ALBERT

Description: Halogenated very short-lived substances (VSLSs), such as bromoform (CHBr3), can be transported to the stratosphere and contribute to the halogen loading and ozone depletion. Given their highly variable emission rates and their short atmospheric lifetimes, the exact amount as well as the spatio-temporal variability of their contribution to the stratospheric halogen loading are still uncertain. We combine observational data sets with Lagrangian atmospheric modelling in order to analyse the spatial and temporal variability of the CHBr3 injection into the stratosphere for the time period 1979–2013. Regional maxima with mixing ratios of up to 0.4–0.5 ppt at 17 km altitude are diagnosed to be over Central America (1) and over the Maritime Continent–west Pacific (2), both of which are confirmed by high-altitude aircraft campaigns. The CHBr3 maximum over Central America is caused by the co-occurrence of convectively driven short transport timescales and strong regional sources, which in conjunction drive the seasonality of CHBr3 injection. Model results at a daily resolution reveal isolated, exceptionally high CHBr3 values in this region which are confirmed by aircraft measurements during the ACCENT campaign and do not occur in spatially or temporally averaged model fields. CHBr3 injection over the west Pacific is centred south of the Equator due to strong oceanic sources underneath prescribed by the here-applied bottom-up emission inventory. The globally largest CHBr3 mixing ratios at the cold point level of up to 0.6 ppt are diagnosed to occur over the region of India, Bay of Bengal, and Arabian Sea (3); however, no data from aircraft campaigns are available to confirm this finding. Inter-annual variability of stratospheric CHBr3 injection of 10 %–20 % is to a large part driven by the variability of coupled ocean–atmosphere circulation systems. Long-term changes, on the other hand, correlate with the regional sea surface temperature trends resulting in positive trends of stratospheric CHBr3 injection over the west Pacific and Asian monsoon region and negative trends over the east Pacific. For the tropical mean, these opposite regional trends balance each other out, resulting in a relatively weak positive trend of 0.017±0.012 ppt Br per decade for 1979–2013, corresponding to 3 % Br per decade. The overall contribution of CHBr3 together with CH2Br2 to the stratospheric halogen loading accounts for 4.7 ppt Br, in good agreement with existing studies, with 50 % and 50 % being injected in the form of source and product gases, respectively.

Description: A suite of oxygenated volatile organic compounds (OVOCs – acetaldehyde, acetone, propanal, butanal and butanone) were measured concurrently in the surface water and atmosphere of the South China Sea and Sulu Sea in November 2011. A strong correlation was observed between all OVOC concentrations in the surface seawater along the entire cruise track, except for acetaldehyde, suggesting similar sources and sinks in the surface ocean. Additionally, several phytoplankton groups, such as haptophytes or pelagophytes, were also correlated to all OVOCs, indicating that phytoplankton may be an important source of marine OVOCs in the South China and Sulu seas. Humic- and protein-like fluorescent dissolved organic matter (FDOM) components seemed to be additional precursors for butanone and acetaldehyde. The measurement-inferred OVOC fluxes generally showed an uptake of atmospheric OVOCs by the ocean for all gases, except for butanal. A few important exceptions were found along the Borneo coast, where OVOC fluxes from the ocean to the atmosphere were inferred. The atmospheric OVOC mixing ratios over the northern coast of Borneo were relatively high compared with literature values, suggesting that this coastal region is a local hotspot for atmospheric OVOCs. The calculated amount of OVOCs entrained into the ocean seemed to be an important source of OVOCs to the surface ocean. When the fluxes were out of the ocean, marine OVOCs were found to be enough to control the locally measured OVOC distribution in the atmosphere. Based on our model calculations, at least 0.4 ppb of marine-derived acetone and butanone can reach the upper troposphere, where they may have an important influence on hydrogen oxide radical formation over the western Pacific Ocean.

Description: Oceanic very short-lived substances (VSLSs), such as bromoform (CHBr3), contribute to stratospheric halogen loading and, thus, to ozone depletion. However, the amount, timing, and region of bromine delivery to the stratosphere through one of the main entrance gates, the Indian summer monsoon circulation, are still uncertain. In this study, we created two bromoform emission inventories with monthly resolution for the tropical Indian Ocean and west Pacific based on new in situ bromoform measurements and novel ocean biogeochemistry modeling. The mass transport and atmospheric mixing ratios of bromoform were modeled for the year 2014 with the particle dispersion model FLEXPART driven by ERA-Interim reanalysis. We compare results between two emission scenarios: (1) monthly averaged and (2) annually averaged emissions. Both simulations reproduce the atmospheric distribution of bromoform from ship- and aircraft-based observations in the boundary layer and upper troposphere above the Indian Ocean reasonably well. Using monthly resolved emissions, the main oceanic source regions for the stratosphere include the Arabian Sea and Bay of Bengal in boreal summer and the tropical west Pacific Ocean in boreal winter. The main stratospheric injection in boreal summer occurs over the southern tip of India associated with the high local oceanic sources and strong convection of the summer monsoon. In boreal winter more bromoform is entrained over the west Pacific than over the Indian Ocean. The annually averaged stratospheric injection of bromoform is in the same range whether using monthly averaged or annually averaged emissions in our Lagrangian calculations. However, monthly averaged emissions result in the highest mixing ratios within the Asian monsoon anticyclone in boreal summer and above the central Indian Ocean in boreal winter, while annually averaged emissions display a maximum above the west Indian Ocean in boreal spring. In the Asian summer monsoon anticyclone bromoform atmospheric mixing ratios vary by up to 50 % between using monthly averaged and annually averaged oceanic emissions. Our results underline that the seasonal and regional stratospheric bromine injection from the tropical Indian Ocean and west Pacific critically depend on the seasonality and spatial distribution of the VSLS emissions.

Description: Halogenated very short-lived substances (VSLSs) are naturally produced in the ocean and emitted to the atmosphere. When transported to the stratosphere, these compounds can have a significant influence on the ozone layer and climate. During a research cruise on RV Sonne in the subtropical and tropical west Indian Ocean in July and August 2014, we measured the VSLSs, methyl iodide (CH3I) and for the first time bromoform (CHBr3) and dibromomethane (CH2Br2), in surface seawater and the marine atmosphere to derive their emission strengths. Using the Lagrangian particle dispersion model FLEXPART with ERA-Interim meteorological fields, we calculated the direct contribution of observed VSLS emissions to the stratospheric halogen burden during the Asian summer monsoon. Furthermore, we compare the in situ calculations with the interannual variability of transport from a larger area of the west Indian Ocean surface to the stratosphere for July 2000–2015. We found that the west Indian Ocean is a strong source for CHBr3 (910 pmol m−2 h−1), very strong source for CH2Br2 (930 pmol m−2 h−1), and an average source for CH3I (460 pmol m−2 h−1). The atmospheric transport from the tropical west Indian Ocean surface to the stratosphere experiences two main pathways. On very short timescales, especially relevant for the shortest-lived compound CH3I (3.5 days lifetime), convection above the Indian Ocean lifts oceanic air masses and VSLSs towards the tropopause. On a longer timescale, the Asian summer monsoon circulation transports oceanic VSLSs towards India and the Bay of Bengal, where they are lifted with the monsoon convection and reach stratospheric levels in the southeastern part of the Asian monsoon anticyclone. This transport pathway is more important for the longer-lived brominated compounds (17 and 150 days lifetime for CHBr3 and CH2Br2). The entrainment of CHBr3 and CH3I from the west Indian Ocean to the stratosphere during the Asian summer monsoon is lower than from previous cruises in the tropical west Pacific Ocean during boreal autumn and early winter but higher than from the tropical Atlantic during boreal summer. In contrast, the projected CH2Br2 entrainment was very high because of the high emissions during the west Indian Ocean cruise. The 16-year July time series shows highest interannual variability for the shortest-lived CH3I and lowest for the longest-lived CH2Br2. During this time period, a small increase in VSLS entrainment from the west Indian Ocean through the Asian monsoon to the stratosphere is found. Overall, this study confirms that the subtropical and tropical west Indian Ocean is an important source region of halogenated VSLSs, especially CH2Br2, to the troposphere and stratosphere during the Asian summer monsoon.

Description: The 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 sinks.

Description: Oceanic emissions of the climate relevant trace gases carbonyl sulfide (OCS) and carbon disulfide (CS2) are a major source to their atmospheric budget. Their current and future emission estimates are still uncertain due to incomplete process understanding and, therefore, inexact quantification across different biogeochemical regimes. Here we present the first concurrent measurements of both gases together with related fractions of the dissolved organic matter (DOM) pool, i.e. solid-phase extractable dissolved organic sulfur (DOSSPE), chromophoric (CDOM) and fluorescent dissolved organic matter (FDOM) from the Eastern Tropical South Pacific (ETSP). These observations are used to estimate in-situ production rates and identify their drivers. We find different limiting factors of marine photoproduction: while OCS production is limited by the humic-like DOM fraction that can act as a photosensitizer, high CS2 production coincides with high DOSSPE concentration. The lack of correlation between OCS production and DOSSPE may be explained by the active cycling of sulfur between OCS and dissolved inorganic sulfide via OCS photoproduction and hydrolysis. In addition, the only existing parameterization for OCS dark production is validated and updated with new rates from the ETSP and the Indian Ocean. Our results will help to predict oceanic concentrations and emissions of both gases on regional and, potentially, global scales.

Description: It is an open question how localized elevated emissions of bromoform (CHBr3) and other very short-lived halocarbons (VSLHs), found in coastal and upwelling regions, and low background emissions, typically found over the open ocean, impact the atmospheric VSLH distribution. In this study, we use the Lagrangian dispersion model FLEXPART to simulate atmospheric CHBr3 resulting from assumed uniform background emissions, and from elevated emissions consistent with those derived during three tropical cruise campaigns. The simulations demonstrate that the atmospheric CHBr3 distributions in the uniform background emissions scenario are highly variable with high mixing ratios appearing in regions of convergence or low wind speed. This relation holds on regional and global scales. The impact of localized elevated emissions on the atmospheric CHBr3 distribution varies significantly from campaign to campaign. The estimated impact depends on the strength of the emissions and the meteorological conditions. In the open waters of the western Pacific and Indian oceans, localized elevated emissions only slightly increase the background concentrations of atmospheric CHBr3, even when 1∘ wide source regions along the cruise tracks are assumed. Near the coast, elevated emissions, including hot spots up to 100 times larger than the uniform background emissions, can be strong enough to be distinguished from the atmospheric background. However, it is not necessarily the highest hot spot emission that produces the largest enhancement, since the tug-of-war between fast advective transport and local accumulation at the time of emission is also important. Our results demonstrate that transport variations in the atmosphere itself are sufficient to produce highly variable VSLH distributions, and elevated VSLHs in the atmosphere do not always reflect a strong localized source. Localized elevated emissions can be obliterated by the highly variable atmospheric background, even if they are orders of magnitude larger than the average open ocean emissions.

Description: Ballast water treatment is required for vessels to prevent the introduction of potentially invasive neobiota. Some treatment methods use chemical disinfectants which produce a variety of halogenated compounds as disinfection by-products (DBPs). One of the most abundant DBP from oxidative ballast water treatment is bromoform (CHBr3) where we find an average concentration of 894 ± 560 nmol L-1 (226 ± 142 μg L-1) in the undiluted ballast water from measurements and literature. Bromoform is a relevant gas for atmospheric chemistry and ozone depletion, especially in the tropics where entrainment into the stratosphere is possible. The spread of DBPs in the tropics over months to years is assessed here for the first time. With Lagrangian trajectories based on the NEMO-ORCA12 model velocity field, we simulate DBP spread in the sea surface and try to quantify the oceanic bromoform concentration and emission to the atmosphere from ballast water discharge at major harbours in the tropical region of Southeast Asia. The exemplary simulations of two important regions, Singapore and the Pearl River Delta, reveal major transport pathways of the DBPs and the anthropogenic bromoform concentrations in the sea surface. Based on our simulations, we expect DBPs to spread into the open ocean, along the coast and also an advection with monsoon-driven currents into the North Pacific and Indian Ocean. Furthermore, anthropogenic bromoform concentrations and emissions are predicted to increase locally around large harbours. In the sea surface around Singapore we estimate an increase in bromoform concentration by 9 % compared to recent measurement. In a moderate scenario where 70 % of the ballast water is chemically treated bromoform emissions to the atmosphere can locally exceed 1000 pmol m-2 h-1 and double climatological emissions. In the Pearl River Delta all bromoform is directly outgassed which leads to an additional bromine (Br) input into the atmosphere of 495 kmol Br (∼42 t CHBr3) a-1. From Singapore ports the additional atmospheric Br input is calculated as 312 kmol Br (∼26 t CHBr3) a-1. We estimate the global anthropogenic Br input from ballast water into the atmosphere of up to 13 Mmol a-1. This is 0.1 % global Br input from background bromoform emissions and thus probably not relevant for stratospheric ozone depletion.

Description: Halocarbons, halogenated short-chained hydrocarbons, are produced naturally in the oceans by biological and chemical processes. They are emitted from surface seawater into the atmosphere, where they take part in numerous chemical processes such as ozone destruction and the oxidation of mercury and dimethyl sulfide. Here we present oceanic and atmospheric halocarbon data for the Peruvian upwelling obtained during the M91 cruise onboard the research vessel Meteor in December 2012. Surface waters during the cruise were characterized by moderate concentrations of bromoform (CHBr3) and dibromomethane (CH2Br2) correlating with diatom biomass derived from marker pigment concentrations, which suggests this phytoplankton group as likely source. Concentrations measured for the iodinated compounds methyl iodide (CH3I) of up to 35.4 pmol L−1, chloroiodomethane (CH2ClI) of up to 58.1 pmol L−1 and diiodomethane (CH2I2) of up to 32.4 pmol L−1 in water samples were much higher than previously reported for the tropical Atlantic upwelling systems. Iodocarbons also correlated with the diatom biomass and even more significantly with dissolved organic matter (DOM) components measured in the surface water. Our results suggest a biological source of these compounds as significant driving factor for the observed large iodocarbon concentrations. Elevated atmospheric mixing ratios of CH3I (up to 3.2 ppt), CH2ClI (up to 2.5 ppt) and CH2I2 (3.3 ppt) above the upwelling were correlated with seawater concentrations and high sea-to-air fluxes. The enhanced iodocarbon production in the Peruvian upwelling contributed significantly to tropospheric iodine levels.

Description: The 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 in top-down approaches. Oceanic emissions have been proposed to close the resulting gap in the atmospheric budget. We present a bottom-up approach including new observations of OCS in surface waters of the tropical Atlantic, Pacific and Indian oceans to show that direct OCS emissions are insufficient to account for the missing source. Extrapolation of our observations using a biogeochemical box model suggests oceanic net uptake instead of emission for the entire tropical ocean area and, further, a global ocean source strength well below that suggested by top-down estimates. This bottom-up estimate of oceanic emissions has implications for using OCS as a proxy for terrestrial CO2 uptake, which is currently hampered by the inadequate quantification of atmospheric OCS sources and sinks.