Near‐surface observations of gas phase dimethyl sulfide, DMS(g), over the central Arctic Ocean display large temporal variability. By using a three‐dimensional numerical model, the atmospheric part ...of COAMPS2.0®, we show that meteorological processes such as transport and mixing cause variability in DMS(g) of the same order as in the observations. The observations used in this study were taken on board the icebreaker Oden that cruised the high Arctic during the following three expeditions: the International Arctic Ocean Expedition 1991, the Arctic Ocean Expedition 1996, and the Arctic Ocean Experiment 2001. Calculation of air‐sea flux and photochemical decay of DMS(g) was added to COAMPS2.0®. A 10‐day period in August 2001 was modeled. The time development of observed DMS(g) is captured by the model, correlation coefficient 0.76, in spite of a simplified treatment of DMS processes. Also, the model results clearly show that DMS(g) is advected over the pack ice in plumes originating from different source areas around the pack ice.
Temperature and particle number concentration profiles were measured at small height intervals above open and frozen leads and snow surfaces in the central Arctic. The device used was a gradient pole ...designed to investigate potential particle sources over the central Arctic Ocean. The collected data were fitted according to basic logarithmic flux-profile relationships to calculate the sensible heat flux and particle deposition velocity. Independent measurements by the eddy covariance technique were conducted at the same location. General agreement was observed between the two methods when logarithmic profiles could be fitted to the gradient pole data. In general, snow surfaces behaved as weak particle sinks with a maximum deposition velocity vd = 1.3 mm s−1 measured with the gradient pole. The lead surface behaved as a weak particle source before freeze-up with an upward flux Fc = 5.7 × 104 particles m−2 s−1, and as a relatively strong heat source after freeze-up, with an upward maximum sensible heat flux H = 13.1 W m−2. Over the frozen lead, however, we were unable to resolve any significant aerosol profiles.
A database of 15,617 point measurements of dimethylsulfide (DMS) in surface waters along with lesser amounts of data for aqueous and particulate dimethylsulfoniopropionate concentration, chlorophyll ...concentration, sea surface salinity and temperature, and wind speed has been assembled. The database was processed to create a series of climatological annual and monthly 1°×1° latitude‐longitude squares of data. The results were compared to published fields of geophysical and biological parameters. No significant correlation was found between DMS and these parameters, and no simple algorithm could be found to create monthly fields of sea surface DMS concentration based on these parameters. Instead, an annual map of sea surface DMS was produced using an algorithm similar to that employed by Conkright et al. 1994. In this approach, a first‐guess field of DMS sea surface concentration measurements is created and then a correction to this field is generated based on actual measurements. Monthly sea surface grids of DMS were obtained using a similar scheme, but the sparsity of DMS measurements made the method difficult to implement. A scheme was used which projected actual data into months of the year where no data were otherwise present.
The summertime high Arctic atmosphere is characterized by extremely low aerosol abundance, such that small natural aerosol inputs have a strong influence on cloud formation and surface temperature. ...The physical sources and the mechanisms responsible for aerosol formation and development in this climate‐critical and changing region are still uncertain. We report time‐resolved measurements of high Arctic Aitken mode (∼20–60 nm diameter) aerosol composition during August–September 2018. During a significant Aitken mode formation event, the particles were composed of a combination of primary and secondary materials. These results highlight the importance of primary aerosol sources for high Arctic cloud formation, and they imply the action of a poorly understood atmospheric mechanism separating larger particles into multiple sub‐particles.
Plain Language Summary
Clouds are an important part of Earth's climate system, in part because they play a role in controlling how much energy passes from the sun to Earth's surface and from Earth's surface out into space. Aerosols, or disperse atmospheric particles, act as seeds onto which water can condense to form cloud droplets. The high Arctic region, near the North Pole, is a unique environment because it often has very low numbers of aerosols, which impacts the formation of clouds and their energy trapping and releasing properties. We set out to learn more about how aerosols form in this remote region, where climate is rapidly changing, with likely impacts on aerosol sources. We measured the composition of small, so‐called ultrafine particles and found that they are made of a combination of sea spray and molecules added onto them as gases from the air. Surprisingly, it seems that sometimes large numbers of particles are formed from the breakup of larger composite particles, a process which has been hypothesized but, if it is occurring, is still not understood.
Key Points
Aitken mode (20–60 nm) particles in the summertime high Arctic were composed of aged, organic‐rich sea spray and secondary compounds
An intense Aitken mode number increase was driven by the atmospheric breakup of pre‐existing aerosol by an unknown mechanism
The potential importance of Aitken mode particles (diameters
∼ 25–80 nm) for stratiform mixed-phase clouds in the
summertime high Arctic (>80∘ N) has been investigated
using two large-eddy simulation ...models. We find that, in both models, Aitken mode particles significantly affect the simulated microphysical and
radiative properties of the cloud and can help sustain the cloud when
accumulation mode concentrations are low (< 10–20 cm−3), even
when the particles have low hygroscopicity (hygroscopicity parameter – κ=0.1). However, the influence of the Aitken mode decreases if the overall liquid water content of the cloud is low, either due to a higher ice fraction or due to low radiative cooling rates. An analysis of the simulated supersaturation (ss) statistics shows that the ss frequently reaches 0.5 % and sometimes even exceeds 1 %, which confirms that Aitken mode particles can be activated. The modelling results are in qualitative agreement with observations of the Hoppel minimum obtained from four different expeditions in the high Arctic. Our findings highlight the importance of better understanding Aitken mode particle formation, chemical properties and emissions, particularly in clean environments such as the high Arctic.
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.
The representation of aerosol properties and processes in climate models is fraught with large uncertainties. Especially at high northern latitudes a strong underprediction of aerosol concentrations ...and nucleation events is observed and can only be constrained by in situ observations based on the analysis of individual aerosol particles. To further reduce the uncertainties surrounding aerosol properties and their potential role as cloud condensation nuclei this study provides observational data resolved over size on morphological and chemical properties of aerosol particles collected in the summer high Arctic, north of 80° N. Aerosol particles were imaged with scanning and transmission electron microscopy and further evaluated with digital image analysis. In total, 3909 aerosol particles were imaged and categorized according to morphological similarities into three gross morphological groups: single particles, gel particles, and halo particles. Single particles were observed between 15 and 800 nm in diameter and represent the dominating type of particles (82 %). The majority of particles appeared to be marine gels with a broad Aitken mode peaking at 70 nm and accompanied by a minor fraction of ammonium (bi)sulfate with a maximum at 170 nm in number concentration. Gel particles (11 % of all particles) were observed between 45 and 800 nm with a maximum at 154 nm in diameter. Imaging with transmission electron microscopy allowed further morphological discrimination of gel particles in "aggregate" particles, "aggregate with film" particles, and "mucus-like" particles. Halo particles were observed above 75 nm and appeared to be ammonium (bi)sulfate (59 % of halo particles), gel matter (19 %), or decomposed gel matter (22 %), which were internally mixed with sulfuric acid, methane sulfonic acid, or ammonium (bi)sulfate with a maximum at 161 nm in diameter. Elemental dispersive X-ray spectroscopy analysis of individual particles revealed a prevalence of the monovalent ions Na+/K+ for single particles and aggregate particles and of the divalent ions Ca2+/Mg2+ for aggregate with film particles and mucus-like particles. According to these results and other model studies, we propose a relationship between the availability of Na+/K+ and Ca2+/Mg2+ and the length of the biopolymer molecules participating in the formation of the three-dimensional gel networks.
Climate change is particularly noticeable in the Arctic. The most common type of cloud at these latitudes is mixed-phase stratocumulus. These clouds occur frequently and persistently during all ...seasons and play a critical role in the Arctic energy budget. Previous observations in the central (north of 80∘ N) Arctic have shown a high occurrence of prolonged periods of a shallow, single-layer mixed-phase stratocumulus at the top of the boundary layer (BL; altitudes ∼ 300 to 400 m). However, recent observations from the summer of 2018 instead showed a prevalence of a two-layer boundary-layer cloud system. Here we use large-eddy simulation to examine the maintenance of one of the cloud systems observed in the summer of 2018 and the sensitivity of the cloud layers to different micro- and macro-scale parameters. We find that the model generally reproduces the observed thermodynamic structure well, with two near-neutrally stratified layers in the BL caused by a low cloud (located within the first few hundred meters) capped by a lower-altitude temperature inversion and an upper cloud layer (based around one kilometer or slightly higher) capped by the main temperature inversion of the BL. The simulated cloud structure is persistent unless there are low aerosol number concentrations (≤ 5 cm−3), which cause the upper cloud layer to dissipate, or high large-scale wind speeds (≥ 8.5 m s−1), which erode the lower inversion and the related cloud layer. The changes in cloud structure alter both the short- and longwave cloud radiative effect at the surface. This results in changes in the net radiative effect of the modeled cloud system, which can impact the surface melting or freezing. The findings highlight the importance of better understanding and representing aerosol sources and sinks over the central Arctic Ocean. Furthermore, they underline the significance of meteorological parameters, such as the large-scale wind speed, for maintaining the two-layer boundary-layer cloud structure encountered in the lower atmosphere of the central Arctic.
The “indirect effect of aerosols” refers to their ability to influence cloud radiative properties, and is considered to be one of the larger uncertainties in climate prediction. Oceans cover about ...70% of the world's surface and the aerosols that they produce are therefore likely to represent an important part of the indirect effect. A description of the ultimate sources of all aerosol constituents and their susceptibility to climate change is then required in order to assess the potential of an aerosol‐cloud‐climate feedback. Here we argue that in the high Arctic in summer, cloud condensation nuclei (CCN) concentration is not determined by the oxidation products of dimethyl‐sulfide as has usually been assumed but by the concentration in the air of small insoluble organic particles derived from the surface microlayer of the ocean by bubble bursting, on which the acid gases condense. Examination of the aerosol over lower latitude oceans suggests that similar processes occur over all oceans.
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