A likely important feature of the poorly understood aerosol‐cloud interactions over the Southern Ocean (SO) is the dominant role of sea spray aerosol, versus terrestrial aerosol. Ice nucleating ...particles (INPs), or particles required for heterogeneous ice nucleation, present over the SO have not been studied in several decades. In this study, boundary layer aerosol properties and immersion freezing INP number concentrations (nINPs) were measured during a ship campaign that occurred south of Australia (down to 53°S) in March–April 2016. Ocean surface chlorophyll a concentrations ranged from 0.11 to 1.77 mg/m3, and nINPs were a factor of 100 lower than historical surveys, ranging from 0.38 to 4.6 m−3 at −20 °C. The INP population included organic heat‐stable material, with contributions from heat‐labile material. Lower INP source potentials of SO seawater samples compared to Arctic seawater were consistent with lower ice nucleating site densities in this study compared to north Atlantic air masses.
Plain Language Summary
The Southern Ocean is known for a prevalence of clouds that contain both liquid and ice, which are one of the most poorly understood cloud regimes in the climate system. A large gap in understanding important processes in these clouds is a lack of knowledge regarding particles (e.g., sea spray) required for forming ice crystals, termed ice nucleating particles. In a ship‐based monthlong field study, several instruments were deployed in efforts to characterize the ice nucleating particles present over the Southern Ocean for the first time in over four decades. Abundances of ice nucleating particles throughout the voyage were extremely low compared to other ocean regions, and concentrations were 2 orders of magnitude lower than the most recent survey conducted in the 1970s. We report that the ocean‐derived ice nucleating particles observed in this study were organic in nature, supporting a hypothesized link between ice nucleating particles and organic particles associated with phytoplankton blooms. The data from this study provide a desperately needed benchmark for constraining the number of ice crystals that may form in the remote and poorly understood clouds occurring over the Southern Ocean.
Key Points
Number concentrations of ice nucleating particles over the Southern Ocean in March 2016 were a factor of 100 lower than historical surveys
The ice nucleating particle source strength of Southern Ocean seawater was lower than previous measurements in northern hemisphere seawater
Ice nucleation site densities were lower over the Southern Ocean compared to measurements of pristine air masses from other ocean basins
Oceans cover over
70 % of the Earth's surface. Ship-based measurements are an important
component in developing an understanding of atmosphere of this vast region. A
common problem that impacts the ...quality of atmospheric data collected from
marine research vessels is exhaust from both diesel combustion and waste
incineration from the ship itself. Described here is an algorithm, developed
for the recently commissioned Australian blue-water research vessel (RV)
Investigator, that identifies exhaust periods in sampled air. The RV
Investigator, with two dedicated atmospheric laboratories,
represents an unprecedented opportunity for high-quality measurements of the
marine atmosphere. The algorithm avoids using ancillary data such as wind
speed and direction, and instead utilises components of the exhaust itself –
aerosol number concentration, black carbon concentration, and carbon monoxide
and carbon dioxide mixing ratios. The exhaust signal is identified within
each of these parameters individually before they are combined and an
additional window filter is applied. The algorithm relies heavily on
statistical methods, rather than setting thresholds that are too rigid to
accommodate potential temporal changes. The algorithm is more effective than
traditional wind-based filters in removing exhaust data without removing
exhaust-free data, which commonly occurs with traditional filters. In
application to the current dataset, the algorithm identifies 26 % of the
wind filter's “clean” data as exhaust, and recovers 5 % of data falsely
removed by the wind filter. With suitable testing, the algorithm has the
potential to be applied to other ship-based atmospheric measurements where
suitable measurements exist.
The Sydney Particle Study involved the comprehensive measurement of
meteorology, particles and gases at a location in western Sydney during
February–March 2011 and April–May 2012. The aim of this ...study was to
increase scientific understanding of particle formation and transformations
in the Sydney airshed. In this paper we describe the methods used to collect
and analyse particle and gaseous samples, as well as the methods employed
for the continuous measurement of particle concentrations, particle
microphysical properties, and gaseous concentrations. This paper also
provides a description of the data collected and is a metadata record for
the data sets published in Keywood et al. (2016a,
https://doi.org/10.4225/08/57903B83D6A5D) and Keywood et al. (2016b,
https://doi.org/10.4225/08/5791B5528BD63).
Abstract 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 semicontrolled 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-Interface 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 resembled 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.
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