Snow photochemical processes drive production of chemical trace gases in snowpacks,
including nitrogen oxides (NO

The emission flux from the snowpack is estimated as the product of the
depth-integrated photolysis rate coefficient,

Field and laboratory experiments over the past 2 decades have
provided evidence that photochemical reactions occurring within snow
lead to the emission of various gaseous compounds from the snowpack
(e.g.

The photolytic lifetime of a chemical species in the snowpack is the
reciprocal of the photolysis rate coefficient (also known as the
photodissociation rate coefficient),

Under clear-sky conditions, a homogeneous snowpack can be separated
into two optical layers based on the propagation of actinic flux from
the surface into the snow: the near-surface layer, i.e. the
top few centimetres of the snowpack, where direct solar radiation is
converted into diffuse radiation. Below the near-surface layer is the
asymptotic zone, where all
solar radiation is diffuse and will decrease
exponentially with depth (

The relationship between actinic flux (and the photolysis rate
coefficient) and depth is complex near the surface of the
snowpack due to rapidly changing contributions from both direct and
diffuse radiation. Enhancement or attenuation of actinic flux in the
near-surface layer compared to above the snow
is dependent on the solar zenith angle
(Fig.

In the asymptotic zone radiation is diffused, and provided that the snowpack
is semi-infinite – i.e. the albedo of the
surface underlying the snow does not affect the calculation of the
actinic flux within the snowpack – the radiation decreases
exponentially according to Beer–Lambert law (

Depth profile within “cold polar snow” (base case:

Reference for quantum yield,

Radiative-transfer (RT) models, such as the TUV-snow model
(

Studies have also demonstrated that photolysis of

The ratio of the depth-integrated photolysis rate coefficients,

The hypothetical homogeneous snowpacks defined in this study were based on
three different types of snow – cold polar, wind-packed and melting snow
(Table

Sensitivity tests calculating

In case 1, snow densities were varied in the range
observed typically in natural snowpack of 0.2–0.6

In cases 2–5, the scattering cross section and
mass ratio of light-absorbing impurities
of the snowpack were varied – both of which have an impact on
the propagation of actinic flux within the snowpack.
The reciprocal of the

The absorption cross section of snowpack is due to wavelength-dependent
absorption by ice,

Properties of snow type studied. Optical and physical properties are based on work by

In case 6, the asymmetry factor,

Optical properties of the snowpacks used. The bold numbers are to highlight the optical property that is varying in that particular case.

Within case 7, column ozone values were varied to cover
the seasonal and spatial variability observed above the polar regions.
The effect of column ozone on the depth-integrated photolysis rate coefficient ratio was
explored as downwelling UV radiation is very sensitive to stratospheric ozone
absorption and the attenuation is a strong function of wavelength.
Typical value of column ozone in Antarctica
(also the global average;

The attenuation of actinic flux with depth was calculated by a coupled
atmosphere–snow radiative-transfer model, TUV 4.4, using an eight-stream
DISORT (Discrete Ordinates Radiative Transfer Program for a Multi-Layered Plane-Parallel Medium) model (

Values of the photolysis rate coefficient,

The

Values of

To determinate the accuracy of the

The study evaluates the accuracy of parameterisation of photolysis rate
coefficient to variation in solar zenith angle, different
photolysis precursors, snowpack properties and total
column ozone. Correction factors were also found for each different
species to improve the performance of the

The

Radiation in the asymptotic layer, i.e. below the first few centimetres of the snow surface (Fig.

Scattering of photons typically occurs at the air–ice interface of a snow grain
and absorption occurs within the snow grain.
A denser snowpack implies more scattering or absorption events
per unit length. A larger scattering cross section will typically reduce the
path length of a photon through the snowpack and reduce the
possibility for absorption by ice or light-absorbing impurities.
Therefore, increases in density, light-absorbing impurities and scattering
cross section result in a smaller

Depth-integrated photolysis rate coefficients of the four chemical
species considered (

When the solar zenith angle is between 0 and
37

The ratio of depth-integrated photolysis rate coefficient,

The effective solar zenith angle,

In reality, only high-altitude glaciers in the tropics, such as those
found in the Himalayas or Andes, would experience the overhead sun or
small solar zenith angles in the summer. In the polar regions, where
snow emission can dominate boundary layer
chemistry (e.g.

The value of the ratio

The

The wavelength of the peak in the action spectrum of a chemical species also has
an impact on its response to changes in column ozone concentration (case 7)
in terms of photolysis rate coefficient.
The surface photolysis rate coefficients for

Despite the value of the photolysis rate coefficient varying with
values of different column ozone, especially for the

Density (case 1), scattering cross section (case 2), light-absorbing impurities
(cases 3–5) and asymmetry factor (case 6)
were considered as the four varying physical properties of the snowpack in this study.
Figure

The effect of different column ozone amount on the photolysis rate
coefficient of

With regard to the density of the snowpack, the photolysis rate coefficient maxima are at a
deeper depth for snowpacks with lower density. That is, the
path length of the photon is longer for less-dense snowpacks. However,
for the range of density values found in natural snow (case 1,

Scattering cross section of the snowpack: lower
values of the scattering cross section
imply longer path length of the photon between individual scattering events.
Hence, the maximum photolysis rate coefficients tend to occur deeper
into the snowpacks, as shown in blue in
Fig.

Photolysis rate coefficient for the

Light-absorbing impurities in the snowpack:
the propagation of actinic flux and the vertical variation of photolysis rate
coefficient within snowpack is dominated by scattering when light-absorbing
impurity contents are low and therefore the absorption properties of the impurity
become unimportant, i.e. there is no difference between the value of

In case 4, the absorption due to HULIS is considerable. A mass ratio of
100

Asymmetry factor of the snowpack:

The difference in the depth-integrated photolysis rate coefficient,

Parameterisation correction for “cold polar and coastal” snowpacks. Values of the correlation coefficient were calculated for two different snowpacks (BaseC, HULIS8 and Comb) with and without applying the correction factors.

Parameterisation correction for “melting and clean” snowpack. Values of the correlation coefficient were calculated for two different snowpacks (Scatt2, HULIS1 and Comb) with and without applying the correction factors.

For snowpacks with a large

The correction was evaluated by comparing the depth-integrated photolysis rate coefficients computed
by the RT method,

The correlation between

There are many factors that might have an impact on the disagreement
between the two methods not taken into account in this
study. Cloudy skies are not taken into account. However,
clouds convert direct radiation into diffuse radiation. Under
a very thickly clouded sky all radiation reaching the ground will be
diffused and the decay of actinic flux within the snowpack would be
exponential. Therefore, on a cloudy day the

The parameterisation of snowpack actinic flux based on the

Depth-integrated photolysis rate coefficient at various solar zenith
angle for different species within snowpack BaseC
(

The discrepancy between the

Depth-integrated photolysis rate coefficient at various solar zenith
angle for different species within snowpack Scatt2
(

The values of

An important approximation of
the

H. G. Chan is funded by the Natural Environment Research Council through Doctoral Studentship NE/L501633/1. Edited by: T. Bartels-Rausch