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The presented data from literature and previously unpublished data from over 23 months of ship-based measurements stretch from the Arctic to the Southern Ocean and include a circumnavigation of Antarctica.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.
Air bubbles bursting on artificial seawater in laboratory experiments have been found to inject numerous particles <200 nm diameter into the atmosphere, some experiments showing copious production of ...particles as small as 10 nm. Some observations of the real marine aerosol support the presence of a large proportion of sea salt <200 nm diameter, while others suggest that it is absent, or nearly so. It is argued here that the observations showing its presence may be misinterpretations. If this is so, modification of currently accepted theories of particle injection into the atmosphere by bursting bubbles would be required. Highly surface active exopolymers produced by bacteria and algae, the microgels formed by them, and large concentrations of submicrometer particulates are known to be present in the ocean. Their possible influence on bubble formation, bubble bursting and particle injection into the atmosphere are discussed. Electron microscopy of individual particles at a number of sites supports the proposal that the exopolymers are involved in these processes. Ultraviolet light and acidification cause structural and chemical changes to exopolymers and their gels exposed to the atmosphere so that marine aerosol will have properties that change with atmospheric residence time.
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.
Large increases in concentration of particles smaller than 20 nm diameter are relatively common over the central Arctic Ocean in summer and have occasionally been observed over lower latitude oceans. ...These events often do not readily fit theoretical models of homogeneous nucleation from known precursor gases. It is shown that aggregates and gels of marine biological origin are often common over remote oceans and have a partially granular structure. Previous work in the central Arctic Ocean in summer has shown evidence of release of particles of the order of 40 nm diameter in the presence of evaporating fogs. It is suggested here that under some circumstances disintegration of the primary particles may be more complete, releasing particles smaller than 5 nm that would then be mistaken for recently nucleated particles. Examination of particles present during an apparent nucleation event at Cape Grim, Tasmania supports this interpretation. Correlation coefficients of concentrations of particles of different sizes during the period before apparent nucleation events suggests a distinct difference between vigorous true nucleation events and those over the central Arctic Ocean in summer where little subsequent growth is observed.
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.
Previous studies have identified regions where the occurrence of rainfall significantly increases or decreases the probability for subsequent rainfall over periods that range from a few days to ...several weeks. These observable phenomena are termed “rainfall feedback” (RF). To better understand the land–atmosphere interactions involved in RF, the behavior of RF patterns was analyzed using data from 1849 to 2016 at ~3000 sites in the contiguous United States. We also considered changes in major land‐use types and applied a geographically weighted regression model technique for analyzing the predictors of RF. This approach identified non‐linear and spatially non‐stationary relationships between RF, climate, land use, and land type. RF patterns in certain regions of the United States are predictable by modeling variables associated with climate, season, and land use. The model outputs also demonstrate the extent to which the effect of precipitation, temperature, and land use on RF depend on season and location. Specifically, major changes were observed for land use associated with agriculture in the western United States, which had negative and positive influences on RF in summer and winter, respectively. In contrast, developed land in the eastern United States correlated with positive RF values in summer but with negative ones in winter. We discuss how changes in climate and land use would be expected to affect land–atmosphere interactions, as well as the possible role that physical mechanisms and rain‐enhanced bioaerosol emissions may play in the spatiotemporal changes observed for historical patterns of rainfall frequency in the United States.
This study analyzed the influence of rainfall on subsequent rainfall (i.e., rainfall feedback) from 1960 to 2010 at ~3000 sites in the contiguous United States using a geographically weighted regression model. The model outputs demonstrate the extent to which the effect of precipitation, temperature, and land use on rainfall feedback depend on season and location.
Marine aerosol was collected in September 1998 and July 2005 on the upwind coast of an island at latitude 15◦S, about 15 km downwind from the outer edge of the Great Barrier Reef, Australia, and ...examined by electron microscopy. Exopolymer gels, aggregates of organic particles, marine micro-organisms and fragments of marine life formed a substantial part of the accumulation mode aerosol. Differences in transparency, firmness of outlines and shape of gels and the influence of organic vapours on them, suggested progressive physical and chemical changes with atmospheric residence time. The organic aggregate components had a size distribution remarkably close to that found in similar particles over the central Arctic Ocean peaking at diameters of 30-40 nm. Single components or small groups of these aggregates were found within at least 75% of particles resembling ammonium sulphate in appearance, indicating that aggregates fragmented in the atmosphere. Sea salt was not detected in particles <200 nm diameter unlike many observations showing it to be a major component, a result that was entirely consistent with the Arctic findings. The deduced sequence of changes to particles entering the atmosphere from the ocean is also very similar to that found in the Arctic, suggesting that it is a common pattern over the oceans. That conclusion would require modification of the parametrization of the marine aerosol used in climate models and of possible climate feedback effects.
The aerosols that influence the initiation and amount of precipitation are cloud condensation nuclei (CCN), giant CCN, and ice nuclei. Aerosols are ever-present, their properties are variable, and ...their abundance is dynamic. Therefore, the extent of their impact on the outcome of meteorological contexts that are favorable for rain are difficult to specify. Rainfall can generate aerosols. Those of biological origin that are generated after rainfall can accumulate in a persistent manner over several weeks. Based on a recently developed index of rainfall feedback that focuses on persistent feedback effects and that represents the a priori sensitivity of rainfall to aerosols—of biological origin in particular—we mapped the intensity and patterns of rainfall feedback at 1,250 sites in the western United States where 100-year daily rainfall data were available and where drought is critically severe. This map reveals trends in feedback related to orographic context, geographical location, and season, among other trends. We describe an open-access tool (<http://w3.avignon.inra.fr/rainfallfeedback/index.html>) for mapping rainfall feedback on a planetary scale to provide a framework for future research to generate hypotheses and to establish rationale to choose field sites for experimentation. This will contribute to the long-term goal of developing a robust understanding of specific and contextual aerosol effects on rainfall applicable to forecasting and to land-use management.
Rainfall is one of the most important aspects of climate, but the extent to which atmospheric ice nuclei (IN) influence its formation, quantity, frequency, and location is not clear. Microorganisms ...and other biological particles are released following rainfall and have been shown to serve as efficient IN, in turn impacting cloud and precipitation formation. Here we investigated potential long-term effects of IN on rainfall frequency and quantity. Differences in IN concentrations and rainfall after and before days of large rainfall accumulation (i.e., key days) were calculated for measurements made over the past century in southeastern and southwestern Australia. Cumulative differences in IN concentrations and daily rainfall quantity and frequency as a function of days from a key day demonstrated statistically significant increasing logarithmic trends (R2 > 0.97). Based on observations that cumulative effects of rainfall persisted for about 20 days, we calculated cumulative differences for the entire sequence of key days at each site to create a historical record of how the differences changed with time. Comparison of pre-1960 and post-1960 sequences most commonly showed smaller rainfall totals in the post-1960 sequences, particularly in regions downwind from coal-fired power stations. This led us to explore the hypothesis that the increased leaf surface populations of IN-active bacteria due to rain led to a sustained but slowly diminishing increase in atmospheric concentrations of IN that could potentially initiate or augment rainfall. This hypothesis is supported by previous research showing that leaf surface populations of the ice-nucleating bacterium Pseudomonas syringae increased by orders of magnitude after heavy rain and that microorganisms become airborne during and after rain in a forest ecosystem. At the sites studied in this work, aerosols that could have initiated rain from sources unrelated to previous rainfall events (such as power stations) would automatically have reduced the influences on rainfall of those whose concentrations were related to previous rain, thereby leading to inhibition of feedback. The analytical methods described here provide means to map and delimit regions where rainfall feedback mediated by microorganisms is suspected to occur or has occurred historically, thereby providing rational means to establish experimental set-ups for verification.
The particulate content of samples of the surface microlayer water collected from the open water between ice floes at latitudes 88° to 89°N in August 2001 was examined by transmission electron ...microscopy. Concentrations varied from 2×10
7 ml
−1 to more than 10
14 ml
−1 although bacterial counts made in the same samples varied by only about 50%. Size distributions of the particles were also very variable with modal diameter sizes of 10 nm in some samples and 50 nm in others, the 50-nm particles appearing to be clusters of the 10 nm ones. A mucus-like material held the particles together in rafts, strings or in balls. The largest particles were compact electron-opaque aggregates of smaller particles. The particles appeared to have very similar characteristics to the “microcolloids” observed in bulk seawater in lower latitude oceans. X-ray analyses of the elements with atomic numbers >16 showed all signals to be weak, suggesting a mainly organic composition. The elements that were most commonly greater than background levels were all those associated with marine biological activity. Rapid aggregation of polymers to form colloids has been noted and is likely to be an important cause of the observed variability of particulate concentrations in the surface microlayer. The possibility of an equally rapid dispersal under the influence of ultraviolet light is raised.
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