Modelling marine emissions and atmospheric distributions of halocarbons and dimethyl sulfide: The influence of prescribed water concentration vs. prescribed emissions

Marine-produced short-lived trace gases such as dibromomethane (CH$_{2}$Br$_{2}$), bromoform (CHBr$_{3}$), methyliodide (CH$_{3}$I) and dimethyl sulfide (DMS) significantly impact tropospheric and stratospheric chemistry. Describing their marine emissions in atmospheric chemistry models as accurately as possible is necessary to quantify their impact on ozone depletion and Earth’s radiative budget. So far, marine emissions of trace gases have mainly been prescribed from emission climatologies, thus lacking the interaction between the actual state of the atmosphere and the ocean. Here we present simulations with the chemistry climate model EMAC (ECHAM5/MESSy Atmospheric Chemistry) with online calculation of emissions based on surface water concentrations, in contrast to directly prescribed emissions. Considering the actual state of the model atmosphere results in a concentration gradient consistent with model realtime conditions at the ocean surface and in the atmosphere, which determine the direction and magnitude of the computed flux. This method has a number of conceptual and practical benefits, as the modelled emission can respond consistently to changes in sea surface temperature, surface wind speed, sea ice cover and especially atmospheric mixing ratio. This online calculation could enhance, dampen or even invert the fluxes (i.e. deposition instead of emissions) of very short-lived substances (VSLS). We show that differences between prescribing emissions and prescribing concentrations (-28%for CH$_{2}$Br$_{2}$ to +11%for CHBr$_{3}$) result mainly from consideration of the actual, time-varying state of the atmosphere. The absolute magnitude of the differences depends mainly on the surface ocean saturation of each particular gas. Comparison to observations from aircraft, ships and ground stations reveals that computing the air–sea flux interactively leads in most of the cases to more accurate atmospheric mixing ratios in the model compared to the computation from prescribed emissions. Calculating emissions online also enables effective testing of different air–sea transfer velocity (k) parameterizations, which was performed here for eight different parameterizations. The testing of these different k values is of special interest for DMS, as recently published parameterizations derived by direct flux measurements using eddy covariance measurements suggest decreasing k values at high wind speeds or a linear relationship with wind speed. Implementing these parameterizations reduces discrepancies in modelled DMS atmospheric mixing ratios and observations by a factor of 1.5 compared to parameterizations with a quadratic or cubic relationship to wind speed

Direct oceanic emissions unlikely to account for the missing source of atmospheric carbonyl sulfide

Lennartz, S.T. ... et. al.-- 18 pages, 5 figures, 5 tables, 1 appendix, supplement https://dx.doi.org/10.5194/acp-17-385-2017-supplementThe climate active trace-gas carbonyl sulfide (OCS) is the most abundant sulfur gas in the atmosphere. A missing source in its atmospheric budget is currently suggested, resulting from an upward revision of the vegetation sink. Tropical oceanic emissions have been proposed to close the resulting gap in the atmospheric budget. We present a bottom-up approach including (i) new observations of OCS in surface waters of the tropical Atlantic, Pacific and Indian oceans and (ii) a further improved global box model to show that direct OCS emissions are unlikely to account for the missing source. The box model suggests an undersaturation of the surface water with respect to OCS integrated over the entire tropical ocean area and, further, global annual direct emissions of OCS well below that suggested by top-down estimates. In addition, we discuss the potential of indirect emission from CS2 and dimethylsulfide (DMS) to account for the gap in the atmospheric budget. This bottom-up estimate of oceanic emissions has implications for using OCS as a proxy for global terrestrial CO2 uptake, which is currently impeded by the inadequate quantification of atmospheric OCS sources and sinksThis work was supported by the German Federal Ministry of Education and Research through the project ROMIC-THREAT (BMBF-FK01LG1217A and 01LG1217B) and ROMIC-SPITFIRE (BMBF-FKZ: 01LG1205C). Additional funding for Christa A. Marandino and Sinikka T. Lennartz came from the Helmholtz Young Investigator Group of Christa A. Marandino (TRASE-EC, VH-NG-819), from the Helmholtz Association through the President’s Initiative and Networking Fund, and from the GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel. Kirstin Krüger acknowledges financial support from the EU FP7 StratoClim project (603557), and Pau Cortes and Rafel Simo acknowledge support from the Spanish MINECO through PEGASO (CTM2012-37615)Peer Reviewe

Carbonyl sulfide as a tracer for terrestrial net primary production: The oceanic perspective

Lennartz, S.T. ... et al.-- 2016 Ocean Sciences Meeting, 21-26 February 2016, New OrleansCarbonyl sulfide (OCS) is the most abundant sulfur gas in the atmosphere, and the ocean is thought to contribute the major part to its atmospheric budget. As OCS is a CO2 analog and is taken up by plants during photosynthesis, it is currently discussed as a tool to constrain terrestrial gross primary production. An important prerequisite for that is a balanced atmospheric budget, but sources and sinks are currently not well explained. One major uncertainty in the atmospheric OCS budget is the role of the ocean. To quantify terrestrial net primary production using carbonyl sulfide as a tracer, the quantification of the ocean’s direct and indirect OCS emissions is crucial. A large missing source indicated by atmospheric modelling studies and satellite measurements has been attributed to the equatorial ocean that requires more than twice the amount of today’s known oceanic OCS emissions. We investigate the role of the equatorial ocean in the global OCS budget using oceanic measurements of OCS and related trace gases (CS2, DMS) together with box modelling. Results from our cruise to the tropical Indian Ocean in summer 2014 show that contrasting to results from previous inverse modelling studies, the tropical open ocean is rather a net sink than a source for atmospheric OCS. This contribution discusses possibilities to close the atmospheric budget with a focus on oceanic emissions and identifies key questions for a better understanding of the global oceanic source strength of OCSPeer Reviewe