Emission of volatile halogenated organic compounds over various Dead Sea landscapes
Authors: Shechner Moshe , Guenther Alex , Rhew Robert , Wishkerman Asher , Li Qian , Blake Donald , Lerner Gil , Tas Eran 
Published: June 2019

Abstract. 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.