In-situ farm-scale measurements are a prerequisite to improve the accuracy of the greenhouse gas inventory and to assess the viability of mitigation options. These measurements are also important in developing and verifying the empirical, as well as process-based, modelling approaches to emission estimates at farm-scales and beyond. Here, we assess the practicalities of implementing process-based modelling methods of quantifying nitrous oxide (N 2 O) emissions through validation and testing at the farm-scale, using environmental inputs and N 2 O emissions data from an emissions measurement campaign on dairy-grazed pastures in New Zealand. The median emissions measured with an atmospheric technique gave an emission factor (EF) of about 0.01, similar to the EF currently used by New Zealand inventory reporting for animal excreta (0.01) or for N fertiliser (0.0125). Emissions were simulated at the farm scale using two process-based models: NZ-DNDC and DayCent. The NZ-DNDC and DayCent models predicted EFs of 0.022 and 0.015, respectively for non-irrigated conditions and 0.041 and 0.032, respectively with irrigation. Lack of information on the grazing (excretal N inputs) and irrigation schedules before the campaign, and on soil ammonium and nitrate levels, was the main limitation in these validations. Identification of these shortcomings has been useful for the design of further integrated work on model development with farm-scale validation.

The goal of the Sea2Cloud project is to study the interplay between surface ocean biogeochemical and physical properties, fluxes to the atmosphere and ultimately their impact on cloud formation under minimal direct anthropogenic influence. Here we present an interdisciplinary approach, combining atmospheric physics and chemistry with marine biogeochemistry, during a voyage between 41 and 47°S in March 2020. In parallel to ambient measurements of atmospheric composition and seawater biogeochemical properties, we describe semi-controlled experiments to characterize nascent sea spray properties and nucleation from gas-phase biogenic emissions. The experimental framework for studying the impact of the predicted evolution of ozone concentration in the Southern Hemisphere is also detailed. After describing the experimental strategy, we present the oceanic and meteorological context including provisional results on atmospheric thermodynamics, composition, and flux measurements. In situ measurements and flux studies were carried out on different biological communities by sampling surface seawater from subantarctic, subtropical and frontal water masses. Air-Sea-Tanks (ASIT) were used to quantify biogenic emissions of trace gases under realistic environmental conditions, with nucleation observed in association with biogenic seawater emissions. Sea spray continuously generated produced sea spray fluxes of 34% of organic matter by mass, of which 4% particles had fluorescent properties, and which size distribution ressembled the one found in clean sectors of the Southern Ocean. The goal of Sea2Cloud is to generate realistic parameterizations of emission flux dependences of trace gases and nucleation precursors, sea spray, cloud condensation nuclei and ice nuclei using seawater biogeochemistry, for implementation in regional atmospheric models.

We have developed a high-precision micrometeorological system capable of measuring emissions of nitrous oxide (N2O) from up to four adjacent pasture plots. The system can be used to compare the influence of environmental factors and management practice on N2O emissions at the paddock scale. The system is capable of determining a minimum detectable N2O difference of the order of 40 pmol/mol, with an ability to resolve flux differences among plots of ~26 µg (N2O-N)/m2.h. So as to independently verify the emission estimates of the micrometeorological system, we developed a calibrated N2O-release system and compared known release rates with the micrometeorological flux estimates. Adjustable release rates up to the equivalent average surface flux of ~500 µg (N2O-N)/m2.h were achieved using mass flow-controlled input of pure N2O in a compressed air stream over two 1.5-ha plots upwind of flux-measurement masts. The comparison of network release rate with measured emission rate was quite variable and complicated by a significant and varying background emissions of N2O from the soil. For optimal steady-wind cases, the ratio of uncorrected measured flux to known release, including the estimated background, was of the order of 0.4–0.5; this ratio is likely to be influenced by the turbulent Schmidt number. Flux estimates for uncorrected flux gradient and WindTrax backward Lagrangian Stochastic method (which includes Schmidt correction) agreed well with a ratio of 0.54. The experiment highlighted the need for accurate estimates of gas eddy diffusivity in the micrometeorological gradient or difference-based flux measurement of N2O.

Abstract. Air-sea fluxes of dimethyl sulphide (DMS) and methanethiol (MeSH) from surface seawater in the remote Southern Pacific Ocean were measured in three Air-Sea Interface Tank (ASIT) experiments during the Sea2Cloud voyage in March 2020. The measured fluxes of 0.78 ± 0.44 ng m-2 s-1 and 0.05 ± 0.03 ng m-2 s-1 for DMS and MeSH, respectively, varied between experiments reflecting the different water mass types investigated, with lowest fluxes with subtropical water and highest with biologically-active water with sub-Tropical water and highest from the sub-Tropical Front. Measured DMS fluxes were consistent with calculated fluxes from a two-layer model using DMS concentration in the ASIT seawater. The experiments also determined the influence of elevated ozone, with one ASIT headspace amended with 10 ppbv ozone while the other provided an unamended control. Elevated ozone resulted in a decrease in DMS flux, corresponding to decreased conversion of dimethylsulfoniopropionate (DMSP) to DMS in the seawater. The MeSH:DMS flux range was 11–18 % across experiments, in line with previous observations, indicating that MeSH represents a significant contribution to the atmospheric sulfur budget. Using the ASIT results in combination with ambient seawater concentrations during Sea2Cloud, significant linear correlations were identified for both DMS and MeSH fluxes with nanophytoplankton cell abundance (rDMS= 0.73 and rMeSH= 0.86), indicating an important role for this phytoplankton size class, and also its potential as a proxy for estimating DMS and MeSH emissions in chemistry-climate models.

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