Mineral dust particles are thought to be an important type of
ice-nucleating particle (INP) in the mixed-phase 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.
Melting of the Greenland Ice Sheet is a leading cause of land-ice mass loss and cryosphere-attributed sea level rise. Blooms of pigmented glacier ice algae lower ice albedo and accelerate surface ...melting in the ice sheet's southwest sector. Although glacier ice algae cause up to 13% of the surface melting in this region, the controls on bloom development remain poorly understood. Here we show a direct link between mineral phosphorus in surface ice and glacier ice algae biomass through the quantification of solid and fluid phase phosphorus reservoirs in surface habitats across the southwest ablation zone of the ice sheet. We demonstrate that nutrients from mineral dust likely drive glacier ice algal growth, and thereby identify mineral dust as a secondary control on ice sheet melting.
Mineral dust is an important component of the climate system, affecting the
radiation balance, cloud properties, biogeochemical cycles, regional
circulation and precipitation, as well as having ...negative effects on
aviation, solar energy generation and human health. Dust size and composition
has an impact on all these processes. However, changes in dust size
distribution and composition during transport, particularly for coarse
particles, are poorly understood and poorly represented in climate models.
Here we present new in situ airborne observations of dust in the Saharan Air
Layer (SAL) and the marine boundary layer (MBL) at the beginning of its
transatlantic transport pathway, from the AERosol
Properties – Dust (AER-D) fieldwork in August 2015,
within the peak season of North African dust export. This study focuses on
coarse-mode dust properties, including size distribution, mass loading,
shape, composition, refractive indices and optical properties. Size
distributions from 0.1 to 100 µm diameter (d) are presented, fully
incorporating the coarse and giant modes of dust. Within the MBL, mean
effective diameter (deff) and volume median diameter (VMD) were 4.6 and 6.0 µm respectively, giant particles with a mode at
20–30 µm were observed, and composition was dominated by quartz and
alumino-silicates at d > 1 µm. Within the SAL, particles
larger than 20 µm diameter were always present up to 5 km altitude, in
concentrations over 10−5 cm−3, constituting up to 40 % of
total dust mass. Mean deff and VMD were 4.0 and 5.5 µm
respectively. Larger particles were detected in the SAL than can be explained
by sedimentation theory alone. Coarse-mode composition was dominated by
quartz and alumino-silicates; the accumulation mode showed a strong
contribution from sulfate-rich and sea salt particles. In the SAL, measured
single scattering albedos (SSAs) at 550 nm representing d < 2.5 µm were
0.93 to 0.98 (mean 0.97). Optical properties calculated for the full
size distribution (0.1 < d < 100 µm) resulted in lower
SSAs of 0.91–0.98 (mean 0.95) and mass extinction coefficients of 0.27–0.35 m2 g−1 (mean 0.32 m2 g−1). Variability in SSA was mainly
controlled by variability in dust composition (principally iron) rather than
by variations in the size distribution, in contrast with previous observations
over the Sahara where size is the dominant influence. It is important that
models are able to capture the variability and evolution of both dust
composition and size distribution with transport in order to accurately
represent the impacts of dust on climate. These results provide a new SAL
dust dataset, fully representing coarse and giant particles, to aid model
validation and development.
Increasing tropospheric ozone levels over the past 150 years have led to a significant climate perturbation; the prediction of future trends in tropospheric ozone will require a full understanding of ...both its precursor emissions and its destruction processes. A large proportion of tropospheric ozone loss occurs in the tropical marine boundary layer and is thought to be driven primarily by high ozone photolysis rates in the presence of high concentrations of water vapour. A further reduction in the tropospheric ozone burden through bromine and iodine emitted from open-ocean marine sources has been postulated by numerical models, but thus far has not been verified by observations. Here we report eight months of spectroscopic measurements at the Cape Verde Observatory indicative of the ubiquitous daytime presence of bromine monoxide and iodine monoxide in the tropical marine boundary layer. A year-round data set of co-located in situ surface trace gas measurements made in conjunction with low-level aircraft observations shows that the mean daily observed ozone loss is ∼50 per cent greater than that simulated by a global chemistry model using a classical photochemistry scheme that excludes halogen chemistry. We perform box model calculations that indicate that the observed halogen concentrations induce the extra ozone loss required for the models to match observations. Our results show that halogen chemistry has a significant and extensive influence on photochemical ozone loss in the tropical Atlantic Ocean boundary layer. The omission of halogen sources and their chemistry in atmospheric models may lead to significant errors in calculations of global ozone budgets, tropospheric oxidizing capacity and methane oxidation rates, both historically and in the future.
Air quality is a major public health threat linked to poor birth outcomes, respiratory and cardiovascular disease, and premature mortality. Deprived groups and children are disproportionately ...affected. Bradford will implement a Clean Air Zone (CAZ) as part of the Bradford Clean Air Plan (B-CAP) in 2022 to reduce pollution, providing a natural experiment. The aim of the current study is to evaluate the impact of the B-CAP on health outcomes and air quality, inequalities and explore value for money. An embedded process and implementation evaluation will also explore barriers and facilitators to implementation, impact on attitudes and behaviours, and any adverse consequences.
The study is split into 4 work packages (WP). WP1A: 20 interviews with decision makers, 20 interviews with key stakeholders; 10 public focus groups and documentary analysis of key reports will assess implementation barriers, acceptability and adverse or unanticipated consequences at 1 year post-implementation (defined as point at which charging CAZ goes 'live'). WP1B: A population survey (n = 2000) will assess travel behaviour and attitudes at baseline and change at 1 year post-implementation). WP2: Routine air quality measurements will be supplemented with data from mobile pollution sensors in 12 schools collected by N = 240 pupil citizen scientists (4 within, 4 bordering and 4 distal to CAZ boundary). Pupils will carry sensors over four monitoring periods over a 12 month period (two pre, and two post-implementation). We will explore whether reductions in pollution vary by CAZ proximity. WP3A: We will conduct a quasi-experimental interrupted time series analysis using a longitudinal routine health dataset of > 530,000 Bradford residents comparing trends (3 years prior vs 3 years post) in respiratory health (assessed via emergency/GP attendances. WP3B: We will use the richly-characterised Born in Bradford cohort (13,500 children) to explore health inequalities in respiratory health using detailed socio-economic data. WP4: will entail a multi-sectoral health economic evaluation to determine value for money of the B-CAP.
This will be first comprehensive quasi-experimental evaluation of a city-wide policy intervention to improve air quality. The findings will be of value for other areas implementing this type of approach.
ISRCTN67530835 https://doi.org/10.1186/ISRCTN67530835.
Knowledge of the temperature-dependent concentration of ice-nucleating particles (INPs) is crucial to understanding the properties of mixed-phase clouds. However, the sources, transport and removal ...of INPs around the globe, and particularly in the Arctic region, are poorly understood. In the Arctic winter and spring, when many local sources are covered by ice and snow, it is not clear which INP types are important. In this study, we present a new dataset of aircraft-based immersion mode INP measurements and aerosol size-resolved composition in the western North American Arctic from 11 to 21 March 2018. Aerosol samples were collected between ∼ 70 and 600 m above the surface on filters that were analysed using both a freezing droplet-based assay and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). The measured INP concentrations were at or close to the limit of detection, with concentrations at −20 ∘C of 1 L−1 or below. The size-resolved composition measurements indicates that the aerosol concentrations were low, dominated mostly by sea spray aerosol and mineral dust. Further analysis shows that mineral dust is important for the ice-nucleating properties of our samples, dominating over the sea spray aerosol particles in the four cases we analysed, suggesting that mineral dust is a relevant type of INP in the Alaskan springtime Arctic. Furthermore, the INP concentrations are more consistent with fertile soil dusts that have an ice-active biological component than what would be expected for the ice-active mineral K-feldspar alone. While we cannot rule out local high-latitude sources of dust, the relatively small size of the mineral dust implies that the dust was from distant sources.
The flux of bioavailable Fe from mineral dust to the surface ocean is controlled not only by the processes in the atmosphere but also by the nature and source of the dust. In this study, we ...investigated how the nature of Fe minerals in the dust affects its potential Fe solubility (Fepsol) employing traditional and modern geochemical, mineralogical, and microscopic techniques. The chemical and mineralogical compositions, particularly Fe mineralogy, in soil samples as dust precursors collected from North African dust source regions were determined. The Fepsol was measured after 3 days of contact with sulfuric acid at pH 2 to simulate acid processes in the atmosphere. Fepsol of the soil dust samples were compared with calculated predictions of Fepsol based on the amount of individual Fe‐bearing minerals present in the samples and Fe solubilities of corresponding standard minerals. The calculated Fepsol deviated significantly from the measured Fepsol of the soil dust samples. We attributed this to the variability in properties of Fe minerals (e.g., size of Fe oxides and heterogeneity of chemical compositions of clay minerals) in soil dusts in comparison to the standard minerals. There were, however, clear relationships between the degree of chemical weathering of North African soils and Fepsol. The Parker index and ratio of ascorbate plus dithionite Fe to total Fe ((FeA+FeD)/FeT) are positively and negatively correlated with Fepsol, respectively. In addition, the ratio of FeA/(FeA+FeD), which decreases with aging of the Fe oxides, was found to be positively correlated with Fepsol in the soil dusts. Overall, our results indicate that there is a significant regional variability in the chemical and Fe mineralogical compositions of dusts across North African sources, as a result of the differences in chemical weathering and aging of Fe oxides. Furthermore, the indices for these weathering processes can provide an estimate of the fraction of Fe which can be solubilized if acid processed in the atmosphere.
Melting of the Greenland Ice Sheet (GrIS) is the largest single contributor to eustatic sea level and is amplified by the growth of pigmented algae on the ice surface, which increases solar radiation ...absorption. This biological albedo-reducing effect and its impact upon sea level rise has not previously been quantified. Here, we combine field spectroscopy with a radiative-transfer model, supervised classification of unmanned aerial vehicle (UAV) and satellite remote-sensing data, and runoff modelling to calculate biologically driven ice surface ablation. We demonstrate that algal growth led to an additional 4.4–6.0 Gt of runoff from bare ice in the south-western sector of the GrIS in summer 2017, representing 10 %–13 % of the total. In localized patches with high biomass accumulation, algae accelerated melting by up to 26.15±3.77 % (standard error, SE). The year 2017 was a high-albedo year, so we also extended our analysis to the particularly low-albedo 2016 melt season. The runoff from the south-western bare-ice zone attributed to algae was much higher in 2016 at 8.8–12.2 Gt, although the proportion of the total runoff contributed by algae was similar at 9 %–13 %. Across a 10 000 km2 area around our field site, algae covered similar proportions of the exposed bare ice zone in both years (57.99 % in 2016 and 58.89 % in 2017), but more of the algal ice was classed as “high biomass” in 2016 (8.35 %) than 2017 (2.54 %). This interannual comparison demonstrates a positive feedback where more widespread, higher-biomass algal blooms are expected to form in high-melt years where the winter snowpack retreats further and earlier, providing a larger area for bloom development and also enhancing the provision of nutrients and liquid water liberated from melting ice. Our analysis confirms the importance of this biological albedo feedback and that its omission from predictive models leads to the systematic underestimation of Greenland's future sea level contribution, especially because both the bare-ice zones available for algal colonization and the length of the biological growth season are set to expand in the future.
In July 2017 three research flights circumnavigating the megacity of London were conducted as a part of the STANCO training school for students and early career researchers organised by EUFAR ...(European Facility for Airborne Research). Measurements were made from the UK's Facility for Airborne Atmospheric Measurements (FAAM) BAe-146-301 atmospheric research aircraft with the aim to sample, characterise and quantify the impact of megacity outflow pollution on air quality in the surrounding region. Conditions were extremely favourable for airborne measurements, and all three flights were able to observe clear pollution events along the flight path. A small change in wind direction provided sufficiently different air mass origins over the 2 d such that a distinct pollution plume from London, attributable marine emissions and a double-peaked dispersed area of pollution resulting from a combination of local and transported emissions were measured. We were able to analyse the effect of London emissions on air quality in the wider region and the extent to which local sources contribute to pollution events.
Low concentrations of ice-nucleating particles (INPs) are thought to be
important for the properties of mixed-phase clouds, but their detection is
challenging. Hence, there is a need for instruments ...where INP concentrations
of less than 0.01 L−1 can be routinely and efficiently determined. The
use of larger volumes of suspension in drop assays increases the sensitivity
of an experiment to rarer INPs or rarer active sites due to the increase in
aerosol or surface area of particulates per droplet. Here we describe and
characterise the InfraRed-Nucleation by Immersed Particles Instrument
(IR-NIPI), a new immersion freezing assay that makes use of IR emissions to
determine the freezing temperature of individual 50 µL droplets each
contained in a well of a 96-well plate. Using an IR camera allows the
temperature of individual aliquots to be monitored. Freezing temperatures are
determined by detecting the sharp rise in well temperature associated with
the release of heat caused by freezing. In this paper we first present the
calibration of the IR temperature measurement, which makes use of the fact
that following ice nucleation aliquots of water warm to the ice–liquid
equilibrium temperature (i.e. 0 ∘C when water activity is
∼1), which provides a point of calibration for each individual
well in each experiment. We then tested the temperature calibration using
∼100 µm chips of K-feldspar, by immersing these chips
in 1 µL droplets on an established cold stage (µL-NIPI) as well
as in 50 µL droplets on IR-NIPI; the results were consistent with one
another, indicating no bias in the reported freezing temperature. In addition
we present measurements of the efficiency of the mineral dust NX-illite and a
sample of atmospheric aerosol collected on a filter in the city of Leeds.
NX-illite results are consistent with literature data, and the atmospheric INP
concentrations were in good agreement with the results from the µL-NIPI instrument. This demonstrates the utility of this approach, which
offers a relatively high throughput of sample analysis and access to low INP
concentrations.
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