Participants from two EUROTRAC subprojects - the modeling project EUMAC and the field project ALPTRAC - have joined for an investigation of meteorology and air pollution over the Alps and their surroundings during 10 days in March 1990. An intensive observation campaign was carried out during this month by ALPTRAC participants at the high mountain observatories Jungfraujoch (JFJ, 3450 m) in the Swiss Alps and Sonnblick (SBK, 3105 m) in the Austrian Alps. The period from 21 to 31 March was simulated with the EURAD model system, consisting of the meteorological model MM5 and the Eulerian chemistry and transport model CTM. Results were carefully analysed and compared with the observations at the mountain stations.
The purpose of this joint case study is, on one hand, to investigate the performance of the modeling system in a high mountain region, and on the other hand, to enhance the understanding of processes leading to observed high pollution episodes on mountain peaks with the aid of model results.
The MM5 model was used in a 2-way nested version, with a resolution of 60 km in the coarser grid and 20 km in the finer grid, and 15 levels in the vertical. The model was initialized every 48 h, initial and boundary conditions were interpolated from the ECMWF analysis (main pressure levels, 1.5 deg resolution). ECMWF data were also used to nudge the horizontal wind components and temperature. Meteorological fields and emissions are fed into the CTM. Emissions are based on the EMEP inventory; they are transferred to the required spatial and temporal resolution by the EURAD Emission Model (EEM), where the spatial distribution of emissions is assumed proportional to the population. The CTM has the same resolution as MM5 and 1-way nesting. It considers gas phase and aqueous chemistry and includes an aerosol chemistry module with the species sulphate, nitrate, and ammonium. The first 48 hours are used as spin-up time for the pollutants.
Measured data which were discussed are filter pack measurements of the major aerosol species (time resolution 12 to 24 hours), and epiphaniometer measurements with 0.5 h resolution. The epiphaniometer gives a signal which is proportional to the so-called Fuchs surface of the aerosol. It reflects mainly surface concentrations of the accumulation mode aerosol. In addition, hourly meteorological data from the mountain observatories have been used.
The simulated period is characterized by a trough approaching the Alps and the subsequent formation of a cut-off low over Italy. A number of pollution events were observed during this time on JFJ and SBK, the first one on March 24. Analyses have up to now focussed on this event. It occurred between a first cold front which had reached the Alps on March 23 and then became stationary, and the second, stronger cold front which crossed the Alps in the morning of March 25.
Fig. 1 shows the observed and modelled aerosol concentrations at SBK during the first 48 hours of the simulation (900323/12-900325/ 12). The correlation between observed and simulated concentrations at the nearest grid point is 0.55; if one searches in a 7x7 subgrid around the station, one can find a grid point (about 60 km to the SSE and one level higher) where the correlation is increased to 0.84, and the two observed peaks are reproduced very well. The peak mainly consisted of ammonium sulphate.
Figure 1. Comparison between observed and modelled aerosol concentrations at Sonnblick. Observed data are epiphaniometer signal (relative units, see text). Modelled data represent the sum of the concentrations of nitrate, sulphate, and ammonium. The plotted values have been transformed to "epiphaniometer units" by means of a linear regression (seperately for both model grid points); the correlation is indicated in the legend.
A careful analysis using horizontal and vertical cross-sections of meteorological variables and of the sulphate fields allowed to understand the processes leading to these peaks. The first cold front had brought fresh, unpolluted air to Sonnblick. It just reached the Po Valley, where vertical motion in the frontal zone picked up aged, polluted boundary layer air. With the flow backing in front of the second cold front, the meridional component of the flow over the Alps changed temporarily from northerly to southerly, thus advecting the polluted air masses to SBK on the main ridge of the Alps. Arrival of the second cold front caused again a strong drop in pollution.
The peak at JFJ was also reproduced well by the model, if a little shift in space was allowed. A first comparison between model and observation (filter pack) for the whole 8 days indicated reasonable model performance for nitrate but a failure for sulphate; this result was obtained with model level 3 and may change when considering higher levels which may be more representative for JFJ.
Earlier arguments that air masses from the Po Valley can cause
high pollution events at high-Alpine stations, and that their
lifting is associated with a cold front, could be confirmed. The
model is able to reproduce some of the quite complex mesoscale
circulations over and around the Alps (though not everything),
including the associated air chemistry. Chemical measurements
at high-Alpine stations thus are an important element in the assessment
of model performance.
MAP Data Centre - April '05 - MAP WebMaster