Photochemistry in Snow: Another surprise

1
Photochemistry in SnowAnother surprise!
2
Snow is not just H2O
3

• Honrath et al. (1999) Suggested photolysis of
  nitrate responsible for elevated levels of NOx
  observed within Summit snowpack

4
Jones et al. (2000) Clear evidence of
photochemical NO and NO2 production within
snowpack
5
So, NO3- NOxWhat else happens?
6
Dibb et al. (2002) Diurnal cycle of HONO in
Summit snow and photochemical link
Via photolysis of nitrate NO3- hv NO3-
NO3- NO2- O (3P) NO3- NO2 O-
NO2 OH OH- followed by 2 NO2 H2O
NO2- NO3- 2H
7
HCHO

• Hutterli et al. (1999) air-snow exchange of HCHO
  driven by temperature
• Sumner Shepson (1999) argue for photochemistry

8
H2O2

• Important source from falling snow. Diurnal cycle
  evident, driven partly by bi-directional fluxes

Hutterli et al. (2001) Jacobi et al. (2002)
9
Alkenes, halocarbons, alkyl nitrates
Swanson et al. (2002) Diurnal measurements in
ambient air (1m and 8m) and in firn air at 10 cm
and 80 cm depth.
10
Alkenes, halocarbons, alkyl nitrates

• Profiles to 2m depth tracers show that air is
  mixed

Swanson et al. (2002)
11
Alkenes, halocarbons, alkyl nitrates

• They argue that elevated concentrations are
  driven by photochemistry

Swanson et al. (2002)
12
What does this mean for HOx?

• Very high OH observed at South Pole Davis et al.
  (2001)
• Mauldin et al. (2001) argue for HO2 NO OH
  NO2 on basis of very high observed NO
• Yang et al. (2002) model Summit a.b.l. based on
  observations of HONO, HCHO. H2O2 etc.
• gt 4x106 molecs/cm3 OH (mean summer daytime)
• gt 30 40 pptv HO2 (midday summer)

ie. HOx is elevated!!
13
Conclusions

• There is lots of photochemistry going on in snow
• This will affect the overlying boundary layer
• Also lots of interest in this subject area See
  AE Special Issue (vol 36, Nos 15-16) Air/Snow/Ice
  interactions in the Arctic