Cloud Studies with the Droplet Aerosol Analyzer

Climate change and atmospheric aerosols are a threat for human health and life. Reducing aerosol emissions would save human lives close to the aerosol source, but could lead to more death due to the implications of a warmer climate. Aerosol particles acting as cloud or ice nuclei can induce a change in cloud properties and thus indirectly induce a change in planetary albedo, which is considered the major reason of changes in planetary albedo associated with global warming. Because of the complexity of the interaction between aerosols and clouds, uncertainties in cloud parametrization remain the major cause of discrepancies between cloud observations and simulations. Thus, improving the understanding of aerosol-cloud-interactions is one of the keys for reducing uncertainty in the estimate of the total anthropogenic radiative forcing and climate sensitivity. Climate sensitivity is important for quantifying risks and probabilities, and the development of adaption strategies. The Droplet Aerosol Analyzer (DAA) was developed to study aerosol-cloud interaction and is unique in providing the number and the direct relationship between cloud droplet and residual particle size. For this purpose a more automatic version with better time resolution (10 min) and an improved and more automated inversion algorithm has been developed to better suit the needs of long-term measurements. Between June and October 2010 aerosol-cloud interaction measurements have been performed at the summit of Mt. Brocken (51.80 N, 10.62 E, 1142 m a.s.l.) in central Germany. For this period the aerosol and cloud properties and the droplet activation regime regarding the ratio between updraft velocity and particle number concentration (w=Ntot), have been determined. The relation between cloud droplet number concentration Nd;tot and total number concentration Ntot, updraft velocity wpred, and size distribution shape R_0.1um has been determined for three overlapping w=Ntot-intervals. As expected, for increasing w=Ntot-ratio (from the transitional regime towards aerosol limited regime) the relative sensitivity ofNd;tot against w decreases while the relative sensitivity of Nd;tot against Ntot increases. The influence of the size distribution shape R_0.1um was examined and the absolute relative sensitivity of Nd;tot against R_0.1um was observed to decrease from the transitional towards the aerosol limited regime. The onset of ’roll-off’, where an increase in Ntot does not lead to a proportional increase in Nd;tot, shifted towards higher total number concentration for increasing w=Ntot-ratio

Continuous stand-alone controllable aerosol/cloud droplet dryer for atmospheric sampling

We describe a general-purpose dryer designed for continuous sampling of atmospheric aerosol, where a specified relative humidity (RH) of the sample flow (lower than the atmospheric humidity) is required. It is often prescribed to measure the properties of dried aerosol, for instance for monitoring networks. The specific purpose of our dryer is to dry cloud droplets (maximum diameter approximately 25 mu m, highly charged, up to 5x10(2) charges). One criterion is to minimise losses from the droplet size distribution entering the dryer as well as on the residual dry particle size distribution exiting the dryer. This is achieved by using a straight vertical downwards path from the aerosol inlet mounted above the dryer, and removing humidity to a dry, closed loop airflow on the other side of a semi-permeable GORE-TEX membrane (total area 0.134m(2)). The water vapour transfer coefficient, k, was measured to be 4.6x10(-7) kgm(-2) s(-1) % RH-1 in the laboratory (temperature 294 K) and is used for design purposes. A net water vapour transfer rate of up to 1.2x10(-6) kg s-1 was achieved in the field. This corresponds to drying a 5.7 L min(-1) (0.35m(3) h(-1)) aerosol sample flow from 100% RH to 27% RH at 293K (with a drying air total flow of 8.7 L min-1). The system was used outdoors from 9 May until 20 October 2010, on the mountain Brocken (51.80 degrees N, 10.67 degrees E, 1142ma.s.l.) in the Harz region in central Germany. Sample air relative humidity of less than 30% was obtained 72% of the time period. The total availability of the measurement system was > 94% during these five months