A global dataset of atmospheric 7 Be and 210 Pb measurements : annual air concentration and depositional flux

7Be and 210Pb air concentration and depositional flux data provide key information on the origins and movements of air masses, as well as atmospheric deposition processes and residence time of aerosols. After their deposition onto the Earth’s surface, they are utilized for tracing soil redistribution processes on land, particle dynamics in aquatic systems, and mixing processes in open ocean. Here we present a global dataset of air concentration and depositional flux measurements of atmospheric 7Be and 210Pb made by a large number of global research communities. Data were collected from published papers between 1955 and early 2020. It includes the annual surface air concentration data of 7Be from 367 sites and 210Pb from 270 sites, the annual depositional flux data of 7Be from 279 sites and 210Pb from 602 sites. When available, appropriate metadata have also been summarized, including geographic location, sampling date, methodology, annual precipitation, and references. The dataset is archived at https://doi.org/10.5281/zenodo.4785136 (Zhang et al., 2021) and is freely available for the scientific community. The purpose of this paper is to provide an overview of the scope and nature of this dataset and its potential utility as baseline data for future research.


Introduction
Naturally occurring beryllium-7 ( 7 Be, T 1/2 : 53.3 d) and lead-210 ( 210 Pb, T 1/2 : 22.3 years) have been widely utilized as tracers to investigate Earth's surface and atmospheric processes . 7 Be, a cosmogenic radionuclide, is produced by the spallation of oxygen and nitrogen nuclei by cosmic rays in the stratosphere and upper troposphere (Lal et al., 1958). The production rate of 7 Be has negligible dependence on longitude or season but depends on the altitude, latitude, and the ∼ 11-year solar cycle Liu et al., 2001;Su et al., 2003). A major Nazaroff, 1992). Vertical profiles of 222 Rn in the atmosphere indicate the highest concentrations occur in the continental boundary layer (CBL, 3-8 Bq m −3 ), while activity that is lower by an order of magnitude (∼ 40 mBq m −3 ) occurs near the tropopause, with decreasing activity with increasing altitude from the CBL (Moore et al., 1977;Liu et al., 1984;Kritz et al., 1993). Consequently, the atmospheric concentration of 210 Pb decreases with increasing altitude and is strongly controlled by the land-sea distribution pattern. After formation, 7 Be, 210 Pb, Rn progeny are all rapidly and irreversibly attached to aerosol particles (Winkler et al., 1998;Elsässer et al., 2011). Subsequently, the fate of 7 Be and 210 Pb is closely linked to that of aerosols. Most of 7 Be and 210 Pb are in accumulation-mode aerosol particles with an aerodynamic diameter of a few hundred nanometers Paatero et al., 2017). Therefore, they are deposited onto the Earth's surface primarily by precipitation because accumulation-mode aerosol particles are too small for gravitational settling and removal and too large to be deposited by Brownian motion.
There have been numerous published datasets with concentrations and depositional fluxes of 210 Pb and 7 Be (directly and indirectly) over the past few decades, particularly under national or international monitoring programs, such as the Environmental Measurements Laboratory (EML) Surface Air Sampling Program , the Sea-Air Exchange (SEAREX) program (Uematsu et al., 1994), the Finnish Meteorological Institute monitoring program , the Radioactivity Environmental Monitoring (REM) network , and the International Monitoring System (IMS) operated by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) (Terzi and Kalinowski, 2017). It is valuable to compile all of these existing data, including those measured in case studies, along with appropriate metadata, in one place for facilitating further data analysis and geological, geochemical, and geophysical applications.
Although several datasets of air concentrations and depositional fluxes of 7 Be Brost et al., 1991) and 210 Pb (Rangarajan et al., 1986;Preiss et al., 1996) have been published, unfortunately, many of the published data are not readily available in a hard data format (e.g., data table), and retrieval of data from published figures is often not precise and can be challenging. To date, only one dataset was published that compiled 7 Be and 210 Pb together (Persson, 2015), but it contained limited data. Therefore, the focus of the present work is to build a new and comprehensive dataset. This dataset is the result of many scientists' efforts in generating the data.

7 Be and 210 Pb air concentrations measurement methodology
Aerosol samples are usually collected by filtering a high volume of air, typically 1.4-1.5 m 3 min −1 , collected typically for a day, through paper filters. Commonly used aerosol collecting equipment include the following instruments: ASS 500 (CLRP Warsaw, Germany), Anderson PM10 (Anderson Ltd., USA), Snow White (Senya Ltd., Finland), and HV-1000F (Shibata Co. Ltd., Japan). The preferred filter membrane materials include glass fiber, cellulose nitrate or acetate, polypropylene fiber, and quartz fiber. The collection efficiency, the percentage of the particles in the air stream that are collected by the air filter, depends on the aerodynamic diameter of aerosol particles and the filter face velocity (average flow velocity of air into the filter) of the airflow. Although the collection efficiency depends on the distribution of particle sizes, generally the collection efficiency varies between 80 % and 100 % for different filter materials. This was shown by overlapping two filters in tandem and comparing the concentrations separately in the top and bottom filters. The sampling frequency is usually set from daily to monthly, with a typical collection time of ∼ 24 h, corresponding to  210 Pb trapped on filters can both be analyzed simultaneously by gamma spectrometry (e.g., McNeary and Baskaran, 2003;Bourcier et al., 2011;Lozano et al., 2012;Mohan et al., 2018), while 210 Pb can also be analyzed by beta counting of its daughter 210 Bi (e.g., Joshi et al., 1969;Poet et al., 1972;Daish et al., 2005) or via alpha counting of in-grown 210 Po from the decay of 210 Pb (e.g., Turekian and Cochran, 1981;Mattsson et al., 1996;Marx et al., 2005).
2.2 7 Be and 210 Pb depositional fluxes measurement methodology The atmospheric depositional fluxes of 7 Be and 210 Pb are commonly measured directly by using rain collectors, such as polyethylene drums or buckets and stainless steel containers, and indirectly by natural archives (e.g., soils, lichens, mosses, snow and ice cores, salt marsh sediments). The direct collecting method is the most reliable technique for the measurement of annual 7 Be and 210 Pb depositional fluxes, and this technique is useful in collecting short timescale (daily, weekly, and monthly) depositional fluxes. In contrast, using natural archives avoids the labor-and time-intensive measurement of the 7 Be and 210 Pb concentration in precipitation and can serve as a complement to fill regional gaps, especially in remote areas. However, these archives are susceptible to being affected by natural processes and anthropogenic activities; thus, the sampling location of these archives should be restricted to undisturbed areas.

Direct 7 Be and 210 Pb flux measurements
Rain collectors are usually placed on the roof of a building so as to prevent contamination from resuspended dust from the ground. Care should be taken to ensure direct overhead atmospheric deposition is collected and that there is no shadowing effect from adjacent structures or buildings or a funneling effect in sample collection. Atmospheric aerosols can be removed not only by precipitation-scavenging but also by settling under the influence of gravitational or electrostatic forces. In most cases, the collectors are continuously exposed over a long enough period, and the bulk (wet + dry) depositional samples are collected periodically (e.g., Hirose et al., 2004;Baskaran and Swarzenski, 2007;Lozano et al., 2011;Du et al., 2015). Fluxes obtained by this method yield the best estimate of the depositional flux. Sometimes, fallout 7 Be and 210 Pb samples are collected only during rainfall, and concentration is measured in the individual rainwater sample (e.g., Cho et al., 2011;Chae and Kim, 2019;Du et al., 2020). In this case, only the bulk depositional flux is obtained for the duration of collection. In rare cases, when only a mean concentration of 210 Pb and/or 7 Be in rainwater is available, the wet flux can be estimated by multiplying by the annual precipitation .
Most of the time, the volume of rainwater sample is large and cannot be directly counted in a gamma-ray spectrometer for the simultaneous measurements of 7 Be and 210 Pb, in which case a preconcentration of the sample is required. Since both 7 Be and 210 Pb have a strong affinity for solid surfaces, it is strongly recommended to add stable Be (commonly 1-5 mg) and stable  in about 1 L of 1 M HCl to the rain collector prior to deployment. Alternatively, the spikes can also be immediately added after sample collection, followed by rinsing of rain collector with 1 L of 1 M HCl rinsing twice and combining the rinses with the collected rainwater. An earlier critical review of earlier atmospheric depositional flux studies by Lal et al. (1979) showed that loss of 7 Be and 210 Pb by sorption onto rain collector walls was observed when pre-acidification of the collector was not done, resulting in the underestimation of depositional flux. In the case of preconcentration by ferric chloride precipitation method, due to variable scavenging efficiency of 7 Be and 210 Pb during the preconcentration method, it is required to add stable Pb and Be as a yield tracer (e.g., . The best chemical procedure to obtain high-quality data is to add acid and stable Be and Pb careers with 1 L of 1 M HCl to the rain collector prior to the start of the sample collection. The final calculation for depositional fluxes would involve chemical yield for 7 Be and 210 Pb and appropriate decay corrections, as outlined in Baskaran et al. (1993) and Du et al. (2015).

Indirect 7 Be flux measurements
Measurements of 7 Be inventory in the upper oceanic water column, from the air-sea interface until the layer where 7 Be activity is below the detection limit (in > 400 L water sample), can indirectly yield the bulk depositional flux of 7 Be . This requires precise determination of the penetration depth of 7 Be in the water column. The only uncertainty is the loss of 7 Be-laden sinking particles from the upper water column where 7 Be is present. After being deposited on the ocean surface, 7 Be is generally mixed uniformly within the surface mixed layer (Young and Silker, 1980;Kadko and Olson, 1996). In open ocean, the particle concentration is generally low, and a major fraction of 7 Be is expected to be in the dissolved phase, thus allowing particle scavenging losses to be ignored (Silker, 1972;Andrews et al., 2008). Therefore, in the absence of physical removal processes other than radioactive decay, the input flux of 7 Be should be balanced by the 7 Be inventory integrated over the water column. In other words, 7 Be flux of atmospheric fallout (Bq m −2 d −1 ) can be obtained from the 7 Be water column inventory (Bq m −2 ) multiplied by the decay constant (0.013 d −1 ) of 7 Be. This method has been proven to be reliable in open ocean due to the relatively short half-life of 7 Be and the constancy of 7 Be deposition over broad latitudinal bands (Young and Silker, 1980;Aaboe et al., 1981). It is expected that the 7 Be inventory is season dependent in areas with large seasonal variations in precipitation (e.g., monsoon-dominated continental and oceanic areas). A time series study in Bermuda has shown that the inventory of 7 Be was relatively constant throughout the year, such that 7 Be inventory measured at any one time is likely representative (to within 20 %) of the instantaneous 7 Be flux (Kadko and Prospeo, 2011;Kodko et al., 2015). This method is not suitable for coastal and estuarine areas where 7 Be is scavenged substantially by particulate matter  and upwelling-dominated areas where 7 Be inventory is diluted by 7 Be "dead" water (Kadko and Johns, 2011;Haskell et al., 2015).
Another potential candidate is undisturbed soil profiles. However, 7 Be inventories in undisturbed soils were reported to vary by more than an order of magnitude within 1 year Kaste et al., 2011;Zhang et al., 2013). Such large variations in depositional fluxes of 7 Be are attributed to the seasonal fluxes of 7 Be. Earlier studies have shown that the atmospheric fluxes are highly dependent upon the amount of precipitation Du et al., 2015). Since seasonal variations will significantly affect the 7 Be inventory in soils, the data of 7 Be soil inventory are not included in our dataset.

Indirect 210 Pb flux measurements
Several archives (soils, snow and ice cores, sediment cores, etc.) have been used to assay the 210 Pb fluxes. Here we only present 210 Pb fluxes estimated from soil profiles and snow and ice cores. The former is the most frequently used, and the latter perfectly fills the regional gap in polar regions and montane permanent snowfields. There are many 210 Pb measurements in sediment cores; however, due to the sediment focusing and erosion, most sediment cores do not provide a reliable estimate of the atmospheric 210 Pb flux (Turekian et al., 1977;Preiss et al., 1996); thus, this type of 210 Pb depositional flux data is not included in our dataset. 210 Pb in surface (upper ∼ 30 cm) soil has two sources: one is generated from the decay of 222 Rn in the soil minerals, known as supported 210 Pb, which is produced from the decay of 238 U, and the other comes from atmospheric deposition as unsupported 210 Pb. The fallout of 210 Pb is retained generally in the organic-rich surface soils, presumably because of the sequestering properties of the organo-mineral complexes (Covelo et al., 2008). When soil CO 2 combines with percolating water, carbonic acid is produced, which can leach some of the sorbed 210 Pb, ultimately resulting in slow migration down to a depth of up to 20-30 cm (Matisoff and Whiting, 2012). As a result, the surface soil layer contains excess 210 Pb compared to that from its equilibrium with 226 Ra . This part of the 210 Pb excess is termed "unsupported" or "excess" 210 Pb ( 210 Pb ex ). 210 Pb ex is the difference between total (measured) 210 Pb and the supported 210 Pb in the soils. Supported 210 Pb is assumed to be the same as 226 Ra activity, under the assumption of a secular equilib-rium between 226 Ra and supported 210 Pb. It can also be obtained by assuming that the supported 210 Pb activity is equal to the total 210 Pb at depths greater than 30 cm in the soil profile where atmospherically delivered 210 Pb has not reached (Matisoff, 2014). The mean residence time of 210 Pb over a large drainage basin is on the order of 2000-3000 years in surface soils (Benninger et al., 1975;Dominik et al., 1987), so the inventory of 210 Pb ex in a soil profile that has not been disturbed by erosion, accumulation, or human activities for about a century can be used to calculate the depositional flux (Graustein and Turekian, 1986). At steady state, the 210 Pb depositional flux can be deduced using the 210 Pb ex inventory multiplied by the decay constant (0.0311 yr −1 ) of 210 Pb. This method has been widely used worldwide (e.g., Nozaki et al., 1978;Turekian, 1986, 1989;Dörr and Munich, 1991;García-Orellana et al., 2006). At undisturbed soil sites, flux values derived from soil profile measurements were consistent with direct atmospheric flux observations (Olsen et al., 1985;Appleby et al., 2002Appleby et al., , 2003, and 210 Pb ex soil inventory showed little discrepancy at different sampling times (Porto et al., 2006. Goldberg (1963) was the first to show that the total 210 Pb activity in a glacier from Greenland decreased with depth, with a possibility of dating ice cores. Subsequently, snow chronology in the Antarctic was determined Picciotto et al., 1964). Since then, this technique has been used in both the large ice caps of Antarctica (e.g., Picciotto et al., 1968;Koide et al., 1979;Nijampurkar et al., 2002) and the Arctic (e.g., Crozaz and Langway, 1966;Koide et al., 1977;Peters et al., 1997) and small montane permanent snowfields (e.g., Windom, 1969;Gäggeler et al., 1983;Monaghan and Holdsworth, 1990). The 210 Pb flux in snow and ice cores is calculated in the same way as for the soil, except that supported 210 Pb in snow and ice cores is very low due to low concentration of 226 Ra in snow and ice core and may be negligible, as the lithogenic dust is the primary source of 226 Ra and its concentration in polar regions is very low . When the snow accumulation rate is known, the depositional flux can also be obtained by using 210 Pb concentration in surface snow multiplied by the accumulation rate (Pourchet et al., 1997;Suzuki et al., 2004). The uncertainty in the depositional flux of 210 Pb from the snow and ice core record is the potential post-depositional movement of the snow and ice due to heavy wind and the possibility of snow melting and percolation.

Data collection
In order to compile the global dataset for annual 7 Be and 210 Pb air concentrations and depositional fluxes comprehensively, we attempted to collect published papers between 1955 and early 2020 in which hard data for their concentrations and depositional fluxes are available or their calculated values are reported. Using a series of keywords or with a search using a combination of words (e.g., 7 Be, 210 Pb,  210 Pb concentration in surface air, (c) 7 Be atmospheric depositional flux, and (d) 210 Pb atmospheric depositional flux. air concentration, depositional flux, fallout radionuclide, atmospheric tracer, or soil erosion), data were retrieved from online literature databases (Web of Science, Science Direct, and China Knowledge Resource Integrated Database). During the literature survey, no a priori criteria (e.g., study area, sampling period, and measurement method) were applied. However, a critical review of the collected literature was conducted to obtain long-term data using the following criteria. For concentrations in air and directly measured fluxes of 7 Be and 210 Pb, only those sites with more than 1 year of data were included. When averaged over a longer period, data are more representative because of the inherent seasonal variations (at least a full year of data) and inter-annual fluctuations (multiyear data). For indirectly measured fluxes of 7 Be and 210 Pb, only those undisturbed sites clearly stated in the original literature were included.
Here we did not include unpublished data, as data quality control could be a potential issue. In the peer-reviewed published data, it is assumed that the authors, reviewers, and/or editors of the original articles have undertaken the necessary steps to verify data quality. All concentrations in air were converted into mBq m −3 and all depositional fluxes were converted into Bq m −2 yr −1 in cases where data were not already reported in these units. When available, the metadata of latitude, longitude, altitude, sampling date, annual precipitation, methodology, and references are also given. A brief description of different variables that could affect the data is summarized in Table 1. In cases where the air concentration and depositional flux data were only available graphically, a computer program (GetData Graph Digitizer) was used to digitize the data from graphics; the same was done for geographical location and annual precipitation. In rare cases, only the locality name of the study site was available, and thus the geographical coordinates were extracted from Google Earth.

Scope of the dataset
From 456 references (Appendix A), we have compiled a comprehensive dataset of atmospheric 7 Be and 210 Pb measurements made by numerous laboratories. The dataset includes 494 annual surface air concentration data of 7 Be covering 367 different sites, 366 annual surface air concentration data of 210 Pb from 270 different sites, 304 annual depositional flux data of 7 Be from 279 different sites, and 645 annual depositional flux data of 210 Pb from 602 different sites. In some cases, data collected from different periods were published in different articles. In these cases, all data from the same site are listed as separate datasets; however, in data analysis and plotting, the data from the same site are merged. The sampling locations of this global dataset show broad geographical coverage (Fig. 1). The sampling maps are mainly composed of continental sites, while the oceanic monitoring sites are limited.
The number of peer-reviewed journal articles published annually containing 7 Be and 210 Pb data from 1955 to 2020 that are included in this article is plotted in Fig. 2. Measurements of 7 Be began in the mid-1950s Rama Thor and Zutshi, 1958), earlier than those of 210 Pb, which began in early 1960s Crozaz et al., 1964;Peirson et al., 1966). The long-term monitoring work started in The field only appears in the 7 Be or 210 Pb annual concentration worksheet. b The field only appears in the 7 Be or 210 Pb annual depositional flux worksheet.
the 1980s with the 7 Be and 210 Pb concentration data generated by the EML Surface Air Sampling Program Larsen et al., 1995). This was followed by more ambitious international programs, such as the REM network  and IMS-CTBTO (Terzi and Kalinowski, 2017). However, in these two programs, 210 Pb concentration measurements were conducted only at a few stations Sangiorgi et al., 2019). In contrast, direct 7 Be and 210 Pb flux measurements were rarely supported by the international program, but there were several national monitoring programs initiated by developed countries like Australia (Bonnyman and Molina-Ramos, 1971), Japan Yamamoto et al., 2006), the United States , and Finland Leppanen, 2019). The measurement of 7 Be inventory in the ocean mixed layer began in the 1970s (Silker, 1972;Silker, 1974, 1980), and the idea proposed by Young and Silker was subsequently developed in the upper 100-200 m to assess surface water subduction, oxygen utilization, and the rate of upwelling (Kadko, 2009; Kadko and Olson, 1996;Kadko and Johns, 2011). The measurement of 210 Pb ex in an undisturbed soil profile was first conducted by Fisenne (1968). Subsequently, Benninger et al. (1975) and Moore and Poet (1976) showed that excess 210 Pb activities in undisturbed soil profiles can be utilized to estimate the atmospheric 210 Pb depositional flux, which resulted in an increase in the measurements of 210 Pb ex in soil profiles in the late 1980s Turekian, 1986, 1989;Dörr and Munnich, 1991). Subsequently, 210 Pb ex was shown to be useful for soil erosion studies on agricultural land Walling et al, 2003). The histogram of sampling durations of 7 Be and 210 Pb measurements is given in Fig. 3. In general, the duration of sampling for 7 Be measurements, especially for air concentration, is longer than that of 210 Pb. Globally, there are 140 sites that monitored 7 Be air concentration for more than 10 years. The long-term (decades) measurements of 7 Be were mainly dedicated to investigating the effect of changes in sunspot number on 7 Be (Megumi et al., 2000; Cannizzaro  210 Pb concentration, (c) 7 Be depositional flux, and (d) 210 Pb depositional flux. For those sites with multiple datasets, we added the sampling duration together, after deducting the overlapping period (if there was one). Note that the 7 Be and 210 Pb depositional flux data plotted here refer to those obtained using rain collectors. Kulan et al., 2006;Pham et al., 2013;Steinmann et al., 2013). Due to the simpler measurement procedure for air concentration, the duration of air concentration measurements, whether for 7 Be or 210 Pb, is generally longer than that of 210 Pb and/or 7 Be depositional flux measurements.

Global variability
The global data of 7 Be and 210 Pb air concentrations and depositional fluxes are presented in Fig. 4. The range of concentrations of 7 Be and 210 Pb are 0.33-17.77 mBq m −3 and 0.003-4.65 mBq m −3 , respectively. The range of depositional fluxes of 7 Be and 210 Pb are 59-6350 and 1-2539 Bq m −2 yr −1 , respectively. The concentrations and depositional fluxes of 7 Be show discernable latitudinal variability ( Fig. 5a and c). In general, 7 Be concentration and flux peak at the mid-latitudes and decrease toward the Equator and poles, as was theoretically predicted by Lal and Pe-   210 Pb concentration, (c) 7 Be depositional flux, and (d) 210 Pb depositional flux plotted by 10 • latitudinal bands.
ters (1967). A symmetric pattern is observed between the Northern Hemisphere and Southern Hemisphere, however, a sharp increase in 7 Be air concentration (lack of flux data) occurred on the Antarctic, which reflects the subsidence of stratospheric air masses over the Antarctic continent (Wagenbach et al., 1988;Elsässer et al., 2011). Although the 210 Pb concentration and depositional flux are expected to heavily depend on the source(s) of air mass(es) and not on the latitude, the 10 • latitudinal variability of 210 Pb concentration and depositional flux is observed in the Northern Hemisphere and Southern Hemisphere ( Fig. 5b and d). The latitudinal variability of 210 Pb flux is similar to the global fallout curve based on 167 global sites . Since most of the 210 Pb data are derived from continental sites, the latitudinal variation is mostly due to differences in the radon emanation rates with latitude. As the area of the landmass in the Southern Hemisphere is smaller than in the Northern Hemisphere, the 210 Pb concentration and depositional flux are much lower there. An asymmetry is observed in the 210 Pb concentration and depositional flux between the Northern Hemisphere and Southern Hemisphere, with the highest values appearing in the mid-latitudes of the Northern Hemisphere.

Fraction of dry deposition and effect of precipitation on 7 Be and 210 Pb depositional flux
It was reported that dry deposition of 7 Be and 210 Pb generally accounts for less than 10 % of the total deposition (Tal- Figure 6. Global distribution of the fraction of dry to total depositional fluxes of (a) 7 Be and (b) 210 Pb. (c) Box and whisker plots showing the comparison between the fraction of dry to total depositional fluxes of 7 Be and the fraction of dry to total depositional fluxes of 210 Pb.
(d) The fraction of dry to total depositional fluxes of 7 Be versus annual precipitation. (e) The fraction of dry to total depositional fluxes of 210 Pb versus annual precipitation. bot and Andren, 1983;Brown et al., 1989;Todd et al., 1989); however, the fraction of dry deposition of 7 Be and 210 Pb is highly variable (McNeary and Baskaran, 2003;Pham et al., 2013). It is likely that the contribution of dry fallout could increase when annual precipitation decreases (McNeary and Baskaran, 2003). The fraction of dry to total depositional flux of 7 Be and 210 Pb are presented in Fig. 6a and b. Globally, the fraction of dry to total depositional flux of 7 Be and 210 Pb ranged from 1 % to 44 % (mean: 12 ± 9 %; n = 29, excluding one extreme site without precipitation) and from 5 % to 51 % (mean: 21 ± 12 %; n = 26), respectively (Fig. 6c). The low fraction of dry to total depositional fluxes of 7 Be and 210 Pb suggest that these nuclides are removed from the atmosphere primarily by precipitation (both rain and snowfall). Our results also support previous studies Benitez-Nelson and Buesseler, 1999) that the fraction of dry deposition is higher for 210 Pb than for 7 Be. The fraction of dry fallout of 7 Be and 210 Pb is plotted against annual precipitation in Fig. 6d and e, and a weak negative correlation is observed, especially for 210 Pb. As precipitation is the primary mechanism of removal of these nuclides from the atmosphere, the annual depositional fluxes generally depend on the amount and frequency of precipitation. In our dataset, the world's lowest 7 Be depositional flux (only 59 Bq m −2 yr −1 , less than 5 % of the global average 7 Be flux) occurred in the Judean Desert, a precipitation-free area in the horse latitudes . The highest 7 Be (6350 Bq m −2 yr −1 ) and 210 Pb (2539 Bq m −2 yr −1 ) depositional flux were observed in heavy rainfall areas: Hokitika, New Zealand (Harvey and Matthews, 1989), and Taiwan (Huh and Su, 2004), respectively. Positive correlations between annual depositional flux and precipitation have been observed on a local scale (e.g., Narazaki et al., 2003;García-Orellana et al., 2006;Sanchez-Cabeza et al, 2007;Leppanen et al., 2019). Here we illustrate the effect of precipitation on annual depositional flux on a global scale (Figs. 7 and 8). As both 7 Be and 210 Pb depositional flux show latitudinal variability, the linear bestfit curve of annual depositional flux to annual precipitation is plotted within the 10 • latitudinal bands (if data are available). The empirical equations and fitting parameters describing the relationships between annual precipitation and 7 Be and 210 Pb depositional fluxes are summarized in Table 2. 7 Be and 210 Pb annual depositional fluxes generally show a good positive correlation with annual precipitation, although the data are limited in some latitudinal bands. Note that the frequency of precipitation is also an important factor that was not considered in earlier studies.

7 Be / 210 Pb ratios and deposition velocities
Considering that some data come from the same station, we further calculated the ratios of 7 Be to 210 Pb and deposition velocities of aerosols using 7 Be and 210 Pb data, as shown in Figs. 9 and 10.
The variations in the 7 Be / 210 Pb ratios reflect both vertical and horizontal transport in the atmosphere Arimoto et al., 1999;Lee et al., 2007;Tositti et al., 2014). Our dataset exhibits similar global patterns of 7 Be / 210 Pb ratio as have been simulated with a threedimensional chemical tracer model , with a positive south poleward gradient and a little variation in the Northern Hemisphere. Globally, the 7 Be / 210 Pb air concentration ratio ranged from 2 to 222, and the 7 Be / 210 Pb depositional flux ratio ranged from 2 to 229. At the 19 sites for which 7 Be / 210 Pb air concentration ratio and depositional flux ratio were available simultaneously, a paired t test indi-  cates that at 0.05 level the 7 Be / 210 Pb air concentration ratio and depositional flux ratio are not significantly different. 7 Be and 210 Pb are excellent tracers for the determination of the deposition velocities of aerosols for two reasons: (1) their depositional fluxes and air concentrations at any given site remain fairly constant over a long period and (2) their size distributions in aerosols are similar to that of many particulate contaminants of interest (McNeary and Baskaran, 2003;Dueñas et al., 2005). When both air concentration (C) and depositional flux (F ) at the same site are available, the average total deposition velocities of aerosols that carry these nuclides (V d ) can be calculated by the following Eq. (1): Thus, the V d obtained from 7 Be and 210 Pb can be used to determine the depositional flux of analog species with a knowledge of their air concentration (Turekian et al., 1983). The V d for 7 Be ranged from 0.2 to 8.4 cm s −1 (mean: 1.3 ± 1.2 cm s −1 ; n = 70), and for 210 Pb it ranged from 0.1 to 12.7 cm s −1 (mean: 1.2 ± 1.7 cm s −1 ; n = 72). The deposition velocity of aerosols collected over a period of 17 months in Detroit, MI, USA, varied over 2 orders of magnitude, from 0.2 to 3.6 cm s −1 (mean: 1.6 cm s −1 ; n = 30) for 7 Be Figure 8. Annual depositional fluxes of 210 Pb are plotted against annual precipitation for 10 • latitudinal bands. and 0.04 to 3.6 cm s −1 (mean: 1.1 cm s −1 ; n = 30) (McNeary and Baskaran, 2003). A summary of deposition velocity from 10 different stations is also given in McNeary and Baskaran (2003). Earlier studies suggested that at continental sites V d of 7 Be will be higher than V d of 210 Pb using the ground level as the reference, which is an artifact in the manner in which the calculation is made (Turekian et al., 1983;Todd et al., 1989;McNeary and Baskaran, 2003). However, later works observed opposite results Lozano et al., 2011;Mohan et al., 2019). The independent t test analysis indicates that at the 0.05 level the V d calculated by 7 Be and 210 Pb are not significantly different in the global dataset, which suggests that 7 Be and 210 Pb attach onto the aerosols by similar mechanisms (Winkler et al., 1998; and are affected by similar deposition processes .

Investigations of global atmospheric dynamics and climate changes
Developing numerical models in which aerosols, chemistry, radiation, and clouds interact with one another and with at-   210 Pb. Note that the color bar below applies to both figures. mospheric dynamics is important for understanding and predicting global climate changes . In such atmospheric dynamic models, the major uncertainty is from the parameterization of subgrid-scale processes such as precipitation scavenging, vertical transport, and radiative effect. Taken together, the cosmogenic 7 Be and terrigenous 210 Pb offer an excellent tool in investigating wet scavenging and vertical transport in global models . A set of data obtained prior to the 1990s was used to compare simulated results in global models such as ECHAM2 Feichter et al., 1991), ECMWF (Rehfeld and Heimann, 1995), chemical transport model (CTM) based on GISS GCM (Balkanski et al., 1993;, GEOS-CHEM , LMDz (Preiss and Genthon, 1997;Heinrich and Pilon, 2013), and GMI CTM . By simulating the ratio of 7 Be / 210 Pb, Koch et al. (1996) eliminated the error associated with the effect of precipitation and provided a better measure of vertical transport. After correcting the cross-tropopause transport, simulation of observed 7 Be and 210 Pb surface concentrations and depositional fluxes with no significant global bias was obtained . Note that the spatial coverage of the dataset used in the previous modeling work was only partial and thus limited the statistical significance of comparisons of simulated and observed results . Additional work with more data is needed for detailed comparison and successful validation of models . The size of the 7 Be and 210 Pb datasets has greatly increased in the last 3 decades, and our new dataset is expected to lay a foundation for developing better parameterizations and to contribute to modeling efforts.
3.6 Soil erosion, aquatic particle dynamics, and ocean surface process studies The atmospheric depositional flux data of 7 Be and 210 Pb are useful in utilizing these nuclides as tracers for soil erosion and redistribution studies in terrestrial environments  and particle dynamics studies in aquatic environments (e.g., Matisoff, 2014). The basic principle involved in using 7 Be or 210 Pb ex as soil tracers are the same, which is to compare the measured inventory of 7 Be or 210 Pb ex (Bq m −2 ) at a sampling point with the inventory at an undisturbed (or reference) site . Depletion of the inventory means that soil erosion has occurred, whereas enrichment provides evidence of accumulation of surficial soil. The first step in these studies is to select a suitable undisturbed site and obtain the reference inventory . However, as human activity intensifies, such undisturbed sites are not always readily available. In aquatic systems (including rivers, lakes, estuaries, and coasts), the mass balance models of 7 Be and 210 Pb ex have become powerful tools to understand the sediment source, transportation and resuspension processes (e.g., Wieland et al., 1991;Feng et al., 1999;Jweda et al., 2008;Huang et al., 2013;Mudbidre et al., 2014). In such models, the atmospheric depositional input of 7 Be and 210 Pb is a required source term. Furthermore, the 7 Be / 210 Pb ex activity ratio can be used to identify the source area of sediments Jweda et al., 2008;Wang et al., 2021), to quantify the age of sediments Saari et al., 2010), and to determine the transport distance of suspended particles (Bonniwell et al., 1999;Matisoff et al., 2002). Thus, the atmospheric depositional flux data of 7 Be and 210 Pb are also important for tracing particle dynamics in aquatic systems. The atmospheric depositional flux (or ocean inventory) data of 7 Be serve as an indispensable parameter for tracing surface ocean process (e.g., subduction, upwelling, and depositional flux of trace metals) Kadko and Olsen, 1996;Kadko et al., 2015). Due to the low activity of 7 Be in open-ocean waters, usually 400-700 L of seawater is needed, which imposes some limitations for sampling, especially for deep layers. This constraint has limited its application. If the 7 Be ocean inventory can be accurately estimated, the collection of a large volume of seawater can be avoided and the application of 7 Be in open ocean will be expanded.
Scientific data are not simply the outputs of research, but they also provide inputs to new hypotheses, extending research and enabling new scientific insights (Tenopir et al., 2011). Our dataset provides a forum in which a large amount of 7 Be and 210 Pb atmospheric depositional flux data for the above-mentioned research communities. This database will help in identifying data gaps and evaluating the empirical relations between 7 Be and 210 Pb depositional fluxes and annual precipitation (Table 2). Researchers can rely on previously collected data in planning their research, without additional monitoring of 7 Be and/or 210 Pb depositional fluxes. Even for those areas with data gaps, the empirical equations between 7 Be and 210 Pb depositional fluxes and annual precipitation provide an empirical method for estimating fluxes, especially for 7 Be, as 7 Be depositional flux is independent of longitude and is constant over broad latitudinal bands. In summary, the atmospheric depositional flux data presented in this dataset, along with the meta-analysis of the data, will be useful in the investigations of soil erosion studies in terrestrial environments, particle dynamics studies in aquatic systems, and surface mixing process studies in open ocean.

Gaps and recommendations
Our dataset is a compilation of most of the published results in international peer-reviewed journal articles. Although the spatial coverage of this dataset is significant, the available sites are unevenly distributed. Compared to spatial and temporal coverage of depositional flux data, the coverage of air concentration data is much larger. The air concentration measurements from deep-ocean sites are limited, but the data from coastal oceanic sites are adequate; however, the depositional flux measurement at oceanic sites is rare. Concerning air concentrations, areas such as Europe, East Asia, eastern Oceania, and the eastern United States are well covered, whereas other areas such as the African continent and northern Asia are limited. A similar spatial coverage pattern exists for the depositional flux of these nuclides, but the regional gaps are more notable, especially for 7 Be flux data, which are almost non-existent for the Antarctic and African continents. In addition, it needs to be emphasized that the number of sampling sites, in which both concentration and flux of 7 Be and 210 Pb were measured simultaneously, are limited.
We recommend that future studies pay more attention to those areas that are currently undersampled or unsampled to better characterize the expected global variability in the 7 Be and 210 Pb air concentrations and depositional fluxes by measuring both nuclides simultaneously to obtain more data, as well as the 7 Be / 210 Pb ratio, and estimate the deposition velocity of aerosols. In areas with very limited precipitation, such as deserts, it is expected that the dry fallout will dominate the bulk depositional flux, and quantification of the role of dry fallout in the removal of these nuclides will provide insights into the removal of other analog species. As mentioned earlier, combining cosmogenic 7 Be with 210 Pb that predominantly originates from the Earth's surface will be useful to trace species that originate from the Earth's surface, such as Hg, SO 2− 4 , NO − 3 , and those that originate in the upper atmosphere, such as O 3 . The size distribution of aerosol particles carrying 7 Be and 210 Pb is crucial for understanding atmospheric behavior and tracing analogues, and such studies also need to be conducted. Besides, the troposphere contains ∼ 99 % of global water vapor with < 1 % in the stratosphere. The deposition velocity of aerosol in the stratosphere is very low (∼ 4 × 10 −3 cm s −1 ; Junge, 1963) with no precipitation. Thus, the 7 Be concentration is governed by local production, zonal and vertical downward transport, and its decay. In the middle and upper troposphere where precipitation is much less frequent, the removal rate of aerosols is also slow. Collection of air samples in that part of the atmosphere will provide useful information on the total deposition velocity of aerosols .
Finally, we acknowledge that the seasonal data of 7 Be and 210 Pb have not been included in the current version of dataset because compiling the seasonal data is more challenging than compiling the annual data. Unlike the annual data, most of the published seasonal data are presented in graphs, without being given in tables, and in some cases the graph quality was poor, and thus the precision of the data extraction is expected to be poor. Besides, in some papers, although seasonal data were measured, only the annual data were provided. Thus, the comprehensive compilation of seasonal data of 7 Be and 210 Pb may need collaboration with, and data sharing from, the scientific community. The compilation of seasonal data is expected to be useful to assess seasonal variability of 7 Be and 210 Pb and understand the relationship between these changes and influencing factors, such as atmospheric dynamics, meteorological conditions, and geographic location, on a global scale. The seasonal data can also be useful in evaluating the seasonality of transport in global atmospheric models.

Data formats and availability
For clarity and convenience, four separate worksheets, each named as 7 Be or 210 Pb annual air concentration and 7 Be or 210 Pb annual atmospheric flux, are available in one Microsoft Excel ® file, although sometimes these data come from the same article. In addition, the data of the 7 Be / 210 Pb air concentration ratio, 7 Be / 210 Pb depositional flux ratio, and deposition velocities for 7 Be and 210 Pb are also presented. The dataset can be downloaded from Zenodo (https://doi.org/10.5281/zenodo.4785136, Zhang et al., 2021). It is free for scientific applications, but the free availability does not constitute a license to reproduce or publish it. The compilation of seasonal data is ongoing, and the dataset will be updated in a future effort.