Oceanic emissions of dimethyl sulfide and methanethiol and their contribution to sulfur dioxide production in the marine atmosphere

Oceanic emissions of dimethyl sulfide (CH3SCH3, DMS) have long been recognized to impact aerosol particle composition and size, the concentration of cloud condensation nuclei (CCN), and Earth’s radiation balance. The impact of oceanic emissions of methanethiol (CH3SH, MeSH), which is produced by the same oceanic precursor as DMS, on the volatile sulfur budget of the marine atmosphere is largely unconstrained. Here we present direct flux measurements of MeSH oceanic emissions using the eddy covariance (EC) method with a high-resolution proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToFMS) detector and compare them to simultaneous flux measurements of DMS emissions from a coastal ocean site. Campaign mean mixing ratios of DMS and MeSH were 72 ppt (28–90 ppt interquartile range) and 19.1 ppt (7.6–24.5 ppt interquartile range) respectively. Campaign mean (and interquartile range) emission fluxes of DMS (FDMS) and MeSH (FMeSH) were 1.13 (0.53–1.61) and 0.21 (0.10–0.31) ppt m s-1 respectively. Linear least squares regression of observed MeSH and DMS flux indicates the emissions are highly correlated with each other (R2 = 0.65) over the course of the campaign, consistent with a shared oceanic source. The campaign mean DMS to MeSH flux ratio (FDMS:FMeSH) was 5.5 ± 3.0 calculated from the ratio of 304 individual coincident measurements of FDMS and FMeSH. Measured FDMS:FMeSH was weakly correlated (R2 = 0.15) with ocean chlorophyll concentrations, with FDMS:FMeSH reaching a maximum of 10.8 ± 4.4 during a phytoplankton bloom period. No other volatile sulfur compounds were observed by PTR-ToFMS to have a resolvable emission flux above their flux limit of detection or to have a gas phase mixing ratio consistently above their limit of detection during the study period, suggesting DMS and MeSH are the dominant volatile organic sulfur compounds emitted from the ocean at this site. The impact of this MeSH emission source on atmospheric budgets of sulfur dioxide (SO2) was evaluated by implementing observed emissions into a coupled ocean-atmosphere chemical box model using a newly compiled MeSH oxidation mechanism. Model results suggest that MeSH emissions lead to afternoon instantaneous SO2 production of 2.5 ppt hr-1, which accounts for 30 % of the instantaneous SO2 production in the marine boundary layer at the mean measured FDMS and FMeSH. This contribution of MeSH to SO2 production is driven by a higher effective yield of SO2 from MeSH oxidation and the shorter oxidation lifetime of MeSH compared to DMS. This large additional source of marine SO2 has not been previously considered in global models of marine sulfur cycling. The field measurements and modeling results presented here demonstrate that MeSH is an important contributor to volatile sulfur budgets in the marine atmosphere, and must be measured along with DMS in order to constrain marine sulfur budgets. This large additional source of marine reduced sulfur from MeSH will contribute to particle formation and growth and CCN abundance in the marine atmosphere, with subsequent impacts on climate.

Monthly resolved modelled oceanic emissions of carbonyl sulphide and carbon disulphide for the period 2000–2019

Carbonyl sulphide (OCS) is the most abundant, long-lived sulphur gas in the atmosphere and a major supplier of sulphur to the stratospheric sulphate aerosol layer. The short-lived gas carbon disulphide (CS2) is oxidized to OCS and constitutes a major indirect source to the atmospheric OCS budget. The atmospheric budget of OCS is not well constrained due to a large missing source needed to compensate for substantial evidence that was provided for significantly higher sinks. Oceanic emissions are associated with major uncertainties. Here we provide a first, monthly resolved ocean emission inventory of both gases for the period 2000–2019 (available at https://doi.org/10.5281/zenodo.4297010) (Lennartz et al., 2020a). Emissions are calculated with a numerical box model (2.8∘×2.8∘ resolution at the Equator, T42 grid) for the oceanic surface mixed layer, driven by ERA5 data from ECMWF and chromophoric dissolved organic matter (CDOM) from Aqua MODIS. We find that interannual variability in OCS emissions is smaller than seasonal variability and is mainly driven by variations in CDOM, which influences both photochemical and light-independent production. A comparison with a global database of more than 2500 measurements reveals overall good agreement. Emissions of CS2 constitute a larger sulphur source to the atmosphere than OCS and equally show interannual variability connected to variability in CDOM. The emission estimate of CS2 is associated with higher uncertainties as process understanding of the marine cycling of CS2 is incomplete. We encourage the use of the data provided here as input for atmospheric modelling studies to further assess the atmospheric OCS budget and the role of OCS in climate.

Monthly resolved modelled oceanic emissions of carbonyl sulfide and carbon disulfide for the period 2000–2019

Carbonyl sulfide (OCS) is the most abundant, long-lived sulphur gas in the atmosphere and a major supplier of sulfur to the stratospheric sulfate aerosol layer. The short-lived gas carbon disulfide (CS2) is oxidized to OCS and constitutes a major indirect source to the atmospheric OCS budget. The atmospheric budget of OCS is not well constrained due to a large missing source needed to compensate for substantial evidence that was provided for significantly higher sinks. Oceanic emissions are associated with major uncertainties. Here we provide a first, monthly resolved ocean emission inventory of both gases for the period 2000–2019 (available at https://doi.org/10.5281/zenodo.4297010) (Lennartz et al., 2020a). Emissions are calculated with a numerical box model (resolution 2.8° × 2.8° at equator, T42 grid) for the surface mixed layer. We find that interannual variability in OCS emissions is smaller than seasonal variability, and is mainly driven by variations in chromophoric dissolved organic matter (CDOM), which influences both photochemical and light-independent production. A comparison with a global database of more than 2500 measurements reveals overall good agreement. Emissions of CS2 constitute a larger sulfur source to the atmosphere than OCS, and equally show interannual variability connected to variability of CDOM. The emission estimate of CS2 is associated with higher uncertainties, as process understanding of the marine cycling of CS2 is incomplete. We encourage the use of the data provided here as input for atmospheric modelling studies to further assess the atmospheric OCS budget and the role of OCS in climate.

Source apportionment of atmospheric mercury in the remote marine atmosphere: Mace Head GAW station, Irish western coast

We examined recent atmospheric mercury concentrations measured with a high temporal resolution of 15 min at Mace Head, a GAW station on the western coast of Ireland. We attributed a direct contribution of 34 % (0.44 ng m−3) to primary sources. Additionally, a steep decline (0.05 ng yr−1) in mercury concentrations was observed between 2013 and 2018. Using a stereo algorithm we reconstructed 99.9 % of the atmospheric mercury. A conservative analysis demonstrated no decrease in total gaseous mercury (TGM) associated with atmospheric species typically used as tracers for oceanic emissions. The results show that the atmospheric mercury mass is mainly loaded in a baseline factor with an ongoing decline. Moreover, we exploit temporal variation and wind pattern effects in the measured atmospheric species; the results show that the diurnal variation and seasonality in TGM observed in Mace Head are closely related to other species linked to primary sources and can be explained by transport from continental areas.

Potential Effect of Halogens on Atmospheric Oxidation in China

<p align="justify"><span lang="en-US">Air pollution has been a hazard in China over recent decades threatening the health of half a billion people. Much effort has been devoted to mitigating air pollution in China leading to a tremendous reduction in primary pollutants emissions from 2013 to 2017, while a continuously worsening trend of surface ozone (O</span><sub><span lang="en-US">3</span></sub><span lang="en-US">, a secondary pollutant and greenhouse gas) was observed over the same period. Atmospheric oxidation, dominated by daytime reactions involving hydroxyl radicals (OH), is the critical process to convert freshly-emitted compounds into secondary pollutants, and is underestimated in current models of China’s air pollution. Halogens (chlorine, bromine, and iodine) are known to profoundly influence oxidation chemistry in the marine environment; however, their impact on atmospheric oxidation and air pollution in China is unknown. In the present study, we report for the first time that halogens substantially enhance the total atmospheric oxidation capacity in polluted areas of China, typically 10% to 20% (up to 87% in winter) and mainly by significantly increasing OH level. The enhanced oxidation along the coast is driven by oceanic emissions of bromine and iodine, and that over the inland areas by anthropogenic emission of chlorine. The extent and seasonality of halogen impact are largely explained by the dynamics of Asian monsoon, location and intensity of halogen emissions, and O</span><sub><span lang="en-US">3</span></sub><span lang="en-US"> formation regime. The omission of halogen emissions and chemistry may lead to significant errors in historical re-assessments and future projections of the evolution of atmospheric oxidation in polluted regions.</span></p>

Marine carbonyl sulfide (OCS) and carbon disulfide (CS₂): a compilation of measurements in seawater and the marine boundary layer

Carbonyl sulfide (OCS) and carbon disulfide (CS2) are volatile sulfur gases that are naturally formed in seawater and exchanged with the atmosphere. OCS is the most abundant sulfur gas in the atmosphere, and CS2 is its most important precursor. They have attracted increased interest due to their direct (OCS) or indirect (CS2 via oxidation to OCS) contribution to the stratospheric sulfate aerosol layer. Furthermore, OCS serves as a proxy to constrain terrestrial CO2 uptake by vegetation. Oceanic emissions of both gases contribute a major part to their atmospheric concentration. Here we present a database of previously published and unpublished (mainly shipborne) measurements in seawater and the marine boundary layer for both gases, available at https://doi.org/10.1594/PANGAEA.905430 (Lennartz et al., 2019). The database contains original measurements as well as data digitalized from figures in publications from 42 measurement campaigns, i.e., cruises or time series stations, ranging from 1982 to 2019. OCS data cover all ocean basins except for the Arctic Ocean, as well as all months of the year, while the CS2 dataset shows large gaps in spatial and temporal coverage. Concentrations are consistent across different sampling and analysis techniques for OCS. The database is intended to support the identification of global spatial and temporal patterns and to facilitate the evaluation of model simulations.

Source apportionment of atmospheric mercury in the remote marine atmosphere: Mace Head GAW station, Irish west coast

We examined recent atmospheric mercury concentrations measured with a high temporal resolution of 15 min. at Mace Head, a GAW station on the west coast of Ireland. We attributed a direct contribution of 34 % (0.44 ng m−3) to primary sources. Additionally, a steep decline (0.05 ng year−1) in mercury concentrations was observed between 2013 and 2018. Using a stereo algorithm we reconstructed 99.9 % of the atmospheric mercury. A conservative analysis demonstrated no decreasing of TGM associated with atmospheric species typically used as tracers for oceanic emissions. The results show that the atmospheric mercury mass is mainly loaded in a baseline factor with an on-going decline. Moreover, we exploit temporal variation and wind pattern effects in the measured atmospheric species, the results show that the diurnal variation and seasonality in TGM observed in Mace Head is closely related to other species linked to primary sources and can be explained by transport from continental areas.

Quantitative detection of iodine in the stratosphere

Oceanic emissions of iodine destroy ozone, modify oxidative capacity, and can form new particles in the troposphere. However, the impact of iodine in the stratosphere is highly uncertain due to the lack of previous quantitative measurements. Here, we report quantitative measurements of iodine monoxide radicals and particulate iodine (Iy,part) from aircraft in the stratosphere. These measurements support that 0.77 ± 0.10 parts per trillion by volume (pptv) total inorganic iodine (Iy) is injected to the stratosphere. These high Iy amounts are indicative of active iodine recycling on ice in the upper troposphere (UT), support the upper end of recent Iy estimates (0 to 0.8 pptv) by the World Meteorological Organization, and are incompatible with zero stratospheric iodine injection. Gas-phase iodine (Iy,gas) in the UT (0.67 ± 0.09 pptv) converts to Iy,part sharply near the tropopause. In the stratosphere, IO radicals remain detectable (0.06 ± 0.03 pptv), indicating persistent Iy,part recycling back to Iy,gas as a result of active multiphase chemistry. At the observed levels, iodine is responsible for 32% of the halogen-induced ozone loss (bromine 40%, chlorine 28%), due primarily to previously unconsidered heterogeneous chemistry. Anthropogenic (pollution) ozone has increased iodine emissions since preindustrial times (ca. factor of 3 since 1950) and could be partly responsible for the continued decrease of ozone in the lower stratosphere. Increasing iodine emissions have implications for ozone radiative forcing and possibly new particle formation near the tropopause.