Aerosol characterization in a Po Valley site

222

Aerosol~ in science, medicine and technolog.~ AEROSOL

CHARACTERIZATION

IN A PO VALLEY

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P. BACCI~. A. LONGHETTO2. G. MARCAZZAN3, A. PIANO2, F. PRODI"~. C. SABBION]i and ~ VE'xrt i<,, C.R.T.N.-ENEL, Mitano: : lstltulo di Fisica Generale-Universlta. Torino: Bologna: ~ SERMA, Milano

CISE, Milano: ~ [s{iudtO F.I.S.B.-\ I ~,NR,

INTRODUCTION Particulate matter local composition and concentration depends on its ortgin (both natural and anthropogemcl and on the meteorological and climatological situation. The present work represents the first step of a study program on inorganic aerosol charactermation at a site tn the western part of the Po Valley in which, in addition to agricultural activities, there are many factories and a large power plant. We chose this site because of our knowledge about its average meteo-climatological conditions which we have studied in many experimental campaigns (the last one was the IVth C.E.C. Campaign on Remote Sensing. held in 1979). METEOROLOGICAL CHARACTERIZATION The Turbigo area (35 km north-west from Milano) follows the Ticino river valley which stretches from the north to the south. The local wind field is marked by a weak circulation in the lower atmospheric layers, which, during spring and summer, is a breeze with a frequency of about 50 Ooin the north-south direction and an average speed Of about 3 m/see. Unfortunately during the first part of the experimental campaign (25-29 May 19811. a large perturbation disturbed the weather with clouds, precipitation and wind field distortions. To obtain the essential meteorological data we used: (a) the meteorological network of the ENEL power plant; (b) some remote instruments like RASS and doppler SODAR; (ct Pilot balloons (aerological balloons with constant vertical lift). PLUME TRAJECTORIES The oil-fired power plant chimneys are the only elevated source at the Turbigo site, and by means of the meteorological data and plume rise formula it is possible to calculate if and how long (persistence) a sampling station at ground level can "feel" the emission. The Briggs plume rise formula, adapted to the local atmospheric stability conditions, allowed us to calculate the effective height of the plume. By dividing the atmosphere into almost constant wind sub layers, and by introducing diffusion, we calculated the axis trajectories of the plume and its vertical and horizontal dispersion parameters, or, a:. The vertical dispersion brings a part of the plume towards the ground so that the aerosol motion is affected by the wind present in the lower atmospheric layers. By means of this process we can draw the X Y projections of the plumes emitted from each chimney and know where they reach the lowest atmospheric layers.

SAMPLING AND ANALYSIS TECHNIQUES Five sampling stations were operating at ground level along the average wind direction, at different distances from the power plant. The measurements were carried out with: (a) Whole filters (0.4/~m in porosity) (b) Dclron single orifice cascade impactors (five stages plus back-up filterl (c) Andersen cascade impactors (eight stages). The impaction surfaces in Delron were coated with a thin layer of Apiezon N. The Whole filters and Dclron iml~tctors samples were collected every six hours in order to follow the evolution of the meteorological phenomena, while the Andersen samples were collected every 24 hr in order to characterize the background and slow local changes. The samples have been analysed by means of the proton induced X-ray fluorescence (PIXE) and atomic absorption spectrophotometry (AAS) techniques. For the last one the samples were completely dissolved in water with acid attack at high temperature and pressure.

ELEMENTAL COMPOSITION The whole filters measure the principal elements: S, K, Ca, Ti, V, Mn, Fe, Cu, Zn and Pb. Their concentrations range from a few tenths of nanograms m - 3 to a few micrograms m - 3. The concentration temporal trends for a single element measured in the different sampling stations, are similar. Generally the concentrations increase Slowly (only the sulphur shows a large rise) in the time starting from the perturbed weather situation. Although the concentration values vary at the different sampling stations, they are lower than the typical ones in urban areas but higher than those of rural sites. Vanadium and Nickel, which are indicated as typical components of oil-fired power plant emissions, presen t *cry low values at ground level, showing very high dilution factors. The Andersen impactor samples, observed in AAS. supplied also the Na. AI, Mg, Mo and Cd concentrations.

The Tenth A n n u a l Conference of the Association for Aerosol Research

223

AVERAGE SIZE D I S T R I B U T I O N S The analysis of impactor samples gave the mass size distributions of the different elements. The distributions averaged over the campaign period seem typical for groups of elements: Ca and Fe, for example, are associated with particles of large diameter, while S and Pb show, as expected, the opposite (Fig. 34). Although the impactors have a different number of stages and different collecting intervals for diameter, the normalized results show good agreement in the dimensional spectra•

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Aerosols m science, medicine and technology DISCL SSION

The aim of the present work was to try to discover the influence of the power ptant emission by establishing the correlation between the plume persistence over a station and the measured aerosol concentration and size distribution change at ground level. Let us consider first the whole filters results. We observe that (Fig. 35): (a) the S total concentration increases a substantial amount; lbl a little modulation v~ith the above defined persistence occurs in a down-wind station when the background is very low rafter rainy weather k ic) when the background increases such a modulation disappears even if the persistence is higher: ld) the average final values in the four stations are almost the same without any plume persistence in two stations; tel the S concentration trend suggests an increase, but the actual final value is lower than the typical ones measured at polluted s'tes. Looking at the dimensional distributions of the average total suspended particles, over the whole period, one sees substantial agreement with long-time averaged distributions at the same site. Since the T S P distributions for each 24 hr measurements period doesn't improve the knowledge about the subject, let us consider now the size distributions of some peculiar elements over 6 hr. The most evident persistence effect concerns the increase of the mass size distribution in the diameter range from 0.5 to 4 ,urn. This p h e n o m e n o n is shown for elements which are associated both with large and small diameter particles {Fig. 36). The last consideration concerns an effect we wanted to see by placing the sampling stations at different distances from the power plant. During persistence, in the nearest sampling station, we collected particles larger than in the other. This phenomenon, which appears reasonable even if it is difficult to be seen in the free atmosphere, is however strong enough to modify the size distribution (Fig. 371.

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The Tenth Annual Conference of the Association for Aerosol Research

225

CONCLUSIONS Although the present experimental results are not conclusive at this stage, it is our opinion that the study program looks promising. We found some evidence for source influence over the ground level measured concentrations and size distributions. A collecting technique improvement at ground level as well as a direct measure of the plume, for example with LIDAR technique, and finally a correlation with the in-stack measures, would allow us to reach more conclusive answers.

REFERENCES Anfossi, D., Bonino, G., Bossa, F. and Richiardone, R. (1978) Atraos. Environ. 12, 1821-1826. Bonino, G., Lombardini, P. P., Longhetto, A. and Trivero, P. (1981) Nature 290, 121-123. Caruso, E, Braga Marcazzan, G. M. and Redaelli, P. (1981) Nuclear Instr. Meth. 181,425. Boneres, L. C. S. and Orsini, C. M. Q (1981) Nuclear Instr. Meth. Igl, 417. Cooper, J. A., (1973) BNWL-SA- 4690.

P A R T I C L E R E T E N T I O N IN A BED OF C R U S H E D R O C K , U N D E R C O N D I T I O N S OF C O N D E N S I N G S T E A M LARS STROM, JAN CHYSSLER and GUNTHER GEBERT

Studsvik Energiteknik AB, S-611 82 Nyk6ping, Sweden Al~traet--The retention of aerosol particles in a bed of crushed rock (stone size 25-30 ram, height 7.5 m) was measured. The carrier gas was an air-steam mixture at 110°C. Steam constituted 0.3 or 0.6 of the total gas flow. The steam condensed in the bed, enhancingparticle deposition. Parallel experiments at room temperature, and with air as carrier gas, permitted the formulation of a theoretical model for particle deposition in the dry case, and served as the basis for estimating the effect of condensing steam. Deposition depended strongly on particle size, gas velocity, and

steam content, and to a lesser degree on particle material (CaO-Fe203), and concentration (4-400 mg/m~). At 5 (10) 20 cm/sec the penetration was for 1/~m particles, at low steam contents 0.1 (0.2) 0.4 and at high steam contents 0.005 (0.01) 0.08, respectively.

1. I N T R O D U C T I O N The filtration of airborne particles by m e a n s o f granular beds is of interest for certain applications, where the filter m e d i u m is exposed to extreme conditions o f temperature, pressure, chemical attack, and suchlike. The efficiency of sand bed filters for gas filtration has been studied theoretically as well as experimentally (Gutfinger et al., 1979; Gebhart et al., 1973). Very high efficiencies have been reported for some beds of fine sarld. Larger grains lead to lower efficiency, and are normally not used. There seems to be no published information on the filtering effect of beds o f coarse materials, such as crushed rock. The investigation reported here was originated by the need for data on the filtration efficiency o f a crushed rock bed, intended for emergency steam condensation and gas cleaning for nuclear reactors (Johansson et al., 1982). The steam and radioactive particles generated during a loss-of-coolant accident would require condensation and filtration, should it be necessary to relieve the pressure in the reactor containment to the atmosphere. If the aerosol carrier gas contains steam, condensation will take place in the stone bed. This is assumed to enhance particle deposition, as there iS a net gas movement to the stone surfaces, and also because water vapour will condense on particles, making them larger and more easily deposited. There is also the thermophoretic effect, moving particles towards cold surfaces. The deposition process is quite complicated because of the high heat and mass transfer rates, strong inhomogenity, and the transient nature of the process, when the gas passes the rather thin condensation zone. The effect of condensing steam can be estimated, however, by comparison with dry bed deposition.

2. E X P E R I M E N T A L C R U S H E D R O C K BED Deposition experiments were performed in a 7.5 m high column, filled with stones of size fraction 25-35 m m , see Fig. 38. Air, steam and the test aerosol were brought together in a mixing chamber, with a throughput of about 200 1/sec. The test aerosol was a resuspended dust from a steel works furnace exhaust. F r o m the mixing chamber the test flow desired for each experiment was conducted to the top of the bed. Immediately above the bed, portholes were arranged for aerosol sampling, which permitted aerosol characterization at the column inlet. Identical portholes and sampling arrangements were installed at the column outlet. Parallel samples were taken, one for total concentration (glass fibre filter), and one for size distribution (Andersen ambient sampler cascade impactor). Before each test the column was dried and brought to r o o m temperature. During the test a constant aerosol flow was admitted to the column from the mixing tank. A condensation zone developed and moved slowly down the column, preceeded by a condensate flow. When static conditions were established at the inlet, sampling was begun and continued until mist appeared at the outlet, when sampling was interrupted.