Andrea I. Flossmann

A Theoretical Study of the Wet Removal of Atmospheric Pollutants. Part I: The Redistribution of Aerosol Particles Captured through Nucleation and Impaction Scavenging by Growing Cloud Drops

Abstract A theoretical model is formulated which allows the processes that control the wet deposition of atmospheric pollutants to be included in cloud dynamic models. The model considers the condensation process and the collision-coalescence process which, coupled together, control the fate of atmospheric aerosol particles removed by clouds and precipitation through nucleation scavenging and impaction scavenging. The model was tested by substituting a simple parcel model for the dynamic framework. In this form the model was used to determine the time evolution of the aerosol particle mass scavenged by drops as well as the aerosol particle mass left unactivated in air as “drop-interstitial” aerosol. In the present computation all aerosol particles are assumed to have the same composition. Our study shows for inside cloud scavenging: 1) collision and coalescencence causes among the various drop size categories a redistribution of the scavenged aerosol particles in such a manner that the main aerosol partic...

The Kleiner Feldberg Cloud Experiment 1990. An overview

An overview is given of the Kleiner Feldberg cloud experiment performed from 27 October until 13 November 1990. The experiment was carried out by numerous European research groups as a joint effort within the EUROTRAC-GCE project in order to study the interaction of cloud droplets with atmospheric trace constituents. After a description of the observational site and the measurements which were performed, the general cloud formation mechanisms encountered during the experiment are discussed. Special attention is given here to the process of moist adiabatic lifting. Furthermore, an overview is given regarding the pollutant levels in the gas phase, the particulate and the liquid phase, and some major findings are presented with respect to the experimental objectives. Finally, a first comparison attempts to put the results obtained during this campaign into perspective with the previous GCE field campaign in the Po Valley.

A 2-D spectral model simulation of the scavenging of gaseous and particulate sulfate by a warm marine cloud

Our 2-D dynamic model including spectral microphysics and a detailed treatment of the scavenging and processing of (NH 4 ) 2 SO 4 and NaCl aerosol particles as well as SO 2 , H 2 O 2 , and O 3 has been evaluated for a warm precipitating convective cloud at Day 261 (September 18, 1974) of the GATE campaign. The model determined the rate at which S(IV) was converted to S(VI) inside the cloud and rain water and the rate at which its pH changed. The rate of drop sulfate formation due to scavenging of particles was compared with the rate of sulfate formation due to the uptake and oxidation of SO 2 . It was found that of the sulfate processed by the cloud 87% was scavenged by nucleation of sulfate containing aerosol particles, 6% of sulfate entered the drop through impaction scavenging below cloud base, almost 7% was produced by oxidation by H 2 O 2 , but only 0.2% by oxidation of O 3 . Averaging over space and time we derived from the model that about 2 mg/l in the rain was due to particulate sulfate and 1 mg/l was due to sulfate from oxidized SO 2 in the drops. Oxidation of SO 2 was accompanied by an average pH of 4.4 in the rain water. For sulfur a washout ratio of 600 and a scavenging coefficient of about 10 −4 s −1 was calcu lated. In addition, a comparison of the aerosol particle and CCN spectrum at one point in the atmosphere before and after the cloud event showed that the size of the aerosol particles had increased and their number decreased such that the new CCN spectrum activates the particles at lower supersaturations

Phase partitioning of aerosol particles in clouds at Kleiner Feldberg

The partitioning of aerosol particles between cloud droplets and interstitial air by number and volume was determined both in terms of an integral value and as a function of size for clouds on Mt. Kleiner Feldberg (825 m asl), in the Taunus Mountains north-west of Frankfurt am Main, Germany. Differences in the integral values and the size dependent partitioning between two periods during the campaign were observed. Higher number and volume concentrations of aerosol particles in the accumulation mode were observed during Period II compared to Period I. In Period I on average 87±11% (±one standard deviation) and 73±7% of the accumulation mode volume and number were incorporated into cloud droplets. For Period II the corresponding fractions were 42±6% and 12±2% in one cloud event and 64±4% and 18±2% in another cloud event. The size dependent partitioning as a function of time was studied in Period II and found to have little variation. The major processes influencing the partitioning were found to be nucleation scavenging and entrainment.

The scavenging of nitrate by clouds and precipitation

AbstractA model with spectral microphysics was developed to describe the scavenging of nitrate aerosol particles and HNO3 gas. This model was incorporated into the dynamic framework of an entraining air parcel model with which we computed the uptake of nitrate by cloud drops whose size distribution changes with time because of condensation, collision-coalescence and break-up. Significant differences were found between the scavenging behavior of nitrate and our former results on the scavenging behavior of sulfate. These reflect the following chemical and microphysical differences between the two systems:(1)nitrate particles occur in a larger size range than sulfate particles.(2)HNO3 has a much greater solubility than SO2 and is taken up irreversibly inside the drops in contrast to SO2.(3)nitric acid in the cloud water is formed directly on uptake of HNO3 gas whereas on uptake of SO2 sulfuric acid is formed only after the reaction with oxidizing agents such as e.g., H2O2 or O3.(4)nitrate resulting from uptake of HNO3 is confined mainly to small drops, whereas sulfate resulting from uptake of SO2 is most concentrated in the largest, oldest drops, which have had the greatest time for reaction. Sensitivity studies showed that the nitrate concentration of small drops is significantly affected by the mass accommodation coefficient.

Interaction of Aerosol Particles and Clouds

With the spectral scavenging and microphysics model DESCAM coupled to a 2D dynamic framework the transport and scavenging of aerosol particles by a medium-sized convective cloud are investigated. For typical marine conditions the author has found a depletion of the marine boundary layer of about 70%. Also, about 70% of the vented up aerosol particle mass entered cloud drops due to nucleation. For these simulations the author found an increase in relative humidity, as well as number concentration of Aitken and large and giant particles near cloud top. The increase in relative humidity and large and giant particle concentration was coupled to cloud outflow regions, while the increase in Aitken particles can be attributed to lifting of free tropospheric air due to cloud-top rising. Sensitivity tests varying the number of free tropospheric sulfate particles changed the number of cloud drops but had little effect on precipitation formation and evolution.

Aerosol dynamics in ship tracks

Ship tracks are a natural laboratory to isolate the effect of anthropogenic aerosol emissions on cloud properties. The Monterey Area Ship Tracks (MAST) experiment in the Pacific Ocean west of Monterey, California, in June 1994, provides an unprecedented data set for evaluating our understanding of the formation and persistence of the anomalous cloud features that characterize ship tracks. The data set includes conditions in which the marine boundary layer is both clean and continentally influenced. Two case studies during the MAST experiment are examined with a detailed aerosol microphysical model that considers an external mixture of independent particle populations. The model allows tracking individual particles through condensational and coagulational growth to identify the source of cloud condensation nuclei (CCN). In addition, a cloud microphysics model was employed to study specific effects of precipitation. Predictions and observations reveal important differences between clean (particle concentrations below 150 cm -3 ) and continentally influenced (particle concentrations above 400 cm -3 ) background conditions: in the continentally influenced conditions there is a smaller change in the cloud effective radius, drop number and liquid water content in the ship track relative to the background than in the clean marine case. Predictions of changes in cloud droplet number concentrations and effective radii are consistent with observations although there is significant uncertainty in the absolute concentrations due to a lack of measurements of the plume dilution. Gas-to-particle conversion of sulfur species produced by the combustion of ship fuel is predicted to be important in supplying soluble aerosol mass to combustion-generated particles, so as to render them available as CCN. Studies of the impact of these changes on the clouds potential to precipitate concluded that more complex dynamical processes must be represented to allow sufficiently long drop activations for drizzle droplets to form.

Microphysics of clouds: Model vs measurements

Abstract In order to study the relation between the initial aerosol particle spectrum at cloud base and the resulting droplet spectrum in cloud for the “Ground-based Cloud Experiment” field campaign at the Great Dun Fell in 1993 numerical model simulations have been performed. The droplet spectra were calculated from a microphysical model coupled to a dynamic air flow model. The resulting droplet spectra were compared with cloud droplet spectra measured with a forward scattering spectrometer probe. The size distribution and chemical composition of the initial aerosol population were derived from a combination of size distribution and size-segregated chemical measurements below cloud base. From this we concluded that the aerosol particles consisted almost entirely of an inorganic salt. As part of the sensitivity studies two different microphysical models were used, as well as the dynamic flow fields from two different air flow models. As in previous studies we found that the measured droplet spectra were broader and contained larger drops than the modelled spectra. From the sensitivity studies we identified fluctuations in the dynamics as the most likely explanation for these differences.

Venting of gases by convective clouds

A two-dimensional dynamic model with spectral microphysics and a spectral treatment of aerosol particle and gas scavenging (DESCAM) was used to estimate the transport of gases from the marine boundary layer to the free troposphere by a medium-sized warm precipitating convective cloud. In the simulation, three gases were considered, covering a range of Henrys law constants: an inert tracer, SO2, and H2O2. SO2 was also used as the inert tracer by artificially suppressing any interaction with the cloud drops. The horizontal and vertical fluxes, their vertical means and the transport across the cloud boundaries were studied. It was calculated that for SO2 as an inert tracer 37 kg, for SO2 as a scavenged gas 34 kg, and for H2O2 12 kg were transported from the marine boundary layer across cloud base to the free troposphere for an estimated three-dimensional cloud. This represents a depletion of the marine boundary layer in the vicinity of the cloud by about 60%. After about half an hour of cloud life time, however, only 75% of the SO2 and only 30% of the H2O2 transported aloft still existed in the cloudy air. These residual gases could eventually participate in a long range transport if the cloud would dissipate. The rest had been scavenged by the cloud.

The Cloud Ice Mountain Experiment (CIME) 1998: experiment overview and modelling of the microphysical processes during the seeding by isentropic gas expansion

The second field campaign of the Cloud Ice Mountain Experiment (CIME) project took place in February 1998 on the mountain Puy de Dome in the centre of France. The content of residual aerosol particles, of H2O2 and NH3 in cloud droplets was evaluated by evaporating the drops larger than 5 μm in a Counterflow Virtual Impactor (CVI) and by measuring the residual particle concentration and the released gas content. The same trace species were studied behind a round jet impactor for the complementary interstitial aerosol particles smaller than 5 μm diameter. In a second step of experiments, the ambient supercooled cloud was converted to a mixed phase cloud by seeding the cloud with ice particles by the gas release from pressurised gas bottles. A comparison between the physical and chemical characteristics of liquid drops and ice particles allows a study of the fate of the trace constituents during the presence of ice crystals in the cloud. In the present paper, an overview is given of the CIME 98 experiment and the instrumentation deployed. The meteorological situation during the experiment was analysed with the help of a cloud scale model. The microphysics processes and the behaviour of the scavenged aerosol particles before and during seeding are analysed with the detailed microphysical model ExMix. The simulation results agreed well with the observations and confirmed the assumption that the Bergeron–Findeisen process was dominating during seeding and was influencing the partitioning of aerosol particles between drops and ice crystals. The results of the CIME 98 experiment give an insight on microphysical changes, redistribution of aerosol particles and cloud chemistry during the Bergeron–Findeisen process when acting also in natural clouds.