Concentration of ice nuclei in continental and maritime air masses in León (Spain)

Atmospheric Research 47–48 Ž1998. 155–167

Concentration of ice nuclei in continental and maritime air masses in Leon ´ ž Spain/ A. Castro a b

a,)

a , J.L. Marcos a , J. Dessens b, J.L. Sanchez , ´ R. Fraile a

Laboratorio de Fısica de la Atmosfera, UniÕersidad de Leon, ´ ´ ´ 24071 Leon, ´ Spain Laboratoire d’Aerologie, UMR CNRSr UPS 5560, 65300 Campistrous, France ´

Abstract Among the objectives of the Plan de Actuacion ´ de Lucha Antigranizo, ŽPALA, Hail Suppression Activity Plan., is the on-going analysis of the mechanisms operating during the formation of storms. There is a relationship between the processes at work in the interior of cloud masses and the concentration of natural ice nuclei ŽIN. at ground level. Through the rigorous analysis of 954 measurements made with an isothermal cloud chamber, it has been possible to establish that a dependence exists between the background concentration of IN and the type of air mass. The work shows that the concentration of IN in the samples analyzed in Leon ´ was, on average, higher for continental than for maritime air masses, with a substantial presence of continental air masses originating from the Sahara desert. Furthermore, from the analysis of 4-day back trajectories, it has been observed that slow-moving maritime air masses gradually become altered during their passage over the peninsula, which causes an increase in the number of active IN. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Ice Nuclei; Isothermal cloud chamber; Continental air masses; Maritime air masses

1. Introduction The processes at work in the ice-formation phase within the atmosphere are highly complex. Various kinds of cloud chamber devices can be used to analyze the concentrations of ice nuclei ŽIN. which, when activated, initiate the nucleation process ŽFletcher, 1962; Gagin, 1972; Huffman and Vali, 1973.. Although the methods available for measuring IN levels have their limitations, it can nevertheless prove useful to establish )

Corresponding author.

0169-8095r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 8 0 9 5 Ž 9 8 . 0 0 0 6 0 - X

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the concentration of natural IN at ground level, in order to go on interpreting the primary nucleation mechanisms which occur in the interior of cloud cells ŽGagin, 1971.. Sources of natural IN, however, are organic and inorganic substances intimately tied to the composition and origins of air masses. The comparative measurement of IN concentrations carried out with identical instruments and under identical conditions of saturation and temperature, demonstrate that the number of active nuclei varies greatly, depending, among other factors, on the type of air mass sampled. Since 1985, a project known as PALA has been under development in Leon ´ ŽSanchez ´ et al., 1994.; one of its objectives has been the analysis of the controlling mechanisms involved in the formation of storm clouds, and the precipitation caused by them. One of the aspects under study, which the present article focuses on, relates to the processes governing the formation of ice crystals, and their variability and relationship with the type of air masses affecting the study zone on each sampling day. For this reason, two measuring campaigns were carried out in 1994 and 1995 involving air samples, which were analyzed in real-time by means of an isothermal cloud chamber at ground level. By means of the latter, two mechanisms of heterogeneous nucleation have been observed: condensation-freezing and contact. Future sampling campaigns to be carried out in the interior of cloud cells are also planned, in order to contrast IN concentration levels in clouds with those at ground. The current study presents, however, the results of the analysis of 954 air samples taken under distinct meteorological conditions and subjected to different temperatures and analyzed in the ground level cloud chamber constructed by the authors ŽCastro et al., 1996..

2. Design of experimental process The nucleation activity of a given mass of atmospheric air can be measured by exposing the nuclei to a near-saturated atmosphere with respect to water in which the temperature has previously been lowered to a determined point. For the present study, an isothermal cloud chamber, made of three identical but independent tanks, with a volume of 11 l each was used. Each tank was equipped with its own refrigerator, operating on a concertina coil on the bottom and sides of the tank, with a thermostat activating the compressor to a precision of "0.18C with respect to the chosen working temperature. Air samples were introduced into the containers through an air intake for 1 min and cooled until the predetermined temperature was reached; water vapour was likewise introduced for 1 min and 45 s until the enclosed air reached saturation with respect to water and the humidity was maintained just above 100%. Previous research studies established suitable working temperatures for air masses, the nucleation period and the methodology to be followed. The chamber used is capable of operation at different temperatures, but in line with other authors ŽRosinski et al., 1987; Perez ´ et al., 1985., specific temperatures were selected for study; in this case y158C, y198C and y238C. When the chamber was in operation at temperatures lower than y238C, the number of crystalsrcm2 growing in the sugar solution was, on many days, so great, that overlapping made accurate counting

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difficult. Meanwhile, at temperatures greater than y158C, the number of active IN was much reduced on a majority of days, falling to zero at temperatures greater than y108C. The specifications recommended by Soulage Ž1964., who used a similar chamber, as well as others developed by the authors, were followed. The following remarks should be emphasized: Ža. the tanks underwent a thorough cleansing after each experiment, and the sides and bottom were thoroughly coated with glycerine to prevent forming frost and crystals breaking away to the solution, which would have falsified measurements, Žb. before the first experiment of the day began, a temperature stabilization period for each tank was established, of 1 h, and of a minimum half hour between experiments, Žc. the air sample was expelled by suction for 2 min, and a new sample introduced by a further air intake of 1-min duration, and Žd. the ice crystals which formed grew on a brass tray containing a solution 3-mm thick of water, sugar and glycerine: 100 g of water, 130 g of sugar and 2 g of glycerine were the quantities used for temperatures of y158C; for y198C, the quantities were 100, 130 and 8 g; and 100, 130 and 11 g for y238C. The crystals grow by means of the solution until they reach a size which allows them to be seen with the naked eye on the surface of the tray, which has a grid with a black background imposed upon it. The time length between the end of the humidification period and the moment when crystals begin to be visible on the sugared solution is scarcely 1–5 s at y238C, while 10–20 s lapse at y198C, and 5–10 min at y158C. The chamber was positioned in a rural area, far away from any potential source of pollution, so that information could be obtained about the nucleating capacity of air in the natural state. All measurements were carried out at an observatory located in the province of Leon ´ ŽNorthwest Spain., some 300 km to the east of the Atlantic coast, 150 km to the south of the North Atlantic and the Bay of Biscay, and 800 km to the northwest of the Mediterranean and at an altitude of 830 m. In Fig. 1, the position of the

Fig. 1. Location of the isothermal cloud chamber in Leon ´ ŽSpain., where IN measurements were carried out, and air masses affecting the Iberian peninsula.

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laboratory within the province of Leon ´ is shown with the different areas from where the air masses which affect the Iberian peninsula come. Samples were taken during the two campaigns in the summers of 1994 and 1995; 318 air samples were analyzed, 240 in 1994 and 78 in 1995, for each temperature Ža total of 954 analyzed samples., and the mean value of the atmospheric pressure at surface level on sampling days was 923 hPa.

3. General characteristics of the data obtained To attempt to count the number of active IN as a function of temperature requires the rigorous control of the conditions under which the nucleation evolves. Vali Ž1975. suggested that each measurement of IN concentration should be linked to the system and instrumentation which measured it. In the present case, the first objective was to ensure that the measurements made were reliably reproducible: in other words, that the concentrations obtained simultaneously for a single air sample in each of the three tanks were the same. Once this was confirmed, the two measuring campaigns were designed as follows: Ža. the 1994 campaign had the objective of determining whether variations in IN concentration existed over short time intervals. Readings were thus taken at 9, 12 and 15 UTC during the months of June through August, for 80 days. For each working temperature, 240 samples were obtained, 720 in total; Žb. the 1995 campaign was carried out over the same months, to establish whether there were appreciable differences from one year to the next, and also to increase the sample size. Measurements were made for 78 days Ž234 in total.. For the duration of the first campaign, readings were taken at 9, 12 and 15 UTC ŽTable 1.. Taking X T , H , Y as the set of measurements for each temperature ŽT ., hour of the day Ž H . and year Ž Y ., it may be seen that nine series are formed:: Xy2 38C, 9 UTC, 1994 , Xy2 38C, 12 UTC, 1994 , Xy238C, 15 UTC, 1994 , Xy198C, 9 UTC, 1994 , Xy198C, 12 UTC, 1994 , Xy198C, 15 UTC, 1994 , Xy158C, 9 UTC, 1994 , Xy158C, 12 UTC, 1994 , Xy158C, 15 UTC, 1994 . However, it could be seen that the number of IN in the atmosphere on each day did not vary greatly between 9 and 15 UTC, so the Kruskal–Wallis test at a significance level of 5% ŽEssenwanger, 1986. was applied to the data to test the homogeneity of the three series Xy2 38C, H, 1994 . In other words, an attempt was made to establish whether the differences observed in IN concentration at three distinct times of the day, each 3 h apart, were statistically significant. If not, it could then be assumed that the readings derived from the same sample, and a single daily reading during the same period Ž9 to 15 UTC. would then

Table 1 Measurements for IN concentration taken at different times and temperatures during the summer of 1994 are indicated by B while those taken in 1995 are indicated by v Timertemperature

y158C

y198C

y238C

9 UTC 12 UTC 15 UTC

B Bv B

B Bv B

B Bv B

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suffice. In this case, the value of statistic H and the corresponding significance level were H s 0.73 Ž a s 69%., indicating that the series were homogenous, and giving therefore the single series Xy2 38C, 1994 , taking the average value of the three measuring times for each day. Thus, the sample is now composed of 80 days, with 80 readings per temperature. Analogously, the homogeneity of the three series Xy1 58C, H, 1994 Žmeasured at y158C. and the three series a Xy1 98C, H, 1994 Žmeasured at y198C. was confirmed, with values of H s 0.89 Ž a s 64%. and H s 1.24 Ž a s 53%., respectively. The single series Xy1 58C, 1994 and Xy198C, 1994 were thus constructed. Because the 1994 data were seen to be independent from the time of day when the measuring took place, in 1995 only one reading was taken daily, at 12 UTC. Thus, the six data series were constructed: Xy2 38C, 1994 , Xy198C, 1994 , Xy158C, 1994 , Xy238C, 1995 , Xy1 98C, 1995 and Xy158C, 1995 . To determine if it would be possible to unite the six series X T , Y into three only, which may be called X T , the Mann–Whitney test ŽEssenwanger, 1986. was applied to the 1994 and 1995 series. Once again, the results showed that they were indeed homogeneous, with no significant differences between the two campaigns. Consequently, the data series were merged, giving the series Xy1 58C , Xy198C and Xy238C , each one composed of 158 data. While, analyzing the IN series, the lowest IN concentration levels were found to be 0 ice nucleirl at y158C, 1 nucleirl at y198C and 7 nucleirl at y238C. On the other hand, a considerable dispersion among the data could be seen ŽTable 2., owing to the high values applying on certain summer days: a dispersion which has also been detected in latitudes similar to the province of Leon ´ ŽAdmirat, 1963 in the southwest of France; Perez ´ et al., 1985 in the province of Valladolid, Northern Spain.. When applying the Kolmogorov–Smirnov test to a significance level of 5%, it became apparent that the distributions were exponential—probability density function of type b expŽyb n. —for the three temperatures of y158C, y198C and y238C. The results are shown in Table 2. Nevertheless, Perez et al. Ž1984. obtained log-normal distributions in Valladolid. ´ Reference will be made to median values of the samples throughout this study, for purposes of interpretation or evaluation. In Figs. 2–4, the active IN concentration per litre is shown, along with the exponential distribution fit at y158C, y198C and y238C, respectively.

Table 2 Distributions of the no. of IN per litre at the three working temperatures; value b of exponential distribution, and value for the minimum, maximum, median and standard deviation for each distribution

No. of data Distribution b Žlitrerno. of IN active. Minimum ŽIN activerlitre. Maximum ŽIN activerlitre. Median ŽIN activerlitre. Standard deviation

y158C

y198C

y238C

158 exponential 0.055 0 179 7 27

158 exponential 0.016 1 356 35 74

158 exponential 0.008 7 527 95 114

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Fig. 2. Concentration of active ice nuclei per litre at a temperature of y158C in the isothermal cloud chamber. Exponential distribution fit.

Fig. 3. Concentration of active ice nuclei per litre at a temperature of y198C in the isothermal cloud chamber. Exponential distribution fit.

Fig. 4. Concentration of active ice nuclei per litre at a temperature of y238C in the isothermal cloud chamber. Exponential distribution fit.

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4. Results Various kinds of analyses were made of the data, and the results may be considered under the following headings. 4.1. Analysis of actiÕe IN as a function of temperature The exponential form of the dependence of IN on temperature has been established by numerous authors Že.g., Hobbs, 1993.. There is a clearly-defined correlation with the temperature, whereby the number of active ice nuclei increases as the temperature of the cloud descends. The results of the present study in Leon ´ reveal an exponential dependence of the form: N s c q exp Ž b 0 q b 1T . where N is the concentration of IN active per litre, and where c, b 0 and b 1 are constants. The values c s y6.09, b 0 s y0.77 and b 1 s y0.238Cy1 are valid for temperatures between y118C and y238C, given that IN were not found at temperatures above y108C. The accounted variance is 99.8% with a coefficient correlation R s 0.999. The median values are 7, 35 and 95 ice nucleirl for the three experimental temperatures of y158C, y198C and y238C, respectively, representing a quite wide dispersion ŽFig. 5., but one which has also been found in the results of other researchers working in rural observation points and at similar latitudes: in Dessens Ž1986., for example, for studies carried out in Lannemezan ŽFrance. where values of between 1 and 100 ice nucleirl were recorded at y218C, and in Perez ´ et al. Ž1985., for studies in

Fig. 5. Concentration of active ice nuclei per litre vs. cloud temperature. The values for the mean and the median are indicated, and between the bars, values are given for the mean"SD.

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Valladolid ŽNorthern Spain. with values of 0.1 ice nucleirl and 315 ice nucleirl at y218C were recorded. Both worked with an isothermal chamber of a similar design to that used in the present study. Reference will be made in the following sections to the results found at y158C only, given the excellent correlation with the temperature which the results show; conclusions for any other temperature of interest can be easily extrapolated. 4.2. Analysis of IN in situations classified according to maritime and continental origin of air mass It is known that the aerosols acting as ice embryos are part of the total population of aerosols in the atmosphere ŽRosinski et al., 1981; Zamurs and Jiusto, 1982., that some are more active than others, and that the behaviour of the ice nuclei may be attributed to the origin of the air mass ŽBertrand et al., 1973.. In looking for a correlation between the number of IN measured in the study zone, and the origin of the air masses crossing it, a difference could be made between continental and maritime air masses. The continental air masses flow predominantly from the northeast and southeast, i.e., they include both the warm and cold polar air masses coming from the European continent ŽcPK and cPW; see Fig. 1. and thus, having accumulated anthropogenic and natural pollutants through crossing the emission zones, and air masses with a tropical origin ŽcTW. which come laden with desert dust and sand from the African Sahara region. The maritime air masses, meanwhile, flow predominantly from the southwest across the Atlantic, from the sub-equatorial tropic ŽmTW., or from the northwest, including those warm and cold polar air masses which have also crossed the Atlantic before reaching the Iberian Peninsula, and which contain mainly a variety of sea salts, such as aerosols. Using the synoptic charts provided by the European Meteorological Bulletin Ž850 hPa level. the origin of the air entering the study zone was analyzed on each day for which IN were measured, at 1200 UTC. The back-trajectory of an air parcel was calculated, following the 850 hPa isohypses which passed over Leon ´ after a trajectory of 3000 km. From the results presented in Table 3, it may be seen that this particular study zone is more affected by maritime than by continental air masses Žon 76% as opposed to 24% of days, respectively. but that the number of active IN is higher under the influence of continental air masses. The median values are, respectively, 5 and 11 active INrl at y158C. At y198C and y238C, more IN are detectable in the continental air masses than in the maritime ones. On applying the non-parametric Mann–Whitney test to compare the maritime samples with the continental ones, the following values for the critical ratio Z and their corresponding significance levels, were found: at y158C, Z s y2.18 and a s 2.9%; at y198C, Z s y2.16 and a s 3%, and at y238C, Z s y1.88 and a s 6%. For a significance level of 5%, the IN concentration of the maritime air masses with respect to the continental ones proved to be different at y158C and at y198C but not y238C; however, seeing that the significance recorded was not excessively greater than 5%, it was maintained in the present study. This poor result at y238C was attributed to the imprecision which occasionally arises in the measurement of high concentrations of IN.

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Table 3 Distributions of the no. of IN per litre at the three working temperatures; value b of exponential distribution, and value for the median, mean and standard deviation of each distribution for days with maritime or continental air masses Maritime

No. of data Exponential distribution b ŽlitrerIN active. Median ŽIN activerlitre. Mean Standard deviation

Continental

y158C

y198C

y238C

y158C

y198C

y238C

120 yes 0.055 5 18 30

120 yes 0.017 29 59 75

120 yes 0.011 92 125 119

38 yes 0.067 11 15 13

38 yes 0.014 45 69 71

38 yes 0.007 107 137 96

As early as 1954, Rau Žcited by Fletcher, 1962. encountered the same pattern confirming what Bertrand et al. Ž1973. later said in their reports on West African projects, particularly in affirming that IN concentration levels are influenced by the origin of air masses. Levi et al. Ž1994. suggest that desert dust is a valuable source of ice nuclei, and that the continental air masses affecting the Iberian Peninsula in summer derive from the Sahara in northern Africa–32 of the 38 days analyzed in his study registered continental Warm Tropics ŽcTW. air masses-corroborating the present authors’ discovery of higher IN levels on days of continental air masses over Leon. ´ On only 6 days, the continental air masses were of European origin, and the median value was 8 nucleirl at y158C, as opposed to 11 nucleirl for continental masses of African origin. Nevertheless, the non-parametric Mann–Whitney test indicates that there are no significant differences, with U s 73 and a s 36% for a significance level of 5%. This would seem to indicate that the air masses arriving over Leon ´ from Africa do not contain a greater number of IN than those of European origin. On 14 days, exceptionally high values for IN were found, exceeding 50 INrl at y158C. The air masses on 12 of those occasions were of the polar maritime variety ŽmP., and of the tropical maritime ŽmT. on only two, with respective median values of 85 and 60 ice nuclei activerl. There were 14 days, for which the number of IN recorded was less than 50rl at y158C, representing 91% of the sample, and registering a median value of only 5 ice nucleirl. So, the breadth of dispersion in the values recorded is seen to be due to the presence of the few days of supermaritime air masses in the sample, when the activity of glaciogenic nuclei is some 17 times higher, at a temperature of y158C. The figures for maritime air masses in general show a broader dispersion than those for the continental ones. When the former reach the Spanish coast, the average distance they have to cross to reach the measuring point, granted that there is a favourable component in the wind from the west, is 300 km. In summer, the average time taken by such an air mass to reach a position over Leon ´ was 2–4 days. During this time, they undergo a certain amount of adulteration, with respect to their original composition, resulting from the importation of glaciogenic nuclei of continental origin, which may be considerable. This additional contribution may be responsible for the greater dispersion

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of the figures found for maritime IN, which is something that will be emphasized in the following section. 4.3. Analysis of IN present in modified maritime air masses The air masses affecting the province of Leon ´ have a long journey to make from their place of origin; the exact trajectory followed by a glaciogenic particle from its point of origin to the sample point is known as a back trajectory. The computation of back trajectories in the present study has been carried out by the French Meteorological Service, using the synoptic wind field and vertical velocities obtained from the analyses of the European Centre for Meteorological Weather Forecasting, Reading, UK ŽMartin et al., 1984.. To make such an evaluation, only the exact location and pressure level at the final point are required. Given this, the position of the particle at previous 6-h periods can be established, for up to 4 days before the collection of the air sample. For the day considered, the trajectories of air masses arriving at the cloud chamber at the specified time were computed for the 900, 500 and 200 hPa levels. For the present study, reference will be made to the back trajectory for the 900 hPa level. General studies have been made to confirm the reliability of this kind of trajectory computation ŽMartin et al., 1987.. In Section 4.2, consideration has been made of the distinction between continental and maritime air masses, without studying the adulteration which the latter may undergo on the phase of their journey from the coast to the isothermal cloud chamber where the IN measurements were made. The surface winds over the Leon ´ province have a predominantly W and SW component ŽSanchez et al., 1994., in other words, they are of ´ Atlantic origin, in accordance with the fact that the summer air masses affecting the region are primarily maritime. The winds from the south and southeast likewise correspond to the trajectories of the north African continental air masses. These seem to be less affected with respect to the composition of their glaciogenic nuclei, as the nuclei accumulated in their trajectories remain continental, independent from their origin, whether man-made Žindustrial pollutants. or natural Žforest fires, etc... The maritime air masses, however, coming from the Atlantic, have the above-described long journey Žminimum 150–300 km. to make, between the crossing of the Iberian coast and the arrival over Leon, ´ incorporating, as they do so, new aerosols of continental origin, which may give rise to a significant modification. Authors such as Elliot et al. Ž1974. and Perez ´ Ž1984. point out that maritime masses should have been more than 36 h en route before they reach the continent, and then reach the relevant observation point in the overland part of their journey in less than 13 h. Elliot et al. Ž1974. considers 36 h for the oceanic passage to be necessary, since this is the estimated time taken for the aerosols of a continental air mass to be changed by coagulation and sedimentation processes into those of a typically maritime mass. Otherwise, they consider that maritime air masses are in fact modified maritime masses. To explore this possible continental factor, the details of 18 back-trajectories were examined. In eight cases, even though at the time of the reading the air masses were classified as maritime, it was noted that after crossing the coast they advanced only

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gradually, taking an average of 67 h Žalmost 3 days. to reach Leon, ´ which implies a compositional transition from that of a maritime air mass to that of a modified maritime mass. In other words, they had been continentalized. On those occasions, the median value of the readings for the sample were 20 INrl at y158C, 56 INrl at y198C and 104 INrl at y238C. Compared with the figures presented in Table 2, these values represent a substantial increase in the concentration of active ice nuclei, so it would appear that maritime masses with a slower approach incorporate a larger number of locally-derived ice-nuclei than those which are faster moving. In Fig. 6, the back trajectory for 01r07r95 at the 900 hPa level is given, showing that the air mass remained over the Iberian peninsula for some 64 h. On the same day, the cloud chamber values for IN concentration recorded were of 16, 56 and 95 ice nuclei activerl at y15, y19 and y238C, respectively: these figures are manifestly higher than the median values evaluated for maritime masses Žsee Table 3.. This case can be interpreted as representing an air mass with an origin over the Atlantic whose slow passage over the peninsula before arriving at the sample point in Leon ´ resulted in the incorporation of continental materials, and the alteration of its original composition. This kind of slowly approaching maritime mass is quite a frequent occurrence in the study zone, and is probably representative of geographically contiguous areas: from which it may be observed that the concept of the maritime mass has to be adapted to the precise kind of trajectory which it has followed.

Fig. 6. Reconstruction, based on data supplied by the European Centre for Meteorological Weather Forecasting, of the trajectory taken by an air mass between 06r27r1995 and its arrival at the sampling point on 07r01r1995. The crosses represent the position of the air mass at 6-h intervals.

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5. Conclusions From this analysis of the data generated after two summers of measurement of the concentration of IN as part of the Leon ´ Weather Modification project, the following conclusions have been drawn: Ž1. the background concentration of IN active at y158C, y198C and y238C fits in all the cases an exponential distribution; Ž2. the dependence found between the concentration of active IN and the temperature is of an exponential nature; Ž3. the concentration of IN in the samples analyzed in Leon ´ were, on average, higher for continental than for maritime air masses; Ž4. the high incidence of air masses originating from the Sahara desert has led to a confirmation of the fact that the number of IN active is higher in these situations than in others. This fact can be accounted for by the thesis that the desert dusts are an important natural source of IN; Ž5. slowly advancing maritime air masses gradually become adulterated during their passage over the peninsula, which causes an increase in the number of active IN; and Ž6. it is convenient to distinguish between maritime and supermaritime air masses, granted that the latter have been found to contain an IN concentration 17 times greater than that of the former.

Acknowledgements This work was supported by Excma. Diputacion ´ Provincial de Leon ´ and Consejerıa ´ de Agricultura ŽJunta de Castilla y Leon ´ . as a part of Plan de Actuacion ´ de Lucha Antigranizo.

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