Cloud Chemistry Monitoring at Mt. Brocken, Germany (1992 - 2009)

Long-term cloud chemistry monitoring started in the early 1990s in Germany at Mt. Brocken/Harz (51.799 degree N, 10.61degree E; 1144 m a.s.l.), the highest elevation in the northern part of Central Europe (Möller et al. 1992, 1993, Acker et al. 1996, 1998a, 2002). From October 1992 until October 2009 a continuous measurement programme were carried out which included cloud water sampling, cloud and meteorological observations and gas measurements (SO2, O3, NO, NO2). From April 1993, we collected and analysed 23.842 1-hour cloud water samples. Liquid water content, cloud base are available for most samples. Additional, 1676 daily rain water samples from a foot-hill site Schierke (612 m a.s.l.) we collected (1992-1995 and 1999-2006). The top of Mt. Brocken is a small plateau above the tree line with subalpine vegetation. Whereas the surface wind in the lower Harz region is strongly influenced by the mountains, i.e. air masses will taken around the Harz massif (Beyrich et al. 1994), the wind field at the Brocken summit it self is relatively uninfluenced and represents the predominant low tropospheric wind. Due to the high occupancy of the summit by clouds (30-50% of total time from April to October) the measurement programme was focused on cloud water chemistry. The station was almost above the boundary layer (Beyrich et al. 1996) and more than 80% of clouds are low passing clouds. It was shown by simulation of different synoptic situations using the nonhydrostatic KAMM model (Adrian and Fiedler 1991) that air masses are steaming over the mountain top and our station provides access to the regional air flow. The measurement station consisted of a wooden house with a platform roof about five meter above the ground where all meteorological, cloud physical, and sampling devices were installed. Meteorological standard parameters (wind direction and velocity, temperature and humidity, global radiation, pressure) were measured with commercial instruments (hygro-thermo transmitter by Thies GmbH), digital barometer AIR-DB 1A, ultrasonic anemometer USA-1 by Metek GmbH). Liquid water content (LWC) of clouds was measured continuously by a laser diffraction technique using a Gerber Particulate Volume Monitor (PVM 100). Since 1995, cloud base high was measured from the foothill station Schierke (51.766 degree N, 10.653 degree E; 614 m a.s.l.) by the Ceilometer CT25K (Vaisala). Continues measurements of O3, SO2, NO, and NO2 were made using commercial automatic analysers (TEI 43S, Dasibi 1108 RS, Ecophysics CLD 770ALppt) up to 1996. As of 1997 gas phase Measurements were carried out by the Saxony-Anhalt State Environment Agency at Mt. Brocken. Cloud water was collected using an automated system, controlled by the PVM signal if LWC arises 25 mg m-3 threshold ("wet only" sampling) and using a passive string collector of ASRC-type (Mohnen collector), in combination with an automatic sampling unit (ISCO 3700). The sample bottles were replaced daily and stored at 4° C until chemical analysis in the Berlin laboratory. Since June 1993 we collected cloud water with 1-h time resolution to be in coincidence with the time scale of synoptic observations (cloud type, cloud frequency, cloud base) and the typical time scale of weather changes. Because for any cloud chemistry statistics it is also imperative to differentiate between precipitating and non-precipitating clouds, a rain detector was working parallel to differ between non-precipitating and precipitating cloud events. LWC measurements and cloud water sampling were carried out only during frost-free periods, as a rule between middle (end of April and middle/end of November. All other measurements were running the whole year. Backward trajectories were computed for selected periods of time by an isentropic model using a mixed dynamic/kinematic procedure (Reimer and Scherer 1991). Further transport parameters (e.g. time of transport in clouds and in the mixing layer) were analysed by our programme TRAP. The samples were analysed by ion chromatography (DIONEX suppressor, SYKAM pump and conductivity detector, JASCO sampler) for anions (Cl-, NO3-, SO42-) and cations (Na+, NH4+, K+, Ca2+, Mg2+), pH and conductivity. The acceptance criteria for ion balance and for conductivity (measured/calculated), respectively, are defined in the Reports 85 (1992) and 102 (1994) of the WMO for all activities within the GAW program and also we did follow rigorously these standard practices. The whole measurement and analytical procedure was laid down in standard operating procedures. Annual samples of the WMO/GAW laboratory intercomparison program organised by the U.S. EPA and since 1996 by QA/SAC Americas have been analysed always showing best results (e.g. Acker et al. 1998). Besides the long-term monitoring, we carried out several campaigns at Mt. Brocken, partly together with other groups. Some key results we found, have been published earlier, such as ozone destruction within clouds (Möller et al. 1994, Acker et al. 1995, Möller et al. 1999), the changing acidity (Möller et al. 1996a, Acker et al. 1998b), the relationship between ionic content and liquid water content (Möller et al. 1996b), formation of nitro phenols in clouds (Lüttke et al. 1997, 1999), occurrence of trace elements in cloud water (Plessow et al. 2001), formation and occurrence of nitrous acid in clouds (Acker et al. 1999, 2001, 2006, 2008), sulphite in clouds (Tian et al. 1999), macromolecular substances (HULIS) in clouds (Feng and Möller 2004), and the relationship between liquid water content and visibility (Chaloupecky et al. 2007, Acker et al. 2010).

The liquid water content of clouds (LWC) has been continuously measured using forward scattering of laser beams by cloud droplets with a Particulate Volume Monitor PVM-100(Gerber Scientific Inc.). This instrument was also used to control an automated "wet-only" sampling unit that opens up and moves a passive string collector (ASRC type) into sampling position when the LWC level exceeds a 25mg/m**3 threshold. The samples were analysed by ion chromatography (DIONEX suppressor, SYKAM pump and conductivity detector, JASCO sampler) for anions ([Cl]-, [NO3]-, [SO4]2-) and cations ([Na]+, [NH4]+, [K]+, [Ca]2+, [Mg]2+), pH and conductivity. The acceptance criteria for ion balance and for conductivity (measured/calculated), respectively, are defined in the Reports 85 (1992) and 102 (1994) of the WMO for all activities within the GAW program and also we did follow rigorously these standard practices. The whole measurement and analytical procedure was laid down in standard operating procedures.