Acidity of raindrop by uptake of gases and aerosol pollutants

Atmospheric Environment 43 (2009) 1571–1577

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Acidity of raindrop by uptake of gases and aerosol pollutants D.M. Chate*, P.C.S. Devara Indian Institute of Tropical Meteorology, Pune 411 008, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 January 2008 Received in revised form 3 June 2008 Accepted 18 June 2008

Below-cloud raindrops acidification simulated with a simple model incorporating gas– liquid equilibriums, gas-phase mass transfer, and catalyzed SO2 oxidation in aqueous phase with uptake of gases and scavenging of particles. Ionic contents of various species in raindrops of different size and pH are computed using one-dimensional time-variant model. The model results are based on SO2 and NH3 absorption and collection of calcium aerosols by raindrops with various collection mechanisms. Aqueous concentrations of (SO2)l and (NH3)l and their ionic components in raindrops are found to be increased with the fall distance from cloud base and decrease of drop size. The overall magnitude of pH enhances with the increase in drop size and transient position of raindrops in the atmosphere below the cloud base. The elevated ionic calcium in raindrops by impaction of calcium aerosols of higher inertia neutralizes the acidic components. Acidic ion contents in smaller droplets are found to be significant and resulted pH of raindrop increases with the size and neutralizing potential of alkaline species. The pH values of rainwater contents of predominant size raindrops in bulk samples corresponding to various rainfall intensities are higher as against the individual non-evaporating smaller raindrops. Results are important in view of the impact of showers on earth surfaces during rain containing large number of smaller droplets as compared to the acidification studies of bulk rainwater. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Acid-rain Collision efficiency Reaction kinetics Oxidation

1. Introduction As a raindrop falls through a cloud and below-cloud base, it will collect fraction of the atmospheric aerosols lying in its path and may absorb and desorbs various atmospheric trace gases. The uptake of atmospheric pollutants by raindrops referred to as precipitation scavenging. Raindrops have chemical contents that are determined from the efficiency of collections of pollutants and compositions from which they are formed in the atmosphere. The acidity of rainwater contents in freely falling raindrops changes with the time-variant uptake of gases and aerosols and corresponding variations in ionic contents of acidic and neutralizing components. Esmen and Fergus (1976) have initiated the measurements of pH of

* Corresponding author. E-mail address: [email protected] (D.M. Chate). 1352-2310/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2008.06.031

individual raindrops using pH paper. Hill and Adamowicz (1977); Adammowicz (1979); Overton et al. (1979) have estimated the chemical composition and pH of rain for the cases of gases pollutants. They considered SO2 uptake with concentrations of not more than 20 ppb. Liu and Huang (1993) and Qin and Huang (2001) have carried out the study of rain acidification processes including uptake of gases and aerosol pollutants by raindrops. They have used approximated collision efficiencies for simulation of subcloud rain acidification for particles of diameters Dp < 0.2 mm and Dp < 2 mm. These results suggest that simulation of uptake of pollutants is useful to examine the ionic compositions and pH of accumulated rain. The parameterizations for wet scavenging proposed by Chang et al. (1987); Berge (1993) and Sportisse and Bois (2002) are widely used in several acid deposition models (Wang et al., 2008; Carmichael et al., 2008). Wang et al. (2008) examined the model – measurement disagreement in model inter-comparison and evaluation of acid deposition and

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suggested that differences in chemical processes, deposition parameterization, and modeled precipitation are responsible for large disparities in results. Large disparity in parameterization of wet scavenging in various acid deposition models is an important factor for differences. Also the mechanisms involved in precipitation scavenging will have certain effects on acid deposition. A parameterization of Sportisse and Bois (2002) considers below-cloud scavenging of gases species by mono-disperse raindrops, while Berge (1993) used mean collection efficiency equal to 0.1 in his parametric study. A wide spread collision efficiency values in droplet– aerosols–gases interactions for poly-dispersed raindrops lead to a significant error in the acid deposition model results. Recent studies show that smaller droplets scavenges significant amount of both gases and aerosols pollutants as they spent longer time in the atmosphere owing to their size-dependent terminal velocities (Chen, 2004). In the present work a proposed simple onedimensional model considers time-variant uptake of gases species and particles of diameter (Dp) by raindrops of sizes (D) with the electro-scavenging, Brownian diffusion, inertial impaction, interception, and thermo-diffusion-phoresis for computations of variable ionic concentrations in raindrops size-dependent pH (Chate et al., 2005). Characterizations of the time-series ionic contents of acidic and alkaline species in raindrops and resulting pH of droplets during drizzle/light rain spread over the longer durations is important in understanding the link between the efficiency with which droplets scavenges pollutants and below-cloud acidification of raindrops. Information about pH with the variable ionic composition of droplets at various transient positions in light rain may aid in the assessment of their impact on taller vegetations over the hilly region. It is advantageous to have time-dependent ionic contents and pH of raindrops using widespread scavenging efficiency of droplets collecting air pollutants in their path. The pHs of droplets as a result of variation in ionic compositions of acidic and neutralizing components in them are determined by combining the processes of gases uptake with the aerosols scavenging mechanisms. We present here the results based on one-dimensional timevariant model which considers SO2 and NH3 absorption and calcium aerosols scavenging by falling raindrops. This study discusses the acidity of individual non-evaporating raindrops attributing to variations in ionic species of acid and alkaline mostly during drizzle spread over the taller vegetations in the hilly regions as against the measured pH of bulk samples of accumulated rainwater. Results may be useful to examine the factors affecting below-cloud scavenging of gases and aerosols by freely falling raindrops. The paucity of comprehensive pollutant measurements, raindrops size spectra, acidity with variable ionic contents of individual raindrops directs a need of a simplified model in current research. 2. Model description We assume the scavenging of SO2, NH3 and aerosols of calcium compounds by falling raindrops and cloud base height to be 1000 m. The initial droplet pH is assumed to be

5.6 representing the acidity due to CO2 absorption and dissociation. Raindrops with known concentrations of various species listed in Table 1 falling vertically through the gases and aerosol pollutants at their terminal velocities. Gas and aqueous phase reactions, reaction rate expressions and constants used in model computations are tabulated in Table 2 quoting the references. The significant internal circulation due to shear on the surface of falling raindrops resulting in rapid mixing inside the drop offers assumption of negligible liquid-phase resistance to mass transfer of pollutants. We assume that there is no advection of pollutants. The inherent assumptions in the model are that the variation in raindrop size due to evaporation, collision and coalescence and temperature below-cloud atmosphere do not affect scavenging processes significantly. SO2 is absorbed into raindrops, undergo dissociation and metal ions catalyses oxidation to produce sulfate (Adammowicz, 1979). NH3 and calcium aerosols are highly basic and serve to neutralize the droplets’ acidity. The model includes the scavenging processes in which raindrops uptake air pollutants and reacts with them and collides with aerosol particles lying in their path of vertical fall. Raindrops falling through the pollutants layer are assumed to fall vertically at their terminal velocities. Diffusion mixing within the drop is taken to be rapid so that no concentration gradients develop there. The large raindrops fall down so quickly that the ion electric neutrality equilibrium cannot be realized. Soluble equilibrium needs a longer residence time, so it is necessary to combine the mass transmission, ionic equilibrium with aqueous phase chemical reaction. The model at this stage has been applied to few gases absorption and single species aerosols scavenging by raindrops falling through the atmospheric layers. The case of multiple components of aerosol captures and gases absorption with chemical reaction in the aqueous phase during thunderstorm and non-thunderstorm rain will be considered in our separate work. The atmosphere below the cloud is assumed to be consisting trace gases (SO2, NH3 etc.) and calcium aerosols. The trace gas concentration distribution in the vertical is assumed to be constant and homogeneous. Weekly Lidar data are available for vertical profiles of total aerosols Table 1 Parameters, equilibrium processes and values of equilibrium constants at 15  C. Parameters

Values

Kinematical viscosity of air 0.133 cm2 s1 Diffusion coefficient 0.166 cm2 s1 for carbon dioxide Diffusion coefficient 0.234 cm2 s1 for ammonia Diffusion coefficient 0.136 cm2 s1 for sulfur dioxide Ambient temperature 20  C Atmospheric pressure 950 mb 2.87  103 ppm [SO2]g [NH3]g 1.3  103 ppm 345 ppm [CO2]g 0.29  105 mole l1; pH ¼ 5.6 [Hþ]l Calcium aerosol size distribution functions (Chate et al., 2003) Coarse particles MMD ¼ 4 mm; sc ¼ 1.5; dM ¼ 7.6 mg m3 Fine particles MMD ¼ 0.62 mm; sc ¼ 2; dM ¼ 5.05 mg m3

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Table 2 Gas and aqueous phase reactions, rate expression and reaction constants. Reaction

Rate expression and constant

Reference

SO2#SO2$H2O(l)

½SO2 $H2 O K1 ¼ ¼ 1:23 M atm1 PSO2  þ ½H ½HSO3  K2 ¼ ¼ 1:23  102 M ½SO2 $H2 O

Smith and Martell (1976)

þ SO2 $H2 O!HSO 3 þH

Smith and Martell (1976)

2 þ HSO 3 !SO3 þ H

K3 ¼

½Hþ ½SO2 3  ¼ 6:61  108 M ½HSO 3

Smith and Martell (1976)

NH3(g)#NH3$H2O

K4 ¼

½NH3 $H2 O ¼ 62 M atm1 PNH3

Seinfeld and Pandis (1998)

 NH3 $H2 O!NHþ 4ðaqÞ þ OH

K5 ¼

 ½NHþ 4 ½OH 

½NH3 $H2 O

¼ 1:7  105 M

Seinfeld and Pandis (1998)

without chemical compositions. Therefore, we have to rely on surface measurements of chemical composition of aerosols. It is assumed that concentrations of gases and aerosols do not change with the time. However mass or number concentrations of aerosols assumed to be decrease with the height (Qin and Huang, 2001, and references therein), as

compositions in raindrops as a function of size and fall distance are numerically computed by integrating the coupled nonlinear differential equations with fourth order Runge–Kutta Gill method.

NðzÞ ¼ N0 expðz=HpÞ

Computations of time-dependent uptake of gaseous pollutants and particulate matter are performed for raindrops in the diameter range 200–6000 mm (in 200 mm steps). The known SO2 4 concentrated droplet starts falling at initial (zero) time. Oxidation catalyzed by trace Fe, Cu and Mn in solution proceeds very rapidly by a complex and not well understood radical chain mechanism (Adammowicz, 1979 and references therein). The rate of in raindrops can be calculated as production of SO2 4 a function of initial concentrations of SO2, NH3, CO2 and Fe. Variation of SO2 4 concentration in rain with known SO2 as drop size changes as a function of fall distance from the contents in rain cloud base is shown in Fig. 1. SO2 4 increases with the decrease of drop diameter from 2 to 0.2 mm. Fig. 1 shows a sharp decrease of SO2 4 in rain in the range of drop size 0.2–2 mm thereafter SO2 4 concentrations do not show much variation in rain for the drop size between 2 mm and 5 mm. Also this pattern reflects clearly for transient positions <500 m. Transient SO2 4

Where N(z) is number concentrations of aerosols at height z, N0 is the concentrations on the ground surface and the scale height Hp ¼ 1410 m. The rate of accumulation of all compounds of the sulfur, carbon, NH3 in each raindrop is assumed to be equal to the rate of mass transfer of these gasses across the gas–water interface of a drop of diameter D. The rate per unit fall distance is expressed in a general dynamic form as (Seinfeld and Pandis, 1998),

 d½Sl 6Kg  ½Sg Hs ½S þ RiVðDÞ ¼ dz DUN

(2)

Where Kg is mass transfer coefficient, Hs is Henry’s law constant, [S]g is concentration of one gaseous species in the atmosphere, [S]l is concentration of one kind of gas in raindrop, Ri is chemical reaction rate, V(D) raindrop volume size distribution function and UN is terminal velocity of raindrops. Terminal velocities for raindrops in various size regimes are computed using formulations of Beard (1976). It is evident from Eq. (2) that the concentration gradients between trace gases concentrations in the raindrop and its surrounding environment is the driving force for gas scavenging processes. The aerosol mass that is collected by raindrop per unit diameter (D) expressed as

  Z N     dm dp 3UN E D; dp m dp ddp VðDÞ ¼ 2D dz 0

(3)

Where m(dp) is mass size distribution function of aerosols of diameter dp and E(D, dp) is collection efficiencies of raindrop and factor 3/2D represents the cross sectional area of raindrops. Chate et al. (2003) reported the collision efficiencies of raindrops scavenging particles of various ionic species. E(D, dp) of raindrops collecting calcium particles of diameter (dp) by electro-scavenging, Brownian diffusion, inertial impaction, interception, and thermodiffusion-phoresis are adopted from Chate et al. (2003) and Chate and Devara (2005) in Eq. (3). Time-variant ionic

2.0

800 m 500 m 100 m

1.5

SO42- µMole

(1)

3. Results

1.0

0.5

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

Drop size (cm) Fig. 1. Variation of SO2 4 concentration in rain with drop size as a function of fall distance.

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Fall distance (m)

1000

D = 0.2 mm D = 1 mm D = 2 mm D = 5 mm

800

600

400

200

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2-µMole

SO4

Fig. 2. Transient SO2 4 concentration in rain as a function of fall distance.

concentration in rain as a function of fall distance is shown in Fig. 2. SO2 4 concentration increases in rain with transient position of raindrops of various sizes below the cloud base with higher SO2 4 in smaller raindrops than that in large raindrops. SO2 4 contents in rain decrease for raindrop sizes in the order 5 mm < 2 mm < 1 mm < 0.2 mm at various transient positions of drops.

a

0.12 0.10

a

100 m 500 m 1000 m

0.08

0.04 0.02

100 m 500 m 1000 m

0.20 0.15 0.10 0.05

0.00 0.1

0.2

0.3

0.4

0.5

0.6

0.00

Drop size (cm)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Drop size (cm)

b 1000

b

D = 0.2 mm D = 1.0 mm D = 2.0 mm

800

Fall distance (m)

Fall distance (m)

0.30 0.25

0.06

(NH4)l µMole

(SO2)l µMole

Aqueous (SO2)l and (NH3)l concentrations as a function of drop size for the position of the drop at 100, 500 and 1000 m below the cloud base is shown in Figs. 3 and 4. Aqueous (SO2)l and (NH4)l contents in raindrops decreases with the increase in drop diameter as seen from Figs. 3a and 4a. Thus it is noticed that the rate of uptake of gas is directly proportional to the difference in vapor pressure between atmosphere and drop surface and changes with diffusion coefficients of gas species. Also, it is known that uptake rate is inversely related to the square of the drop size (Eq. 2). Taking into account that smaller drop has lower terminal velocities it is expected that the pollutants concentration in rainwater of raindrops increase with decrease of drop size. The rate at which SO2 enters in a drop of a given size and hence the fall distance at which equilibrium chemical compositions in raindrops is reached is determined by the raindrops volume (6 Kg mgD1) in Eq. (2). It evidently happens that the ranges of raindrop sizes, terminal velocities and mass transfer coefficients are such that equilibrium contents in raindrops are predicted to occur for well-mixed small drops over fall distances which are less than physically significant mixed layer heights. The transient aqueous (SO2)l and (NH3)l concentrations in falling raindrops of 0.2, 1 and 5 mm diameter as a function of fall distance are depicted in Figs. 3b and 4b for a mixed layer containing gaseous pollutants such as SO2 and NH3. Concentration of aqueous SO2 and NH3 in rain increases with fall distance (Figs. 3b and 4b). It is evident from Figs. 2, 3b and 4b that the small raindrops reach their

600 400 200 0 0.00

0.02

0.04

0.06

0.08

0.10

0.12

(SO2)l µMole/litter Fig. 3. Aqueous (SO2)l is shown as function of drop size for transient positions.

1000 D = 0.2 mm D = 1.0 mm D = 2.0 mm

800 600 400 200 0 0.00

0.05

0.10

0.15

0.20

0.25

(NH4)l µMole Fig. 4. Same as Fig. 3 for (NH4)l.

0.30

0.35

D.M. Chate, P.C.S. Devara / Atmospheric Environment 43 (2009) 1571–1577

equilibrium composition very rapidly while the larger raindrops are required to fall over a long distance (1000 m) before coming to within 5–20% of their equilibrium composition. This result from the characteristic differences of the three raindrops (0.2, 1 and 5 mm) due to residence times in atmosphere for raindrops of different sizes. This behavior illustrates that there is a critical residence time in the atmosphere for each raindrop at which the rate of attainment of its equilibrium composition is a maximum. This critical residence time of the falling raindrops is naturally, a function of the size of the drop, its potential for uptake of trace gases and capture of aerosols exist in the mixed layer and the initial composition of the raindrops. The consequence of the uptake of gaseous and aerosol pollutants by vertically falling raindrops is that small drops would be acidic and large drops basic. Timedependent pH with the raindrop size at successive transient positions below the cloud base and of different drop sizes as a function of fall distance is discussed in the following paragraph. Fig. 5 shows variations in pH with the raindrops size at transient positions 100, 500 and 800 m in Fig. 5a and as a function of fall distance for 0.2, 1 and 5 mm raindrops in Fig. 5b. The pH increases exponentially with the increase in raindrop size and corresponding size-dependent fall distance of raindrops from the cloud base. Fig. 5a shows that the pH of rain changes with the drop size for the case of gas scavenging and gases–aerosol–rain interaction at fall

a

6.0 5.5

800 m 500 m

pH

5.0

100 m

4.5

Ca2+ scavenging Gases uptake

4.0 3.5 0.1

0.2

0.3

0.4

0.5

0.6

0.7

Drop size (cm)

b

6.5 5 mm

6.0

1 mm

pH

5.5 0.2 mm

5.0 4.5 4.0 3.5 200

400

600

800

1000

1200

Fall distance (m) Fig. 5. Variation of pH with drop size with uptake of gasses pollutants and with capture of aerosols by raindrops.

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distances of 100, 500 and 800 m below the cloud base. Smaller droplets fall slowly in the atmosphere owing to their size-dependent terminal velocities scavenges significant amount of uniformly distributed gases pollutants. On the contrary observations confirm that the concentration of aerosols decreases with vertical distance from the Earth’s surface to cloud levels. This is expected from the atmospheric density profile and also because the surface constitutes the major source of aerosols, while aerosol removal mechanisms operate continuously throughout the atmosphere (Seinfeld and Pandis, 1998). As a result at transient positions <500 m below the cloud base, smaller droplets are highly concentrated with acidic components due to rapid oxidation of SO2 with ionic catalyst and relatively less concentrations of alkaline aerosols of higher inertia available for impaction scavenging (Fig. 5a). An appreciable amount of acidic radicals produced in the smaller raindrops (D  2 mm) exhibits effective pH variations with the drop size as a function of fall distances at 100, 500 and 800 m (Fig. 5a). On the other hand, larger drops between 2 mm and 5 mm do not show much variation in pH with the increase in drop size. Also, Fig. 5b illustrates significant variation in pH with the fall distance for drop of diameters 0.2, 1 and 5 mm. Time-series variation in pH for 0.2, 1 and 5 mm raindrops with the fall distance from the cloud base indicates that pH for 0.2 mm raindrops with the fall distance are lower than that for 1 and 5 mm raindrops. The fact of the size-dependent scavenging behavior of raindrops is stressed by several investigators (e.g. Ebert et al., 1998). In general this processes leads to an increase of the Hþ concentration in raindrops and hence to a pH decrease. It means uptake capacities of pollutants by raindrops corresponding to their size-dependent terminal velocities and vertical distributions of pollutants play a major role in determining the pH of rainwater amounts in raindrops of different sizes. Acidic proportion of raindrops due to SO2 oxidation decreases with the increase in fall distance from the cloud base as they get neutralized with NHþ 4 in case of gases uptake of falling drops. Additionally time-series droplet–aerosol interaction elevates the pH as a function of raindrop transient positions and size with the accumulation of ionic alkaline species (Ca2þ) and subsequent neutralization of existing acidic components in raindrops. The contribution of calcium aerosols to Hþ in enhancement of the pH with drop size and fall distance is seen in Fig. 5 for neutralizing potential of raindrops for a typical aerosol distribution at Pune (Chate et al., 2003). The broken curves in Fig. 5 show the neutralizing effect of Ca2þ in the pH of rain. The present results indicates that rain acidification processes should be assessed by number concentration of raindrops instead of mass concentration because the former correlate with the prevalence and extent of smaller raindrops. Number concentration most closely reflects the acidity of raindrops as large fractions of the raindrops during drizzle and low rainfall intensities are in the smaller size (D  2 mm). The large number of smaller acidic raindrops when showers on biosphere pose a great problem of acid-rain on tall plantations at elevated earth surfaces even in Indian subcontinents where acid deposition is a remote possibility due to very high concentrations of calcium and other alkaline compounds in the ambient atmosphere. The measurements of pH of individual raindrops at their transient positions of

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vertical fall below the cloud base in near future will support the model results. The presented results suggest that water contents of raindrops of different sizes have different pH in accordance with the time-variant ionic concentrations determined by uptake of pollutants. Observations show that most raindrops which reach the ground without much evaporation have equivalent diameter D  0.2 mm (Pruppacher and Klett, 1997). Despite the importance of non-evaporating acidic raindrops of diameter D  0.2 mm, discussion on comparison with the observed pH of bulk rainwater samples is expected to examine the impact of smaller droplets on vegetations in hilly region. Marshall Palmer size distributions is n(D) ¼ n0exp(LD), where L ¼ 4.1R0.21 mm1, D for drop size, R is rainfall intensity and n0 ¼ 8  103 m3 mm1. The predominant drop diameters Dp, liquid water content W and number concentrations of predominant drops are computed for various rainfall intensities assuming constant n0. These rainfall parameters (predominant drop diameter Dp, liquid water content W and number of predominant drops) are tabulated in Table 3. The hydrogen ion concentration [Hþ] reflects the acidity of raindrops after neutralizing by Ca and NH4. Therefore the initial acidity, in the absence of Ca and NH4 can be estimated for [Hþ] þ [Ca2þ] þ [NHþ 4 ] to compare with the acidity due to [Hþ]. For various rainfall intensities the acidity [Hþ] of predominant drops is shown in Table 3. The major source of calcium in Indian precipitation is wind-blown soil dust particles from the Earth’s surface. Ammonium originates from ammonia from domestic animals, loss from fertilizer applications to agricultural sector and biomass burning. The animal source has a distinct maximum in the northeastern parts of India. Both the ionic concentrations of acidic and neutralizing components deplete with the progress of the rainfall so that pH of raindrop should stabilizes accordingly. The computed pH of raindrops of sizes from 0.2 to 3.0 mm vary between 4.35 and 5.41 for [Hþ] concentrations and between 4.28 and 5.34 in the absence of neutralizing components. Verma (1989) reported the highest frequency of bulk rainwater pH around 6.5–7.0 over Indian stations. The highest frequency of pH in this range for bulk rainwater was reported for long-term samples (1984 to 2002) over Pune (Safai et al., 2004). The Pune site (IITM) is about 6 km west of the city center of Pune. There is a hilly part towards west and south which may partly shield the observation site from pollution from the central part of the city. Bulk rainwater samples collected at Sinhagad during monsoon 2002 and 2003 and post-monsoon 2002 were alkaline with pH varying from 5.8 to 7.1 with average value 6.4 (Momin et al., 2005). Only one sample was found acidic with pH of 5.2. The Sinhagad site is about 100 km east of the Arabian

Sea at an altitude of 1400 m in the mountains of the Western Ghats, 18 km southwest of the Pune. It is located on a small plateau surrounded by steep cliffs and valleys covered with sparse trees and the area is very sparsely populated. Wet-only rainwater samples collected at Sinhagad during the year 2002–2003 were alkaline with an average pH of 6.2. The sample collected on 11.10.2003 was acidic with pH 4.9 (Momin et al., 2005). High concentration   of non-sea-salt SO2 4 and NO3 and excess Cl from a source other than sea salt (anthropogenic activity) seems to be contributed to acidic pH of rainwater. The rainwater mass contents for bulk samples in the atmosphere during rain is significantly due to water contents from predominant raindrops at various rainfall intensities (Table 3). The ionic concentration of both acidic and neutralizing components in raindrops decreases with the increase in raindrops size. Also the ionic concentrations of neutralizing components (Ca and NH4) are roughly two times to those of acidic components (SO4 and NO3) in bulk rainwater samples over Pune (Safai et al., 2004). The dilution effects due to major liquid water contents from the predominant raindrops (Dp) and raindrops  Dp in bulk rainwater samples maintains the higher pH (6.5–7.0) as against the computed pH of individual raindrops (
Table 3 Rainfall parameters for various rainfall intensities (R). R (mm h1) Dp (mm) W (mg m3) Np (m3) pH

0.3 0.57 33.2 339 4.85

0.5 0.63 49.6 381 4.9

0.7 0.68 65.9 406 4.93

0.9 0.71 81.1 429 4.96

1.0 0.72 88.8 457 4.97

1.5 0.8 125.0 478 5.02

2.0 0.84 159.1 516 5.04

2.5 0.9 192.0 529 5.07

3.0 0.93 223.7 550 5.1

D.M. Chate, P.C.S. Devara / Atmospheric Environment 43 (2009) 1571–1577

Overall pH of raindrops enhances with fall distance and drop sizes due to Ca2þ neutralization. Scavenging of gases and aerosols by falling raindrops, leads to an increase in concentration of various ionic contents in them. The predicted values of pH  5 for raindrops  0.2 mm and smaller than raindrops of predominant sizes corresponding to various rainfall intensities. The current results are important for highly acidic raindrops in these size ranges while they showers on taller vegetations in the hilly region. Because of the simplified nature of the model adopted, the uncertainties in the data and the very limited sets of observations of chemical species used to evaluate the relevant wet scavenging (including chemical and physical) processes in the atmosphere during precipitation, the results should be viewed as preliminary. In spite of these limitations, however we believe that they serve to illustrate the complex interactions that can occur in gaseous–aerosol–aqueous– phase media suspended in the atmosphere and the effects of size on the variations of chemical contents in falling raindrops. The predicted ionic contents and pH of raindrops are important in visualizing the acid-rain problem in view of the lack of instrumental techniques for the measurements of acidity of freely falling individual raindrops at their respective terminal velocities in the below-cloud atmosphere at various transient positions. The results are useful to understand the impacts of showers having large number of smaller raindrops on the biosphere which may have concentrations of acidic components much higher than the bulk samples of rainwater collected during rain events. A more complete model should be developed which include both in-cloud and below-cloud uptake of all the gases and aerosol pollutants present in the atmosphere during thunderstorm and non-thunderstorm conditions. The aforementioned issue deserves further investigations in near future. Acknowledgements Authors express their gratitude to the Prof. B. N. Goswami, Director, IITM, Pune for encouragement. References Adammowicz, R.F., 1979. A model for the reversible washout of sulfur dioxide, ammonia and carbon dioxide. Atmospheric Environment 13, 105–121. Beard, K.V., 1976. Terminal velocity and shape of cloud and precipitation drops aloft. Journal of Atmospheric Sciences 33, 851–864. Berge, E., 1993. Coupling of wet scavenging of sulfur to clouds in a numerical weather prediction model. Tellus 45B, 1–22.

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Chang, J.S., Brost, R.A., Isaksen, I.S.A., Middleton, P., Stockwell, W.R., Walcek, C.J., 1987. A three-dimensional Eluerian acid deposition model: physical concepts and formulation. Journal of Geophysical Research 92, 14681–14700. Carmichael, G.R., et al., 2008. MICS-Asia II: the model inter-comparison study for Asia Phase II methodology and overview of findings. Atmospheric Environment 42, 3468–3490. Chate, D.M., Devara, P.C.S., 2005. Parametric study of scavenging of atmospheric aerosols of various chemical species during thunderstorm and non-thunderstorm rain events. Journal of Geophysical Research 110 D23208. Chate, D.M., Rao, P.S.P., Naik, M.S., Momin, G.A., Safai, P.D., Ali, K., 2003. Scavenging of aerosols of different chemical species by rain. Atmospheric Environment 37, 2477–2484. Chate, D.M., Ali, K., Rao, P.S.P., Momin, G.A., Safai, P.D., Praveen, P.S., Devara, P.C.S., 2005. Study of acidity of raindrop by uptake of gases and aerosol pollutants during rain. December, 13–16. In: Conference of the Asian Aerosol Research Assembly. BARC, Mumbai, pp. 377–378. Chen, W.H., 2004. Atmospheric ammonia scavenging mechanisms around a liquid droplet in convective flow. Atmospheric Environment 38, 1107–1116. Ebert, P., Kibler, M., Mainka, A., Tenberken, B., Baechmann, K., Frank, G., Tschiersch, J., 1998. A field study of particle scavenging by raindrops of different sizes using mono-disperse trace aerosols. Journal of Aerosol Science 29, 173–186. Esmen, N.A., Fergus, R.B., 1976. Rain water acidity pH spectrum of individual drops. Science of Total Environment 6, 223–226. Hill, F.B., Adamowicz, R.F., 1977. A model for rain composition and the washout of sulfur dioxide. Atmospheric Environment 11, 917–927. Liu, S., Huang, M., 1993. The effects of below-cloud aerosol on the acidification processes of rain. Journal of Atmospheric Chemistry 17, 157–178. Momin, G.A., Kaushar, Ali, Rao, P.S.P., Safai, P.D., Chate, D.M., Praveen, P.S., Rodhe, H., Granat, L., 2005. Study of chemical composition of rain water at an urban (Pune) and a rural (Sinhagad) location in India. D08302. Journal of Geophysical Research 110, 1–10. doi:10.1029/ 2004jd004789. Overton Jr., J.H., Aneja, V.P., Durham, J.L., 1979. Production of sulfate in rain and rain drops in polluted atmosphere. Atmospheric Environment 13, 355–367. Pruppacher, H.R., Klett, J.D., 1997. Microphysics of Clouds and Precipitation. Kluwer Academic Publishers, London, p. 954. Qin, G., Huang, M., 2001. A study on rain acidification processes in ten cities of China. Water Air and Soil Pollution 130, 163–174. Safai, P.D., Rao, P.S.P., Momin, G.A., Ali, K., Chate, D.M., Praveen, P.S., 2004. Chemical composition of precipitation during 1984–2002 at Pune, India. Atmospheric Environment 38, 1705–1714. Seinfeld, J.H., Pandis, S.N., 1998. Atmospheric Chemistry and Physics. A Wiley-Inter Science Publication, John Wiley & Sons, Inc., New York, p. 1326. Smith, R.M., Martell, A.E., 1976. Critical State Constants: Inorganic Complexes. Plenum Press, New York. Sportisse, B., Bois, L., 2002. Numerical and theoretical investigation of a simplified model for the parameterization of below-cloud scavenging by falling raindrops. Atmospheric Environment 36, 5719–5727. Verma, G.S., 1989. Background trends of pH of precipitation over India. Atmospheric Environment 23, 747–751. Wang, Z., Xie, F., Sakurai, T., Ueda, H., Han, Z., Carmichaelc, G.R., Streets, D., Engardt, M., Holloway, T., Hayamig, H., Kajino, M., Thongboonchoo, N., Bennet, C., Park, S.U., Fung, C., Chang, A., Sartelet, K., Amann, M., 2008. MICS-Asia II: model inter-comparison and evaluation of acid deposition. Atmospheric Environment 42, 3528–3542.