Since many years TROPOS takes part in national and international cloud field campaigns, which are part of research projects investigating the aerosol cloud interactions in warm clouds. The objectives of these field experiments are:
- Identification of the chemical and microphysical properties of aerosol particles, which form cloud drops in the real atmosphere (particularly in differentiation to the artificial activation in cloud condensation nuclei counters)
- Quantification of the distribution of atmospheric aerosol substances in the liquid and interstitial phase
- Determination of the effect of cloud processing on atmospheric aerosol particles, like changes in the chemical composition and size
- Provision of input and validation parameters for cloud chemistry and cloud microphysics models
- Investigation of all mentioned bullet points for different cloud types: orographic clouds, tropical trade wind cumuli, cold but liquid strato-cumulus clouds
The experimental in-situ cloud-studies were carried out until now at the ground, i.e. on mountain top measurement stations. The drops and interstitial aerosol were separated collected by means of a counterflow virtual impactor (CVI) and an interstitial inlet, respectively. After the airborne drying of the drops within the CVI, their residuals as well as the interstitial particles are analyzed micro-physically (number, size, morphology, electrical charge) and chemically (composition, mixing state) in collaboration with working groups of other research institutes. Current research projects where atmospheric clouds are investigated are HCCT 2010 (Hill Cap Cloud Thuringia, Schmücke, Deutschland, follow up project of FEBUKO), PRADACS (Puerto Rico African Dust and Cloud Study, East Peak, Puerto Rico) and ACRIDICON-Zugspitze (Schneefernerhaus, Germany).
The major results from these cloud field measurements are:
- Mineral dust accelerates the formation of sulfate in clouds. Since a major part of the sulfate is situated on the large mineral dust particles, the life time of sulfate decreases, i.e. the indirect aerosol forcing of sulfate requires a re-evaluation (Harris et al., 2013)
- Size resolved activation curves show a significant correlation with the soluble fraction of the aerosol particles in front of the cloud and have 50 % activation diameters between 100 and 200 (Mertes et al, 2005a)
- A mass production rate of 1.16 µg m-3 h-1 due to cloud processing was empirically determined by the comparison of particle size distributions in front and behind the cloud, which is in the same order of magnitude like corresponding model calculations (Mertes et al, 2005b)
- Experimental evidence was found, that long-range transported Saharan dust changes the cloud microphysical state of Caribbean trade wind clouds in clean air masses. The drop concentration increased whereby the effective drop radius decreased
- In cold, liquid alpine clouds 20 to 30 % of the abundant aerosol particles became activated and more than 50 % of the soot mass was transferred into the liquid phase
- Activation diameters were found to be at about 200 nm, which indicates that lower super-saturation exists in these clouds than in lower altitudes
The research in the named projects was and still is supported by the German Research Foundation DFG (grant ME 3534/1-2)
Harris et al (2013) , Sulfur isotopes highlight the importance of transition metal ion-catalysed oxidation of SO2 in clouds, Science 340: 727-730.
S. Mertes, K. Lehmann, A. Nowak, A. Massling, A. Wiedensohler (2005a), Link between aerosol hygroscopic growth and droplet activation observed for hill-capped clouds at connected flow conditions during FEBUKO, Atmospheric Environment 39(23-24): 4247-4256.
S. Mertes, D. Galgon, K. Schwirn, A. Nowak, K. Lehmann, A. Massling, A. Wiedensohler, W. Wieprecht (2005b), Evolution of particle concentration and size distribution observed upwind, inside and downwind hill cap clouds at connected flow conditions during FEBUKO, Atmospheric Environment 39(23-24): 4233-4245.