Measuring and identifying ice-nucleating particles in the atmosphere

A fraction of aerosol particles known as Ice-Nucleating Particles (INPs) has the potential to trigger ice formation in supercooled liquid droplets, dramatically altering the properties of mixed-phase clouds. However, our current knowledge on the way these particles are distributed in the atmosphere is still limited. This thesis aimed to expand our understanding of the sources and concentrations of INPs in the atmosphere at various locations at mid- and high-latitudes. This was done by performing immersion mode INP and aerosol size-resolved composition measurements on board of a research aircraft in three different locations (North-Western Europe, Iceland and the Western North American Arctic). Aerosol measurements were performed on board of the FAAM BAe-146 research aircraft. Hence, I first characterised the biases of the filter inlet system available on board of this aircraft both theoretically and experimentally, providing recommendations on how to operate the system. I also implemented a methodology to study the size-resolved composition of aerosol samples collected on top of filters using Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS). This technique has been applied in parallel with a droplet freezing assay to measure INPs as well as aerosol size-resolved composition in aerosol samples collected on board of the FAAM BAe-146 in different locations. The first area of study was North-Western Europe. The INP concentration was dominated by the presence of mineral dust at the lower end of the temperature spectrum, while an additional source (likely biological) was the main component at higher temperatures. The second area of study was Iceland, where the INP concentrations were dominated by the presence of local dust. The used methodology allowed me to derive a parameterization of the ice-nucleation ability of airborne Icelandic dust. This parameterization was combined with a global aerosol model which included the Icelandic dust emissions, showing that Icelandic dust significantly contributes to the INP population over the North Atlantic and some areas of the Arctic. The INP concentrations in the Western North American Arctic were the lowest I detected, being compatible with the limit of detection in most cases. The SEM-EDS analysed revealed that mineral dust is more important than sea spray aerosol for the INP population. Overall, across the campaigns, I observed large variability in aerosol concentration, aerosol composition and INP concentration. A part of this variability in the INPs can be explained by the presence of surface area of aerosol and dust in the samples

The ice-nucleating ability of quartz immersed in water and its atmospheric importance compared to K-feldspar

Mineral dust particles are thought to be an important type of ice-nucleating particle (INP) in the mixedphase cloud regime around the globe. While K-rich feldspar (K-feldspar) has been identified as being a particularly important component of mineral dust for ice nucleation, it has been shown that quartz is also relatively ice-nucleation active. Given quartz typically makes up a substantial proportion of atmospheric desert dust, it could potentially be important for cloud glaciation. Here, we survey the ice-nucleating ability of 10 α-quartz samples (the most common quartz polymorph) when immersed in microlitre supercooled water droplets. Despite all samples being α-quartz, the temperature at which they induce freezing varies by around 12 ◦C for a constant active site density. We find that some quartz samples are very sensitive to ageing in both aqueous suspension and air, resulting in a loss of ice-nucleating activity, while other samples are insensitive to exposure to air and water over many months. For example, the ice-nucleation temperatures for one quartz sample shift down by ∼ 2 ◦C in 1 h and 12 ◦C after 16 months in water. The sensitivity to water and air is perhaps surprising, as quartz is thought of as a chemically resistant mineral, but this observation suggests that the active sites responsible for nucleation are less stable than the bulk of the mineral. We find that the quartz group of minerals is generally less active than K-feldspars by roughly 7 ◦C, although the most active quartz samples are of a similar activity to some K-feldspars with an active site density, ns(T ), of 1 cm−2 at −9◦C. We also find that the freshly milled quartz samples are generally more active by roughly 5 ◦C than the plagioclase feldspar group of minerals and the albite end member has an intermediate activity. Using both the new and literature data, active site density parameterizations have been proposed for freshly milled quartz, K-feldspar, plagioclase and albite. Combining these parameterizations with the typical atmospheric abundance of each mineral supports previous work that suggests that K-feldspar is the most important ice-nucleating mineral in airborne mineral dust

Newly identified climatically and environmentally significant high-latitude dust sources

Dust particles from high latitudes have a potentially large local, regional, and global significance to climate and the environment as short-lived climate forcers, air pollutants, and nutrient sources. Identifying the locations of local dust sources and their emission, transport, and deposition processes is important for understanding the multiple impacts of high-latitude dust (HLD) on the Earth\u27s systems. Here, we identify, describe, and quantify the source intensity (SI) values, which show the potential of soil surfaces for dust emission scaled to values 0 to 1 concerning globally best productive sources, using the Global Sand and Dust Storms Source Base Map (G-SDS-SBM). This includes 64 HLD sources in our collection for the northern (Alaska, Canada, Denmark, Greenland, Iceland, Svalbard, Sweden, and Russia) and southern (Antarctica and Patagonia) high latitudes. Activity from most of these HLD sources shows seasonal character. It is estimated that high-latitude land areas with higher (SI ≥0.5), very high (SI ≥0.7), and the highest potential (SI ≥0.9) for dust emission cover >1 670 000 km$^{2}$, >560 000 km$^{2}$, and >240 000 km$^{2}$, respectively. In the Arctic HLD region (≥60$^{∘}$ N), land area with SI ≥0.5 is 5.5 % (1 035 059 km$^{2}$), area with SI ≥0.7 is 2.3 % (440 804 km$^{2}$), and area with SI ≥0.9 is 1.1 % (208 701 km$^{2}$). Minimum SI values in the northern HLD region are about 3 orders of magnitude smaller, indicating that the dust sources of this region greatly depend on weather conditions. Our spatial dust source distribution analysis modeling results showed evidence supporting a northern HLD belt, defined as the area north of 50$^{∘}$ N, with a “transitional HLD-source area” extending at latitudes 50–58∘ N in Eurasia and 50–55$^{∘}$ N in Canada and a “cold HLD-source area” including areas north of 60$^{∘}$ N in Eurasia and north of 58$^{∘}$ N in Canada, with currently “no dust source” area between the HLD and low-latitude dust (LLD) dust belt, except for British Columbia. Using the global atmospheric transport model SILAM, we estimated that 1.0 % of the global dust emission originated from the high-latitude regions. About 57 % of the dust deposition in snow- and ice-covered Arctic regions was from HLD sources. In the southern HLD region, soil surface conditions are favorable for dust emission during the whole year. Climate change can cause a decrease in the duration of snow cover, retreat of glaciers, and an increase in drought, heatwave intensity, and frequency, leading to the increasing frequency of topsoil conditions favorable for dust emission, which increases the probability of dust storms. Our study provides a step forward to improve the representation of HLD in models and to monitor, quantify, and assess the environmental and climate significance of HLD

Newly identified climatically and environmentally significant high-latitude dust sources

Peer reviewe

Data for 'Aircraft ice-nucleating particle and aerosol composition measurements in the western North American Arctic'

Dataset associated to the publication "Aircraft ice-nucleating particle and aerosol composition measurements in the western North American Arctic". The dataset contains the sampling locations, INP concentrations (both concentrations and freezing temperatures obtained in the experiments) and SEM-EDS data (size distributions from both SEM and optical probes, as well as EDS size-resolved composition fractions).shown in the publication. It additionally contains the HYSPLIT backtrajectories shown in the SM

Dataset associated with 'Contributions of biogenic material to the atmospheric ice-nucleating particle population in North Western Europe'

Results from the study used to create figures in pape

Iceland is an episodic source of atmospheric ice-nucleating particles relevant for mixed-phase clouds

Ice-nucleating particles (INPs) have the potential to remove much of the liquid water in climatically important mid- to high-latitude shallow supercooled clouds, markedly reducing their albedo. The INP sources at these latitudes are very poorly defined, but it is known that there are substantial dust sources across the high latitudes, such as Iceland. Here, we show that Icelandic dust emissions are sporadically an important source of INPs at mid to high latitudes by combining ice-nucleating active site density measurements of aircraft-collected Icelandic dust samples with a global aerosol model. Because Iceland is only one of many high-latitude dust sources, we anticipate that the combined effect of all these sources may strongly contribute to the INP population in the mid- and high-latitude northern hemisphere. This is important because these emissions are directly relevant for the cloud-phase climate feedback and because high-latitude dust emissions are expected to increase in a warmer climate.ISSN:2375-254

A major combustion aerosol event had a negligible impact on the atmospheric ice-nucleating particle population

Clouds containing supercooled water are important for both climate and weather, but our knowledge of which aerosol particle types nucleate ice in these clouds is far from complete. Combustion aerosols have strong anthropogenic sources and if these aerosol types were to nucleate ice in clouds they might exert a climate forcing. Here, we quantified the atmospheric ice-nucleating particle (INP) concentrations during the UK’s annual Bonfire Night celebrations, which are characterised by strong anthropogenic emissions of combustion aerosol. We used three immersion mode techniques covering more than six orders of magnitude in INP concentration over the temperature range from −10 °C to homogeneous freezing. We found no observable systematic change in the INP concentration on three separate nights, despite more than a factor of 10 increase in aerosol number concentrations, up to a factor of 10 increase in PM10 concentration and more than a factor of 100 increase in black carbon (BC) mass concentration relative to pre-event levels. This implies that BC and other combustion aerosol such as ash did not compete with the INPs present in the background air. Furthermore, the upper limit of the ice-active site surface density, ns(T), of BC was shown to be consistent with several other recent laboratory studies, showing a very low ice-nucleating activity of BC. We conclude that combustion aerosol particles similar to those emitted on Bonfire Night are at most of secondary importance for the INP population relevant for mixed-phase clouds in typical mid-latitude terrestrial locations