Volatile halogenated organic compounds (VHOCs), such as methyl
halides (CH3X; X is Br, Cl and I) and very short-lived halogenated
substances (VSLSs; bromoform – CHBr3, dibromomethane
– CH2Br2, bromodichloromethane – CHBrCl2, trichloroethylene
– C2HCl3, chloroform – CHCl3 – and dibromochloromethane
– CHBr2Cl) are well known for their significant influence on ozone
concentrations and oxidation capacity of the troposphere and stratosphere
and for their key role in aerosol formation. Insufficient characterization
of the sources and the emission rate of VHOCs limits our ability to understand
and assess their impact in both the troposphere and stratosphere. Over the
last two decades, several natural terrestrial sources for VHOCs, including
soil and vegetation, have been identified, but our knowledge of emission
rates from these sources and their responses to changes in ambient
conditions remains limited. Here we report measurements of the mixing ratios
and fluxes of several chlorinated and brominated VHOCs from different
landscapes and natural and agricultural vegetated sites at the Dead Sea
during different seasons. Fluxes were generally positive (emission into the
atmosphere), corresponding to elevated mixing ratios, but were highly
variable. Fluxes (and mixing ratios) for the investigated VHOCs ranged as
follows: CHBr3 from -79 to 187 nmol m-2 d-1 (1.9 to 22.6 pptv), CH2Br2 from
-55 to 71 nmol m-2 d-1 (0.7 to 19 pptv), CHBr2Cl from -408 to 768 nmol m-2 d-1 (0.4 to 11 pptv),
CHBrCl2 from -29 to 45 nmol m-2 d-1 (0.5 to 9.6 pptv),
CHCl3 from -577 to 883 nmol m-2 d-1 (15 to 57 pptv),
C2HCl3 from -74 to 884 nmol m-2 d-1 (0.4 to 11 pptv),
methyl chloride (CH3Cl) from -5300 to 10,800 nmol m-2 d-1
(530 to 730 pptv), methyl bromide (CH3Br) from -111 to 118 nmol m-2 d-1 (7.5 to 14 pptv) and methyl iodide
(CH3I) from -25 to
17 nmol m-2 d-1 (0.4 to 2.8 pptv). Taking into account statistical
uncertainties, the coastal sites (particularly those where soil is mixed
with salt deposits) were identified as sources of all VHOCs, but this
was not statistically significant for CHCl3. Further away from the
coastal area, the bare soil sites were sources for CHBrCl2,
CHBr2Cl, CHCl3, and probably also for CH2Br2 and
CH3I, and the agricultural sites were sources for CHBr3,
CHBr2Cl and CHBrCl2. In contrast to previous reports, we also
observed emissions of brominated trihalomethanes, with net molar fluxes
ordered as follows: CHBr2Cl > CHCl3 > CHBr3 > CHBrCl2
and lowest positive flux incidence
for CHCl3 among all trihalomethanes; this finding can be explained by
the soil's enrichment with Br. Correlation analysis, in agreement with
recent studies, indicated common controls for the emission of CHBr2Cl
and CHBrCl2 and likely also for CHBr3. There were no indications
for correlation of the brominated trihalomethanes with CHCl3. Also in
line with previous reports, we observed elevated emissions of CHCl3 and
C2HCl3 from mixtures of soil and different salt-deposited
structures; the flux correlations between these compounds and methyl halides
(particularly CH3I) suggested that at least CH3I is also emitted
via similar mechanisms or is subjected to similar controls. Overall, our
results indicate elevated emission of VHOCs from bare soil under semiarid
conditions. Along with other recent studies, our findings point to the
strong emission potential of a suite of VHOCs from saline soils and salt
lakes and call for additional studies of emission rates and mechanisms of
VHOCs from saline soils and salt lakes.
Introduction
Volatile halogenated organic compounds (VHOCs), such as methyl halides
(CH3X; X is Br, Cl and I) and very short-lived halogenated substances
(VSLSs; lifetime < 6 months), contribute substantially to the loading
of tropospheric and lower stratospheric reactive halogen species containing
Cl, Br or I, and their oxides (Carpenter et al., 2013, 2014; Derendorp et al., 2012). Reactive halogen species, in turn, lead to
ozone (O3) destruction, changes in atmospheric oxidation capacity and
radiative forcing (Simpson et al., 2015). Depletion of O3
in the stratosphere is associated with damage to biological tissues owing to
an increase in transmittance of UVB radiation (Rousseaux et al., 1999).
In the troposphere, O3 destruction is of great importance, given that
O3 is toxic to humans, plants and animals; is a greenhouse gas; and
plays a key role in the oxidation capacity of the atmosphere.
The lifetimes of VHOCs vary significantly (see summary in Table S1 in the Supplement), which
in turn affects their influence in both the troposphere and the
stratosphere. Owing to their relatively short lifetimes (< 6 months), the transport of VSLSs to the stratosphere occurs primarily in the
tropics, where deep convection is frequent. Brominated VSLSs originate
primarily from the ocean, whereas chlorinated VSLSs, except for chloroform
(CHCl3) and chloroethane, originate primarily from anthropogenic
sources (Carpenter et al., 2014). Methyl iodide
(CH3I), having a relatively short lifetime, is also classified as a
VSLS and contributes significantly to tropospheric O3 destruction in
the marine boundary layer (MBL; Carpenter et al.,
2014) and also, indirectly, to the formation of cloud condensation nuclei
(O'Dowd et al., 2002). It is now well established that emission of
brominated (e.g., bromoform – CHBr3, methylene bromide
– CH2Br2 – and dibromochloromethane – CHBr2Cl) and iodinated
(e.g., CH3I) VSLSs tends to be much greater in coastal areas than in
the open ocean (Carpenter et al., 2000, 2009; Liu et al.,
2011; Bondu et al., 2008; Manley and Dastoor, 1988; Quack and Wallace, 2004),
since in the former, they can also be emitted from macroalgae under oxidative
stress at low tide (Pedersen et al., 1996). The ocean is also a
major source of methyl bromide (CH3Br) and a significant
(∼19 %) source of methyl chloride (CH3Cl; Carpenter et al., 2014), as they originate from
phytoplankton, bacteria and detritus.
Despite the numerous efforts made in recent years to evaluate halocarbon
budgets, uncertainties still exist concerning the strengths of both their
sources and their sinks. The budgets of CH3Br and CH3Cl are
unbalanced, with sinks outweighing sources by ∼32 % and
∼17 %, respectively (Carpenter et al.,
2014). Uncertainties in the global budgets of naturally occurring VSLSs are
large, with discrepancies having a factor of ∼2–3 between
top-down and bottom-up emission inventories (Carpenter
et al., 2014). This results largely from poor characterization of emission
sources (Warwick et al., 2006; Hossaini et al., 2013; Ziska et al., 2013).
Studies over the past few decades have clearly demonstrated that terrestrial
sources also constitute a major fraction of the atmospheric budget for both
methyl halides and VSLSs (Carpenter et
al., 2014). Many terrestrial plants have been identified as sources of
CH3Cl (Yokouchi et al., 2007), and the results of recent
modeling indicate that about 55 % of the global sources of CH3Cl
originate from tropical lands (Xiao et al., 2010; Carpenter et al., 2014).
It has also been suggested that natural terrestrial sources of CH3Br,
especially emissions from terrestrial vegetation, must account for a large
part of the missing sources (Gebhardt et al., 2008; Yassaa et al.,
2009; Warwick et al., 2006; Gan et al., 1998; Yokouchi et al., 2002; Moore,
2006; Rhew et al., 2001; Wishkerman et al., 2008), and emissions have been
observed from peatlands, wetlands, salt marshes, shrublands, forests and
some cultivated crops (Gan et al., 1998; Varner et al., 1999; Lee-Taylor
and Holland, 2000). CHCl3 has also been found to be emitting from
various terrestrial sources, including rice, soil, tundra, forest floor and
different types of microorganisms, such as fungi and termites (see
Dimmer et al., 2001 and Rhew et al., 2008).
The importance of VHOC emission from soil, sediments and salt lake deposits
has been recently recognized (see Kotte et al., 2012;
Ruecker et al., 2014, and references therein). For example,
Keppler et al. (2000) revealed natural abiotic emission of
CH3Br, CH3Cl and CH3I as well as additional chlorinated
VHOCs from soil and sediments harboring an oxidant such as Fe(III), halides,
or organic matter (OM), while Weissflog et al. (2005) found that salt
lake sediments can be a source for several C1 and C2 chlorinated species,
including CHCl3 and trichloroethylene (C2HCl3), induced by halobacteria in the presence of dissolved Fe. Huber
et al. (2009) identified abiotic natural emission of trihalomethanes from
soil, including CHCl3, bromodichloromethane (CHBrCl2) and
CHBr2Cl, induced by oxidation of OM by Fe(III) and hydrogen peroxide,
while Hoekstra et al. (1998) identified natural emission of
CHBr3 following enrichment of the soil by potassium bromide. In
addition, Carpenter et al. (2005) identified CHBr3
emission from peatland or another terrestrial source at Mace Head
(in Ireland). Albers et al. (2017) revealed that CHCl3, CHBrCl2
and potentially also other trihalomethanes can be emitted from soils,
probably induced by hydrolysis of trihaloacetyl compounds. Several other
studies have reported strong emissions of CH3Cl, CH3Br and
CH3I from coastal marsh vegetation and to a lesser extent from the
marsh soil (Rhew et al., 2000, 2002,
2014; Wishkerman et al., 2008), with significant importance
on a global scale (Deventer et al., 2018; Manley et al., 2006). In
addition, peatland has been indicated as an important source for CH3Br,
CH3Cl, CH3I and CHCl3 (Simmonds et al., 2010; Khan et al.,
2012; Dimmer et al., 2001; Carpenter et al., 2005), and Sive et
al. (2007) identified a globally significant source of CH3I from
midlatitude vegetation and soil.
Accordingly, the need for improved understanding of VHOC emissions from
saline environments and their potential importance on the global scale have
been highlighted by recent studies (Weissflog et al., 2005; Kotte et al.,
2012; Ruecker et al., 2014; Deventer et al., 2018). Moreover, due to global
warming, saline environments are likely to become more prevalent (IPCC
2007; Ruecker et al., 2014; Jiao et al., 2018). The present study is aimed at improving our
knowledge of the emission of VHOCs from salt lake environments by
quantifying the flux and mixing ratios of methyl halides and halogenated
VSLSs from different sites in the area of the Dead Sea.
The Dead Sea is unique because it is the lowest point on the Earth's
surface, about 430 m below sea level, with water salinity 12 times higher
and a ratio of bromide (Br-) to chloride (Cl-; Br-/Cl-)
that is 7.5 times higher than in normal ocean waters. Fast evaporation from the sea
leads to a variety of newly exposed sea deposits. Despite the high salinity,
emission of VHOCs via biotic processes at the Dead Sea is also potentially
feasible. The unicellular green alga Dunaliella parva has been found to be active in Dead Sea
water (Oren and Shilo, 1985), while additional bacteria and fungi that
have been isolated from the sea could also potentially be active under the
Dead Sea's extreme conditions (Oren et al., 2008; Jacob et al.,
2017; Buchalo et al., 1998). Mycobiota, including fungi and biota, have also
been detected in the Dead Sea's hypersaline soil and coastal sand
(Pen-Mouratov et al., 2010; Kis-Papo et al., 2001; Jacob et al., 2017).
Studying the emission of VHOCs at the Dead Sea is also fundamental for
understanding local surface O3-depletion events (Hebestreit et al.,
1999; Tas et al., 2003, 2006; Matveev et al., 2001; Zingler and Platt, 2005) as well as mercury-depletion events (Tas et al., 2012; Obrist
et al., 2011) in the boundary layer of this area. Emissions of brominated
and iodinated VHOCs can potentially lead to formation of the reactive iodine
and bromine species that are responsible for these processes.
Summary of VHOC samplings at the Dead Sea. Shown are the date, time
and site name (and abbreviation) for the sample, sampling height, total
number of samplings for each experiment and whether the sample could
potentially have been influenced by emission from the seawater and by
precipitation prior to sampling.
a The suffixes “s” and “w” refer to samples taken during the spring and
winter, respectively. SD, MD and LD refer to relatively short, medium and
long distance from the coastline, respectively (see Sect. 2.1).
b “+”, “–” and “+/–” indicate that the samplings were, could
not be or may be influenced by emission from the seawater, respectively.
c Values indicate the number of days before sampling on which
precipitation occurred. Additional abbreviations: MSD – Masada, MSMR – Mishmar, KLY – Kalya, ET – Ein
Tamar, KDM – Kedem, EGD – Ein Gedi, BARE – bare soil site, COAST – coastal
soil–salt mixture site, WM – agricultural cultivated watermelon site,
TMRX –
natural site with Tamarix – and SEA – sampling near the seawater (see Sect. 2.1.1).
d Samples exclude one CH3Cl measurement in TMRX–ET-1 (see Sect. 2.1.2).
e Samples exclude one measurement for all VHOCs (see Sect. 2.1.2).
MethodsField measurements and sampling
Field measurements were taken at selected sites along the Dead Sea to
measure the mixing ratios and evaluate the vertical flux of VHOCs over
different land-use types, seasons and distance from the seawater, as
summarized in Table 1. Soil samples from the various sites were analyzed, and
meteorological measurements were performed in situ.
Measurement sites
All measurements were taken in the Dead Sea area. The Dead Sea's
geographical position is between 31∘00′ N and 31∘50′ N
at 35∘30′ E, about 430 m below sea level. It is located in a
semiarid area, with mean daily maximum temperatures for summer and winter of
∼40 and ∼21∘C,
respectively. The Dead Sea has low rates of freshwater inflow and
precipitation (20–50 mm yr-1; Shafir and Alpert, 2010), while seawater
evaporation rates are high, estimated at about 400 cm yr-1 (Alpert et al., 1997). As a result, the water salinity is 12 times
higher than the average salinity of ocean water. Dead Sea water contains on
average 5.6 g L-1Br- and 225 g L-1Cl-
(Br-/Cl-≈0.025; Niemi et al., 1997), whereas
normal ocean water contains 0.065 g L-1Br- and 19 g L-1Cl- (Br-/Cl-≈0.0034; Sverdrup et al., 1942).
The main anthropogenic emission source in the area, apart from local
transportation and a few small settlements, is the Dead Sea Works, a potash
plant located to the south of most of the measurement sites (see Fig. 1).
Agricultural fields, which are mostly concentrated in the north near Kalya,
in the south near Ein Tamar and near Ein Gedi (see Fig. 1), are also
potential sources for the emission of VHOCs in the area. To the best of our
knowledge, there are no wastewater facilities near the Dead Sea area, which
could otherwise also contribute to the emission of VHOCs such as CHCl3
and CHBr3.
Location and satellite image of the Dead Sea measurement sites
(see Sect. 2.1.2) and Dead Sea Works (DSW). (a) Location of the Dead Sea.
(b) Zoomed view of the area of the measurement sites.
All measurement sites were nearly flat, homogeneous and located either along
or near the Dead Sea coast (see Fig. 1). Sites were classified according to
surface cover: bare soil sites at Mishmar (BARE–MSMR) and at Masada
(BARE–MSD), coastal sites that are mixtures of soil and salt deposits at
Ein Gedi (COAST–EGD) and Tzukim (COAST–TKM), natural Tamarix vegetation at Ein
Tamar (TMRX–ET), irrigated agricultural watermelon field at Kalya
(WM–KLY), and seawater at Kedem (SEA–KDM). Note that at SEA–KDM, we
did not evaluate fluxes. Based on in situ wind-direction measurements, the
sampled air masses at SEA–KDM were transported over the seawater from the
east (see Fig. 1) at least 1 h prior to sampling and during the sampling. To
study the effect of distance from the seawater on emission rates,
measurements at both COAST–EGD and COAST–TKM were taken at three and two
different distances from the sea, respectively. The shorter, middle and
longer distances from the seawater were termed, respectively, SD, MD and LD.
Emission rates at both COAST–EGD and COAST–TKM could potentially be
affected by distance from the seashore; there are several reasons for this,
including changes across the sites in soil salt and water content and
changes in density of the extremely sparse vegetation cover. In addition,
depending on the local wind direction at COAST–TKM-SD and COAST–EGD-SD,
direct emission and uptake from the seawater can potentially affect the
samples.
In the following, we briefly describe the different measurement sites;
additional information about the sites and measurements is provided in Table 1. BARE–MSMR has bare soil consisting of loess and a small fraction of
drifted soil covered with small stones and extremely sparse vegetation and
is located in a valley 1.5 km to the west of the Dead Sea shore. BARE–MSD
has bare Hamada soil, with small stones and loess, and is located 2.1 km to
the west of the Dead Sea. COAST–EGD-SD has dried-out bare saline soil,
mixed with salty beds and rocks, with a small contribution of freshwater
inflow. COAST–EGD-MD has a dried-out seabed of bare saline soil, mixed
with salty beds and rocks, and is located 0.3 km west of the Dead Sea shore. COAST–EGD-LD
is a dried-out seabed of loess saline bare soil, mixed with drifted soil, and
is located 0.8 km from the Dead Sea shore. COAST–TKM-SD is wetted bare soil with
salt deposits, groundwater inflow from the Dead Sea and minor (< 5 %) freshwater inflow lines covered with perennial grasses found in
wetlands (e.g., Phragmites sp.) and is located about 0.5 km from the shore. COAST–TKM-LD is a flat
rocky loess area about 1.5 km from the shore, with patchy salts and sparse
mixed shallow vegetation, mostly small Atriplex sp., Tamarix sp. and Retama raetam. TMRX–ET is a
moderately dense Tamarix grove, of 4–5 m average height, with an area of ∼2.25 km2, a 60 %–70 % vegetation cover fraction and sandy soil, located
1.7 km south of the southern tip of the Dead Sea evaporation ponds (see Fig. 1). Lastly, WM–KLY is a well-irrigated and flat 700 m×350 m
agricultural field with cultivated watermelon surrounded by a larger
agricultural area of ∼3 km2, located 2.5 km northwest of
the Dead Sea shore (Fig. 1). The watermelon crop had an average height of
∼0.67 m and 95 %–99 % vegetation cover.
Field measurements and sampled air analysis
Air was sampled at each site by placing three different canisters at
specified heights (see Table 1) along a meteorological tower. The samples
were used to quantify the mixing ratios of different VHOCs in the air, and
their corresponding fluxes were calculated by applying the flux-gradient
method (see Stull, 1988; Maier and Schack-Kirchner, 2014; Meredith et al.,
2014). By default, the differences in height between the canisters
increased exponentially with height, considering the typical decrease in the
vertical gradient of emitted species in the surface layer (Stull, 1988). All
canisters were placed high enough above the ground to ensure that all
sampling was performed within the inertial sublayer, except for the lowest
canister at TMRX–ET. In all cases, the sample footprint fell inside the
target fetch, except for the sampling at COAST–EGD, for which the sample
footprint included a narrow strip of the seawater (estimated at about 40 %
of the footprint). To minimize non-synchronized air sampling by the three
canisters, we constructed a special sampling system that allows almost
simultaneous filling of the canisters. For each sample, air was drawn into a
1.9 L stainless-steel canister via passive grab samplers (Restek
Corporation, PA, USA), resulting in a sampling duration of 20 min and
internal canister pressures higher than 600 Torr. Meteorological parameters,
including temperature and relative humidity, wind speed and direction, and
global solar radiation, were all continuously measured, starting at least 30 min before air sampling was initiated (summarized in Table S6). All
canisters were sent to the Blake–Rowland group, University of California,
Irvine, where they were analyzed by techniques similar to those described in
Colman et al. (2001). Analyses were performed using gas
chromatography combined with mass spectrometry, flame-ionization detection
and electron-capture detection to quantify the air mixing ratios of
CHBr3, C2HCl3, CH2Br2, CHBr2Cl, CHBrCl2,
CHCl3, CH3I, CH3Br and CH3Cl. For all gases, accuracy
ranged from 1 % to 10 %, and analytical precision ranged from 1 %
to
5 % (see Table S2). Note that the lower-height canister analysis for
COAST–TKM-LD-s and the mid-height canister analysis of TMRX–ET-1
indicated an outlier mixing ratio for all VHOCs and for CH3Cl,
respectively (p≪0.01; Grubbs test; Grubbs and
Beck, 1972). We therefore excluded the lower-height COAST–TKM-LD-s
measurement from all of our calculations and used only the lowest and
highest canisters in the flux calculation for TMRX–ET-1, as indicated in
all relevant figures and tables.
Vertical flux evaluation
The vertical flux, Fc, of a species, c, was evaluated according to the
gradient approach using the vertical gradient of c, ∂C∂z, and a constant, Kc:
Fc≡-Kc∂C∂z.Kc represents the rate of turbulent exchange in Eq. (1) and was evaluated
on the basis of the Monin–Obukhov similarity theory (MOST) described by
Lenschow (1995):
φζKC(z)=u∗KZC,
where u∗ is the friction velocity, K is the Von Kármán
constant, Z is the measurement height and φc is a universal
function of the dimensionless parameter ζ. According to MOST,
vertical fluxes in the surface layer can be evaluated on the basis of the
dimensionless length parameter, ζ, according to
ζ=(z-d)/L,
where z, d and L are the vertical coordinate, zero displacement and the
Monin–Obukhov length, respectively (Schmugge and André, 1991).
We relied on the commonly used assumption that φC is similar
to φh for chemical species with a relatively
long lifetime (Dearellano et al., 1995) and calculated
φh using the following equation for the relationship between
φh and ζ, which was found to be valid for 0.004≤-z/L≤4 (Dyer and Bradley, 1982; Yang et al., 2001):
φζh=(1-14)-1/2.
We derived L from the Pasquill and Gifford stability class
(Pasquill and Smith, 1971) and roughness length (z0)
according to Golder (1972). z0 was evaluated based on the
specific surface characteristics at each site using information provided by
Jarraud (2008). The stability class was evaluated using the solar radiation and wind
speed measured in situ (Gifford, 1976; Pasquill and Smith, 1971). u∗ was derived
from the logarithmic wind profile according to MOST, using
the following equation:
u(z)=u∗klnz-dz0,
where u(z) is the wind speed at height z, and ψm is a correction
for diabatic effect on momentum transport. Using the measured u at a height
of 10 m, we calculated the wind speed at each measurement height according
to Gualtieri and Secci (2011):
u2=u1ln(z2/z0)-ψm(z2/L)ln(z1/z0)-ψm(z1/L),
where ψm is calculated using
Ψm(Z/L)=2ln(1+X/2)+ln(1+X2/2)7-2arctan(X)+π/2,
and
X=1-15ZL1/4.
Soil analyses
Soil samples at each site were collected up to a depth of 5 cm during the
summer, at least 3 months after any rain event in the Dead Sea area, to
ensure no impact on the samples by recent drift and percolation. The samples
were analyzed for Br, Cl, I, OM, moisture and Fe in the soil as well as for
soil pH. Prior to halide quantification, extractions for each sample were
prepared using HNO3 (BSI, 1990). Total Br and I were quantified
using inductively coupled plasma mass spectrometry (ICP–MS). Total Cl was
quantified by potentiometric titration against AgNO3.
To quantify Fe in the soil, microwave-assisted digestion with reverse aqua
regia was used, and Fe concentration was determined by inductively coupled
plasma optical emission spectrometry (ICP–OES). A batch of each sample
(∼300 mg of dry soil) was digested in reverse aqua regia
(HNO3 – 65 % – to HCl – 30 %; 3:1 mixture, v/v). Digestion was allowed
to proceed in quartz vessels using a Discover sample digestion system at
high temperature and pressure (CEM Corporation, NC, USA). The vessels were
cooled and the volume was brought to 20 mL with deionized water. Element
concentrations were measured in clear solutions using high-resolution
dual-view ICP–OES PlasmaQuant PQ 9000 Elite (Analytik Jena, Germany). The
reported values represent the lower limit because the samples were not
completely dissolved. Soil water content and OM were determined by weight
loss under dry combustion at 105 and 400 ∘C,
respectively. Soil pH was measured in 1:1 (v/v) soil-to-water extracts with
a model 420 pH meter (Thermo Orion, MA, USA).
Results and discussionVHOC flux and mixing ratio
Overall, the measurements at the Dead Sea boundary layer revealed higher
mixing ratios for all investigated VHOCs than their expected levels at the
Mediterranean Sea and Red Sea MBL, indicating higher local emissions from
the Dead Sea area. No association was observed between the measured mixing
ratios and the air masses flowing from the direction of the Dead Sea Works
(see Sect. S5 for anthropogenic impact), a potash plant located to the
northwest of the TMRX–ET site and to the south of all other measurement
sites (see Fig. 1), that is the main anthropogenic source in the area under
investigation. Furthermore, the correlation analysis (Table S5) revealed
that only C2HCl3 was associated with C2Cl4, a well-known
anthropogenic VHOC. The absence of any other associations suggested
dominance of natural sources for the VHOCs in the studied area. The measured
mixing ratios for the different species at the measurement sites are
summarized and compared with mixing ratios from the MBL in Table S3 and in
Fig. 2. The figure indicates that median mixing ratios measured at the Dead
Sea were generally higher than the corresponding mixing ratios in the MBL.
Our calculations suggest that the mixing ratios at the Dead Sea are higher
by factors of 1.2–8.0 for brominated and chlorinated VSLSs and
∼1.5, 1.3 and 1.1 for CH3I, CH3Br and CH3Cl,
respectively. It should be noted, however, that while Fig. 2 implies
elevated VHOC emission from the Dead Sea, comparison of mean or median
mixing ratios of VHOCs for the Dead Sea with those for the MBL is not
straightforward, considering that VHOC mixing ratios in the MBL are
sensitive to several factors, including season and latitude. Moreover, the
measurement height can play a significant role in affecting the mixing
ratios due to decreasing mixing ratios with height over areas where local
emissions occur. Hence, we also compared the measured fluxes and mixing
ratios with their corresponding values measured in coastal areas, where the
highest mixing ratios in the MBL were generally measured due to stronger
emissions. The measured mixing ratios and fluxes at the Dead Sea were in
most cases comparable to or higher than in coastal areas.
Comparison of VHOC mixing ratios (in pptv) measured at the Dead
Sea with their corresponding values at the marine boundary layer (MBL). For
the Dead Sea sites, boxes indicate median, upper and lower quartiles, and
bars show minimum and maximum VHOC mixing ratios (see Table 1 for site
abbreviations; n specifies the number of samples for each site). For the
MBL, boxes indicate the median, minimum and maximum mixing ratios reported
by Carpenter et al. (2014). a Values for
CH3Cl and CH3Br represent mean and range for 2012 based on flask
measurements by the US National Oceanic and Atmospheric Administration
(NOAA; http://www.esrl.noaa.gov/gmd/dv/site/,
last access: 2019) and in situ
measurements by the Advanced Global Atmospheric Gases Experiment (AGAGE; http://agage.eas.gatech.edu/,
last access: 2019), which were performed at ground
stations, not in all cases representing the MBL. * Related CH3Cl
measurement excludes one sample at TMRX–ET-1 (see Sect. 2.1.2). ** Related
measurements exclude one sample for all VHOCs (see Sect. 2.1.2).
Owing to their large contribution to stratospheric Br, CHBr3 and
CH2Br2 are the most extensively studied VSLSs in the MBL
(Hossaini et al., 2010). The mixing ratios of CHBr3 and
CH2Br2 that we measured at the Dead Sea ranged from 1.9 to 22.6
pptv and from 0.7 to 18.6 pptv, respectively, higher than most of their
reported mixing ratios in coastal areas where the highest mixing ratios have
typically been measured. For example, Carpenter et al. (2009)
reported elevated mixing ratios for CHBr3 and CH2Br2 along
the eastern Atlantic coast, ranging from 1.9 to 4.9 and from 0.9 to 1.4 ppt,
respectively, and Mohd Nadzir et al. (2014) reported mixing ratios of
0.82–5.25 pptv and 0.90–1.92 ppt for CHBr3 and CH2Br2,
respectively, for several tropical coastal areas, including the Strait of
Malacca, the South China Sea and the Sulu–Sulawesi Sea. Somewhat higher
mixing ratios for CHBr3 have been measured in only a few locations,
including some in coastal areas near New Hampshire (Zhou et al., 2008),
San Cristóbal Island (Yokouchi et al., 2005; O'Brien et al., 2009), Cabo Verde (O'Brien et al., 2009), Borneo (Pyle et al.,
2011), Cape Point (Kuyper et al., 2018; Butler et al., 2007) and at the
Atmospheric Observing Station at Thompson Farm (TF) in New Hampshire, USA,
during the summer (Zhou et al., 2005), whereas the range (and average)
concentrations at those locations were 0.2–37.9 pptv (5.6–6.3),
4.2–43.6 pptv (14.2), 2.0–43.7 pptv (4.3–13.5), 0.2–60 pptv
(1.3–1.7), 4.4–64.6 pptv (24.8) and 0.6–37.9 pptv (2.6–5.9),
respectively. For CH2Br2, the corresponding mixing ratios were
reported as 1.3–2.3, 0.5–4.1, 0.7–8.8 and 0.4–4.2 pptv
in New Hampshire, San Cristóbal Island, Cabo Verde and TF, respectively,
which are comparable with the mixing ratios measured at the Dead Sea.
VHOC fluxes for the different measurement sites. Shown is the
measured flux (nmol m-2 d-1) obtained for the different
measurements. Values in bold and in parentheses indicate that the related
measurement site is a significant (p < 0.05) or non-significant (p > 0.15)
net source or sink for the specific VHOC based on
one-sample t test. Additional categories are defined below. These
calculations assume COAST–EGD-SD and COAST–TKM-SD to be the same source (see
Sect. 2.1.2). Also shown are the average flux (mean) and average positive
flux (mean positive) for all species, as well as the percentage of incidence
of positive flux (X) out of total measured fluxes, individually for each site
and each VHOC (See Table 1 for abbreviations of the different measurement
sites). All presented values, including mean, mean positive and X include
only fluxes associated with p < 0.05 (bolded; S) and values associated
with p≥0.05 (presented in parentheses; NS), based on one-sample
t test.
a 0.05 < p < 0.1 for a measurement site as net source
or sink for a specific species.
b 0.1 < p < 0.15 for a measurement site as a net source
or sink for a specific species; S and NS indicate p < 0.05 and p > 0.05, respectively.
c Flux calculation excludes one measurement for all VHOCs (see Sect. 2.1.2).
d Flux calculation excludes one CH3Cl sample (see Sect. 2.1.2).
Table 2 presents the measured fluxes of all VHOCs studied alongside the
corresponding statistical significance for a specific species' emission or
depletion to a specific site. Note that considering the similar
characteristics of the two SD sites, and of the two BARE sites, we assumed there to be a
common emission source from the two sites, in both cases, in evaluating the
statistical significance for these sites as a net source or net sink for
the studied species. Considering the small number of measurements at each
site, the table classifies the statistical significance of the fluxes'
negative or positive values at a specific site into four different
categories. While p values < 0.05 are used here to indicate
statistical significance, p values of < 0.1 and < 0.15 are
also indicated when present.
VHOC fluxes at the different measurement sites. Fluxes associated
with p values < 0.05 are marked by colored circles to indicate
measurements during spring, winter and summer, with solid colored circles
indicating measurements up to 3 d after a rain event in spring
(Spring-prec.), up to 6 d after a rain event in winter (Winter-prec.) and
in the evening in summer (Summer-eve.). Gray and black shapes indicate fluxes
associated with no clear statistical significance (p > 0.1 and
0.05 < p < 0.1, respectively). At the center of each graph,
the small black circles and error bars represent the average and standard
error of the mean (SEM), respectively, for each measurement site. Dashed
lines represent zero flux. In each box, the numbers indicate the mean flux
and SEM (in parentheses) for each site and species. Additional information
is provided for measurement conditions (Tables 1 and S6), measurement
abbreviations (Table 1) and statistical analysis (Table 2). * Calculation of
CH3Cl flux mean and SEM excludes one sample at TMRX–ET-1 (see Sect. 2.1.2).
** Calculation of mean flux and SEM excludes one sampling canister at
COAST–TKM-LD (see Sect. 2.1.2).
Figure 3 presents the measured fluxes of all VHOCs studied individually, for
statistically significant and non-significant fluxes emitted or depleted to
a specific site. Non-significant fluxes are marked with black and gray for
0.05 < p < 0.1 and p > 0.1, respectively. It can be
seen that for all species, at least one of the six studied areas could be
classified as a net source, with slightly fewer sites being statistically
significant net sources for CHCl3, C2HCl3 and CH3I. Note
that as explained above, C2HCl3 was found to be affected by
anthropogenic emission, which could explain the relatively less-frequent
identified emissions for this species. Figure 3 clearly demonstrates that
the COAST sites, and particularly the SD sites, are associated with the
highest number of VHOCs with positive flux. These sites were also found to be a
source for CHCl3, C2HCl3 and CH3I. Figure 3 does not
indicate elevated VHOC emissions from the vegetated sites (WM–KLY and
TMRX–ET) compared to the BARE sites.
The flux magnitudes for CHBr3 and CH2Br2 were greater than
for most reported emissions in the MBL (e.g., CHBr3, 25.2–62.88 nmol m-2 d-1 for the Mauritanian upwelling – Quack et al.,
2007; CH2Br2, 0.14–0.29 nmol m-2 d-1 for the New
Hampshire coast – Zhou et al., 2008) but were smaller than the
corresponding average fluxes estimated by Butler et al. (2007) for global
coastal areas (∼220 and 110 nmol m-2 d-1,
respectively) and than the average flux from the New Hampshire coast as
reported by Zhou et al. (2005; ∼620±1370 and 113±130 nmol m-2 d-1, respectively).
Relatively high positive CHCl3 fluxes were measured for BARE–MSMR (247 nmol m-2 d-1), TMRX–ET-2 (213 nmol m-2 d-1) and
COAST–EGD-SD-s (883 nmol m-2 d-1), although the latter two sites
were not identified as a net source for CHCl3 (Table 2). For
comparison, the emission from BARE–MSMR-1 was similar to the maximum
emission found for tundra peat by Rhew et al. (2008), whereas
the average emissions from COAST–EGD-SD-s and TMRX–ET-2 were comparable to
those from temperate peatlands (∼496 nmol m-2 d-1
as measured by Dimmer et al., 2001). Whereas emissions for
COAST–EGD-SD-s and TMRX–ET-2 might have been affected by seawater and
vegetation, respectively, the emission for BARE–MSMR can be completely
attributed to soil. The latter emission flux in BARE–MSMR was higher than
the maximum emission rate in arctic and subarctic soils (∼115 nmol m-2 d-1) reported by Albers et al. (2017).
All investigated site types, except for the natural vegetation (TMRX–ET),
were identified as net sources for CHBr2Cl and CHBrCl2 (Fig. 3).
The mixing ratios of CHBr2Cl and CHBrCl2 were higher by factors of
∼4–14 and ∼5–11, respectively, than the
average reported values for the MBL and were also higher than the mixing
ratios measured in nearby coastal areas, except for the extremely high
CHBr2Cl mixing ratios attributed to emission from a rock pool at Gran
Canaria (ranging from 19 to 130 ppt; Ekdahl et al., 1998). For
example, Brinckmann et al. (2012) found mean mixing ratios for
CHBr2Cl and CHBrCl2 in coastal areas of the Sylt Islands
(North Sea) of up to 0.2 and 0.1 ppt, respectively, while Mohd Nadzir et al. (2014) found CHBr2Cl and CHBrCl2 mixing ratios of 0.07–0.15
and 0.15–0.22 ppt, respectively, in the tropics. The measured CHBr2Cl
fluxes for the Dead Sea were also higher than the reported value of 0.8 nmol m-2 d-1
(range of -1.2–10.8 nmol m-2 d-1) at coastal areas sampled during
the Gulf of Mexico and East Coast Carbon Cruise (GOMECC; Liu et al., 2011). Typically, the net CHBrCl2 flux at
the Dead Sea was significantly higher than corresponding fluxes from arctic
and subarctic soils, as recently reported by Albers et al. (2017),
ranging from 0.03–5.27 nmol m-2 d-1.
COAST–TKM and COAST–EGD-SD were found to be the only net source sites for
CH3Cl. The highest positive fluxes were measured at COAST–EGD-SD and
COAST–TKM-SD, with maximum net fluxes of ∼10800 and 4900 nmol m-2 d-1, respectively. These fluxes are comparable in
magnitude to those reported for several terrestrial sources, such as
tropical forests (∼4520 nmol m-2 d-1), by
Gebhardt et al. (2008) or by Yokouchi et al. (2002) and
for other tropical or subtropical vegetation (Yokouchi et al.,
2007), and they are higher than emissions from dryland ecosystems, including
shortgrass steppe or shrublands (Teh et al., 2008). In some cases, the
measured fluxes were higher than average emissions from salt marshes (e.g.,
∼7300 nmol m-2 d-1; Deventer et
al., 2018) but significantly smaller than the maximum fluxes from salt
marshes (e.g., 570 000 nmol m-2 d-1; Rhew et al.,
2000).
Both COAST–TKM and COAST–EGD sites were identified as net sources,
while with less statistical significance (p < 0.1),
BARE–MSMR was also identified as a net source of CH3Br (Table 2). In contrast to CH3Cl, emissions
of CH3Br at the Dead Sea were significantly lower than the average
reported emissions from marshes (e.g., ∼600 nmol m-2 d-1; Deventer et al., 2018). The fluxes measured
at the Dead Sea were also lower than the reported emission from a coastal
beach on a Japanese archipelago island (∼53000 nmol m-2 d-1) but higher, in most cases, than in other dryland ecosystems (see
Rhew et al., 2001).
Similar to CH3Br and CH3Cl, for CH3I, COAST–TKM and
COAST–EGD, and particularly the SD sites, were identified as net sources
(Table 2). BARE–MSMR was also identified as a net source for CH3I.
Positive measured net fluxes of this compound were in most cases comparable
to other reported fluxes over soil and vegetation. For example,
Sive et al. (2007) reported a CH3I flux of
∼18.7 nmol m-2 d-1 over soil and vegetation at TF
and a somewhat lower emission (∼12.6 nmol m-2 d-1)
in Duke Forest, NC, USA. While the elevated flux at COAST–EGD-SD-s (17.0 nmol m-2 d-1) could potentially have been affected by flow of the
sampled air over the seawater, the positive net fluxes at BARE–MSMR (1.00
and 4.42 nmol m-2 d-1) indicate significant emission from bare
soil at the Dead Sea. The positive fluxes measured at BARE–MSMR were
similar to the measured soil-emission fluxes of CH3I reported by
Sive et al. (2007) at Duke Forest, averaging ∼0.27 nmol m-2 d-1 (range of ∼0.11–4.1 nmol m-2 d-1).
Only COAST–EGD and COAST-TKM-SD sites were found to be statistically
significant sources (p < 0.05, see Table 2) for C2HCl3,
suggesting that the elevated mixing ratios for this species in the Dead Sea
area result mostly from local anthropogenic emissions. This possibility is
supported by the high correlations with C2Cl4 (Table S5).
Emissions from a more distant natural source, such as the Mediterranean Sea
or Red Sea, are unlikely given their large distance (∼90 and ∼160 km, respectively).
Bar graphs of VHOC fluxes from the different site types, organized
by relative orientation to the Dead Sea and with visual indicators of
surface cover type. Colored bars represent measured fluxes associated with
p values < 0.05. Gray and black bars indicate fluxes associated with
no clear statistical significance (black for 0.05 < p < 0.1
and gray for p > 0.1). Circles with drop lines are used to mark
fluxes with relatively low values. Different colors refer to different VHOCs
as indicated at the top of the figure. The different site types are
indicated in the legend. S, M and L indicate short, medium and long distance
of the measurement site from the seawater for the coastal sites (SD, MD and
LD, respectively; see Sect. 2.1.1). See Table 1 for measurement sites and
measurement abbreviations. *CH3Cl flux calculation excludes one sample
in TMRX–ET-1 (see Sect. 2.1.2). ** Flux calculation excludes one sampling
canister in COAST–TKM-LD (see Sect. 2.1.2).
Factors controlling VHOC fluxSeasonal, meteorological and spatial effects
The results presented in Sect. 3.1 showed elevated mixing ratios and net
fluxes for all investigated VHOCs, with relatively less-frequent positive
fluxes for CH3I, CHCl3 and C2HCl3. For all of the
investigated VHOCs, a positive flux was measured for at least one of the two
bare soil sites, BARE–MSMR and BARE–MSD, which are located a few
kilometers from the Dead Sea water. For several VHOCs (CH2Br2,
CHBr2Cl, CHBrCl2 and CHCl3), at least one of these sites was
identified as a significant net source (p < 0.05, Table 2).
Additional measurements are required to determine whether the other VHOCs
are also emitted from these bare soil sites. Note that for all VHOCs except
C2HCl3 and CH3Cl, measured mixing ratios were highest over at
least one of these bare soil sites (Table S3). Figure 4 further provides the
spatial distribution of the investigated VHOCs at the various sites.
Elevated positive fluxes are seen at the coastal sites, with a general
tendency toward higher positive net fluxes closer to the seashore. Figure 4
also demonstrates relatively high positive fluxes for the natural vegetation
in TMRX–ET, higher than for WM–KLY. However, additional measurements are
required to decipher whether this site can be classified as a statistically
significant source for VHOCs (see Table 2).
No clear impact of meteorological conditions on the measured net flux rates
or mixing ratios was observed. We could not identify any clear association
between flux magnitude and any parameter, including solar radiation
intensity, measurement time, temperature and daytime relative humidity.
Our findings on the effects of season and distance from the sea on the
measured fluxes are presented in Fig. 3, which shows the measured fluxes for
spring and winter and for different distances from the sea at COAST–EGD
and COAST–TKM. Differences in VHOC emissions between winter and spring may
arise from the generally much higher temperature and lower precipitation
during the latter; further considering the high evaporation rate in this
area, the soil water content is expected to be generally lower in spring
compared to winter (Sect. 2.1.1; see also Table S6). Figure 3 suggests that
there were no clear differences in VHOC fluxes between spring and winter, as
supported by statistical analysis, except for CH3I and CH2Br2, for which fluxes were higher in the spring, with moderate
statistical significance (0.05 < p < 0.1).
No clear impact of distance from the seawater on the measured net fluxes
could be detected, including in cases where a significant fraction of the
footprint included the seawater, such as for COAST–EGD-SD-w and
COAST–EGD-SD-s. However, owing to variations in soil properties, the
emissions near the seawater tended to be more frequent and more intense (see
Sect. 3.2.2, 3.2.3).
Seasonal and spatial influences on measured mixing ratios of VHOCs
for coastal sites only. (a) Measured VHOC mixing ratios are presented vs.
vertical height above surface level, separately for winter (blue) and spring
(orange). Black filled circles and error bars represent average and standard error of the mean (SEM),
respectively. LD, MD and SD indicate long, medium and short distance from
the seawater, respectively, while SEA–KDM is located at the seawater (see
Sect. 2.1.1). Values above and to the right of the figure indicate the
percentages of higher average mixing ratios in spring (left box) or winter
(right box) individually, for each site (SEA–KDM, COAST–TKM and EGD sites)
and for each specific species. (b) For each species, the
average mixing ratios over all sites (SEA–KDM, COAST–EGD and COAST–TKM)
are presented (All), and the corresponding percentage of higher average
mixing ratios in spring and in winter are also presented. See Table 1 for
measurement site abbreviations. Species with no observed difference between
seasons were excluded (see Fig. S1 in the Supplement for complete information); y axes for
sites in the same coastal area (COAST–TKM or COAST–EGD) are evenly scaled.
* Measurements exclude one sampling canister at COAST–TKM-LD (see Sect. 2.1.2).
Figure 5 compares the mixing ratios of the measured VHOCs at different
distances from the seawater individually for winter and spring. Note that
differences in sampling heights at different sites can lead to a biased
comparison between mixing ratios at different sites; nevertheless, in most
cases, differences across measurement sites were larger than across vertical
heights. No clear impact of season or distance from the seawater on the
mixing ratios can be discerned in this figure, also based on the sampling
over SEA–KDM, which directly represents air masses over the seawater (Sect. 2.1.1). Nevertheless, further investigation, using direct flux measurements
over the Dead Sea water, is needed to study the potential emission of VHOCs
from this water body. While no clear impact of season on mixing ratios was
observed, for most sites, differences between two measurement sets resulted
in consistent differences in mixing ratios such that one measurement set
resulted in higher mixing ratios for all or most species than the other.
This suggests that other factors play a significant role in emission rates
of all or most VHOCs in the studied area. Only the CH3I results
indicated moderate statistical significance (0.05 < p < 0.1)
for higher mixing ratios in the spring vs. winter, in agreement with
seasonal trends for its flux, as discussed above.
Impact of specific site characteristics and ambient conditions
The formation of VHOCs requires a chemical interaction between OM and
halides, induced by biogeochemical, biochemical or macrobiotic processes
(Kotte et al., 2012; Breider and Albers, 2015). Despite the extreme
salinity, biotic activity was detected in both the water and the soil of the
Dead Sea (see Sect. 1), demonstrating that biotic activity can potentially
contribute to VHOC emission in this area. Previous studies on emission of
VHOCs from soil and sediments revealed that OM content and type, halide ion
concentrations, pH, and the presence of an oxidizing agent (most frequently
referred to as Fe(III)) also play important roles in the emission rate of
VHOCs (see Kotte et al., 2012).
Soil properties – OM, soil water content (SWC), I, Br, Cl and Fe
fraction of dry weight and pH. Analyses were performed for a single mixture
of samples at each site. See Table 1 for measurement site abbreviations.
SitepHOMSWCIBrClFe(%)(%)(mg kg soil dw-1)(g kg soil dw-1)(g kg soil dw-1)(mg kg soil dw-1)BARE–MSMR7.461.961.902.240.0076.70> 20 800BARE–MSD7.413.613.612.790.02741.2> 7450COAST–EGD-SD7.612.281.790.241.47202> 1120COAST–EGD-MD7.930.350.350.570.29337.4> 3140COAST–EGD-LD7.703.672.581.030.00826.1> 5950COAST–TKM-SD7.4324.133.73.193.93169> 12 500COAST–TKM-LD7.803.401.641.140.18619.5> 10 600TMRX–ET7.883.142.972.690.47485.2> 10 100WM–KLY7.644.101.401.690.0131.12> 7680
Table 3 provides a basic representation of the soil composition parameters.
The results presented in Table 3 show substantial enrichment of Cl and Br in
the sites closest to the seawater (COAST–EGD-SD and COAST–TKM-SD) and
lower concentrations at greater distances from the seawater. For comparison,
both Br and Cl concentrations were generally much higher than those reported
by Kotte et al. (2012) for various saline soils and sediments
(0.12–0.32 and 6.1–120 g kg-1, respectively) but lower
for Br at BARE–MSMR and BARE–MSD and for both Cl and Br at WM–KLY. No
enrichment of I in the soil samples was observed (e.g., Keppler et
al., 2000; Kotte et al., 2012). The OM content of the samples was generally
higher than would be expected in desert soil. For comparison, forest floors
typically contain 1 %–5 % OM (Osman, 2013). Detection of VHOC
emissions from the soil is, in some cases, associated with higher soil OM
(e.g., Albers et al., 2017; Keppler et al., 2000) and, in some cases, with
lower soil OM (e.g., Kotte et al., 2012; Huber et al., 2009) than that
reported here. Table 3 provides only a lower limit of the total Fe, rather
than Fe(III), in the samples. Note, however, that soil Fe content similar to
that reported here as a low-limit value corresponds with that associated
with the finding of small amounts of VHOC emissions, while the emission
rates become saturated when enrichment with Fe(III) is relatively minor
(Keppler et al., 2000). Saturation at relatively low soil Fe
concentrations was also reported by Huber et al. (2009). Hence,
variations in Fe across different sites may play a minor role in affecting
emission rates.
While the number of samples collected at each site was limited, Table 2 and
Fig. 4 indicate elevated positive fluxes for the SD sites, and to some
extent also at COAST–EGD-MD, with respect to both statistically
significant and non-statistically significant positive fluxes. Moreover, for
both COAST–EGD and COAST–TKM, during both spring and winter, the
occurrence of positive fluxes was correlated with proximity to seawater
(i.e., COAST–EGD-SD > COAST–EGD-MD > COAST–EGD-LD and COAST–TKM-SD > COAST–TKM-LD). All of
these COAST sites contain mixtures of soil and salt-deposited structures
(see Sect. 2.1.1), and Table 3 indicates that soil concentrations of both Br
and Cl correlated with proximity to seawater at both COAST–EGD and
COAST–TKM. The concentration of I in the soil showed a similar trend only
at the COAST–TKM sites (see Table 3). The association between the
magnitude and incidence of the positive net flux and soil halide
concentrations points to an increase in VHOC emission with salinity, even
under the hypersaline conditions of the Dead Sea area. This interpretation
is supported by the fact that whereas for COAST–TKM-SD, both soil water
and OM content were relatively high, for COAST–EGD-SD, no other measured
parameter which could limit the emission of VHOCs, except for the soil
halide concentration, was higher than for both COAST–EGD-MD and
COAST–EGD-LD (Table 3). The fact that emission rates for COAST–TKM
tended to be similar or lower in terms of incidence and magnitude compared
to COAST–EGD (Table 2) suggests, in view of the apparently lower Fe
content for the latter (Table 3), that the emission of VHOCs from these
sites is not significantly limited by the availability of Fe(III) in the
soil.
COAST–EGD-SD-s was associated with the highest incidence of both
statistically significant and non-significant positive fluxes. Fluxes at
COAST–EGD-SD-w were generally lower and with a smaller incidence of
positive fluxes. Based on the wind direction, in both cases, the sampling
footprint included both the seawater and a narrow strip of bare soil mixed
with salty beds (estimated at about 60 % of the footprint) very close to
the seawater. The main notable difference between the two measurement days
was that precipitation occurred just before the COAST–EGD-SD-w
measurement, whereas there was no precipitation event for several weeks
prior to the COAST–EGD-SD-s measurement (Table 1). Rain events also
occurred ∼1.5 and ∼2.5 d before
BARE–MSD-1 and BARE–MSD-2 measurements, respectively. Note that the
emission fluxes for BARE–MSD-1 were lower and more negative for most of
the species than those for BARE–MSD-3 or BARE–MSD-4. In addition, the
occurrence of positive net fluxes tended to increase according to the order
BARE–MSD-1 < BARE–MSD-2 < BARE–MSD-3 (see Table 2).
The analyses for both COAST–EGD and BARE–MSD suggest that increased soil
water content caused by rain events can decrease the emission rates or
enhance soil-uptake rates of certain VHOCs.
A reduction in net flux rates following rain events did not occur for all
species and was not clearly consistent across the BARE–MSD and
COAST–EGD-SD sites. Thus, further research on the effects of rain on the
various VHOCs and ambient conditions is required. Nevertheless, the analyses
clearly demonstrate that strong emission rates do not depend on rain
occurrence, in agreement with findings by Kotte et al. (2012).
The lower emission fluxes following the rain event may be attributable to
the low infiltration rate of VHOCs into the soil, to salt dilution and
washout, or both.
Our measurements suggested an elevated contribution of natural vegetation to
some of the investigated VHOCs (Fig. 4), but with no statistical
significance for this site being a source of any of the investigated VHOCs
(Table 2). This might reflect the fact that only a few measurements are
available for this site. No clear contribution of the agricultural
vegetation to the emission fluxes was found in this study.
Factors controlling the flux of specific VHOCs
Trihalomethanes. Differently from previous studies,
brominated VHOCs had relatively higher overall incidence of positive fluxes
than chlorinated VHOCs (Table 2). The overall average net flux of
trihalomethanes decreased according to CHBr2Cl > CHCl3
> CHBr3 > CHBrCl2, while CHCl3 showed
the lowest incidence of positive and highest mean positive fluxes among all
trihalomethanes.
Natural emission of trihalomethanes from soil has been shown to occur
without microbial activity, induced via oxidation of OM by an electron
acceptor such as Fe(III) (Huber et al., 2009) or via hydrolysis
of trihaloacetyl compounds (Albers et al., 2017). The soils studied by
Albers et al. (2017) were significantly richer in OM than the soils at
the Dead Sea, except for COAST–TKM-SD. Hence, the apparently higher
emission from the Dead Sea soil may indicate either a different mechanism
leading to the release of trihalomethanes from the soil or only a weak
dependency on availability of soil OM. The latter explanation may be
supported by the fact that Albers et al. (2017) did not find any
correlation between the CHCl3 emission rate and organic Cl in the soil.
Furthermore, our study points to higher emission rates and incidence of
VHOCs, and generally also of trihalomethanes, closer to the seawater
(COAST–EGD and COAST–TKM sites), which suggests higher sensitivity to
soil halide content than OM (Sect. 3.2.2).
While trihalomethane formation via OM oxidation has been reported to occur
more rapidly at low pH, and specifically at pH < ∼3.5
(Huber et al., 2009; Ruecker et al., 2014), its formation via hydrolysis
of trihaloacetyl is expected to occur more rapidly at the relatively high pH
≥7 (Hoekstra et al., 1998; Albers et al., 2017). Yet according to
Ruecker et al. (2014), in hypersaline sediments, the formation of
VHOCs via OM oxidation involving Fe(III) can occur at pH > 8 for
biotic processes. Therefore, given the relatively high pH (∼7.4–7.9; Table 3) at the SD sites as well as the BARE and WM–KLY sites,
the high trihalomethane-emission rates from both bare and agricultural field
sites support the work by Albers et al. (2017) concerning the emission of
trihalomethanes from the soil following trihaloacetyl hydrolysis.
Albers et al. (2017) showed that their proposed mechanism supports the
emission of CHCl3 and CHBrCl2 from soil and suggested that
additional halomethanes with a higher number of Br atoms can be expected to
be emitted via this mechanism but at much lower rates. Hence, the elevated
net fluxes for CHBr2Cl and CHBr3 at the Dead Sea (Table 2) could
occur either because of the markedly higher composition of Br in the Dead
Sea soil (see Table 3) or because another mechanism is also playing a role
in the emission; note that agriculture could potentially be a source for the
emission of CHBr2Cl and CHBr3 for WM–KLY but not for the other
sites (Sect. 2.1.1). The finding of Hoekstra et al. (1998) that Br
enrichment mainly enhances the emission of CHBr3 and CHBr2Cl,
rather than that of CHBrCl2, supports the former possibility, namely,
relatively elevated emission of CHBr2Cl and CHBr3 due to higher Br content in the soil. While both Cl and Br soil contents are
relatively high for both COAST SD sites and COAST–EGD-MD, where emission of
brominated trihalomethanes was higher than that of chlorinated
trihalomethanes (see Table 2), a remarkably high Br/Cl value (1:43) relative
to other sites was found at COAST–TKM-SD. Table 2 does not indicate a
clear difference in the flux magnitude of the brominated compared to
chlorinated trihalomethanes for this site, suggesting that the main reason
for the relatively elevated brominated trihaloethanes at the SD sites and
COAST–EGD-MD is the high Br content rather than the Br/Cl ratio.
The relatively elevated net flux of brominated trihalomethanes from BARE and
WM–KLY indicates that relatively high rates of emission of these species
can also occur from soils that are much less rich in Br than the SD sites
and the COAST–EGD-MD site (see Tables 2, 3). Yet the emission rates of
CHBrCl2 at the Dead Sea were generally higher than those observed by
Albers et al. (2017), probably reflecting the higher soil Cl content at
the Dead Sea.
Methyl halides. A relatively high incidence of
negative fluxes was observed for CH3Br, and more statistically
significantly so for CH3Cl and CH3I, implying high rates of both
emission and deposition, at least for the latter two, in the studied area
(Table 2). The average positive flux of CH3Cl was the highest of all
VHOCs investigated, indicating strong emission and deposition for this
species at the Dead Sea. Several studies have indicated that soil tends to
act as a sink for CH3Cl (Rhew et al., 2003). The relatively
high positive net fluxes of CH3Cl and CH3Br at WM–KLY-1 (983 and
53.5 nmol m-2 d-1, respectively) may point to emission of this
species from the local agricultural field, in agreement with previous
studies (Sect. 1), potentially by microbially induced or fungus-induced emission
(Moore et al., 2005; Watling and Harper, 1998), but this should be further
investigated, considering the lack of statistical significance.
Positive net fluxes for CH3I were not significantly higher than those
obtained in previous studies (Sect. 3.1), a finding that might be attributed
to the small concentration of I in the soil relative to those of the other
halides. At Duke Forest, Sive et al. (2007) observed a
soil-emission CH3I flux of ∼0.27 nmol m-2 d-1
on average (ranging from ∼0.11 to 0.31 nmol m-2 d-1) under precipitation conditions in June and higher emission
rates (0.8 and 4.1 nmol m-2 d-1) under warmer and dryer conditions
in September. In agreement with those findings, although in general our
analyses did not indicate clear seasonal effects, we found that in all
cases, net CH3I fluxes were higher in spring than in winter, except for
COAST–TKM-SD (Fig. 3). As discussed in Sect. 3.2.1, the mixing ratios of
CH3I also tended to be higher in magnitude in spring compared to
winter, with moderate statistical significance (0.05 < p < 0.1 in both cases; Figs. 3, 5).
Relatively high fluxes of CH3Cl and CH3Br and, to a lesser
extent,
of CH3I were observed at the COAST–TKM and COAST–EGD sites,
particularly from the sites closest to the seawater (Fig. 4). According to
Keppler et al. (2000), the presence of Fe(III), OM and halide
ions is basically enough to result in emission of methyl halides from
both soil and sediments by a natural abiotic process (Sect. 1). The strong
emission of methyl halide from the COAST–TKM and COAST–EGD sites
indicates that these species can be emitted at high rates from saline soil
that is not rich in OM. The strongest emissions occurred from COAST–TKM-SD
and COAST–EGD-SD, which may indicate high sensitivity of methyl halide
emission to soil OM and/or halide content (see Table 3). The fact that the
emission of methyl halides, particularly CH3Br and CH3Cl, from
COAST–TKM-SD, where soil OM is substantially higher than at all other
investigated sites, was not higher than the emission from COAST–EGD-SD-s
may indicate that emission of methyl halides was not sensitive to soil OM in
our study. Note that the lower fluxes for EGD-SD-w compared to EGD-SD-s can
be associated to a prior rain event for the former (Sect. 3.2.2).
In controlled experiments to study emissions of the three methyl halides
from soil, Keppler et al. (2000) found a decrease in the
efficiency of methyl halide emission according to CH3I > CH3Br > CH3Cl (10:1.5:1; mole fractions).
We estimated
the emission efficiencies of the different methyl halides based on the ratio
between their fluxes and the concentrations of halide in the soil. To
maintain consistency with the calculations of Keppler et al. (2000), our calculation was also based on mole fractions and took into
account only positive fluxes, on the assumption that they are closer in
magnitude to emission. This corresponded with measured soil halide
concentration proportions for Cl:Br:I of 2.4E5:1.5E3:1, and the evaluated
emission efficiency proportions were 15:1.4:1 for CH3I, CH3Br and
CH3Cl, respectively, when two outliers were excluded from the
calculations. These calculations confirmed the increasing efficiency of
methyl halide emission following CH3Cl < CH3Br < CH3I, in agreement with Keppler et al. (2000), suggesting
that at least the methylation and emission of CH3Br and CH3Cl in
our study were controlled by abiotic mechanisms similar to those reported by
Keppler et al. (2000). The apparently higher relative efficiency
of CH3I emission may indicate emissions of CH3I via other
mechanisms in the studied area, as discussed in Sect. 3.3. It should be
noted, however, that the fluxes that we used for the methyl halide emission
efficiencies were based on measured net flux rather than measured emission
flux. This might also explain the inconsistency between the relative
CH3I-emission efficiency calculated by Keppler et al. (2000) and by us.
C2HCl3.
C2HCl3 had the second-lowest incidence of positive fluxes, with
statistically significant (p < 0.05) positive fluxes only from the
COAST SD sites and COAST–EGD-MD (Table 2). These sites are mixtures of salt
beds and deposits with salty soil, and therefore the elevated emissions of
C2HCl3 at these sites appear to support previous evidence for the
emission of this gas by halobacteria from salt lakes, as reported by
Weissflog et al. (2005). Additional chlorinated VHOCs, including
CHCl3 and CH3Cl, also demonstrated increased emission from this
site, in line with the findings of Weissflog et al. (2005). Note that the
net measured fluxes for most of the VHOCs investigated at the
COAST–EGD-SD-w site were smaller than those at COAST–EGD-SD-s, as
discussed in Sect. 3.2.2.
CH2Br2.
CH2Br2 showed positive fluxes from all site types, with a positive
average net flux from most sites (see Fig. 3), but its fluxes over the
vegetated and agricultural sites were not statistically significant.
Correlation of CH2Br2 with trihalomethanes will be discussed in
Sect. 3.3.
Correlations between the mixing ratios of VHOCs. Shown is the
Pearson correlation coefficient (r) between each VHOC pair for the measured
mixing ratio, when calculated over all sites excluding SEA–KDM (NO-KDM),
all sites (ALL), bare soil sites (BARE), coastal sites (COAST), short
distance from the sea at the coastal sites (SD) and the vegetated sites
(VEG). Correlations were calculated for mean mixing ratios at each site. The
p value for r being significantly different from zero is indicated based on
one-sample t test, in four categories. For values in bold, p<0.05. For values in
parentheses, p > 0.15. ap≪0.1. bp < 0.15.
a Correlation calculation for COAST–TKM-LD excluded one sampling canister
(see Sect. 2.1.2).
b Correlation calculation for CH3Cl excluded one sample for
TMRX–ET-1 (see Sect. 2.1.2).
Flux and mixing ratio correlations between VHOCs
Table 4 presents the Pearson correlation coefficients (r) between the
measured mixing ratios of VHOCs at the Dead Sea, separately for all sites
and for the terrestrial sites only as well as separately for BARE, COAST,
and the natural vegetation and agricultural field sites (VEG). For COAST, r
is also presented individually for the two sites which were closest to the
seawater (SD). The correlations' significance levels are also indicated. In
most cases, the correlations between species over all terrestrial sites were
low but were substantially higher for the brominated trihalomethanes
(CHBr3–CHBrCl2 – r=0.79; CHBr2Cl–CHBrCl2
– r=0.87; CHBr2Cl–CHBr3 – r=0.85),
supporting a common source mechanism for these species. High correlations
between these three trihalomethanes can be attributed to high correlations
at the BARE and VEG sites. Relatively high correlations were also obtained,
although to a lesser extent, between methyl halides, particularly between
CH3Cl and CH3Br (r=0.75), which can be attributed to
correlations at the COAST sites, particularly at the SD sites. For COAST, and
particularly for SD, a high correlation was observed between
C2HCl3 and CHCl3. Correlations were in most cases either
similar or smaller when we included measurements from the seawater site
SEA–KDM, which may reinforce the notion that emission from the seawater
does not contribute significantly to VHOC mixing ratios in the area of the
Dead Sea.
Correlations between the measured net fluxes of VHOCs. The table
records the Pearson correlation coefficient (r) for the measured net flux
between each VHOC pair, calculated over all sites except SEA–KDM (ALL),
bare soil sites (BARE), coastal sites (COAST), short distance from the sea
at the coastal sites (SD) and the vegetated sites (VEG). The p value for
r,
being significantly different from zero, is indicated based on t test in
four categories. By default, for bolded values, p < 0.05. For values in parentheses,
p > 0.15. ap < 0.1. bp < 0.15.
a Correlation calculations for COAST–TKM-LD excluded one sampling canister
(see Sect. 2.1.2).
b Correlation calculation for CH3Cl excluded one sample for
TMRX–ET-1 (see Sect. 2.1.2).
Table 5 shows the correlations between the measured VHOC fluxes, separately
for all sites (ALL), BARE sites, VEG sites, the TMRX–ET site and WM–KLY site, and COAST–TKM and COAST–EGD sites. For the latter two sites,
correlations are also presented separately for the SD sites. Note that the
table compares net flux rather than emission flux, and therefore the
reported correlations are expected to be affected by both sinks and sources
for the different VHOCs.
The results in Table 5 show moderate to high positive correlations in most
cases when all sites are included in the calculation, whereas in many cases,
the correlations were significantly higher when calculated for sites of the
same type, suggesting common emission mechanisms or controls. High
correlations were obtained for VEG between CHBrCl2, CHBr2Cl and
CHBr3 (r≥0.94; p < 0.05), except for the correlation
between CHBr2Cl and CHBr3 (r=0.82; p > 0.15). Note
that these correlations can potentially be attributed to agricultural
emission, considering that WM–KLY, but not TMRX–ET, was identified as a
statistically significant source for the three trihalomethanes. At the BARE
sites, high positive correlations between the fluxes of the three brominated
trihalomethanes were observed which were all associated with p values
< 0.05, except for a lower correlation between CHBr3 and
CHBrCl2 (r=0.72; p < 0.1). Furthermore, high correlations
between the mixing ratios of the three trihalomethanes were obtained for
these two sites, although relatively low statistical significance was
obtained for the correlation between CHBr3 and CHBrCl2 at these
sites (see Table 4). This further supports the notion that the three
brominated trihalomethanes are emitted via similar mechanisms or controls.
Moderately low p values for the correlations between CH2Br2 and
both CH2Br2Cl (p < 0.15) and CHBrCl2 (p < 0.1)
at these sites further suggests common controls for CH2Br2 and the
brominated trihalomethanes (see Table 5).
Correlation of CH2Br2 with CHBr2Cl at the SD sites was
strongly negative (r=-0.93; p < 0.1), similar to the negative
correlation between CHBr2Cl and the other brominated trihalomethanes,
CHBrCl2 (r=-0.98; p < 0.05) and CHBr3 (r=-0.65; p > 0.15), at these sites.
This, together with the fact that the
measured fluxes of these three species were generally positive over the SD
sites, suggests competitive emission between CHBr2Cl and CHBrCl2
and potentially also CHBr3, at least at the SD sites. This is supported
by the analysis in Sect. 3.2.2 and 3.2.3, which demonstrated that the
halide content of the soil appears to play a major role in controlling the
emission rates of VHOCs under the studied conditions.
Table 5 also indicates overall low correlations between CHCl3 and all
of the brominated trihalomethanes, mostly resulting from negative
correlations at the BARE sites. The anticorrelation of CHCl3 with
trihalomethanes increased in the order CHBrCl2 < CHBr2Cl < CHBr3. The incidence of the chlorinated trihalomethanes
(CHCl3 and CHBrCl2), compared to the less chlorinated ones
(CHBr3 and CHBr2Cl) also tended to be higher at the BARE sites
compared to the other sites (Table 2). Hence, the negative correlation
between CHCl3 and the brominated trihalomethanes at the bare soil sites
may indicate competitive emission between the more chlorinated and more
brominated trihalomethanes. The situation at the BARE sites resembles
previous reports of predominant emission of CHCl3 at the expense of the
more brominated species (e.g., Albers et al., 2017; Huber et al., 2009),
particularly CHBr3 and CHBr2Cl, and was expected given the higher
Cl / Br ratio at these sites (see Table 3). We should emphasize that even at
the BARE sites, we observed relatively high positive fluxes of brominated
trihalomethanes, particularly of CHBr2Cl and CHBrCl2, which would not
generally be expected (Albers et al., 2017) and can be attributed to the
relatively high Br enrichment in the soil.
Interestingly, in agreement with Table 4, Table 5 also shows relatively high
correlations between CHCl3 and all methyl halides, particularly for the
BARE sites (CH3I, r=0.68, p < 0.15; CH3Br, r=0.83,
p < 0.05; CH3Cl, r < 0.86, p < 0.05) and SD
sites (CH3I, r=0.99, p < 0.05; CH3Br, r=0.59, p > 0.15;
CH3Cl, r=0.91, p < 0.1). Remarkably, a
high correlation was found for CH3I with CHCl3 and
C2HCl3 at the SD sites (r=0.99, p < 0.05 in both
cases). Positive fluxes of the three species were observed at the SD sites
in most cases, although with only moderate statistical significance for
CHCl3 (Table 2). Weissflog et al. (2005) found that emission of
C2HCl3, CHCl3 and other chlorinated VHOCs can occur from salt
lakes via the activity of halobacteria in the presence of dissolved Fe (III)
and crystallized NaCl. The strong correlations of CHCl3,
C2HCl3 and CH3I at the SD sites, where statistically
significant fluxes were frequently measured for these species, reinforce the
co-located emissions of CHCl3 and C2HCl3 from salt lake
sediments, as indicated by Weissflog et al. (2005), and suggest that
CH3I can be emitted in a similar fashion. The fact that the relative
emission efficiency of CH3I in our study was much higher than under the
conditions used by Keppler et al. (2000) supports the possibility
that mechanisms other than the abiotic emission pathway proposed by
Keppler et al. (2000) influence the emission of CH3I at the
Dead Sea (Sect. 3.2.3).
Summary
The results of this study demonstrate high emission rates of the
investigated VHOCs in the Dead Sea region, corresponding with mixing ratios
which, in most cases, are significantly higher than typical values in the
MBL. Overall, our measurements indicate a generally elevated incidence of
positive fluxes of brominated vs. chlorinated VHOCs compared to previous
studies. The high incidence of the former can be attributed primarily to the
relatively large amount of Br in the soil rather than the Br/Cl ratio. We
did not detect any clear effect of meteorological parameters, emission from
the seawater or the season, other than – in agreement with Sive
et al. (2007) – apparently higher emission of CH3I in spring vs.
winter. Three of the investigated site types – bare soil, coast and
agricultural field – were identified as being statistically significant (p < 0.05) sources for at least some of the investigated VHOCs. The
fluxes, in general, were highly variable, showing changes between sampling
periods, even for a specific species at a specific site. The coastal sites,
particularly at a short distance from the sea (SD sites) where soil is mixed
with salt deposits, were sources for all of the investigated VHOCs but not
statistically significantly for CHCl3. Further from the coastal area,
the bare soil sites were sources for CHBrCl2, CHBr2Cl, CHCl3
and apparently also for CH2Br2 and CH3I, and the agricultural
vegetation site was a source for CHBr3, CHBr2Cl and CHBrCl2.
Our measurements reinforce reports of CHCl3 and CHBrCl2 emission
from bare soil but indicate that such emission can also occur under
relatively low soil organic content. To the best of our knowledge, we report
here for the first time strong emission of CHBr2Cl and emission of
CH2Br2 from hypersaline bare soil, at least a few kilometers from
the Dead Sea. We could not identify the contribution of either natural or
agricultural vegetation to the emission of the investigated VHOCs.
The highest emissions from the SD sites were associated with maximum
salinity and clearly showed an increased incidence of positive flux with
proximity to the seawater, pointing to the sensitivity of VHOC emission
rates to salinity, even under hypersaline conditions. The measurements did
not indicate either increased or reduced emissions of VHOCs from the
seawater itself. Emission of VHOCs has been shown to occur from dry soil
under semiarid conditions during the summer, in agreement with the finding
from other geographical locations that soil water does not seem to be a
limiting factor in VHOC emission (Kotte et al., 2012). Rain
events appeared to attenuate the emission rates of VHOCs at the Dead Sea.
Measurements at a bare soil site suggested a decrease in VHOC emission rates
for 1–3 d after a rain event.
Both flux and mixing ratio correlation analyses pointed to common formation
and emission mechanisms for CHBr2Cl and CHBrCl2, in line with
previous studies, for the agricultural watermelon-cultivation field and bare
soil sites. These analyses further strongly suggest common formation and
emission mechanisms for CHBr3 with these two trihalomethanes. Whereas
Albers et al. (2017) suggested that CHBr3 and CHBr2Cl are
emitted from soil only in relatively small amounts compared to CHCl3,
our results indicated their high emission via common mechanisms with the
other trihalomethanes. The overall average net flux of the trihalomethanes
decreased according to CHBr2Cl > CHCl3 > CHBr3 > CHBrCl2,
while CHCl3 showed the lowest
incidence of positive fluxes among all trihalomethanes. The enhanced
emission of brominated trihalomethanes probably reflects enrichment of the
Dead Sea soil with Br, in line with findings by Hoekstra et al.
(1998).
We identified the coastal sites as being a probable source for all methyl halides,
whereas neither the agricultural field nor natural vegetation site was
identified as net sink or net source for these species, except for the
agricultural field being a net sink for CH3I. Our analysis
demonstrated, however, much higher efficiencies of CH3I emission than
of CH3Br and CH3Cl emissions as a function of halides in the soil,
compared to those reported by Keppler et al. (2000), pointing to
emission of CH3I via other mechanisms. The strong correlation between
both fluxes and mixing ratios of CH3I, CHCl3 and C2HCl3,
particularly at the SD sites, strongly suggests that the coastal area of the
Dead Sea acts as an emission source for CHCl3, C2HCl3 and
CH3I via similar mechanisms, although these sites were associated with
only moderate statistical significance (p<0.1) as a net source for
CHCl3. The emission of CHCl3 and C2HCl3 from these sites
is in line with findings by Weissflog et al. (2005) of emission of
various chlorinated VHOCs, including CHCl3 and C2HCl3, from
salt lake sediments. Weissflog et al. (2005) reported that the emission
of chlorinated VHOCs in their study was induced by microbial activity.
Keppler et al. (2000) reported the involvement of an abiotic
process in the formation of alkyl from soil and sediments, and the observed
correlation between methyl halides and between CH3I and both CHCl3
and C2HCl3 may indicate that the two processes occur
simultaneously in the coastal area of the Dead Sea.
Although relatively high, the CHBr3 fluxes and mixing ratios that we
measured at the Dead Sea cannot be directly related to the high mixing
ratios of reactive bromine species that were found at the Dead Sea (e.g.,
Matveev et al., 2001; Tas et al., 2005) via its
photolysis. Similarly, if CH3I photolysis is the only source of
reactive I species, the measured fluxes and elevated mixing ratios of
CH3I are not high enough to account for the high iodine monoxide in
this area. Given their relatively fast photolysis, however, CH3I and
CHBr3, as well as CH2Br2, may well have roles in the
initiation of reactive bromine and iodine formation in this area.
Overall, along with other studies, the findings presented here highlight the
potentially important role of saline soil and salt lakes in VHOC emission
and call for further research on VHOC emission rates and controlling
mechanisms and implications on stratospheric and tropospheric chemistry.
Data availability
Data are available upon request from the corresponding
author Eran Tas (eran.tas@mail.huji.ac.il).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-7667-2019-supplement.
Author contributions
ET, AG, RR and AW designed the experiments. MS,
GL and QL carried out the field measurements, and DB carried out the sampled
air analyses. GL contributed to designing and constructing a special
mechanism for the simultaneous lifting and dropping of sampling canisters.
Data curation and formal analysis were performed by ET and MS, with support
from RR. ET and MS prepared the paper, with contributions from all
co-authors.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
We thank the United States–Israel Binational Science
Foundation (grant no. 2012287) for funding this study. Eran Tas holds the Joseph H. and Belle R. Braun
Senior Lectureship in Agriculture.
Financial support
This research has been supported by the United States–Israel Binational Science Foundation (grant no. 2012287).
Review statement
This paper was edited by James Roberts and reviewed by two anonymous referees.
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