Journal of Atmospheric and Solar-Terrestrial Physics 97 (2013) 1–10

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Journal of Atmospheric and Solar-Terrestrial Physics

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7Be in Finland during the 1999–2001 Solar maximumand 2007–2009 Solar minimum

Ari-Pekka Leppanen a,n, Jussi Paatero b

a Radiation and Nuclear Safety Authority, Regional Laboratory in Northern Finland, Lahteentie 2, 96400 Rovaniemi, Finlandb Finnish Meteorological Institute, Observation Services, P.O. Box 503, FI-00101 Helsinki, Finland

a r t i c l e i n f o

Article history:

Received 1 September 2012

Received in revised form

10 December 2012

Accepted 18 January 2013Available online 8 February 2013

Keywords:7Be

Surface air

Deposition

Solar cycle

26/$ - see front matter & 2013 Elsevier Ltd. A

x.doi.org/10.1016/j.jastp.2013.01.007

esponding author. Tel.: þ358 40 7641788.

ail address: ari.leppanen@stuk.fi (A.-P. Leppan

a b s t r a c t

The surface air 7Be concentrations and deposition data were considered for the time periods of 1999–

2001 and 2007–2009. The time period of 1999–2001 was the maximum of the 23rd solar cycle, while

the 2007–2009 time period was the minimum of the 24th solar cycle. The observed mean change in the

surface air 7Be concentrations from 1999–2001 to 2007–2009 varied from �19% to 39%, while in

deposition the change was �7%. This is different and nearly opposite of the calculated increase of

60–100% in 7Be production from solar maximum to solar minimum (solar modulation effect). This

indicates that in Finland atmospheric effects are significantly more important than the solar modula-

tion determining the overall changes in the surface air 7Be concentrations and in deposition. The

surface air 7Be concentration time series was analyzed using a wavelet transform method and an

intraseasonal periodicity with a 45–90-day period was found. This periodicity was intermittent when it

was only observable from April to October and had varying power making it unobservable in some

years. The 45–90-day periodicity was attributed to Meridional Wind Oscillations and/or Artic

Oscillations occurring in stratosphere. Based on the deposition and air concentration data, the 7Be

deposition velocities were calculated and found to be slightly lower in Northern Finland than those in

Southern Finland. The deposition velocities ranged from 0.004 to 0.035 m/s, which were slightly lower

than those in mid-latitudes.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Many countries routinely monitor radioactive substances inambient air within the framework of radiation protection. As aresult, 7Be is routinely monitored in many countries around theworld. The cosmogenic isotope 7Be (half-life of 53.3 days) proved tobe a useful tracer for atmospheric dynamics. After formation in theatmosphere, beryllium is adsorbed on predominantly small sub-micron size aerosols and is transported to the ground level viaatmospheric vertical mixing (Papastefanou and Ioannidou, 1995;Winkler et al., 1998). Accordingly, 7Be has been proposed as anatural atmospheric tracer (Lal and Peters, 1962; Brost et al., 1991;Vecchi and Valli, 1997). The concentrations of cosmogenic nuclidesin ground-level air are, despite production changes, dependent onfour factors: wet scavenging, stratosphere-to-troposphere exchange,vertical transfer in troposphere and horizontal advection (Feelyet al., 1989). About 75% of 7Be is produced in the stratosphere and25% in the upper troposphere in the spallation of atmospheric atomsfrom galactic cosmic rays (Johnson and Viezee, 1981; Usoskin and

ll rights reserved.

en).

Kovaltsov, 2008). The flux of galactic cosmic rays (GCR) near theEarth is modulated by solar magnetic activity. As a result, morecosmic rays are expected around solar minima and vice versa (e.g.,Forbush, 1954; Parker, 1965; Vainio et al., 2009). Therefore, tem-poral variations in atmospheric 7Be measured near the ground levelare potential tracers of tropospheric dynamics, stratosphere–tropo-sphere coupling and cosmic ray flux variations due to solar activity.The 7Be production maximum is in the lower stratosphere (around20 km in altitude) and decreases on moving downwards (e.g., Laland Peters, 1967). Many studies have found connections betweensolar activity and 7Be concentration in near-surface air (e.g., Azahraet al., 2003; Talpos et al., 2005; Yoshimori, 2005; Doering and Akber,2008; Kikuchi et al., 2009).

In addition, connections between air mass transport and surfaceair 7Be concentration have been identified. These include seasonalvariations caused by stratosphere–troposphere exchange (STE),vertical air mass transport and transport from mid-latitudes to polarregions (Feely et al., 1989; Aldahan et al., 2001; Usoskin et al., 2009).The 7Be surface air concentration and deposition variations atintraseasonal scales have been connected to large-scale climaticphenomena (Koch and Mann, 1996; Gerasopoulos et al., 2003;Leppanen et al., 2010, 2012) but observations of intraseasonalvariations are scarce. When Sakurai et al. (2005) and Kikuchi et al.

A.-P. Leppanen, J. Paatero / Journal of Atmospheric and Solar-Terrestrial Physics 97 (2013) 1–102

(2009) collected daily 7Be concentration data in surface air in Japanduring 2000–2001 and 2006–2007, they observed intraseasonalperiodicities between 18 and 36 days which they attributed to solaror atmospheric variations. In earlier studies in Finland, Leppanenand Grinsted (2008) observed a 50–60-day periodicity in powerspectrum analysis of surface air 7Be concentrations in Ivalo wheredata were collected during 1989–2007.

This study concentrates on two time periods 1999–2001 and2007–2009. The first period, 1999–2001, marks the solar max-imum of the 23rd solar cycle when the sun was considered to bevery active. The 2007–2009 time period marks the minimumof 24th solar cycle when the sun was exceptionally calm, thusmaking these two time periods ideal for comparing the solarmodulation effect on 7Be concentrations. The 7Be concentration inthe deposition data was included in this study to observe theeffect of solar modulation on the amount of 7Be in deposition.

Table 1Fallout sampling stations in Finland.

Location Period of data collection used in this study

Helsinki 1999–2001, 2007–2009

Imatra 1999–2001, 2007–2009

Ivalo 1999–2001, 2007–2009

Jokioinen 1999-2001

Jyvaskyla 1999–2001

Kajaani 1999–2001, 2007–2009

Kotka 1999–2001, 2007–2009

Kuopio 2007–2009

Lakiala 1999–2001, 2007–2009

Maarianhamina 1999–2001

Niinisalo 1999–2001

Rovaniemi 1999–2001, 2007–2009

Sodankyla 1999–2001, 2007–2009

Taivalkoski 1999–2001

Vaasa 1999–2001

2. Materials and methods

2.1. Aerosol sample collection

The Finnish Radiation and Nuclear Safety Authority (STUK)maintains and operates an aerosol sampler network in eightstations. These monitoring stations produce data on airborneradioactivity from a daily to a weekly basis. For this study, datafrom the Ivalo, Rovaniemi and Helsinki airborne radioactivitymonitoring stations were used. The objective is to study short-term variations, e.g., at 20–30 day scales observed in previousstudies, in the surface air 7Be concentrations. The sampling ratesvary from 500 m3/h to 900 m3/h. A high volume JL-900 ‘SnowWhite’ aerosol sampler is used in Rovaniemi, an automated‘Cinderella’ sampler in Helsinki and a JL-150 ‘Hunter’ samplerin Ivalo. In Cinderella there is a 3-day cycle where a sample iscollected for 24 h, ‘cooled down’ for another 24 h and finallymeasured for 24 h. In Snow White, there is a 3-hour collectioncycle where the sampler is on for two hours and off for an hour. InSnow White, the filter change and measurements are donemanually, otherwise the procedure is similar to that of Cindrella’swith respect to the sample collection, cooling and measurementtimes when on daily data collection frequency. In Hunter filter ischanged twice a week and filters from one week is combined intoone weekly sample. In Snow White, aerosols are captured bymicrofibre glass Whatman GF/A filters, 570�460 mm2 in size,while in Cinderella Camfill PADS A500 GH-HIY filters,570�285 mm2 in size, were used. The filters have a pore size of122 mm and they have a very high retention capability, about 98%of the aerosol particles being captured (Mattsson et al., 1965).Typical 1-day sample volumes were 14,500 m3 and 16,500 m3 ofair for Rovaniemi and Helsinki, respectively. The measurement ofair volume is done by measuring pressure difference over acalibrated flange in the outflow channel. The air volume uncer-tainty is 2.5%. All filters are measured with a HPGe detector with arelative efficiency of 30–50% for g emitting radionuclides. Themeasured spectra were analyzed using Gamma-99 or Unisampo/Shaman analysis software.

2.2. Deposition sample collection

In conjunction with airborne radioactivity monitoring, radio-active substances in dry and wet deposition are monitored inseveral stations in Finland. Data from all available stations wereused in this study. Wet and dry deposition are monitored usingthe Ritva 300 samplers by Senya Oy. The Ritva sampler collectsdry and wet depositions in a cylindrical shaped funnel with anarea of 0.07 m2. In the bottom of the funnel, there is a hose

leading to the container where the actual fallout is collected. Thecontainer is changed monthly, however, samples from a 3-monthperiod (every quarter of the year) are combined into one sample.The collected water sample is placed into a beaker coated withcellophane and evaporated under infrared lights. The remnantsare then ashed in an oven at 450 1C and then measured forg-emitting radionuclides with a HPGe detector housed inside alead shielding typically for � 72 h.

Table 1 shows the sampling stations and periods of data thatwere used in this study. In 2006, the sampling network wasreduced from 15 to 9 stations: eight operated by STUK and oneoperated by the Finnish Defence Forces. Similar to the airborneradioactivity data, two time periods of deposition data were usedin the periods of 1999–2001 and 2007–2009. As shown in Table 1the number of deposition collection stations was different in1999–2001 period than in 2007–2009. Fig. 1 shows the location ofaerosol and fallout sampling stations on a map: aerosol stationsare indicated with triangles and fallout stations with circles.

The monthly 7Be activity concentrations in precipitation fromthe 15 deposition monitoring stations were extrapolated overFinland by using the inverse distance weighing (IDW) method(Bonham-Carter, 1996). We used weights inversely proportionalto the third power of the distance to strongly enhance thesignificance of the closest observations. A 7Be concentration valuegrid between the latitudes 601N and 701N and the longitudes 201Eand 311E was calculated with a resolution of 0.51 in the north/south direction and 1.01 in the east/west direction. Next, a similargrid was calculated for the monthly precipitation amounts usingobservations from all the available meteorological and hydrolo-gical monitoring stations in Finland, several hundreds alltogether. Finally, a deposition estimate was calculated at eachgrid node by multiplying the 7Be concentration value with theprecipitation amount (Paatero et al., 2010).

3. Results and discussion

3.1. Variation in 7Be production

Cosmogenic isotopes are produced in the spallation reactionsbetween galactic cosmic ray particles and atmospheric atoms. Theflux of the cosmic rays is modulated by solar activity and hencethe concentrations of cosmogenic isotopes at ground level are inantiphase with the solar activity, i.e., higher concentrations areproduced during solar minimum (see e.g., Talpos et al., 2005;Aldahan et al., 2008). Sodankyla Geophysical Observatory (SGO)operates and manages a neutron monitor at Oulu. The neutron

Fig. 2. Sodankyla Geophysical Observatory’s Oulu neutron monitor count rate

data (counts per minute) from 1999–2001 and from 2007–2009. The variations in

the count rate depict the variations in the GCR flux which produce 7Be in the

upper atmosphere. In the figure lower line (red) indicates mean value for 1999–

2001 time period while upper line (blue) indicates mean value for 2007–2009

time period. (For interpretation of the references to color in this figure legend, the

reader is referred to the web version of this article.)

Kotka

Kajaani

Rovaniemi

Ivalo

Arctic Circle

Imatra

Kuopio

Jyväskylä

Vaasa

Maarian-hamina

Lakiala

Niinisalo

Sodankylä

Taivalkoski

Helsinki

Jokioinen

lat 70°

lat 65°

lat 60°

long

3 0°

lon

g25

°

lon

g20

°

Fig. 1. Locations of fallout and aerosol sample collection stations. Aerosol sample

collection stations are indicated with triangles and fallout collection stations with

circles.

A.-P. Leppanen, J. Paatero / Journal of Atmospheric and Solar-Terrestrial Physics 97 (2013) 1–10 3

monitor detects secondary neutrons from nuclear reactions inducedby the GCRs in the atmosphere. This monitor follows the flux of theGRSs that produce 7Be in the atmosphere. The monitor data areshown in Fig. 2, which also depict the changes in 7Be production inthe upper atmosphere. In previous studies, the CRAC:7Be model(CRAC¼cosmic rays, atmospheric cascade) has been used to calcu-late 7Be production in the atmosphere from GCR and it was foundthat the production is dependent not only on the solar cycle, but alsoon latitude and atmospheric depth (Usoskin and Kovaltsov, 2008).The CRAC:7Be model calculation results have been obtained fromLeppanen et al. (2012) and they are shown in Table 2.

According to Table 2, the increase in 7Be production in the upperatmosphere in polar regions between solar maximum and minimumis 104%. In polar regions, the 7Be production doubles while globallythe increase is � 60%. In theory, the same increase should beobserved in the mean 7Be concentrations in surface air and indeposition from 1999–2001 (solar maximum) to 2007–2009 (solarminimum).

3.2. Precipitation and surface wind conditions during 1999–2001

and 2007–2009

The surface air 7Be concentrations are also strongly modulatedby weather phenomena such as temperature (effects the verticalmixing), precipitation (scavenging of 7Be carrier aerosols) andhumidity (see e.g., Ioannidou, 2011; Pham et al., 2011; PineroGarcıa et al., 2012). The size of the carrier aerosols varies withhumidity, in humid conditions the carrier aerosols are larger

compared to those in low humidity conditions (Ioannidou, 2011).The large-scale atmospheric and ocean circulations are known toaffect 10Be deposition and surface air 7Be concentrations and 10Beconcentrations in deposition measured from ice cores (Velascoand Mendoza, 2008; Leppanen et al., 2010, 2012). Table 3 showsthe statistics on rain from Helsinki, Rovaniemi and Ivalo during1999–2001 and 2007–2009.

As it can be seen from the Table 3, there are clear changes inthe rain conditions. In Helsinki (Southern Finland), there was anincrease in precipitation by 6.3% and an increase in the number ofrainy days by 7.1%. In Northern Finland, the situation is theopposite, in Rovaniemi there was a decrease in the total amountof precipitation by 3.5% and in rainy days by 3.5%. In Ivalo thechanges are strongest were precipitation was decreased by 11.3%and the rainy days by 26.4%. In Ivalo, all years during 2007–2009are systematically lower compared to 1999–2001. In Finlandduring 1999–2001 and 2007–2009, the weather conditions wereclearly different which can be expected to influence surface air7Be concentrations.

Another contributing factor can be the direction of the wind.Fig. 3 shows the wind roses from Helsinki (Harmaja station) andRovaniemi (airport). There were only small but similar differencesin Helsinki and in Rovaniemi. It should be noted that the missingwesterly winds in Rovaniemi are due to a lee too close to theanemometer location. In Helsinki (panels (a) and (c)) in 2007–2009, there is a reduction in the easterlies, particularly in thesoutheast direction, and increase in the westerlies, particularly inthe southwest direction. In Rovaniemi, changes are similar toHelsinki, the reduction in the south or southeast direction is clearbut also there is also a reduction in easterly wind speeds.According to the wind roses, during 1999–2001 there was moreinfluence from air masses originating from continental Russia,while in 2007–2009 there was more influence from marine airfrom the Atlantic Ocean.

3.3. Variations of 7Be in surface air

A distinct seasonal cycle in 7Be concentrations can be observedin Fig. 4 that 7Be concentrations begin to rise in April, and fall inSeptember, indicating a period of stronger atmospheric mixingdue to solar heating between these months. This has beenobserved to increase surface air 7Be concentrations and alter

Table 2Differences in 7Be production between solar maximum and minimum (in units of at/cm2/s). The 7Be production values have been adopted from Leppanen et al. (2012).

Solar activity Polar Global

Troposphere Stratosphere Sum Troposphere Stratosphere Sum

Solar max. 0.016 0.087 0.102 0.018 0.032 0.051

Solar min. 0.026 0.182 0.208 0.025 0.057 0.082

Solar modulation (%) 63 109% 104% 39 78 61

Solar modulation is the increase in 7Be production from solar maximum to solar minimum.

Table 3Statistics on precipitation from Helsinki, Rovaniemi and Ivalo during 1999–2001 and 2007–2009.

Location Year Precipitation (mm) Rainy days (rr40) Year Precipitation (mm) Rainy days (rr40) Diff. (pre./r.d.)

Helsinki 1999 623.4 172 2007 723.0 196 þ6.3%/þ7.1%

2000 706.4 178 2008 786.0 208

2001 708.7 187 2009 658.8 171

Rovaniemi 1999 636.6 221 2007 643.3 220 �7.4%/�3.5%

2000 753.9 217 2008 738.9 217

2001 697.9 212 2009 551.6 190

Ivalo 1999 594.6 212 2007 508.7 166 �11.3%/�26.4%

2000 489.9 195 2008 464.8 148

2001 489.2 191 2009 422.2 126

Diff., change in total values from 1999–2001 to 2007–2009; pre., precipitation; r.d., rainy days.

A.-P. Leppanen, J. Paatero / Journal of Atmospheric and Solar-Terrestrial Physics 97 (2013) 1–104

isotopic ratios such as 7Be/10Be and 7Be/22Na (Bourcier et al.,2011; Aldahan et al., 2001; Leppanen et al., 2012).

Fig. 4 shows surface air 7Be concentrations from Ivalo, Rova-niemi and Helsinki. In Ivalo, surface air samples were collected ona weekly basis, while in Helsinki, air samples were collected on adaily basis. During the International Polar Year (IPY) 2007–2008the sampling frequency of the Rovaniemi station was changedfrom one week to one day in an attempt to observe possibleperiodicities at a scale of 20–30 days. The source areas for 7Be-rich air masses observed in Finland are in Central Russia and inmid-latitudes southwest of Finland, e.g., in Germany and in UK(Paatero and Hatakka, 2000). For Central Europe, Gerasopouloset al. (2001) found a source region for 7Be rich air in the latitudeband 521N–601N over western Europe and the North Atlantic.Kulan et al. (2006) showed that surface air 7Be concentration hasdistinctive meridional dependence where maximum values areobserved at mid-latitudes and lower one at high latitudes. Thereis a strong vertical gradient in 7Be concentrations,where concen-trations in the stratosphere are roughly 100 times higher thanthose at ground level (Jordan et al., 2003; Simon et al., 2009).

Kulan et al. (2006) reported the influence of the 22nd and 23rdsolar cycles on surface air 7Be concentrations between latitudes471N and 681N. In mid-latitudes, the effect of solar modulation tosurface air 7Be concentrations was 30–40% while at higherlatitudes (55–681N) the effect was only 15–20%. The mean surfaceair 7Be concentrations during 1999–2001 and 2007–2009 areshown in Table 4. The differences shown in Table 4 are calculatedfrom the geometric mean. The Ivalo data were the only ones thatshowed increase in the mean surface air concentrations. TheRovaniemi data showed a 19% decrease and the Helsinki datashowed 1% decrease. According to Table 2 the calculated increasein the production was 60–100%. The removal of 7Be from surfaceair via scavenging can partly explain the 1% decrease in Helsinkidata since during 2007–2009 there was more rain and rainy dayscompared to 1999–2001. The discussion related to precipitationduring the observed time periods is shown in Section 3.2. The 39%increase in Ivalo data is the only station which is comparable tofindings of Kulan et al. (2006). Compared to 1999–2001, during2007–2009 there was significantly less rain and rainy days during

which can partly explain the relatively high increase in the 7Beconcentrations. The lack of rain and rainy days may be due toincreased influence of continental or Arctic air masses. The 19%decrease in Rovaniemi data cannot be explained by rain andnumber of rainy days since they are relatively constant between1999–2001 and 2007–2009. However, as shown in Fig. 3 during2007–2009 there is a reduction in the easterly winds which bring7Be rich air from the Eurasian continent and this can partlyexplain the reduction. Clearly, the surface air 7Be concentrationsin Finland are mainly determined by atmospheric effects whilesolar modulation plays a small or an insignificant role. Theatmospheric effects include variations in the large-scale atmo-spheric circulation, vertical transport of the air, the amount ofprecipitation, etc. The difference in the origin of air massesinfluencing the stations may explain differences between differ-ent stations. The Ivalo station, being the most northern one, ismost sensitive to air masses originating from the Arctic and polarregion, while Helsinki is the most sensitive to air masses originat-ing from Central Europe and Continental Russia. Based on obser-vations in this study the surface air 7Be concentrations in highlatitudes are highly dependent on atmospheric effects rather thansolar modulation.

3.3.1. Intraseasonal periodicities in 7Be concentrations

Wavelet analysis was used to analyze periodicities from thetime series shown in Fig. 4. The method described in Grinstedet al. (2004) was used. Before wavelet analysis, the data were de-trended for the annual period, normalized, and the time step ofthe data was averaged as required. Several different waveletanalyses were with varying damping parameters (o) and withvarying time steps. Fig. 5 shows the results of wavelet analysiswhere 8-day time step and o¼ 6 was used. As can be seen fromFig. 5, an intraseasonal and an intermittent periodicity with aperiodicity between 45 and 90 days with varying strength wasobserved in all three stations. In the Ivalo data, this periodicityduring solar maximum was weak in 1999, strong in 2000 andagain weaker in 2001, while during the 2007–2009 solar mini-mum it was totally absent. The Rovaniemi data show that during

Fig. 3. Windroses from Helsinki and Rovaniemi from 1999–2001 and 2007–2009. Panel (a) Helsinki 1999–2001, panel (b) Rovaniemi 1999–2001, (c) Helsinki 2007–2009

and panel (d) Rovaniemi 2007–2009.

Fig. 4. Concentrations of 7Be in surface air in Helsinki, Rovaniemi and Ivalo during

1999–2001 solar maximum and during 2007–2009 solar minimum.

Table 4Observed surface air 7Be concentrations in Ivalo, Rovaniemi and Helsinki. The

difference is calculated using geometric mean values.

Time period Ivalo (a.m./g.m.) Rovaniemi (a.m./g.m.) Helsinki (a.m./g.m.)

1999–2001 1570/1410 1740/1620 2130/1810

2007–2009 2120/1960 1450/1320 2130/1790

Diff. 39% �19% �1%

a.m., arithmetic mean; g.m., geometric mean.

diff., difference in surface air 7Be concentrations from 1999–2001 to 2007–2009.

A.-P. Leppanen, J. Paatero / Journal of Atmospheric and Solar-Terrestrial Physics 97 (2013) 1–10 5

solar maximum the periodicity was present in 1999 and 2000 butabsent in 2001, while during solar minimum it was present in2007, 2008 and 2009. In the Helsinki data, the intraseasonalvariability is less pronounced and only visible during solar

minimum in 2007 and 2009. The periodicity of this variation isnot well defined; the Ivalo 1999–2001 data suggest periodicitiesbetween 60 and 90 days, while the Rovaniemi data, for the sametime period, suggest 45–80 day periodicities. During 2007–2009,the Rovaniemi and Helsinki data suggest periodicities between 50and 80 days. The observed intraseasonal variations were typicallyvisible from April until late October when vertical mixing of theatmosphere is stronger, and were absent during winter, whenatmospheric mixing was weak. This implies that the cause of thisperiodicity lies higher in the atmosphere. The spatial dependenceof the intraseasonal oscillation, where the data from Rovaniemiwere most consistent, may be due to the different source regions

Fig. 5. Wavelet analysis of surface air 7Be concentrations in Ivalo, Rovaniemi and Helsinki during solar maximum of 1999–2001 and solar minimum 2007–2009. An

intermittent intraseasonal 45–90 day periodicity with varying strength was observed in all three data sets.

A.-P. Leppanen, J. Paatero / Journal of Atmospheric and Solar-Terrestrial Physics 97 (2013) 1–106

of surface air. The Ivalo station is most sensitive to air originatingfrom Arctic regions, while Helsinki is sensitive to air massesoriginating from southwest of Finland and Central Russia.

Sakurai et al. (2005) performed a similar study to this study byobserving daily surface air 7Be concentrations in Japan in 2000–2001 during the 23rd solar cycle maximum. They observed aperiodicity of 26 days, which they assigned to the solar rotationalperiod, but also two atmospheric periodicities, 18 days and 30days. Kikuchi et al. (2009) also studied daily 7Be concentrations inJapan and observed a 36-day period that occurred between springand summer in 2006 and 2007. This was accounted for bystratosphere–troposphere exchange. None of these periodicitieswere observed in this study.

Singh et al. (2012) analyzed cosmic ray intensities observed bythe Oulu neutron monitor data from 1996 to 2008 and foundintraseasonal periodicities at 27, � 66, � 144 and � 262 days.The 66-day periodicity is similar to that observed in this study,however, one could also expect to observe 144- and 262-dayperiodicities too if this period is of cosmic ray origin. Both 66-dayand 144-day (Reiger) periods had maximum power during solarmaximum and in the declining phase of the solar cycle. However,the 144-day Reiger period has nearly double the power of the 66-day period. Only the Ivalo data follow the temporal behaviour ofthe 66-day and 144-day periodicities found in the cosmic rayintensities; there must also be a strong atmospheric componentinvolved. In addition, by comparing Figs. 2 and 5, the observedintraseasonal variation seems to be independent of the solar

cycle; there is no clear or systematic strengthening or weakeningin the periodicity between solar maximum and minimum.

Atmospheric intraseasonal oscillations with a period of20–100 days are typically observed at equatorial regions, e.g., inzonal winds Guharay and Sekar (2012) but they have also beenobserved in polar mesospheric clouds (Bailey et al., 2005). In theNorthern Hemisphere, there are two important modes of oscilla-tions with periods of 23 and 48 days from which the 48-dayperiod is the most important having both standing and travellingcomponents (Ghil and Kingtse, 1991). In a recent study, Studeret al. (2012) observed intraseasonal periodicities with a 10–60day period in stratospheric ozone above Switzerland. In addition,Studer et al. (2012) studied the North Atlantic Oscillation (NAO)index and found a broad spectral peak at 30 days and also aweaker peak at 80 days. It has been shown that surface air 7Beconcentrations inversely follow the NAO index in Scandinavia atinterannual time scales (Leppanen et al., 2010, 2012). If the45–90-day periodicities found in this study were caused byNAO, one would expect to find a much stronger 30-day periodi-city, but it is absent in the wavelet analysis as seen in Fig. 5. Awavelet coherence analysis was done in order to find coherencewith the daily NAO index and daily surface air 7Be concentrationsin Rovaniemi and Helsinki, but no coherence was found atinterannual time scales.

In the numerical spectral simulations of Mayr et al. (2003)meridional wind oscillations (MWO) with periods of about twomonths were observed to be generated by momentum deposition

Fig. 7. Cumulative 7Be deposition map, during 2007–2009.

Table 5Statistics concerning the gridded 7Be deposition values (Bq/m2/yr) in Finland.

Parameter 1999–2001 2007–2009

Minimum 483 408

P10 637 583

P25 740 755

Median 911 952

P75 1041 1073

P90 1486 1174

Maximum 3932 1302

Arith. mean 1019 908

A.-P. Leppanen, J. Paatero / Journal of Atmospheric and Solar-Terrestrial Physics 97 (2013) 1–10 7

from small-scale gravity waves propagating north or south. In amore recent study, Mayr et al. (2009) found individual oscillationswith periods between 1.7 and 3 months that represent, in part,global oscillations. In addition to MWO, Northern Annular Mode(or Arctic oscillation) has also a period of 40–50 days at the altitudeof 20–30 km during the months of May–July (Baldwin andDunkerton, 2005). The periods of the AO and MWO agree withthe periodicity of 45–90 days found in 7Be concentrations inthis study (see Fig. 5). Baldwin and Dunkerton (2005) (and thereferences therein) discussed the coupling of stratosphere andtroposphere. Perturbations originating in the tropical upper strato-sphere may be transmitted to higher latitudes and to loweraltitudes by the dynamical mechanism of wave forcing. Waveforcing and induced mean meridional circulation in the lowerstratosphere are responsible for perhaps half of the surfacepressure signal (Baldwin and Dunkerton, 2005). The maximum7Be production from GCRs is calculated to occur in the polarstratosphere (10 km and above), however, the maximum surfaceair concentrations are observed at mid-latitudes (Kulan et al.,2006; Usoskin and Kovaltsov, 2008; Usoskin et al., 2009). In thestratosphere, the 7Be concentrations are roughly two orders ofmagnitude higher than the surface air concentrations (Jordan et al.,2003; Simon et al., 2009). According to Heikkila et al. (2009) 65% ofthe 10Be deposition in the polar regions are of stratospheric origin.If we assume that 7Be � 10Be, and 65% of 7Be in Finland is ofstratospheric origin, then oscillations occurring in the stratosphereshould be visible in surface air 7Be concentrations. The 45–90-dayperiodicities in 7Be concentrations observed in this study are mostlikely caused by stratospheric MWO and/or AO related oscillations.

Geom. mean 942 877

3.4. Variations of 7Be in deposition

In Figs. 6 and 7, the regional deposition pattern of 7Be in Finlandpresents a negative gradient northwards. This can be explained bytwo factors. Firstly, the annual amount of precipitation in Finlanddecreases towards the north (Pirinen et al., 2012). Most of theairborne 7Be is attached to accumulation mode aerosol particles,and wet deposition is the dominant mechanism for removingparticles of this size range from the atmosphere (Winkler et al.,1998). Secondly, the highest activity concentrations of 7Be in theair in Finland are associated with air masses coming from CentralRussia and, especially in spring, from southwest of Finland due tothe transfer of stratospheric air masses through the tropopausealong the polar front between the 40th and 50th latitudes (Paatero,2000). Thus, the influence of these 7Be source areas decreases asthe distance from them increases.

The statistics concerning the gridded 7Be deposition values(Table 5) indicate that depending on the location in Finland, the

Fig. 6. Cumulative 7Be deposition map, during 1999–2001.

annual 7Be deposition can vary during solar maximum by a factorof eight, from 483 to 3932 Bq/m2. The corresponding figures forthe solar minimum are a factor of three with a range from 408to 1302 Bq/m2. Even though the maximum deposition varies asmuch as by a factor of three from solar maximum to solarminimum, the mean values differ only by about 10%. From1999–2001 to 2007–2009 the geometric mean value is reducedby 7%. Similar to surface air 7Be concentrations, less 7Be wasdeposited during solar minimum than solar maximum. Thevariation in the surface air 7Be concentrations (see Section 3.3)and its deposition is of the same order of magnitude. However,the reduction in surface air 7Be concentrations and in 7Bedeposition is the opposite than what could expected from thesolar modulation and from previous studies. As shown in Section3.1 and Table 2 there should be a clear increase in deposition. Thisindicates that a major factor determining the amount of 7Be indeposition is also atmospheric effects instead of the productionrate variations due to the solar cycle. The atmospheric effectsinclude variations in the large-scale atmospheric circulation,vertical transport of the air, the amount of precipitation, etc.The amount of precipitation affects the scavenging of 7Be from airand thus to the amount of deposition. For information aboutprecipitation during 1999–2001 and 2007–2009 see Table 3.

The average annual 7Be deposition in Finland during 1999–2001 was 1019 Bq/m2 and during 2007–2009 was 908 Bq/m2.Finland consists of an area of 338,424 km2, which means thatduring 1999–2001 3.4�1014 Bq of 7Be was deposited annually.During the solar minimum of 2007–2009, the 7Be deposition of3.1�1014 Bq per year occurred. For comparison, the total 90Srdeposition in Finland due to the 1986 Chernobyl accident was5.3�1013 Bq (Paatero et al., 2010). According to the Table 2, the7Be production rate in polar regions during solar minimum is0.208 at/cm2/s which translates into an annual 7Be production of9890 Bq/m2. When this is compared with the observed average

Table 67Be deposition and rainfall in various locations. For comparison of rainfall in Finland see Table 3.

Location 7Be deposition (Bq/m2/yr) Avg. rainfall (mm/yr) Reference

New Haven, CT, USA 3783 1386 Turekian et al. (1983)

Bermuda 2850 1699 Turekian et al. (1983)

Norfolk, VA, USA 2000–2150 1315 Todd et al. (1989)

Solomons, MD, USA 2267 960 Dibb (1989)

Hokitika, New Zealand 6350 2800 Harvey and Matthews (1989)

Thessaloniki, Greece 483–841 479 Papastefanou and Ioannidou (1991)

Canberra, Australia 1030 660 Wallbrink and Murray (1994)

Galveston, TX, USA 2000–3937 1235 Baskaran (1995)

Malaga, Spain 188–585 308 Duenas et al. (2002)

Rokkasho, Japan 2160–3300 1541 Akata et al. (2008)

Brisbane, Australia 1070–1362 857 Doering and Akber (2008)

Samarkand, Uzbekistan 216–1200 444 Azimov et al. (2011)

Niteroi, Brazil 498 865 Sanders et al. (2011)

Table 7Mean 7Be fluxes and activity concentrations in air during solar maximum 1999–

2001 and solar minimum 2007–2009. The 7Be deposition velocities are calculated

according to flux and activity concentration values.

Period Rovaniemi/Helsinki

Mean 7Be flux

(F, mBq=m2=s)

Mean 7Be in air

(A, mBq=m3)

Velocity

(VD , m/s)

January–March

1999–2001

6.2/24 1670/1690 0.005/0.014

April–June

1999–2001

46/48 2210/2640 0.021/0.019

July–September

1999–2001

36/45 1830/2630 0.020/0.017

September–December

1999–2001

12/46 1190/1470 0.010/0.032

January–March

2007–2009

17/25 1290/1550 0.013/0.016

April–June

2007–2009

34/43 1800/3070 0.019/0.014

July–September

2007–2009

25/53 1700/2610 0.016/0.021

September–December

2007–2009

20/46 920/1310 0.022/0.035

Fig. 8. Mean 7Be deposition velocities in Helsinki and in Rovaniemi per each

quarter of the year.

A.-P. Leppanen, J. Paatero / Journal of Atmospheric and Solar-Terrestrial Physics 97 (2013) 1–108

annual deposition of 908 Bq/m2 (2007–2009), the observed aver-age annual 7Be deposition is only 9% of the 7Be produced aboveFinland. The annual 7Be deposition in Finland is of the same orderas values observed in other countries when taking into considera-tion the amount of precipitation, see Table 6. The average amountof precipitation decreases from 750 mm/yr in Southern Finland to450 mm/yr in the Northern Finland (Pirinen et al., 2012).

3.5. Deposition velocity

The deposition velocity can be calculated using the followingequation:

VD ¼F ðBq=m2=sÞ

Aair ðBq=m3Þ, ð1Þ

where VD is the deposition velocity (m/s), F is the total depositionflux (Bq/m2/s) and Aair is the mean activity concentration insurface air (Bq/m3).

The deposition velocities were calculated for Helsinki andRovaniemi stations according to Eq. (1). The F can be obtainedfrom the fallout measurements done during 2007–2009. Table 7shows the mean 7Be flux calculated from deposition measure-ments where sampling frequency is three months, as mentionedin Section 2.2. The mean three-month 7Be concentration in sur-face air was obtained from measurements shown in Fig. 4.

The mean 7Be deposition velocities during 1999–2001 and2007–2009 were 0.021/0.014 m/s and 0.022/0.017 m/s, for Hel-sinki and Rovaniemi, respectively. Table 7 shows the depositionfluxes and velocities in Rovaniemi and in Helsinki during 1999–2001 and 2007–2009. The mean deposition velocities were lowerin both time periods in Northern Finland than in SouthernFinland, which is due to the smaller amount of rainfall in North-ern Finland.

Fig. 8 shows the average deposition velocities in Helsinki and inRovaniemi per quarter year. In the whole country, the depositionvelocities were in the range of 0.01–0.04 m/s. The 7Be depositionvelocities in Finland were systematically slightly lower than thedeposition velocities observed in Monaco in mid-latitudes (Phamet al., 2011). The difference in deposition velocities between Monacoand Finland was larger during winter and smaller during summer.In Greece, the average annual deposition velocity of 7Be variedbetween 0.003 m/s and 0.008 m/s (Papastefanou and Ioannidou,1995). These values are lower than the results of this study. This canbe explained by the relatively high atmospheric concentration of 7Bearound the 40th latitude and a lower amount of precipitation.

Monthly average deposition velocities of total beta activity(� 210Pb) ranged between 0.006 m/s and 0.022 m/s in Helsinki,and 0.0025 m/s and 0.028 m/s in Ivalo (Paatero and Hatakka,1997). The seasonal variation of the total deposition velocities

showed a clear maximum in summer in Ivalo. In Helsinki, theseasonal variation was much less pronounced. These are lowervalues compared with 7Be deposition velocity values observed inthis study and can be explained by the different sources of 210Pb

A.-P. Leppanen, J. Paatero / Journal of Atmospheric and Solar-Terrestrial Physics 97 (2013) 1–10 9

and thus opposite height profiles between 210Pb and 7Be concen-trations. A significant part of 7Be is produced in the upper tropo-sphere where it is within or close to rain-forming layers of theatmosphere. For the same reason, the 7Be concentration in the airdecreases from the upper troposphere towards ground level. 210Pband its predecessor 222Rn, on the other hand, have to be trans-ported from ground level through the atmospheric boundary layerto the free troposphere to be incorporated in the precipitation.

4. Conclusions

This study concentrated on 7Be concentrations in surface airand in deposition during the maximum of the 23rd solar cycle in1999–2001 and during the exceptionally calm solar minimum inthe beginning of the 24th solar cycle in 2007–2009. The solarcycle is known to modulate 7Be production in the upper atmo-sphere where the 7Be production is in anti-phase with the solarcycle. This has been shown by many calculations and observa-tions. In this study, there was an enhancement of �19 to 39% inthe surface air 7Be concentrations from 1999–2001 to 2007–2009,i.e., from minimum production (solar maximum) to maximumproduction (solar minimum). The Rovaniemi and Helsinki surfaceair data showed different behaviour than the 60–100% increase inthe calculated production. Thus, the factors affecting 7Be surfaceair concentrations in Finland are mainly of atmospheric origin andthe observed differences in 7Be concentrations in surface air aremainly caused by the different climate/weather patterns duringthe time of observations.

In this study, we did not observe the 20–30 day periodicitywhich has been observed in surface air 7Be concentrations inother countries in previous studies. However, a periodicity of45–90-days was observed in Ivalo, Rovaniemi and Helsinki datasets. This periodicity was intermittent and visible only from Apriluntil October; it had a varying strength and it was not visibleevery year in all data sets. The cause of the periodicity wasthought to be the oscillations in the stratosphere possible inducedby the to MWO propagating northwards in stratosphere and/or bythe AO.

In deposition, the 1999–2001 and 2007–2009 time periodspresented spatially different fallout patterns. However, a decreaseof 7% was observed in the total deposition from 1999–2001 to2007–2009. Similar to surface air concentrations, the decreasein deposition from 1999–2001 to 2007–2009 is opposite thanexpected from the solar modulation effect. Higher concentrationsshould have been observed solar minimum, i.e., during 2007–2009.Thus, factors affecting 7Be deposition in Finland are mainly ofatmospheric origin. During 1999–2001 there were more easterlywinds compared to 2007–2009 in Helsinki and in Rovaniemi.Generally, the easterlies bring 7Be rich air from Russia. Both timeperiods, 1999–2001 and 2007–2009, showed different spatiallydistributed 7Be deposition due to different transport patterns of airmasses and/or precipitation patterns. There were changes in theamount of precipitation and in the number of rainy days from1999–2001 to 2007–2009 so that in Helsinki precipitation andnumber of rainy days increased while in Ivalo there was significantdecreases in precipitation and in number of rainy days. Duringboth time periods less 7Be was deposited in Northern Finland thanin Southern Finland. This is due to smaller 7Be concentrations inthe air and due to smaller amount of precipitation.

The observed deposition velocities ranged from 0.003 to 0.035 m/swhere slightly higher velocities were observed in Southern Finland.There was also a weak seasonal pattern were the highest velocitieswere observed during the months of October–December. On average,the deposition velocities observed in Finland were smaller comparedto the deposition velocities in mid-latitudes in the Mediterranean

region. Even though 7Be deposition has been measured for severaldecades in Finland, this is the first quantitative analysis of the regionaland temporal variation of 7Be deposition in Finland. The data mayfind use in, e.g., soil erosion studies (Mabit et al., 2008).

Acknowledgements

The authors acknowledge the help of the staffs of STUK’sRegional Laboratory in Northern Finland and the LaboratoryEnvironmental Surveillance and Emergency Preparedness forcollecting, preparing and providing the surface air and deposition7Be data used in this study. The help of M.Sc. Murat Buyukay ofthe Finnish Meteorological Institute for the 7Be fallout analysisgreatly appreciated. The staff of the Sodankyla GeophysicalObservatory is acknowledged for making the Oulu neutronmonitor data available at http://cosmicrays.oulu.fi. And finally,the comments of the two anonymous reviewers are greatlyappreciated.

Appendix A. Supplementary materials

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org.10.1016/j.jastp.2013.01.007.

These data include Google map of the most important areasdescribed in this article.

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