Biological ice nucleating particles at tropospheric cloud height

Airborne ice nucleating particles (INPs) promote the freezing of cloud droplets, which is relevant for the radiative properties of clouds and for the development of precipitation. A quantitative assessment of the impact of INPs on cloud processes and on their responsiveness to climate and land use change is still missing. This is particularly true for INPs of biological origin. They are made of molecules produced by bacteria, fungi, plants, lichens, which promote the freezing of droplets at temperatures above -15 °C. Bottom-up modelling studies based on the release of ice nucleation active cells from crops and plants have excluded any chance for biological INPs to impact climate. Nevertheless, recent observations point at the ubiquity across ecosystems of species capable of producing INPs and at the fact that such INPs can be released from cells and maintain their activity for instance when linked to soil particles. Here we employed a top-down approach to improve our understanding of the variability of biological INPs in precipitation. 16 sampling campaign were organised between 2012 and 2014 and over 100 precipitation samples were collected at the High Altitude Research Station Jungfraujoch (3580 m a.s.l., Switzerland). They have been analysed for their content in INPs active at moderate supercooling directly in field with our new immersion freezing apparatus LINDA. Several environmental parameters have been studied to derive more information on the most relevant factors responsible for the variability of INPs. The abundance of bacterial cells and the presence of the nucleation active plant pathogen bacterium Pseudomonas syringae have been determined as well, to know more on the nature of biological INPs in precipitation. By means of stable water isotopes, we demonstrate that INPs are rapidly and selectively removed by precipitating clouds. Focusing on INPs active at -8 °C (INPs-8), their concentrations varied between 0.21 and 434 INPs-8 mL-1. Up to 75% of this large temporal variability can be modelled and predicted by multiple linear regression models based on the combination of a few environmental parameters. These models point at the interaction of “source” (uptake) and “sink” (removal) processes as relevant to determine the variability of INPs-8. Large abundance of INPs-8 can be best expected with a coincidence of high wind speed and little precipitation lost from an air mass prior to sampling. Bacterial cells present more constant concentrations than INPs, from 2.4·103 to 6.8·104 cells mL-1, with their numbers increasing mainly under high wind speed. INPs are more efficiently removed than bacterial cells by precipitation, which implies a larger variability, a shorter residence time in the atmosphere and shorter lengths of dispersal for INPs rather than for bacterial cells. P. syringae has been successfully isolated at high-altitude and its presence seems to be influenced by uptake and removal processes, as it happens for INPs-8. This study constitutes a strong improvement of our understanding of the abundance, variability and nature of biological INPs in precipitation and points at the potential for this group of INPs to impact cloud processes. In fact, a coincidence of high wind speed and first precipitation often occurs at the passage of a front, where the meteorological conditions are also favourable to precipitation. This can be the ideal and frequent context where to expect large numbers of INPs-8 and to study their effects on cloud processes. Furthermore, bacterial cells can contribute to the number of INPs-8, but a large fraction of INPs-8 is potentially due to cellular fragments and macromolecules, both freely floating and attached to mineral and soil dust. This multiplies the possibility for biological INPs to be released and be present in the atmosphere

Noble gas constraints on the fate of arsenic in groundwater

Groundwater contamination of geogenic arsenic (As) remains a global health threat, particularly in south-east Asia. The prominent correlation often observed between high As concentrations and methane (CH$_{4}$) stimulated the analysis of the gas dynamics in an As contaminated aquifer, whereby noble and reactive gases were analysed. Results show a progressive depletion of atmospheric gases (Ar, Kr and N$_{2}$) alongside highly increasing CH$_{4}$, implying that a free gas phase comprised mainly of CH$_{4}$ is formed within the aquifer. In contrast, Helium (He) concentrations are high within the CH$_{4}$ (gas) producing zone, suggesting longer (groundwater) residence times. We hypothesized that the observed free (CH$_{4}$) gas phase severely detracts local groundwater (flow) and significantly reduces water renewal within the gas producing zone. Results are in-line with this hypothesis, however, a second hypothesis has been developed, which focuses on the potential transport of He from an adjacent aquitard into the (CH$_{4}$) gas producing zone. This second hypothesis was formulated as it resolves the particularly high He concentrations observed, and since external solute input from the overlying heterogeneous aquitard cannot be excluded. The proposed feedback between the gas phase and hydraulics provides a plausible explanation of the anti-intuitive correlation between high As and CH$_{4}$, and the spatially highly patchy distribution of dissolved As concentrations in contaminated aquifers. Furthermore, the increased groundwater residence time would allow for the dissolution of more crystalline As-hosting iron(Fe)-oxide phases in conjunction with the formation of more stable secondary Fe minerals in the hydraulically-slowed (i.e., gas producing) zone; a subject which calls for further investigation

Carbon and methane cycling in arsenic-contaminated aquifers

Geogenic arsenic (As) contamination of groundwater is a health threat to millions of people worldwide, particularly in alluvial regions of South and Southeast Asia. Mitigation measures are often hindered by high heterogeneities in As concentrations, the cause(s) of which are elusive. Here we used a comprehensive suite of stable isotope analyses and hydrogeochemical parameters to shed light on the mechanisms in a typical high-As Holocene aquifer near Hanoi where groundwater is advected to a low-As Pleistocene aquifer. Carbon isotope signatures (δ$^{13}$C-CH$_{4}$, δ$^{13}$C-DOC, δ$^{13}$C-DIC) provided evidence that fermentation, methanogenesis and methanotrophy are actively contributing to the As heterogeneity. Methanogenesis occurred concurrently where As levels are high (>200 µg/L) and DOC-enriched aquitard pore water infiltrates into the aquifer. Along the flowpath to the Holocene/Pleistocene aquifer transition, methane oxidation causes a strong shift in δ$^{13}$C-CH$_{4}$ from -87‰ to +47‰, indicating high reactivity. These findings demonstrate a previously overlooked role of methane cycling and DOC infiltration in high-As aquifers

Spatial and temporal evolution of groundwater arsenic contamination in the Red River delta, Vietnam: Interplay of mobilisation and retardation processes

Geogenic arsenic (As) contamination of groundwater poses a major threat to global health, particularly in Asia. To mitigate this exposure, groundwater is increasingly extracted from low-As Pleistocene aquifers. This, however, disturbs groundwater flow and potentially draws high-As groundwater into low-As aquifers. Here we report a detailed characterisation of the Van Phuc aquifer in the Red River Delta region, Vietnam, where high-As groundwater from a Holocene aquifer is being drawn into a low-As Pleistocene aquifer. This study includes data from eight years (2010–2017) of groundwater observations to develop an understanding of the spatial and temporal evolution of the redox status and groundwater hydrochemistry. Arsenic concentrations were highly variable (0.5–510 μg/L) over spatial scales of <200 m. Five hydro(geo)chemical zones (indicated as A to E) were identified in the aquifer, each associated with specific As mobilisation and retardation processes. At the riverbank (zone A), As is mobilised from freshly deposited sediments where Fe(III)-reducing conditions occur. Arsenic is then transported across the Holocene aquifer (zone B), where the vertical intrusion of evaporative water, likely enriched in dissolved organic matter, promotes methanogenic conditions and further release of As (zone C). In the redox transition zone at the boundary of the two aquifers (zone D), groundwater arsenic concentrations decrease by sorption and incorporations onto Fe(II) carbonates and Fe(II)/Fe(III) (oxyhydr)oxides under reducing conditions. The sorption/incorporation of As onto Fe(III) minerals at the redox transition and in the Mn(IV)-reducing Pleistocene aquifer (zone E) has consistently kept As concentrations below 10 μg/L for the studied period of 2010–2017, and the location of the redox transition zone does not appear to have propagated significantly. Yet, the largest temporal hydrochemical changes were found in the Pleistocene aquifer caused by groundwater advection from the Holocene aquifer. This is critical and calls for detailed investigations

A comprehensive characterization of ice nucleation by three different types of cellulose particles immersed in water

We present the laboratory results of immersion freezing efficiencies of cellulose particles at supercooled temperature (T) conditions. Three types of chemically homogeneous cellulose samples are used as surrogates that represent supermicron and submicron ice-nucleating plant structural polymers. These samples include microcrystalline cellulose (MCC), fibrous cellulose (FC) and nanocrystalline cellulose (NCC). Our immersion freezing dataset includes data from various ice nucleation measurement techniques available at 17 different institutions, including nine dry dispersion and 11 aqueous suspension techniques. With a total of 20 methods, we performed systematic accuracy and precision analysis of measurements from all 20 measurement techniques by evaluating T-binned (1 ∘C) data over a wide T range (−36 ∘C <T<−4 ∘C). Specifically, we intercompared the geometric surface area-based ice nucleation active surface site (INAS) density data derived from our measurements as a function of T, ns,geo(T). Additionally, we also compared the ns,geo(T) values and the freezing spectral slope parameter (Δlog(ns,geo)/ΔT) from our measurements to previous literature results. Results show all three cellulose materials are reasonably ice active. The freezing efficiencies of NCC samples agree reasonably well, whereas the diversity for the other two samples spans ≈ 10 ∘C. Despite given uncertainties within each instrument technique, the overall trend of the ns,geo(T) spectrum traced by the T-binned average of measurements suggests that predominantly supermicron-sized cellulose particles (MCC and FC) generally act as more efficient ice-nucleating particles (INPs) than NCC with about 1 order of magnitude higher ns,geo(T)

The study of atmospheric ice-nucleating particles via microfluidically generated droplets

Ice-nucleating particles (INPs) play a significant role in the climate and hydrological cycle by triggering ice formation in supercooled clouds, thereby causing precipitation and affecting cloud lifetimes and their radiative properties. However, despite their importance, INP often comprise only 1 in 10³–10⁶ ambient particles, making it difficult to ascertain and predict their type, source, and concentration. The typical techniques for quantifying INP concentrations tend to be highly labour-intensive, suffer from poor time resolution, or are limited in sensitivity to low concentrations. Here, we present the application of microfluidic devices to the study of atmospheric INPs via the simple and rapid production of monodisperse droplets and their subsequent freezing on a cold stage. This device offers the potential for the testing of INP concentrations in aqueous samples with high sensitivity and high counting statistics. Various INPs were tested for validation of the platform, including mineral dust and biological species, with results compared to literature values. We also describe a methodology for sampling atmospheric aerosol in a manner that minimises sampling biases and which is compatible with the microfluidic device. We present results for INP concentrations in air sampled during two field campaigns: (1) from a rural location in the UK and (2) during the UK’s annual Bonfire Night festival. These initial results will provide a route for deployment of the microfluidic platform for the study and quantification of INPs in upcoming field campaigns around the globe, while providing a benchmark for future lab-on-a-chip-based INP studies

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

A minute fraction of atmospheric particles exert a disproportionate effect on the phase of mixed-phase clouds by acting as ice-nucleating particles (INPs). To understand the effects of these particles on weather and climate, both now and into the future, we must first develop a quantitative understanding of the major INP sources worldwide. Previous work has demonstrated that aerosols such as desert dusts are globally important INPs, but the role of biogenic INPs is unclear, with conflicting evidence for their importance. Here, we show that at a temperate site all INPs active above −18 °C at concentrations >0.1 L−1 are destroyed on heating, consistent with these INPs being of biological origin. Furthermore, we show that a global model of desert dust INPs dramatically underestimates the measured INP concentrations, but is consistent with the thermally-stable component. Notably, the heat sensitive INPs are active at temperatures where shallow cloud layers in Northern Europe are frequently observed to glaciate. Hence, we suggest that biogenic material is important for primary ice production in this region. The prevalence of heat sensitive, most likely biogenic, INPs in this region highlights that, as a community, we need to quantify the sources and transport of these particles as well as determine their atmospheric abundance across the globe and at cloud altitudes

Freezing nucleation apparatus puts new slant on study of biological ice nucleators in precipitation

For decades, drop-freezing instruments have contributed to a better understanding of biological ice nucleation and its likely implications for cloud and precipitation development. Yet, current instruments have limitations. Drops analysed on a cold stage are subject to evaporation and potential contamination. The use of closed tubes provides a partial solution to these problems, but freezing events are still difficult to be clearly detected. Here, we present a new apparatus where freezing in closed tubes is detected automatically by a change in light transmission upon ice development, caused by the formation of air bubbles and crystal facets that scatter light. Risks of contamination and introduction of biases linked to detecting the freezing temperature of a sample are then minimized. To illustrate the performance of the new apparatus we show initial results of two assays with snow samples. In one, we repeatedly analysed the sample (208 tubes) over the course of a month with storage at +4 °C, during which evidence for biological ice nucleation activity emerged through an increase in the number of ice nucleators active around −4 °C. In the second assay, we indicate the possibility of increasingly isolating a single ice nucleator from a precipitation sample, potentially determining the nature of a particle responsible for a nucleation activity measured directly in the sample. These two seminal approaches highlight the relevance of this handy apparatus for providing new points of view in biological ice nucleation research