Influence of rainfall on atmospheric deposition fluxes of 7Be and 210Pb in Mangaluru (Mangalore) at the Southwest Coast of India

Atmospheric Environment 202 (2019) 281–295

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Influence of rainfall on atmospheric deposition fluxes of 7Be and Mangaluru (Mangalore) at the Southwest Coast of India

210

Pb in

T

M.P. Mohana, Renita Shiny D'Souzaa, S. Rashmi Nayaka, Srinivas S. Kamatha, Trilochana Shettya, K. Sudeep Kumaraa, Y.S. Mayyaa,b, N. Karunakaraa,∗ a b

Centre for Advanced Research in Environmental Radioactivity, Mangalore University, Mangalagangotri, 574 199, India Department of Chemical Engineering, Indian Institute of Technology-Bombay, Mumbai, 400 076, India

A R T I C LE I N FO

A B S T R A C T

Keywords: 7 Be 210 Pb Deposition flux Rainfall Tracer Washout ratio

The depositional fluxes of 7Be and 210Pb were measured at Mangalore (12.82° N, 74.92° E) on the South West Coast region, which is one of the highest rainfall regions in India. This region receives pristine air masses from the Southern Indian Ocean (oceanic) and polluted air from Asia (continental). The activity concentration for individual rain events collected during the monsoon season varied between 0.15 and 4.3 Bq L−1 (with geometric mean, GM = 1.1 Bq L−1, geometric standard deviation, GSD = 2.6) and 0.01–0.71 Bq L−1 (GM = 0.12 Bq L−1, GSD = 2.8) for 7Be and 210Pb, respectively. The GM value for monthly (dry period) 7Be and 210Pb concentrations in air were 6.9 mBq m−3 (GSD = 1.3) and 1.2 mBq m−3 (GSD = 2.1), respectively. The dry deposition flux of 7 Be varied in the range of 0.081–0.184 Bq m−2 d−1 (GM = 0.12 Bq m−2 d−1, GSD = 1.3) and that of 210Pb varied in the range of 0.026–0.293 Bq m−2 d−1 (GM = 0.11 Bq m−2 d−1, GSD = 1.8). The GM value of the wet deposition flux of 7Be was higher by two orders of magnitude when compared with that of the dry deposition flux, whereas, that in the case of 210Pb it was higher by an order of magnitude. The mean annual deposition fluxes of 7Be and 210Pb were 2026 Bq m−2 y−1 and 221 Bq m−2 y−1, respectively. The large variability observed in the wet deposition fluxes was attributed to the large variations in the intensity of the rainfall. The wet deposition velocities of the aerosols, estimated based on the concentrations of these radionuclides in air and their corresponding deposition fluxes, were found to be 6.8 cm s−1 for 7Be and 6.5 cm s−1 for 210Pb. Although the two radionuclides originate from distinct sources, a strong positive correlation was observed between the temporal variability of their deposition fluxes. In addition to this, a high degree of correlation was observed between the deposition fluxes of both 7Be and 210Pb and rainfall. This points at the dominating effect of scavenging by rainfall in controlling their fluxes.

1. Introduction Beryllium-7 (7Be, T1/2 = 53.12 d) and Lead-210 (210Pb, T1/ = 22.3 y) are two naturally occurring airborne radionuclides, which 2 have been used as tracers to study the atmospheric processes. 7Be is produced by the interaction of cosmic rays with atmospheric nitrogen and oxygen (through spallation reaction) in the stratosphere (∼70%) and upper troposphere (∼30%) (Lal et al., 1958). Lal and Peters (1962, 1967) estimated the production rate of 7Be in the stratosphere and the troposphere from the experimental data on the cosmic ray produced proton and neutron fluxes and the spallation reactions and obtained a global average of 810 atoms m−2 s−1. Several other investigators have also reported the average production rate of 7Be values ranging from 100 to 1100 atoms m−2 s−1 (Arnold and Al-Salih, 1955; Benioff, 1956;

∗

Cruikshank et al., 1956; Lal et al., 1958, 1960a; O'Brien, 1979; Masarik and Reedy, 1995; Masarik and Beer, 1999; Nagai et al., 2000; Yoshimori et al., 2003; Yoshimori, 2005). The concentration of 7Be in the troposphere is dependent on the season, altitude, and latitude. 7Be undergoes temporal variation over a 11-year solar cycle (Lal and Peters, 1962; Papastefanou and Ioannidou, 2004). It exhibits latitudinal variation due to the deflection of the incoming cosmic rays as the earth's magnetic field is stronger above the equator (Terzi and Kalinowski, 2017). Due to its shorter half-life, when compared with the residence time of stratospheric aerosols (∼one year), most of the 7Be produced in the stratosphere does not reach the troposphere, except during spring and summer. The thinning of the tropopause at mid-latitudes during spring and summer results in intense convective mixing of the air between the stratosphere and the troposphere (Rastogi and Sarin, 2008),

Corresponding author. E-mail addresses: [email protected], [email protected] (N. Karunakara).

https://doi.org/10.1016/j.atmosenv.2019.01.034 Received 12 August 2018; Received in revised form 19 January 2019; Accepted 23 January 2019 Available online 29 January 2019 1352-2310/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Map showing (a) the geographical location of Mangalore, and. (b) wind rose diagram for Mangalore region (downloaded from https://www.meteoblue.com/ en/weather/forecast/modelclimate/mangalore_india_1263780; Mohan et al., 2018).

which leads to higher input of 7Be into the troposphere during these seasons. The abundance of 7Be undergoes both temporal and spatial variations in the surface air based on the atmospheric vertical mixing and regional meteorology (Lal and Peters, 1967; Feely et al., 1989; Dibb et al., 1994). 210 Pb is a terrigenic radionuclide, which is produced in the atmosphere by the radioactive decay of Radon-222 (222Rn, T1/2 = 3.82 d) (Poet et al., 1972; Tokieda et al., 1996). About 99% of the 222Rn in the atmosphere is due to emanation, mainly from the continents. It has been estimated that about 1–10% of the 222Rn produced from the decay of 226Ra in the upper one metre of the soil is released to the atmosphere (Turekian et al., 1989; Lozano et al., 2011). The median concentration of 226Ra in the soil is 35 Bq kg−1 (UNSCEAR, 2000a), whereas it is ∼1.3 mBq kg−1 in the surface ocean water (Lozano et al., 2011; Baskaran, 2011). The global 222Rn flux from the continent ranges from 1300 to 1800 Bq m−2 d−1 and it is ∼17 mBq m−2 d−1 from oceanic areas (Samuelsson et al., 1986; Nazaroff, 1992; Baskaran, 2011). Considering the emanation rate of 222Rn as 0.75 atom cm−2 s−1 (Wilkening and Clements, 1975) for the overall land area (1.50 × 1018 cm2), the corresponding 210Pb flux can be estimated to be 3.50 × 1016 Bq y−1, which is in good agreement with the reported value of 3.50 × 1016 Bq y−1 by Baskaran (2016) under the assumption that the upward flux of 222 Rn is equal to the downward flux of 210Pb. The concentration of 210 Pb in the air varies widely in both the continental and marine settings. Baskaran (2011) has stated that the factors that affect the concentration of 210Pb in the air are seasons, atmospheric pressure variations, height of the atmospheric boundary layer, temperature inversions, diurnal and seasonal variations of meteorological parameters, soil moisture content, frequency and amount of precipitation, presence of snow cover, etc. The major removal mechanism for 210Pb from the atmosphere is washout by precipitation (Cannizzaro et al., 1999; Hirose et al., 2011; Baskaran, 2011). Koch et al. (1996) have studied the vertical transport and removal mechanism of 210Pb and 7Be from the troposphere. Their study indicated that the removal of these

radionuclides from the troposphere is controlled by the rainfall scavenging process in which, ∼88% of 210Pb is removed by rainfall and 12% by dry deposition, whereas 68% of 7Be is removed by rainfall, 3% by dry deposition, and the remaining 28% decays in the atmosphere itself due to its shorter radioactive half-life. When modelling the atmospheric transport of cosmogenic radionuclides like 7Be and 10Be with general circulation models (GCMs) (Brost et al., 1991; Field et al., 2006; Koch et al., 1996; Land and Feichter, 2003; Liu et al., 2001), it turned out that the stratosphere to troposphere exchange (STE) as well as wet scavenging are uncertain factors in these models and need to be adequately understood. There is a need for deposition flux observations to improve the wet scavenging routines of the models. In order to validate the models related to atmospheric transport of cosmogenic radionuclides such as GCMs, a regional database on cosmogenic radionuclides fluxes of 7Be and 10Be is essential (Heikkila et al., 2008). The availability of databases for different geographical regions with distinct atmospheric and meteorological conditions is of paramount importance in the validation of the models. A review of the literature by Baskaran (2016) regarding various aspects of 210Pb in the atmosphere brings out the fact that although extensive studies have been reported for 20˚-50° N latitude regions, studies for the Indian environs (10˚-20° N) are very sparse. Limited number of studies was carried out in Mumbai (Arabian Sea) (Rangarajan and Gopalakrishnan, 1970 and 1990; Sarin et al., 1999; Rengarajan and Sarin, 2004), Kaiga (James et al., 2010), and Ahmedabad regions of India (Rastogi and Sarin, 2008). Also, there is a general paucity of data in the regions corresponding to the northern latitudes of 0˚-20˚N (Baskaran, 2011; Mohan et al., 2018). In the previous publication (Mohan et al., 2018), the temporal variations of 7Be and 210 Pb for the west coast region of India (where pristine air masses from the Southern Indian Ocean and polluted air from the Asian land mass meet during winter and spring seasons) was presented and influence of meteorological parameters on the concentrations of these radionuclides in atmospheric air was discussed in detail. In this paper, we present: (i) 282

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the seasonal variability of dry and wet depositional fluxes of 7Be and 210 Pb, (ii) the effect of rainfall intensity on total deposition fluxes of 7Be and 210Pb, (iii) the deposition velocity of aerosols, (iv) the washout ratios, (v) precipitation-normalized depositional fluxes of 7Be and 210 Pb, (vi) mass balance approach between 222Rn and 210Pb. 2. Materials and methods

6M HCl (McNeary and Baskaran, 2003). To evaluate the deposition velocity of aerosols, a total of 42 aerosol samples (2016–17) (particle sizes of ≤10 μm) were collected by drawing atmospheric air through a pre-weighed glass filter paper (Whatman, GF/A) of dimension 20.3 × 25.4 cm2 using a high volume air sampler (Envirotech, APM, 460 BL) at a flow rate of 1.13 m3 min−1. Details of air sampling were published elsewhere (Mohan et al., 2018).

2.1. Study area

2.3. Radiochemical processing of dry and wet fallout samples

Mangalore (12.82° N, 74.92° E, 22m amsl), a city on the South West Coast of India was chosen for this study (Fig. 1a). The sampling location at Mangalore was situated in the Mangalore University campus, which is about 2 km aerial distance from the Arabian Sea to its west and the Western Ghats (mountain range) to its east. The South West Coast region of India is influenced by two types of air masses– continental and oceanic. In the previous publication, air mass movements were identified to explain the temporal variability of the concentrations of 7Be and 210 Pb in aerosols by constructing the backward trajectories from National Oceanographic and Atmospheric Administration (NOAA) (Mohan et al., 2018). These backward trajectories reveal that, during the monsoon season, the maritime air masses arrive predominantly from the Arabian sea (Southwest) and contribute a major rainfall in the study region, while during the winter season, the air masses arrive predominantly from the continents (Northerly winds). The wind rose diagram for Mangalore region is shown in Fig. 1b. The Arabian Sea has a unique weather pattern in view of the Indian monsoon and the associated winds that reverse its direction seasonally. As mentioned in the previous section, the pristine air masses from the Southern Indian Ocean and the polluted air from Asia meet in this region during the winter and spring seasons and hence it is a very interesting area for aerosol studies (Ramanathan et al., 2001). Mangalore has a tropical monsoon climate because of the influence of the Arabian Sea. During the rainy season, the major wind direction is from the south-west. About 95% of the rainfall is received during a period of six months from May to October and the region generally experiences dry spell from December to March. The average annual rainfall for the last 10 years has been ∼3500 mm. The maximum rainfall during the southwest monsoon at the study site is attributed to orographic effect and the occurrence of this effect is due to the presence of long coastal chain with parallel Western Ghats (Manjunatha et al., 2015). It is a humid region with an annual average relative humidity of ∼75%, with maximum humidity reaching up to 93% during May, June, and July. The geology of the study area is characterized by hilly regions with laterite along the land surface and sandy soil along the seashore.

The rainwater and dry fallout samples, collected in polyethylene containers, were filtered using the Whatman filter paper (#42), which was rinsed with 6M HCl. The filtered sample was evaporated to reduce the total volume to 50 mL. It was then transferred to a 100 mL volumetric flask and made up to 100 mL with 2N HCl. This 100 mL solution was dried completely and the obtained residue was transferred to a 20 mL polypropylene vial (McNeary and Baskaran, 2003) and subjected to gamma spectrometry. 2.4. Determination of 7Be and spectrometry

210

Pb activity concentrations by gamma

The 7Be and 210Pb activity concentrations in the filter papers and rainwater samples were determined by gamma spectrometry using 50% relative efficiency p-type broad energy HPGe detector with a carbon composite window (BEGE-5030, Canberra Industries Inc., USA). The details of the gamma spectrometer and efficiency calibration were presented in the previous publication (Mohan et al., 2018; Karunakara et al., 2013). The counting time for the aerosol and rainwater samples was 60000s and 80000s, respectively. The activity concentration was estimated from the 477.7 keV and 46.5 keV gamma lines of 7Be and 210 Pb, respectively. The minimum detectable activity (MDA) for 7Be and 210Pb were 0.15 Bq and 0.23 Bq, respectively, for a counting time of 60,000s (Mohan et al., 2018). From the gamma counts obtained, the activity concentration was estimated using the following equation, which takes into account decay correction during sampling and the intervening period between sampling and counting (McNeary and Baskaran, 2003).

A 7Be =

{

N λt1 λt2 e × T 1−e− λt2

}

1 (V×E×BR)

(1)

where, N is the total counts under the photopeak of 7Be and 210Pb, T is the counting time (s), λ is the decay constant of 7Be (0.0130 d−1), V is the volume of rainwater collected (L) or air passed (m3), E is the photo-peak efficiency (cpm/dpm ratio obtained using the radioactive reference material used for detector calibration in a particular geometry), BR is the branching ratio of the radionuclide of interest (10.5% for 7 Be and 4% for 210Pb), t1 is the time between the end of sample collection and mid-counting (d), and t2 is the sampling duration (d).

2.2. Collection of wet and dry fallout, and aerosol samples A rainwater collector (polyethylene container of area 1017 cm2) was deployed on the terrace of the laboratory building at about 5m from the ground. Prior to deployment, the walls of the collector were acidified with concentrated HCl to avoid the adsorption of 7Be and 210 Pb onto it (McNeary and Baskaran, 2003). The rain sample was collected during a major rainfall event. In case of low intensity rainfall, it was collected after the accumulation of 2–5 L from several such events. This sampling program was conducted for two years (2016–2017) and a total of 24 wet fallout samples were collected. The rainfall data were obtained from the nearest meteorological station at the International Airport, Mangalore, and these data were supplied by India Meteorological Department (IMD), Pune. The dry deposition samples were collected using stainless steel trays, which had been exposed over a period of one month in an open area. Prior to deployment, the steel tray was acidified with concentrated HCl. A layer of grease was applied to the inner sides of the steel tray to avoid the loss of the deposited dust particles due to wind. After the collection, the sample was transferred to a polythene container by repeated rinses of the tray with

The decay during counting was considered as negligible in view of the fact that the half-life of both the radionuclides is much larger than the counting duration (∼24 h). The deposition fluxes (F) were calculated by the following equation (McNeary and Baskaran, 2003),

F=

A S×T

where, 283

(2)

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Table 1a 7 Be and 210Pb concentrations in air and deposition fluxes for the dry season at Mangalore. Be concentration in air (mBq m−3)

Be fluxa (Bq m−2 d−1)a

Month and year

7

7

January-2016 February-2016 March-2016 April-2016 October-2016 November-2016 December-2016 January-2017 February-2017 March-2017 April-2017 May-2017 Range GM GSD

5.8 ± 2.1 7.0 ± 2.9 5.7 ± 2.2 5.7 ± 0.5 6.5 ± 0.6 4.7 ± 1.9 10.7 ± 0.3 7.1 ± 3.8 9.2 ± 1.4 7.4 ± 3.9 10.5 ± 2.8 5.8 ± 0.8 4.7–10.7 6.9 1.3

0.100 0.121 0.098 0.098 0.112 0.081 0.184 0.123 0.159 0.128 0.181 0.100 0.081–0.184 0.12 1.3

a b

Pb concentration in air (mBq m−3)

Pb fluxb (Bq m−2 d−1)b

Be/210Pb flux ratio

210

210

7

1.0 ± 0.2 2.2 ± 0.2 1.1 ± 0.1 < 0.16 1.3 ± 0.1 1.5 ± 0.7 3.4 ± 0.2 1.5 ± 0.2 1.9 ± 0.6 1.3 ± 0.7 0.98 ± 0.28 1.0 ± 0.5 < 0.16–3.4 1.2 2.1

0.086 0.190 0.095 0.026 0.112 0.129 0.293 0.130 0.164 0.112 0.085 0.086 0.026–0.293 0.11 1.8

1.16 0.62 1.03 3.80 1.00 0.63 0.63 0.95 0.97 1.14 2.13 1.16 0.63–3.8 1.1 1.7

Estimated using dry deposition velocity, Vd = 0.02 cm s−1 (UNSCEAR, 2000b). Estimated using dry deposition velocity,Vd = 0.1 cm s−1 (Rastogi and Sarin, 2008).

Pb with corresponding GM values of 0.12 Bq m−2 d−1 and 0.11 Bq m d−1 (Table 1a). The annual fluxes calculated from the GM values of dry deposition fluxes of 7Be and 210Pb, considering average dry deposition for 157 d (for both the years), were found to be 18.8 Bq m−2 y−1 and 18.4 Bq m−2 y−1, respectively. Direct measurements of the dry deposition fluxes were performed using the fallout collectors as mentioned in Section 2.2. It is of interest to compare the deposition fluxes determined from the air concentration and the literature based dry deposition velocity values with those of the direct measurements, and Table 1b presents this comparison. The directly measured dry deposition fluxes of 7Be varied in the range of 0.119–0.173 Bq m−2 d−1 with a GM value of 0.14 Bq m−2 d−1, and that of 210Pb varied in the range of 0.060–0.276 Bq m−2 d−1 with a GM value of 0.11 Bq m−2 d−1 (Table 1b). The comparison revealed that the estimated and measured dry depositional fluxes for both the radionuclides matched very well. From the direct measurements of the deposition fluxes and the concentration of these radionuclides in air, sitespecific dry deposition velocity values were estimated and were found to be 0.023 cm s−1 for 7Be and 0.085 cm s−1 for 210Pb. These values matched well with those given in UNSCEAR (2000b) as well as the values reported in the literature, discussed in the previous paragraph.

A is the total activity (Bq) due to the sample of the radionuclide (7Be or 210Pb), T is the time duration of sampling (d), and S is the surface area of the collector (m2).

210

−2

3. Results 3.1. Activity concentrations of 7Be and and flux ratio

210

Pb in air, dry deposition fluxes,

The activity concentrations of 7Be and 210Pb in the air are presented in Table 1a. Statistical tests indicated that the data sets of 7Be and 210Pb are log-normally distributed with geometric standard deviation (GSD) of 1.5 and 1.7, respectively with the correlation coefficients R > 0.95. Further, Shapiro-Wilk normality test rejected the hypothesis that all data sets come from a normal distribution with 5% decision level. This was also confirmed by the skewness and kurtosis values. Hence, the geometric mean (GM) values are presented as central values for all the entities in Table 1a. The overall monthly mean concentrations of 7Be and 210Pb varied in the range of 4.7–10.7 mBq m−3 and < 0.16–3.4 mBq m−3 with corresponding GM values of 6.9 mBq m−3 and 1.2 mBq m−3 (Table 1a). The activity ratio of 7Be and 210Pb in aerosols was estimated in the study region through a three years (2014–2017) extensive study and it varied between 1.7 and 26 with a GM value of 5.8 (Mohan et al., 2018). One can estimate the deposition fluxes directly from the activity concentrations of these radionuclides in the air and the corresponding deposition velocities as under (Baskaran, 2016; Rastogi and Sarin, 2008): Fd = Cair × Vd(dry)

3.2. Specific activity concentrations in rain In the monsoon season, the rainfall in the Southwest Coast of India spans for about five months (June–October). The pre-monsoon showers Table 1b Comparison of measured and estimated dry deposition fluxes of 7Be and210Pb for the dry seasons.

(3)

Where, Fd is the dry deposition flux of Be or 7

210

Pb (Bq m

−2

d

Collection month and year

−1

),

Cair is the concentration of 7Be or 210Pb in the air (Bq m−3), and Vd(dry) is the dry deposition velocity of 7Be or 210Pb (cm s−1).

March – 2016 December 2016 March – 2017 May – 2017 Range GM GSD

The dry deposition velocity for 210Pb was measured in Mumbai, India, as 0.14 ± 0.07 cm s−1 by Anand and Rangarajan (1990). James et al. (2010) estimated the dry deposition velocity (7Be) for the Kaiga, India region to be 0.03 cm s−1. Both these locations are on the West Coast of India. In the present study, the calculation of fluxes was performed using deposition velocity values of 0.1 cm s−1 for 210Pb (Rastogi and Sarin, 2008) and 0.02 cm s−1 for 7Be (UNSCEAR, 2000b). The dry deposition fluxes thus estimated (Equation (3)) varied in the range of 0.081–0.184 Bq m−2 d−1 for 7Be and 0.026–0.293 Bq m−2 d−1 for

Be dry deposition flux (Bq m−2 d−1) 7

Pb dry deposition flux (Bq m−2 d−1) 210

Measured

Estimateda

Measured

Estimatedb

0.122 0.173

0.098 0.184

0.060 0.276

0.095 0.293

0.162 0.119 0.119–0.173 0.14 1.2

0.128 0.100 0.098–0.184 0.12 1.3

0.098 0.095 0.060–0.276 0.11 1.9

0.112 0.086 0.086–0.293 0.13 1.8

a Estimated using dry deposition velocity,Vd = 0.02 cm s−1 (UNSCEAR, 2000b). b Estimated using dry deposition velocity,Vd = 0.1 cm s−1 (Rastogi and Sarin, 2008).

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L−1 for 7Be and 0.06 Bq L−1 for 210Pb, for Huelva province (Lozano et al., 2011), 1.26 Bq L−1 for 7Be and 0.31 Bq L−1 for 210Pb, for Shanghai, China (Du et al., 2015), 1.90 Bq L−1 for 7Be and 0.15 Bq L−1 for 210Pb, for College Station, Texas (Baskaran et al., 1993). The corresponding values reported for Southeastern Michigan, USA, were higher by a factor of ∼4 for 7Be (2.86 Bq L−1) and by a factor of ∼5 for 210 Pb (0.35 Bq L−1) (McNeary and Baskaran, 2003). 3.4. Wet deposition fluxes and flux ratio The statistical summary of results of wet deposition fluxes of 7Be and 210Pb corresponding to individual rainfall event are also presented in Table 2. The overall wet deposition fluxes of 7Be varied in the range of 1.6–134.6 Bq m−2 d−1 with a GM of 16.9 Bq m−2 d−1, and that of 210 Pb, varied in the range of 0.10–20.9 Bq m−2 d−1 with a GM of 1.7 Bq m−2 d−1. Rastogi and Sarin (2008) studied the wet deposition flux of 210 Pb in Ahmedabad, India, and reported values were in the range of 0.06–19 Bq m−2 d−1 for individual rain events during 2000–2002. Their study showed that wet scavenging to be a major removal pathway for 210Pb from the atmosphere. From an eight-year (2000–2008) study, James et al. (2010) reported that the 7Be wet deposition rate in the Kaiga region of India varied between 0.8 Bq m−2 d−1 and 105.4 Bq m−2 d−1, which is similar to the values observed in the present study. The 7Be/210Pb flux ratio (for wet deposition) ranged from 3.9 to 31.2 with a GM of 9.3 for the present study.

Fig. 2. Monthly averages of rainfall at Mangalore during the period 2016–2017.

start in the middle of May and maximum rainfall occurs during June–July months, and then decreases. Individual rainwater samples were collected at Mangalore during the southwest monsoon seasons in the years 2016 and 2017 and analysed for 7Be and 210Pb. The rainfall received in the study region during these years has been plotted in Fig. 2. The results of specific activities of both the radionuclides in the rainwater samples are presented in Table 2, along with statistical parameters. As presented in Table 2, the specific activity concentration of 7Be ranged from 0.15 to 4.3 Bq L−1 with a GM value of 1.1 Bq L−1 and that of 210Pb ranged from 0.01 to 0.71 Bq L−1 with a GM value of 0.12 Bq L−1. Lal et al. (1979) reported the mean values of concentration in the rain samples of Mumbai, India, as 0.515 Bq L−1 and 0.098 Bq L−1, respectively, for 7Be and 210Pb. Rastogi and Sarin (2008) measured the volume weighted mean concentration of 210Pb in the rains of Ahmedabad, India, during 2000–2002, and reported the value as 0.074 Bq L−1. The GM values of specific activities of 7Be (1.1 Bq L−1) and 210Pb (0.12 Bq L−1) in the rainwater, observed in Mangalore region, are higher when compared to the values reported by others as cited above.

4. Discussion 4.1. Dry depositional fluxes and flux ratio Correlation studies between 7Be and 210Pb were performed in order to understand the factors governing the dry depositions. The dry deposition fluxes of these radioisotopes exhibited a positive correlation (Fig. 3) with R = 0.60, which is statistically significant at 95% confidence level (P = 0.02, N = 14). This may be attributed to the fact that (i) both these radionuclides attach to aerosol particles in the accumulation mode (Papastefanou and Ioannidou, 1995; Grundel and Porstendorfer, 2004; Rengarajan and Sarin, 2004), and (ii) removal mechanism for both the radionuclides are the same (Ioannidou et al., 2005; Rengarajan and Sarin, 2004). From Table 1a, it can be observed that the dry deposition flux values are higher during summer (March–May) and lower during winter (November–February) for 7Be, and for 210Pb high concentration values are observed during the winter season and lower during summer. The higher flux of 7Be during summer is due to the injection of stratospheric air into the troposphere, which in turn, is due to the thinning of the tropopause at mid-latitudes (Rastogi and Sarin, 2008). During the winter season, the prevailing wind is from the northeast (NE), which carries continental air masses (rich in 210Pb) to the coastal region. This, as well as the low mixing height, may be responsible for the higher concentration of 210Pb in the winter.

3.3. Volume-weighted concentrations The volume-weighted concentrations of 7Be and 210Pb were estimated as the ratio of the annual activity of individual radionuclide deposited on the rain collector (Bq m−2) to the annual precipitation (L m−2) in the rain collector (McNeary and Baskaran, 2003; Lozano et al., 2011). The GM values of the volume weighted concentration, thus estimated, for Mangalore was 0.68 Bq L−1 for 7Be and 0.07 Bq L−1 for 210 Pb. Rastogi and Sarin (2008) have reported this value to be 0.07 Bq L−1for 210Pb for Ahmedabad, India. Other reported values are 0.83 Bq

Table 2 Specific activities in air (mBq m−3), specific activities (Bq L−1), wet deposition fluxes (Bq m−2 d−1), wet deposition velocities (cm s−1), and wet flux ratio of 7Be and210Pb in individual rainfall events (24 samples) at Mangalore, Southwest Coast of India during 2016–2017. Parameters

Range Median Mean Standard deviation (SD) Coefficient of variation (CV) Geometric mean (GM) Geometric standard deviation (GSD) a

Specific activities in air

Specific activities in rainwater

Wet deposition fluxes

Wet deposition velocity

7

210

7

7

210

7

0.05–1.0 0.30 0.34 0.20 0.59 0.29 1.8

0.15–4.3 1.2 1.6 1.2 0.76 1.1 2.6

1.6–134.6 20.1 29.4 32.1 1.1 16.9 3.2

0.10–20.9 2.3 3.3 4.4 1.3 1.7 3.7

0.57–48.7 8.3 10.8 10.6 0.98 6.8 2.9

Be

1.3–8.0 3.2 3.1 1.5 0.46 2.9 1.5

Pb

Be

210

Pb

0.01–0.71 0.13 0.18 0.17 0.91 0.12 2.8

Be

The flux ratio value (85) is not taken for any statistical analysis and it is considered as an outlier. 285

Pb

Be

7

Be/210Pb Wet flux ratio

210

Pb

0.37–44.9 8.9 11.2 10.7 0.96 6.5 3.4

3.9–85a 8.8 10.6 5.9 0.56 9.3 1.7

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(AMAD) of 0.90 μm (Papastefanou and Ioannidou, 1995), whereas, Pb has an AMAD of 0.56 μm (Winkler et al., 1998; Reineking and Porstendorfer, 1995). Hence, 7Be and 210Pb bearing aerosols are considered as accumulation mode aerosols (Papastefanou and Ioannidou, 1995; Paatero et al., 2017). During the precipitation event, the wet deposition of 7Be and 210Pb bearing aerosols occurs through the following two processes: (i) washout (below cloud scavenging, BCS), and (ii) rainout (in-cloud scavenging, ICS). Washout predominates during the early stages of the rainfall and quickly cleanses the 7Be and 210Pb aerosols in the lower troposphere, whereas, rainout process delivers these radionuclides from within the cloud layer to earth's surface and is active throughout the rainfall event (Ioannidou and Papastefanou, 2006; Ishikawa et al., 1995; Wallbrink and Murray, 1994). Both BCS and ICS depends on the size, chemical composition and concentration of aerosol, and the presence of electric charge (Andronache, 2003). In addition to this, BCS depends on the droplet size distribution and rainfall intensity, whereas, ICS also depends on relative humidity and saturation conditions (Zikova and Zdimal, 2016). During the premonsoon, the rain is sharp and intense and gets over for the day, after one spell, whereas southwest monsoon brings longer spells of rain which are continual in nature. Rainwater with low precipitation (as in the case of pre-monsoon) is known to exhibit higher specific activity concentrations of 7Be and 210Pb since small raindrops have a much larger surface area, which results in higher removal of these radionuclides on each droplet (McNeary and Baskaran, 2003). These may be the reasons for the higher concentrations of 7Be and 210Pb in rainwater observed during the pre-monsoon when compared to monsoon and post-monsoon seasons. To study the effect of rainfall on the specific concentrations of these two radionuclides, linear regression analysis was performed (Fig. 4 and Fig. 5). This analysis revealed that a weak correlation existed between the activity concentrations of both the radionuclides with the corresponding rainfall events (for 7Be: R = 0.22, P = 0.30, Fig. 4 and for 210 Pb: R = 0.1, P = 0.6, Fig. 5). Majority of the studies have reported similar observation (weak or moderate correlation) (Brown et al., 1989; Todd et al., 1989; Baskaran et al., 1993; Kim et al., 2000; BenitezNelson and Buesseler, 1999). McNeary and Baskaran (2003) reported a linear decrease in the specific concentrations of both these radionuclides with increasing precipitation. A similar observation was reported for the south-eastern Virginia Coast by Todd et al. (1989). A good correlation was reported with a logarithmic plot for 7Be and 210Pb with rainfall at Galveston, Texas, but a weak correlation was observed in College Station, Texas (Baskaran et al., 1993). The strong correlation between the 7Be activity concentration and the amount of rainfall is be 210

Fig. 3. Correlation between 7Be and 210Pb dry deposition fluxes (flux estimated considering Vd = 0.1 cm s−1 for 210Pb and 0.02 cm s−1 for 7Be, as given in UNSCEAR, 2000b).

The dry deposition fluxes of 7Be observed in the present study are similar to the values (16.3 Bq m−2 y−1) reported for Kaiga region (James, 2007). The reported values of 210Pb dry deposition fluxes over Ahmedabad, India, was 22 Bq m−2 y−1, 23 Bq m−2 y−1 and 23 Bq m−2 y−1 for the year 2000, 2001 and 2002, respectively (Rastogi and Sarin, 2008). Rengarajan and Sarin (2004) estimated the dry deposition fluxes of 7Be and 210Pb in the Arabian Sea considering the deposition velocity of 0.9 cm s−1 and it was found to be 2155 Bq m−2 y−1 and 215 Bq m−2 y−1, respectively. For the Bay of Bengal, these values were 1560 Bq m−2 y−1 for 7Be and 275 Bq m−2 y−1 for 210Pb. The dry deposition fluxes observed in the present study for 7Be and 210Pb were two orders and one order of magnitude lower, respectively than the values reported by Rengarajan and Sarin (2004). This discrepancy could be explained due to the use of higher effective deposition velocity value of 0.9 cm s−1 (for both 7Be and 210Pb) by the above -referred authors when compared to those used in the present study (0.02 cm s−1 for 7Be and 0.1 cm s−1 for 210Pb). Rengarajan and Sarin (2004) have stated that the flux values estimated by them may vary by an order of magnitude due to the uncertainties in Vd values used. The 7Be/210Pb flux ratio (for dry deposition) for Mangalore varied between 0.63 and 3.8 with a GM of 1.1 (Table 1a). This is about a factor of 3 lower when compared with the mean value of 3.55 reported for Southeastern Michigan (McNeary and Baskaran, 2003). This may be attributed to increased dust suspension, containing higher specific activity of 210Pb when compared to 7Be, during winter and summer which results in lower 7Be/210Pb flux ratio (Lozano et al., 2011; Hirose et al., 2004; Duenas et al., 2005). 4.2. Specific activity concentrations in rainwater The large variability in the activity concentrations of both the radionuclides, observed in the present study, may be attributed to many factors like cloud height, extent and duration of rainfall, the intensity of the rainfall, cloud type, and vertical mixing of the air masses at the sampling location (McNeary and Baskaran, 2003; Rastogi and Sarin, 2008). High specific activity concentrations of 7Be and 210Pb were observed in the pre-monsoon rain sample (the first few days of the rainy season). It was well known that both 7Be and 210Pb attaches to sub-micron sized aerosols (Young et al., 1975). About 85% of the 7Be is accompanied with the particle sizes < 2.1 μm in diameter, and the remaining 15% is associated to particle sizes in the range of 2.1–9.0 μm in diameter with a mean value of activity median aerodynamic diameter

Fig. 4. Correlation between the specific activity concentration of 7Be in individual rainfall events and intensity of rain. 286

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Fig. 5. Correlation between the specific activity concentration of dividual rainfall events and intensity of rain.

210

Fig. 6. Monthly variations of deposition fluxes of 7Be and 210Pb. Total fluxes for each month are plotted.

Pb in in-

Baskaran, 2003; Duenas et al., 2017; Lozano et al., 2011). The deposition flux of 7Be was maximum during the spring at Shangai and Huelva regions, spring and summer seasons for Michigan, whereas for Malaga and Japan, it was during the winter season. In the case of 210Pb, the maximum deposition fluxes were recorded during the winter season for Shangai, Malaga, and Japan but for Michigan and Huelva, it was during summer and spring seasons, respectively. The deposition fluxes of both 7Be and 210Pb had the lowest value during summer months at all the above mentioned sites, except at Michigan, for which it was during winter. Such seasonal trends in deposition fluxes were not observed in the present study since, as mentioned in section 2.1, the West Coast region of India is a tropical region and experiences only two distinct seasons: (i) wet season (June–September), and (ii) dry season (October–May), with no snow and rainfall during winter. McNeary and Baskaran (2003) have reported that snow is more efficient in removing 7 Be and 210Pb than rain from the atmosphere and it is due to larger surface area of snow flurries compared to rain droplets. In addition to this, strong winter monsoon and associated air mass movements containing high 7Be and 210Pb concentrations also contribute to the higher deposition fluxes during winter in temperate regions (Yamamoto et al., 2006; Du et al., 2015). The seasonal variations in the depositional fluxes of 7Be and 210Pb, are caused by seasonal variation in the amount and frequency of precipitation, increased stratospheric to tropospheric exchange (STE)

attributed to the efficient removal of 7Be by rainfall below the cloud cover, whereas the weak correlation is attributed to insufficient time for the build-up of 7Be below the cloud cover due to its slower replenishment from the air masses above the cloud cover (McNeary and Baskaran, 2003). 4.3. Monthly and seasonal variability on the wet deposition fluxes Monthly wet depositional fluxes of 7Be and 210Pb were determined to study the variations of these fluxes indifferent months of rainy season during 2016 and 2017 (Table 3). Temporal variations in the fluxes of these radionuclides for the full year are plotted in Fig. 6. Both the radionuclides show similar trends in which the maximum deposition occurs during July, and thereafter it decreases (Fig. 6). The wet deposition fluxes of these two radionuclides were further categorized to pre-monsoon, monsoon, and post-monsoon (Table 4) in order to delineate variations in the deposition flux during the monsoon season. The deposition fluxes of both 7Be and 210Pb were higher during the premonsoon when compared with the monsoon and post-monsoon, and it was in the order pre–monsoon > monsoon > post-monsoon. Studies reported for other regions such as Japan, Shangai (China), Southeastern Michigan (USA), Malaga (Spain), and Huelva (Spain) have shown marked seasonal variabilities in depositional fluxes of these radionuclides (Yamamoto et al., 2006; Du et al., 2015; McNeary and

Table 3 Monthly specific activities (Bq L−1) and wet deposition fluxes (Bq m−2 month−1) of 7Be and during 2016–2017. Precipitation (mm month−1)

Sample details Year

Month

2016

July (5)a August (6) September (3) GM GSD

2017

May (1) June (4) July (3) August(2)

210

795 491 196

13.6 9.3 3.0 7.2 2.2

151 844 737 738

3.7 3.6 4.7 0.8 2.7 2.2

Be

a b

The number in the parenthesis represents the number of samples analysed. Represents the total specific activity and wet depositional fluxes of 7Be and

210

287

Pb in rainwater measured at Mangalore, Southwest Coast of India Depositional fluxesb

Specific activityb 7

GM GSD

210

7

Be/210Pb flux ratio

7

210

1.07 1.07 0.53 0.84 1.5

1735 515 107 457 4.0

136.3 53.7 14.3 47 3.1

12.7 9.6 7.5 9.7 1.3

0.71 0.45 0.50 0.08 0.3 2.7

92 644 788 134 281 2.9

21 82.2 83.8 12.8 36.9 2.6

4.3 7.8 9.4 10.5 7.6 1.5

Pb

Be

Pb corresponding to each month.

Pb

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Table 4 Variation of wet deposition fluxes of 7Be and Statistical parameters

210

Pb during different periods of monsoon.

Pre- monsoon (May–June) 7

Range GM

−2

Be (Bq m

−1

d

210

)

11.4–91.4 28.8

Monsoon (July–August) Pb (Bq m

−2

d

−1

7

)

2.3–20.9 4.5

−2

Be (Bq m

1.6–134.6 19.8

Table 5 Summary of results of annual dry, wet and total depositional fluxes of 7Be and 210 Pb for Mangalore over a 2-year period. Year

2016 2017 Average

Dry deposition (Bq m−2 y−1)

Wet deposition (Bq m−2 y−1)

Total deposition (Bq m−2 y−1)

7

210

7

210

7

210

19.7 17.1 18.4

2357 1658 2008

204.3 199.8 202

2374 1678 2026

224 217 221

Be

17.2 20.3 18.8

Pb

Be

Pb

Be

d

)

Post- monsoon (September-October) 210

−2

Pb (Bq m

0.1–8.9 1.6

d

−1

)

7

Be (Bq m−2 d−1)

1.8–8.2 4.8

Pb (Bq m−2 d−1)

210

0.2–1.9 0.49

α = Cw,s/Cw,t

(5)

where, Cw,s is the radionuclide concentration in the water for a given season, and Cw,t is the average concentration for one full year. One can also estimate the α for both 7Be and 210Pb using Equation (5). In the present study, α was estimated using this equation (5) for the monsoon season since the rainfall is limited only to this season. The GM value of α thus estimated for Mangalore was found to be 0.72 for 7Be and 0.66 for 210Pb, which suggested the depletion of the radionuclide fluxes due to precipitation. Many authors have observed α values to be less than 1 during fall (Duenas and Fernandez, 2002; Lozano et al., 2011; McNeary and Baskaran, 2003; Du et al., 2015). Others have determined this value for both the radionuclides in all the four seasons: winter, spring, summer, and fall using Equation (4) (Turekian et al., 1983; Olsen et al., 1985; Baskaran, 1995; Duenas and Fernandez, 2002; McNeary and Baskaran, 2003). The minimum and maximum values of α were observed during the winter and summer for both 7Be and 210Pb, respectively (Lozano et al., 2011). McNeary and Baskaran (2003) have observed the α values to be < 1 during fall for 7Be and 210Pb and spring for 210Pb only, whereas, the values were > 1 during the summer and winter. On the other hand, Du et al. (2015) have found that α values were generally > 1 during the winter and spring and < 1 during the summer and fall seasons. Duenas et al. (2017) have recorded highest α values for 7Be and 210Pb during spring and lowest values during autumn for 7Be and winter for 210Pb. The alpha values > 1 during the spring and summer seasons were due to intrusion of air from the stratosphere to troposphere and dominance of dry fallout during these periods, whereas, for alpha values < 1 during the fall and winter were due to the increase in the amount and frequency of precipitation (Lozano et al., 2011; McNeary and Baskaran, 2003).

Pb

during late winter and early spring season, and increased vertical transport rate of 7Be from the upper troposphere to the middle and lower troposphere during the summer months (Baskaran, 1995). 4.4. Annual deposition fluxes The total annual average deposition fluxes (dry + wet) of both these radionuclides were estimated and presented in Table 5. The total annual deposition fluxes, considering the two years data, had mean values of 2026 Bq m−2 y−1 for 7Be and 221 Bq m−2 y−1 for 210Pb (Table 5). The ratio between the deposition fluxes of these radionuclides (9.2) is in agreement with their corresponding air concentration ratio for the region (Section 3.1). The contributions of dry deposition fluxes of these radionuclides to the total deposition fluxes were < 1% for 7Be and 9% for 210Pb. These percentage values are in good agreement with the global model estimates of dry deposition fluxes of these radionuclides (3% for 7Be and 12% for 210Pb reported by Koch et al., 1996 and < 1% and 12% for 7Be and 210Pb, respectively reported by Benitez-Nelson and Buesseler, 1999). Rastogi and Sarin (2008), evaluated the contribution of dry deposition for 210Pb to the total deposition flux in Ahmedabad (semi-arid region), India and they found that dry deposition contributes about 23–45% to the total removal of atmospheric 210 Pb at Ahmedabad, India. Other authors have reported that the major removal pathway for 210Pb was wet deposition and it contributes > 80% (Todd et al., 1989; McNeary and Baskaran, 2003) to the total deposition flux.

4.6. Correlation between wet deposition fluxes of 7Be and individual rainfall events

210

Pb for

A linear regression was performed between the 7Be and 210Pb wet deposition fluxes for individual rain events. A significant positive correlation was observed between the deposition fluxes of these two radionuclides with linear regression coefficient, R = 0.94 (N = 23, P= < < 10−4, Fig. 7). This suggests that the wet deposition of both the radionuclides to the earth's surface is governed by the same process, even though their origins are different and so, both the radionuclides cannot be used as independent atmospheric tracers (Mohan et al., 2018). This trend was also observed in different stations around the world, both continental and oceanic stations (Baskaran et al., 1993; Kim et al., 1999, 2000; McNeary and Baskaran, 2003; Caillet et al., 2001; Huh et al., 2006).

4.5. Precipitation-normalized depositional fluxes To explain the seasonal variations and to quantify the importance of the amount of precipitation in the depositional fluxes, Baskaran (1995) has defined the precipitation-normalized enrichment factor (α) for each season as follows: α = (Fs × Rt) / (Ft × Rs)

−1

(4)

where, Fs and Ft represents respectively the seasonal and total annual depositional fluxes of 7Be or 210Pb, and Rs and Rt represents the amount of rainfall during a particular season and in one full year, respectively. Alpha values greater than 1.0 indicate that the depositional fluxes are higher than that expected from the amount of rainfall, and values less than 1.0 indicate the depletion of the radionuclide fluxes (Baskaran, 1995). Since the specific activity concentration in the water is given by Cw = F/R (where, F is the depositional fluxes of the radionuclides and R is the intensity of the rainfall), the α can well be interpreted as:

4.7. Effect of rainfall intensity on total deposition fluxes The effect of rainfall intensity on the deposition fluxes was evaluated through correlation studies. The correlation coefficient value between monthly 7Be flux and rainfall intensity was R = 0.89 (N = 12, P= < 10−4, Fig. 8a), and for 210Pb, it was R = 0.91 (N = 12, P= < 10−4, Fig. 8b). These are significant at 95% confidence level, which suggests that rainfall plays an important role in the removal of 7 Be and 210Pb from the troposphere. Similar observations were reported 288

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Fig. 7. Correlation between 7Be and rain events.

210

Pb wet deposition fluxes for individual

in other studies (Turekian et al., 1983; Dibb, 1989; Baskaran et al., 1993; Benitez-Nelson and Buesseler, 1999; Kim et al., 2000). The variations in the relationship between 7Be and 210Pb deposition fluxes arise from the difference in the air-mass sources, scavenging rates, and radioactive decay (Benitez-Nelson and Buesseler, 1999). 4.8. Mass balance approach As we mentioned earlier, the source of 210Pb in air is the exhalation of Rn from soil and this, in turn, depends on the 226Ra activity concentration in the soil, in addition to different soil parameters such as physical properties of mineral grains, porosity, water content of the soil (which implies the frequency and amount of rainfall), atmospheric pressure and wind velocity. The data on 226Ra activity concentration in soil and 222Rn exhalation rate is important to quantify the source term for 210Pb. In a previous study, extensive measurements of 222Rn exhalation from soil surface, 222Rn concentration in air, 226Ra and 210Pb activity concentrations in soil, atmospheric parameters, such as temperature and humidity, were carried out in entire South West Coast of India (Siddappa et al., 2000; Karunakara et al., 2008). These included 5 years of measurements of the above parameters in the sampling locations selected for the present study. The results of these studies are summarized in Table 6. The 222Rn exhalation rate in West Coast of India varied in the range of 0.6–156.1 mBq m−2 s−1 with a mean value of 85.6 mBq m−2 s−1. The 226Ra activity concentration varied in the range of 12.0–63.9 Bq kg−1 with a mean value of 31.7 Bq kg−1. For Mangalore University campus, i.e. the sampling station for the present study, the 222Rn exhalation rate varied from 15.0 mBq m−2 s−1 to 116.6 mBq m−2 s−1 with the mean of 45.1 mBq m−2 s−1, which is equivalent to 2.14 222Rn atom cm−2 s−1. The long-term measurements on global 222Rn fluxes from continental regions range in 0.0056–2.0 atom cm−2 s−1 (Turekian et al., 1977; Turekian and Graustein, 2003). The global 222Rn flux reported in UNSCEAR (2000a) for dry soil is 33 mBq m−2 s−1 (1.57 atom cm−2 s−1), the predicted global average value reported by Schery and Wasiolek (1998) is 34 mBq m−2 s−1 (1.62 atom cm−2 s−1) for ice-free land and the corresponding value reported by Wilkening and Clements (1975) is 15.75 mBq m−2 s−1 (0.75 atom cm−2 s−1). Thus, the 222Rn exhalation rate, measured in the present study, in terms of 222Rn atoms can be considered as higher than the reported values. The mean residence time of 210Pb in the atmosphere is short when compared with its half-life, so it can be assumed that the upward flux of 222 Rn equals the downward flux of 210Pb (210Pb flux = 222Rn flux × [mean life of 222Rn/mean life of 210Pb]). Baskaran (2016) found 222

Fig. 8. Correlation between the monthly deposition fluxes (dry + wet) of (a) 7 Be, and (b) 210Pb with monthly rainfall (data for January to December are considered for plot). Table 6 222 Rn concentration in air and its exhalation rate along Southwest Coast of India. Parameters

222

Rn concentration in air (Bq m−3) 222 Rn exhalation rate (mBq m−2 s−1) 226 Ra concentration in soil (Bq kg−1)

Entire South West Coast of India [6]a

Mangalore

Range

Mean

Mean

9.0–62.1

15.7

10.5

0.6–156.1

85.6

45.1

12.0–63.9

31.7

46.0

a Number of sampling stations in 280 km coastal stretch starting from Mangalore to Karwar, West Coast of India.

289

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(N = 27, P = < 10−4, Fig. 9b). The total annual deposition fluxes (dry + wet) were considered for the regression analysis. The high degree of positive correlation coefficients suggested that the rainfall was the main governing factor for the deposition of these radionuclides from the atmosphere to the earth's surface. Beks et al. (1998) found the positive correlation between the 210Pb deposition fluxes with the number of heavy rains and thunderstorms at two sites in Netherlands. The authors attributed this to the variations in the cloud condensation height during normal rainfall and thunderstorms, the type of air masses (continental versus oceanic), and weather systems. The above analyses of the results indicate that in the case of 7Be, out of the several values reported for other countries, the annual deposition flux reported for Japan, Bermuda, New Haven, Massachusetts and China are higher than that observed for West Coast of India even though the rainfall received in those locations are lower when compared to the West Coast of India (Table 7). It must be admitted that while rainfall is an important factor, other factors such as production rate, stratospheric to tropospheric exchange (STE) and snow precipitation could also affect the fluxes. It has been argued (Yamamoto et al., 2006) that the cause for high average annual flux for 7Be (5300 Bq m−2 y−1) and 210Pb (840 Bq m−2 y−1) in Japan was due to high deposition during strong winter monsoon (November to March) and snowfall, and associated air mass movements containing high 7Be and 210 Pb concentrations in air. Similarly, The higher depositional fluxes of 7 Be at Bermuda (2853 Bq m−2 y−1), and New Haven, Connecticut (3780 Bq m−2 y−1), may be due to (i) latitudinal variations, and (ii) variations in frequency and amount of rainfall (Du et al., 2015). The reason attributed for higher annual 7Be deposition flux in Shangai was the stratosphere-troposphere exchange of air masses in the midlatitudes and the high production of 7Be in the stratosphere which results in increased 7Be concentration in the troposphere during late winter and spring (Lal and Peters, 1967), and partly due to the intense jet stream (speeds of 60–100 m s−1) of air masses (Du et al., 2015). On the other hand, the reasons attributed for higher annual deposition flux of 210Pb were: (i) increased deposition of this radionuclide during winter, when compared to other seasons, due to the air masses derived from large landmasses in the west and west-northwest of China, which have much higher 222Rn concentrations, and (ii) the northwest monsoon during winter (Du et al., 2015).

the above assumption to be reasonable as 100% of the 222Rn, which undergoes decay in the atmosphere, is brought down to the earth's surface and there is no loss of 222Rn or its progenies in this process. Hence, from the annual depositional flux of 210Pb one can estimate the exhalation rate of 222Rn from the earth's surface (Baskaran, 2016). Baskaran (2016), has reviewed published data on 222Rn flux, estimated from long-term (> 1 year) measurements on the atmospheric depositional fluxes of 210Pb, from different sites across the world. From these measurements, the estimated 222Rn flux varied in the range of 0.19–2.70 atom cm−2 s−1 (Du et al., 2015; Baskaran, 2016). The 222Rn flux estimated in the present study from the measured total annual depositional flux of 210Pb (221 Bq m−2 y−1) was 0.71 atom cm−2 s−1, which is well within the range of values compiled by Baskaran (2016). However, it is interesting to note that the measured 222Rn flux at Mangalore is higher by a factor of 3 when compared to that estimated from the downward 210Pb flux. The difference in the measured and estimated 222Rn flux values may be due to the inter-annual variability of the 210Pb depositional flux (∼ ± 10–15%) depending on the frequency and amount of rainfall (Baskaran, 2016). Also, the study region being a coastal site, the oceanic air-masses are depleted of 222Rn and hence will cause considerable dilution thereby lowering 210Pb flux. 4.9. Comparison of deposition fluxes with the reported values Many researchers have studied the deposition fluxes of 7Be and Pb across different countries. Tables 7 and 8 respectively, presents a comparison of the deposition fluxes of 7Be and 210Pb reported for the different parts of the world with those observed for Mangalore, India. The values are tabulated in the increasing order of latitudes for better visualisation. In Section 3.2, the variation of specific activity concentrations of these radionuclides with rain was discussed, in particular, their variation with pre-monsoon, monsoon, and post-monsoon. Now, an attempt is made to study the effect of rainfall on the annual 7Be and 210Pb fluxes (dry + wet) on a global scale by considering the values reported for different regions of the world. Linear regression analysis was performed to study the relationship between these fluxes and rainfall. The correlation coefficient value between 7Be deposition fluxes reported for different regions and the corresponding annual rainfall was R = 0.80 (N = 20, P = < 10−4, Fig. 9a) and that for 210Pb was R = 0.81 210

Table 7 Comparison of total 7Be deposition fluxes. Location

Lat

Long

Year of measurement

Annual rainfall (mm)

7

Be flux Bq m−2 y−1

Reference

Kodaikanal, Tamilnadu Mangalore, India Kaiga, India Arabian Sea Bay of Bengal Bombay, Maharashtra Bombay, Maharashtra Xiamen, China Shanghai, China Shanghai, China Kumamoto, Japan Damascus, Syria Bermuda Mediterranean coastal station (Malaga) Granada, Spain Tatsunokuchi, Japan Huelva, Spain New Haven, Connecticut Woods Hole, Massachusetts Southeastern Michigan, USA Brisbane, Australia Canberra, Australia

10° N 12.82° N 14° 86′ N 5-20° N 5-20° N 19˚- 20° N 19° N 24.5° N 31˚13′39″ N 31˚13′39″ N 32° 48′N 33˚N 33˚N 36° 43′ 40″ N 36˚N 36.4˚N 37˚N 41˚N 41˚32′N 42° 25′ N 27° 29′ S 35˚S

– 74.92° E 74˚44′E – 64° E – 72° 55′ E 118˚E 121° 23′56″ E 121° 23′56″ E 130° 43′ E – – 4° 28′ 8″ W – 136.8˚E 7˚W – 70˚42′W 83° 1′ W 153° 2′ E –

1956 2016–2017 2000–2004 1997–1999 1997–1999 1970–1971 1956 2011–2013 2005–2006 2006–2013 2001–2003 1995–1997 1977–1978 2005–2015 1995–1998 1991–2002 2009–2010 1977–1978 1996–1998 1999–2001 2004–2006

1750 2954 3500 – – 2277 980 1092 1140 1168 1298 153 1700 417 452 2208 1000 1240 848 760 795.6 660

767 2026 1254 2155a 1560a 1267 677 685 2070 1467 1717 528 2850 1215 469 5300 834 3780 2133 1900 1362 1030

Rama and Zutshi (1958) This study James (2007) Rengarajan and Sarin (2004) Rengarajan and Sarin (2004) Lal et al. (1979) Rama and Zutshi (1958) Zhang et al. (2016) Du et al. (2008) Du et al. (2015) Momoshima et al. (2006) Othman et al. (1998) Turekian et al. (1983) Duenas et al. (2017) Gonzalez-Gomez et al. (2006) Yamamoto et al. (2006) Lozano et al. (2011) Turekian et al. (1983) Benitez- Nelson and Buesseler (1999) McNeary and Baskaran (2003) Doering and Akber (2008) Wallbrink and Murray (1994)

a

- The authors have reported dry deposition flux of 7Be only. 290

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Table 8 Comparison of total

210

Pb deposition fluxes.

Location

Lat

Long

Year of measurement

Annual rainfall (mm)

210 Pb Flux Bq m−2 y−1

Reference

Ootacamund, Tamilnadu Bangalore, Karnataka Mangalore, India Bombay, Maharastra Bombay, Maharastra Arabian Sea Bay of Bengal Nagpur, Maharastra Calcutta, West Bengal Ahmedabad, Gujarat Gangtok, Delhi Srinagar Xiamen, China Galveston, Texas College Station, Texas Shanghai, China Shanghai, China Nagasaki, Japan Bermuda Tsukuba, Japan Tatsunokuchi, Japan Mediterranean coastal station (Malaga) Huelva, Spain Still Pond New Heaven Woods Hole, Massachussets Southeastern Michigan, USA Monaco

11° 23′ N 12˚57′ N, 12.82° N 18˚57′ N 19˚-20˚N 5-20° N 5-20˚N 21˚12′N 23˚34′N 23.0° N 27˚12′N 28˚45′N 18° 57′N 24.4˚N 29.3° N 30.6° N 31˚13′39″ N 31˚13′39″ N 32.8˚N 33° N 36˚N 36.4˚N 36° 43′ 40″ N 37˚N 39˚N 41° N 41˚32′N 42° 25′ N 43.83˚N

76° 40′ E 77° 30′ E 74.92° E 72° 55′ E – – 64° E 79° 04′ E 88° 25′ E 72.6° E 88° 23′ E 77° 20′ E 72° 55′ E 118.1˚E 94.8° W 96.4° W 121° 23′56″E 121° 23′56″E 129.8˚E – 140.1˚E 136.8˚E 4° 28′ 8″ W 7˚W 70.1˚W – 70˚42′W 83° 1′ W 7.5˚E

1962–1966 1962–1966 2016–2017 1962–1966 1970–1971 1997–1999 1997–1999 1962–1966 1962–1966 2000–2002 1962–1966 1962–1966 1962–1966 2001–2003 1988–1992 1989–1992 2005–2006 2006–2013 2001 1977–1978 2000–2001 1991–2002 2005–2015 2009–2010 1995–1997 1977–1978 1996–1998 1999–2001 1998–2010

1326 1000 2954 2167 2211 – – 975 1337 603 3052 739.7 728.2 1618 1167 1220 1140 1168 1471 1700 1305 2208 417 1000 1260 1480 848 760 622

91.3 87.5 221 239 233.3 215a 275a 102 105.8 68.3 229.8 112 182.2 150 172 179 479 366 234 115 179 840 144 59 130 200 238 235 205

Joshi et al. (1969) Joshi et al. (1969) This study Joshi et al. (1969) Lal et al. (1979) Rengarajan and Sarin (2004) Rengarajan and Sarin (2004) Joshi et al. (1969) Joshi et al. (1969) Rastogi and Sarin (2008) Joshi et al. (1969) Joshi et al. (1969) Joshi et al. (1969) Jia et al. (2003) Baskaran et al. (1993) Baskaran et al. (1993) Du et al. (2008) Du et al. (2015) Hirose et al. (2004) Turekian et al. (1983) Hirose et al. (2004) Yamamoto et al. (2006) Duenas et al. (2017) Lozano et al. (2011) Kim et al. (2000) Turekian et al. (1983) Benitez- Nelson and Buesseler (1999) McNeary and Baskaran (2003) Pham et al. (2013)

a

- The authors have reported dry deposition flux of

210

Pb only.

(Table 2). The GM values of the dry deposition velocities of 7Be and 210 Pb were 0.023 cm s−1 and 0.085 cm s−1, respectively (Section 3.1), for Mangalore, which are negligible when compared with their wet deposition velocities. Hence, the wet deposition velocity can be considered as the representative value of the total annual deposition velocity of aerosol for the present study region. The process of wet deposition is controlled by the scavenging mechanism of the radionuclides by raindrops. In the course of the fall of a raindrop, the aerosol particles containing radioactivity are captured by the drops by various collection mechanisms such as diffusion, impaction, and interception (Anand and Mayya, 2015). One therefore expects that the coefficient representing this mechanism, the wet deposition velocity, should be similar for both 7Be and 210Pb since both attach to aerosol particles. The present data on wet deposition velocities of 7Be and 210Pb support this deduction. On the other hand, McNeary and Baskaran (2003) have observed that the total deposition velocity of 7Be is higher than that of 210Pb from their compiled data (Turekian et al., 1983; Todd et al., 1989). Todd et al. (1989) have mentioned that the deposition velocity of 7Be is about a factor of 2 higher than that of the deposition velocity of 210Pb. The reason attributed to this is that since the source of 210Pb is the earth's surface, and hence, the concentration of this radionuclide in the air close to the earth's surface is expected to be higher, and therefore, the total deposition velocity is expected to be lower (McNeary and Baskaran, 2003). Excellent correlation was observed between the wet deposition velocities of 7Be and 210Pb (R = 0.94, P= < < 10−4) as presented in Fig. 10. It can be noted, that the slope, which is the ratio of deposition velocities of two radionuclides, is about 0.93 which again indicates that the deposition velocities are the same for both the radionuclides. This suggests that the attachment mechanism of both the radionuclides onto the aerosols is similar, irrespective of their differences in sources (Winkler et al., 1998; Papastefanou, 2006; Lozano et al., 2011; McNeary and Baskaran, 2003). A positive intercept value of

4.10. Total deposition velocity of aerosols The process of deposition removes the radionuclides along with aerosols from the atmosphere and deposits them on the earth's surface (Lozano et al., 2011; Duenas et al., 2005). Therefore, one can estimate the total aerosol deposition velocity using 7Be and 210Pb. Usually, the deposition fluxes are characterized by the deposition velocity. The deposition velocity (Vd) for any radionuclide is estimated as follows (Papastefanou and Ioannidou, 1991; McNeary and Baskaran, 2003; Lozano et al., 2011): Vd = F/Ca

(6)

where, F is the total flux of a radionuclide to the earth's surface (Bq m−2 d−1), and Ca is the activity concentration of that radionuclide in the surface air (Bq m−3). The use of 7Be and 210Pb to determine the total deposition velocity of the aerosols have several advantages, such as, ease of measurement of these radionuclides with high degree of accuracy, their production rates are constant at any given location, similar size distributions of these radionuclides when compared to the other particulate contaminant, and, therefore, can be used as tracers to determine the fluxes of contaminants to the earth's surface (McNeary and Baskaran, 2003; Duenas et al., 2005; Lozano et al., 2011). In Section 3.1, the dry deposition velocity was estimated by taking into consideration the radionuclide flux and the concentration of that nuclide in the air for the dry season. Now, the wet deposition velocity is estimated by taking into account the total wet deposition flux during the monsoon season. Since rainfall is limited to the monsoon season in the South West Coast of India, the wet deposition velocity estimated for the monsoon season can be considered as the total annual wet deposition velocity for the region. The wet deposition velocity, thus estimated for 7Be varied between 0.57 cm s−1 to 48.7 cm s−1 with a GM of 6.8 cm s−1 and that for 210Pb varied between 0.37 and 44.9 cm s−1 with a GM of 6.5 cm s−1 291

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Fig. 10. Correlation between deposition velocities of 7Be and

210

Pb.

The reason for the weak correlation is that aerosol mass is not a controlling factor for the amount of 7Be or 210Pb scavenged since a major portion of the aerosols are not involved in the removal of particle reactive nuclides (McNeary and Baskaran, 2003). 4.11. Washout ratios The washout ratio, W, is a parameter that relates the average concentration of 7Be or 210Pb in the surface level precipitation to its average concentration in the unwashed surface level air. This was calculated from the following relationship (McNeary and Baskaran, 2003; Lozano et al., 2011): W = ρ Crain/Cair Where, ρ is the density of air in standard conditions (1.2 kg m

(7) −3

),

Crain is the 7Be or 210Pb concentration in bulk deposition (Bq kg−1), and Cair is the 7Be or 210Pb concentration in the air (Bq m−3). The washout ratio is based on the assumption that the specific 210Pb activity of the air in the precipitating cloud is the same as that measured at the surface level in the aerosol (Baskaran, 2016; Lozano et al., 2011). Considering the wet deposition flux for both the radionuclides during the rainy season of 2016 and 2017, the washout ratios were estimated, and it varied between 158 and 2510 (GM = 553) for 7Be and 440–1096 (GM = 629) for 210Pb (Table 10). These values can be compared with the values for other regions: mean values of 496 and 534 for 7Be and 210Pb, respectively, for the Huelva province (Lozano et al., 2011); 948 for 7Be and 637 for 210Pb in Southeastern Michigan (McNeary and Baskaran, 2003), and 290 for 210Pb in Ahmedabad (Rastogi and Sarin, 2008). Lozano et al. (2011) mentioned that the washout ratios of 210Pb were similar to that of 7Be for the Huelva province. These authors have attributed this to the fact that both the radionuclides attach to aerosols of the same size, and thus, both were effectively removed from the atmosphere by rain. On the other hand, McNeary and Baskaran (2003) have attributed the lower value of washout ratio for 210Pb, when compared to 7Be, to the origins of these radionuclides - the 210Pb originates from the earth's surface, its concentration is expected to be low at the point where the cloud condensation takes place whereas in the case of 7Be it is vice versa.

Fig. 9. Correlation between deposition fluxes of (a) 7Be, (b) 210Pb with the rainfall across different locations of the world. Data compiled in Du et al., (2015) are taken for plotting the correlation. Details of the data set are given in Tables 7 and 8

0.31 cm s−1 (Fig. 10) was obtained, which indicates that even if the wet deposition velocity for 210Pb is zero, 7Be has a finite deposition velocity. A similar positive intercept (0.4 cm s−1) was observed by McNeary and Baskaran (2003) for South-eastern Michigan, USA. In Table 9, the total deposition velocities of 7Be and 210Pb reported by previous researchers for different places around the globe are compared. The values for Mangalore region are significantly higher when compared with most of the values reported by other investigators, except for the Kaiga region in the West Coast of India. The deposition velocity for 7Be as reported for Kaiga (7.4 cm s−1) is comparable with that observed in the present study, and the authors have attributed this high value to the high rainfall of ∼3500 mm y−1 (James et al., 2010). The correlation between the deposition velocities derived from either 7Be or 210Pb and the total particulate matter load collected onto the filter paper was attempted. A weak correlation between the deposition velocities and total particulate matter (N = 24, R = 0.06, and P = 0.78 for 7Be; N = 24, R = 0.026, and P = 0.9 for 210Pb) was observed, and similar findings were reported earlier (McNeary and Baskaran, 2003).

5. Conclusions

• The 292

present study bridges the gap in the available database

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Table 9 Comparison of total deposition velocities (Vd) of 7Be and210Pb. Sampling station

Latitude

Longitude

Vd for 7Be (cm s−1)

Vd for

Mangalore, India Kaiga, India Palermo, Italy Thessaloniki, Greece Rokkasho, Japan Dijon, France Munich, Germany Munich, Germany Prague, Czech Republic Groningen, Netherlands Roskilde, Denmark Ljungbyhed, Sweden Grindsjon, Sweden Kiruna, Sweden Hawaii Western North Atlantic Bermuda Bermuda Eastern and Mid-western U.S Norfolk, Virginia Malaga, Spain Malaga, Spain Huelva, Spain Detroit, Michigan Quillayute, Washington

12° N 14° 86′ N 39° N 40° N 40° 57′ N 47° N 48° N 48° N 50° N 53° N 53° N 56° N 59° N 68° N 21° N 31˚- 42˚N 33° N 33° N 35˚- 44° N 35° N 36° N 36° 43′ N 37° N 42° N 47° N

74° E 74° 44′ E 13° E 23° E 141° 21′ E 5° E 11° E 11° E 14° E 6° E 6° E 13° E 18° E 20° E 157° W 60° - 74° W 64° W 64° W 72° - 105° W 84° W 4° W 4° 28′ W 7° W 83° W 124° W

6.8 7.4 0.4 0.4 2.3 0.5 1.53 – 0.5 – 1.0 0.5 0.5 0.5 3.0 – 4.0 2.8 – 1.3 0.4 0.91 0.5 1.6 1.0

6.6 – – – 2.7 – – 1.0 – 1.0 – – – – – 1.9 – 1.0 0.6 0.7 1.5 0.96 0.5 1.1 0.95

• • • •

(Baskaran, 2016) of atmospheric radionuclides over the Southeast Asian region. More specifically, data generated on the depositional characteristics of 7Be and 210Pb for Mangalore, South West Coast region of India provides valuable input to understand the effect of atmospheric phenomena, in a tropical region with distinct weather patterns and air mass movements. These data are useful for validating various global circulation models (GCM). The GM values of the dry depositional fluxes for the study region were: 0.12 Bq m−2 d−1 (GSD = 1.3) for 7Be and 0.13 Bq m−2 d−1 (GSD = 1.8) for 210Pb. The deposition fluxes of these radionuclides showed similar, but distinct, seasonal variations, in which the higher values are observed during the rainy season and lower during dry periods. Detailed correlation study with rainfall data clearly demonstrates that these variations are mainly controlled by precipitation rate. The annual average deposition fluxes (dry + wet) of 7Be and 210Pb were 2026 Bq m−2 y−1 and 221 Bq m−2 y−1, respectively. The flux ratio between 7Be and 210Pb are approximately in agreement with the corresponding air concentration ratio measured in this region (Mohan et al., 2018). The contribution of dry deposition flux to the total annual deposition flux is < 1% in the case of 7Be and 9% in the case of 210Pb. These percentage values observed in the present study are in good agreement with the global model estimates of dry deposition fluxes

•

•

•

210

Pb (cm s−1)

Reference This study James et al. (2010) Kulan et al. (2006) Papastefanou and Ioannidou (1991) Akata et al. (2008) Kulan et al. (2006) Rosner et al. (1996) Winkler and Rosner (2000) Kulan et al. (2006) Beks et al. (1998) Fogh et al. (1999) Kulan et al. (2006) Kulan et al. (2006) Kulan et al. (2006) Turekian et al. (1983) Hussain et al. (1998) Turekian et al. (1983) Turekian et al. (1983) Graustein and Turekian (1986) Todd et al. (1989) Duenas et al. (2005) Duenas et al. (2017) Lozano et al. (2011) McNeary and Baskaran (2003) Crecelius (1981)

of these radionuclides (Koch et al., 1996). Thus, in high rainfall region, one can generally neglect the dry deposition flux. The average wet deposition velocities, estimated considering solely the rainy day events, were 6.8 cm s−1 for 7Be and 6.5 cm s−1 for 210 Pb. The individual deposition velocities of 7Be and 210Pb are also well correlated. This suggests that the attachment mechanisms of both the radionuclides onto the aerosols are similar in the atmosphere. The washout ratio (which indicates the enrichment due to the attachment mechanism with the raindrop) of 7Be (553) and 210Pb (629), observed in the present study, are well comparable with the values of other regions (496 for 7Be and 534 and 210Pb; Lozano et al., 2011). Approximately, similar values of washout ratio observed for both the radionuclides in the study could be attributed to the attachment of these radionuclides to the same size of aerosols (sub-micron sized aerosols). Precipitation-normalized enrichment factors (α) were found to be less than unity for both the radionuclides thereby indicating the seasonal depletion of the radionuclide fluxes as compared to their annual average.

Declaration of interest None.

Table 10 Washout ratios for 7Be and

210

Pb.

Sample ID

Month and year of sampling

Mean activity of 7Be in rain (Bq L−1)

Mean activity of 7Be in air (mBq m−3)

7 Be washout ratio

Mean activity of in rain (Bq L−1)

AR-1 AR-2 AR-3 AR-4 AR-5 AR-6 AR-7

July 2016 Aug 2016 September 2016 May 2017 June 2017 July 2017 Aug 2017

2.72 1.79 0.83 3.72 0.89 1.56 0.42

1.3 2.6 2.95 5.8 2.93 3.67 3.2

2510 826 338 770 365 510 158 158–2510 553 2.3

0.21 0.19 0.16 0.71 0.11 0.17 0.04

Range GM GSD

293

210

Pb

Mean activity of 210Pb in air (mBq m−3)

210

0.23 0.4 0.4 1.0 0.3 0.28 0.1

1096 570 480 852 440 729 480 440–1096 629 1.4

Pb washout ratio

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Acknowledgement

Gonzalez-Gomez, C., Azhara, M., Lopez-Penalver, J.J., Camacho-Garcia, A., Bardouni, T.E., Boukhal, H., 2006. Seasonal variability in 7Be depositional fluxes at Granada, Spain. Appl. Radiat. Isot. 64, 228–234. Graustein, W.C., Turekian, K.K., 1986. 210Pb and 137Cs in air and soils measure the rate and vertical profile of aerosol scavenging. J. Geophys. Res. 91, 14335–14366. Grundel, M., Porstendorfer, J., 2004. Differences between the activity size distributions of the different natural radionuclide aerosols in outdoor air. Atmopsheric Environment 38, 3723–3728. Heikkila, U., Beer, J., Alfimov, V., 2008. Beryllium-10 and beryllium-7 in precipitation in dubendorf (440 m) and at jungfraujoch (3580 m), Switzerland (1998-2005). J. Geophys. Res. 113, D11104. Hirose, K., Honda, T., Yagishita, S., Igarashi, Y., Aoyama, M., 2004. Deposition behaviours of 210Pb, 7Be and thorium isotopes observed in Tsukuba and Nagasaki, Japan. Atmos. Environ. 38, 6601–6608. Hirose, K., Kikawada, H., Doi, T., Su, C.-C., Yamamoto, M., 2011. 210Pb deposition in the far East Asia: controlling factors of its spatial and temporal variations. J. Environ. Radioact. 102, 514–519. Huh, C.A., Su, C.C., Shiau, L.J., 2006. Factors controlling temporal and spatial variations of atmospheric depositions of 7Be and 210Pb in northern Taiwan. J. Geophys. Res. 111 D16304. Hussain, N., Church, T.M., Veron, A.J., Larson, R.E., 1998. Radon-daughter disequilibria and lead systematic in the western North Atlantic. J. Geophys. Res. 103, 16059–16071. Ioannidou, A., Manolopoulou, M., Papastefanou, C., 2005. Temporal changes of 7Be and 210 Pb concentrations in surface air at temperate latitudes (40˚N). Appl. Radiat. Isot. 63, 277–284. Ioannidou, A., Papastefanou, C., 2006. Precipitation scavenging of 7Be and 137Cs radionuclides in air. J. Environ. Radioact. 85, 121–136. Ishikawa, Y., Murakami, H., Sekine, T., Yoshihara, K., 1995. Precipitation scavenging studies of radionuclides in air using cosmogenic 7Be. Journal of Environmnetal Radioactivity 26, 19–36. James, J.P., 2007. Studies on Environmental Transportation of Natural Radionuclides in Kaiga Region. .Ph.D. Thesis.. D-900, Department of Chemistry, Mangalore University, Mangalore, India. James, J.P., Ravi, P.M., Joshi, R.M., Hegde, A.G., Sarkar, P.K., 2010. Estimation of sitespecific deposition velocities and mass interception factor using 7Be and the prediction of deposition pattern of radionuclides at Kaiga site, India. Radiat. Protect. Dosim. 141, 248–254. Jia, C., Liu, G., Yang, W., Zhang, L., Huang, Y., 2003. Atmospheric depositional fluxes of 7 Be and 210Pb at Xiamen. J. Xiamen Univ. 42 (3), 352–357. Joshi, L.U., Rangarajan, C., Smt Gopalakrishnan, S., 1969. Measurement of lead-210 in surface air and precipitation. Tellus XXI, 1. Karunakara, N., Somashekarappa, H.M., Siddappa, K., 2008. Summary of studies on radon and thoron in West Coast region of India. In: DAE-BRNS Theme Meeting, Radon-2008, pp. 64–67. Karunakara, N., Chetan, R., Ujwal, P., Yashodhara, I., Sudeep, K., Ravi, P.M., 2013. Soil to rice transfer factors for 226Ra, 228Ra, 210Pb, 40K and 137Cs: a study on rice grown in India. Journal of Environmnetal Radioactivity 118, 80–92. Kim, G., Alleman, L.Y., Church, T.M., 1999. Atmospheric depositional fluxes of trace elements, 210Pb, and 7Be to the Sargasso Sea. Glob. Biogeochem. Cycles 13, 1183–1192. Kim, G.N., Hussain, N., Scudlark, J.R., Church, T.M., 2000. Factors influencing the atmospheric depositional fluxes of stable Pb, 210Pb and 7Be into Chesapeake Bay. J. Atmos. Chem. 36, 65–79. Koch, D.M., Jacob, D.J., Graustein, W.C., 1996. Vertical transport of tropospheric aerosols as indicated by 7Be and 210Pb in a chemical tracer model. J. Geophys. Res. 101 (D13), 18651–18666. Kulan, A., Aldahan, A., Possnert, G., Vintersved, I., 2006. Distribution of 7Be in surface air of Europe. Atmos. Environ. 40, 3855–3868. Lal, D., Arnold, J.R., Honda, M., 1960. Cosmic-ray production of Be7 in oxygen, and P32, P33, S35 in argon at mountain altitudes. Phys. Rev. 118 (6), 1626–1632. Lal, D., Malhotra, P.K., Peters, B., 1958. On the production of radioisotopes in the atmosphere by cosmic radiation and their application to meteorology. J. Atmos. Terr. Phys. 12, 306–328. Lal, D., Nijampurkar, V.N., Rajagopalan, G., Somayajulu, B.L.K., 1979. Annual fallout of 32 Si, 210Pb, 22Na, 35S and 7Be in rains in India. In: Proc. Indian Academy of Sciences 88 A. pp. 29–40 Number 1. Lal, D., Peters, B., 1962. Cosmic ray produced isotopes and their applications to problems in geophysics. In: Progress in Cosmic Ray and Elementary Particles, vol. 6. pp. 3–74 North- Holland, Newyork. Lal, D., Peters, B., 1967. Cosmic ray produced radioactivity on the Earth. In: sitte, K. (Ed.), Encyclopaedia of Physics. Springer-Verlag, New York, pp. 551–612. Land, C., Feichter, J., 2003. Stratosphere-troposphere exchange in a changing climate simulated with the general circulation model MAEC-HAM4. J. Geophys. Res. 108 (D12), 8523. Liu, H., Jacob, J., Bey, I., Yantosca, R.M., 2001. Constrains from 210Pb and 7Be on wet deposition and transport in a global three-dimensional chemical tracer model driven by assimilated meteorological fields. J. Geophys. Res. 106, 12109–12128. Lozano, R.L., San Miguel, E.G., Bolivar, J.P., Baskaran, M., 2011. Depositional fluxes and concentrations of 7Be and 210Pb in bulk precipitation and aerosols at the interface of Atlantic and Mediterranean coasts in Spain. J. Geophys. Res. 116 D18213. Manjunatha, B.R., Balakrishna, K., Krishnakumar, K.N., Manjunatha, H.V., Avinash, K., Mulemane, A.C., Krishna, K.M., 2015. Increasing trend of rainfall over Agumbe, Western Ghats, India in the scenario of global warming. Open Oceanogr. J. 8, 39–44. Masarik, J., Beer, J., 1999. Simulation of particle fluxes and cosmogenic nuclide production in the Earth's atmosphere. J. Geophys. Res. 104 (D10), 12099–12111.

Authors would like to thank Mangalore University for encouraging this study. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.atmosenv.2019.01.034. References Akata, N., Kawabata, H., Hasegawa, H., Sato, T., Chikuchi, Y., Kono, K., Hisamatsu, S., Inaba, J., 2008. Total deposition velocities and scavenging ratios of 7Be and 210Pb at Rokkasho, Japan. J. Radioanal. Nucl. Chem. 277, 347–355. Anand, S., Mayya, Y.S., 2015. Coagulation effect on the activity size distributions of long lived radon progeny aerosols and its application to atmospheric residence time estimation techniques. J. Environ. Radioact. 141, 151–163. Anand, S.J.S., Rangarajan, C., 1990. Studies on the activity ratios of polonium-210 to lead- 210 and their dry-deposition velocities at Bombay in India. J. Environ. Radioact. 11, 235–250. Andronache, C., 2003. Estimated variability of below-cloud aerosol removal by rainfall for observed aerosol size distributions. Atmos. Chem. Phys. 3 (1), 131–143. Arnold, J.R., Al-Salih, A., 1955. Beryllium-7 produced by cosmic rays. Science 121, 451–453. Baskaran, M., 1995. Research for the seasonal variability on the deposition fluxes of 7Be and 210Pb. J. Geophys. Res. 100, 2833–2840. Baskaran, M., 2011. Po-210 and Pb-210 as atmospheric tracers and global atmospheric Pb- 210 fallout: a Review. J. Environ. Radioact. 102, 500–513. Baskaran, M., 2016. Radon: A Tracer for Geological, Geophysical and Geochemical Studies. Springer International Publishing, Switzerland. Baskaran, M., Coleman, C.H., Santschi, P.H., 1993. Atmospheric depositional fluxes of 7Be and 210Pb at Galveston and College Station, Texas. J. Geophys. Res. 98, 20555–20571. Beks, J.P., Elisma, D., Van Der Plichit, J., 1998. A record of atmospheric 210Pb deposition in The Netherlands. Sci. Total Environ. 222, 35–44. Benioff, P.A., 1956. Cosmic-ray production rate and mean removal time of beryllium-7 from the atmosphere. Phys. Rev. 104 (4), 1122–1130. Benitez-Nelson, C.R., Buesseler, K.O., 1999. Phosphorus 32, phosphorus 33, beryllium 7, and lead 210: atmospheric fluxes and utility in tracing stratosphere/troposphere exchange. J. Geophys. Res. 104, 11745–11754. Brost, R.A., Feichter, J., Heimann, H., 1991. Three dimensional simulation of 7Be in a global climate model. J. Geophys. Res. 96, 22423–22445. Brown, L., Stensland, G.J., Klein, J., Middleton, R., 1989. Atmospheric deposition of 7Be and 10Be. Geochemica and Cosmochemica Acta 53, 135–152. Caillet, S., Arpagaus, P., Monna, F., Dominik, J., 2001. Factors controlling 7Be and 210Pb atmospheric deposition as revealed by sampling individual rain events in the region of Geneva, Switzerland. J. Environ. Radioact. 53, 241–256. Cannizzaro, F., Greco, G., Raneli, M., Spitale, M.C., Tomarchio, E., 1999. Determination of 210Pb concentration in the air at ground-level by gamma-spectrometry. Appl. Radiat. Isot. 51, 239–249. Crecelius, E.R., 1981. Prediction of marine atmospheric deposition rates using total 7Be deposition velocities. Atmos. Environ. 15, 579–582. Cruikshank, A.J., Cowper, G., Grumitt, W.E., 1956. Production of 7Be in the atmosphere. Can. J. Chem. 34, 214–219. Dibb, J.E., 1989. Atmospheric deposition of beryllium 7 in the Chesapeake Bay region. J. Geophys. Res. 94 (D2), 2261–2265. Dibb, J.E., Meeker, D.L., Finkel, R.C., Southon, J.R., Caffee, M.W., Barrie, L.A., 1994. Estimation of stratospheric input to the Arctic troposphere: 7Be and 10Be in aerosols at Alert, Canada. J. Geophys. Res. 99 (D6), 12855–12864. Doering, C., Akber, R., 2008. Beryllium-7 in near-surface air and deposition at Brisbane, Australia. J. Environ. Radioact. 99, 461–467. Du, J., Du, J., Baskaran, M., Bi, Q., Huang, D., Jiang, Y., 2015. Temporal variations of atmospheric depositional fluxes of 7Be and 210Pb over 8 years (2006-2013) at Shanghai, China, and Synthesis of global fallout data. J. Geophys. Res.: Atmosphere 120, 4323–4339. Du, J., Zhang, J., Zhang, J., Wu, Y., 2008. Deposition patterns of atmospheric 7Be and 210 Pb in coast of east China sea, Shangai, China. Atmos. Environ. 42 (20), 5101–5109. Duenas, C., Fernandez, M.C., 2002. Atmospheric deposition of 7Be at a coastal Mediterranean station. J. Geophys. Res. 106, 34059–34065. Duenas, C., Fernandez, M.C., Carretero, J., Liger, E., Canete, S., 2005. Deposition velocities and washout ratios on a coastal site (south-eastern Spain) calculated from 7Be and 210Pb measurements. Atmos. Environ. 39, 6897–6908. Duenas, C., Gordo, E., Liger, E., Cabello, M., Canete, S., Perez, M., De la Torre-Luque, P., 2017. 7Be, 210Pb and 40K depositions over 11 years in Malaga. J. Environ. Radioact. 178–179, 325–334. Feely, H.W., Larsen, R.j., Sanderson, C.G., 1989. Factors that cause seasonal variations in Beryllium-7 concentrations in air. J. Environ. Radioact. 9, 223–249. Field, C.V., Schimdt, G.A., Koch, D., Salyk, C., 2006. Modelling, production and climate related impacts on 10Be concentration in ice-cores. J. Geophys. Res. 111, D15107. Fogh, C.L., Roed, J., Anderson, K.G., 1999. Radionuclide resuspension and mixed deposition at different heights. J. Environ. Radioact. 46, 67–75.

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Atmospheric Environment 202 (2019) 281–295

M.P. Mohan et al.

Rosner, G., Hotzl, H., Winkler, R., 1996. Continuous wet-only and dry-only deposition measurements of 137Cs and 7Be: an indicator of their origin. Appl. Radiat. Isot. 47, 1135–1139. Samuelsson, C., Hallstadius, B., Persson, B., Hedvall, R., Holm, E., 1986. 222Rn and 210Pb in the Arctic summer air. J. Environ. Radioact. 3, 35–54. Sarin, M.M., Rengarajan, R., Krishnaswami, S., 1999. Aerosol NO-3 and 210Pb distribution over the central-eastern Arabian sea and Bay of bengal. Tellus 51B, 749–758. Schery, S.D., Wasiolek, M.A., 1998. Modelling radon flux from the earth's surface. In: Proceedings of Seventh Towa University International Symposium, Fukuoka, Japan. 981-02-3443-0, pp. 207–217. Siddappa, K., Somashekarappa, H.M., Narayana, Y., Karunakara, N., Avadhani, D.N., Mahesh, H.M., 2000. Studies on Radioactivity in Aquatic and Atmospheric Environs of Coastal Karnataka, Kaiga and Goa. Final Report Submitted to BRNS. DAE, India. Terzi, L., Kalinowski, M., 2017. World-wide seasonal variation of 7Be related to largescale atmospheric circulation dynamics. J. Environ. Radioact. 178–179, 1–15. Todd, J.F., Wong, G.T.F., Olsen, C.R., Larsen, I.L., 1989. Atmospheric depositional characteristics of beryllium 7 and lead 210 along the southeastern Virginia coast. J. Geophys. Res. 94, 11106–11116. Tokieda, T., Yamanaka, K., Harada, K., Tsunogai, S., 1996. Seasonal variations of residence time and upper atmospheric contribution of aerosols studied with Pb-210, Bi210, Po- 210 and Be-7. Tellus 48B, 690–702. Turekian, K.K., Benninger, L.K., Dion, E.P., 1983. 7Be and 210Pb total depositional fluxes at New Haven, Connecticut and at Bermuda. J. Geophys. Res. 88 (C9), 5411–5415. Turekian, K.K., Graustein, W.C., 2003. Natural radionuclides in the atmosphere. In: Treatise in Geochemistry, vol. 4. pp. 261–279. Turekian, K.K., Graustein, W.C., Cochran, J.K., 1989. Lead-210 in the SEAREX program: an aerosol tracer across the Pacific. In: In: Duce, R.A. (Ed.), Chemical Oceanography, vol. 10. Academic Press, pp. 51–80. Turekian, K.K., Nozaki, Y., Benninger, L.K., 1977. Geochemistry of atmospheric radon and radon and radon products. Annu. Rev. Earth Planet Sci. 5, 227–255. UNSCEAR, 2000a. Report to the General Assembly, Annex B. Exposure from Natural Radiation Sources. Sources and Effects of Ionising radiation. United Nations Scientific Committee on Effects on the Effects of Atomic Radiation, vol. 1 pp. 98–99 and 116, New York. UNSCEAR, 2000b. Report to the General Assembly, Annex A. Dose Assessment methodologies. Sources and Effects of Ionising radiation. United Nations Scientific Committee on Effects on the Effects of Atomic Radiation, vol. 1. pp. 38 New York. Wallbrink, P.J., Murray, A.S., 1994. Fallout of 7Be in the south eastern Australia. J. Environ. Radioact. 25, 213–228. Wilkening, M.H., Clements, W.E., 1975. Radon-222 from the ocean surface. J. Geophys. Res. 80, 3828–3830. Winkler, R., Dietl, F., Frank, G., Tschiersch, J., 1998. Temporal variation of 7Be and 210Pb size distributions in ambient aerosol. Atmos. Environ. 32 (6), 983–991. Winkler, R., Rosner, G., 2000. Seasonal and long-term variation of 210Pb concentration in air, atmospheric deposition rate and total deposition velocity in south Germany. Sci. Total Environ. 263, 57–68. Yamamoto, M., Sakaguchi, A., Sasaki, K., Hirose, K., Igarashi, Y., Kim, C.K., 2006. Seasonal and spatial variation of atmospheric 210Pb and 7Be deposition: features of the Japan Sea side of Japan. J. Environ. Radioact. 86 (1), 110–131. Yoshimori, M., 2005. Production and behaviour of beryllium 7 radionuclide in the upper atmosphere. Adv. Space Res. 36, 922–926. Yoshimori, M., Hirayama, H., Mori, S., Sasaki, K., Sakurai, H., 2003. Be-7 nuclei produced by galactic cosmic rays and solar energetic particles in the Earth's atmosphere. Adv. Space Res. 32 (12), 2691–2696. Young, J.A., Tanner, T.M., Thomas, C.W., Wogman, N.A., Petrsen, M.R., 1975. Concentrations and rates of removal of contaminants from the atmosphere in and downwind of st Louis.Pacific northwest laboratory annual report for 1974 to the USAEC division of biomedical and environmental research. Atmopsheric Sciences 70–76 Part 3. Zhang, L., Yang, W., Chen, M., Wang, Z., Lin, P., Fang, Z., Qiu, Y., Zheng, M., 2016. Atmospheric deposition of 7Be in the Southeast of China: a case study in Xiamen. Aerosol and Air Quality Research 16, 105–113. Zikova, N., Zdimal, V., 2016. Precipitation scavenging of aerosol particles at a rural site in the Czech Republic. Tellus B: Chem. Phys. Meteorol. 68 (1), 27343.

Masarik, J., Reedy, R.C., 1995. Terrestrial cosmogenic-nuclide production ystematic calculated from numerical simulations. Earth Planet. Sci. Lett. 136, 381–395. McNeary, D., Baskaran, M., 2003. Depositional characteristics of 7Be and 210Pb in southeastern Michigan. J. Geophys. Res. 108 (D7) 3-1-3-15. Mohan, M.P., D'Souza, R.S., Nayak, S.R., Kamath, S.S., Shetty, T., Sudeep Kumara, K., Yashodhara, I., Mayya, Y.S., Karunakara, N., 2018. A study of temporal variations of 7 Be and 210Pb concentrations and their correlations with rainfall and other parameters in the South West Coast of India. J. Environ. Radioact. 192, 194–207. Momoshima, N., Nishio, S., Kusano, Y., Fukuda, A., Ishimoto, A., 2006. Seasonal variations of atmospheric 210Pb and 7Be concentrations at Kumamoto, Japan and their removal from the atmosphere as wet and dry depositions. J. Radioanal. Nucl. Chem. 268, 297–304. Nagai, H., Tada, W., Kobayashi, T., 2000. Production rates of 7Be and 10Be in the atmosphere. Nucl. Instrum. Methods Phys. Res. B 172, 796–801. Nazaroff, W.W., 1992. Radon transport from soil to air. Rev. Geophys. 30, 137–160. O'Brien, K., 1979. Secular variations in the production of cosmogenic isotopes in the Earth's atmosphere. J. Geophys. Res. 84 (A2), 423–431. Olsen, C.R., Larsen, I.L., Lowry, P.D., Cutshall, N.H., Todd, J.F., Wong, G.T.F., Casey, W.H., 1985. Atmospheric fluxes and marsh-soil inventories of 7Be and 210Pb. J. Geophys. Res. 90, 10487–10495. Othman, I., Al-Masri, M.S., Hassan, M., 1998. Fallout of 7Be in damascus city. J. Radioanal. Nucl. Chem. 238 (1–2), 187–199. Papastefanou, C., 2006. Residence time of tropospheric aerosols in association with radioactive nuclides. Appl. Radiat. Isot. 64, 93–100. Papastefanou, C., Ioannidou, A., 1991. Depositional fluxes and other physical characteristics of atmospheric Beryllium-7 in the temperate zones (40˚N) with dry (precipitation free) climate. Atmos. Environ. 25 A, 2335–2343. Papastefanou, C., Ioannidou, A., 1995. Aerodynamic size association of 7Be in ambient aerosols. J. Environ. Radioact. 26, 273–282. Papastefanou, C., Ioannidou, A., 2004. Beryllium-7 and solar activity. Appl. Radiat. Isot. 61, 1493–1495. Paatero, J., Ioannidou, A., Ikonen, J., Lehto, J., 2017. Aerosol particle size distribution of atmospheric lead-210 in northern Finland. J. Environ. Radioact. 172, 10–14. Pham, M.K., Povinec, P.P., Nies, H., Betti, M., 2013. Dry and wet deposition of 7Be, 210Pb and 137Cs in Monaco air during 1998-2010: seasonal variation of depositional fluxes. J. Environ. Radioact. 120, 45–57. Poet, S.E., Moore, H.E., Martell, E.A., 1972. 210Pb, 210Bi, and 210Po in the atmosphere: accurate ratio measurement and application to aerosol residence time determination. J. Geophys. Res. 77, 6515–6527. Rama, T., Zutshi, P.K., 1958. Annual deposition of cosmic ray produces Be7 at equatorial latitudes. Tellus X, 1. Ramanathan, V., Crutzen, P.J., Lelieveld, J., Mitra, A.P., Althausen, D., Anderson, J., Andreae, M.O., Cantrell, W., Cass, G.R., Chung, C.E., Clarke, A.D., Coakley, J.A., Collins, W.D., Conant, W.C., Dulac, F., Heintzenberg, J., Heymsfield, A.J., Holben, B., Howell, S., Hudson, J., Jayaraman, A., Kiehl, J.T., Krishnamurti, T.N., Lubin, D., McFarquhar, G., Novakov, T., Ogren, J.A., Podgorny, I.A., Prather, K., Priestley, K., Prospero, J.M., Quinn, P.K., Rajeev, K., Rasch, P., Rupert, S., Sadourny, R., Satheesh, S.K., Shaw, G.E., Sheridan, P., Valero, F.P.J., 2001. Indian ocean experiment: an integrated analysis of the climate forcing and effects of the great Indo-Asian haze. J. Geophys. Res. 106, 28371–28398. Rangarajan, C., Eapen, C.D., 1990. The use of natural radioactive tracers in a study of atmospheric residence times. Tellus 42B, 142–147. Rangarajan, C., Gopalakrishnan, S.M.T.S., 1970. Seasonal variation of beryllium-7 relative to Caesium-137 in surface air at tropical and sub-tropical latitudes. Tellus 22, 115–121. Rastogi, N., Sarin, M.M., 2008. Atmospheric 210Pb and 7Be in ambient aerosols over lowand high-altitude sites in semiarid region: temporal variability and transport processes. J. Geophys. Res. 113, D11103. Reineking, A., Porstendorfer, J., 1995. Time variations of size distributions of aerosolattached activities of 212Pb, 210Pb and 7Be in the outdoor atmosphere. In: Book of Abstracts, National Radiation Environmnet, Montreal, Canada, 5-9 June, 1995, vol. 199 Clarkson University, Potsdam, NY. Rengarajan, R., Sarin, M.M., 2004. Atmospheric deposition fluxes of 7Be and 210Pb and chemical species to the Arabian sea and Bay of bengal. Indian J. Mar. Sci. 33 (1), 56–64.

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