Atmos. Chem. Phys., 24, 3421–3443, 2024
https://doi.org/10.5194/acp-24-3421-2024
© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
Observations of cyanogen bromide (BrCN) in the global
troposphere and their relation to polar
surface O
3
destruction
James M. Roberts
1
, Siyuan Wang
1,2
, Patrick R. Veres
1,a
, J. Andrew Neuman
1,2
, Michael A. Robinson
1,2
,
Ilann Bourgeois
1,2,b
, Jeff Peischl
1,2
, Thomas B. Ryerson
1
, Chelsea R. Thompson
1
, Hannah M. Allen
3
,
John D. Crounse
4
, Paul O. Wennberg
4,5
, Samuel R. Hall
6
, Kirk Ullmann
6
, Simone Meinardi
7
,
Isobel J. Simpson
7
, and Donald Blake
7
1
NOAA Chemical Sciences Laboratory, Boulder, CO, USA
2
Cooperative Institute for Research in Environmental Sciences, CIRES, University of Colorado,
and NOAA, Boulder, CO, USA
3
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA
4
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
5
Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA
6
Atmospheric Chemistry Observations & Modeling Laboratory, National Center for Atmospheric Research,
Boulder, CO, USA
7
Department of Chemistry, University of California Irvine, Irvine, CA, USA
a
now at: Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA
b
now at: Université Savoie Mont Blanc, INRAE, CARRTEL, 74200 Thonon-Les Bains, France
Correspondence:
James M. Roberts (james.m.roberts@noaa.gov)
Received: 28 April 2023 – Discussion started: 30 May 2023
Revised: 16 January 2024 – Accepted: 18 January 2024 – Published: 20 March 2024
Abstract.
Bromine activation (the production of Br in an elevated oxidation state) promotes ozone destruction
and mercury removal in the global troposphere and commonly occurs in both springtime polar boundary layers,
often accompanied by nearly complete ozone destruction. The chemistry and budget of active bromine com-
pounds (e.g., Br
2
, BrCl, BrO, HOBr) reflect the cycling of Br and affect its environmental impact. Cyanogen
bromide (BrCN) has recently been measured by iodide ion high-resolution time-of-flight mass spectrometry
(I
−
CIMS), and trifluoro methoxide ion time-of-flight mass spectrometry (CF
3
O
−
CIMS) during the NASA
Atmospheric Tomography Mission second, third, and fourth deployments (NASA ATom), and could be a pre-
viously unquantified participant in active Br chemistry. BrCN mixing ratios ranged from below the detection
limit (1.5 pptv) up to as high as 36 pptv (10 s average) and enhancements were almost exclusively confined to
the polar boundary layers in the Arctic winter and in both polar regions during spring and fall. The coincidence
of BrCN with active Br chemistry (often observable BrO, BrCl and O
3
loss) and high CHBr
3
/
CH
2
Br
2
ratios
imply that much of the observed BrCN is from atmospheric Br chemistry rather than a biogenic source. Likely
BrCN formation pathways involve the heterogeneous reactions of active Br (Br
2
, HOBr) with reduced nitrogen
compounds, for example hydrogen cyanide (HCN
/
CN
−
), on snow, ice, or particle surfaces. Competitive reac-
tion calculations of HOBr reactions with Cl
−
/
Br
−
and HCN
/
CN
−
in solution, as well as box model calculations
with bromine chemistry, confirm the viability of this formation channel and show a distinct pH dependence, with
BrCN formation favored at higher pH values. Gas-phase loss processes of BrCN due to reaction with radical
species are likely quite slow and photolysis is known to be relatively slow (BrCN lifetime of
∼
4 months in
midlatitude summer). These features, and the lack of BrCN enhancements above the polar boundary layer, imply
that surface reactions must be the major loss processes. The fate of BrCN determines whether BrCN production
fuels or terminates bromine activation. BrCN reactions with other halogens (Br
−
, HOCl, HOBr) may perpetuate
Published by Copernicus Publications on behalf of the European Geosciences Union.
3422
J. M. Roberts et al.: Observations of cyanogen bromide (
BrCN
) in the global troposphere
the active Br cycle; however, preliminary laboratory experiments showed that BrCN did not react with aqueous
bromide ion (
<
0.1 %) to reform Br
2
. Liquid-phase reactions of BrCN are more likely to convert Br to bromide
(Br
−
) or form a C–Br bonded organic species, as these are the known condensed-phase reactions of BrCN and
would therefore constitute a loss of atmospheric active Br. Thus, further study of the chemistry of BrCN will be
important for diagnosing polar Br cycling.
1 Introduction
Photochemically active halogen species in the lower tropo-
sphere affect the oxidizing capacity of the atmosphere via
radical reactions that may either produce or destroy ozone
(Saiz-Lopez and von Glasow, 2012; Simpson et al., 2015).
In addition, active halogens play an important role in O
3
destruction when transported to the upper troposphere and
lower stratosphere (UT/LS). Active halogen species are de-
fined as halogen atoms, dihalogen compounds, or species that
have an effective oxidation state above
−
1 (e.g., hypobro-
mous acid, HOBr). A complete understanding of this chem-
istry requires accounting for reactions that produce or de-
stroy active halogens in a variety of environments, ranging
from the polar ice caps and the ocean surface layer to pol-
luted continental regions.
Bromine chemistry in polar surface regions has been of
particular interest since it was discovered that active bromine
compounds were responsible for ozone destruction in those
regions (Barrie et al., 1988). The chemistry behind the initi-
ation and propagation of active bromine chemistry has since
been the subject of numerous experimental and numerical
modeling studies and several review articles (Abbatt et al.,
2012; Simpson et al., 2015; Simpson et al., 2007), and ozone
destruction is a widespread and persistent phenomenon in the
Arctic boundary layer (Jacobi et al., 2010; Halfacre et al.,
2014; Ridley et al., 2003). The review articles describe how
ozone destruction follows from the production of Br
2
or BrCl
on ice, snow, or particle surfaces, volatilization into the gas
phase, and subsequent photolysis to two halogen atoms that
react rapidly with O
3
to create halogen oxide (XO, where
X
=
Cl, Br, or I). Halogen oxides have a number of pathways
to form hypohalous acids (HOX) that can amplify the halo-
gen chemistry through surface reactions with halide ions: for
example,
HOBr
+
Br
−
+
H
+
→
Br
2
+
H
2
O
.
(R1)
Several studies have shown that Br
2
tends not to form on
sea ice surfaces (Pratt et al., 2013) because those surfaces
are buffered at relatively high pH (Wren and Donaldson,
2012). The phenomenon represented by Reaction (R1) has
been termed the “bromine explosion” when it accompanies
the destruction of ozone (Wennberg, 1999; Platt and Janssen,
1995), and the participation of Br atoms in the photochemical
cycle has been confirmed by direct observation of Br atoms
(Wang et al., 2019). One of the keys to achieving a quantita-
tive understanding of this Br chemistry is assessing the other
heterogeneous reactions that HOBr can participate in. For ex-
ample, there is evidence that HOBr reacting with dissolved
organic matter (DOM), either on surface snow/ice or on
particles, could be responsible for bromoform (CHBr
3
) en-
hancements observed in polar environments (Carpenter et al.,
2005; Gilman et al., 2010) and CHBr
3
and other organic
bromine compounds observed in sea ice in the Antarctic win-
ter (Abrahamsson et al., 2018). Reaction of HOBr with re-
duced nitrogen species to form cyanogen bromide (BrCN)
could divert active bromine from the O
3
bromine explosion
cycle.
The existence of cyanogen halides has been known for
some time (e.g., ClCN was discovered in 1851; Wurtz, 1851),
but to our knowledge they have not been reported in the
ambient atmosphere. Note that cyanogen fluoride (FCN) is
known, but FCN will not be formed in the environment due
to the lack of chemical pathways that can form active F (e.g.,
HOF, F
2
). In the environment, XCN compounds are formed
from the reaction of active halogens, X
2
, or HOX (where
X
=
Cl, Br, I) with reduced nitrogen species. For example,
X
2
(HOX)
+
HCN
→
XCN
+
HX(H
2
O)
.
(R2)
This chemistry is well known in water and wastewater
treatment processes where Cl
2
, NaOCl, or chloramines are
added and reduced nitrogen species are present (Shah and
Mitch, 2012; Yang and Shang, 2004). ClCN, and to some
extent BrCN, has been measured in chlorination systems, in-
cluding swimming pools, spas (Daiber et al., 2016), and wa-
ter treatment systems (Heller-Grossman et al., 1999) because
the reaction of active Cl compounds with bromide ions pro-
duces active Br. For example,
HOCl
+
Br
−
→
HOBr
+
Cl
−
.
(R3)
As in the case of Cl chemistry, a range of N-containing
substrates may be involved in BrCN formation. The solu-
bility of XCN compounds has recently been found to range
from only slightly soluble for ClCN to moderately soluble for
ICN (Roberts and Liu, 2019). Thus, cyanogen halides will
easily volatilize from solution and should be considered pos-
sible important participants in active halogen chemistry in
the troposphere.
In addition to the above purely abiotic mechanisms, ClCN,
BrCN, and ICN are known to be produced in biological
systems through photosynthetic reactions involving H
2
O
2
,
Atmos. Chem. Phys., 24, 3421–3443, 2024
https://doi.org/10.5194/acp-24-3421-2024