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Some physical factors influencing scavenging ratios
Atmospheric Environment Vol. 24A, No. 12, pp. 3073-3083, 1990 Printed in Great Britain.
0004-45981/90 $3.00+0.00 Pergamon Press plc
SOME PHYSICAL FACTORS I N F L U E N C I N G SCAVENGING RATIOS JEAN-LUC JAFFREZO* a n d JEAN-LOUIS COLIN Laboratoire de Physico-Chimie de l'Atmospbere, Universit6 Paris VII, 2, Place Jussieu, 75 251 Paris Cedex 05, France and JEAN-MIcHEL GROS Mgt6orologie Nationale, SCEM/ES, 2, avenue Rapp, 75 230 Paris Cedex 07, France (First received 1 Auoust 1989 and in final form 7 June 1990)
Almtraet--A better understanding of the causes of scavengingratio variation is required if one is to use them more preciselyfor predicting the chemicalconcentrations in precipitations. For this purpose, a set of 82 rain and associated aerosol samples were obtained during a 1-yearsampling program in Paris. Scavengingratios are calculated for C1, SO2 -, Na, K, Mg, Ca, Zn, Al, Si and Fe. First, it is shown that scavengingratios enable us to describe the scavengingprocesses correctly,even on an urban site:classificationof air masses according to back-trajectories indicates similar long range transport impact on both aerosol and precipitation. Scavenging ratios are then shown to vary with the volume of rain: this is interpreted as a dilution effect related to the depletion of the aerosol source. On the other hand, average rain-rate has little effect, if any, on the scavengingparameter. Finally, the role of the cloud mass type (stratiform or convective)is investigated, in relation with differences in nucleation capabilities. Key word index: Scavenging ratios, air mass trajectory, rain volume, rain-rate, stratiform situation, convective situation, scavenging mechanisms.
INTRODUCTION The atmospheric aerosol is the main source of most of the non-reactive mineral species found in precipitations. The concentration variations in rain or snow water can therefore be attributed to two major causes: the concentration variation in the aerosol scavenged, and the variations in the scavenging processes. Many physico-chemical models have been developed to account for interphase coupling, but they are generally too complex to be readily applied to real cases. Correspondingly, field studies describe the relationship between the aerosol and the wet phase, for each of the particulate species, by means of a simplified parameter, the scavenging ratio (W) (Chamberlain, 1960; Engelmann, 1970). This is defined as the dimensionless ratio of the concentrations measured in each phase. By normalizing the concentration observed in the liquid phase with respect to that in the associated aerosol, one intends to take into account the variations in the source term, the aerosol. Thus, despite the many assumptions included in this simplification (Slinn, 1983), the effect of certain processes or physicochemical parameters on the transfer of species between phases can be investigated (Scott, 1981; Barrie, 1985; Jaffrezo and Colin, 1988a). *Present address: Department of Civil Engineering, Carnegie-Mellon University, Pittsburgh, PA 15213, U.S.A.
However, in addition to its role in the study of precipitation and aerosol relationships, the scavenging ratio potentially constitutes a tool for evaluating the wet deposition of species if the concentrations in the aerosol are known (Arimoto et al., 1985; Uematsu et al., 1985; Davidson et al., 1985; Keene et al., 1986). This aspect is particularly important, insofar as local studies on aerosol are more abundant, more complete and easier than those on wet deposition, because of the stochastic nature of the latter. With a view to this application, it is therefore necessary to achieve a better understanding of the factors, in particular meteorological, which lead to the observed variability in the coupling parameter. It should then be possible to determine more closely the field of application of scavenging ratios and the conditions of their use. This type of study has already been undertaken, albeit for a limited number of factors. We present here a more in-depth study with data acquired during a 1-year program on an urban site. Thanks to the large number of data, it is possible to observe trends throughout the variety of situations encountered, and to compare them with theoretical expectations. First of all, however, it is necessary to validate the calculation of scavenging ratios from our chemical data. In order to obtain parameters depicting some physical reality, it is essential to ensure that the samples were obtained from interacting phases. In other words, we have to check that the aerosol collected does come
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JEAN-LUC JAFFREZO et al.
from the air mass which enriched the precipitation. This step is not usually discussed in studies dealing with scavenging ratios. It is all the more important here because of the urban nature of the sampling site, but also because of the very short sampling times, used for the first time in this sort of study.
SAMPLING AND ANALYSIS
The general field experiment has already been presented elsewhere (Jaffrezo and Colin, 1988a). This study concerns samples collected from May 1984 to April 1985 in downtown Paris. The sampling site is located on the roof of the University Paris VII, about 25 m from the ground to represent an average urban atmosphere and to minimize contamination by resuspension of road dust by the traffic. Precipitation is collected by means of an automatic sequential sampler (sampling area 1 m 2, giving 100 ml of water for fractions of 0.1 mm of rain depth). Dry deposition is avoided by a cover which protects the collector when it is not raining and is removed automatically at the onset of each event. These are thus individualized, with a discrimination time 10rain. The polyethylene funnel is carefully washed before each rain event. The rain sampling equipment is described in more detail elsewhere (Jaffrezo and Colin, 1987). The sample storage and analysis techniques have also been described elsewhere (Colin et al., 1987). After filtration on 0.4 #m pore size Nuclepore film, the rain samples are stored at 4°C in the polyethylene bottle used for collection. The soluble fractions are analyzed within 10 days for CI-, N O r and SO 2- (ion chromatography) and Na, K, Mg, Ca and Zn (by flame atomic absorption spectrometry). The insoluble elements AI, Si and Fe are analyzed by X-ray fluorescence spectrometry (Elichegaray et al., 1981). The total concentrations given here for each event are obtained either by analysis of the recombined fractions, or by calculation of the volume-weighted means after analysis of the entire series. Total aerosols were sampled continuously throughout the year. They were collected on filters consisting of 0.4/~m pore size Nuclepore film mounted on a Nylon ring (diameter 42 mm) fitting the geometry of the X-ray fluorescence apparatus. Four identical sampiing lines, each including a filter-holder, a pump and a volumetric meter are programmed to operate automatically in sequential 6-h intervals. This short sampiing time is possible despite the low flow rate (16 ~'min- 1) because the concentrations of the major species found in Paris are generally high (Dutot et al., 1983; Koutrakis, 1984) and because of the high sensitivity of the X-ray fluorescence apparatus (Losno et al., 1987). The selected filters are analyzed for CI, S, K, Ca, Zn, A1, Si and Fe. They are then removed from their support, suspended in 20 ml of Milli-Q water and
sonicated for 2 h. The solution is filtered through a 0.4/~m pore size Nuciepore filter prior to analysis for Na and Mg by flame atomic absorption spectrometry according to the same procedure used for the rain samples. With a continuous survey using a 6-h sampling interval, we are thus able to associate each rain event with the corresponding aerosol, by choosing the aerosol filter fraction collected just before the precipitation. Moreover, this short sampling time seems better suited to the description of scavenging phenomena than the daily period that is generally used. Daily sampling may encompass a succession of several meteorological situations not relevant to the rain sampled (Jaffrezo and Colin, 1987); furthermore, it would give only an average value over the rapid changes of atmospheric concentrations due to aerosol scavenging (Sisterson et al., 1985; Jaffrezo and Colin, 1988b). It should be noted that the techniques of sampling and analysis are designed to describe the coupling phenomena as closely as possible, by selecting only the interacting phases: (i) exclusion of dry deposition; (ii) separation of successive rains, since differences in concentrations can be very large even between two close events; (iii) aerosol collection over short periods; (iv) choice of the appropriate aerosol fraction. Finally, the automation of the sampling station enabled us to collect 82 rain and related aerosol samples, out of a total of 95 rain events collected; we have therefore data corresponding to a variety of situations spanning a whole year.
PHYSICALAND METEOROLOGICALDATA Additional physical and meteorological data are associated with each of these paired samples. The total volume of each precipitation is measured by weighing the fractions. The average rain-rate over the entire event can then be calculated exactly from the times of collector opening and closing which are recorded electronically. Air mass trajectories
The geographical origin of the precipitating system associated with each sample is determined from the air mass trajectories. These three-dimensional trajectories have been calculated by analyzing the wind fields from the European Centre for Medium Range Weather Forecasts (Lorenc et al., 1977; Martin et al., 1987). Trajectories can be supplied for every 6 h (00(X), 0600, 1200 and 1800h TU), for nine different pressure levels (100050 hPa), for the previous 5 days before arriving on site. For this routine survey, we used 925 hPa as the final pressure level. Back-trajectories were calculated for
Factors influencing scavenging ratios periods both before and after each event to check for possible changes in wind direction during the sampling. The few cases where the back-trajectories varied during a sampling period have been discarded. Events have then been classified according to a geographical zoning of the origin of their associated air mass. The definition of sectors adopted for this study is based mainly on the distribution of the main emission sources around the site, and leads to the definition of three geographical sectors (W, N, E and S), as shown in Fig. 1. Such a zoning system is similar to that of previous studies conducted in Paris, on the aerosol only (Elichegaray, 1980; Dutot et al., 1983; Koutrakis, 1984). It agrees quite well with the compilation of data on industrial emissions given by Semb (1978) for SO2 and Pacyna et al. (1984) for heavy metals over Europe. As long as the purpose of this classification is not to characterize extensively the impact of each source in each zone, such broad sectors are precise enough to achieve a sufficient geochemical differentiation of the events. Types of meteorolooical situations The events have then been classified according to the meteorological nature of the cloud mass, stratiform or convective. The purpose of this distinction is to roughly separate cases with strong (convective) or weaker (stratiform) exchanges between the cloud and the underlying air; such a distinction corresponds to that used in the M E T R O M E X program (Gatz, 1977) or that proposed by Slinn (1983) or Sisterson et al. (1985). The situations are classified in this way on the basis of satellite images (visible and infrared channels),
Fig. 1. Geographical zoning used for the classification of the sources of air masses according to the backtrajectories.
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meteorological observations from the Paris region and surface level synoptic weather charts. Clouds with a marked vertical development, such as cumulus or cumulonimbus (whose summits are clearly seen on satellite images), and also stratocumulus and altocumulus are characteristic of convectively unstable air layers. The associated air mass conditions are then described as 'convective'. Conversely, stratiform clouds such as stratus, altostratus, nimbostratus (which appear as broad sheets on satellite pictures) are typical of stable air masses. These corresponding situations are then described as 'stratiform'. Finally, the presence of unstable clouds (cumulus or cumulonimbus) in a stratiform mass (of nimbostratus or altostratus) puts the related events in the convective category, as the association of stratiform and cumuliform clouds indicates the rising of a warm convectively unstable air mass in contact with a colder one. The type of cloud observed for each situation is due to the thermodynamic characteristics of the air mass; the distinction between convective and stratiform situations on the basis of this criterion appears therefore to be suitable for the classification of the events in terms of the amount of exchange between the air mass below the cloud and the cloud itself.
INTER-PHASE RELATIONSHIPS
In a previous publication we showed that for most of the chemical species these aerosol and wet deposition data gave correlations at the 1% confidence level between the concentrations measured in each of the two phases (Jaffrezo and Colin, 1988a). These relationships had to be established in order to calculate scavenging ratios with some physical meaning. They seemed sufficient to confirm the interdependency of the two phases collected and thus justified the collection procedure. However, further precautions are to be taken, due to the urban nature of the site, for the purpose of the present study. Thus, one could imagine that this rain/aerosol relationship exists only locally, if the species were mainly emitted by the urban area and were incorporated essentially by below-cloud scavenging of the aerosol. Under these circumstances, the samples would still represent closely related media, but the scavenging processes involved would be fundamentally different from those occurring during simultaneous transport from distant sources. Now, it is important to know the phenomena actually taking place in the interaction of the two phases sampled, in order to better delineate the scope of application of the calculated scavenging ratios. Classification of the air mass trajectories shows that the relationship between the two phases is not only local but, on the contrary, that the aerosols collected have travelled with the precipitating systems.
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JEAN-Luc JAFFREZOet al.
Figure 2 presents the evolution of the average concentrations for each of the species in the two phases. In each of these drawings the length of each wedge (corresponding to the different major source areas previously defined) is proportional to the geometric mean of the concentrations. First of all, it can be seen that the criteria used to define the geographical zones show up clearly in the results. The West sector is characterized by high
concentrations of marine species (Na, Mg and CI), while the other species are at their lowest concentrations. The South sector is known to be the source of dust episodes on Paris (Jaffrezo and Colin, 1988b). In the southeast of France, in Corsica, it has been shown that this phenomenon is due to long-range transport at high altitude of dust originating from sandstorm in North Africa (Loye-Pilot et al., 1986; Bergametti et aL, 1988). It results in high average concentrations of
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Fig. 2. Geometric mean concentrations in rains (shaded areas) and associated aerosols (plain areas) as a function of the geographical sectors of the air masses. The length of each wedge is proportional to the concentration.
Factors influencing scavenging ratios species of mainly crustal origin (A1, Si, Fe, Ca and K), predominating over marine and anthropogenic components. Finally, as it has been defined, the third sector (N and E) covers a very wide geographical area with some marine and crustal components, but with maximal concentrations for anthropogenic species. These general results are very comparable to those found for a rural site in the east of France (Colin et al., 1989). But, secondly, and much more importantly in the context of this study, one can see that the impact of these major sources is clearly similar in both media, rain and aerosol. It indicates therefore that the aerosol over Paris does not result solely from local emissions but that the contributions of medium and long-range transport are also very important. Moreover, the average profile of the aerosol found in the associated precipitation shows that interphase exchanges must occur during these transport processes. Thus, even for such a large and powerful emitter site as Paris, the scavenging ratio takes into account both the nucleation phenomena occurring during transport of the air masses and the local scavenging of the aerosol under the precipitating cloud. Figure 3 clearly shows that the scavenging ratio make it possible to overcome the problem of aerosol variation in the studies of scavenging processes. Despite the concentration differences between the various sectors, the scavenging ratios are practically equivalent from one zone to another for those species originating from remote sources, indicating that the scavenging processes are depicted in much the same way. The case of zinc, which behaves differently from the other elements (cf. Figs 1 and 2) lends support to this idea. Koutrakis (1984) has shown that the zinc observed in Paris comes largely from incineration plants located in the nearby suburbs, one in the south-southeast, the other in the west. For these two sectors, we are therefore in the same situation as Gatz (1977), with the main source being too close for significant incorporation, occurring primarily by nucleation in the
Scavenging ratios 7345
West
1500
Northeast
South
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cloud. On the other hand, for the North east sector, the more distant emissions allow sufficient time for this type of incorporation and one finds a higher scavenging ratio, indicating more complete transfer between the two phases. The case of chlorine appears to be similar to that of zinc for the South and West sectors; but its behavior is complicated in the North sector by the possible gas phase (not measured) produced by aerosol degassing, a phenomenon that depends upon the acidity (Hitchcock et al., 1980), itself mainly linked to anthropogenic activity. This geographical sectorization of the major source areas indicates therefore that scavenging ratios can adequately be used as an approximate measurement of scavenging process, even at an urban site. It is, however, essential to note the specificity of such a site and the consequent limitations. Thus, it is possible that concentration gradients with altitude introduce a systematic error in the estimation of the nucleation phenomena. This distortion which is inherent in all measurements at ground level is probably exaggerated at an urban site, even if the local component of the aerosol is not necessarily predominant. On the contrary, such a situation certainly allows a closer look at the enhanced below-cloud-scavenging. This is the topic of a wider study, which includes the results presented here, and takes into account different types of receptor site, the comparison pointing out the hallmarks of each of them (Colin and Jaffrezo, 1988). IMPACT OF RAIN VOLUME AND RATE ON SCAVENGING RATIOS Precipitation volume has been the factor most often studied in relation to scavenging ratio variation. This is due partly to the availability of this datum, easily measured during sampling. But, more importantly, it is known that the concentrations generally decrease with the amount of rain for a single event, because of a dilution effect (Junge, 1963; Dawson, 1978; Hicks and Shannon, 1979; MAP3S, 1982). It seems therefore reasonable to look for a similar relationship for the scavenging ratios. Table 1 gives the coefficients A1, B1 and the correlations calculated for each species in regressions of the type: log(IV) = log(B1) + A 1 • log(Vol) (Vol given in mm).
CI
SO~-Na
K Mg Ca Zn AI Si
Fe
Fig. 3. Geometric mean scavenging ratios according to the geographical sector of air masses.
Except for the sulfates, correlations are significant at the 1% confidence level. These results are comparable with those of Gatz (1977), Barrie 0985) and Savoie et al. (1987), who report similar relationships for different elements and receptor sites. All these studies give scavenging ratios decreasing with the precipitation volume (negative A 1 coefficients). In our case, the slopes of the relationships vary little from one element to another; except for chlorine and
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JEAN-Luc JAFFREZOet al.
sulfate (the value for the latter will not be considered since the correlation coefficient is not significant), the slopes range only from -0.31 to -0.40. Such a behavior can indeed be explained by a dilution effect that proves to be similar for all the species. The curves for concentration variation with the amount of rain in single events sampled sequentially are all alike. The initial decrease, accounting for the greater part of the total concentration, is completed, regardless of the species, after 0.7-0.9 mm of rainfall. The end of the rain event often represents most of the volume but usually contains only low concentrations, a factor of 10-15 lower than these of the initial fractions. Such a pattern is shown in Fig. 4 for chlorine and aluminium in an event collected on the 7 November 1984 in Paris. Other studies of sequential sampling on other sites give similar results (Asman, 1980; Seymour and Stout, 1983; Wilson et al., 1983). One can see nevertheless in Fig. 4 that the initial decrease of the concentration is much steeper for chlorine than for aluminium and, indeed, than for any
other species considered in this study. This feature is also commonly observed with sequential sampling. The effect of a possible gaseous fraction of chlorine is suspected. With a very high solubility and Henry's Law constant (Clegg and Brimblecombe, 1985), this fraction could be readily and quickly dissolved at the beginning of the precipitation. This effect remains to be clearly demonstrated. Whatever the cause, the sharp initial decrease observed for chlorine concentrations on an event basis certainly accounts for the higher slope for scavenging ratios of chlorine vs volume (coefficient A1 in Table 1). Barrie (1985) and Savoie et al. (1987) attempted to compare their results for these A1 coefficients with theoretical values derived by Scott (1978) and Slinn (1983). The various experimental data are similar (varying between - 0 . 2 6 and -0.38), and are comparable with values derived by theory ( - 0 . 2 7 and -0.25). However, the theoretical calculations relate the scavenging ratios and the rain-rate, not the volume of precipitation. One can therefore ask whether the experimental relationship between W and volume is due to a direct dilution phenomena or, as suggested by Table 1. Coefficients A1, B1 and correlations for the rela- the values of Scott (1978) and Slinn (1983), to processes mostly related to the rain-rate. In this last case, the tions: log(W) = log(Bl) + A 1 * log(Vol) (Vol in mm) relationship between W and volume could be an n B1 A1 Corr. artefact of the volume and rain-rate being closely coupled (Drufuca and Zawadzki, 1975). This latter CI 78 3928 -0.625 -0.452 relationship is seen in our own data: SO282 784 -0.116 -0.190 Na 81 515 -0.340 -0.326 log (Int) = -2.31 +0.347 • log (Vol) K 82 1096 -0.311 -0.416 Mg 81 711 -0.405 -0.514 Int in m m h - 1 ; Vol in mm; n=82; corr=0.446. Ca 82 1220 -0.335 -0.407 To answer this question, it should be first pointed Zn 69 879 -0.336 -0.389 AI 82 367 -0.334 -0.412 out that several objections can be raised about the Si 82 460 -0.317 -0.451 comparison of these theoretical and experimental Fe 82 235 -0.400 -0.512 values. The value of - 0 . 2 7 taken from Scott's work is
] en 10"8M 500
AI
300
100
200
600
1000
1400
V ml
Fig. 4. Concentrations variations for chlorine and aluminium during a rain event collected sequentially in Paris on 11 July 1984.
Factors influencing scavenging ratios valid only for a particular range of rainfall rate and a particular type of precipitation, with wide variations for different situations. Furthermore, the two theoretical calculations assume a steady state system, with two consequences: the aerosol scavenged comes from an infinite reservoir and the rain-rate is constant throughout the event. These two conditions are by no means satisfied in real cases. It is known that the aerosol is scavenged in the precipitating cloud itself (Radke et al., 1980) and also by below-cloud-scavenging (Sisterson et al., 1985). This depletion of the source leads to the observed dilution effect of the concentrations in precipitation, with an exponential pattern. This effect is observed in rains collected sequentially (Asman, 1980; Jaffrezo and Colin, 1988b) and is reproduced by modeling (Stensland and de Pena, 1975; Altwicker and Wang Teng Tsai, 1987). Variations in the rain-rate, observed or modeled, generally cause only variations in the slope of the exponential decrease. The assumption of a steady state on which the theoretical calculations are based contradicts the experimental data. Particularly, by keeping the concentrations in the aerosol phase constant, any dilution effect is neglected. It seems therefore that the apparent agreement of the theoret-
Table 2. Coefficients A2, B2 and correlations for the relations: log(W) = log(B2) + A2 * log (Int) (Int in mm h - 1)
C1 SO~ Na K Mg Ca Zn A1 Si Fe
n
B2
A2
Corr.
78 82 81 82 81 82 69 82 82 82
3193 748 469 991 647 1128 827 342 419 204
-0.328 -0.024 -0.231 -0.171 -0.342 -0.303 -0.326 -0.393 -0.250 -0.421
-0.183 -0.031 -0.174 -0.177 -0.341 -0.287 -0.301 -0.380 -0.278 -0.396
~
0.5
0.3
0.1
Cl SO~," Na
K
Volume
Mg
Ca
Zn ~
AI
Si
p <0,01 1.:82)
Fe
Raln- rata
Fig. 5. Correlation coefficients for the regressions: log(W)=log(B1)+Al*log(Vol) (Vol in ram); log(W) = log(B2) + A2 * log(Int) (Int in mm h- 1). AE(A)
21:12-L
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ical and experimental data for the A 1 coefficient is only fortuitous. Nonetheless, one can try to find a relationship between scavenging ratios and rain-rate. Table 2 gives the coefficients A2, B2 and the correlations calculated for equations of the type: log (W) = log (B2) + A2 * log (Int) (Int in mm h - 1). The exponents A2 are again negative but cover a much greater range than A1. The correlation coefficients are systematically smaller than those found previously; they are not significant at the 1% level for CI, S, Na and K, and are close to this threshold for Ca, Zn and Si (cf. Fig. 5). Scott (1981) also failed to demonstrate a significant correlation between W and precipitation rate for snow samples. This absence of correlation strongly supports the importance of the volume effect as compared to a possible implication of the rain-rate on the scavenging ratios. Finally, it seems logical that the value of rain-rate averaged over a complete event is not suitable to describe what appears to be a second order phenomena in comparison to the main process, the depletion of the aerosol source. IMPACT OF THE TYPE OF PRECIPITATION ON THE SCAVENGING RATIOS
Independently of the relationship with the volume or the rain-rate, the nature of the precipitating system should be able to affect the scavenging ratios by modifying the incorporation mechanisms. Convection, which causes more rapid and extensive exchanges between the cloud and the underlying air, should play an important role. In addition to this increased mixing efficiency, the effect of convection is found in several theoretically described processes. Firstly, there is a direct increase of the efficiency of particle collection by precipitating drops or cloud droplets due to the turbulence associated with convection (Grover and Pruppacher, 1985). For convective situations, greater incorporation should therefore result in an increase of the scavenging ratio for a given volume of rain. Again, the turbulence associated with convection leads to faster growth of the cloud droplets by collision and coalescence (Pruppacher and Klett, 1980). This improves their chances of being incorporated by precipitating drops or of becoming such drops themselves. Furthermore, supersaturation reached within clouds increases in parallel with the updraft, speed. This phenomenon allows the activation of less soluble particles, or smaller particles of equivalent solubility, for convective cases compared to activation with more stable air masses (Jensen and Charlson, 1984). Scavenging is more efficient for activated particles of greater size (Pruppacher and Klett, 1980); this effect should also lead to an increase in scavenging ratios in
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JEAN-Luc JAFFREZOet al.
convective cases. There is, however, little difference for speeds greater than 20 cm s-1 (Jensen and Charlson, 1984). Conversely, Slinn (1983) indicates that scavenging ratios should be greater for stable frontal systems, which allow more time for the growth of droplets after the activation of the particles. These larger droplets are then more easily scavenged. He interprets in this way the only comparison of experimental data on the subject, contrasting the results of Gatz (1977) with those of Cawse (1974). It will be noted first of all that the two studies are difficult to compare. The very unlike sampling conditions (daily in the case of Gatz (1977), monthly for Cawse (1974)), as well as the statistical parameters used (volume-weighted means and arithmetic means, respectively) are quite enough to introduce differences by a factor of two in the scavenging ratios (Jaffrezo and Colin, 1988a). Even more important, it will be remarked that the two studies are nonspecific for the phenomena followed: thus Gatz's data would arise mainly after 'sampling essentially all convective storm precipitation', while in the case of Cawse 'much of the data is probably for frontal storm' (Slinn, 1983). It appears therefore very risky under these conditions to ascribe the differences observed between the two sets of data to the more or less convective character of the meteorological situations. It is possible to investigate the effect of convection on a same set of data by classifying the events collected during this study into two groups by means of the observations and criteria stated above. F o r each of these groups, we have calculated the regressions between scavenging ratios and volumes of the events, in order to allow for the dilution effect. The correlation coefficients and the parameters A (slope) and B (value of the scavenging ratio for 1 mm of rain depth) are presented for all the elements in Figs 6a-c. Only sulfate is absent, the correlations not being significant at the 1% level. The other species have been grouped according to their behavior. There is the group of elements of predominantly marine origin (Na, Mg and C1), a group of mainly soluble nonmarine elements (K, Ca and Zn) and, finally, a group of mainly crustal elements which appear essentially in the insoluble phase of the precipitation (Colin et al., 1990). It can be seen that correlations are found between volume and scavenging ratio for all the cases considered (apart from Na and A1 in the stratiform situations). The very large increase in these correlations for the marine elements in the convective cases is to be noted, whereas few fluctuations appear for the other two groups (cf. Fig. 6a). Figure 6b reveals a certain contrast between the marine species and the other soluble elements. For the former, the slopes are smaller for stratiform situations while it is the opposite for K, Ca and Zn: their scavenging ratios decrease more rapidly with the volume of precipitation in these cases.
Correlations
0.75
0.50 )<0,01 n = 3 5 )< 0,01 n : 4 5 0.25
K Ca' Zn
CI Na Mg
(a)
Strst (n = 45}
AI Si
Fe
I---] Cony In = 35) Coefficients
A
0.75
0.60
0.26
Cl
(b)
Na Mg
I~
K
Ca Zn
Stret (n : 451 Coefficients
f~l
AI $1 Fo Cony
In : 35) B
,.°°0iji" I000~--~
CI
(C)
.d
Na Mg
R~
K Ca Zn Strat
(n = 45)
f~l
AI Si
Fc
Cony
(n : 35)
Fig. 6. Coefficients A, B and correlations for the relations: log (W) =log(B)+ A * Iog(Vol) for stratiform situations (n=45), shaded areas, for convective situations (n = 35), plain areas. Volume expressed in mm of rain depth. Fig. 6a: correlations. Fig. 6b: coefficient A. Fig. 6c: coefficient B. Figure 6c shows higher B coefficients in stratiform than convective situations for these same elements, but little differences from one case to another for the marine species.
Factors influencing scavenging ratios The scavenging ratio is supposed to describe a scavenging efficiency of species by precipitation with the assumption, amongst others, that it is independent of the amount of rainfall. Since this condition is not fulfilled, it is necessary to take into account the relationship between W and volume of the precipitation. This could be done with a volume-weighted scavenging ratio, in the case of a linear relationship. According to the kind of relationship previously calculated, it seems better to use coefficient B, which can be represented as a scavenging ratio for a given volume of 1 mm of rain depth. The coefficient A, the slope, expresses the rate of exhaustion of the aerosol source: the steeper the slope, the quicker the dilution. It follows therefore that the scavenging of K, Ca and Zn is more efficient in situations described as 'stratiform' (cf. Fig. 6c), in combination with a more rapid exhaustion of the aerosol source (cf. Fig. 6b). This would support the hypothesis proposed by Slinn (1983), for these soluble species that can act as condensation nuclei. The case of elements of mainly crustal origin also tends towards the same conclusion, with the involvement of the nucleation capabilities of the aerosol particles and the modification in behavior it implies. Both the slopes (cf. Fig. 6b) and the scavenging ratios (coefficients B, cf. Fig. 6c) show trends identical with those of K, Ca and Zn but considerably reduced. Now it is known that these elements borne by mainly insoluble particles are much less active as condensation nuclei (Hiinel, 1976; Pruppacher and Klett, 1980). Their behavior therefore differs little from one cloud mass to another. Marine species exhibit a very different pattern from other soluble species, with comparable scavenging in both types of situation, but high slopes for convective cases. It does not seem possible at present to give a fully comprehensive explanation of these differences between stratiform and convective situations for marine species. But one can point out several factors that could cause different behavior as compared to the other soluble elements. The most obvious of these is that the source of marine species disappears once the air mass leading to the precipitating system is inland, while other species are more or less replenished in the same conditions. Another reason is also linked to the difference in sources between the two groups of species. It is very likely, particularly in stratiform cases, that over Paris species of marine origin are present in the system for much longer than any other element. If Slinn's (1983) hypothesis is correct, it would imply a greater opportunity for their activation and growth, leading to their precipitation with cloud drops. At this point, one should note that Noone et al. (1988) demonstrated chemical differences between cloud droplets of different sizes, in the case of a stratus of marine origin. They found that large droplets contain mainly sea-salt-like compounds, while smaller ones contain more sulfate. For these reasons, it seems probable that large
3081
differences in the scavenging characteristics can be expected between marine and other soluble species over continental areas. A better insight has to be gained into the relationship between scavenging ratios and the transport of aerosols from remote sources before one can fully explain differences observed from one meteorological situation to another.
CONCLUSION Thanks to the collection strategy employed, we have been able to obtain valuable data for the study of aerosol scavenging by precipitation over Paris. In a previous publication, strong relationships between the collected aerosol and precipitation phases were calculated, with high correlation coefficients for most of the chemical species studied. This observation is complemented here; the impact of the major aerosol sources, tracked with back-trajectories, proves to be very similar in both air and rain. The predominant role of long- and medium-range transport is thus demonstrated, even over a large urban area. As an implication, scavenging ratios can reflect incorporation mechanisms occurring during this transport (i.e. nucleation scavenging), as well as below-cloud-scavenging over the sampling site. The authors are nevertheless aware of the potential enhancement of this last process due to the larger aerosol concentrations at ground level. This feature can also lead to scavenging ratios slightly lower than those expected over sites with weaker gradients of aerosol concentration with altitude, with less opportunity for nucleation scavenging. It is then possible to calculate relationships between scavenging ratios and rain volume. The decrease observed for the scavenging parameter can be ascribed to dilution of the species in the rain as the aerosol feeding the precipitating cloud is depleted. A much weaker dependency, if any, with the rain-rate can be found; compared to aerosol depletion, the rain-rate variation seems to have a second-order effect on the concentrations in rain water. Its effect should be evaluated more efficiently with sequential sampling of rain events. For the first time, scavenging ratios have been extensively investigated in term of the type of meteorological situation, stratiform or convective. For most of the chemical species studied, higher scavenging ratios (for a given volume of rain) and quicker dilution (assumed to reflect the rapidity of the aerosol depletion process) are associated with stratiform situations. Species of marine origin (CI, Na and Mg) behave differently. More data are needed to evaluate the role of atmospheric transport (particularly the lack of sources over the continent) as regards this difference. Acknowledgements--This work was made possible by the
financial support of the convention no. 82138 of the Ministry of the Environment. The authors are grateful to E. Bon Nguyen for technical assistance.
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REFERENCES Altwicker E. R. and Wang Teng Tsai (1987) A wet removal study of air pollutants using a one-dimensional eulerian transport/transformation/removal model. In Proceedings of "Acid Rain: Scientific and Technical Advances" (edited by R. Perry, R. M. Harrison, J. N. B. Bell and J. N. Lester), pp. 159-166. Selper, London. Arimoto R., Duce R. A., Ray B. J. and Unni C. K. (1985) Atmospheric trace elements at Enewetak atoll--2. Transport to the ocean by wet and dry deposition. J. oeophys. Res. 90, 2391-2408. Asman W. H. A. (1980) Draft, construction and operation of a sequential rain sampler. War. Air Soil Pollut. 13, 235-245. Barrie L. A. (1985) Scavenging ratios, wet deposition and incloud oxidation: an application to the oxides of sulphur and nitrogen. J. geophys. Res. 90, D3, 5789-5799. Bergametti G., Dutot A. L., Buat-Menard P., Losno R. and Remoudaki E. (1989) Seasonal variability of the elemental composition of atmospheric aerosol particles over the northwestern Mediterranean. Tellus 41B, 353-361. Cawse P. A. (1974) A survey of atmospheric trace elements in the U.K. AERE HarweU Rept. R 7669, HMSO, London. Chamberlain A. C. (1960) Aspects of the deposition of radioactive and other gases and particles. Int. J. Air Pollut. 3, 63-88. Clegg S. L. and Brimblecombe P. (1985) Potential degassing of hydrogen chloride from acidified sodium chloride droplets. Atmospheric Environment 19, 465-470. Colin J. L. and Jaffrezo J. L. (1988) Rain/aerosol coupling in maritime, urban and rural areas: scavenging ratios measurements in relation with transport. 10th lntern. Cloud Phys. Conf. Bad-Homburg (F.R.G.) 15-20 September 1988. Colin J. L., Jaffrezo J. L. and Gros J. M. (1990) Solubility of major species in precipitations: factors of variation. Atmospheric Environment 24A, 537-544. Colin J. L., Jaffrezo J. L., Pinart J. and Roulette-Cadene S. (1987) Sequential sampling of snow in a rural area. Experimentation and identification of the acidifying agents. Atmospheric Environment 21, 1147-1157. Colin J. L., Renard D., Lescoat V., Jaffrezo J. L., Gros J. M. and Straus B. (1989) Relationship between rain and snow acidity and air mass trajectory in Eastern France. Atmospheric Environment 23, 1487-1494. Davidson C. I., Santhanam S., Fortman R. C. and Olson M. P. (1985) Atmospheric transport and deposition of trace elements onto the Greenland Ice Sheet. Atmospheric Environment 19, 2065-2081. Dawson G. A. (1978) Ionic composition of rain during sixteen convective showers. Atmospheric Environment 12, 19911999. Drufuca G. and Zawadzki I. I. (1975) Statistics of rain gauge data. J. appl. Met. 14, 1419-1429. Dutot A. L., Elichegaray C. and Vi6 le Sage R. (1983) Application de ranalyse des correspondances/t l'6tude de la composition physico-chimique de l'a6rosol urbain. Atmospheric Environment 17, 73-78. Elichegaray C. (1980) Contribution ~i l'6tude du comportemerit physico-chimique de i'a6rosol urbain. Th6se de 36me cycle. Universit6 Paris VII. Elichegaray C., Dutot A. L., Grubis B. and Vi6 le Sage R. (1981) Dosage par fluorescence X des a6rosols atmosph6riques: d6termination des facteurs de correction. Analusis 9, 492-497. Engelmann R. J. (1970) Scavenging prediction using ratios of concentrations in air and precipitation. In: Precipitation Scavenoino (1970). AEC Symposium Series 22, CONF 700601 NTIS, Springfield, 475-485. Gatz D. F. (1977) Scavenging ratio measurements in METROMEX. In Precipitation Scavenoino (1974) (edited
by R. W. Beadle and R, G. Semonin). ERDA Symposium series 41, CONF 741003 NTIS Springfield, 71-87. Grover S. N. and Pruppacher H. R. (1985) The effect of vertical turbulent fluctuations in the atmosphere on the collection of aerosol particles by cloud drops. J. atmos. Sci. 42, 2305-2318. H~nel G. (1976) In Vol. 19 of Advances in Geophysics (edited by Landsberg H. E. and Van Mieghem J.), pp. 73-188. Academic Press, New York. Hicks B. B. and Shannon J. D. (1979) A method for modeling the deposition of sulfur by precipitation over regional scales. J. appl. Met. 18, 1415-1420. Hitchcock D. R., Spiller L. L. and Wilson W. E. (1980) Sulfuric acid aerosols and HC1 release in coastal atmosphere. Atmospheric Environment 14, 165-182. Jaffrezo J. L. and Colin J. U (1987) Construction and exploitation of an automatic sequential wet-only rain sampler. Envir. Technol. Lett. 8, 467-474. Jaffrezo J. L. and Colin J. L. (1988a) Rain-aerosol coupling in urban area: scavenging ratio measurements and identification of some transfer processes. Atmospheric Environment 22, 929-935. Jaffrezo J. L. and Colin J. L. (1988b) Study of aerosol scavenging by precipitation for an episode of Saharan dust input collected sequentially in Paris. Fifth European Symposium on Physico-chemical Behaviour of Atmospheric Pollutants. COST 611.3-4 May 1988, Villefranche sur Mer, France. Jensen J. B. and Charlson R. J. (1984) On the efficiency of nucleation scavenging. Tellus 36B, 367-375. Junge C. E. (1963) Air Chemistry and Radioactivity. Academic Press, New York. Keene W. C., Pszenn A. A. P., Galloway J. N. and Hawley M. E. (1986) Sea-salt corrections and interpretation of constituent ratios in marine precipitation. J. geophys. Res. 91, D6, 6647-6658. Koutrakis P. (1984) Physico-chimie de ra6rosol urbain: identification et quantification des principales sources par analyse multivariable. Th6se de 36me cycle. Universit6 Paris VII. Lorenc A., Rutherford I. and Larsen G. (1977) The E.C.M.W.F. analysis and data assimilation scale analysis of mass and wind fields. ECMWF Technical report no. 6, Reading, England. Losno R., Bergametti G. and Mouvier G. (1987) Determination of optima conditions for atmospheric aerosol analysis by X-ray fluorescence. Envir. Technol. Lett. 8, 77-86. Loye-Piiot M. D., Morelli J., Martin J. M., Gros J. M. and Strauss B. (1986) L'impact des poussi6res sahariennes sur l'acidit6 des pluies dans l'atmosph6re m6diterran6enne. In Physico-Chemical Behaviour of Atmospheric Pollutants (edited by G. Angeletti and G. ResteUi), pp. 511-519. D. Reidel, Dordrecht. Martin D., Mithieux C. and Strauss B. (1987) On the use of the synoptic vertical wind component in a transport trajectory model. Atmospheric Environment 21, 45-52. Noone K. J., Charlson R. J., Covert D. S., Ogren J. A. and Heintzenberg J. (1988) Cloud droplets: solute concentration is size dependant. J. geophys. Res. 93, D8, 94779482. Pacyna J. M., Semb A. and Hanssen J. E. (1984) Emission and long-range transport of trace elements in Europe. Tellus 36B, 163-178. Pruppacher H. R. and Klett J. D. (1980) Microphysics of Clouds and Precipitations. D. Reidei, Dordrecht. Radke L. F., Hobbs P. V. and Elgroth M. W. (1980) Scavenging of aerosol particles by precipitation. J. appl. Met. 19, 715-722. Savoie D. L., Prospero J. M. and Nees R. T. (1987) Washout ratios of nitrate, non-sea-sulfate and sea-salt on Virginia Key, Florida and on American Samoa. Atmospheric Environment 21, 103-112.
Factors influencing scavenging ratios Scott B. C. (1978) Parameterization of sulfate removal by precipitation. J. appl. Met. 17, 1375-1389. Scott B. C. (1981) Sulfate washout ratios in winter storm. J. appl. Met. 20, 619-625. Semb A. (1978) Sulphur emissions in Europe. Atmospheric Environment 12, 455-460. Seymour M. D. and Stout T. (1983) Observations on the chemical composition of rain using short sampling times during a single event. Atmospheric Environment 17, 14831487. Sisterson D. L., Johnson S. A. and Kumar R. (1985) The influence of humidity on fine particle aerosol dynamics and precipitation scavenging. Aerosol Sci. Technol. 4, 287-300. Slinn W. G. N. (1983) Air-to-sea transfer of particles. In Air-Sea Exchange of Gases and Particles (edited by P. S. Liss and W. G. N. Slinn), pp. 299-405. D. Reidel, Dordrecht.
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Stensland G. J. and de Pena R. G. (1975) A model of belowcloud precipitation scavenging of NaCI. J. geophys. Res. 80, 3410-3418. The MAP3S/RAINE Research Community (1982) The MAP3S/RAINE precipitation chemistry network: statistical overview for the period 1976-1980. Atmospheric Environment 16, 1603-1631. Uematsu M., Duce R. A. and Prospero J. M. (1985) Deposition of atmospheric mineral particles in the North Pacific Ocean. J. atmos. Chem. 3, 123-138. Wilson J., Graham R. and Robertson J. (1983) Correlation of intrastorm sequential precipitation chemistry with storm meteorology. In Precipitation Scavenging, Dry Deposition and Resuspension (edited by H. R. Pruppacher, R. G. Semonin and W. G. N. Slinn), pp. 229-237. Elsevier, New York.
0004-45981/90 $3.00+0.00 Pergamon Press plc
SOME PHYSICAL FACTORS I N F L U E N C I N G SCAVENGING RATIOS JEAN-LUC JAFFREZO* a n d JEAN-LOUIS COLIN Laboratoire de Physico-Chimie de l'Atmospbere, Universit6 Paris VII, 2, Place Jussieu, 75 251 Paris Cedex 05, France and JEAN-MIcHEL GROS Mgt6orologie Nationale, SCEM/ES, 2, avenue Rapp, 75 230 Paris Cedex 07, France (First received 1 Auoust 1989 and in final form 7 June 1990)
Almtraet--A better understanding of the causes of scavengingratio variation is required if one is to use them more preciselyfor predicting the chemicalconcentrations in precipitations. For this purpose, a set of 82 rain and associated aerosol samples were obtained during a 1-yearsampling program in Paris. Scavengingratios are calculated for C1, SO2 -, Na, K, Mg, Ca, Zn, Al, Si and Fe. First, it is shown that scavengingratios enable us to describe the scavengingprocesses correctly,even on an urban site:classificationof air masses according to back-trajectories indicates similar long range transport impact on both aerosol and precipitation. Scavenging ratios are then shown to vary with the volume of rain: this is interpreted as a dilution effect related to the depletion of the aerosol source. On the other hand, average rain-rate has little effect, if any, on the scavengingparameter. Finally, the role of the cloud mass type (stratiform or convective)is investigated, in relation with differences in nucleation capabilities. Key word index: Scavenging ratios, air mass trajectory, rain volume, rain-rate, stratiform situation, convective situation, scavenging mechanisms.
INTRODUCTION The atmospheric aerosol is the main source of most of the non-reactive mineral species found in precipitations. The concentration variations in rain or snow water can therefore be attributed to two major causes: the concentration variation in the aerosol scavenged, and the variations in the scavenging processes. Many physico-chemical models have been developed to account for interphase coupling, but they are generally too complex to be readily applied to real cases. Correspondingly, field studies describe the relationship between the aerosol and the wet phase, for each of the particulate species, by means of a simplified parameter, the scavenging ratio (W) (Chamberlain, 1960; Engelmann, 1970). This is defined as the dimensionless ratio of the concentrations measured in each phase. By normalizing the concentration observed in the liquid phase with respect to that in the associated aerosol, one intends to take into account the variations in the source term, the aerosol. Thus, despite the many assumptions included in this simplification (Slinn, 1983), the effect of certain processes or physicochemical parameters on the transfer of species between phases can be investigated (Scott, 1981; Barrie, 1985; Jaffrezo and Colin, 1988a). *Present address: Department of Civil Engineering, Carnegie-Mellon University, Pittsburgh, PA 15213, U.S.A.
However, in addition to its role in the study of precipitation and aerosol relationships, the scavenging ratio potentially constitutes a tool for evaluating the wet deposition of species if the concentrations in the aerosol are known (Arimoto et al., 1985; Uematsu et al., 1985; Davidson et al., 1985; Keene et al., 1986). This aspect is particularly important, insofar as local studies on aerosol are more abundant, more complete and easier than those on wet deposition, because of the stochastic nature of the latter. With a view to this application, it is therefore necessary to achieve a better understanding of the factors, in particular meteorological, which lead to the observed variability in the coupling parameter. It should then be possible to determine more closely the field of application of scavenging ratios and the conditions of their use. This type of study has already been undertaken, albeit for a limited number of factors. We present here a more in-depth study with data acquired during a 1-year program on an urban site. Thanks to the large number of data, it is possible to observe trends throughout the variety of situations encountered, and to compare them with theoretical expectations. First of all, however, it is necessary to validate the calculation of scavenging ratios from our chemical data. In order to obtain parameters depicting some physical reality, it is essential to ensure that the samples were obtained from interacting phases. In other words, we have to check that the aerosol collected does come
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from the air mass which enriched the precipitation. This step is not usually discussed in studies dealing with scavenging ratios. It is all the more important here because of the urban nature of the sampling site, but also because of the very short sampling times, used for the first time in this sort of study.
SAMPLING AND ANALYSIS
The general field experiment has already been presented elsewhere (Jaffrezo and Colin, 1988a). This study concerns samples collected from May 1984 to April 1985 in downtown Paris. The sampling site is located on the roof of the University Paris VII, about 25 m from the ground to represent an average urban atmosphere and to minimize contamination by resuspension of road dust by the traffic. Precipitation is collected by means of an automatic sequential sampler (sampling area 1 m 2, giving 100 ml of water for fractions of 0.1 mm of rain depth). Dry deposition is avoided by a cover which protects the collector when it is not raining and is removed automatically at the onset of each event. These are thus individualized, with a discrimination time 10rain. The polyethylene funnel is carefully washed before each rain event. The rain sampling equipment is described in more detail elsewhere (Jaffrezo and Colin, 1987). The sample storage and analysis techniques have also been described elsewhere (Colin et al., 1987). After filtration on 0.4 #m pore size Nuclepore film, the rain samples are stored at 4°C in the polyethylene bottle used for collection. The soluble fractions are analyzed within 10 days for CI-, N O r and SO 2- (ion chromatography) and Na, K, Mg, Ca and Zn (by flame atomic absorption spectrometry). The insoluble elements AI, Si and Fe are analyzed by X-ray fluorescence spectrometry (Elichegaray et al., 1981). The total concentrations given here for each event are obtained either by analysis of the recombined fractions, or by calculation of the volume-weighted means after analysis of the entire series. Total aerosols were sampled continuously throughout the year. They were collected on filters consisting of 0.4/~m pore size Nuclepore film mounted on a Nylon ring (diameter 42 mm) fitting the geometry of the X-ray fluorescence apparatus. Four identical sampiing lines, each including a filter-holder, a pump and a volumetric meter are programmed to operate automatically in sequential 6-h intervals. This short sampiing time is possible despite the low flow rate (16 ~'min- 1) because the concentrations of the major species found in Paris are generally high (Dutot et al., 1983; Koutrakis, 1984) and because of the high sensitivity of the X-ray fluorescence apparatus (Losno et al., 1987). The selected filters are analyzed for CI, S, K, Ca, Zn, A1, Si and Fe. They are then removed from their support, suspended in 20 ml of Milli-Q water and
sonicated for 2 h. The solution is filtered through a 0.4/~m pore size Nuciepore filter prior to analysis for Na and Mg by flame atomic absorption spectrometry according to the same procedure used for the rain samples. With a continuous survey using a 6-h sampling interval, we are thus able to associate each rain event with the corresponding aerosol, by choosing the aerosol filter fraction collected just before the precipitation. Moreover, this short sampling time seems better suited to the description of scavenging phenomena than the daily period that is generally used. Daily sampling may encompass a succession of several meteorological situations not relevant to the rain sampled (Jaffrezo and Colin, 1987); furthermore, it would give only an average value over the rapid changes of atmospheric concentrations due to aerosol scavenging (Sisterson et al., 1985; Jaffrezo and Colin, 1988b). It should be noted that the techniques of sampling and analysis are designed to describe the coupling phenomena as closely as possible, by selecting only the interacting phases: (i) exclusion of dry deposition; (ii) separation of successive rains, since differences in concentrations can be very large even between two close events; (iii) aerosol collection over short periods; (iv) choice of the appropriate aerosol fraction. Finally, the automation of the sampling station enabled us to collect 82 rain and related aerosol samples, out of a total of 95 rain events collected; we have therefore data corresponding to a variety of situations spanning a whole year.
PHYSICALAND METEOROLOGICALDATA Additional physical and meteorological data are associated with each of these paired samples. The total volume of each precipitation is measured by weighing the fractions. The average rain-rate over the entire event can then be calculated exactly from the times of collector opening and closing which are recorded electronically. Air mass trajectories
The geographical origin of the precipitating system associated with each sample is determined from the air mass trajectories. These three-dimensional trajectories have been calculated by analyzing the wind fields from the European Centre for Medium Range Weather Forecasts (Lorenc et al., 1977; Martin et al., 1987). Trajectories can be supplied for every 6 h (00(X), 0600, 1200 and 1800h TU), for nine different pressure levels (100050 hPa), for the previous 5 days before arriving on site. For this routine survey, we used 925 hPa as the final pressure level. Back-trajectories were calculated for
Factors influencing scavenging ratios periods both before and after each event to check for possible changes in wind direction during the sampling. The few cases where the back-trajectories varied during a sampling period have been discarded. Events have then been classified according to a geographical zoning of the origin of their associated air mass. The definition of sectors adopted for this study is based mainly on the distribution of the main emission sources around the site, and leads to the definition of three geographical sectors (W, N, E and S), as shown in Fig. 1. Such a zoning system is similar to that of previous studies conducted in Paris, on the aerosol only (Elichegaray, 1980; Dutot et al., 1983; Koutrakis, 1984). It agrees quite well with the compilation of data on industrial emissions given by Semb (1978) for SO2 and Pacyna et al. (1984) for heavy metals over Europe. As long as the purpose of this classification is not to characterize extensively the impact of each source in each zone, such broad sectors are precise enough to achieve a sufficient geochemical differentiation of the events. Types of meteorolooical situations The events have then been classified according to the meteorological nature of the cloud mass, stratiform or convective. The purpose of this distinction is to roughly separate cases with strong (convective) or weaker (stratiform) exchanges between the cloud and the underlying air; such a distinction corresponds to that used in the M E T R O M E X program (Gatz, 1977) or that proposed by Slinn (1983) or Sisterson et al. (1985). The situations are classified in this way on the basis of satellite images (visible and infrared channels),
Fig. 1. Geographical zoning used for the classification of the sources of air masses according to the backtrajectories.
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meteorological observations from the Paris region and surface level synoptic weather charts. Clouds with a marked vertical development, such as cumulus or cumulonimbus (whose summits are clearly seen on satellite images), and also stratocumulus and altocumulus are characteristic of convectively unstable air layers. The associated air mass conditions are then described as 'convective'. Conversely, stratiform clouds such as stratus, altostratus, nimbostratus (which appear as broad sheets on satellite pictures) are typical of stable air masses. These corresponding situations are then described as 'stratiform'. Finally, the presence of unstable clouds (cumulus or cumulonimbus) in a stratiform mass (of nimbostratus or altostratus) puts the related events in the convective category, as the association of stratiform and cumuliform clouds indicates the rising of a warm convectively unstable air mass in contact with a colder one. The type of cloud observed for each situation is due to the thermodynamic characteristics of the air mass; the distinction between convective and stratiform situations on the basis of this criterion appears therefore to be suitable for the classification of the events in terms of the amount of exchange between the air mass below the cloud and the cloud itself.
INTER-PHASE RELATIONSHIPS
In a previous publication we showed that for most of the chemical species these aerosol and wet deposition data gave correlations at the 1% confidence level between the concentrations measured in each of the two phases (Jaffrezo and Colin, 1988a). These relationships had to be established in order to calculate scavenging ratios with some physical meaning. They seemed sufficient to confirm the interdependency of the two phases collected and thus justified the collection procedure. However, further precautions are to be taken, due to the urban nature of the site, for the purpose of the present study. Thus, one could imagine that this rain/aerosol relationship exists only locally, if the species were mainly emitted by the urban area and were incorporated essentially by below-cloud scavenging of the aerosol. Under these circumstances, the samples would still represent closely related media, but the scavenging processes involved would be fundamentally different from those occurring during simultaneous transport from distant sources. Now, it is important to know the phenomena actually taking place in the interaction of the two phases sampled, in order to better delineate the scope of application of the calculated scavenging ratios. Classification of the air mass trajectories shows that the relationship between the two phases is not only local but, on the contrary, that the aerosols collected have travelled with the precipitating systems.
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Figure 2 presents the evolution of the average concentrations for each of the species in the two phases. In each of these drawings the length of each wedge (corresponding to the different major source areas previously defined) is proportional to the geometric mean of the concentrations. First of all, it can be seen that the criteria used to define the geographical zones show up clearly in the results. The West sector is characterized by high
concentrations of marine species (Na, Mg and CI), while the other species are at their lowest concentrations. The South sector is known to be the source of dust episodes on Paris (Jaffrezo and Colin, 1988b). In the southeast of France, in Corsica, it has been shown that this phenomenon is due to long-range transport at high altitude of dust originating from sandstorm in North Africa (Loye-Pilot et al., 1986; Bergametti et aL, 1988). It results in high average concentrations of
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Fig. 2. Geometric mean concentrations in rains (shaded areas) and associated aerosols (plain areas) as a function of the geographical sectors of the air masses. The length of each wedge is proportional to the concentration.
Factors influencing scavenging ratios species of mainly crustal origin (A1, Si, Fe, Ca and K), predominating over marine and anthropogenic components. Finally, as it has been defined, the third sector (N and E) covers a very wide geographical area with some marine and crustal components, but with maximal concentrations for anthropogenic species. These general results are very comparable to those found for a rural site in the east of France (Colin et al., 1989). But, secondly, and much more importantly in the context of this study, one can see that the impact of these major sources is clearly similar in both media, rain and aerosol. It indicates therefore that the aerosol over Paris does not result solely from local emissions but that the contributions of medium and long-range transport are also very important. Moreover, the average profile of the aerosol found in the associated precipitation shows that interphase exchanges must occur during these transport processes. Thus, even for such a large and powerful emitter site as Paris, the scavenging ratio takes into account both the nucleation phenomena occurring during transport of the air masses and the local scavenging of the aerosol under the precipitating cloud. Figure 3 clearly shows that the scavenging ratio make it possible to overcome the problem of aerosol variation in the studies of scavenging processes. Despite the concentration differences between the various sectors, the scavenging ratios are practically equivalent from one zone to another for those species originating from remote sources, indicating that the scavenging processes are depicted in much the same way. The case of zinc, which behaves differently from the other elements (cf. Figs 1 and 2) lends support to this idea. Koutrakis (1984) has shown that the zinc observed in Paris comes largely from incineration plants located in the nearby suburbs, one in the south-southeast, the other in the west. For these two sectors, we are therefore in the same situation as Gatz (1977), with the main source being too close for significant incorporation, occurring primarily by nucleation in the
Scavenging ratios 7345
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1500
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South
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cloud. On the other hand, for the North east sector, the more distant emissions allow sufficient time for this type of incorporation and one finds a higher scavenging ratio, indicating more complete transfer between the two phases. The case of chlorine appears to be similar to that of zinc for the South and West sectors; but its behavior is complicated in the North sector by the possible gas phase (not measured) produced by aerosol degassing, a phenomenon that depends upon the acidity (Hitchcock et al., 1980), itself mainly linked to anthropogenic activity. This geographical sectorization of the major source areas indicates therefore that scavenging ratios can adequately be used as an approximate measurement of scavenging process, even at an urban site. It is, however, essential to note the specificity of such a site and the consequent limitations. Thus, it is possible that concentration gradients with altitude introduce a systematic error in the estimation of the nucleation phenomena. This distortion which is inherent in all measurements at ground level is probably exaggerated at an urban site, even if the local component of the aerosol is not necessarily predominant. On the contrary, such a situation certainly allows a closer look at the enhanced below-cloud-scavenging. This is the topic of a wider study, which includes the results presented here, and takes into account different types of receptor site, the comparison pointing out the hallmarks of each of them (Colin and Jaffrezo, 1988). IMPACT OF RAIN VOLUME AND RATE ON SCAVENGING RATIOS Precipitation volume has been the factor most often studied in relation to scavenging ratio variation. This is due partly to the availability of this datum, easily measured during sampling. But, more importantly, it is known that the concentrations generally decrease with the amount of rain for a single event, because of a dilution effect (Junge, 1963; Dawson, 1978; Hicks and Shannon, 1979; MAP3S, 1982). It seems therefore reasonable to look for a similar relationship for the scavenging ratios. Table 1 gives the coefficients A1, B1 and the correlations calculated for each species in regressions of the type: log(IV) = log(B1) + A 1 • log(Vol) (Vol given in mm).
CI
SO~-Na
K Mg Ca Zn AI Si
Fe
Fig. 3. Geometric mean scavenging ratios according to the geographical sector of air masses.
Except for the sulfates, correlations are significant at the 1% confidence level. These results are comparable with those of Gatz (1977), Barrie 0985) and Savoie et al. (1987), who report similar relationships for different elements and receptor sites. All these studies give scavenging ratios decreasing with the precipitation volume (negative A 1 coefficients). In our case, the slopes of the relationships vary little from one element to another; except for chlorine and
3078
JEAN-Luc JAFFREZOet al.
sulfate (the value for the latter will not be considered since the correlation coefficient is not significant), the slopes range only from -0.31 to -0.40. Such a behavior can indeed be explained by a dilution effect that proves to be similar for all the species. The curves for concentration variation with the amount of rain in single events sampled sequentially are all alike. The initial decrease, accounting for the greater part of the total concentration, is completed, regardless of the species, after 0.7-0.9 mm of rainfall. The end of the rain event often represents most of the volume but usually contains only low concentrations, a factor of 10-15 lower than these of the initial fractions. Such a pattern is shown in Fig. 4 for chlorine and aluminium in an event collected on the 7 November 1984 in Paris. Other studies of sequential sampling on other sites give similar results (Asman, 1980; Seymour and Stout, 1983; Wilson et al., 1983). One can see nevertheless in Fig. 4 that the initial decrease of the concentration is much steeper for chlorine than for aluminium and, indeed, than for any
other species considered in this study. This feature is also commonly observed with sequential sampling. The effect of a possible gaseous fraction of chlorine is suspected. With a very high solubility and Henry's Law constant (Clegg and Brimblecombe, 1985), this fraction could be readily and quickly dissolved at the beginning of the precipitation. This effect remains to be clearly demonstrated. Whatever the cause, the sharp initial decrease observed for chlorine concentrations on an event basis certainly accounts for the higher slope for scavenging ratios of chlorine vs volume (coefficient A1 in Table 1). Barrie (1985) and Savoie et al. (1987) attempted to compare their results for these A1 coefficients with theoretical values derived by Scott (1978) and Slinn (1983). The various experimental data are similar (varying between - 0 . 2 6 and -0.38), and are comparable with values derived by theory ( - 0 . 2 7 and -0.25). However, the theoretical calculations relate the scavenging ratios and the rain-rate, not the volume of precipitation. One can therefore ask whether the experimental relationship between W and volume is due to a direct dilution phenomena or, as suggested by Table 1. Coefficients A1, B1 and correlations for the rela- the values of Scott (1978) and Slinn (1983), to processes mostly related to the rain-rate. In this last case, the tions: log(W) = log(Bl) + A 1 * log(Vol) (Vol in mm) relationship between W and volume could be an n B1 A1 Corr. artefact of the volume and rain-rate being closely coupled (Drufuca and Zawadzki, 1975). This latter CI 78 3928 -0.625 -0.452 relationship is seen in our own data: SO282 784 -0.116 -0.190 Na 81 515 -0.340 -0.326 log (Int) = -2.31 +0.347 • log (Vol) K 82 1096 -0.311 -0.416 Mg 81 711 -0.405 -0.514 Int in m m h - 1 ; Vol in mm; n=82; corr=0.446. Ca 82 1220 -0.335 -0.407 To answer this question, it should be first pointed Zn 69 879 -0.336 -0.389 AI 82 367 -0.334 -0.412 out that several objections can be raised about the Si 82 460 -0.317 -0.451 comparison of these theoretical and experimental Fe 82 235 -0.400 -0.512 values. The value of - 0 . 2 7 taken from Scott's work is
] en 10"8M 500
AI
300
100
200
600
1000
1400
V ml
Fig. 4. Concentrations variations for chlorine and aluminium during a rain event collected sequentially in Paris on 11 July 1984.
Factors influencing scavenging ratios valid only for a particular range of rainfall rate and a particular type of precipitation, with wide variations for different situations. Furthermore, the two theoretical calculations assume a steady state system, with two consequences: the aerosol scavenged comes from an infinite reservoir and the rain-rate is constant throughout the event. These two conditions are by no means satisfied in real cases. It is known that the aerosol is scavenged in the precipitating cloud itself (Radke et al., 1980) and also by below-cloud-scavenging (Sisterson et al., 1985). This depletion of the source leads to the observed dilution effect of the concentrations in precipitation, with an exponential pattern. This effect is observed in rains collected sequentially (Asman, 1980; Jaffrezo and Colin, 1988b) and is reproduced by modeling (Stensland and de Pena, 1975; Altwicker and Wang Teng Tsai, 1987). Variations in the rain-rate, observed or modeled, generally cause only variations in the slope of the exponential decrease. The assumption of a steady state on which the theoretical calculations are based contradicts the experimental data. Particularly, by keeping the concentrations in the aerosol phase constant, any dilution effect is neglected. It seems therefore that the apparent agreement of the theoret-
Table 2. Coefficients A2, B2 and correlations for the relations: log(W) = log(B2) + A2 * log (Int) (Int in mm h - 1)
C1 SO~ Na K Mg Ca Zn A1 Si Fe
n
B2
A2
Corr.
78 82 81 82 81 82 69 82 82 82
3193 748 469 991 647 1128 827 342 419 204
-0.328 -0.024 -0.231 -0.171 -0.342 -0.303 -0.326 -0.393 -0.250 -0.421
-0.183 -0.031 -0.174 -0.177 -0.341 -0.287 -0.301 -0.380 -0.278 -0.396
~
0.5
0.3
0.1
Cl SO~," Na
K
Volume
Mg
Ca
Zn ~
AI
Si
p <0,01 1.:82)
Fe
Raln- rata
Fig. 5. Correlation coefficients for the regressions: log(W)=log(B1)+Al*log(Vol) (Vol in ram); log(W) = log(B2) + A2 * log(Int) (Int in mm h- 1). AE(A)
21:12-L
3079
ical and experimental data for the A 1 coefficient is only fortuitous. Nonetheless, one can try to find a relationship between scavenging ratios and rain-rate. Table 2 gives the coefficients A2, B2 and the correlations calculated for equations of the type: log (W) = log (B2) + A2 * log (Int) (Int in mm h - 1). The exponents A2 are again negative but cover a much greater range than A1. The correlation coefficients are systematically smaller than those found previously; they are not significant at the 1% level for CI, S, Na and K, and are close to this threshold for Ca, Zn and Si (cf. Fig. 5). Scott (1981) also failed to demonstrate a significant correlation between W and precipitation rate for snow samples. This absence of correlation strongly supports the importance of the volume effect as compared to a possible implication of the rain-rate on the scavenging ratios. Finally, it seems logical that the value of rain-rate averaged over a complete event is not suitable to describe what appears to be a second order phenomena in comparison to the main process, the depletion of the aerosol source. IMPACT OF THE TYPE OF PRECIPITATION ON THE SCAVENGING RATIOS
Independently of the relationship with the volume or the rain-rate, the nature of the precipitating system should be able to affect the scavenging ratios by modifying the incorporation mechanisms. Convection, which causes more rapid and extensive exchanges between the cloud and the underlying air, should play an important role. In addition to this increased mixing efficiency, the effect of convection is found in several theoretically described processes. Firstly, there is a direct increase of the efficiency of particle collection by precipitating drops or cloud droplets due to the turbulence associated with convection (Grover and Pruppacher, 1985). For convective situations, greater incorporation should therefore result in an increase of the scavenging ratio for a given volume of rain. Again, the turbulence associated with convection leads to faster growth of the cloud droplets by collision and coalescence (Pruppacher and Klett, 1980). This improves their chances of being incorporated by precipitating drops or of becoming such drops themselves. Furthermore, supersaturation reached within clouds increases in parallel with the updraft, speed. This phenomenon allows the activation of less soluble particles, or smaller particles of equivalent solubility, for convective cases compared to activation with more stable air masses (Jensen and Charlson, 1984). Scavenging is more efficient for activated particles of greater size (Pruppacher and Klett, 1980); this effect should also lead to an increase in scavenging ratios in
3080
JEAN-Luc JAFFREZOet al.
convective cases. There is, however, little difference for speeds greater than 20 cm s-1 (Jensen and Charlson, 1984). Conversely, Slinn (1983) indicates that scavenging ratios should be greater for stable frontal systems, which allow more time for the growth of droplets after the activation of the particles. These larger droplets are then more easily scavenged. He interprets in this way the only comparison of experimental data on the subject, contrasting the results of Gatz (1977) with those of Cawse (1974). It will be noted first of all that the two studies are difficult to compare. The very unlike sampling conditions (daily in the case of Gatz (1977), monthly for Cawse (1974)), as well as the statistical parameters used (volume-weighted means and arithmetic means, respectively) are quite enough to introduce differences by a factor of two in the scavenging ratios (Jaffrezo and Colin, 1988a). Even more important, it will be remarked that the two studies are nonspecific for the phenomena followed: thus Gatz's data would arise mainly after 'sampling essentially all convective storm precipitation', while in the case of Cawse 'much of the data is probably for frontal storm' (Slinn, 1983). It appears therefore very risky under these conditions to ascribe the differences observed between the two sets of data to the more or less convective character of the meteorological situations. It is possible to investigate the effect of convection on a same set of data by classifying the events collected during this study into two groups by means of the observations and criteria stated above. F o r each of these groups, we have calculated the regressions between scavenging ratios and volumes of the events, in order to allow for the dilution effect. The correlation coefficients and the parameters A (slope) and B (value of the scavenging ratio for 1 mm of rain depth) are presented for all the elements in Figs 6a-c. Only sulfate is absent, the correlations not being significant at the 1% level. The other species have been grouped according to their behavior. There is the group of elements of predominantly marine origin (Na, Mg and C1), a group of mainly soluble nonmarine elements (K, Ca and Zn) and, finally, a group of mainly crustal elements which appear essentially in the insoluble phase of the precipitation (Colin et al., 1990). It can be seen that correlations are found between volume and scavenging ratio for all the cases considered (apart from Na and A1 in the stratiform situations). The very large increase in these correlations for the marine elements in the convective cases is to be noted, whereas few fluctuations appear for the other two groups (cf. Fig. 6a). Figure 6b reveals a certain contrast between the marine species and the other soluble elements. For the former, the slopes are smaller for stratiform situations while it is the opposite for K, Ca and Zn: their scavenging ratios decrease more rapidly with the volume of precipitation in these cases.
Correlations
0.75
0.50 )<0,01 n = 3 5 )< 0,01 n : 4 5 0.25
K Ca' Zn
CI Na Mg
(a)
Strst (n = 45}
AI Si
Fe
I---] Cony In = 35) Coefficients
A
0.75
0.60
0.26
Cl
(b)
Na Mg
I~
K
Ca Zn
Stret (n : 451 Coefficients
f~l
AI $1 Fo Cony
In : 35) B
,.°°0iji" I000~--~
CI
(C)
.d
Na Mg
R~
K Ca Zn Strat
(n = 45)
f~l
AI Si
Fc
Cony
(n : 35)
Fig. 6. Coefficients A, B and correlations for the relations: log (W) =log(B)+ A * Iog(Vol) for stratiform situations (n=45), shaded areas, for convective situations (n = 35), plain areas. Volume expressed in mm of rain depth. Fig. 6a: correlations. Fig. 6b: coefficient A. Fig. 6c: coefficient B. Figure 6c shows higher B coefficients in stratiform than convective situations for these same elements, but little differences from one case to another for the marine species.
Factors influencing scavenging ratios The scavenging ratio is supposed to describe a scavenging efficiency of species by precipitation with the assumption, amongst others, that it is independent of the amount of rainfall. Since this condition is not fulfilled, it is necessary to take into account the relationship between W and volume of the precipitation. This could be done with a volume-weighted scavenging ratio, in the case of a linear relationship. According to the kind of relationship previously calculated, it seems better to use coefficient B, which can be represented as a scavenging ratio for a given volume of 1 mm of rain depth. The coefficient A, the slope, expresses the rate of exhaustion of the aerosol source: the steeper the slope, the quicker the dilution. It follows therefore that the scavenging of K, Ca and Zn is more efficient in situations described as 'stratiform' (cf. Fig. 6c), in combination with a more rapid exhaustion of the aerosol source (cf. Fig. 6b). This would support the hypothesis proposed by Slinn (1983), for these soluble species that can act as condensation nuclei. The case of elements of mainly crustal origin also tends towards the same conclusion, with the involvement of the nucleation capabilities of the aerosol particles and the modification in behavior it implies. Both the slopes (cf. Fig. 6b) and the scavenging ratios (coefficients B, cf. Fig. 6c) show trends identical with those of K, Ca and Zn but considerably reduced. Now it is known that these elements borne by mainly insoluble particles are much less active as condensation nuclei (Hiinel, 1976; Pruppacher and Klett, 1980). Their behavior therefore differs little from one cloud mass to another. Marine species exhibit a very different pattern from other soluble species, with comparable scavenging in both types of situation, but high slopes for convective cases. It does not seem possible at present to give a fully comprehensive explanation of these differences between stratiform and convective situations for marine species. But one can point out several factors that could cause different behavior as compared to the other soluble elements. The most obvious of these is that the source of marine species disappears once the air mass leading to the precipitating system is inland, while other species are more or less replenished in the same conditions. Another reason is also linked to the difference in sources between the two groups of species. It is very likely, particularly in stratiform cases, that over Paris species of marine origin are present in the system for much longer than any other element. If Slinn's (1983) hypothesis is correct, it would imply a greater opportunity for their activation and growth, leading to their precipitation with cloud drops. At this point, one should note that Noone et al. (1988) demonstrated chemical differences between cloud droplets of different sizes, in the case of a stratus of marine origin. They found that large droplets contain mainly sea-salt-like compounds, while smaller ones contain more sulfate. For these reasons, it seems probable that large
3081
differences in the scavenging characteristics can be expected between marine and other soluble species over continental areas. A better insight has to be gained into the relationship between scavenging ratios and the transport of aerosols from remote sources before one can fully explain differences observed from one meteorological situation to another.
CONCLUSION Thanks to the collection strategy employed, we have been able to obtain valuable data for the study of aerosol scavenging by precipitation over Paris. In a previous publication, strong relationships between the collected aerosol and precipitation phases were calculated, with high correlation coefficients for most of the chemical species studied. This observation is complemented here; the impact of the major aerosol sources, tracked with back-trajectories, proves to be very similar in both air and rain. The predominant role of long- and medium-range transport is thus demonstrated, even over a large urban area. As an implication, scavenging ratios can reflect incorporation mechanisms occurring during this transport (i.e. nucleation scavenging), as well as below-cloud-scavenging over the sampling site. The authors are nevertheless aware of the potential enhancement of this last process due to the larger aerosol concentrations at ground level. This feature can also lead to scavenging ratios slightly lower than those expected over sites with weaker gradients of aerosol concentration with altitude, with less opportunity for nucleation scavenging. It is then possible to calculate relationships between scavenging ratios and rain volume. The decrease observed for the scavenging parameter can be ascribed to dilution of the species in the rain as the aerosol feeding the precipitating cloud is depleted. A much weaker dependency, if any, with the rain-rate can be found; compared to aerosol depletion, the rain-rate variation seems to have a second-order effect on the concentrations in rain water. Its effect should be evaluated more efficiently with sequential sampling of rain events. For the first time, scavenging ratios have been extensively investigated in term of the type of meteorological situation, stratiform or convective. For most of the chemical species studied, higher scavenging ratios (for a given volume of rain) and quicker dilution (assumed to reflect the rapidity of the aerosol depletion process) are associated with stratiform situations. Species of marine origin (CI, Na and Mg) behave differently. More data are needed to evaluate the role of atmospheric transport (particularly the lack of sources over the continent) as regards this difference. Acknowledgements--This work was made possible by the
financial support of the convention no. 82138 of the Ministry of the Environment. The authors are grateful to E. Bon Nguyen for technical assistance.
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JEAN-Luc JAFFREZOet al.
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Factors influencing scavenging ratios Scott B. C. (1978) Parameterization of sulfate removal by precipitation. J. appl. Met. 17, 1375-1389. Scott B. C. (1981) Sulfate washout ratios in winter storm. J. appl. Met. 20, 619-625. Semb A. (1978) Sulphur emissions in Europe. Atmospheric Environment 12, 455-460. Seymour M. D. and Stout T. (1983) Observations on the chemical composition of rain using short sampling times during a single event. Atmospheric Environment 17, 14831487. Sisterson D. L., Johnson S. A. and Kumar R. (1985) The influence of humidity on fine particle aerosol dynamics and precipitation scavenging. Aerosol Sci. Technol. 4, 287-300. Slinn W. G. N. (1983) Air-to-sea transfer of particles. In Air-Sea Exchange of Gases and Particles (edited by P. S. Liss and W. G. N. Slinn), pp. 299-405. D. Reidel, Dordrecht.
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Stensland G. J. and de Pena R. G. (1975) A model of belowcloud precipitation scavenging of NaCI. J. geophys. Res. 80, 3410-3418. The MAP3S/RAINE Research Community (1982) The MAP3S/RAINE precipitation chemistry network: statistical overview for the period 1976-1980. Atmospheric Environment 16, 1603-1631. Uematsu M., Duce R. A. and Prospero J. M. (1985) Deposition of atmospheric mineral particles in the North Pacific Ocean. J. atmos. Chem. 3, 123-138. Wilson J., Graham R. and Robertson J. (1983) Correlation of intrastorm sequential precipitation chemistry with storm meteorology. In Precipitation Scavenging, Dry Deposition and Resuspension (edited by H. R. Pruppacher, R. G. Semonin and W. G. N. Slinn), pp. 229-237. Elsevier, New York.