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Fog chemistry during EUMAC joint cases. Analysis of routine measurements in southern Germany and model calculations

: Forkel, R.; Seidl, W.; Ruggaber, A.; Dlugi, R.


Meteorology and atmospheric physics 57 (1995), S.61-86 : Ill., Lit.
ISSN: 0177-7971
Fraunhofer IFU; 2002 in Helmholtz-Gesellschaft integriert

The role of fog events for acid production and moist deposition in southern Germany during three EUMAC Joint Cases has been investigated by analysis of routine measurements and simulations with a one-dimensional fog-chemistry model. To identify the chemical and meteorological conditions, routine measurements by different institutions have been interpreted. The periods under consideration include a smog episode with low photooxidant concentrations during the 'Winter Case' in February 1982. The 'Wet Case' in spring 1986 represents a period with higher photooxidant concentrations. Conditions which are mostly characterized by low SO2 and oxidant concentrations and comparatively high pH-values in the fog are given during the 'SANA 1' case in autumn 1990. Fog mostly occurs as a subscale phenomenon, but sometimes it can also cover large areas and it can contribute significantly to moist deposition. The model results indicate that the liquid phase sulfate production in the fog layer may even e xceed the gas phase production during 24 hours within a layer of the same height occasionally. On the other hand, during the SANA 1 case the sulfate production in the fog was extremely low at night due to lack of oxidants and SO2. Depending on the dissipation time of the fog a remarkable effect on the photolysis rates is possible. Since a significant amount of particulate mass is lost by moist deposition during fog, it is evident that fog events can have a noticeable effect on some of the gas phase constituents which are easily soluble.