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Physico-chemical characterization of air, particles, and cloud water in cloud experiments (part of the FEBUKO project in AFO 2000)

In the period of 01/2001 to 12/2003 the following joint project is worked within the atmosphere research cluster (AFO 2000):

FEBUKO - subproject cloud- and aerosol chraracterization, FEBUKO - Website

To multiphase modeling connected with FEBUKO field experiments:

MODMEP - subproject multiphase modeling, MODMEP - Website

Within the project two aerosol-cloud interaction field campaign experiments took place in the Thüringer Wald (Germany) in October 2001 and 2002 (Figure 1). Experimentally, a variety of measurement methods were deployed to probe the gas phase, particles and cloud droplets at three sites upwind, downwind and within an orographic cloud (Figure 2) with special emphasis an the budgets and interconversions of organic gas and particle phase constituents. Out of a total of 14 sampling periods within 30 cloud events three events are selected for detailed interpretation. An impact of the cloud process on particle chemical composition such as on th organic compounds content, sulphate and nitrate and also on particle size distributions and particle mass was observed. Moreover, direct phase transfer of polar organic compounds from the gas phase was found to be very important for the understanding of cloud water composition. For the interpretation of the chemical and physical results a cloud model (MODMEP project) was developed, which combines a complex multiphase chemistry with detailed microphysics.

The results of the research combination FEBUKO/MODMEP were published in a Special Issue of the journal Atmospheric Environment 2005 (39) .

Fig. 1: Measurement tower
at the cloud site.

Fig. 2: Schematic view of the measurement sites.

Precipitation chemistry

Changes in concentration of pollutants in rain water caused by changes in emissions were investigated by wet-only samplers at the outskirts of Leipzig (L) and the rural site Melpitz (M, Research Measurement Site of the IfT) beginning in 1992.
The emission situation of the former GDR was characterized by high amounts of SO₂, NO_x, NH₃ and dust. After the German reunification in 1990 the emission of these pollutants could increased substantially, but not at the same time, which caused positive and negative effects on the rain water components.
Figure 1 shows the trend of the annual mean volume-weighted concentration of the components sulfate (SO₄^2-), nitrate (NO₃^-), ammonium (NH₄⁺), and calcium (Ca₂⁺) and in Figure 2 the acidity as well as the pH-value in the precipitation.
The changes of acidity of precipitation were mainly influenced by the changes of main ionic components as sulfate (precursor SO₂), nitrate (precursor NO_x), ammonium (precursor NH₃) and calcium (from brown coal ash). In 1992 sulfate (L: 195 µeq/l, M: 140 µeq/l) and calcium (L: 124 µeq/l, M: 72 µeq/l) were the components with the highest concentrations in the precipitation (brown coal burning). In 2005 ammonium has the highest concentration (about 70 µeq/l) followed of sulfate and/or nitrate (about 45 µeq/l] and calcium (about 27 µeq/l) at both sites in rain water. The nitrate and ammonium concentration remained roughly stable.
The acidity and pH-value, respectively, of precipitation follows of the ratio of acidic (sulfate and nitrate) and basic (ammonium and calcium) components. An increase of extreme acidity values in rain water was observed at both sites in the summer months up to 1996 (see poster). The maximum acidity was measured at site L in 1996 (468 µeq/l - pH-value: 3.33). The mean annual pH-value increases from about 4.35 (1992) to about 4,8 (2005).