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Supplement of Atmos. Chem. Phys., 24, 3421–3443, 2024
https://doi.org/10.5194/acp-24-3421-2024-supplement
© Author(s) 2024. CC BY 4.0 License.
Supplement of
Observations of cyanogen bromide (BrCN) in the global troposphere and
their relation to polar surface O
3
destruction
James M. Roberts et al.
Correspondence to:
James M. Roberts (james.m.roberts@noaa.gov)
The copyright of individual parts of the supplement might differ from the article licence.
1
Supplement to: BrCN
in the global troposphere
S
1 Formation of BrCN from HOBr chemistry
The formation of XCN compounds from reactions of HCN/CN ̄ in the condensed phase is well known, so deserves
5
consideration in the context of the polar boundary layer. The major sources of HCN to the atmosphere are biomass burning
and vehicle exhaust, two sources that favor the northern hemisphere. HCN is a very weak acid (pKa=9.
4
)
,
is not very soluble
in water
(Yoo et al., 1986)
, and its gas phase loss processes are quite slow (several years against reaction with OH).
Consequently, HCN is widely distributed throughout the troposphere as is apparent in the ATom data shown in the main text.
Vertical profiles of HCN imply a surface
sink and the effective lifetime of HCN has been estimated to be about 5 months
(Li
10
et al., 2003)
. The data from ATom
-
3&4, including the vertical profile and inter
-
hemisphere distribution, are for the large part
in agreement with this.
There are many parameters that go into a model description of polar bromine chemistry, some of them fairly uncertain
(for example rate constants at cold temperatures). One way to isolate and examine how BrCN formation from HCN might
impact the active
-
brom
ine cycle is to compare the rates of liquid phase chemistry that perpetuates the Br cycle, to the chemistry
15
that would remove active Br by making BrCN
(Gerritsen et al., 1993)
:
Br
2
+ HCN
®
BrCN + Br ̄ + H
+
(
k
= 800 M
-
1
s
-
1
)
(S1)
HOBr + HCN
®
BrCN + H
2
O
(
k
= 10 M
-
1
s
-
1
)
(S2)
20
OBr ̄ + CN ̄ + H
2
O
®
BrCN + 2OH ̄
(
k
= 5.7
́
10
7
M
-
1
s
-
1
)
(S3)
HOBr + CN ̄
®
BrCN + OH ̄
(
k
= 4.2
́
10
9
M
-
1
s
-
1
)
(S4)
25
So, the formation of BrCN from HCN is highly pH dependent, slower at lower pHs
. This is a result both of the slower rate
constants of the parent compounds compared to the conjugate anions, and the lower solubility of HCN (pKa=9.4) at
atmospheric pHs. The main liquid
-
phase reactions of HOBr that perpetuate the bromine explosion are t
he acid
-
assisted
mechanism
(Roberts et al., 2014)
:
30
HOBr + X ̄
«
HOBrX ̄
k
1
, k
-
1
(S5)