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 therefore the region is under-represented in existing parameterisations. The Surface Ocean Aerosol Production (SOAP) voyage was a 23 d 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.
Forests offer high potential for the fight against climate change. However, forests are faced with increased deforestation. REDD+ is a financial mechanism that offers hope to developing countries for ...tackling deforestation. Aboveground (AGB) estimation, however, is necessary for such financial mechanisms. Remote sensing methods offer various advantages for AGB estimation. A study, therefore, was conducted for the estimation of AGB using a combination of remote sensing Sentinel-1 (S1) and Sentinel-2 (S2) satellite data and field inventorying. The mean AGB for Sub-tropical Chir Pine Forest was recorded as 146.73 ± 65.11 Mg ha
, while for Sub-tropical Broadleaved Evergreen Forest it was 33.77 ± 51.63 Mg ha
. Results revealed weak associations between the S1 and S2 data with the AGB. Nonetheless, S1 and S2 offer advantages such as free data resources that can be utilized by developing countries for forest biomass and carbon monitoring.
Weather and climate models are challenged by uncertainties and biases in simulating Southern Ocean (SO) radiative fluxes that trace to a poor understanding of cloud, aerosol, precipitation, and ...radiative processes, and their interactions. Projects between 2016 and 2018 used in situ probes, radar, lidar, and other instruments to make comprehensive measurements of thermodynamics, surface radiation, cloud, precipitation, aerosol, cloud condensation nuclei (CCN), and ice nucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase clouds common to this pristine environment. Data including soundings were collected from the NSF–NCAR G-V aircraft flying north–south gradients south of Tasmania, at Macquarie Island, and on the R/V Investigator and RSV Aurora Australis. Synergistically these data characterize boundary layer and free troposphere environmental properties, and represent the most comprehensive data of this type available south of the oceanic polar front, in the cold sector of SO cyclones, and across seasons. Results show largely pristine environments with numerous small and few large aerosols above cloud, suggesting new particle formation and limited long-range transport from continents, high variability in CCN and cloud droplet concentrations, and ubiquitous supercooled water in thin, multilayered clouds, often with small-scale generating cells near cloud top. These observations demonstrate how cloud properties depend on aerosols while highlighting the importance of dynamics and turbulence that likely drive heterogeneity of cloud phase. Satellite retrievals confirmed low clouds were responsible for radiation biases. The combination of models and observations is examining how aerosols and meteorology couple to control SO water and energy budgets.
Southern Ocean (SO) shortwave (SW) radiation biases are a common problem in contemporary general circulation models (GCMs), with most models exhibiting a tendency to absorb too much incoming SW ...radiation. These biases have been attributed to deficiencies in the representation of clouds during the austral summer months, either due to cloud cover or cloud albedo being too low. The problem has been the focus of many studies, most of which utilised satellite datasets for model evaluation. We use multi-year ship-based observations and the CERES spaceborne radiation budget measurements to contrast cloud representation and SW radiation in the atmospheric component Global Atmosphere
(GA) version 7.1 of the HadGEM3 GCM and the MERRA-2 reanalysis. We find that
the prevailing bias is negative in GA7.1 and positive in MERRA-2. GA7.1
performs better than MERRA-2 in terms of absolute SW bias. Significant errors
of up to 21 W m−2 (GA7.1) and 39 W m−2 (MERRA-2) are present
in both models in the austral summer. Using ship-based ceilometer observations,
we find low cloud below 2 km to be predominant in the Ross Sea and the
Indian Ocean sectors of the SO. Utilising a novel surface lidar simulator
developed for this study, derived from an existing Cloud Feedback Model Intercomparison Project (CFMIP) Observation Simulator Package (COSP) – active remote sensing simulator (ACTSIM) spaceborne lidar
simulator, we find that GA7.1 and MERRA-2 both underestimate low cloud and fog
occurrence relative to the ship observations on average by 4 %–9 % (GA7.1) and
18 % (MERRA-2). Based on radiosonde observations, we also find the low cloud
to be strongly linked to boundary layer atmospheric stability and the sea
surface temperature. GA7.1 and MERRA-2 do not represent the observed
relationship between boundary layer stability and clouds well. We find that
MERRA-2 has a much greater proportion of cloud liquid water in the SO in
austral summer than GA7.1, a likely key contributor to the difference in the SW
radiation bias. Our results suggest that subgrid-scale processes (cloud and
boundary layer parameterisations) are responsible for the bias and that in
GA7.1 a major part of the SW radiation bias can be explained by cloud cover
underestimation, relative to underestimation of cloud albedo.
Marine sulfate aerosols in the Southern Ocean are critical to the global radiation balance, yet the sources of sulfate and their seasonal variations are unclear. We separately sampled marine and ...ambient aerosols at Baring Head, New Zealand for 1 year using two collectors and evaluated the sources of sulfate in coarse (1–10 μm) and fine (0.05–1 μm) aerosols using sulfur isotopes (δ34S). In both collectors, sea‐salt sulfate (SO42−SS) mainly existed in coarse aerosols and nonsea‐salt sulfate (SO42−NSS) dominated the sulfate in fine aerosols, although some summer SO42−NSS appeared in coarse particles due to aerosol coagulation. SO42−NSS in the marine aerosols was mainly (88–100%) from marine biogenic dimethylsulfide (DMS) emission, while the SO42−NSS in the ambient aerosols was a combination of DMS (73–79%) and SO2 emissions from shipping activities (~21–27%). The seasonal variations of SO42−NSS concentrations inferred from the δ34S values in both collectors were mainly controlled by the DMS flux.
Plain Language Summary
Marine sulfate aerosols are critical to the global radiation balance through directly scattering sunlight or forming clouds; however, their feedback effects are poorly quantified because their sources and size distributions are unclear. We investigated the origins and size distributions of sulfate aerosols from the Southern Ocean as well as the ambient environment at Baring Head, New Zealand. We found that the sulfate in coarse (>1 um) aerosols was dominated by sea‐salt sulfate; while the fine aerosol (<1 um) sulfate, which could act as cloud condensation nuclei, was mostly formed via atmospheric oxidation of sulfur‐bearing gases. The origin of the secondary sulfate was then identified using sulfur isotopic analysis: Dimethylsulfide emitted by phytoplankton contributed over 90% of the secondary sulfate on the Southern Ocean; it also contributed ~73–79% of secondary sulfate aerosols in the ambient air at Baring Head, while the remainder was from anthropogenic sulfur emissions. Our work suggest that marine biological activity is an important factor that controls the amount of sulfate aerosols in remote marine atmosphere, which is of great importance to global climate models.
Key Points
Source distribution and seasonal variation of size‐aggregated sulfate aerosols collected at Baring Head, New Zealand were investigated
Coarse aerosol sulfate was mainly from sea salt; sulfate in fine aerosols was mostly secondary sulfate
Most secondary sulfate was originated from marine DMS emission and should explain the seasonal variations of Southern Ocean CCN
The SOLAS Air‐Sea Gas Exchange (SAGE) Experiment was conducted in the western Pacific sector of the Southern Ocean. During SAGE, gas transfer velocities were determined using the 3He/SF6 dual gas ...tracer technique, and results were obtained at higher wind speeds (16.0 m s−1) than in previous open ocean dual tracer experiments. The results clearly reveal a quadratic relationship between wind speed and gas transfer velocity rather than a recently proposed cubic relationship. A new parameterization between wind speed and gas transfer velocity is proposed, which is consistent with previous 3He/SF6 dual tracer results from the coastal and open ocean obtained at lower wind speeds. This suggests that factors controlling air‐sea gas exchange in this region are similar to those in other parts of the world ocean, and that the parameterization presented here should be applicable to the global ocean.
Ambient concentrations of ice-forming particles measured during ship expeditions are collected and summarised with the aim of determining the spatial distribution and variability in ice nuclei in ...oceanic regions.
The presented data from literature and previously unpublished data from over 23 months of ship-based measurements stretch from the Arctic to the Southern Ocean and include a circumnavigation of Antarctica. In comparison to continental observations, ship-based measurements of ambient ice nuclei show 1 to 2 orders of magnitude lower mean concentrations. To quantify the geographical variability in oceanic areas, the concentration range of potential ice nuclei in different climate zones is analysed by meridionally dividing the expedition tracks into tropical, temperate and polar climate zones. We find that concentrations of ice nuclei in these meridional zones follow temperature spectra with similar slopes but vary in absolute concentration. Typically, the frequency with which specific concentrations of ice nuclei are observed at a certain temperature follows a log-normal distribution. A consequence of the log-normal distribution is that the mean concentration is higher than the most frequently measured concentration. Finally, the potential contribution of ship exhaust to the measured ice nuclei concentration on board research vessels is analysed as function of temperature. We find a sharp onset of the influence at approximately −36 ∘C but none at warmer temperatures that could bias ship-based measurements.
Atmospheric methanethiol (MeSH.sub.a ), dimethyl sulfide (DMS.sub.a) and acetone (acetone.sub.a) were measured over biologically productive frontal waters in the remote southwest Pacific Ocean in ...summertime 2012 during the Surface Ocean Aerosol Production (SOAP) voyage. MeSH.sub.a mixing ratios varied from below the detection limit ( 10 ppt) up to 65 ppt and were 3 %-36 % of parallel DMS.sub.a mixing ratios. MeSH.sub.a and DMS.sub.a were correlated over the voyage (R.sup.2 =0.3, slope = 0.07) with a stronger correlation over a coccolithophore-dominated phytoplankton bloom (R.sup.2 =0.5, slope 0.13). The diurnal cycle for MeSH.sub.a shows similar behaviour to DMS.sub.a with mixing ratios varying by a factor of ⼠2 according to time of day with the minimum levels of both MeSH.sub.a and DMS.sub.a occurring at around 16:00 LT (local time, all times in this paper are in local time). A positive flux of MeSH out of the ocean was calculated for three different nights and ranged from 3.5 to 5.8 µmol m.sup.-2 d.sup.-1, corresponding to 14 %-24 % of the DMS flux (MeSH / (MeSH + DMS)). Spearman rank correlations with ocean biogeochemical parameters showed a moderate-to-strong positive, highly significant relationship between both MeSH.sub.a and DMS.sub.a with seawater DMS (DMS.sub.sw) and a moderate correlation with total dimethylsulfoniopropionate (total DMSP). A positive correlation of acetone.sub.a with water temperature and negative correlation with nutrient concentrations are consistent with reports of acetone production in warmer subtropical waters. Positive correlations of acetone.sub.a with cryptophyte and eukaryotic phytoplankton numbers, and high-molecular-weight sugars and chromophoric dissolved organic matter (CDOM), suggest an organic source. This work points to a significant ocean source of MeSH, highlighting the need for further studies into the distribution and fate of MeSH, and it suggests links between atmospheric acetone levels and biogeochemistry over the mid-latitude ocean.
The atmosphere is the least understood biome on Earth despite its critical role as a microbial transport medium. The influence of surface cover on composition of airborne microbial communities above ...marine systems is unclear. Here we report evidence for a dynamic microbial presence at the ocean-atmosphere interface of a major marine ecosystem, the Great Barrier Reef, and identify that recent air mass trajectory over an oceanic or continental surface associated with observed shifts in airborne bacterial and fungal diversity. Relative abundance of shared taxa between air and coral microbiomes varied between 2.2 and 8.8% and included those identified as part of the core coral microbiome. We propose that this variable source of atmospheric inputs may in part contribute to the diverse and transient nature of the coral microbiome.
Direct measurements of marine dimethylsulfide (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 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
atmospheric pressure chemical ionization–mass spectrometry (API-CIMS) and
the gradient flux (GF) technique from an autonomous catamaran platform.
Catamaran flux measurements are relatively unaffected by airflow 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 Coupled Ocean–Atmosphere Response Experiment gas transfer algorithm (COAREG). 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, as well as in
the stable periods when the observations are not well predicted by the COAREG
model.
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