ALBERT

Description: The aerosol driven radiative effects on marine low-level cloud represent a large uncertainty in climate simulations, in particular over the Southern Ocean, which is also an important region for sea spray aerosol production. Observations of sea spray aerosol organic enrichment and the resulting impact on water uptake over the remote southern hemisphere are scarce, and are therefore the region is under-represented in existing parameterisations. The Surface Ocean Aerosol Production (SOAP) voyage was a 23 day voyage which sampled three phytoplankton blooms in the highly productive water of the Chatham Rise, east of New Zealand. In this study we examined the enrichment of organics to nascent sea spray aerosol and the modifications to sea spray aerosol water uptake using in-situ chamber measurements of seawater samples taken during the SOAP voyage. Primary marine organics contributed up to 23 % of the sea spray mass for particles with diameter less than approximately 1 μm, and up to 87 % of the particle volume in the Aitken mode. The composition of the organic fraction was consistent throughout the voyage and was largely comprised of a polysaccharide-like component, characterised by very low alkane to hydroxyl concentration ratios of approximately 0.1–0.2. The enrichment of organics was compared to the output from the chlorophyll-a based sea spray aerosol parameterisation suggested by Gantt et al. (2011) and the OCEANFILMS models. OCEANFILMS improved on the representation of the organic fraction predicted using chlorophyll-a, in particular when the co-adsoprtion of polysaccharides was included, however the model still under predicted the proportion of polysaccharides by an average of 33 %. Nascent sea spray aerosol hygroscopic growth factors averaged 1.93 ± 0.08, and did not decrease with increasing sea spray aerosol organic fractions. The observed hygroscopicity was greater than expected from the assumption of full solubility, particularly during the most productive phytoplankton bloom (B1), during which organic fractions were greater than approximately 0.4. The water uptake behaviour observed in this study is consistent with that observed for other measurements of phytoplankton blooms, and was attributed to the surface partitioning of the organic components which leads to a decrease in particle surface tension and an increase in hygroscopicity. The compressed film model was used to estimate the influence of surface partitioning and the error in the modelled hygroscopicity was low only when the entire organic fraction was available to partition to the particle surface. The modelled sea spray aerosol hygroscopicity at high organic fractions was underestimated when only a portion of the organic component was available to be partitioned to the surface. The findings from the SOAP voyage highlight the influence of biologically-sourced organics on sea spray aerosol composition, these data improve the capacity to parameterise sea spray aerosol organic enrichment and water uptake.

Description: Establishing the relationship between marine boundary layer (MBL) aerosols and surface water biogeochemistry over the remote ocean is required to understand aerosol and cloud production processes, and also represent them accurately in Earth System Models and global climate projections. This was addressed in the SOAP (Surface Ocean Aerosol Production) campaign, which examined air-sea interaction over biologically-productive frontal waters east of New Zealand. This overview details the objectives, regional context, sampling strategy, and provisional findings of a pilot study, PreSOAP, in austral summer 2011, and the following SOAP voyage in late austral summer 2012. Both voyages characterised surface water and MBL composition in three phytoplankton blooms of differing species composition and biogeochemistry, with significant regional correlation observed between chlorophyll-a and DMSsw. Surface seawater dimethylsulfide (DMSsw) and associated air-sea DMS flux showed spatial variation during the SOAP voyage, with maxima of 25 nmol L−1 and 100 µmol m−2 d−1, respectively, recorded in a dinoflagellate bloom. Inclusion of SOAP data in a regional DMSsw compilation confirmed that the current climatological mean is an underestimate for this region of the South-west Pacific. Estimation of the DMS gas transfer velocity (kDMS) by independent techniques of eddy covariance and gradient flux showed good agreement, although both exhibited periodic deviations from model estimates. Flux anomalies were related to surface warming and sea surface microlayer enrichment, and also reflected the heterogeneous distribution of DMSsw and the associated flux footprint. Other aerosol precursors measured included the halides and various volatile organic carbon compounds, with the first measurements of the short-lived gases glyoxal and methylglyoxal in pristine Southern Ocean marine air indicating an unidentified local source. The application of a real-time clean-sector, contaminant markers, and a common aerosol inlet facilitated multi-sensor measurement of uncontaminated air. Aerosol characterisation identified variable Aitken mode, and consistent sub-micron sized accumulation and coarse modes. Sub-micron aerosol mass was dominated by secondary particles containing ammonium sulfate/bisulfate under light winds, with an increase in sea-salt under higher wind-speeds. MBL measurements and chamber experiments identified a significant organic component in primary and secondary aerosols. Comparison of SOAP aerosol number and size distributions reveals an underprediction in GLOMAP-mode aerosol number in clean marine air masses, suggesting a missing marine aerosol source in the model. The SOAP data will be further examined for evidence of nucleation events, and also for relationships between MBL composition and surface ocean biogeochemistry with the aim of identifying potential proxies for aerosol precursors and production.

Description: Direct measurements of marine DMS fluxes are sparse, particularly in the Southern Ocean. The Surface Ocean Aerosol Production (SOAP) voyage in February–March 2012 examined the distribution and flux of dimethylsulfide (DMS) in a biologically-active frontal system in the southwest Pacific Ocean. Three distinct phytoplankton blooms were studied with oceanic DMS concentrations as high as 25 nmol L−1. Measurements of DMS fluxes were made using two independent methods: the eddy covariance (EC) technique using API-CIMS chemical ionization mass spectrometry, and the gradient flux technique (GF) from an autonomous catamaran platform. Catamaran flux measurements are relatively unaffected by air flow distortion and are made close to the water surface where gas gradients are largest. Flux measurements were complemented by near-surface hydrographic measurements to elucidate physical factors influencing DMS emission. Individual DMS fluxes derived by EC showed significant scatter and, at times, consistent departures from the COARE gas exchange parameterization. A direct comparison between the two flux methods was carried out to separate instrumental effects from environmental effects, and showed good agreement with a regression slope of 0.96 (r2 = 0.89). A period of abnormal downward atmospheric heat flux enhanced near-surface ocean stratification and reduced turbulent exchange, during which GF and EC transfer velocities showed good agreement but modelled COAREG values were significantly higher. The transfer velocity derived from near surface ocean turbulence measurements on a spar buoy compared well with the COAREG model in general, but showed less variation. This first direct comparison between EC and GF fluxes of DMS provides confidence in compilation of flux estimates from both techniques, and also in the stable periods when the observations are not well-predicted by the COAREG model.