We provide new estimates of the air-sea flux of CCl4 using simulations from a global ocean biogeochemistry model (NEMO-PlankTOM) in combination with depth-resolved CCl4 observations from global oceanic databases. Estimates of global oceanic CCl4 uptake are derived from a range of model analyses, including prescribed parameterizations using reported values on hydrolysis and degradation, and analyses optimized using the global observational databases. We evaluate the sensitivity of our results to uncertainties in air-sea gas exchange parameterization, estimation period, and circulation processes. Our best constrained estimate of ocean CCl4 uptake for the period 1996-2000 is 20.1Gg/year (range 16.6-22.7), corresponding to estimates of the partial atmospheric lifetime with respect to ocean uptake of 124 (110-150) years. This new oceanic lifetime implies higher emissions of CCl4 than currently estimated and therefore a larger missing atmospheric source of CCl4. Plain Language Summary Carbon tetrachloride (CCl4) is a trace gas emitted to the atmosphere during the industrial manufacture of refrigerants and solvents. CCl4 is also an ozone-depleting agent affecting loss of the atmosphere's protective ozone layer, and CCl4 emissions are now controlled by the Montreal Protocol. However, recent analyses of observed atmospheric CCl4 concentration conclude that the current rate of atmospheric decline is slower than expected, and inconsistent with recent estimates of CCl4 emissions to and removal from the atmosphere. Estimated oceanic uptake of CCl4 has been noted as potentially the most uncertain of the atmospheric loss processes, and here we provide new quantification of this flux. Our methodology employs a novel approach and uses a global ocean biogeochemistry model which includes representation of known physical and biogeochemical processes influencing oceanic CCl4 uptake, together with a compiled global ocean database of CCl4 observations. The observational database is used to evaluate model simulations, and to derive optimized estimates of the oceanic uptake flux by minimizing the discrepancies between the model and observations. We also evaluate the sensitivity of our estimates of ocean CCl4 uptake to known uncertainties in the model representation of processes, including air-sea gas transfer and oceanic chemical and biogeochemical loss. Our best constrained estimate of ocean CCl4 uptake for the 1996-2000 period is 20.1Gg/year (range 16.6-22.7), which is greater than previous recent estimates, and implies a larger missing source to the atmosphere of CCl4.

Isoprene surface ocean concentrations and vertical distribution, atmospheric mixing ratios, and calculated sea‐to‐air fluxes spanning approximately 125° of latitude (80°N–45°S) over the Arctic and Atlantic Oceans are reported. Oceanic isoprene concentrations were associated with a number of concurrently monitored biological variables including chlorophyll a (Chl a), photoprotective pigments, integrated primary production (intPP), and cyanobacterial cell counts, with higher isoprene concentrations relative to all respective variables found at sea surface temperatures greater than 20°C. The correlation between isoprene and the sum of photoprotective carotenoids, which is reported here for the first time, was the most consistent across all cruises. Parameterizations based on linear regression analyses of these relationships perform well for Arctic and Atlantic data, producing a better fit to observations than an existing Chl a‐based parameterization. Global extrapolation of isoprene surface water concentrations using satellite‐derived Chl a and intPP reproduced general trends in the in situ data and absolute values within a factor of 2 between 60% and 85%, depending on the data set and algorithm used.

Accurately accounting for new particle formation (NPF) is crucial to our ability to predict aerosol number concentrations in many environments and thus cloud properties, which is in turn vital in simulating radiative transfer and climate. Here we present an analysis of NPF events observed in the Greenland Sea during the summertime as part of the Aerosol-Cloud Coupling And Climate Interactions in the Arctic (ACCACIA) project. While NPF events have been reported in the Arctic before, we were able, for the first time, to detect iodine in the growing particles using an Aerosol Mass Spectrometer (AMS) during a persistent event in the region of the coastal sea-ice near Greenland. Given the potency of iodine as a nucleation precursor, the results imply that iodine was responsible for the initial NPF, a phenomenon that has been reported at lower latitudes and associated with molecular iodine emissions from coastal macroalgae. The initial source of iodine in this instance is not clear, but it was associated with air originating approximately 1 day previously over melting coastal sea-ice. These results show that atmospheric models must consider iodine as a source of new particles in addition to established precursors such as sulfur compounds.

The oceans are a key source of a number of atmospherically important volatile gases. The accurate and robust determination of trace gases in seawater is a significant analytical challenge, requiring reproducible and ideally automated sample handling, a high efficiency of seawater–air transfer, removal of water vapour from the sample stream, and high sensitivity and selectivity of the analysis. Here we describe a system that was developed for the fully automated analysis of dissolved very short-lived halogenated species (VSLS) sampled from an under-way seawater supply. The system can also be used for semi-automated batch sampling from Niskin bottles filled during CTD (conductivity, temperature, depth) profiles. The essential components comprise a bespoke, automated purge and trap (AutoP & T) unit coupled to a commercial thermal desorption and gas chromatograph mass spectrometer (TD-GC-MS). The AutoP & T system has completed five research cruises, from the tropics to the poles, and collected over 2500 oceanic samples to date. It is able to quantify >25 species over a boiling point range of 34–180 °C with Henry's law coefficients of 0.018 and greater (CH22l, kHcc dimensionless gas/aqueous) and has been used to measure organic sulfurs, hydrocarbons, halocarbons and terpenes. In the eastern tropical Pacific, the high sensitivity and sampling frequency provided new information regarding the distribution of VSLS, including novel measurements of a photolytically driven diurnal cycle of CH22l within the surface ocean water.