Radon 222 as a Tracer of Atmospheric Motions: Research in Lombardy. A contribution to subproject LOOP

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1 Radon 222 as a Tracer of Atmospheric Motions: Research in Lombardy A contribution to subproject LOOP L Sesana*, L Barbieri*, U. Facchini* and G. LanzanP ^Istituto difisica Generate Applicata, Via G. Celoria 16, Milano, Italy ^Amministrazione Provinciale di Como, ViaBorgovico 148, Como, Italy Introduction Measurements of atmospheric concentrations of radon 222 supply an index of atmospheric stability, of the formation of thermal inversions, of convective turbulence, and of the movement of air masses, and hence are extremely relevant to the understanding of the diffusion of atmospheric pollutants. In 1996 our Institute set up three stations for radon 222 concentration measurements in the town of Milan and in the prealps area (Facchini and Sesana, 1996; Facchini and Sesana, 1997; Sesana et al., in press). A map showing the sites of the radon stations is given in Fig. 1. O Saronno Fig. 1: The map shows the sites of radon stations, a) Milan; b) Alpe Vicere; c) Ghisallo Basin. Proceedings ofeurotrac Symposium '98 Editors: P.M. Borrell and P. Borrell 1999: WITPKESS, Southampton

2 Radon 222 as a Tracer 901 One station is located in Milan, namely in the garden of the Physics Institute. The other two stations are located in the prealps: * at Alpe Vicere, at forty kilometres north of Milan: this is a prealp upland at a height of 900 metres above sea level, overlooking the plain; * in the Ghisallo Basin, south of the Ghisallo pass in the high Vallassina, at height of m. Measurement of the radon concentration in the atmosphere was performed with a high-volume alpha particle detector. Results and discussion Milan station Fig. 2 shows the radon 222 concentrations measured in the month of April The figure shows the wide variations in radon concentrations on single days and between different days. In the late morning up until the afternoon radon concentrations were modest, in the order of a few Bq/nf; while two different patterns are observed at night: on clear windless nights following sunny days radon accumulates to high levels of up to 30 Bq/nf, whereas on overcast, rainy or windy nights little or no radon accumulates. These observations were true for all the months in which measurements were made. 10 II II II 14 l> I* 17 II It JO J M 27 2t 21 JO Fig. 2: Radon, Milan station, April 1997.

3 902 L. Sesana et al. On clear days there are two typical situations involving the layer and its variation. a) Night trend A temperature inversion often occurs overnight at ground level. This is due to radiative cooling of the ground and causes the formation of a Nocturnal Stable Layer. Motions are not completely suppressed in the Nocturnal Stable Layer. The model proposed by Fontan and coworkers (Fontan et al, 1980) and then adjusted by Fujinami and Esaka (Fujinami and Esaka, 1987) assumes that radon mixing occurs in a layer of height he inside the NSL. The authors define this height as the "Equivalent Mixing Height" (EMH). The formula proposed makes use of the rise in radon concentration at night, starting from the minimum value in the afternoon, as follows: h, = [# (1 -e'^)] / [X (Q - Qe'^)] = (O At) / (Cf - C,) where Cf and C, are the maximum and minimum concentration values, during the rise time At, O is the radon flow rate from the ground, 1 is the decay constant of radon 222. Three basic hypotheses are made: 1) radon concentration is uniform in the box of height he; 2) radon flow rate from the soil is nearly constant over a large surface; in Milan this was found to be about 72 Bq/(nf h); 3) radon concentration variations are related solely to the vertical stability; there is no advection of air masses with different radon concentrations. Application of the formula to reference days when night accumulation marked gives values of he of m. b) Day layer At daybreak solar radiation warms the ground and the temperature inversion is destroyed; vertical thermal mixing takes place and the mixing layer rises upwards. In summer this layer can rise to a height of more than 2000 metres over the Lombardy plain. After the nocturnal increase in the EMH, radon concentration decreases reaching its minimum value in the afternoon around 6 p.m. was

4 Radon 222 as a Tracer 903 Radon and ozone at themilan station The study of radon concentration gives direct information on the diffusive properties of the low troposphere. The discussion of the ozone-radon relation will consider daytime and night hours separately, starting with what happens on a clear day followed by a still windless night. (a) Fig. 3: Radon and ozone, Milan station, (a) April 17th-19th, 1997; in the night radon accumulates, ozone level is minimum; (b) March 21st-23rd, 1997; in the night radon does not accumulate, nocturnal peak of ozone is present.

5 904 L. Sesana et al. On a clear day with intense solar radiation, the chain of photochemical reactions between nitrogen oxides and VOC activate the production of ozone during daylight hours, at the same time as radon disperses. The dispersion of radon marks the start of the vertical turbulence, which carries the radon along with the ozone precursors and ozone itself to high quotas. At night a temperature inversion with a base at ground level occurs. The PEL is divided into two layers: the nocturnal stable layer and the upper residual layer. This structure is directly indicated by the accumulation of radon at ground level. The pollutants, NO, NO2 and VOC, accumulate in the lower layer, however minimum levels of the nightly ozone concentration are observed. The accumulation of radon signals a low concentration of the nocturnal ozone (Fig. 3a). Let us consider conditions in which radon did not accumulate at night and remained at low concentrations. These nights are characterised by high levels of ozone, which peaked sharply in thefirsthours after midnight (Fig. 3b). A hypothesis on the origin of high nocturnal levels of ozone can be made: nocturnal ozone peaks occur when vertical instability is present, as indicated by the failure of radon to accumulate; hence, nocturnal ozone is transported downwards from height. Alpe Vlcere station Concentrations measured at Alpe have different trends to those measured in the plain (Fig. 4), the nocturnal accumulation of radon observed in the plain being absent here. Alpe station is situated at the level of the residual layer. /V v J Fig. 4: Radon and wind speed, Alpe Vicere station, September 4th-7th, 1997.

6 Radon 222 as a Tracer 905 Two points should be noted: * on the whole, the base line of radon concentrations measured at Alpe Vicere is on par with the minimum values obtained in Milan. This correspondence is interesting, because the Milan minimum corresponds to the mixing of radon in the PEL; * radon peaks were observed between 10:00-12:00 a.m. and 4:00 p.m. at Alpe Vicere. Fig. 4 shows the correspondence between the peaks of radon and the initiation of the valley breeze. Ghlsallo basin station Ghisallo is situated at about 800 m a.s.l. in a basin surrounded by mountains. Findings obtained from the Ghisallo station show cycles of accumulation and dispersion in the radon concentration similar to those observed at the Milan station. Accumulation takes place at night, beginning in the early hours of the evening or later towards midnight, the accumulation being less well defined than it is in the plain. The nocturnal accumulation of radon indicates the formation of layers of limited height which are stable nocturnally and which dissolve at sunrise, carrying the radon to height. Conclusions The study of radon concentration in the atmosphere and the trend in concentration during the day show interesting features which are generally correlated with the conditions of stability or turbulence in the atmosphere either thermal and dynamical. References Facchini, U., L. Sesana; Atmospheric turbulence: radon as a tracer of thermal and gravitational motions of the atmosphere. Air Pollution IV, Tolosa 1996, pp Facchini, U., L. Sesana; A research by radon measurements in Milan and in the Prealps. Air Pollution V, Bologna 1997, pp Sesana, L., L. Barbieri, U. Facchini, G. Marcazzan; Radon 222 as a tracer of atmospheric motions: a research in Milan. Radiation Protection Dosimetry, in press. Fontan, J., D Guedalia, A. Ntsila, A. Druilhet; Monitoring of atmospheric stability above an urban and suburban site using sodar and radon measurements. J. Applied Meteorology 19 (1980) Fujinarm, N., S. Esaka; Variations in radon 222 daughter concentrations in surface air with atmospheric stability. J. Geophys. Res. 92 (1987)