Post on 14-Jan-2016
description
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)
0
1 0
2 0
3 0
4 0
5 0
6 0
7 0
8 0
9 0
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0
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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50
100
150
200
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300
350
0
2
4
6
8
10
4/1 4/6 4/12 4/18 4/24 4/30
2008 Phase 1
7 Be fCi/scm
210
Pb fCi/scm
0
2
4
6
8
10
0 50 100 150 200 250 300 350
210Pb = 0.026
7Be + 0.86
R2 = 0.81
210
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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50
100
150
200
0
1
2
3
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6
6/6 6/10 6/14 6/18 6/22 6/26 6/30 7/4 7/8
2008
7 Be fCi m
-3210
Pb fCi m
-3
0
5
10
15
20
0
1
2
3
4
5
6
6/6 6/10 6/14 6/18 6/22 6/26 6/30 7/4 7/8
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.
2 0 x 1 0
6
1 5
1 0
5
OH (moleucles cm
-3
)
6 / 2 9 / 2 0 0 3 7 / 3 / 2 0 0 3 7 / 7 / 2 0 0 3 7 / 1 1 / 2 0 0 3
L o c a l T i m e ( W G S T )
5 x 1 0
8
4
3
2
1peroxy (molecules cm
-3
)
1 2
8
4
0
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
0 2 4 6 8 10 12Model OH (1E6 molec./cm3)
0
5
10
15
20
25
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)
0
5
10
15
20
25
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)
1 0 x 1 0
6
8
6
4
2
0
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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10
5
0
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
0.4
0.3
0.2
0.1
0.0
-0.1
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
6
10
8
6
4
2
0
OH(molec.cm
-3
)
5/16/2007 5/21/2007 5/26/2007 5/31/2007 6/5/2007 6/10/2007
Time(GMT)
200
100
0
-100
-200
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
10
8
6
4
2
0
BrO (pptv)
6/11/2008 6/16/2008 6/21/2008 6/26/2008 7/1/2008 7/6/2008
GMT
60
50
40
Ozone (ppbv)
BrO_10min_avg Ozone_10min_avg
0
0.5
1
1.5
2
5/10 5/18 5/26 6/3 6/11 6/19 6/27 7/5
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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0.1
0.2
0.3
0.4
0.5
0.6
0:00 6:00 12:00 18:00 24:00
2008 Mean (std error)
AmbientFirn Air
Soluble Br
- ppt
0
0.1
0.2
0.3
0.4
0.5
0.6
0:00 6:00 12:00 18:00 24:00
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?
0
5
10
15
20
6/6 6/10 6/14 6/18 6/22 6/26 6/30 7/4 7/8
2008 Surface SnowMean (stdev)
Br- nmol kg
-1
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
6/6 6/10 6/14 6/18 6/22 6/26 6/30 7/4 7/8
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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60
62
64
200
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700
DC
-8 S
pira
l
Tem
pera
ture
K
O3
pp
b
Pre
ssu
re m
b
Coincident DC-8 spiral down over Summit and ARCIONS sonde launch, 5 April, 2008.
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DC
-8 S
pira
l
Tem
pera
ture
K
O3
ppb
Pre
ssure
mb
Coincident DC-8 spiral down over Summit and ARCIONS sonde launch, 7 July, 2008.