Events of new particle formation (NPF) were analyzed in a 10-year data set of hourly particle size distributions recorded on Mt. Zeppelin, Spitsbergen, Svalbard. Three different types of NPF events ...were identified through objective search algorithms. The first and simplest algorithm utilizes short-term increases in particle concentrations below 25 nm (PCT (percentiles) events). The second one builds on the growth of the sub-50 nm diameter median (DGR (diameter growth) events) and is most closely related to the classical banana type of event. The third and most complex, multiple-size approach to identifying NPF events builds on a hypothesis suggesting the concurrent production of polymer gel particles at several sizes below ca. 60 nm (MEV (multi-size growth) events). As a first and general conclusion, we can state that NPF events are a summer phenomenon and not related to Arctic haze, which is a late winter to early spring feature. The occurrence of NPF events appears to be somewhat sensitive to the available data on precipitation. The seasonal distribution of solar flux suggests some photochemical control that may affect marine biological processes generating particle precursors and/or atmospheric photochemical processes that generate condensable vapors from precursor gases. Notably, the seasonal distribution of the biogenic methanesulfonate (MSA) follows that of the solar flux although it peaks before the maxima in NPF occurrence. A host of ancillary data and findings point to varying and rather complex marine biological source processes. The potential source regions for all types of new particle formation appear to be restricted to the marginal-ice and open-water areas between northeastern Greenland and eastern Svalbard. Depending on conditions, yet to be clarified new particle formation may become visible as short bursts of particles around 20 nm (PCT events), longer events involving condensation growth (DGR events), or extended events with elevated concentrations of particles at several sizes below 100 nm (MEV events). The seasonal distribution of NPF events peaks later than that of MSA and DGR, and in particular than that of MEV events, which reach into late summer and early fall with open, warm, and biologically active waters around Svalbard. Consequently, a simple model to describe the seasonal distribution of the total number of NPF events can be based on solar flux and sea surface temperature, representing environmental conditions for marine biological activity and condensation sink, controlling the balance between new particle nucleation and their condensational growth. Based on the sparse knowledge about the seasonal cycle of gel-forming marine microorganisms and their controlling factors, we hypothesize that the seasonal distribution of DGR and, more so, MEV events reflect the seasonal cycle of the gel-forming phytoplankton.
ABSTRACT
Transmission electron microscopy photographs of airborne particles are compared with those of particles found in the surface microlayer of the open water between ice floes in the central ...Arctic Ocean in summer. The similarity in morphology, physical properties, X‐ray spectra and a chemical reaction of the numerous aggregates and their building blocks predominantly smaller than 70 nm diameter, and of bacteria and other micro‐organisms found in both, strongly suggests that the airborne particles were ejected from the water by bursting bubbles. The shape of the size distribution of aggregates in the air is very similar to that in the water, each with a well‐defined Aitken mode but shifted towards smaller sizes. Diffuse electron‐transparent material joining and surrounding the heat resistant and non‐hygroscopic particulates in both the air and water is shown to have properties consistent with the exopolymer secretions (EPS) of microalgae and bacteria in the water. EPS are highly surface‐active, highly hydrated molecules that can spontaneously assemble into gels. They are broken down by ultraviolet light or acidification. These properties provide an explanation for the different resistance to dehydration of bacteria from air and water samples when subjected to a vacuum, and the apparent absence of sea salt on airborne bacteria and aggregates. The difference in size distribution between the air and water samples is also explained. The role of EPS and particulate matter from the open lead surface microlayer in the production of the airborne Aitken mode particles and cloud condensation nuclei is examined and concluded to be very important.
In recent years, sea spray as well as the biological material it contains has received increased attention as a source of ice-nucleating particles (INPs). Such INPs may play a role in remote marine ...regions, where other sources of INPs are scarce or absent. In the Arctic, these INPs can influence water–ice partitioning in low-level clouds and thereby the cloud lifetime, with consequences for the surface energy budget, sea ice formation and melt, and climate. Marine aerosol is of a diverse nature, so identifying sources of INPs is challenging. One fraction of marine bioaerosol (phytoplankton and their exudates) has been a particular focus of marine INP research. In our study we attempt to address three main questions. Firstly, we compare the ice-nucleating ability of two common phytoplankton species with Arctic seawater microlayer samples using the same instrumentation to see if these phytoplankton species produce ice-nucleating material with sufficient activity to account for the ice nucleation observed in Arctic microlayer samples. We present the first measurements of the ice-nucleating ability of two predominant phytoplankton species: Melosira arctica, a common Arctic diatom species, and Skeletonema marinoi, a ubiquitous diatom species across oceans worldwide. To determine the potential effect of nutrient conditions and characteristics of the algal culture, such as the amount of organic carbon associated with algal cells, on the ice nucleation activity, Skeletonema marinoi was grown under different nutrient regimes. From comparison of the ice nucleation data of the algal cultures to those obtained from a range of sea surface microlayer (SML) samples obtained during three different field expeditions to the Arctic (ACCACIA, NETCARE, and ASCOS), we found that they were not as ice active as the investigated microlayer samples, although these diatoms do produce ice-nucleating material. Secondly, to improve our understanding of local Arctic marine sources as atmospheric INPs we applied two aerosolization techniques to analyse the ice-nucleating ability of aerosolized microlayer and algal samples. The aerosols were generated either by direct nebulization of the undiluted bulk solutions or by the addition of the samples to a sea spray simulation chamber filled with artificial seawater. The latter method generates aerosol particles using a plunging jet to mimic the process of oceanic wave breaking. We observed that the aerosols produced using this approach can be ice active, indicating that the ice-nucleating material in seawater can indeed transfer to the aerosol phase. Thirdly, we attempted to measure ice nucleation activity across the entire temperature range relevant for mixed-phase clouds using a suite of ice nucleation measurement techniques – an expansion cloud chamber, a continuous-flow diffusion chamber, and a cold stage. In order to compare the measurements made using the different instruments, we have normalized the data in relation to the mass of salt present in the nascent sea spray aerosol. At temperatures above 248 K some of the SML samples were very effective at nucleating ice, but there was substantial variability between the different samples. In contrast, there was much less variability between samples below 248 K. We discuss our results in the context of aerosol–cloud interactions in the Arctic with a focus on furthering our understanding of which INP types may be important in the Arctic atmosphere.
The representation of aerosol properties and processes in climate models is fraught with large uncertainties. Especially at high northern latitudes a strong under-prediction 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 3903 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 nm 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 accompanied by a minor fraction of ammonium (bi)sulfate with a maximum in number concentration at 170 nm. Gel particles (11% of all particles) were observed between 45 nm and 800 nm with a maximum in number concentration at 154 nm. Imaging with transmission electron microscopy allowed further morphological discrimination of gel particles in âaggregateâ particles, âaggregate with filmâ particles and âmucus-likeâ particles.
Rainwater samples for chemical analysis were collected over the Indian Ocean during the Indian Ocean Experiment (INDOEX) campaign January–March 1999 on board the research vessels Ronald H. Brown and ...Sagar Kanya. Samples were analyzed for major ions and some trace metals. The rainwater data are interpreted in terms of transport from potential source regions in Asia using air mass trajectories covering 10 days. A comparison is also made between the rainwater data and the concentration of aerosol components measured simultaneously on the ships. The concentrations of nonsea‐salt (nss)‐SO42−, NO3−, NH4+, nss‐K+, and nss‐Ca2+ in rainwater over the Indian Ocean, while a factor of 2 to 3 lower than over the Indian continent, were still clearly influenced by pollution and soil sources in Asia. The concentration of nss‐Ca2+ decreased more rapidly as the air moved southward from the continent out over the ocean, whereas the concentration of nss‐SO42− became relatively more abundant. This was consistent with the observed higher acidity of the rainwater over the ocean (pH in the range 4.8 to 5.4) than over the Indian subcontinent, with NH4+ as the main cation (rather than Ca2+, as over land). Variations in the concentration of Al and Fe correlated well with those of nss‐Ca2+, indicating a crustal source for these elements. The relation between Na+, Cl−, and Br− in the rainwater was close to that of seawater, implying no excess or deficit of the two halogen ions. The ratio between the concentration in rainwater and the concentration in surface air was systematically larger for aerosol components that exist in the coarse mode of sea‐salt origin (Na+, Mg2+, and Cl−) than those in the fine mode (NH4+, nss‐K+, and nss‐SO42−), indicating that fine‐mode particles are scavenged mainly by in‐cloud processes whereas coarse‐mode sea‐salt particles are scavenged also by falling raindrops under the clouds. Nss‐Ca2+ and NO3− fall in a category in between, indicating that these compounds are not as effectively removed by below‐cloud scavenging as sea‐salt aerosols.
ABSTRACT
Within the framework of SWEDARP (Swedish Antarctic Program) 92/93 an aerosol sampling program was carried out on board of M/S Polarbjörn which carried staff and material to the Nordic ...Antarctic field exercises during the Austral summer 1992/1993. The cruise started 11 November 1992 from Oslo, went via Cape Town to Antarctica, and then back to Cape Town where the ship arrived on 4 January 1993. During the cruise, a meridional profile of physical and chemical submicrometre aerosol properties was derived covering the East Atlantic Ocean from 60°N to 70°S. The multicomponent aerosol data set combined with a trajectory analysis revealed a systematic meridional distribution of aerosol sources over the Atlantic that covered European and African continental plumes and, south of 15°S, a largely biologically controlled marine aerosol. Median number concentrations calculated over the whole cruise spanned a factor of 20 between 2000 and 100 cm−3, while total analyzed mass concentrations ranged between 7800 and 40 ng m3. From the biologically dominated subset of the data in the southern hemisphere, relationships were developed that allowed an apportionment of the observed sulfate and ammonium concentration to biogenic and anthropogenic sources over the whole meridional aerosol profile.
Concentrations of cloud condensation (CCN) and ice forming nuclei (IFN) were measured throughout an expedition by icebreaker around the central Arctic Ocean including the North Pole from July 15 to ...September 23, 1996. Daily median CCN concentrations at 0.25% supersaturation were typically in the range 15 to 50 cm−3, but concentrations varied by 3 orders of magnitude over the expedition and commonly by an order of magnitude within a day. They were highest near the ice edge and fell by almost an order of magnitude in the first 36 hours of transport from the open sea into the pack ice region. For longer transport times they increased again indicating a local source, suggested to be drops injected into the air by bubbles bursting on open leads. Median concentrations of IFN ranged from 18 m−3 just inside the pack ice at the beginning of the expedition to 1 m−3 at the end. The differences with transport time from the ice edge were less marked than for CCN. Comparison of CCN measurements with simultaneously measured number size distributions showed that the median concentration active at a given supersaturation was only 71% of the number expected if all the particles had been composed of pure ammonium sulfate. Transmission electron microscope observations of individual particles suggested an evolution of many CCN from nonvolatile, nonhygroscopic particles <50 nm by acquisition of the oxidation products of dimethyl sulfide. Cloud processing added further mass. The largest primary source within the pack ice region was deduced to be film drops from bursting bubbles yielding liquid particles with a high organic content and surfactant properties. A relationship between CCN number and sulfur mass is derived which extends previous results to lower CCN numbers and indicates a greater sensitivity to change in sulfate mass than at lower latitudes. Bacteria and probable submicron fragments of marine organisms were identified in the samples and suggested to be the source of IFN.
ABSTRACT
Recent results from summer measurement campaigns over the partly ice covered central Arctic Ocean show that the high Arctic aerosol has a larger organic fraction than previously thought. We ...use a Lagrangian parcel model to infer the properties of the unexplained organic aerosol fraction that is necessary for reproducing the observed concentrations of cloud condensation nuclei (CCN). With increasing distance from the open ocean a highly surface‐active Aitken mode, associated with particles found in the open lead surface microlayer, becomes increasingly important for cloud droplet formation. The presence of such an Aitken mode population increases the high Arctic indirect aerosol effect (added cooling) relative to just a marine source of CCN from oxidation products of dimethyl sulfide (DMS) released from phytoplankton.
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