Greenland Environmental Observatory ~ Summit

Cooperating Agencies:

GEOSummit Science Coordination OfficeJack Dibb

John BurkhartRoger Bales

Summit Camp

• Manned year-round since 2003 by at least 4

• Operated by US NSF OPP under MOU with Denmark and Greenland

• Site of GISP2 and GRIP ice cores drilled 1989 - 1993

• Special issue of Atmos. Env. on recent Summit studies in 2007

Dome protecting the ice core drill

72o34’N, 38o29’W

3,250 m.a.s.l.

Year Round Sampling of Trace Gases

• Opposing secular trends in CO2 and δ13C reflect anthropogenic input, antiphase seasonality due to C uptake by terrestrial biosphere.

• Methane and CO show little recent change, yet clear seasonality.

• N2O and SF6 (minor GH gases) show anthropogenic increases.

NOAA ESRL/GMD sampling at Summit demonstrates the value of the site as a unique, high latitude, free troposphere observatory.

Hydrocarbon Seasonality

• Seasonal cycles of hydrocarbons mainly reflect anthropogenic emissions and a weaker winter OH sink

• Reproducibility of cycles over 8 years indicates source/sink balance, providing sensitive baseline to detect/quantify future changes

(Whole air samples analyzed by the D. R. Blake group at UC Irvine)

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C a l e n d a r D a y

Ozone (ppbv)

S u m m i t B a r r o w Z e p p e l i n f j e l l e t

2004 at Summit

All Summit data 2000-2004

Profiles at 4 polar stations are the mean of all available sonde data 1995-2004, X marks the median mixing ratio at Summit.

From Helmig et al., 2007

7Be and 210Pb Radionuclides

• Summer peak in 7Be unique at Summit among Arctic sites, indicating significant seasonal influence from the upper troposphere/lower stratosphere

• 210Pb concentrations decrease with increasing elevation to Summit. Distinct lack of a winter peak confirms that Arctic Haze rarely extends to 3 km altitude over Summit

• (From Dibb, 2007)

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2008 Phase 1

7 Be fCi/scm

210

Pb fCi/scm

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210Pb = 0.026

7Be + 0.86

R2 = 0.81

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Pb fCi/scm

7Be fCi/scm

Filter sampling with 48 hour integration time for the radio-tracers was conducted at Summit in the 1997-98 pilot “overwinter campaign”, again for the 2000-2002 experiment, and since August 2003 to present. Dibb (2007) reported on results through July 2005.

Here are observations during the first phase of ARCTAS/ARCIONS. Note that the peak of 7Be in late April is the highest seen since the 2-day sampling interval was initiated in 1997! Unfortunately, NASA airplanes went home, and I believe ARCIONS took a break, on 19 or 20 April.

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2008

7 Be fCi m

-3210

Pb fCi m

-3

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2008

Surface Snow Br

- nmol kg

-1

210

Pb fCi m

-3

Everyone came back in the summer for phase 2.

Coupling between 7Be and 210Pb even more striking than in April.

What does bromide have to do with any of this? Mostly, means I am getting ahead of myself, but for reactive gases it is now well established that Summit is not really a great “free trop” observatory.

In 1998 production of NOx in snow was discovered at Summit (also Alert and South Pole)

“Snow Photochemistry” is now a hot topic at Summit, and elsewhere.

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 XO

Turbulent EddyDiffusion

Windpumping/Turbulent Diffusion

Modeling

PhotochemicalModeling

Adsorption/Desorption

PhotochemicalModeling

Advection/Diffusion

Incorporation into Ice

Actinic FluxModeling

More like this model Than this!

RONO2

OH H2Ohν

hν

hνhνOH

O3 HO2 RO2H+hν

OH

RO2

HNO3NO2HONONO

RONO2

NO2̄NO3̄

HNO3NO2

NOHONO

Surface Gas PhaseIce

RONO2

Organics(Carbonyls)

hν

Alkenes

hν

O3

Monocarboxylic Acids

hν

Peroxy Radicals

O2

OH

Formation Mechanisms??

Oxidation of the abundant (but poorly characterized) supply of organic compounds in the snow appears to result in production of a slew of reactive hydrocarbons.

Clearly, OH is enhanced above the snow and is likely even higher in the upper part of the pack.

Studies in both polar regions, and seasonal snowpacks have confirmed that photolysis of nitrate in snow releases NOx 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.

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1 5

1 0

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OH (moleucles cm

-3

)

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L o c a l T i m e ( W G S T )

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1peroxy (molecules cm

-3

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1 2

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Wind Speed (m/s)

p e r o x y m e a s u r e d

p e r o x y p r e d i c t e d

O H m e a s u r e d

O H p r e d i c t e d

W i n d S p e e d

HOx measurements above polar snow are limited, so far, to South Pole, Summit, and Halley.

Levels highest at Summit, particularly for OH (summer median values of OH about 4 times higher at Summit than South Pole).

Standard models, constrained by observations of presumed main precursors, reasonably simulate HO2 at Summit, but can not capture OH enhancements.

Note that model underestimates OH most severely in high winds.

From Sjostedt et al. 2007 (in the special issue)

HOx Measurements vs. Model

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Obs. OH (1E6 molec./cm3)

NO (pptv)200+160 to 200

120 to 16080 to 120

40 to 800 to 40

0 2 4 6 8 10 12Model OH (1E6 molec./cm3)

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Obs. OH (1E6 molec./cm3)

Wind Speed (m/sec)10+8 to 10

6 to 84 to 6

2 to 40 to 2

Ob

s. H

O (

1E6

mo

lec.

/cm

3)

Ob

s. H

O (

1E6

mo

lec.

/cm

3)

Note the different scales! Obs. > Model consistently

HO Measurements vs. Model(Sjostedt et al., 2007)

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OH (molecules cm

-3

)

2 01 51 05

L o c a l T i m e ( W G S T )

m o d e l e d

o b s e r v e d

Composite 24 hr measurements and model predictions of OH during 2003 summer campaign. Predicted values obtained from NMHC model constrained by H2O2, CH2O and HONO from Sjostedt et al., 2007.

O3 vertical profile measured at Summit,

Greenland by Helmig D.et al.,2007

Ozone production rate due to standard photochemistry

from Summit 2003 data

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Ozone Production (ppbv/day)

6/27/2003 6/29/2003 7/1/2003 7/3/2003 7/5/2003 7/7/2003 7/9/2003 7/11/2003

Local time

BL

RO2 + NO RO+NO2

A couple more puzzling observations at Summit, particularly when compared to South Pole.

Note that Detlev has much more O3 flux data now, confirms the snow/surface is often a sink at Summit.

Effect of Bromine Chemistry on Summit Photochemistry

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Normalized Shift

OH HO2 CH3O2 RO2 HO2+RO2

BrO= 2 pptv BrO= 4 pptv BrO= 10 pptv

• BrO can significantly shift the HO2 and OH partition while has a limited impact on total peroxy radical levels.

• BrO does not have significant impact on NO2/NO ratio and is a minor sink for CH2O.

• Model estimated net ozone production rates ranges from 0.6, -0.8, to -6 ppbv/day, corresponding to 2, 4, and 10 pptv of BrO.

• Near constant O3 observations is consistent with a few pptv of BrO.

2. Key bromine chemical cycles in polar BL

BrO

Br2

Aqueous Phase

Deposit

GEM (Hgo)

RGM

Gaseous Phase

NO

hv

O3

HO2

HCHOHO2

hv

BrO

HOBr

Br

HBr

OH

Bromine chemical cycles that induce ozone depletion, mercury depletion and impact the RO2/OH ratio.

hv

Br- + H+ HOBr + Br- + H+

Species Method Research Group

Hg (GEM, RGM, FPM) Tekran NOAA - Brooks

OH, RO2, H2SO4 CIMS Ga. Tech - Huey

HCl, HO2NO2, BrO,SO2, etc CIMS Ga. Tech - Huey

BrO, HCHO DOAS UCLA - Stutz

Soluble Ions (Br-, Cl-, etc.)

Aerosol #, area distribution

MC/IC, Filters, Snow

SMPS

UNH - Dibb

Radiation, J-values

BL profiling (met and O3)

Spectrometer

Tower/Tether Sonde

UH - Lefer

Whole Air Samples GC-MS UCI – Blake

NO, O3, Dewpoint, etc commercial Ga. Tech - Huey

G-SHOx May-June ‘07, June-July ‘08Greenland – Summit Halogens and HOx

12x10

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OH(molec.cm

-3

)

5/16/2007 5/21/2007 5/26/2007 5/31/2007 6/5/2007 6/10/2007

Time(GMT)

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O3

3(ppbv) and RGM(pg.m

-3)

OHmeas

OHpred

RGM

O3 3 times

Comparison of OH predictions from the HOx-NOx-CH4 model to OH measurements, plotted with

RGM and O3 concentrations. Note the model has trouble when RGM (Br?) is enhanced.

2007 LP-DOAS/CIMS intercomparison

BrO observation by CIMS in 2008

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BrO (pptv)

6/11/2008 6/16/2008 6/21/2008 6/26/2008 7/1/2008 7/6/2008

GMT

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Ozone (ppbv)

BrO_10min_avg Ozone_10min_avg

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2007

Ambient soluble Br

- pptv

2008

Gas phase soluble Br- in the air just above snow was also generally lower in 2008 than 2007.

May be a seasonal feature, but can not rule out interannual variability in transport and/or ambient conditions.

Note that soluble Br- shows modest increase in first 2008 CIMS BrO event, but is quite low 20 - 21 June.

Simultaneous measurements above the snow and at 10 cm depth in the snowpack were made on 2 days in 2007.

Both cases the mid-day peak was more pronounced in the firn.

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2008 Mean (std error)

AmbientFirn Air

Soluble Br

- ppt

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2008 Median

AmbientFirn Air

Soluble Br

- ppt

In 2008 such measurements were made on 2/3 of all sampling days.

Mixing ratios in the firn air were lower in 2008 than 2007, but tended to be higher than those above the snow.

Again, firn air enhancement largest in early afternoon.

Is the snow one of the sources of Br at Summit?

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2008 Surface SnowMean (stdev)

Br- nmol kg

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Sea Salt Ratio =

~ 1.8 x 10-3

Br- /Na

+

In the surface layer of snow bromide was always quantifiable, albeit low compared to major ions.

If 5 nmol Br kg-1 in the top cm of snow was released to a well mixed 100 m deep boundary layer as a gas the mixing ratio would be ~165 ppt (compare to soluble Br- mainly < 2 ppt).

Note that bromide in snow was greatly in excess of the sea salt ratio (based on sodium).

~ 300m

FT

aerosol, gases

partly halogen processed airpartly cleanpartly marinepartly BB

inversion

T

gases

Br-

summer, year -1

summer, year -2

gases trapped at nightmixed during day

Br reservoir is snowatmosphere ~ 1%

Br reactivity

Cl reactivity

• strong diurnal variation• long range air mass changes• intermittent BL venting

Hg

fog blowing snow

1st year sea ice; frost flowers

Early spring

no transport

no Br/Cl influence on VOCno Arctic Haze

no elevated Br- in snow pack

Bromine explosionArctic Haze

autumn/winter: - no Br/Cl influence on VOC - no elevated Br- in snow pack

Late spring/early summer

long-range transport

Convection(open leads)

Bromine explosion

deposition of Bry via snow fall: - Br- peak in summer snow pack - ambient gas and aerosol Bry is “low”

active photochemistry

snow pack photochemistry: - release of NOx to air - high snow pack Br-

- high Br VOC-reactivity in snow

“cold”: production of RGM“warm”: no RGM production

reduction and release of RGM during summer no summer peak in Hg deeper in firn

in low elevation sites, snow-bound RGM might get into aquatic ecosystems during snowmelt

processes in blue are critical, need checking!!

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Coincident DC-8 spiral down over Summit and ARCIONS sonde launch, 5 April, 2008.

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Coincident DC-8 spiral down over Summit and ARCIONS sonde launch, 7 July, 2008.