The influence of transformed Reynolds number suppression on gas transfer parameterizations and global DMS and CO2 fluxes

Eddy covariance measurements show gas transfer velocity suppression at medium to high wind speed. A wind-wave interaction described by the transformed Reynolds number is used to characterize environmental conditions favoring this suppression. We take the transformed Reynolds number parameterization to review the two most cited wind speed gas transfer velocity parameterizations: Nightingale et al. (2000) and Wanninkhof (1992, 2014). We propose an algorithm to adjust k values for the effect of gas transfer suppression and validate it with two directly measured dimethyl sulfide (DMS) gas transfer velocity data sets that experienced gas transfer suppression. We also show that the data set used in the Nightingale 2000 parameterization experienced gas transfer suppression. A compensation of the suppression effect leads to an average increase of 22% in the k vs. u relationship. Performing the same correction for Wanninkhof 2014 leads to an increase of 9.85 %. Additionally, we applied our gas transfer suppression algorithm to global air-sea flux climatologies of CO2 and DMS. The global application of gas transfer suppression leads to a decrease of 11% in DMS outgassing. We expect the magnitude of Reynolds suppression on any global air-sea gas exchange to be about 10

1. Wochenbericht SO234/2

Wochenbericht 08.07.-13.07.2014 (Durban/Südafrika - ~27°S, 24°E

2. Wochenbericht SO234/2

Wochenbericht vom 14.07.-20.07.2014 (Port Louis/Mauritius

RV SONNE SO243 Cruise Report / Fahrtbericht Guayaquil, Ecuador: 05. October 2015 Antofagasta, Chile: 22. October 2015 SO243 ASTRA-OMZ: AIR SEA INTERACTION OF TRACE ELEMENTS IN OXYGEN MINIMUM ZONES

The ASTRA-OMZ SO243 cruise on board the R/V Sonne took place between the 5th and 22nd October 2015 from Guayaquil, Ecuador to Antofagasta, Chile. Scientists from Germany, the U.S.A, and Norway participated, spanning chemical, biological, and physical oceanography, as well as atmospheric science. The main goal of the cruise was to determine the impact of low oxygen conditions on trace element cycling and distributions, as well as to determine how air-sea exchange of trace elements is influenced by high productivity conditions. The subsequent impact of trace element ocean-atmosphere exchange on atmospheric chemistry and climate will be determined. A summary of the main preliminary results is below: - a strong source of nitrous oxide (N2O) and carbon dioxide (CO2) was detected from surface waters in the Peruvian upwelling, particularly in the near-coastal area between 9°S and 18°S - generally, surface N2O during the SO-243 cruise was lower than previously observed, probably due to the reduced extent of upwelling events because of El Niño conditions - less dimethyl sulphide (DMS) (< 2nmol L-1) and isoprene (at 20-30 pmol L-1) than on board previous cruises in the coastal upwelling region (8°-12°S) were detected, likely due to suppressed upwelling off of Peru because of the El Niño conditions - a strong source for atmospheric carbonyl sulphide (OCS) was observed, as well as a strong correlation with oxygen. OCS decreased below the detection limit in oxygen depleted zones. - a strong contrast between normal and El Niño conditions were detected for the halocarbon compounds. Both surface and deeper water was characterized by much larger concentrations of bromocarbons than of iodocarbons during ASTRA-OMZ, which stands in contrast to the previous M91 cruise during neutral conditions. - it appears that the direct flux eddy covariance method was successful for sea-to-air flux measurements of N2O (for the first time) - a pronounced atmospheric inversion layer at approximately 1 km altitude was striking, which was accompanied by an accumulation of high relative humidity and moderate to fresh southerly winds below this inversion. Convective activity was limited and very few precipitation events were detected. Tropospheric ozone levels reveal distinct fluctuations within 9.5°S and 16.5°S latitude. - the oxygen distribution measured at about 9°S showed that the upwelling in October 2015 was very weak. Low oxygen water with less than 5 μmol kg-1 was located only below 250 m in October 2015 - higher oxygen distribution in 2015, as well as the changes in water temperature, salinity and density indicate the influence of El Niño. We have already published our first paper related to El Niño during SO243 (Stramma et al. 2016)

Space-based retrievals of air-sea gas transfer velocities using altimeters: Calibration for dimethyl sulfide

This study is the first to directly correlate gas transfer velocity, measured at sea using the eddy-correlation (EC) technique, and satellite altimeter backscattering. During eight research cruises in different parts of the world, gas transfer velocity of dimethyl sulfide (DMS) was measured. The sample times and locations were compared with overpass times and locations of remote sensing satellites carrying Ku-band altimeters: ERS-1, ERS-2, TOPEX, POSEIDON, GEOSAT Follow-On, JASON-1, JASON-2 and ENVISAT. The result was 179 pairs of gas transfer velocity measurements and backscattering coefficients. An inter-calibration of the different altimeters significantly reduced data scatter. The inter-calibrated data was best fitted to a quadratic relation between the inverse of the backscattering coefficients and the gas transfer velocity measurements. A gas transfer parameterization based on backscattering, corresponding with sea surface roughness, might be expected to perform better than wind speed-based parameterizations. Our results, however, did not show improvement compared to direct correlation of shipboard wind speeds. The relationship of gas transfer velocity to satellite-derived backscatter, or wind speed, is useful to provide retrieval algorithms. Gas transfer velocity (cm/hr), corrected to a Schmidt number of 660, is proportional to wind speed (m/s). The measured gas transfer velocity is controlled by both the individual water-side and air-side gas transfer velocities. We calculated the latter using a numerical scheme, to derive water-side gas transfer velocity. DMS is sufficiently soluble to neglect bubble-mediated gas transfer, thus, the DMS transfer velocities could be applied to estimate water-side gas transfer velocities through the unbroken surface of any other gas Key Points: - Show relations between altimeter data and field values of air-sea gas transfer - DMS gas transfer velocity can be used to estimate direct gas transfer of any gas - Direct gas transfer velocity (for Sc = 660) is roughly double 10 m wind spee

A chemical ionization mass spectrometer for continuous underway shipboard analysis of dimethylsulfide in near-surface seawater

A compact, low-cost atmospheric pressure, chemical ionization mass spectrometer ("mini-CIMS") has been developed for continuous underway shipboard measurements of dimethylsulfide (DMS) in seawater. The instrument was used to analyze DMS in air equilibrated with flowing seawater across a porous Teflon membrane equilibrator. The equilibrated gas stream was diluted with air containing an isotopically-labeled internal standard. DMS is ionized at atmospheric pressure via proton transfer from water vapor, then declustered, mass filtered via quadrupole mass spectrometry, and detected with an electron multiplier. The instrument described here is based on a low-cost residual gas analyzer (Stanford Research Systems), which has been modified for use as a chemical ionization mass spectrometer. The mini-CIMS has a gas phase detection limit of 220 ppt DMS for a 1 min averaging time, which is roughly equivalent to a seawater DMS concentration of 0.1 nM DMS at 20°C. The mini-CIMS has the sensitivity, selectivity, and time response required for underway measurements of surface ocean DMS over the full range of oceanographic conditions. The simple, robust design and relatively low cost of the instrument are intended to facilitate use in process studies and surveys, with potential for long-term deployment on research vessels, ships of opportunity, and large buoys

A time series of incubation experiments to examine the production and loss of CH3I in surface seawater

In order to investigate production pathways of methyl iodide and controls on emissions from the surface ocean, a set of repeated in vitro incubation experiments were performed over an annual cycle in the context of a time series of in situ measurements in Kiel Fjord (54.3°N, 10.1°E). The incubation experiments revealed a diurnal variation of methyl iodide in samples exposed to natural light, with maxima during day time and losses during night hours. The amplitude of the daily accumulation varied seasonally and was not affected by filtration (0.2 µm), consistent with a photochemical pathway for CH3I production. The methyl iodide loss rate at nighttime correlates with the concentration accumulated during daytime suggesting a first-order loss mechanism (R2 = 0.29, p << 0.01). Daily (24 h) net production (Pnet) was similar in magnitude between in vitro and in situ mass balances. However, the estimated gross production (Pgross) of methyl iodide ranged from −0.07 to 2.24 pmol L−1 d−1 and was up to 5 times higher in summer than Pnet calculated from the in situ study. The large excess of Pgross over Pnet in summer revealed by the incubation experiments is a consequence of large losses of CH3I by as-yet uncharacterized processes (e.g., biological degradation or chemical pathways other than Cl− substitution)