ARTICLE IN PRESS

1352-2310/$ - se

doi:10.1016/j.at

�Correspondfax: +8522788

E-mail addr

Atmospheric Environment 39 (2005) 7872–7879

www.elsevier.com/locate/atmosenv

Observational and modeling studies of chemical speciesconcentrations as a function of raindrop size

K.M. Wai, C.W.F. Tam, P.A. Tanner�

Department of Biology and Chemistry, City University of Hong Kong, Tat Chee, Avenue, Kowloon, Hong Kong SAR, PR China

Received 9 December 2004; received in revised form 18 May 2005; accepted 1 September 2005

Abstract

The Guttalgor method has been used to determine the chemical species concentrations in size-selected raindrops in nine

rain events at Hong Kong from 1999 to 2001. The curve (concentration against raindrop radius) patterns for all the species

are similar but depend on the starting time of sampling within a rain event. In these plots, the maximum concentration

occurs at the same range of droplet radius, irrespective of the species, and this indicates the importance of coalescence and

breakup processes. The maximum is located at a smaller droplet radius than was found in previous studies in Germany. All

results show almost constant concentrations with size for large raindrops, and these indicate the in-cloud contributions.

The pH of raindrops of similar size is linearly correlated with a function of the sulfate, nitrate, acetate, formate, calcium

and ammonium ion species concentrations. Within a single raindrop, chloride depletion is not significant, and sulfate,

ammonium and hydrogen ions are found in ratios compatible with the precursor solid-phase mixture of ammonium sulfate

and ammonium bisulphate. When simulated by a below-cloud model, good agreement between the modeled and measured

sodium and sulfate concentrations has been found. Below-cloud sulfur dioxide scavenging contributes at most 60% of the

sulfate concentration in a single raindrop.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Guttalgor; Concentration and radius; Below-cloud; scavenging

1. Introduction

It has been shown that analyte concentrations arefairly similar in different size precipitation dropletsat the base of the cloud (Bachmann et al., 1993,1996a). Below-cloud atmospheric particles areremoved by raindrops through Brownian diffusion,interception, impaction and phoretic effects. Theconcentration of aerosol in a droplet is determined

e front matter r 2005 Elsevier Ltd. All rights reserved

mosenv.2005.09.017

ing author. Tel. +852 2788 7840;

7406.

ess: bhtan@cityu.edu.hk (P.A. Tanner).

partly by the collision efficiency, which is a functionof both the droplet size and the aerosol size(Pruppacher and Klett, 1997). Gases can also bescavenged by the falling droplets and the (non-equilibrium) rate of gas uptake is inversely propor-tional to the square of the raindrop radius, r

(Bachmann et al., 1992). Although the chemicalcompositions of bulk rainwater samples have beenintensively studied, few experiments have investi-gated the chemistry of individual raindrops. Georgiiand Wotzel (1970, and references therein) observeddecreasing concentrations (c) with r for given

.

ARTICLE IN PRESSK.M. Wai et al. / Atmospheric Environment 39 (2005) 7872–7879 7873

aerosol radius, a. The most thorough studies of thec/r dependence have been made by Bachmannet al. (1992, 1993, 1996a,b). In general, two shapesof the c/r curves were found: (i) a continuousdecrease in concentration with increasing raindropradius and (ii) the formation of a maximumconcentration for a certain droplet radius. Theformer type of curve was especially observed at thebeginning of a rain event and was attributed to theeffects of evaporation (Bachmann et al., 1992) and/or to the decreasing scavenging efficiency of gasesand aerosol for large droplets (Pruppacher andKlett, 1997). The type (ii) behavior results from thecollisional breakup of large raindrops; and/or fromthe effects of maxima for collision efficiency E inplots of E against r for given a, and of E against a

for a given r, in the regime of ro500 mm; and/orfrom the distinctions in height of the aerosolparticles. The breakup of large raindrops is evi-denced by the fact that single raindrops of a givensize which are simultaneously collected show arange of concentrations of a certain analyte(Tenberken and Bachmann, 1996). In some experi-ments with tracers the distinction between types (i)and (ii) was clearly attributed to the difference inparticle size of the chemical species scavenged by thedroplets (Bachmann et al., 1995, 1996b; Ebert et al.,1998) whereas a clear identification of the dominantmechanism could not be made for the sampling ofraindrops well below the cloud level (Bachmannet al., 1996a).

The objective of this paper is to study the c/r-dependence of size-classified rain droplets in HongKong, since the chemical and physical character-istics of the rainfall events are markedly differentfrom those in Germany. First, the composition ofrainwater differs considerably at the two locations,since, for example, zinc and formate are present inmoderate concentrations in Germany but are tracein coastal Hong Kong where seawater species areabundant. Second, the intensity of rainfall and theraindrop sizes are greater; the temperature is higher,and the ambient concentrations of suspendedparticulate matter are considerably greater insubtropical Hong Kong. Other differences fromGermany are discussed in the later part of thispaper. We have, therefore, completed nine sam-plings of rain events in Hong Kong and in thefollowing we highlight data from two of these eventswhich are representative of the whole set: i.e. thetwo cases (i) and (ii) above, and attempt to modelthese results using a below-cloud model.

2. Methodology

2.1. Experimental

Nine size-selected raindrop samplings were per-formed between April 1999 and March 2001 atground level in the campus at City University ofHong Kong (latitude ¼ 221 210N, longitude ¼ 1141100 E), which is in a residential area and has nonearby large point emission sources. We focus hereupon the rain events on 1, 4 and 24 August 2000 and1 March 2001, when wind speeds were less than2m s�1 so that the raindrop path did not have alarge departure from the vertical. The samplingprocedures have been described in detail elsewhere,e.g., Bachmann et al. (1992, 1993, 1996a,b). Briefly,the sampling unit (the Guttalgor) consists of adewar vessel filled with liquid nitrogen. The dewar issurrounded by an inert gas (nitrogen) box to avoidcontamination from the environment. Individualraindrops fall into liquid nitrogen during the briefsampling period, then freeze and sink down into thedewar where they are separated as size fractions bysieves of different mesh widths (1400, 900, 800, 710,600, 500, 410, 300, 265 and 102 mm). The size-selected raindrops were analyzed for cations andanions using capillary electrophoresis (Tam et al.,2001). A Ross semi-micro combination electrodewas used to measure pH. Routine quality assuranceprocedures included replicate analyses and theanalysis of simulated rainwater.

2.2. Model description and input data

The c/r-dependence of the inert species Na+ fromaerosol and the reaction product SO2�

4 from gaseousSO2 and particulate SO2�

4 have been modeled in theraindrops by the physico-chemical below-cloudmodel which has previously been used to study theacidification processes under the cloud in rural andurban areas of China (Lei et al., 1997; Qin andHuang, 2001). The description and assumptions ofthe model are detailed in Liu and Huang (1993) andLei et al. (1997). Briefly, it consists of: (i) a physicalmodel to calculate the mass transfer of atmosphericgases to the raindrop at the terminal velocity of theraindrop; (ii) a chemical model to take into accountthe aqueous phase reversible/irreversible chemicalreactions within the CO2–SO2–NH3–HNO3–H2Osystems, as well as SO2 to SO2�

4 oxidation (viaO3 and H2O2) and below-cloud gaseous phaseCO2, SO2, O3, NH3, HNO3, H2O2 and particulate

ARTICLE IN PRESS

Table 1

Model inputs of the number distribution [d N/d lg(a)] for each

particle size bin for species Naþ and SO2�4 in ambient aerosol

Range of particle radius (mm) Number distribution [d N/d

lg(a), cm�3]

Species Na+

0.05–0.2 70

40.2–0.3 5

40.3–0.4 2

4 0.4–1 1

41–2 0.1

42–4 0.04

44–6 0.002

Species SO2�4

0.025–0.035 4000

40.035–0.05 2000

40.05–0.09 500

40.09–0.2 100

40.2–0.4 40

40.4–0.75 1

40.75–2 0.1

42–5 0.01

K.M. Wai et al. / Atmospheric Environment 39 (2005) 7872–78797874

phase SO2�4 , NO�3 , NHþ4 are included in this system;

(iii) an aerosol model to determine the contributionof chemical species of the aerosol to the raindrop byconsidering the Gamma-Junge particle size distribu-tion, the collection/collision efficiency between theraindrop and aerosol, and the aerosol mass per unitlength collected by the raindrop. Considering all ofthe above, a set of non-linear differential equationsis formed and solved numerically to obtain solu-tions of the unknown concentrations of SO2�

4 , NO�3 ,NHþ4 , etc., in single raindrops.

In the present study, modifications to the aerosolmodel have been made as follows: (i) the collection/collision efficiency E is calculated using Slinn’sformula (1983), which considers the contribution ofthree important removal mechanisms: interception,inertial impaction and diffusion;

E ¼ 4=ðRe ScÞ½1þ 0:4 Re1=2 Sc1=3

þ 0:16 Re1=2 Sc1=2�

þ F½1=oþ ð1þ 2Re1=2ÞF�

þ ½ðSt� S�Þ=ðSt� Sþ 2=3Þ�3=2,

where Re is the Reynolds number of a raindrop, Sc

is the Schmidt number of the collected particle, F isthe ratio of diameter, o is the viscosity ratio, St isthe Stokes number of the particle and S* ¼ [1.2+1/12 ln(1+Re)]/[1+ln(1+Re)]; (ii) The Gamma-Junge particle size distribution has been replacedby specific distributions of the chemical species. Theoriginal formulae for collision/collection efficiencyE are based on research works in the 1960s and1970s and should be updated by the well-knownSlinn’s equation (1983) which has been discussedand applied in many more recent research papers(e.g. Mircea et al., 2000; Garcıa Nieto et al., 1994).The use of the Gamma-Junge particle size distribu-tion in the original version of the model showsdifferences concerning the size distribution for aspecific chemical species. Therefore, this part of themodel should be modified. Table 1 shows the valuesof the number distribution [d N/d lg(a)] used foreach particle size bin for the species Na+ and SO2�

4 ,assuming the densities of the Na+ and SO2�

4 speciesare 2.2 and 1.8 g cm�3, respectively. For Na+, thedata sets adopted here are based on those fromZhuang et al. (1999a) and Fitzgerald (1991, andreferences therein). It is noted that (i) the numberconcentrations with respect to each size bin varywith each sample (though the modes are at similarparticle sizes) which may be due to the different

wind speeds on the sampling dates and the air massorigins, (ii) Zhuang et al. (1999a) reported thenumber concentrations of relatively few samplesonly, and (iii) although the number distributions forfine particles are comparable between the two, thevalues of Zhuang et al. (1999a) for the giant particlesize range (r41 mm) are about one order ofmagnitude smaller than those by Fitzgerald (1991).It is noted that the former authors worked at a moreinland site and particles with larger sizes (havinglarger settling velocity) are then dissipated quicker,which explains this discrepancy. Therefore, the datasets of Fitzgerald (1991) and Zhuang et al. (1999a)are considered as the upper and lower limits ofmodel inputs, respectively. For SO4

2�, a ‘‘typical’’(from 20 samples) size distribution is adopted fromthe work of Zhuang et al. (1999b).

Table 2 shows the model input data for theconcentrations of ambient pollutants. The typicalaverage HNO3 and NH3 values are obtained fromYao et al. (2001); H2O2 data from Tanner andWong (1998); SO2 and O3 data from a routinemonitoring station (�1 km away from the samplingsite) operated by the Hong Kong EnvironmentalProtection Department. The SO2�

4 , NO�3 and NHþ4in-cloud concentrations were estimated as 0.8 of thespecies concentration at the 425 mm raindrop size,which is justified at the end of Section 3.2.

ARTICLE IN PRESS

Table 2

Concentration input data to the below-cloud model

Species Concentration on 1 August 2000a

H2O2 14.4

HNO3 0.05

NH3 1.85

SO2 5.2

O3 2.2

SO2�4

26.2

NO�3 20.9

NHþ4 21.5

aThe concentrations of the ambient species SO2, O3, NH3,

HNO3, and H2O2 are expressed in ppbv; in-cloud SO2�4 , NO�3 ,

NHþ4 are in meq l�1.

Table 3

Average concentrations (meq l�1) of ions in simultaneous bulk

deposition samples for nine rainfall event samplings

Iona Bulk deposition (Std. deviation)

Cl� 42.379.1

SO2�4

38.873.0

NO�3 30.1714.1

Fo� 5.476.4

Ac� 2.572.7

Na+ 38.2717.3

NHþ4 21.679.8

Ca2+ 16.8713.7

H+ 7.774.0

Mg2+ 6.572.7

K+ 1.378.0

aFo� formate; Ac� acetate; std. Standard.

K.M. Wai et al. / Atmospheric Environment 39 (2005) 7872–7879 7875

3. Results and discussion

3.1. General characteristics of experimental results

The concentrations of bulk deposition simulta-neously sampled during the size-selected Guttalgorsamplings are shown in Table 3 as a guide to therainwater composition. On two occasions: 24August 2000 and 1 March 2001, the H+ concentra-tions were measured and therefore the ionicbalance, i.e. (sum of anion concentrations)/(sum ofcation concentrations), in these two samplings canbe calculated. The values are 0.99 and 1.07. A highcorrelation (r ¼ 0:98, po0:01, N ¼ 16) exists be-tween the H+ concentration and the sum of theconcentrations of sulphate, nitrate, formate, acetateminus those of ammonium and calcium ion species,by lumping all species concentrations at eachraindrop size within the two events. Species withhigh inter-correlations also include Na+ and Cl�

(r ¼ 0:96, po0:01, N ¼ 65) and NHþ4 and SO2�4

(r ¼ 0:66, po0:01, N ¼ 65) by lumping all thesamples from the nine rainfall events. The averagemole ratio for the former (Cl�/Na+) is 1.0, showingtheir sea salt origin, and the ratio for the latter(NHþ4 /SO

2�4 ) is 1.4, suggesting mixed forms of

NH4HSO4 and (NH4)2SO4 in the precursor solidphase.

3.2. The characteristic patterns of c/r curves

Table 4 reports a summary of the sampling of thenine rainfall events for the species Na+ and SO2�

4 .The rain intensity at each sampling has also beenincluded. The raindrop spectrum may be calculated

according to the Marshall–Palmer (M–P) distribu-tion [N(d) ¼ n exp (�ld)]. However, a modified M–Pdistribution (Peng and Qin, 1992) was adopted here,where N(d) ¼ raindrop concentration (m�3mm�1);n ¼ 3000m�3mm�1; l ¼ 2.27R�0.15mm�1; R ¼

rainfall rate (mmh�1); d ¼ raindrop diameter(mm). These authors reported the spectrum inGuangdong Province, China (wherein Hong Kongis situated), using 552 rainfall samples, and it differsmuch from that of M–P distribution. It should benoted that the adoption of this average spectrumcan only provide rough information about theactual situation during the samplings. Only twospecies are shown in this table since the c/r curvepatterns are more or less similar irrespective of thespecies for each sampling exercise. In most cases fora given species, the concentrations of raindrops withr larger than about 200 mm do not show a significantdifference. In the following we focus upon the twotypes of representative results from all samplings,i.e. types (i) and (ii) described in the Introduction, asexemplified by the samplings on 1 and 4 August2000. Fig. 1 plots the c/r curves for anions andcations on 1 August 2000. Each data point in thefigure represents the species concentration within arange of radii of the raindrop. For example, thesmallest range of radii is from 51 to 133 mm insteadof the single point at 92 mm. This sampling started atabout the beginning of the rainfall event. All speciesshow continuous decrease in their concentrationswith raindrop radius just as in the findings fromBachmann et al. (1993). This is explained by thescavenging of ‘‘giant’’ aerosol particles (ra-dius41 mm) and/or raindrop evaporation, which isparticularly important for smaller raindrop sizes

ARTICLE IN PRESS

Table

4

Summary

ofconditionsandresultsforspeciesNa+

andSO

2�4

indropsofdifferentsizesduringninerainfallevents

Event

Start

ofsampling

after

eventbegins

(min)

Sampling

Duration

(min)

Rain

intensity

(mmhr�

1)

Ion

Ionconcentration(meq

l�1)in

dropsof

differentsizes

Curveshape

Rem

arks(r

inmm

)

92mm

(given

size,

mm)

575mm

1Middle

ofevent

10

8.3

Na+

—62(178)

59

Nomaxim

um

Nodata

for

ro150,due

tonotenoughsample

(11April99)

SO

2�4

—50(178)

37

220

—0.2

Na+

—30(275)

38

Nomaxim

um

Nodata

for

ro250,due

tonotenoughsample

(26April99)

SO

2�4

33(275)

29

33

20

0.8

Na+

—86(141)

33

Nomaxim

um

Nodata

for

ro133,due

tonotenoughsample

(25June99)

SO

2�4

87(141)

33

4Middle

ofevent

10

1.0

Na+

—46(141)

35

Nomaxim

um

Nodata

for

ro133,due

tonotenoughsample

(28June99)

SO

2�4

36(141)

22

50

25

8.5

Na+

84

35

cdecreasesas

rincreases

(type(i))(see

Fig.1)

—

(1August

00)

SO

2�4

88

33

610

20

6.0

Na+

34

49(141)

35

cmaxim

um

with

rat

141mm

(type(ii))(see

Fig.2)

—

(4August

00)

SO

2�4

19

40(141)

22

7Middle

ofevent

15

13.9

Na+

—69(141)

31

cmaxim

um

with

rat

141mm

forSO

2�4

(type

(ii))

Nodata

forcationswith

ro133,dueto

not

enoughsample

(24August

00)

SO

2�4

30

56(141)

33

820

15

0.1

Na+

—80(141)

24

cmaxim

um

with

rat

141mm

forSO

42�(type

(ii))

Nodata

forcationswith

ro133dueto

notenough

sample

(1March01)

SO

2�4

48

74(141)

36

95

50.5

Na+

—59(228)

49

Nomaxim

um

Nodata

for

ro205,due

tonotenoughsample

(11March01)

SO

2�4

—54(228)

45

K.M. Wai et al. / Atmospheric Environment 39 (2005) 7872–78797876

ARTICLE IN PRESS

Fig. 1. Species concentrations (meq l�1): (a) cations (’ Na+; E NHþ4 ; m Ca2+; * Mg2+; � K+); (b) anions (’ SO2�4 ; E Cl�; � NO�3 ; m

Fo�; *Ac�) against raindrop radius (mm), on 1 August 2000.

K.M. Wai et al. / Atmospheric Environment 39 (2005) 7872–7879 7877

(Bachmann et al., 1996a). Gas scavenging of atmo-spheric gases (e.g. SO2, NH3, HNO3, etc.) can alsolead to a continuous increase of solute concentra-tion with decreasing drop size since the uptake ratefunction decreases with raindrop radius (Bachmannet al., 1996b) but this is irrelevant for species suchas Na+.

In Fig. 1, the decay of Na+ matches well the Cl�

one. Unfortunately, no measurements for H+ areavailable to verify if it accounts for the SO2�

4 +NO�3extended decay, not matched by the sum of theother cations that flatten- out at 178 mm.

Fig. 2 plots the c/r curves for anions and cationson 4 August 2000, with the addition of the curve forH+ ion measured on 24 August 2000 (since it wasnot measured on 4 August). The sampling started atabout 10min after the start of the rainfall event. Allspecies show maximum concentrations at the141 mm raindrop size range (i.e. radii from 133 to150 mm) and then exhibit a decrease in theirconcentrations with increasing raindrop radius.Whereas the curve patterns agree with the study of

Bachmann et al. (1993), the maxima of speciesconcentrations presented here are located at asmaller drop radius. Nevertheless, the maxima inc/r curves are interpreted as due to the coalescenceand breakup processes, which depend upon theraindrop size and rain intensity (Pruppacher andKlett, 1997 and references therein; Bachmann et al.,1996a), and which more strongly influence thespecies originating from higher levels above theground (Bachmann et al., 1996b). We stress that thisis the major reason for the appearance of maxima inthe c/r curves in our experiments [instead of thescavenging of ‘‘large’’ aerosol particles: 0.1 mmoradiuso1mm (Bachmann et al., 1996a and referencestherein)], since all species irrespective of their particle

size show similar curve patterns. Our argument issupported by the ground level experiment of Ebertet al. (1998), which showed that curve maxima atraindrop radii from 150 to 350mm are only created bymonodisperse particles with radii ranging from 0.3 to1.5mm, as explained by the collision efficiencies ofdifferent particles and raindrop sizes.

ARTICLE IN PRESS

Fig. 2. Species concentrations (meq l�1): (a) cations (’ Na+; E NHþ4 ; m Ca2+; * Mg2+; � K+;+H+), (b) anions (’ SO2�4 ; E Cl�; �

NO�3 ; m Fo�; *Ac�) against raindrop radius (mm), on 4 August 2000 (except for H+ sampled on 24 August 2000).

Fig. 3. Modeled concentrations (meq l�1) against raindrop radius

(mm) on 1 August 2000: (a) Na+ (solid line for simulated results;

dots for measurements), (b) SO2�4 (solid line for simulated results,

dots for measurements, dotted line for gas contributions).

K.M. Wai et al. / Atmospheric Environment 39 (2005) 7872–78797878

It is observed that, from Figs. 1 and 2, the speciesconcentrations do not decrease indefinitely withincreasing raindrop size but become fairly constantwith the larger sizes. This asymptote indicatesmainly the in-cloud species concentrations at verylarge raindrop sizes since the in-cloud concentra-tions are virtually independent of the drop radius.

3.3. Modeling results

Only the curve patterns of type (i) mentioned inthe Introduction (i.e. as exemplified by the experi-ment performed on 1 August 2000) for Na+ andSO2�

4 are modeled here since the below cloud modeldoes not consider the coalescence and breakupprocesses. The modeled Na+ and SO2�

4 concentra-tions (Figs. 3(a) and (b), respectively) plottedagainst raindrop radius are well consistent withthe experimental data (discussed above), eventhough there are some discrepancies as shown forthe smallest raindrop size range. SO2 contributes atmost 60% of the modeled raindrop SO2�

4 concen-tration at small drop size and down to 20% in largeraindrop size range, Fig. 3(b).

ARTICLE IN PRESSK.M. Wai et al. / Atmospheric Environment 39 (2005) 7872–7879 7879

4. Concluding remarks

Results of chemical properties of nine size-selected raindrop samplings in Hong Kong havebeen presented. The coalescence and breakupprocesses govern the raindrop radius where themaximum concentration (if existing) is locatedirrespective of the chemical species. Such a max-imum concentration is located at a smaller dropletradius than was found in previous studies inGermany. All results show almost constant con-centrations with size for large raindrops, and theseindicate the in-cloud contributions. A modelingexercise has been attempted to simulate the curvepatterns for two species. This is one of the firststudies outside Germany dealing with the analysis(by capillary electrophoresis) of size-selected rain-drops and there exist some technical and logisticlimitations such as: (1) only one sampling perrain event could be performed, (2) the modelingwork was performed only for one rainfall event, and(3) no record about the frequency of drops collectedin each size class and the change in the drop spectrawith time during each sampling was available. Suchlimitations should be taken into account in newstudies of a similar kind.

Acknowledgements

PAT acknowledges the travel grants from theHong Kong—Germany Research Grant fund(9050106). We are grateful to the Hong KongEnvironmental Protection Department for supply-ing air quality monitoring data. This paper isdedicated to Professor Knut Bachmann for hisoutstanding contributions to the area of analyticalatmospheric chemistry.

References

Bachmann, K., Roder, A., Haag, I., 1992. Determination of

anions and cations in individual raindrops. Atmospheric

Environment 26A, 1795–1797.

Bachmann, K., Haag, I., Roder, A., 1993. A field-study to

determine the chemical content of individual raindrops as a

function of their size. Atmospheric Environment 27A,

1951–1958.

Bachmann, K., Haag, I., Steigerwald, K., 1995. Determination of

transition metals in size-classified rain samples by atomic

absorption spectrometry. Atmospheric Environment 29,

175–177.

Bachmann, K., Ebert, P., Haag, I., Prokop, T., 1996a. The

chemical content of raindrops as a function of drop radius I.

Field measurements at the cloud base and below the cloud.

Atmospheric Environment 30, 1019–1025.

Bachmann, K., Ebert, P., Haag, I., Prokop, T., Steigerwald, K.,

1996b. The chemical content of raindrops as a function of

drop radius II. Field experimental study on the scavenging of

marked aerosol particles by raindrops sampled as a function

of drop size. Atmospheric Environment 30, 1027–1033.

Ebert, P., Kibler, M., Mainka, A., Tenberken, B., Bachmann, K.,

Grank, G., Tschiersch, J., 1998. A field study of particle

scavenging by raindrops of different sizes using monodisperse

trace aerosol. Atmospheric Environment 32, 767–771.

Fitzgerald, J.W., 1991. Marine aerosols—a review. Atmospheric

Environment 25A, 533–545.

Garcıa Nieto, P.J., Arganza Garcıa, B., Fernandez Dıaz, J.M.,

Rodrıguez Brana, M.A., 1994. Parametric study of selective

removal of atmospheric aerosol by below-cloud scavenging.

Atmospheric Environment 28, 2335–2342.

Georgii, H.W., Wotzel, D., 1970. On relation between drop size

and concentration of trace elements in rainwater. Journal of

Geophysical Research 75, 1727–1731.

Lei, H.C., Tanner, P.A., Huang, M.Y., Shen, Z.L., Wu, Y.X.,

1997. The acidification process under the cloud in southwest

China: observation results and simulation. Atmospheric

Environment 31, 851–861.

Liu, S.R., Huang, M.Y., 1993. The effects of below-cloud aerosol

on the acidification process of rain. Journal of Atmospheric

Chemistry 17, 157–178.

Mircea, M., Stefan, S., Fuzzi, S., 2000. Precipitation scavenging

coefficient: influence of measured aerosol and raindrop size

distributions. Atmospheric Environment 34, 5169–5174.

Peng, H., Qin, Y., 1992. The parameterization of the influence of

precipitation on the washout of aerosol particles,. Scientia

Atmopherica Sinica 16 (5), 622–630 (in Chinese).

Pruppacher, H.R., Klett, J.D., 1997. Microphysics of Clouds and

Precipitation. Kluwer Academic Publishers, Boston.

Qin, G.Y., Huang, M.Y., 2001. A study on rain acidification

processes in ten cities of China. Water Air and Soil Pollution

130, 163–174.

Slinn, W.G.N., 1983. Precipitation scavenging. In: Atmospheric

Sciences and Power Production. US Department of Energy,

Washington, DC.

Tam, W.F.C., Tanner, P.A., Law, P.T.R., Bachmann, K.,

Potzsch, S., 2001. Use of capillary electrophoresis in the

analysis of aerosol and bulk/dry deposition collected on a

daily basis. Analytica Chimica Acta 427, 259–269.

Tanner, P.A., Wong, A.Y.S., 1998. Spectrophotometric determi-

nation of hydrogen peroxide in rainwater. Analytica Chimica

Acta 370, 279–287.

Tenberken, B., Bachmann, K., 1996. Analysis of individual

raindrops by capillary zone electrophoresis. Journal of

Chromatography A 755, 121–126.

Yao, X.H., Fang, M., Chan, C.K., 2001. Experimental study of

the sampling artifact of chloride depletion from collected sea

salt aerosols. Environmental Science and Technology 35,

600–605.

Zhuang, H., Chan, C.K., Fang, M., Wexler, A.S., 1999a.

Formation of nitrate and non-sea-salt sulfate on coarse

particles. Atmospheric Environment 33, 4223–4233.

Zhuang, H., Chan, C.K., Fang, M., Wexler, A.S., 1999b. Size

distributions of particulate sulfate, nitrate, and ammonium at

a coastal site in Hong Kong. Atmospheric Environment 33,

843–853.