Nitric acid on water ice:
the chemical-physics
behind a key pollutant
of the Earth’s cryosphere
Nitric acid HNO
3
is an important player
in many environmental heterogeneous
processes involving airborne icy particulates
and snow. For example, nitric acid is a
precursor of snow-bounded nitrate anion
NO
3
-
, an important photochemical source
of NO
x
and OH radicals in Polar Regions [1].
As the photolysis rates of HNO
3
and NO
3
-
at
actinic wavelength (>300 nm) are strongly
different [2], whether nitric acid adsorbs
molecularly or is dissociated at the surface
of environmental ices is fundamental
in the NO
x
and OH atmospheric budget.
NEXAFS spectroscopy performed on the
TEMPO beamline revealed that the known
propensity for HNO
3
to be extensively
dissociated in aqueous solutions is
preserved upon adsorption onto ice at
cryogenic temperatures and concentration
regimes relevant to environmental chemistry
processes. Nitric acid should thus be
expected to behave as a strong acid
at the surface of supercooled aerosols
and in the quasi-liquid layer
of environmental ices.
Given its major role in many environmental
processes, the dissociative adsorption
and acid−base chemistry of HNO
3
at
aqueous interfaces continues to attract
tremendous interest from both theoretical
and experimental perspectives. Indeed,
our understanding of several important
atmospheric chemistry processes hinges
on a quantitative description of the factors
and parameters that control whether HNO
3
exists in its molecular or its dissociated
form at the surface of atmospheric
aerosols and of ice. For instance, this
would impact our interpretation of the
formation, stability, and reactivity of
nitrates in urban particulate matter, the
atmospheric reactive nitrogen budget,
the NO
x
photochemical fluxes from the
snowpack, and the formation and lifetime
of cirrus clouds.
HNO
3
and NO
3
-
have different electronic
structures leading to distinct NEXAFS
spectra [Figure
➊
], especially around 409
eV, where HNO
3
(red) displays a strong
resonance (arrow) specific to the hydroxyl
group OH, absent in NO
3
-
(blue). Therefore,
investigating the conversion of nitric
acid HNO
3
into nitrate NO
3
-
on water ice
is particularly convenient. It allows the
study of the extent of ionization of HNO
3
on the surface of a thin water ice film
(100 monolayers thick) deposited on a
gold substrate. Figure
➋
(
a
)
displays the
evolution of the N K-edge signal for 0.4
monolayer of HNO
3
adsorbed on ice at 23
K, then warmed to 150 K. The progressive
disappearance of the resonance at 409 eV
with the temperature indicates the gradual
conversion of HNO
3
into NO
3
-
. By fitting the
NEXAFS spectra with a linear combination
of the HNO
3
and NO
3
-
spectra presented
Figure
➊
, the relative abundance of these
two species can be estimated as function
of the temperature [Figure
➋
(
b
)
]. Although
the spectrum at 23 K looks like that of
pure HNO
3
, the fit indicates that actually
25 % of nitric acid is already converted
in NO
3
-
. This shows that ionic dissociation
of HNO
3
at the surface of ice occurs with
no thermal activation barrier. This occurs
when HNO
3
sits on a favorable adsorption
site, i.e. with enough water molecules to
dissociate HNO
3
and solvate the nitrate
anion. The molecular portion (75%@23 K)
corresponds to HNO
3
molecules sitting
on other surface sites with lower solvation
capabilities. These molecules are in a
metastable state and are progressively
converted into NO
3
-
when increasing
the surface temperature, which provides
the necessary energy for the optimization
of the solvation shell and thus leads
to dissociation. Around 110 K, the steep
increase in the conversion rate is due
to the diffusion of nitric acid molecules
in the bulk of ice where they find enough
water molecules for ionization and
solvation. Those findings indicate that
the known propensity for HNO
3
to be extensively dissociated in aqueous
solutions is preserved upon adsorption
onto ice at cryogenic temperatures and
concentration regimes relevant
to environmental chemistry processes.
HNO
3
should thus be expected to behave
as a strong acid at the surface of
supercooled aerosols and in the quasi-
liquid layer of environmental ices.
Atmospheric chemistry of nitric acid
depends on how it adsorbs on ice
Nitric and HNO
3
easily converts into nitrate on ice
SURFACES, INTERFACES AND NANOSYSTEMS
22
SOLEIL
HIGHLIGHTS
2013
