Sabine Wurzler

Modification of mineral dust particles by cloud processing and subsequent effects on drop size distributions

Dust particles coated with soluble materials, such as sulfate, are frequently observed in the Mediterranean. Thus far, the processes responsible for the sulfate coating of dust particles have still not been identified. One possible explanation is that the formation of the sulfate-coated aerosols is related to cloud processing of dust particles. In this process the scavenging of aerosol particles and gases, such as SO2, O3, and H2O2, by the droplets and the subsequent impaction scavenging of mineral dust particles followed by evaporation could release into the atmosphere dust particles coated with soluble materials. These modified particles can serve as giant cloud condensation nuclei and thus can have a significant impact on the microphysical development of other clouds. Using an air parcel model with detailed microphysics, it is shown that cloud processing of dust particles is a possible effective pathway to form soluble coatings on these particles. Furthermore, the simulations show that after one or two cycles of particles through convective clouds the contribution of gas uptake by drops and subsequent liquid phase oxidation add considerable mass of soluble material to particles in the size range of 0.05 μm. On the other hand, this process adds about 1 order of magnitude less mass to the larger particles as compared to the contribution made by coagulation of drops containing soluble aerosols.

The water‐soluble fraction of atmospheric aerosol particles and its influence on cloud microphysics

The water-soluble fraction of atmospheric aerosol particles is a major property dividing the particle population into cloud condensation nuclei and interstitial particles during cloud formation. Likewise, this property influences cloud microphysics and chemistry as well as direct and indirect aerosol forcing of climate. Up to now this important parameter has been only poorly determined experimentally. Here, we present SoFA, a new method to determine the water-Soluble Fraction of large and giant Aerosol particles in five narrow size bands with geometrical radii of 0.4, 0.6, 0.9, 1.3, 1.8 μm and one band for particles with radius larger than 2.3 μm. First results show three different types of aerosol particles (AP I, II, III). AP I were characterized by 9% water-soluble material, and AP II by 50% and correspond to those found in earlier studies for small particles. The new AP III consists of 88% water-soluble material. In those size bands, where all three particle types could be detected (0.9, 1.3, and 1.8 μm; because of detection limits of SoFA, only two types of particles could be detected in the remaining size bands), about 50% of the total number of analyzed particles belong to AP III, whereas about 25% belong to both AP II and I. These numbers lead to 59% water-soluble material on average. Particles of AP III are assumed to be cloud-processed particles, those of AP I might be of biological origin. The impact of the water-soluble fraction of particles on cloud microphysics is investigated by comparing two cloud situations using an entraining air parcel model with detailed microphysics. The model simulations show an influence on nucleation scavenging, cloud interstitial aerosol, and in particular, drop size population. Most notable is that the same aerosol particle number distribution causes either a precipitating or a non precipitating cloud, depending only upon different water-soluble fractions of the particles.

Cloud processing of continental aerosol particles: Experimental investigations for different drop sizes

Atmospheric aerosol particles are activated and grow into drops during the formation of a cloud. Subsequently, they are delivered from the dissipating cloud drops back to the atmosphere. During the cloud lifetime, the drops scavenge water-soluble trace gases, leading to an increase in size and solubility of the particles emerging from the evaporating cloud drops. This processing of aerosol particles by clouds has an influence on the microphysics of the following cloud and its probability to rain as well as on the cooling effect of the direct and indirect aerosol forcing of climate. To measure the cycling history (particle activation and gas scavenging in drops followed by processing of the activated particles followed by emerging processed particles) of continental aerosol particles passing through cloud drops of different sizes, a new method is developed and applied during three field experiments carried out on the Mount Kleiner Feldberg/Ts., Germany, in 1990, 1993, and 1995. The typical droplet spectra of most of the observed stratus clouds is weakly bimodal, with mode 1 at drop sizes between 3 and 5 μm and mode 2 between 5 and 10 μm radius. Cloud drops in this overall size range are subject to growth only by condensation, while coalescence can be neglected. Therefore the observed processing is related solely to gas scavenging and in-cloud chemical reactions. We found that the processing of particles is different for the two modes of the cloud drop size spectrum: Small activated particles mostly grow to the small drops of mode 1, while larger particles can grow further to the larger drops of mode 2. Likewise, the mass of scavenged gas is, on average, lower for the small than for the larger drops. Vice versa, the ratio of scavenged gas to particle mass, the parameter quantifying the particle processing, is, on average, found to be higher in the small drop mode containing the smaller particles. The reason is that the degree of processing is mainly inversely linked to the mass of the activated particles. Therefore the strongest modification of particles takes place in smaller drops and affects mainly the smaller activated particles (r ap ≤ 0.1 μm). Their radii can increase by up to a factor of 3 and, consequently, their nucleation as well as radiative properties change significantly. The consequence for the aerosol climate forcing is that the cooling can be, to an unknown extent, intensified with increasing atmospheric amount of water-soluble trace gases such as HNO3, NH3, and SO2, counteracting the warming effect of the greenhouse gases.

The scavenging of nitrogen compounds by clouds and precipitation:: Part II. The effects of cloud microphysical parameterization on model predictions of nitric acid scavenging by clouds

Model predictions of nitric acid scavenging by an air parcel model and a two-dimensional dynamic cloud model with detailed microphysics are compared with corresponding predictions for parameterized microphysics for the same initial conditions. In general the gas uptake proceeds slower in the model with parameterized microphysics than in the model with detailed microphysics and, the larger the drop size, results in a slower mass transfer. This behaviour can be explained by the inverse drop-radius dependency of the mass transfer coefficient and the time delays were a direct result of the microphysical parameterization. With the air parcel model for the scavenging of nitric acid, time delays in the range of several seconds to several minutes for the model with parameterized microphysics have been found in comparison with the predictions of the model with detailed microphysics. This may not seem significant. However it has an impact on the scavenging behaviour of rapidly developing clouds, as the results from the two-dimensional model show. Here the parameterized solution predicted approximately a 50% lower rainfall rate, scavenging efficiency and wet-deposition efficiency of nitric acid than the detailed model. The results of the present study suggest that parameterized cloud models tend to underestimate the scavenging of gases and their wet deposition.

Implementation of a Numerical Solution of the Multicomponent Kinetic Collection Equation (MKCE) on Parallel Computers

Two different numerical solutions of the two-component kinetic collection equation were implemented on parallel computers. The parallelization approach included domain decomposition and MPI commands for communications. Four different parallel codes were tested. A dynamic decomposition based on an occupancy function provided the optimum balance between time performance and flexibility for any number of processors. The occupancy function was defined according to the number of calculations required at each grid point in the domain. Speed-up performance depended very much on the parallel code used and in some cases very good results were obtained for up to 32 processors.

Effects of electric vehicles on air quality in street canyons

Road traffic is one of the main causes of poor air quality in European cities. Electric vehicles (EV) are ofte n presented as solution for air quality problems in c ities. In addition they are claimed to be a climate friendly means of transport owing to their non-existent exhaust emissions. The German government has formulated the ambitious goal to increase the amount of electric vehicles in Germany to 1 million in 2020 and 6 mill ion in 2030. The aim of this paper is to investigate ho w much of this claim is true. While traffic is a majo r source of air pollutants in street canyons, it is often fo rg tten that electricity production nowadays is a significa nt source of air pollutants on the regional scale. Focus of the present study is the air quality in street canyons, to be more precise the PM10 and NO 2 concentrations. We concentrate our investigation on road traffic, taking the fleet composition into account. We have investigated the following questions: Is the goal of the German government effective wi th regard to an improvement of the air quality? What share of electric vehicles would be needed t o a) significantly reduce the annual immission load (5 μg/m3 for NO2 and 3 μg/m3 for PM10)? b) comply with the EC limit values? Which vehicle type is most promising with regard to an efficient immission reduction? E.g., passenger cars ? Busses? Heavy duty vehicles? What is the impact of the reduced traffic emissio n n the regional background concentration of PM10 and NO2? How much electricity is consumed by the vehicles? What is the resulting effect on power plant emissi on ? We present results for two street canyons in Germany with moderate EC air quality limit value exceedances: Gladbecker Street in Essen, and Friedr ichEbert-Street in Mönchengladbach. The air quality analyses were carried out using the canyon plume bo x model Immis for the contribution of local traffic, combined with field observations of air quality and model results of the chemical transport model EURAD (Memmesheimer et al., 2004). The change in power plant emissions was estimated based on the emission inventory of LANUV NRW. The energy consumption by the vehicles was estimated based on values from the recent literature, such as product information (Schöllnhammer, 2012). As already mentioned the German government wants to increase the amount of electric vehicles i n Germany. An example of the effects of this program on the air quality is shown in Table 1 for the annual average PM10 concentration. Reference year is 2009. We have assumed that the replacement of vehicles with combustion engines by electric vehicles would happe n instantaneously. The proportion of electric vehicle s in our street canyon was assumed to be the same as for the vehicle fleet in Germany. The findings are rather daunting for PM10, the results for NO 2 are somewhat better. The good news for these scenarios is that t he effect of the increased electricity production on t he regional air quality is leveled out by the emission reduction by the vehicle fleet on the regional scal e.