contributed to the development of the Earths atmosphere to a large extent. Today, the sulfur budget of the atmosphere is still much influenced by volcanic degassing. Even though the total flux of sulfur gases from quiescent and small eruptive volcanic sources is small compared with anthropogenic release, the geographical distribution and the height of the main sources lead to comparable effects in total atmospheric sulfur budget and also for the important radiative effects. In the upper troposphere the volcanic sources even determine the sulfur content of the atmosphere. When estimated volcanic sulfur emissions are processed and transported in an atmosphere chemistry–climate model, where the contribution of different sources is trace marked, one can estimate the relative contribution of the emittents to total sulfate aerosol radiative forcing. In the global mean the volcanic (−0.15 W/m 2 ) is comparable with the anthropogenic (−0.17 W/m 2 ) direct backscatter effect of sulfate aerosol. Only in the Northern hemisphere winter, when anthropogenic emissions have their maximum, do they have a stronger (−0.25 W/m 2 ) effect than volcanic contributions (−0.15 W/m 2 ).

Indonesian smoke aerosols from peat fires and the contribution from volcanic sulfur emissions

The island arc volcanoes in and around Indonesia have been permanently degassing for thousands of years, thereby contributing significantly to the total emissions of sulfur species in that region. The hot and wet tropical weather conditions with high solar irradiation and regular daily precipitation during the wet season lead to efficient removal of oxidised sulfate by wet deposition. This is accumulated in the Indonesian peat areas, which serve as natural sponges, soaking up rain during the wet season and slowly releasing moisture into the atmosphere during the dry season. When peat forests are drained for land clearing purposes, the peat quickly dries out and becomes extremely flammable. When ignited, the composition of the burning peat mainly determines the fire aerosol chemical composition and microphysical properties. In this paper we investigate the contribution of volcanic sulfur emissions to wet deposition of sulfur in Indonesian peat swamp areas based on numerical simulations carried out with a global atmospheric circulation model including the tropospheric sulfur cycle. Our study suggests that the observed hygroscopicity and elevated sulfur content of the Indonesian peat fire aerosols is due to accumulated volcanic sulfur.

Radiative forcing of climate by sulfate aerosols as determined by a regional circulation chemistry transport model

Abstract H 2 SO 4 /SO 2- 4 aerosols have the potential to modify the radiation budget of the atmosphere. Under clear-sky condition they scatter solar radiation back to space, reducing absorption of solar irradiance (direct effect). The capability of sulfate particles to act as cloud condensation nuclei, thus influencing cloud droplet number concentration, cloud albedo and the development of precipitation is referred to as indirect effect. Evidence has been presented that sulfate aerosol climate forcing is sufficiently large to reduce significantly the positive forcing by anthropogenic greenhouse gases regionally, especially in the Northern Hemisphere. Until now, only coarse grid global models with rather simple chemistry modules have been applied to estimate the radiative forcing of sulfate aerosols. In this paper we would like to ascertain the short wave sulfate forcing over Europe, one of the main anthropogenic source regions. For this purpose the three-dimensional European sulfate distribution was generated by a regional climate model in combination with a complex chemistry transport model. Then a computationally efficient radiation transfer model was applied. It estimates the direct and indirect short wave forcing of sulfate aerosols on the basis of the variable sulfate mass distribution and meteorological input data. For comparison coarse grid global model results will be shown. Regional and global model results are comparable concerning the distribution and amount of sulfate burden and radiative forcing over Europe. Hence, for the estimation of the European budgets, the global model produces sufficiently precise information. As expected, the advantage of the higher resolution regional model is to show smaller scale phenomena, which cannot be resolved by the global model. Different predictions of the amount of clouds by the regional and global model modify the forcing significantly, emphazising the role of clouds in estimating the direct (and, of course, the indirect) short wave forcing of sulfate aerosols. It is further interesting to note that in the global model results about 30% of the sulfate burden and its short wave radiative forcing over Europe, one of the main anthropogenic source regions, is caused by sulfate from natural sources (DMS and volcanoes). Thus, the long-range transport of primary and secondary pollutants from outside the regional model domain contributes significantly to the limited area model’s atmospheric load. Therefore, the initial and boundary chemical composition of the atmosphere for the limited area model should be investigated in more detail in the future.

The influence of the global photochemical composition of the troposphere on European summer smog, Part I: Application of a global to mesoscale model chain

Elevated mixing ratios of ozone in the lower troposphere are a major summer time air pollution issue in Europe. Photochemical in-situ production is the most important source of ozone in the planetary boundary layer and has been studied extensively. However, the contributions of background ozone due to stratospheric intrusions, lightning nitrogen oxide followed by ozone production, convective mixing and intercontinental transport are still poorly quantified. We analyze in this paper the influence of the large-scale ozone background on near-surface ozone throughout a summer smog period in July 1994 over Europe. For this purpose a chain of global to mesoscale models is applied with a nesting procedure coupling the individual model simulations. It is found that background ozone as determined by the global model dominates the results of the higher resolution limited area models increasingly with height. But improvements of limited area model results are not only restricted to the free troposphere. Strong convective events like thunderstorms couple free tropospheric and planetary boundary layer air masses so that ozone from above is injected into the planetary boundary layer contributing an amount of 5–10 ppbv to nearsurface ozone in the afternoon hours. A decrease in the same range of 5–10 ppbv in maximum near-surface ozone over Central Europe is found in a model simulation where European anthropogenic emissions are reduced by 25%, an amount equal to the reported emission trends in Germany from 1994 to 2000. We conclude that intercontinental transport of pollution can obscure the results of local efforts to reduce critical exposure levels of ozone.

The chemistry of the polluted atmosphere over Europe: Simulations and sensitivity studies with a regional chemistry-transport-model

Abstract A model environment has been established, which allows an estimation of the influence of global climate change on the chemistry of the polluted atmosphere over Europe. For this purpose the regional chemistry-transport-model of the EURAD-system has been modified and made adaptable for input data from a regional climate model, which is nested in a global atmospheric circulation model. Thus, the dynamical aspect of a possible global temperature increase as well as enhanced water vapour concentrations and background concentrations of carbon monoxide and methane can be considered. By substituting the meteorological driver model the main problems arise from different vertical grids and physical parameterization schemes. In particular, the parameterization of cloud processes has to be checked to avoid inconsistencies between the chemistry-transport-model and its meteorological driver model. As the length of a simulation period is mainly limited by the large amount of computer time spent for the determination of chemical transformation rates, the gas-phase chemistry module has been optimized concerning computer time and numerical stability. For validation studies of the new model system two episodic simulations were investigated, one for a summer photo-oxidants period in July 1990, the other one for January 1991.

Aerosol modeling over Europe: 2. Interannual variability of aerosol shortwave direct radiative forcing

[1] Aerosol distribution over Europe and its direct radiative forcing have been simulated with a regional atmosphere-chemistry model and an off-line radiation transfer model. Primary and secondary organic and inorganic aerosols have been considered. The simulation was conducted for meteorologically different years 2002 and 2003 to analyze the spatial and temporal variability of the aerosol distribution and the direct forcing. The accompanying paper focuses on the aerosol distribution, while radiative forcing is discussed in this paper. The mixing state of aerosols, externally or internally, is shown to influence the strength, regional distribution and sign of radiative forcing, thereby regulating the forcing efficiency. Positive top-of-the-atmosphere forcing was simulated over eastern and southeastern Europe in spring and winter because of contribution of black carbon. Its strength varies from +0.2 to + 1 W m -2 , depending on aerosol mixing assumptions. Sensitivity studies show a mean European direct forcing of -0.3 W m -2 in winter and -2.5 W m -2 in summer, regionally ranging from -5 to + 4 W m -2 .

On the Importance of Reliable Background Concentrations of Ozone for Regional Scale Photochemical Modelling

Ozone throughout the troposphere is subject of significant temporal and spatial variability due to photochemical production in the planetary boundary layer and free troposphere, stratospheric intrusions, convective events and long range transport. However, high resolving observations of ozone in the troposphere are generally rare today. That is of special disadvantage for limited area models, which represent mathematically a differential equation system with an initial and boundary problem. As ozone concentrations usually increase from the earth surface to the stratosphere, a proper choice of the background ozone concentrations is necessary to reproduce or even predict the amount and distribution of ozone in a specific region of interest.