S. Pahl

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.

The size-dependent chemical composition of cloud droplets

Abstract Size-dependent cloud droplet solute concentrations were measured using a two-stage fog water impactor at the summit station of Great Dun Fell (GDF) in the north of England. The measurements showed mostly higher concentrations in the small-droplet fraction. During one cloud event, however, higher solute concentrations were found in the larger-droplet fraction. In order to identify the factors governing the size dependence of cloud droplet solute concentrations, sensitivity studies by means of a diffusional growth model were performed. The time available for the droplets to grow was identified to be of great importance for the size dependence of solute concentrations. In cases when higher solute concentrations were found in the fraction containing the bigger droplets, the cloud droplets were relatively young having been formed by orographic lifting of the air at the GDF summit. For the other events the evidence indicates that the cloud was already formed far upwind from the summit site. Our experimental and model results imply that, after an initially strong decrease of solute concentrations with droplet size we would observe: • ⊎|increasing solute concentrations with increasing diameters during the initial stage of a cloud, e.g. near cloud base where the droplets have just been formed. The primary factors contributing to this behaviour are high peak supersaturations, large numbers of coarse aerosol particles, and high solubility of the aerosol particles. • ⊎|decreasing solute concentrations with increasing diameters in aged cloud parcels, such as those which can be observed high above the cloud base in cumuliform clouds or are advected to the observation point in the case of stratiform clouds. The primary factors contributing to this behaviour are low peak supersaturations, low numbers of coarse particles, and low solubility of the aerosol particles.

Henry’s Law and the Behavior of Weak Acids and Bases in Fog and Cloud

Experimental data from two field experiments on ground based clouds were used to study the distribution of formic acid, acetic acid, ammonia and S(IV) species between liquid and gas phase. The ratio of the concentrations of these compounds between the phases during concurrent measurements was compared to ratios expected according to Henrys law (considering the pH influence). Large discrepancies of several orders of magnitude were seen. Three hypotheses have been investigated to explain the observed discrepancies: The existence of a microscale equilibrium which does not persist in a bulk sample, a thermodynamic shift of the equilibrium due to competing reactions, and nonequilibrium conditions due to mass transfer limitations. Approximate quantitative calculations show that none of these hypotheses is sufficient to explain all of the discrepancies, so a combination of different effects seems to be responsible for this observation. The same theoretical considerations also suggest that mass transfer limitation may be an important factor for highly soluble compounds. The data presented here indicates that it is not possible to simply extrapolate interstitial gas phase composition from measured bulk liquid phase concentrations of a fog or cloud.

Multiphase chemistry and acidity of clouds at Kleiner Feldberg

The chemistry of cloud multiphase systems was studied within the Kleiner Feldberg Cloud Experiment 1990. The clouds encountered during this experimental campaign could be divided into two categories according to the origin of air masses in which the clouds formed. From the chemical point of view, clouds passing the sampling site during the first period of the campaign (26 October-4 November) were characterized by lower pollutant loading and higher pH, as compared to clouds during the final period of the experimental campaign (10–13 November). The study of multiphase partitioning of the main chemical constituents of the cloud systems and of atmospheric acidity within the multiphase systems themselves (gas + interstitial aerosol + liquid droplets) are presented in this paper. A general lack of gaseous NH3 was found in these cloud systems, which caused a lack of buffer capacity toward acid addition. Evidence supports the hypothesis that the higher acidity of the cloud systems during this final period of the campaign was due to input of HNO3. Our measurements, however, could not determine whether the observed input was due to scavenging of gaseous HNO3 from the air feeding into the cloud, or to heterogeneous HNO3 formation via NO2 oxidation by O3 to NO3 and N2O5. Sulfate in cloud droplets mainly originated from aerosol SO42− scavenging, since S(IV) to S(VI) liquid phase conversion was inhibited due to both lack of H2O2 and low pH of cloud droplets, which made O3 and metal catalyzed S(IV) oxidation inefficient.

METEOROLOGY OF THE GREAT DUN FELL CLOUD EXPERIMENT 1993

Synoptic and local meteorological conditions during the Spring 1993 Ground-based Cloud Experiment on Great Dun Fell are described, including cloud microphysics, general pollution levels and sources of air, especially for five case studies selected for detailed analysis. Periods when air was flowing across the hill are identified and the extent to which air mixed into the cloud from above reached the ground is estimated. To aid the interpretation of cloud chemistry and microphysics measurements, the horizontal and vertical extent of the cloud are used to estimate droplet lifetimes and to comment on the influence of complex terrain on peak supersaturation.

The reduced nitrogen budget of an orographic cloud

Field data collected during the GDF 93 project indicated that during polluted conditions (SO2(g) > 2 ppbv, NH3(g) > 0.5 ppbv), sulphate and ammonium concentrations in air increased through cloud chemistry by as much as 25%. Similarly, ammonia was seen to be consumed by cloud processing and decreased by up to 20%. In comparatively clean conditions (SO2(g) < 0.5 ppbv, NH3(g) < 0.5 ppbv), the sulphate loading of the aerosol was seen to remain constant, and ammonium was lost from the aerosol and outgassed as ammonia, increasing ambient ammonia concentrations by as much as 0.5 ppbv. An ideal cloud chemistry model predicted up to 20% more sulphate production than is implied by the bulk aerosol data set. A non-ideal cloud chemistry model was used to estimate the final ammonium loading of the aerosol, which is determined by the transformation from wet cloud droplet to dry aerosol particles below their deliquescence point. The non-ideal model showed that in three of the four cases ammonia outgassed from evaporating cloud droplets, consistent with field observations, but at variance with the ideal chemistry model. The results indicate that in low pollution conditions clouds act to re-equilibrate reduced nitrogen in the aerosol phase with gaseous ammonia. The outgassed ammonia will then be rapidly deposited to semi-natural ecosystems downwind of such clouds.

Deposition of trace substances via cloud interception on a coniferous forest at Kleiner Feldberg

A resistance model to calculate the deposition of cloud droplets on a coniferous forest and some improved parameterizations of the indispensable input parameters are described. The deposition model is adapted to the coniferous forest at the Kleiner Feldberg site and verified by the data of a drip water monitoring station below the forest canopy. The measurements of liqud water content, wind speed and trace substance compounds in cloud water of the Ground-based Cloud Experiment (GCE) at Kleiner Feldberg in 1990 are used to calculate the cloud water deposition fluxes and the deposition of trace substances via cloud water interception. The calculated deposition of trace substances via cloud water interceptions is three to six times higher than via rain during the experiment. On a long term data basis the yearly amount of cloud water deposition is 180 mm year−1 at Kleiner Feldberg site (840 m a.s.l.) while the precipitation amount is 1030 mm year−1. Due to higher trace substance concentrations in cloud water compared to rain the ionic deposition via cloud water interception and via precipitation were assessed to be of comparable magnitude.

The budget of oxidised nitrogen species in orographic clouds

Abstract The transformation of NOx as it passed through a hill-top cap cloud was investigated by measuring (i) NOx (NO and NO2) and particulate nitrate concentrations on both sides of the hill, (ii) nitrite and nitrate in cloud, and (iii) nitrous and nitric acid concentrations below cloud base. Results from four periods during April and May 1993 are presented as part of a EUROTRAC GCE (Ground-based Cloud Experiment) experimental campaign at Great Dun Fell, in Cumbria, England. The overall change in NOx in all four periods was less than 20%. On 22 April the air flow was from west to east, and NOx concentrations were very much larger than particulate nitrate concentrations. There was evidence for loss of NOx and production of nitrous acid as air passed through the cloud. During May the air flow was from east to west; NOx concentrations were much smaller, and similar to concentrations of particulate nitrate. Differences in NOx concentrations before and after cloud sometimes showed a loss of NOx which was greater than the combined measurement uncertainty. Changes in particulate nitrate concentrations were close to the combined measurement errors. Concentrations of nitrate in cloud, however, were greater than could be accounted for by particles alone, suggesting a significant input of HNO3 Measurements below cloud showed that as much as 25% of the cloud nitrate could be released as HNO3 as cloud droplets evaporated. These field measurements show that chemical processing of oxidised nitrogen does occur in clouds, involving consumption of NOx and production of gaseous HNO3 as cloud droplets evaporate. The methods used to measure particulate nitrate concentrations, however, were not adequate to construct a complete budget of oxidised nitrogen across the hill.

Vertical gradients of dissolved chemical constituents in evaporating clouds

Abstract Vertical gradients in cloud-water composition were investigated during the Ground-based Cloud Experiment at Great Dun Fell (GDF) 1993. The cloud-water measurements were performed at two heights above the cloud base. The observed changes in cloud-water concentration were not only induced by dilution or concentration due to an increasing or decreasing liquid water content (LWC), but also by loss or uptake of chemical compounds, and, under appropriate meteorological conditions (downslope flow), by evaporation of small droplets between the two heights. The observed vertical gradients were found to be ion-specific. Higher amounts of total dissolved material were measured at greater distances above the cloud base, e.g. SO42− during most of the time of the monitored cloud events. Thus, vertical gradients may be important for deposition calculations of trace substances onto vegetation via cloud-water interception. In any case, the cloud base is a very important parameter relevant for the cloud chemical studies, because it is of importance for data interpretation.