# Presentation: 164093

## CO2 MEASUREMENTS ON THE PIRATA NETWORK

**CO****2**** MEASUREMENTS ON THE PIRATA NETWORK**
- Nathalie Lefèvre, IRD LOCEAN
*Nathalie.lefevre@locean-ipsl.upmc.fr*

*PIRATA 12 meeting, Miami, 1-3 November 2006*

- Presented by Fabrice Hernandez, IRD/Mercator Ocean

## Scientific objectives

_**European project CARBOOCEAN (theme 1, WP4)**_
- Estimate the flux of CO2 at the air-sea interface by developing an observational network for CO2

**Network for the tropical Atlantic:**
-  2 merchant ships (France-Brazil, *red line*, France-French Guiana, *green line*) equipped with an automated CO2 system to measure atmospheric and oceanic CO2 underway by infrared detection.
-  2 CO2 sensors for PIRATA moorings at **6****o****S, 10****o****W** and **8****o****N, 38****o****W** (*black dots*) using a colorimetric method (CARIOCA buoy technology).

*EGEE 3 cruise (in blue)*

**Notes:**
The CO2 network is part of the European project CARBOOCEAN. One of the objectives of this project is to better estimate the CO2 flux at the air-sea interface and its variability. The approach used is the development of an observational network for the Atlantic ocean and to a lesser extent for the Southern ocean. At IRD LOCEAN we have proposed to equip 2 merchant ships (France-Brazil and France- French Guiana) and 2 moorings to assess the CO2 flux and its variability in the tropical Atlantic. This work is funded under CARBOOCEAN and the network is being setup.

## Choice of the PIRATA moorings for installing CO2 sensors

- SeaWiFs, Septembre 2003

- Impact of the Amazon outflow on the CO2 distribution during the retroflection of the North Brazil Current (NBC)
- Advection far East in the North Equatorial Counter-Current (NECC)

- _CO__2__ sensors on PIRATA moorings:_
**6****o****S, 10****o****W**: CO2 time-series in the SEC
**8****o****N, 38****o****W**: CO2 time-series in the NECC

- 10W

- 40W

- 10S

**Notes:**
Surface circulation is shown: 2 main current systems, the NECC and the SEC. The plan is to have one CO2 sensor in each current, eastern and western parts of the tropical Atlantic. The CO2 variability is affected by SST, S, biological activity. We know nothing about the river impact on the CO2 distribution (freshwater input & biological activity). Amazon waters propagate very far east when the NECC is at its maximum (autumn). In the east the impact of the upwelling supplying CO2-rich waters is monitored at PIRATA 6S, 10W.

## Installation of the first CO2 sensor on the PIRATA mooring at 6oS, 10oW during the EGEE 3 cruise (June 2006)

- Box with electronics

- Argos antenna

- CO2 sensor and oxygen optode

- CO2 sensors built at DT INSU (*Antoine Guillot, Th**é**o Danguy, Laurence Beaumont*)

**Notes:**
We measure the partial pressure (or more correctly the fugacity) of CO2 (CO2 sensor) and the dissolved oxygen concentration (optode Aanderaa 3835 that can operate down to 300m).

## Data of the CO2 sensor since the 7th of June 2006

- SST decrease due to the upwelling (June-September)

- CO2 increase

- Hourly measurements of CO2, SST and O2 sent by ARGOS

- pCO2 =  -19.54 * SST + 895.84           r2 = -0.88

**367 micro-atm**

**Notes:**
The upwelling starts at the end of June so the temperature decreases and the surface waters, initially undersaturated in CO2 (below the atmospheric value of 367 atm, becomes supersaturated.

## Correlation between pCO2 and SST and SSS

*Detrended and normalized hourly pCO**2**, sea surface temperature and sea surface salinity (interpolated from PIRATA daily records) as a function of time (in days).*
*Data from 8 June to 7 October 2006.*

- Colder and fresher seawater associated with higher CO2 concentrations

## Diurnal cycle consistent with biological activity

- Dissolved inorganic carbon (DIC) calculated (does not depend on temperature)
- Decrease of DIC during the day
- (photosynthesis)
- Increase of DIC during the night
- (convection & respiration)

## Air-sea CO2 flux

- Calculation of the flux using the formula:
**F = k.S.(pCO****2 ocean ****– pCO****2 atm****)**
**- k** is a gas exchange coefficient depending on the wind speed (PIRATA wind speed converted to a 10m wind speed), it is calculated using the formula of Wanninkhof (1992)
**- S** is the solubility of CO2 (Weiss et al., 1974)
**- pCO****2 atm** is the atmospheric pCO2 measured during EGEE 3 (367 atm)

**Source of CO****2** : 4.56 m.mol.m-2.d-1 (*8 June – 7 October 2006*)
- ranging from 1.19 in June to 6.82 m.mol.m-2.d-1 in September

**Notes:**
The flux (expressed in milli mole per square meter and per day) is positive which means an outgassing of CO2 from the ocean to the atmosphere. It was lower in June then it gradually increased to be maximum in September. The wind was quite steady with an average of 7.3 m/s during that period.

## Conclusions & perspectives

** ****Time-series set up at the PIRATA mooring at ****6****o****S, 10****o****W**** to monitor the CO****2**** variability in the eastern part of the tropical Atlantic (in the South Equatorial Current).**
** ****Replace the sensor (lifetime of 1 year) during ****EGEE 5 (June 2007)**** to continue the time-series (the existing one is sent for calibration, renewal of the dye at the DT INSU while a spare one is installed).**
****** ****Funding obtained to install a CO****2**** sensor at ****8****o****N, 38****o****W**** during the PIRATA Brazil cruise in 2007 (NECC and monitoring of the Amazon discharge in the tropical Atlantic).**

**Notes:**
Satisfactory results at the PIRATA mooring, no biofouling detected on the CO2 sensor, good CO2 data in agreement with the physical properties (water mass variability and impact of the upwelling). The sensor needs to be replaced each year during the PIRATA cruise. The existing one is sent for calibration, renewal of the dye and a spare one is installed on the mooring with new batteries. The year after, we swap the sensors. 
We hope to extend the CO2 network by equipping other PIRATA moorings...