B. Vogel

Soil-air exchange of nitric oxide: An overview of processes, environmental factors, and modeling studies

Terrestrial ecosystems with their main elements soil and plants may act, in principle, as both source and sink for atmospheric nitric oxide (NO). The net exchange between ecosystems and the atmosphere, however, is globally dominated by biogenic emissions of NO from soils. Consequently the soil–air exchange of NO is the focus of the following overview. Particular emphasis is placed on the major processes that are responsible for NO production in soils (nitrification, denitrification) and their regulation by environmental factors (nitrogen availability, soil water content, soil temperature, ambient NO concentration). It is shown that interactions of these factors are a major reason for the broad range that exists in published data on NO fluxes. This variability makes it difficult to predict the magnitude of NO fluxes on relevant spatial and temporal scales. To overcome the problem various generalization procedures for scaling up in space and time have been developed, and the potential and limitations of the different approaches is discussed.

A Particle-Surface-Area-Based Parameterization of Immersion Freezing on Desert Dust Particles

AbstractIn climate and weather models, the quantitative description of aerosol and cloud processes relies on simplified assumptions. This contributes major uncertainties to the prediction of global and regional climate change. Therefore, models need good parameterizations for heterogeneous ice nucleation by atmospheric aerosols. Here the authors present a new parameterization of immersion freezing on desert dust particles derived from a large number of experiments carried out at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud chamber facility. The parameterization is valid in the temperature range between −12° and −36°C at or above water saturation and can be used in atmospheric models that include information about the dust surface area. The new parameterization was applied to calculate distribution maps of ice nuclei during a Saharan dust event based on model results from the regional-scale model Consortium for Small-Scale Modelling–Aerosols and Reactive Trace Gases (COSMO-ART). The ...

Intercomparison of the gas-phase chemistry in several chemistry and transport models

An intercomparison of nine chemical mechanisms (e.g. ADOM, CBM-IV, EMEP, RADM2) as used by 12 contributing groups was conducted. The results for three scenarios are presented covering remote situations with a net O3 loss of around 2.7 ppb (LAND and FREE) and a moderately polluted situation with O3 formation of around 100 ppb (PLUME1) over a 5 day simulation period. The overall tendencies (i.e. the total net production/loss over 5 days) for O3 show a r.m.s. error of 38, 15 and 16%; for H2O2 the errors are 76, 23 and 30% (for LAND, FREE, PLUME1). In terms of ozone production in PLUME1, the most productive mechanisms are EMEP and IVL, the RADM-type mechanisms lie in the mid-range and the CBM-IV type mechanisms fall at the bottom of the range. The differences in H2O2 can partly be explained by an incorrect use of the HO2 + HO2 rate constant and by differences in the treatment of the peroxy radical interactions. In the PLUME1 case the r.m.s. error of the PAN tendency was found to be 29%. Differences between mechanisms for the HO radical are 10, 15 and 19% and for the NO3 radical 35, 16 and 40% (for LAND, FREE, PLUME1) in terms of the r.m.s. error of the results for a 12 h time period centred around the last noon (HO), respectively, a 8 h time period centred around the last midnight (NO3) of simulation. Especially for NO3 some differences are due to different numerical treatment of photolytic processes in the models. Large differences between mechanisms are observed for higher organic peroxides and higher aldehydes with a r.m.s. error of around 50% for the final concentration in PLUME1. The protocol of the intercomparison is given in the appendix, so that the comparison could be repeated for the purpose of mechanism development and sensitivity studies.

Influence of topography and biogenic volatile organic compounds emission in the state of Baden‐Württemberg on ozone concentrations during episodes of high air temperatures

A nonhydrostatic mesoscale model which is coupled with a transport and diffusion model and the gas phase mechanism RADM2 is used to study the influence of biogenic volatile organic compounds (VOC) emission on the ozone concentration during episodes of high air temperatures in the state of Baden-Wurttemberg, Germany. All model parts, including the determination of the biogenic VOC emissions, are run in a coupled mode. The results of the model simulations are compared with observations for a summer smog episode which occurred in August 1990. Simulations which are carried out without biogenic VOC emissions show a maximum difference in the ozone concentration of 18 ppb, while the maximum ozone values are of the order of 100 ppb. This shows that the biogenic VOC emissions play an important role when high temperatures are present in Baden-Wurttemberg. Simulations which are carried out with homogeneous terrain height show differences in the biogenic emissions caused by changes in the temperature distribution. Large differences are also found for the concentration distributions. The differences in the results which are obtained with and without biogenic VOC emissions are decreasing ; the maximum difference is about 10 ppb. This demonstrates the necessity of a sufficient treatment of topographic effects in the regional scale.

Numerical modelling of pollen dispersion on the regional scale

New parameterisations were developed to calculate the emission and resuspension flux of pollen grains. These new parameterisations were included in the comprehensive mesoscale model system KAMM/DRAIS. Two types of simulations were performed. In the first case, horizontally homogeneous meteorology was assumed. In this case, pollen concentration rapidly decreased with the distance from the source. In the second case, where the fully three-dimensional model was applied, atmospheric lifetime of the pollen grains increased remarkably. This was mainly caused by the vertical wind speeds induced by topographic effects. Consequently, the pollen grains could travel much larger distances until they were deposited and finally removed from the atmosphere. This is an important finding with respect to the problem of cross pollination. Due to the lack of measurements, a number of parameters had to be assumed. However, the parameterisation proposed may well serve as a starting point of a daily pollen forecast with numerical models.

Aspects of the convective boundary layer structure over complex terrain

Abstract Measurements are presented of the development of the convective boundary layer in the transition zone from the Upper Rhine valley to the Northern Black Forest during one special observation period of the TRACT campaign conducted in September 1992. The data used in this study were obtained from airborne instruments as well as from ground-based stations. The analysed boundary layer structure shows a strong influence of the underlying terrain. Until noon, a nearly terrain following capping inversion developed. However, advective processes proved to play an important role in the boundary layer structure over the hilly terrain. So, the large-scale air flow caused suppression of the convective boundary layer growth at the mountain ridge by forcing the capping inversion towards the elevation of the terrain. A mountain induced secondary circulation system was observed on the western facing slopes of the Black Forest. This secondary circulation system affected the heat budget and therefore the growth of the convective boundary layer over the mountain slopes. The advection of cold air by up-slope winds lowered the heating rate near the ground and was able to generate an inversion above the up-slope wind layer. In the late afternoon, the terrain following structure of the capping inversion diminished and the capping inversion tended to form a horizontal plane. Prognostic formulae for boundary layer growth are discussed for different sites of the terrain. While in the valley good agreement is found between calculated and observed boundary layer depths, the calculations for the mountain ridge overestimate the obserations by up to a factor of 5 if a boundary layer growth equation derived for homogeneous terrain is used.

Numerical simulation of birch pollen dispersion with an operational weather forecast system.

We included a parameterisation of the emissions of pollen grains into the comprehensive model system COSMO-ART. In addition, a detailed density distribution of birch trees within Switzerland was derived. Based on these new developments, we carried out numerical simulations of the dispersion of pollen grains for an episode that occurred in April 2006 over Switzerland and the adjacent regions. Since COSMO-ART is based on the operational forecast model of the German Weather Service, we are presenting a feasibility study of daily pollen forecast based on methods which have been developed during the last two decades for the treatment of anthropogenic aerosol. A comparison of the model results and very detailed pollen counts documents the current possibilities and the shortcomings of the method and gives hints for necessary improvements.

Findings on H2O2/HNO3 as an indicator of ozone sensitivity in Baden‐Württemberg, Berlin‐Brandenburg, and the Po valley based on numerical simulations

[i] The usefulness of the indicator H 2 O 2 /HNO 3 to separate NO x -sensitive and VOC-sensitive regions in the polluted planetary boundary layer on the regional scale is investigated using numerical simulations. The results of box model runs are applied to highlight individual definitions of the NO x -VOC-ozone sensitivity. In particular, different ways to determine the transition value of the indicators H 2 O 2 /HNO 3 and NO y are explained. Three-dimensional model runs are performed for two model domains in Germany and one domain in the Po valley (Italy). These domains differ significantly in the spatial distribution of the emissions, in the absolute values of the emissions, and in the composition of the VOC. The domains also differ in their meteorological conditions and hence in the involved transport processes. Nevertheless, the transition values of H 2 O 2 /HNO 3 found for the individual domains differ from 0.2 by less than 15%. This transition value is applied to an emission reduction based on the assumption of all aromatic compound emissions being switched off. It shows that this transition value and the horizontal distribution of H 2 O 2 /HNO 3 of the base case allow a forecast of those regions where ozone is affected by this emission reduction. This offers the possibility to apply the indicator H 2 O 2 /HNO 3 and the transition value of 0.2 to observations.

A model of dust transport applied to the Dead Sea Area

We applied a parameterization for the emission of mineral dust particles which takes into account the relevant processes such as saltation and combines previous, physically based parameterizations. The size distribution of the soil particles is taken into account to describe the saltation. The emitted particles are described by three log-normal distributions with fixed standard deviations. A comparison of the results of a stand alone version of our parameterization with observations shows that despite tuning of model parameters there are still differences. Finally, we included the parameterization within our three-dimensional mesoscale model system for the area of the Dead Sea. The channelling effect of the Jordan Valley and stable stratification during the day modifies the horizontal distribution of the dust particles. At greater distances the size and the mass distributions of the particles is shifted towards smaller diameters due to sedimentation which is important for radiative feedback mechanisms. Sensitivity runs show the advantage of the parameterization which allows a time dependent ratio of the saltation and the emission flux at each grid point.

Comparison of measured and calculated motorway emission data

Abstract A comprehensive field campaign was carried out to check the quality of modelled on-road emissions of NOx and CO by comparing them with real-world emissions on a motorway in the southwestern part of Germany. Three different sets of emission data were determined for a section of the motorway A 656 that connects Mannheim and Heidelberg. First, emission data were pre-calculated, that means before the field campaign took place, using literature data for traffic volume, types of vehicles and driving speeds. Second, detailed traffic measurements during two intensive measurement phases were used to calculate improved emission data based on the actual traffic situation. Third, real-world emission data were determined by meteorological and chemical measurements. It shows that the differences of the pre-calculated emission data and the emission data improved by the traffic measurements differ by less than 15% in case of CO and approx. by 35% in case of NOx. A comparison of the observed emissions with the calculated ones gives an agreement for NOx within the error bars. For CO, however, a discrepancy by a factor of two was found. The real-world emissions were much higher than calculated.