Jack Dibb University of New Hampshire Snow Photochemistry at Summit, Greenland Funding from the Arctic Natural Sciences Program in the Office of Polar Programs at the National Science Foundation

Motivations To understand the chemistry of the atmosphere over remote, snow-covered regions In light of recent measurements February Up to 50% of northern hemisphere land surface is snow- covered in winter

Summit Summit is at the peak of the Greenland ice sheet. No melting occurs Accumulation rate is ~65 cm/yr

Summit Summit is at the peak of the Greenland ice sheet. No melting occurs Accumulation rate is ~65 cm/yr. Summit was the site of GISP2 and GRIP ice cores drilled in the 1980s. The GISP2 core was 3,053 m long, dating back more than 110,000 years). Dome protecting the ice core drill

Ice core records from Summit: NO 3 - and SO 4 = From Legrand and Mayewski, 1997.

Ice core records from Summit: NO 3 - and SO 4 = From Legrand and Mayewski, “ Study of the current atmosphere is essential to the interpretation of the paleoatmospheric record in ice cores”

Ice core records from Summit: NO 3 - and SO 4 = From Legrand and Mayewski, “ Study of the current atmosphere is essential to the interpretation of the paleoatmospheric record in ice cores” Original question: In what form is nitrate carried to Summit?

Field observations and lab experiments have shown that NO x is produced from illuminated NO 3 - -containing snow: From [Honrath et al., 2000] NO (ppbv) J NO2 NO  NO <100 pptv Low NO 3 - snow

Field observations and lab experiments have shown that NO x is produced from illuminated NO 3 - -containing snow: From [Honrath et al., 2000] NO (ppbv) J NO2 NO  NO <100 pptv  NO >600 pptv Low NO 3 - snow High NO 3 - snow

Getting to Summit The special ski-equipped NY Air Guard LC-130 Hercules planes take us from Kangerlussuaq, to Summit, landing on a 4-mile long runway of groomed snow.

Studying Snow Photochemistry There are five major parts of our project: Measuring radicals. We measure these very reactive chemicals, including hydroxyl radical (OH).

Studying Snow Photochemistry There are five major parts of our project: Measuring radicals. We measure these very reactive chemicals, including hydroxyl radical (OH). Measuring radical precursors We also need to measure the precursors that form these radicals in the presence of sunlight..

Studying Snow Photochemistry There are five major parts of our project: Measuring radicals. We measure these very reactive chemicals, including hydroxyl radical (OH). Measuring radical precursors We also need to measure the precursors that form these radicals in the presence of sunlight.. Characterizing sunlight in the snowpack. Sunlight is a crucial ingredient so we also have to understand the amounts and wavelengths of light that penetrate the snowpack.

Studying Snow Photochemistry There are five major parts of our project: Measuring radicals. We measure these very reactive chemicals, including hydroxyl radical (OH). Measuring radical precursors We also need to measure the precursors that form these radicals in the presence of sunlight.. Characterizing sunlight in the snowpack. Sunlight is a crucial ingredient so we also have to understand the amounts and wavelengths of light that penetrate the snowpack. Determining the physical structure of the snowpack. The dry snow at Summit is very porous so winds blowing into and out of the snowpack play a key role in releasing chemicals from snow. We measure things like the sizes of snow grains, how tightly packed the snow is, and how temperature varies throughout the snowpack.

Studying Snow Photochemistry There are five major parts of our project: Measuring radicals. We measure these very reactive chemicals, including hydroxyl radical (OH). Measuring radical precursors We also need to measure the precursors that form these radicals in the presence of sunlight.. Characterizing sunlight in the snowpack. Sunlight is a crucial ingredient so we also have to understand the amounts and wavelengths of light that penetrate the snowpack. Determining the physical structure of the snowpack. The dry snow at Summit is very porous so winds blowing into and out of the snowpack play a key role in releasing chemicals from snow. We measure things like the sizes of snow grains, how tightly packed the snow is, and how temperature varies throughout the snowpack. Putting it all together. Combine our observations into a computer model of photochemistry in the snowpack. This model would allow us to predict the impacts of snow reactions on the composition of the atmosphere, the snowpack, and resulting ice.

UNH Work at Summit Snow sampling Air sampling Sarah collecting snow samples for chemical analysis (for water soluble ions, such as H +, NH 4 +, SO 4 2-, Cl -, Na +, Mg +, K +, Ca 2+ that are deposited from the atmosphere) Jack running a mist chamber to sample nitrous acid (HONO), and nitric acid (HNO 3 )

Traditional firn air sampling is done via air drawn through a tube in the snow. Even with perfect contact between the tube and the snow, in the top-most meter the air sampled is ambient air drawn directly to the inlet, yielding firn air concentrations that are diluted by ambient concentration. How does the sampling technique affect the concentrations?

Even with perfect contact between the tube and the snow, in the top-most meter the air sampled is ambient air drawn directly to the inlet, yielding firn air concentrations that are diluted by ambient concentration. Use of a bonnet makes the air flow through more snow before reaching the inlet, yet increasing mixing ratios are also apparent with deeper sampling. Traditional firn air sampling is done via air drawn through a tube in the snow. How does the sampling technique affect the concentrations?

Special firn air probe allows for: different instruments to simultaneously sample the same point in the firn sampling at different depths (down to several meters) Firn Air Sampling Probe

Special firn air probe allows for: different instruments to simultaneously sample the same point in the firn sampling at different depths (down to several meters) deployment from a sled to minimize disturbance of the natural snow Firn Air Sampling Probe

Close-up of Firn Air Sampling Probe thermocouple array Thermocouple array (orange wire) measures temperature depth profile.

Close-up of Firn Air Sampling Probe fiber optic cable thermocouple array Thermocouple array (orange wire) measures temperature depth profile. Fiber optic cable (blue wire) connected to a radiometer to measure light levels in snow.

Shallow Sampling How does the sampling technique affect the concentrations?

Shallow Sampling Deeper Sampling How does the sampling technique affect the concentrations?

Where does the firn air come from? Where does the air come from? Finite element calculations using measured firn properties allow the computation of flow field and concentrations under a variety of conditions. Depth in firn Air flow vectors with inlet at -5cm

Where does the firn air come from? Depth in firn Air flow vectors with inlet at -5cm Flowfield for inlet at -50 cm (homogeneous firn) Where does the air come from? Finite element calculations using measured firn properties allow the computation of flow field and concentrations under a variety of conditions. Here the difference in flow field for several inlet depths under the bonnet are shown. The inlet depth controls residence time & flow route, and flow within the hood itself is significant even with assumed perfect firn-hood contact.

A Shading Experiment: The firn air sampling probe is shaded to block direct visible and UV light - Manually “turn off” the sun.

The vertical brown line indicates the start time that the bonnet was shaded by cardboard, the blue represents an OP-2 shading, and the vertical yellow indicates unshaded. In sunny conditions, the bonnet provided a greenhouse effect in the near-surface snow. In the top few centimeters the bonnet and shading affected the temperature profile by as much as several degrees, with minimal effect at 10 cm depth and no effect at 20 cm depth or below. Shading experiments investigate sampling effects To gather non-reactive scalar data for transport model verification, vertical arrays of fine-gauge thermocouples were installed to record firn temperatures.

Finite element calculations of the temperature profile is compared to observations at 2 cm and 10 cm depth within the bonnet in the figure above. The locations of the measurements were at points I20-2 and I20-10, corresponding to the data in the plot immediately to the left. Boundary conditions on the temperature field were the measured temperatures at the surface and at 100 cm depth. Measurements and modeling compare well. Firn temperature measurements provide model verification

Windy Conditions

Calm Conditions (winds <2 m/s)

Comparison of HO 2 +RO 2 : Model vs. Observation (all data) For filtered dataset: R 2 = 0.81 and Median Ratio = 1.03 Obs. HO2 (1E8 molec./cm3)

HO 2 and OH Correlations Variable Wind Case vs. Calm Wind Case Variable WindCalm Wind HO2 (1E8 molec./cm3)

Comparison of OH: Model vs. Observation (all data) Obs. HO (1E6 molec./cm3)

Ozone Production Ozone formation ~ HO 2, RO 2 + NO Ozone loss – J O( 1 D) and HO 2 + O 3 Net ozone production above 10 pptv NO Calculated net production of 1-2 ppbv/day very similar to SP Observations consistently show boundary layer and especially firn air to be depleted relative to free troposphere [Helmig et al.; Peterson and Honrath]

Shading experiments show fast response HONO and NO x respond to light/dark cycles in minutes

Snowpack Hydroxyl Measurements Andreas Beyersdorf & Nicola Blake, UC Irvine Because of its high reactivity, hydroxyl cannot be measured directly in the snowpack. A hydroxyl estimate can be made by measuring the decay of doped hydrocarbons in a snow-filled chamber. Hydrocarbons Doped Sampling Probes Pump Attached UV-transparent Chamber

Decay of Butenes (July 11 3:40PM) Slope of graph is related to [OH] in the snowpack Increasing decay 1-Butene i-Butene cis-2-Butenetrans-2-Butene

Hourly Averages for July 10-12, 2003 Error bars (when present) show standard deviation when more than one measurement was made.

RONO 2 Organics (Carbonyls) h Alkenes h O3O3 Monocarboxylic Acids h Peroxy Radicals O2O2 OH Formation Mechanisms?? Oxidation of the abundant (but poorly characterized) supply of organic compounds in the snow appears to result in production of monocarboxylic acids and their precursors. Clearly, OH is enhanced above the snow and is likely even higher in the upper part of the pack. Our work at Summit, in Michigan, and ISCAT, and that of others at Alert and Neumayer has confirmed that photolysis of nitrate in snow releases NO x into the firn air. It is not yet clear whether subsequent cycling of N oxides is dominated by homogeneous reactions (in and above the snow) or is mediated by surface chemistry in the porous snowpack.

Halogens? The link between halogens and ozone depletion is well established in polar regions especially BrO. Halogens can cycle HO 2 to OH especially BrO and IO XO + HO 2 + light -> OH + X 10 pptv of BrO would significantly improve model/measurement agreement

Spring 2004 Experiment J (NO 3 ) increased rapidly (shown here from 25-Mar to 13- Apr) (Plot provided by Barry Lefer)

Spring 2004 Experiment J (NO 3 ) increased rapidly (shown here from 25-Mar to 13- Apr) Daytime firn NO also increased rapidly (shown here from 27-Mar to 13-Apr) (Plot provided by Jeff Peischl) (Plot provided by Barry Lefer)

We have learned.....that the way impurities get incorporated into snow (and ice cores) is much a more complex process than we thought!

We have learned.....that the way impurities get incorporated into snow (and ice cores) is much a more complex process than we thought! Atmospheric Boundary Layer Deposition Snow Firn Ice X XO X Turbulent Eddy Diffusion Windpumping/ Turbulent Diffusion Modeling Photochemical Modeling Adsorption/Desorption Photochemical Modeling Advection/Diffusion Incorporation into Ice Actinic Flux Modeling More like this model

We have learned.....that the way impurities get incorporated into snow (and ice cores) is much a more complex process than we thought! Atmospheric Boundary Layer Deposition Snow Firn Ice X XO X Turbulent Eddy Diffusion Windpumping/ Turbulent Diffusion Modeling Photochemical Modeling Adsorption/Desorption Photochemical Modeling Advection/Diffusion Incorporation into Ice Actinic Flux Modeling More like this model Than this!

Atmospheric Boundary Layer Deposition Snow Firn Ice X XO X Turbulent Eddy Diffusion Windpumping/ Turbulent Diffusion Modeling Photochemical Modeling Adsorption/Desorption Photochemical Modeling Advection/Diffusion Incorporation into Ice Actinic Flux Modeling When sunlight shines onto the snow surface, some of the impurities in the snow are released (or desorbed). Sunlight also breaks apart larger molecules into smaller, reactive compounds known as “radicals”. We have learned...

Atmospheric Boundary Layer Deposition Snow Firn Ice X XO X Turbulent Eddy Diffusion Windpumping/ Turbulent Diffusion Modeling Photochemical Modeling Adsorption/Desorption Photochemical Modeling Advection/Diffusion Incorporation into Ice Actinic Flux Modeling When sunlight shines onto the snow surface, some of the impurities in the snow are released (or desorbed). Sunlight also breaks apart larger molecules into smaller, reactive compounds known as “radicals”. These radicals initiate a chain of additional reactions. Some reactive gases diffuse or are blown by the wind into the atmosphere above the snow. We have learned...

Atmospheric Boundary Layer Deposition Snow Firn Ice X XO X Turbulent Eddy Diffusion Windpumping/ Turbulent Diffusion Modeling Photochemical Modeling Adsorption/Desorption Photochemical Modeling Advection/Diffusion Incorporation into Ice Actinic Flux Modeling When sunlight shines onto the snow surface, some of the impurities in the snow are released (or desorbed). Sunlight also breaks apart larger molecules into smaller, reactive compounds known as “radicals”. These radicals initiate a chain of additional reactions. Some reactive gases diffuse or are blown by the wind into the atmosphere above the snow. This photochemistry can pollute the air and change what ice cores tell us. So, we are examining how these pollutants are released and how this affects the air, snow and ice core records We have learned...

Summer Team: 20 scientists from Arizona, California, Colorado, New Hampshire, Georgia, Washington DC, and Switzerland Gas Phase Measurements – OH, HO 2 +RO 2, H 2 SO 4, NO, O 3, H 2 O, HNO 3, HONO, Soluble Bromine, J values, HCHO, HOOH

View of mobile lab from the main satellite lab

Our science area (in foreground) is 1 km away from the main camp in order to minimize the amount of pollution that reaches us from the camp generators, etc.

Motorized transport allowed near camp and away from science area BUT - ONLY low pollution transport allowed near the science area!

Motorized transport allowed near camp and away from science area BUT - ONLY low pollution transport allowed near the science area!

Summit Pics Midnight sun

The Big House The dome covers antennas, which gave Summit Internet access to the world for the first time in 2001.

The Big House The Big house is the kitchen and dining hall The dome covers antennas, which gave Summit Internet access to the world for the first time in 2001.

The Big House The Big house is the kitchen and dining hall Living area The dome covers antennas, which gave Summit Internet access to the world for the first time in 2001.

Links: summit.ucdavis.edu Acknowledgements We thank: National Science Foundation VECO Polar Resources Fellow scientists and students at Summit

Summit Camp – some other buildings The Generator Building (Electricity and Water) The “Greenhouse” (Staff Quarters and Laboratory)

Inside the Science Tent - a place to work and to send . Nicola outside the Science Tent

Living at Summit We sleep in 8’ x 8’ tents (“Arctic Ovens”) Warm during day if sunny Not so warm at night! A rural bathroom experience. Chilly!

Popular down-time activities Golf Croquet igloo-building Skiing Cooking

Getting to Greenland We take NY Air Guard planes (LC-130 Hercules) from New York to Kangerlussuaq, a small town on the coast of Greenland, then take another flight a few days later to Summit

Kangerlussuaq

Kangerlussuaq - Wildlife Caribou Musk Oxen