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

Sources of N2O to the atmosphere

Denman et al., 2007

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

Global distribution of ΔpN2O

(Suntharalingam & Sarmiento 2000)

-20 0 10 20 30 40 50 60 150

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”

diurnal stratification in the equatorial Atlantic

Ekman vertical velocities from QuikScat wind fields

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

Diurnal stratification in the equatorial Atlantic Ocean

Kerstin Richter, p.c.

Effect of surfactants on N2O gas exchange – laboratory results

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