Effect of surfactants on N2O emissions from biologically productive regions
Annette Kock, Jens Schafstall, Tim Fischer, Marcus Dengler, Peter Brandt and Hermann W. Bange
Effects of N2O in the atmosphere
N2O as greenhouse gas Ozone depletion in the stratosphere
100 yr global warming potential
298 x CO2
Atmospheric lifetime 114 yr
(Intergovernmental Panel on Climate Change (IPCC), 2007)
2
Ravishankara et al., 2009: „Nitrous Oxide (N2O): The dominant ozone-depleting substance in the 21st century.“
http://agage.eas.gatech.edu/data.htm
310
300N2O
Mol
e Fr
actio
n [p
pb] 320
1980 1990 2000 2010Year
N2O in the marine N cycle
4
suboxic/anoxic
NH4+
NO2-
NO3-
NH2OHN2
-III
+V
±0
Denitrificati
on
Org. N
Oxidation state
oxic
N2O
Nitrification
N2O distribution off Mauritania
0
50
100
150
200
400
600
800
1000
200
Dep
th [m
]
21°W 20°W 19°W 18°W 17°W
15
25
35
5
40
30
20
10
N2O [nmol L-1]P347Jan 07
P348Feb 07
ATA3Feb 08
N2O mixed layer budget in the Mauritanian upwelling
Atmosphere
Subsurface ocean
Pycnocline
Gas exchange
Diapycnal fluxVertical advection
N2O production
dzdKF O][N2
ON2
)][]([ 22ON2 eqsurfacew ONONkF
Mixed layer
Continent
0
50
100
150
200
Dep
th [m
]
Kρ [m2 s-1]
1e-006 1e-005 0.0001 0.001 0.01
18°W 17°W 16.8°W 16.6°W 16.5°W 16.4°W 16.3°W
N2O fluxes off Mauritania
• Diapycnal flux calculated by microstructure measurements of small scale turbulence in combination with N2O-profiles from the water column.
• Quantification of sea-to-air flux from surface N2O concentrations and in-situ wind speeds.
21°N
20°N
19°N
16°N
24°W 22°W 16°W20°W 18°W
ΔN2O [nmol L-1] 12
10
8
6
4
2
0
18°N
17°N
Projection of all Stations to 18°N
Diapycnal fluxSea-to-air flux
Wat
er d
epth
[m] 0
2000
400024°W 22°W 20°W 18°W 16°W
Comparison of sea-to-air and diapycnal flux
N2O mixed layer budget in the Mauritanian uwpelling
Atmosphere
Pycnocline
Sea-to-air flux
Diapycnal fluxVertical advection
N2O production Mixed Layer
Continent
25m +0.048 nmol m-2 s-1
Flux difference~60 nmol L-1 yr-1
Potential N2O production rate
in the mixed layer
+0.002 nmol m-2 s-1 +0.019 nmol m-2 s-1
-0.069 nmol m-2 s-1
Subsurface ocean
N2O Production in the mixed layer?
Potential N2O production rate:
• Evidence for nitrification in the euphotic zone -> N2O production in the mixed layer (e.g. Yool et al. 2007, Clark et al. 2008).
• Previous mixed layer budget calculations for N2O indicate strong near-surface production (e.g. Dore & Karl, 1996; Morell et al., 2001)
PRO• N2O production rate below the mixed
layer: <3.3 nmol L-1 yr -1 (Freing et al., 2012).• N2O yield increases with decreasing
oxygen concentrations (Goreau et al., 1980;
Löscher et al., 2012).• Measurements of nitrification rates in
the Mauritanian upwelling: higher nitrification rates at greater depths than in the surface (Rees et al., 2011).
• Surface N2O distribution linked to upwelling
CONTRA
~60 nmol L-1 yr-1
Effect of surfactants on N2O gas exchange in laboratory studies
20
20
20
K. Richter, p.c., see poster „The Schmidt Number Dependency of Air-SeaGas Exchange with Varying Surfactant Coverage”
x3
x3
x3
Nightingale et al. 2000Tsai & Liu 2003, surfactant-influenced
• Good conditions for occurrence of surfactants in upwelling areas (Wurl et al.,
2011; Gasparovic et al., 1998 ).
Similar findings:Calculations of net community production based on CO2/N2O O2/N2O (Steinhoff et al., 2012; see poster „Biological productivity in the Mauritanian upwelling estimated with a triple gas approach”)
Recalculation of sea-to-air flux using gas exchange parameterization of Tsai & Liu (2003).
Effect of surfactants on N2O gas exchange in the field?
N2O mixed layer budget with reduced gas exchange
Atmosphere
Pycnocline
Diapycnal fluxVertical advection
N2O production Mixed layer
Continent
-0.001 nmol m-2 s-1
Flux difference
+0.002 nmol m-2 s-1 +0.019 nmol m-2 s-1
-0.020 nmol m-2 s-1
Sea-to-air flux (Parameterization Tsai & Liu 2003)
Subsurface ocean
Diurnal stratification in the equatorial Atlantic
5:00 10:00 15:00 20:00Time
temperature diurnal cycle 0°N 10°W
5
10
0:00
Dep
th [m
]
0
→ Evolution of a diurnal stratification within the mixed layer
→ Supersaturated N2O
concentrations in the deeper mixed layer cut off from the sea-surface while near-surface waters may quickly equilibrate.
Glider measurements of temperature during MSM 18-2 & MSM 18-3, equatorial Atlantic, May-July 2011
Intense solar irradiation:
Consequences?
data from Denman et al., 2007
Favorable conditions for surfactants:Areas with high primary productivity, high solar irradiation, low to moderate wind speeds. → Most eastern boundary upwelling systems, equatorial upwelling, coastal
areas.
Summary & Conclusions
• Mixed layer budget off Mauritania reveals large discrepancy between sea-to-air flux and supply from subsurface layer.
• Mixed layer source of N2O would require extremely high production rates to compensate discrepancy.
• Reduced gas exchange in line with findings by Steinhoff et al., 2012 and K. Richter.
• Potential for reduced N2O emissions from other upwelling areas, too.
• Effect of surfactants on N2O emissions from other productive regions needs to be investigated.
• Other possible causes for budget imbalance need to be considered! Poster: „Physical processes controlling greenhouse gas emission in
upwelling regions of the ocean: a N2O case study”
Future activities
R/V Meteor Cruise 91 (Peruvian upwelling), December 2012:
Evaluate potential processes that influence the mixed layer budget of N2O:
• High resolution N2O profiles of mixed layer• Microstructure measurements• High resolution underway pCO2/pN2O
• Determination of the role of the organic matter and gel particles in the surface microlayer for the air-sea gas exchange of trace gases. • Air-sea gas echange processes and atmos. fluxes • Determination of the upwelling velocity
Advances in measurement techniques
Better precision of underway measurements using OA-ICOS N2O analyzer allows flux calculation even at low ΔN2O
Arevalo, 2012
MSM 18-2,equatorial Atlantic May/June 2011
N2O in the marine N cycle
27
suboxic/anoxic
Nitrification
• Bacterial and archaeal ammonium oxidation• N2O production during archaeal nitrification (Santoro et al. 2011, Löscher et al. 2012)• N2O production increases with decreasing oxygen (Goreau et al. 1980, Löscher et al., 2012)• Light inhibition of nitrification (e.g. Guerrero & Jones, 1996) challenged by in-situ measurements of nitrification rates (e.g. Clark et al. 2008)
NH4+
NO2-
NO3-
NH2OHN2
-III
+V
±0
Denitrificati
on
Org. N
Oxidation state
oxic
N2O
Nitrification