ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-17-1061-2017Processes controlling the seasonal variations in 210Pb and 7Be at
the Mt. Cimone WMO-GAW global station, Italy: a model analysisBrattichErikahttps://orcid.org/0000-0001-8402-7611LiuHongyuhongyu.liu-1@nasa.govhttps://orcid.org/0000-0002-2164-6383TosittiLaurahttps://orcid.org/0000-0002-1778-672XConsidineDavid B.CrawfordJames H.Department of Chemistry “G Ciamician”, Alma Mater Studiorum University
of Bologna, 40126 Bologna (BO), ItalyNational Institute of Aerospace, Hampton, VA 23666, USANASA Headquarters, Washington, DC 20546, USANASA Langley Research Center, Hampton, VA 23681, USAHongyu Liu (hongyu.liu-1@nasa.gov)24January20171721061108030June20164August201620December201622December2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/17/1061/2017/acp-17-1061-2017.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/17/1061/2017/acp-17-1061-2017.pdf
We apply the Global Modeling Initiative (GMI) chemistry and transport model
driven by NASA's MERRA assimilated meteorological data to simulate the
seasonal variations in two radionuclide aerosol tracers (terrigenous
210Pb and cosmogenic 7Be) at the WMO-GAW station of Mt. Cimone
(44∘12′ N, 10∘42′ E; 2165 m a.s.l.; Italy), which is
representative of free-tropospheric conditions most of the year, during 2005
with an aim to understand the roles of transport and precipitation scavenging
processes in controlling their seasonality. The total precipitation field in
the MERRA data set is evaluated with the Global Precipitation Climatology
Project (GPCP) observations, and generally good agreement is found. The
model reproduces reasonably the observed seasonal pattern of 210Pb
concentrations, characterized by a wintertime minimum due to lower 222Rn
emissions and weaker uplift from the boundary layer and summertime maxima
resulting from strong convection over the continent. The observed seasonal
behavior of 7Be concentrations shows a winter minimum, a summer maximum,
and a secondary spring maximum. The model captures the observed 7Be
pattern in winter–spring, which is linked to the larger stratospheric
influence during spring. However, the model tends to underestimate the
observed 7Be concentrations in summer, partially due to the sensitivity
to spatial sampling in the model. Model sensitivity experiments indicate a
dominant role of precipitation scavenging (vs. dry deposition and convection)
in controlling the seasonality of 210Pb and 7Be concentrations at
Mt. Cimone.
Introduction
The use of atmospheric radionuclides to understand atmospheric dynamics,
pollution transport and removal processes has a long history (e.g., Junge,
1963; Reiter et al., 1971; Gäggeler, 1995; Arimoto et al., 1999; Turekian
and Graustein, 2003; WMO-GAW, 2004; Dibb, 2007; Rastogi and Sarin, 2008;
Froehlich and Masarik, 2010; Lozano et al., 2012). It has been recognized
that natural radionuclides are useful in a global monitoring network for
atmospheric composition to support global climate change and air quality
research, and therefore they are measured at many of the regional, global, and
contributing-partner stations in the Global Atmosphere Watch (GAW) network of
the World Meteorological Organization (WMO) (WMO-GAW, 2004). In particular,
terrigenous 210Pb and cosmogenic 7Be natural radionuclides are
helpful in the understanding of the roles of transport and/or scavenging in
controlling the behaviors of radiatively active trace gases and aerosols
(Feichter et al., 1991; Balkanski et al., 1993; Koch et al., 1996), as well
as their anthropogenic (vs. natural) origin (e.g., Graustein and Turekian,
1996; Arimoto et al., 1999; Liu et al., 2004; Cuevas et al., 2013). They are
routinely monitored at WMO-GAW stations around the world (Lee et al., 2004).
Although 210Pb and 7Be have long (1998–2011) been measured at the
Global WMO-GAW station of Mt. Cimone (Italy), their seasonal behavior has not
been thoroughly elucidated (Lee et al., 2007; Tositti et al., 2014). Here we
apply a state-of-the-art global chemistry and transport model (CTM) to the
simulation of 210Pb and 7Be, with an objective to better understand
the roles of transport and precipitation scavenging processes in controlling
their seasonal variations at Mt. Cimone.
Because of their contrasting natural origins, 210Pb and 7Be have
been used as a pair to study the vertical transport and scavenging of
aerosols (Koch et al., 1996). 210Pb (half-life τ1/2=22.3 years)
is the decay daughter of 222Rn (τ1/2=3.8 days), which is
emitted from soils by decay of 226Ra. The oceanic input of 222Rn is
about 2 orders of magnitude less than the continental input and, because of
the continental origin of 222Rn, 210Pb is considered a tracer of
air masses with continental origin (Baskaran, 2011). 7Be (τ1/2=53.3 days) is a cosmogenic radionuclide generated by cosmic ray spallation
reactions with nitrogen and oxygen (Lal et al., 1958). Most
(∼ 67 %) of 7Be is produced in the stratosphere and the
remaining (∼ 33 %) is generated in the troposphere, particularly in
the upper troposphere (Johnson and Viezee, 1981; Usoskin and Kovaltsov,
2008). 7Be is thus considered a tracer of stratospheric influence
(Viezee and Singh, 1980; Dibb et al., 1992, 1994; Liu et al., 2004, 2016) and
subsidence (Feely et al., 1989; Koch et al., 1996; Liu et al., 2004). Once
produced, both radionuclides rapidly attach to ubiquitous submicron aerosol
particles in the ambient air (Papastefanou and Ioannidou, 1995; Winkler et
al., 1998; Gaffney et al., 2004; Ioannidou et al., 2005) and are removed from
the atmosphere mainly by wet and secondarily dry deposition (Kulan et al.,
2006). The concentrations of these radionuclides in surface air thus depend
on their sources, transport, wet and dry removal, and radioactive decay (in
the case of 7Be). Rainfall scavenging processes are generally more
effective on 210Pb than on 7Be concentrations (Koch et al., 1996;
Caillet et al., 2001; Likuku, 2006; Dueñas et al., 2009; Lozano et al.,
2012).
Observational studies have previously been conducted to examine the factors
influencing surface 210Pb and 7Be concentrations in Europe, the
Middle East, and northern Africa. Different synoptic and mesoscale patterns are
associated with the ranges of 210Pb and 7Be activity
concentrations (Lozano et al., 2012, 2013). In southwestern Spain (El
Arenosillo), for instance, low 210Pb values are strongly linked to air
masses from the Atlantic Ocean, whereas the highest values are associated
with air masses clearly under the influence of continents, such as the
Iberian Peninsula and North of Africa (Lozano et al., 2012). As for
7Be, the highest 7Be activity concentrations over the southwestern
Iberian Peninsula are related to the arrival of air masses from middle
latitudes, and in particular from the Canary Islands, the western Mediterranean
Basin, and the north of Africa (Dueñas et al., 2011; Lozano et al.,
2012).
With respect to 210Pb and 7Be spatial variability, 210Pb
concentrations in surface air are strongly dependent on whether it is located
over land or ocean, whereas 7Be concentration is mainly latitudinally
dependent, due to their different production mechanisms. Generally speaking,
in the Northern Hemisphere, higher 7Be concentrations are present at
middle latitudes (20–50∘ N) because of the mixing of stratospheric
air into the upper troposphere along the tropopause discontinuity in
midlatitude regions and subsequent convective mixing within the troposphere,
which brings 7Be-rich air masses into the planetary boundary layer and
to the Earth's surface (Kulan et al., 2006). Lower 7Be concentrations
are towards the pole and towards the Equator (Kulan et al., 2006; Steinmann
et al., 2013).
Many studies examined the seasonal behavior of 210Pb and 7Be at
European midlatitude surface sites (e.g., Cannizzaro et al., 2004; Ioannidou
et al., 2005; Daish et al., 2005; Todorovic et al., 2005; Likuku, 2006;
Dueñas et al., 2009; Pham et al., 2011; Carvalho et al., 2013; Steinmann
et al., 2013). High levels of 210Pb during summer and low levels in
winter were found, reflecting the differing rates of 222Rn emanation
from soil above the European land mass during winter (wet or snow-covered
soil) and summer (dry soil) (Hötzl and Winkler, 1987; Caillet et al.,
2001; Daish et al., 2005; Ioannidou et al., 2005). At low-elevation sites,
monthly 7Be averages are characterized by a well-defined annual cycle
with lower values during winter and higher values during summer. Generally,
the increase in 7Be in ground level air from March to May is ascribed to
the more efficient and higher frequency stratosphere–troposphere exchange
(STE), whereas the further increase in 7Be during summer is due to the
stronger convective mixing and higher tropopause (Ioannidou et al., 2014).
The higher tropopause height is associated with anticyclonic conditions,
which results in downward transport from the upper troposphere and reduced
wet scavenging during these conditions (Gerasopoulos et al., 2001, 2005;
Ioannidou et al., 2014). In fact, compensating subsidence associated with
convective mixing enhances downward transport of 7Be from the upper
troposphere (rather than direct input of stratospheric air) down to the lower
troposphere and ground level (Zanis et al., 1999; Gerasopoulos et al., 2001,
2005; Ioannidou et al., 2005; Likuku, 2006; Steinmann et al., 2013).
High-elevation sites such as Jungfraujoch (Switzerland), Zugspitze (Germany),
and Mt. Cimone (Italy), typically lying above the planetary boundary layer
(PBL), are characterized by lower 210Pb concentrations and higher
7Be due to direct influences of air masses from the free troposphere
(Zanis et al., 2000). The observed seasonal 210Pb pattern at the
high-altitude sites of Puy de Dôme (1465 m a.s.l., France) and Opme
(660 m a.s.l., France) is characterized by maximum concentrations in spring
and autumn and minimum concentrations in winter. This is due to higher radon
emissions during the dry season (summer) than during the wet season (winter),
and lower PBL height during winter (Bourcier et al., 2011). The latter
results in weaker upward transport of 222Rn and 210Pb at
high-altitude sites. Similar to low-elevation sites, higher 7Be values
are observed in summer due to convection-forced exchange with the upper
troposphere and the higher tropopause height, which leads to more efficient
vertical transport from the upper to lower troposphere (Reiter et al., 1983;
Gerasopoulos et al., 2001; Bourcier et al., 2011). At high-altitude sites a
secondary maximum of 7Be during cold months (December–March) is
generally observed and attributed to the increase in
stratosphere-to-troposphere events during this season (e.g., James et al.,
2003; Stohl et al., 2003; Trickl et al., 2010). The higher frequency of rapid
subsidence in winter at Northern Hemisphere midlatitudes can be ascribed to
the intensity of baroclinic systems, which is greatest in the wintertime. In
fact, well-developed tropopause folds and rapid deep intrusions are most
likely to occur in the wake of intense cyclogenesis, usually limited to the
wintertime storm track regions (James et al., 2003).
Numerical models have been used to analyze 210Pb and 7Be
observations at high-elevation sites. One-dimensional model simulations of surface
7Be have shown higher concentrations at high-elevation sites (Jasiulionis
and Wershofen, 2005; Simon et al., 2009) but have also suggested that the
diffusion of 7Be was affected by the seasonal variation in
meteorological conditions. Balkanski et al. (1993) examined the transport of
210Pb in a global 3-D model and reported a weak decrease in 210Pb
concentrations between the continental mixed layer and the free troposphere:
simulated concentrations at 6 km altitude were about 50 % of those in
the continental mixed layer over much of the Northern Hemisphere in summer,
and over large areas of the tropics year around, a result consistent with the
few observations available for the free troposphere at that time (Moore et
al., 1973). Rehfeld and Heimann (1995) compared the 3-D model simulated
seasonal pattern of surface 210Pb and 7Be concentrations with the
observations at several sites in both hemispheres. At Mauna Loa
(19.47∘ N, 155.6∘ W; 3400 m a.s.l.; Hawaii) 210Pb
seasonality was characterized by high concentrations in spring and summer and
lower ones in winter, as opposed to the seasonal pattern found at higher
latitudes, where the 210Pb maximum concentrations in winter are
attributed to the advective transport of 210Pb aerosols from
midlatitudes. This behavior is due to the elevation of the site,
representative of the conditions of the free troposphere rather than those of
the PBL. As for 7Be, the comparison between the model and the
observations at Rexburg (43.8∘ N, 111.83∘ W;
1483 m a.s.l.; USA) showed systematically lower model values, due to the
much higher precipitation rates in the model.
Previous studies have examined surface 7Be observations at Mt. Cimone
with respect to the role of STE in surface ozone increases (Bonasoni et al.,
1999, 2000a, b; Cristofanelli et al., 2003, 2006, 2009a, 2015; Lee et al.,
2007) within the framework of European projects such as VOTALP (Vertical
Ozone Transport in the Alps) and STACCATO (influence of
Stratosphere-Troposphere exchange in A Changing Climate on Atmospheric
Transport and Oxidation capacity). These studies led to the assessment of a
higher incidence of STE events during the period from October to February
relative to the warm season, when thermal convection and the rising of the
tropopause promote vertical mixing, which acts as a confounding factor in STE
detection. Lee et al. (2007) and Tositti et al. (2014) reported the seasonal
patterns and frequency distributions of 210Pb and 7Be measured at
Mt. Cimone, and highlighted higher concentrations of both radionuclides in
the summertime due to the higher mixing height and horizontal transport by
regional airflows. During winter, a general increase in 7Be is
associated with a decrease in 210Pb, due to the dominant effect of STE
and subsidence in the free troposphere. At the time of this work, no model
analyses of 210Pb and 7Be observations at the site have been
conducted.
In this paper, we conduct simulations of 210Pb and 7Be at
Mt. Cimone with a state-of-the-art global 3-D chemistry and transport model
(GMI CTM) driven by assimilated meteorological fields for the year of 2005.
Our objectives are a better elucidation of the seasonal variations in
210Pb and 7Be concentrations and an improved understanding of the
roles of transport and precipitation scavenging processes in their
seasonalities at Mt. Cimone.
The remainder of this paper is organized as follows.
Section 2 describes the measurement site, the radioactivity
measurements at Mt. Cimone, and the GMI CTM. Section 3 evaluates the model
performance in reproducing the observed wind and precipitation fields.
Section 4 evaluates the seasonal 210Pb and 7Be concentrations in
the model with those observed. Section 5 examines the sources and seasonal
variations in the simulated radionuclide activities, followed by a summary
and conclusions in Sect. 6.
Data and methodsRadionuclide measurements at Mt. Cimone
Mt. Cimone station (44∘12′ N, 10∘42′ E,
2165 m a.s.l.) is a global WMO-GAW station managed by the Meteorological
Office of the Italian Air Force, which hosts the research platform “Ottavio
Vittori” of the Institute of Atmospheric and Climate Science of the National
Council of Research (ISAC-CNR). The station is located on top of the highest
peak of the Italian Northern Apennines, with a 360∘ free horizon and
an elevation such that the station lies above the PBL during most of the
year: the Mt. Cimone measurements are considered representative of the
southern Europe/Mediterranean free troposphere (Bonasoni et al., 2000a;
Fischer et al., 2003; Cristofanelli et al., 2007), although during the warmer
months an influence of PBL air can be detected due both to convective
processes and to mountain/valley breeze regimes (Fischer et al., 2003; van
Dingenen et al., 2005; Tositti et al., 2013). Note in this framework that
southern Europe and the Mediterranean Basin are considered a hot-spot region
in terms of both climate change (e.g., Forster et al., 2007) and air quality
(Monks et al., 2009), as well as a major crossroad of different air mass
transport processes (Li et al., 2001; Lelieveld et al., 2002; Millàn et
al., 2006; Duncan et al., 2008; Tositti et al., 2013).
At Mt. Cimone station, 210Pb, 7Be, and aerosol mass load in the
form of PM10 have been regularly measured in the period of 1998–2011
with a Thermo Environmental PM10 high-volume sampler. PM10 is
sampled on rectangular glass fiber filters (Whatman, 20.3 cm × 25.4 cm, with an effective exposure area of about 407 cm2), which
were manually changed every 2–3 days, depending on weather conditions,
failures of the sampling equipment, and/or of the power supply and personnel
on site. The average flow rate was about 1.13 m3 min-1 at
standard temperature and pressure, with an average volume of air
collected on each filter equal to 3000–4000 m3 (about 48 h of
sampling, 115–175 samples per year).
Airborne radionuclides travel attached to particulate matters and, as a
consequence of their physical origin, tend to populate the fine fraction
(< 1.0 µm) (Winkler et al., 1998; Gaffney et al., 2004). The
PM10 samples were subjected to non-destructive high-resolution γ spectrometry for the determination of airborne radiotracers 210Pb and
7Be. The characteristics of the two hyper-pure germanium crystal
detectors (HPGe) detectors are as follows: one p-type coaxial detector by
Ortec/Ametek with a relative efficiency of 32.5 % and full width at half maximum (FWHM) 1.8 at
1332 keV and one planar DSG detector with an active surface of
1500 mm2 and FWHM of 0.73 at 122 keV, for higher and lower energy ranges
(100–2000 and 0–900 keV), respectively.
Spectra were accumulated for at least 1 day to optimize peak analysis and
then processed with a specific software package (GammaVision-32, version
6.07, Ortec). Efficiency calibration was determined on both detectors with a
blank glass fiber filter traced with accurately weighted aliquots of a
standard solution of mixed radionuclides (QCY48, Amersham) supplemented with
210Pb, homogeneously dispersed dropwise over the filter surface. Once
dried under a hood under ambient conditions, the calibration filter was
folded into a polystyrene container in the same geometry as the unknown
samples. Quantitative analysis on samples was carried out by subtracting the
spectrum of a blank filter in the same geometry, while uncertainty on peaks
(k=1, 68 % level of confidence) was calculated propagating the
combined error over the efficiency fit previously determined with the
counting error. Minimum detectable activity was calculated making use of the
traditional ORTEC method (ORTEC, 2003) with a peak cut-off limit of 40 %.
Activity data were corrected to the midpoint of the time interval of
collection and for the decay during spectrum acquisition. For our analysis,
we used monthly averages of 210Pb and 7Be data at Mt. Cimone in
2005.
GMI model
The Global Modeling Initiative (GMI, http://gmi.gsfc.nasa.gov) is a
NASA-funded project aiming at improving assessments of anthropogenic
perturbations to the Earth system; in this framework, a CTM appropriate for
stratospheric assessments was developed (Rotman et al., 2001). It was firstly
used to evaluate the potential effects of stratospheric aircraft on the
global stratosphere (Kinnison et al., 2001) and on the Antarctic lower
stratosphere (Considine et al., 2000). The recent version of the GMI CTM
includes a full treatment of both stratospheric and tropospheric
photochemical and physical processes and is also capable of simulating
atmospheric radionuclides 222Rn, 210Pb, 7Be, and 10Be
throughout the troposphere and stratosphere (Considine et al., 2004, 2005;
Rodriguez et al., 2004; Liu et al., 2016). Details of the model are described
in Duncan et al. (2007, 2008), Strahan et al. (2007), and Considine et
al. (2008).
In this work, we simulate 222Rn, 210Pb, 7Be, and 10Be
using a version of the GMI model with the same basic structure as described
by Considine et al. (2005) and Liu et al. (2016), including parameterizations
of the important tropospheric physical processes such as convection, wet
scavenging, dry deposition, and planetary boundary layer mixing.
Meteorological data used to drive the CTM at 2∘ latitude by
2.5∘ longitude resolution, e.g., horizontal winds, convective mass
fluxes, and precipitation fields, are the Modern-Era Retrospective analysis
for Research and Applications (MERRA) assimilated data set from the NASA
Global Modeling and Assimilation Office (GMAO) (Rienecker et al., 2011).
The flux-form semi-Lagrangian advection scheme and a convective transport
algorithm from the CONVTRAN routine in NCAR CCM3 physics package are used in
the model. The wet deposition scheme is that of Liu et al. (2001): it
includes scavenging in wet convective updrafts, as well as first-order rainout and
washout from both convective anvils and large-scale precipitations. The
gravitational settling effect of cloud ice particles included in Liu et
al. (2001) is not considered here. Dry deposition of aerosols is computed
using the resistance-in-series approach. For the simulations of
radionuclides, each simulation was run for 6 years, recycling the MERRA
meteorological data for 2005, to equilibrate the lower stratosphere as well
as the troposphere (Liu et al., 2001). The sixth-year output was used for
analysis.
A uniform 222Rn emission of 1.0 atom cm-2 s-1 from land
under nonfreezing conditions is assumed (Liu et al., 2001). Following Jacob
and Prather (1990), the flux is reduced by a factor of 3 under freezing
conditions. The flux from oceans and ice is null. Although a large
variability in 222Rn emission from land is observed, the above emission
estimate is thought to be accurate to within 25 % globally (Turekian et
al., 1977) and to within a factor of 2 regionally (Wilkening et al., 1975;
Schery et al., 1989; Graustein and Turekian, 1990; Nazaroff, 1992; Liu et
al., 2001).
Following Brost et al. (1991) and Koch et al. (1996), we used the Lal and
Peters (1967) 7Be source for 1958 (solar maximum year), as it best
simulated stratospheric 7Be concentrations measured from aircraft (Liu
et al., 2001). The rates of 7Be production reported more recently by
Usoskin and Kovaltsov (2008) broadly agree with those of Lal and
Peters (1967) with slightly (about 25 %) lower global production rate and
will be tested in a separate model study. The Lal and Peters (1967) source is
represented as a function of latitude and altitude (pressure) and does not
vary with season (see Fig. 1 of Koch et al., 1996). No interannual
variability in the 7Be source is considered in the model (Liu et al.,
2001). This may lead to an underestimate of tropospheric 7Be
concentrations, especially at high latitudes during a solar minimum (or near
minimum) year. Lal and Peters (1967) reported that the relative amplitude of
the 7Be production rate over a 11-year solar cycle is about 13 %
below 300 hPa at latitudes above 45∘.
Because of the coarse horizontal resolution of the model (2∘ latitude
by 2.5∘ longitude), the model representation of the topography at the
site is poor. The elevation of Mt. Cimone in the model is only 298 m,
whereas in reality the mountain is 2165 m (a.s.l.) high (Fig. 1). For this
reason, the model output was not sampled at ground level but instead at the grid box
corresponding to the elevation of the site. In order to see the sensitivity
of model–observation comparisons to spatial sampling, the model was sampled
not only for the grid corresponding to the latitude and longitude of
Mt. Cimone but also for the eight adjacent grids. To better understand the
sources and seasonality of radiotracers in the model, we examine model output
not only for 210Pb, 7Be, and their ratio (7Be /210Pb, an indicator of vertical transport; Koch et al., 1996), which can be
directly compared to the measurements taken at Mt. Cimone, but also for other
radiotracers and quantities, e.g., 222Rn and 10Be /7Be
(a STE tracer; Zanis et al., 2003).
Year 2005 was chosen for analysis because of the availability of the
observational data and model output at the time of this work. As discussed
later, the seasonal behavior of 210Pb and 7Be radionuclides during
year 2005 was “typical” for Mt. Cimone. Monthly averages of 210Pb and
7Be data at Mt. Cimone were calculated for comparison with model
results. To better compare the seasonalities of 210Pb and 7Be
between the model and the observations, monthly percentage deviations from the annual mean concentration
were also calculated.
Surface elevations (km) in the model. The white dot indicates the
location of Mt. Cimone (44∘12′ N, 10∘42′ E;
2165 m a.s.l.).
Seasonal variations in transport and precipitation at Mt. Cimone:
observations vs. model simulations
Mt. Cimone is the windiest meteorological station in Italy and the prevailing
local winds blow from SSW and NNE
directions (Ciattaglia, 1983; Ciattaglia et al., 1987; Colombo et al., 2000).
The wind observations at Mt. Cimone during the period of 1998–2011, when
radionuclide measurements were performed at the station (Tositti et al.,
2014), agree with the climatology of local wind intensity and direction
during the period of 1946–1999 as reported by the Italian Air Force (Colombo
et al., 2000). NNE directions are
more significant during the cold period, and fluxes from SW are more typical
of the warm period. While winds blowing from the SSW sector generate a sea air inflow, a continental air inflow
is observed when winds come from the NNE sector (Ciattaglia et al., 1987).
Simulated monthly mean (a)210Pb concentrations and
(b)7Be concentrations, at the elevation of Mt. Cimone. Arrows
represent the seasonality of winds in the MERRA meteorological data. The
white dot indicates the location of Mt. Cimone (44∘12′ N,
10∘42′ E; 2165 m a.s.l.).
Comparison of the MERRA total precipitation (0–75∘ N,
90∘ W–90∘ E) during January and July 2005 with that in the
GPCP observations. The white dot indicates the location of Mt. Cimone
(44∘12′ N, 10∘42′ E; 2165 m a.s.l.).
However, when considering the lifetimes of 210Pb (about one week) and
7Be (about 3 weeks) aerosols (Liu et al., 2001), it is apparent that
regional and long-range transport
has a much more important role than local transport. On a large scale, about
70 % of background air masses reaching Mt. Cimone in the period of
1996–1998 came from Atlantic and Arctic areas, with a smaller contribution
from the Mediterranean Basin and the eastern area, as estimated by Bonasoni
et al. (2000b). A more recent and extended study of advection patterns at
Mt. Cimone (Brattich et al., 2017), analyzing clusters of 4-day kinematic
back-trajectories calculated for the period of 1998–2011 with the HYSPLIT
(HYbrid Single-Particle Lagrangian Integrated Trajectory) model driven by the
NCEP/NCAR (National Centers for Environmental Prediction/National Center for
Atmospheric Research) meteorological reanalysis, shows that the air masses
advected to Mt. Cimone (55 %) arrive from the western Atlantic–North
America sector, while the remaining air masses (from the Arctic, eastern
Mediterranean, and
Mediterranean Basin–northern Africa) together represent 45 % of
trajectories. Seasonal transport to Mt. Cimone in the model is shown in
Fig. 2, representing winds at the elevation of Mt. Cimone (winds are weaker
at the model bottom layer). In agreement with the description of advection
patterns at the site, prevailing model winds (Fig. 2) blow from the western
Atlantic sector. Slow summer winds suggest the stronger influence of
regional/local transport at Mt. Cimone during the period (e.g., Lee et al.,
2007; Marinoni et al., 2008; Tositti et al., 2013, 2014; Brattich et al.,
2015).
In the model, Mt. Cimone appears to be in a location where there is a large
horizontal gradient of wind (transport) during 2005. Long-range transport
from western Europe, North America, and the Arctic region prevail during the
cold period, while regional transport appears more important in summer. The
model is able to capture relevant features of pressure systems and seasonal
circulation patterns of the North Atlantic, Mediterranean, and African regions, such as
the semi-permanent high-pressure system located in the North Atlantic with
different positions during different seasons (Bermuda/Azores High), a
semi-permanent system of high pressure centered in northeastern Siberia
during the colder half of the year (Siberian High), and the Intertropical
Convergence Zone (ITCZ) in the summer/autumn season. However, due to the
coarse resolution of the global meteorological reanalysis that we use to
construct the model winds, the more than 50 local-scale wind systems present
in the Mediterranean and surrounding regions are not resolved (Burlando,
2009). In northern Europe, in fact, there are approximately two main states
for the atmosphere, the westerly or zonal flows modulated by the advection of
Atlantic lows, and the long-lived blocking anticyclonic configurations over
North Sea or Scandinavia (easterly) (Burlando et al., 2008).
In the Mediterranean region, the main cyclones during winter are essentially
sub-synoptic lows triggered by the major North Atlantic synoptic systems
affected by the local topography of the northern Mediterranean coast (Trigo
et al., 2002), whereas in summer cyclones develop because of thermal effects,
orography (e.g., the Atlas Mountains), and increase in low-level thermal
gradients (Trigo et al., 2002; Campins et al., 2006). Again, due to the
coarse resolution of the meteorological data we use, these sub-synoptic
processes are not resolved. For instance, northern African lows and Sahara
depressions (also referred to as Atlas lee depressions) and the resulting
SSW wind (sirocco) (Reiter, 1975), potentially linked to 210Pb
variations at Mt. Cimone, appear to be an important feature missing in the
degraded MERRA data, where they appear only during October/November. However,
MERRA is able to capture the summertime north-northeasterly winds in the
eastern Mediterranean (Aegean Sea), known as the Etesians. The Etesians are
the most persistent localized wind system in the world as a result of a sharp
east–west pressure gradient manifested by large-scale circulation features
(i.e., low pressure over the eastern Mediterranean/Middle East and high
pressure over central and southeastern Europe) (Dafka et al., 2016).
Comparison of the seasonal precipitation at Mt. Cimone in the MERRA
meteorological data set with that in the GPCP observations for
(a) the model grid box (“ij”) corresponding to the location of
Mt. Cimone, (b) the model grid box (“ij - 1”) to the west of
“ij”, (c) the model grid box (“i - 1j - 1”) to the
southwest of “ij”, and (d) the model grid box
(“i + 1j + 1”) to the northeast of “ij”.
We evaluate the MERRA precipitation with that from the GPCP (Global
Precipitation Climatology Project, https://precip.gsfc.nasa.gov/)
satellite and surface observations in 2005. Figure 3 shows the MERRA and GPCP
monthly precipitation for the region defined by 0–75∘ N and
90∘ W–90∘ E. Good agreement between the MERRA and the GPCP
precipitations averaged over the region was found. In particular, summer
precipitation patterns are very similar. The geographical distribution of
precipitation in MERRA shows some important features in agreement with the
observed climatology precipitations: the desert climate in North Africa, with
very low precipitation all year long; the ITCZ, with high precipitation
during the summer and autumn seasons; the North Atlantic region, with high
precipitation especially during the winter and autumn seasons; and Europe,
where the seasonal pattern of precipitation is similar to that in the North
Atlantic region but precipitation is lower.
Comparison of GMI-simulated (black dotted line) monthly
(a)210Pb and (b)7Be activities with those
observed at Mt. Cimone (solid lines) in 2005. Also shown are GMI-simulated
monthly activities of (c)222Rn,
(d)10Be /7Be ratios, and (e) stratospheric
7Be / total7Be ratios. Model values are for the “ij” grid box
corresponding to the location of Mt. Cimone. Vertical bars indicate the
uncertainty in observed activities.
Figure 4 shows the comparison of the GPCP and MERRA precipitation seasonality
at Mt. Cimone. Since Mt. Cimone is located in a region with a large
horizontal gradient in precipitation, we also show in the figure the
comparisons for three adjacent grid boxes. The MERRA precipitation is
generally lower than that of GPCP at two grid boxes (except for summer,
Fig. 4a, b), but in good agreement at the other two grid boxes (Fig. 4c, d).
The agreement between the MERRA and GPCP precipitation seasonality is
reasonable, with the squared correlation coefficient R2 varying between
0.56 (at the grid to the northwest of “ij”) and 0.89 (at the grid to the
southeast of “ij”). Large differences between the MERRA precipitation and
that locally observed at the station are instead present. While the daily
mean observed 2005 precipitation is 0.81 mm, which is close to the
corresponding precipitation (0.73 mm) in MERRA at the “ij” grid (i.e., a
negative bias of -0.08 mm), the model bias is positive and much higher
(0.31–1.28 mm) at adjacent grids. This bias may very well reflect again the
fact that the observed surface precipitation is localized, whereas the
satellite and MERRA precipitations correspond to a much larger scale (about
200 km). Moreover, as Colombo et al. (2000) previously pointed out,
different from the surrounding area, where the climate is defined as
temperate-continental, the climate at the mountaintop is classified as alpine
because of the high elevation. In fact, in agreement with the GPCP
precipitation in 2005, the observed climatology in the region shows a maximum
during November (secondary maximum in spring) and an absolute minimum in July
(secondary minimum in January), whereas on the top of the mountain the
precipitation is maximal during summer. The MERRA precipitation shows
increased amounts during April and August–December, with minimum in
June–July. As the local precipitation at the site is important to the
scavenging of radionuclide aerosol tracers, this difference between the local
and regional precipitation could contribute to any biases in our simulations.
However, as we will show below, the ratio 7Be /210Pb may
cancel out the errors associated with precipitation scavenging (Koch et al.,
1996).
Low 210Pb concentrations are seen over the Atlantic Ocean, due to the
negligible emissions of 222Rn from the oceans and strong precipitation
scavenging, and in northern and western Europe, especially during the cold
season (Fig. 2a). High 210Pb concentrations appear over the Sahara and
northern Africa, as a result of low precipitation in this area, and also over
the Middle East and southern Asia. 210Pb concentrations over southern
Europe appear higher during the transition seasons, especially autumn, and
peak during summer when the minimum precipitation and slow winds from west
are observed in the region. Low 7Be concentrations are simulated along
the equator where convective scavenging is strongest (Fig. 2b). High 7Be
concentrations are seen over the Sahara due to a combination of low
precipitation and subsidence in this region. Elevated values also occur over
the Middle East, North America, and Greenland. 7Be concentrations over
southern Europe appear higher during spring and peak during winter, when
model winds are stronger and transport 7Be aerosols from North America
and Greenland regions, where 7Be production is
highest (Beer et al., 2012).
Seasonal variations in 210Pb and 7Be at Mt. Cimone: observations vs. model simulations
The seasonality and frequency distributions of 210Pb and 7Be
concentrations measured at the Mt. Cimone station were previously examined by
Lee et al. (2007), while more recent analyses of the 12-year record were
presented in Tositti et al. (2014) and Brattich et al. (2016). Generally,
both radionuclides show a marked seasonal maximum in the summertime, a
behavior shared by PM10 (Tositti et al., 2013) and O3 (Bonasoni et
al., 2000b). The 210Pb summer maximum is mainly due to the higher mixing
height and enhanced uplift from the boundary layer as a result of thermal
convection. The seasonal fluctuation of 7Be is more complex and
characterized by two relative maxima, one during the cold season associated
with stratosphere-to-troposphere transport, and the other during the warm
season mainly associated with tropospheric subsidence balancing
lower-tropospheric air masses ascent occasionally accompanied by STE (Tositti
et al., 2014). The 210Pb and 7Be measurements in 2005 are
consistent with this description (Fig. 5): 210Pb concentrations are
characterized by two maxima during the warm period (July and September);
7Be concentrations are characterized by one absolute maximum during
summer (July) and one secondary maximum during spring (March).
Same as Fig. 5a and b but for the “ij - 1” to the south of
“ij” (left column) and “i - 1j - 1” to the southwest of “ij”
(right column) grids, respectively.
Figure 5a and b compare the simulated monthly 210Pb and 7Be
activities with the observations at Mt. Cimone in 2005. The comparisons for
the monthly percentage deviations from the annual mean concentration are
available in the Supplement (Figs. S1–S2). The seasonality of 210Pb is
well captured by the model. The model reproduces the presence of two seasonal
maxima in the 210Pb observations, with the maximum observed in July
shifted to June in the simulation. The squared correlation coefficient
R2 between observed and simulated 210Pb activities is equal to 0.83
at the “ij” grid and varies between 0.42 and 0.82 for adjacent grid boxes
(to the north and to the west of “ij”, respectively), confirming the good
performance of the model in reproducing the 210Pb seasonal pattern.
As for 7Be, the model well captures the March maximum (i.e., secondary
maximum in the observations) and the month-to-month variation during the cold
and transition seasons (January–April, October–December). However, during
the warm period, the simulated 7Be concentrations are lower by a factor
of 2 than the observed. A better agreement was found at some adjacent model
grid boxes (e.g., to the south and to the southwest of “ij”; Fig. 6 vs.
Fig. 5). The correlation between observed and simulated monthly 7Be
activities also increases from R2=0.03 at “ij” to R2=0.11–0.60 at adjacent model grid boxes. The largest value of R2=0.6
was obtained at the “ij - 1” grid box to the south of “ij” (Fig. 6).
This improvement is due to the large horizontal gradient in the simulated
7Be concentrations near the site (Fig. 2).
Sources and seasonality of 210Pb and
7Be at Mt. Cimone: a model analysis
In this section, we quantify the sources of 210Pb and 7Be and
determine the processes governing their seasonality in the GMI model.
Additional tracers as simulated by the model are used to aid in the
interpretation. Model sensitivity experiments are conducted to examine the
roles of transport and precipitation scavenging in the seasonality.
As discussed in Sect. 4, the model reproduces the 210Pb
seasonality well, with minimum in the cold period and maximum in the warm period.
The 210Pb seasonality (Fig. 5a) can be linked with the seasonal pattern
of its precursor 222Rn (Fig. 5c). It is seen that the summer 210Pb
maximum is due to stronger (thermal) convection, which uplifts more
222Rn out of the boundary layer (e.g., Lee et al., 2007; Tositti et al.,
2014; Brattich et al., 2015). This uplift of 222Rn from the boundary
layer is minimum in the cold period, and the minimal level of 210Pb in
this period can be considered representative of the free troposphere. The
210Pb summer increase appears to be associated with short-range and
regional transport, as suggested by the model simulations (Fig. 2a). As
expected, long-range transport is more typical of the winter/spring seasons
because of stronger horizontal winds, while regional effects are more
important during summer, when convection gets stronger.
In a similar manner, the source of the 7Be March maximum can be
investigated with model tracer simulations. Figure 5d and e also show the
simulated seasonal patterns of the 10Be /7Be activity ratio
and of the fraction of 7Be originating from the stratosphere (strat
7Be / total 7Be). The simulated seasonal pattern of the
10Be /7Be ratio is very similar to the observations at
Jungfraujoch (Switzerland, 3580 m a.s.l.) (Zanis et al., 2003),
characterized by a clear seasonal cycle with peak ratios in spring. The
usefulness of 10Be /7Be ratio as a stratospheric tracer is due
to the fact that both 10Be and 7Be cosmogenic radionuclides attach
to the same aerosols and share therefore the same removal mechanism.
Moreover, due to the much longer physical half-life of 10Be
(τ1/2=1.5×106 years) compared to 7Be (τ1/2=53.3 days), their concentration ratios in the stratosphere (about 3–4) are
much higher than in the troposphere (about 2 or even less) (Koch and Rind,
1998). The simulated 10Be /7Be ratio behavior indicates that
deep stratosphere-to-troposphere (STT) peaks during winter, while shallower
STT has a spring maximum, consistent with previous analyses of stratospheric
intrusions at Mt. Cimone (Cristofanelli et al., 2006, 2009a), and more
generally with the climatology of STE at the Northern Hemisphere midlatitudes
(James et al., 2003). Altogether the simulated high strat
7Be / total 7Be, high 7Be /210Pb (Fig. 7), and
low 10Be /7Be ratios during December–January indicate
strongest STE during this period, followed by spring with slightly weaker
stratospheric influence on surface 7Be. However, the model tends to
overestimate the observed 7Be concentrations and
7Be /210Pb ratios during December–February, suggesting that
stratospheric influence and/or subsidence in the model is probably too strong
in this region at this time of the year. It is noted that globally integrated
STT mass fluxes in the MERRA reanalysis are actually smaller than in some
other reanalyses, e.g., ERA-Interim, JRA-55, and MERRA-2 (Boothe and Homeyer,
2016).
Comparison between GMI-simulated monthly 7Be /210Pb
ratios at the “ij” and “ij - 1” grids (black dotted line) and those
from the observations at Mt. Cimone (green solid line). Vertical bars
indicate the uncertainty in observed activities.
The use of the 7Be production rate of Lal and Peters (1967) for a solar
maximum year (1958) may partly explain the lower annual mean 7Be in the
model (3.4 mBq m-3 annual mean at the “ij” grid) than in the
observations (4.2 mBq m-3). In fact, the sunspot number in 2005 (29.8)
was quite low (slowly decreasing from 2000, a solar maximum year, and
reaching minimum in 2008), especially compared to the 1958 value of 184.8.
Sunspot number data are available from the World Data Center for the
production, preservation and dissemination of the international sunspot
number (Sunspot Index and Long-term Solar Observation, SILSO, Royal
Observatory of Belgium, Brussels, http://sidc.oma.be/sunspot-data/,
2016).
Comparison of GMI-simulated monthly 210Pb and 7Be
activities at Mt. Cimone between the standard (black dotted line) and the
sensitivity runs (“ij - 1” grid). The sensitivity runs are those
without convective transport/scavenging (red dotted line), without dry
deposition (blue dotted line), and without scavenging (orange dotted line;
y axis on the right). The observations are shown as a green solid line.
Vertical bars indicate the uncertainty in observed activities.
GMI-simulated differences of 210Pb concentrations at the
elevation of Mt. Cimone between a sensitivity run without convection (i.e.,
without transport and scavenging in convective updrafts) and the standard
run. Arrows denote MERRA winds. The white dot indicates the location of
Mt. Cimone (44∘12′ N, 10∘42′ E; 2165 m a.s.l.).
During the winter period, associated with the simulated and observed 7Be
increases (Figs. 5–6), strong long-range transport was dominant in the
European region (Fig. 2b). Transport from higher-latitude regions (Arctic,
northern Europe, and North America) appears particularly important during
this period (Fig. 2b); such transport from high-latitude regions, where the
7Be production rate is highest (Beer et al., 2012), has typically been
observed during STE events at Mt. Cimone in many studies (e.g., Bonasoni et
al., 1999, 2000a, b).
The discrepancy between the simulated and the observed 7Be
concentrations during the warm period is partly due to the sensitivity to
spatial sampling in the model. As seen from the map plots of 210Pb and
7Be concentrations at the elevation of Mt. Cimone (Fig. 2), the sampling
site appears to be located in a region where the N–S gradient of
concentrations is large (especially for 7Be). An elevated gradient in
the region surrounding Mt. Cimone was also seen for winds, as transport plays
a critical role in determining the distributions of these tracers. The
sensitivity to spatial sampling in the model is therefore ascribed to this
observed strong gradient in the N–S direction. In fact, while the grids to
the south and southwest of “ij” are better for summer 7Be comparisons
(Fig. 6), the grids to the northeast, north, and northwest of “ij” are
better for winter (not shown).
The model underestimate of 7Be levels in the warm months may also
suggest the mixing of air masses between the PBL and the lower free
troposphere is likely too weak. Previous observational analyses indicated
that such mixing is higher in summer at Mt. Cimone due to enhanced convection
and mountain wind breeze (e.g., Fischer et al., 2003; Cristofanelli et al.,
2007). Weaker entrainment of free-tropospheric air into the PBL would result
in lower 7Be concentrations at the surface.
Same as Fig. 9 but for a sensitivity simulation where wet
scavenging is turned off.
GMI-simulated differences of 7Be concentrations at the
elevation of Mt. Cimone between a sensitivity run without convection and the
standard run. Arrows denote MERRA winds. The white dot indicates the location
of Mt. Cimone (44∘12′ N, 10∘42′ E; 2165 m a.s.l.).
The model annual average biases are about 8 % for 210Pb and about
19 % for 7Be, respectively. By contrast, the model average bias for
7Be /210Pb ratios is about -13 % (Fig. 7). The smaller
model bias for 7Be /210Pb ratios than for 7Be
concentrations reflects the fact that the ratio cancels out the errors in
precipitation scavenging (Koch et al., 1996) that contribute to the
underestimate of 210Pb and 7Be activities. On the other hand, the
negative model bias for the 7Be /210Pb ratio again points to
weak downward mixing from the free troposphere.
If one compares the month-to-month variation in 210Pb and 7Be
(Figs. 5 and 6) and precipitation in the model (Fig. 4), the maxima/minima of
precipitation appear to be in phase with those of both radionuclides'
activities. This reflects the effects of precipitation scavenging on
radionuclide aerosols.
We conducted model sensitivity experiments where either convection
(transport and scavenging), wet scavenging due to both large-scale and
convective precipitation, or dry deposition processes are turned off to
examine the roles of these processes in controlling the seasonality of
210Pb and 7Be at Mt. Cimone. Figure 8 shows the results for the
standard and sensitivity runs at the “grid to the south of “ij”, for which
the simulated tracer seasonal variations are similar to those observed, while
the monthly percentage deviations from the annual mean concentrations are
shown in Fig. S3. Figures 9–12 show maps of simulated changes in 210Pb
and 7Be concentrations when convection or wet scavenging is turned off.
Same as Fig. 11 but for the difference between a sensitivity run
without wet scavenging and the standard run.
Turning off dry deposition does not significantly change the simulated
210Pb and 7Be concentrations, partly due to sampling the higher
vertical grid box in the model (larger effects are seen at the bottom model
layer). With convection turned off (i.e., with neither convective transport nor
convective scavenging), the simulated 7Be seasonality also remains
nearly the same. This suggests the compensating effects between subsidence
(increasing 7Be) associated with convective transport and scavenging
(decreasing 7Be) due to convective precipitation. In the case of
210Pb, turning off convection does not change the seasonal pattern but
generally results in larger 210Pb concentrations and particularly during
summer/autumn, when convective transport is more important at the site. In
fact, no convective transport of 222Rn (Fig. S5) results in less
222Rn (and 210Pb) being transported to the free troposphere, as well as more 210Pb being available in PBL lifted to the free troposphere by
large-scale vertical transport; on the other hand, the lack of convective
scavenging of 210Pb increases its concentration in the free troposphere.
Turning off convection therefore results in an increase in 210Pb
concentrations in the free troposphere. Both surface 222Rn
concentrations at the elevation of Mt. Cimone (Fig. S4), as well as a map of
changes in 210Pb concentrations due to convection in the model (Fig. 9)
show that convection in the region is more important during summer and
autumn but is not negligible during spring, possibly due to thermal inertia.
The model run without scavenging suggests that, apart from downward transport
from the upper troposphere and lower stratosphere, wet scavenging is mainly
responsible for the seasonal variation in 7Be (Fig. 8, bottom panel).
None of our simulations is able to describe the observed 7Be summertime
peak, suggesting that local and regional circulations in this region with
complex topography may not be resolved by the coarse-resolution model. For
210Pb (Fig. 8, top panel), it appears that wet scavenging plays a more
important role during August–December than during January–July. This
appears to be associated with the seasonality of precipitation, which shows
prolonged elevated values during August–December, as well as a maximum
during April, as previously discussed (Fig. 5). A plot of changes in
210Pb concentrations due to scavenging in the model (Fig. 10) confirms
that the scavenging effect is larger during autumn and, to a lesser extent,
during summer. At Mt. Cimone, the scavenging effect is not minimal during
July (month of minimum precipitation, Fig. 4), suggesting the influence of
precipitation scavenging elsewhere in the region on the site.
Summary and conclusions
We have used a global 3-D model (GMI CTM) driven by the MERRA assimilated
meteorological data from NASA's GMAO to simulate the 210Pb and 7Be
observations from the Mt. Cimone (44∘12′ N, 10∘42′ E;
2165 m a.s.l.; Italy) WMO-GAW station in 2005. The two natural atmospheric
radionuclides originate from contrasting source regions (lower troposphere
and upper troposphere/lower stratosphere, respectively), attach to submicron
particles, and are removed from the troposphere mainly by wet deposition. Our
objective was to examine the roles of horizontal advection, vertical
transport (large-scale and convection), and wet scavenging in determining the
seasonality of 210Pb and 7Be at Mt. Cimone. The observed 210Pb
concentrations are characterized by maxima in summer and minima during the
cold period. The seasonality of 7Be is more complex, with a major peak
in summer, a secondary peak in spring and a minimum in winter. This is the
first modeling study of 210Pb and 7Be observations at Mt. Cimone.
This site is representative of free-tropospheric southern
Europe/Mediterranean conditions most of the year, and thus the comparison
between measurements and simulations can serve as an indication of
shortcomings in the model or in the meteorological data.
Precipitation and wind fields are important to the model's performance in
representing the transport and scavenging processes. We evaluated the MERRA
precipitation field used by GMI CTM against the GPCP satellite and surface
observations, and generally good agreement was found. The seasonality of
precipitation at Mt. Cimone shows increased amounts during April and the
period of August–December, and minimum in June–July. The MERRA assimilated
winds at the low-resolution version we used captured the main circulation
patterns (e.g., location of the Azores high-pressure system, location of the ITCZ)
in the Northern Hemisphere. However, some local-scale winds and pressure
systems, which are important for transport to the sampling site, were likely
not well resolved at the coarse resolution we used. Generally good agreement
was found between the MERRA assimilated wind fields and the main advection
patterns at the site (e.g., prevalence of long-range transport from western
Europe, North America, and the Arctic region during the cold season, as opposed to
the prevailing regional transport during the warm season).
The model reproduced the observed 210Pb seasonality well: 210Pb
maxima during the warm period were attributed to the stronger (thermal)
convection, which uplifts more 222Rn (and 210Pb) from the boundary
layer. The model is less successful in reproducing the observed 7Be
seasonality. 7Be was better represented during the cold period, while
the observed summer 7Be maximum was underestimated by the model. The
model underestimate of 7Be levels in the warm months is partly due to
the sensitivity to spatial sampling in the model, but also suggests that the
mixing of air masses between the PBL and the lower free troposphere (e.g.,
via convection and compensating subsidence) is likely too weak during summer,
when the Mt. Cimone station is located within the PBL. This suggests that
additional work comparing the model results with more surface observations is
needed in order to better understand this effect. The simulated lower annual
average 7Be concentration relative to the observation is also partly
attributed to the fact that the model used the 7Be production rate for a
solar maximum year, while in 2005 (our simulation year) the solar activity
was rather low.
By examining the wind fields and horizontal distribution of radiotracers in
the model, we noted that the sampling site is in a location where there is a
large gradient, especially in the north–south direction. Accordingly, we
investigated the sensitivity of model results to spatial sampling. A better
agreement between the model and the observations at some adjacent grid boxes
was found. The 7Be March maximum was linked to the large stratospheric
influence during winter/spring. The model tends to underestimate the
summertime 210Pb and 7Be but better simulates the
7Be /210Pb ratio because the model errors due to precipitation
scavenging appear to be canceled out in the ratio.
We have conducted a series of model sensitivity experiments to further
examine and quantify the roles of wet scavenging, dry deposition, and
convection (transport and scavenging) in controlling the seasonality of
210Pb and 7Be at Mt. Cimone. Dry deposition does not have a
significant effect on the magnitude and seasonality of 210Pb and
7Be concentrations at the site. The relatively weak combined effects of
convective transport and convective scavenging on the radiotracer seasonality
were attributed to the compensating effects of convective transport and
convective scavenging on tracer concentrations in the lower free troposphere
(at the elevation of Mt. Cimone). Convection appears to be more important to
the regional distribution of both radiotracers during summer and autumn,
although it is also significant during spring. Finally, scavenging is found
to be the most important process controlling the seasonal variations in
210Pb and 7Be at Mt. Cimone. For 210Pb, precipitation plays a
more important role during August–December than during January–July. This
was attributed to the seasonality of local and regional precipitation, which
shows prolonged elevated values in the period of August–December.
While our simulations demonstrated some capabilities of the model to
reproduce the seasonality of 210Pb and 7Be, they highlight the
weaknesses of the model in reproducing local features, presumably due to its
coarse resolution. Model simulations at a higher resolution would improve
this model analysis of 210Pb and 7Be observations at Mt. Cimone, a
high-elevation site. The understanding of downward transport associated with
convection during summer also requires improving. Therefore, 210Pb and
7Be tracers will prove to be very useful in our understanding of
seasonal behaviors of other environmentally important trace gases and
aerosols at Mt. Cimone. Since other aerosols and trace gases (e.g., black
carbon, CO, O3) are also measured at the station, we plan to conduct
comparisons between model simulations and those measurements to corroborate
or contrast with the radionuclide results.
Data availability
A description of the observational data and model output used in this paper
can be found in Sect. 2 and they are available upon request by contacting
Laura Tositti (laura.tositti@unibo.it) and Hongyu Liu
(hongyu.liu-1@nasa.gov), respectively.
The Supplement related to this article is available online at doi:10.5194/acp-17-1061-2017-supplement.
Acknowledgements
Italian Air Force Meteorological Office (IAFMS) and ISAC-CNR are gratefully
acknowledged for their precious technical support at the Mt. Cimone station.
In particular, ISAC-CNR is gratefully acknowledged for providing
infrastructural access at the WMO-GAW Global Station Italian Climate
Observatory “O. Vittori” at Mt. Cimone. IAFMS is gratefully acknowledged
for providing meteorological observations at Mt. Cimone station. The Italian
Climate Observatory “O. Vittori” is supported by MIUR and DTA-CNR
throughout the Project of National Interest “NextData”. Erika Brattich
thanks the National Institute of Aerospace (NIA) Visitor Program for hosting
her one-month visit, and the Department of Biological, Geological and Earth
Sciences of the University of Bologna for grant support during her PhD study.
Hongyu Liu is supported by NASA Modeling and Analysis Program (MAP), NASA
Atmospheric Composition Modeling and Analysis Program (ACMAP), and NASA
Atmospheric Composition Campaign Data Analysis and Modeling (ACCDAM) program.
The GMI activity is managed by José Rodriguez and Susan Strahan (NASA
GSFC). Stephen Steenrod, Megan Damon, and Jules Kouatchou (GSFC) are
acknowledged for programming support. NASA Center for Computational Sciences
(NCCS) provided supercomputing resources. We thank the two anonymous reviewers
for their comments, which improved the quality of our
work. Edited by: J. Ma Reviewed by: two anonymous
referees
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