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This study investigates the chemical composition of PM2.5 collected at a central location in Beijing, China, during winter 2016 and summer 2017. The samples were characterised using direct-infusion negative-nano-electrospray-ionisation ultrahigh-resolution mass spectrometry to elucidate the composition and the potential primary and secondary sources of the organic fraction. The samples from the two seasons were compared with those from a road-tunnel site and an urban background site in Birmingham, UK, analysed in the course of an earlier study using the same method. There were strong differences in aerosol particle composition between the seasons, particularly regarding (poly-)aromatic compounds, which were strongly enhanced in winter, likely due to increased fossil fuel and biomass burning for heating. In addition to the seasonal differences, compositional differences between high- and low-pollution conditions were observed, with the contribution of sulfur-containing organic compounds strongly enhanced under high-pollution conditions. There was a correlation of the number of sulfur-containing molecular formulae with the concentration of particulate sulfate, consistent with a particle-phase formation process.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous pollutants in air, soil, and water and are known to have harmful effects on human health and the environment. The diurnal and nocturnal variations of 17 PAHs in ambient particle-bound PAHs were measured in urban Beijing (China) and Delhi (India) during the summer season using gas-chromatography–quadrupole time-of-flight mass spectrometry (GC-Q-TOF-MS). The mean concentration of particles less than 2.5 µm (PM2.5) observed in Delhi was 3.6 times higher than in Beijing during the measurement period in both the daytime and night-time. In Beijing, the mean concentration of the sum of the 17 PAHs (∑17 PAHs) was 8.2 ± 5.1 ng m−3 in daytime, with the highest contribution from indeno[1,2,3-cd]pyrene (12 %), while at night-time the total PAHs was 7.2 ± 2.0 ng m−3, with the largest contribution from benzo[b]fluoranthene (14 %). In Delhi, the mean ∑17 PAHs was 13.6 ± 5.9 ng m−3 in daytime and 22.7 ± 9.4 ng m−3 at night-time, with the largest contribution from indeno[1,2,3-cd]pyrene in both the day (17 %) and night (20 %). Elevated mean concentrations of total PAHs in Delhi observed at night were attributed to emissions from vehicles and biomass burning and to meteorological conditions leading to their accumulation from a stable and low atmospheric boundary layer. Local emission sources were typically identified as the major contributors to total measured PAHs in both cities. Major emission sources were characterized based on the contribution from each class of PAHs, with the four-, five- and six-ring PAHs accounting ∼ 95 % of the total PM2.5-bound PAHs mass in both locations. The high contribution of five-ring PAHs to total PAH concentration in summer Beijing and Delhi suggests a high contribution from petroleum combustion. In Delhi, a high contribution from six-ring PAHs was observed at night, suggesting a potential emission source from the combustion of fuel and oil in power generators, widely used in Delhi. The lifetime excess lung cancer risk (LECR) was calculated for Beijing and Delhi, with the highest estimated risk attributed to Delhi (LECR = 155 per million people), which is 2.2 times higher than the Beijing risk assessment value (LECR = 70 per million people). Finally, we have assessed the emission control policies in each city and identified those major sectors that could be subject to mitigation measures.

The “Carnegie curve” describes the diurnal variation of the global atmospheric electric circuit. It was originally found from atmospheric electric potential gradient (PG) measurements made on the Carnegie, effectively a floating atmospheric electrical observatory, which undertook global cruises between 1915 and 1929. These measurements confirmed that the single diurnal cycle PG variation, previously obtained in both polar regions, was global in extent. The averaged diurnal PG variation, represented by derived harmonic fits, provides a characteristic variation known as the “Carnegie curve”, against which modern measurements are still compared. The ocean air PG measurements were extensively described in reports of the Carnegie Institution of Washington (CIW) but widely used secondary sources of the Carnegie curve contain small differences, arising through approximations and transcription errors. Investigations using the historical CIW data show that the original harmonic fit coefficients are reproducible. Despite the inconsistencies, the secondary sources nevertheless mostly yield diurnal variations which fall within the variability of the original historical data.

Ionisation in planetary atmospheres resulting from cosmic rays fragments atmospheric molecules resulting in the formation of free ions. The rate at which ions are produced varies with altitude and is determined by a combination of the cosmic ray flux and atmospheric density. The altitude at which this ion production rate peaks is known as the Pfotzer-Regener maximum which, on Earth, occurs at around 15-20 km. On Venus this maximum occurs at ~63 km, coinciding with the main cloud deck. This study investigates the effects enhanced ionisation may have on cloud droplets and their behaviour. Interactions between the ions produced and cloud droplets may have many consequences, including activation at lower saturation ratios, enhanced droplet coalescence and, for large charges, droplet breakup by Rayleigh instability.This work explores the effects of ionisation on water droplets in the laboratory and also simulates some of the conditions occurring in the clouds of Venus. The main element of the experimental apparatus is an acoustic levitator that can allow individual droplets to be electrically isolated and observed. Measurements are taken by a CCD camera and processed using LabView image acquisition software. The droplets can be subjected to enhanced ionisation from a corona source and perturbed by using a 10 kV/m electric field placed across the droplet causing it to be deflected relative to its charge. The principal findings on water droplets were that higher charge led to a slower evaporation rate; however, higher charge also led to increased incidence of Rayleigh explosions which were observed during several of the experiments. Overall, the effect of charge slowing evaporation did not lead to a longer droplet lifetime due to mass loss occurring from the periodic Rayleigh instabilities. In order to simulate conditions more like the clouds of Venus, sulphuric acid droplets were also examined. It was found that even very dilute sulphuric acid was extremely resistant to evaporation, suggesting that the clouds of Venus may have very long-lived droplet lifetimes. This has wide-reaching implications as cloud droplets on Venus have been suggested to act as a substrate for possible microbial life in the clouds.

We present a pioneering report of the meteorological effects of a partial solar eclipse across a wide region. Using large networks of official weather stations and a regional climate model simulation, we detected meteorological effects of the 10 June 2021 eclipse in the UK, Iceland and Greenland. A meteorological model was used to show that the eclipse decreased the daily Greenland Ice Sheet surface melt by ~10%. Solar photovoltaic and wind renewable energy production across the UK were also affected.

Funder: Cambridge Trust (Cambridge Commonwealth, European & International Trust); Id: http://dx.doi.org/10.13039/501100003343, We report the characterization of anthropogenic magnetic particulate matter (MPM) collected on leaves from roadside Callistemon (bottlebrush) trees from Lahore, Pakistan, and on known sources of traffic‐related particulates to assess the potential of first‐order reversal curve (FORC) diagrams to discriminate between different sources of anthropogenic magnetic particles. Magnetic measurements on leaves indicate the presence of surface‐oxidized magnetite spanning the superparamagnetic (

In atmospheric science, measurements above the surface have long been obtained by carrying instrument packages, radiosondes, aloft using balloons. Whilst occasionally used for research, most radiosondes – around one thousand are released daily – only generate data for routine weather forecasting. If meteorological radiosondes are modified to carry additional sensors, of either mass-produced commercial heritage or designed for a specific scientific application, a wide range of new measurements becomes possible. Development of add-on devices for standard radiosondes, whilst retaining the core meteorological use, is described here. Combining diverse sensors on a single radiosonde helps interpretation of findings, and yields economy of equipment, consumables and effort. A self-configuring system has been developed to allow different sensors to be easily combined, enhancing existing weather balloons and providing an emergency monitoring capability for airborne hazards. This research programme was originally pursued to investigate electrical properties of extensive layer clouds, and has expanded to include a wide range of balloon-carried sensors for solar radiation, cloud, turbulence, volcanic ash, radioactivity and space weather. For the layer cloud charge application, multiple soundings in both hemispheres have established that charging of extensive layer clouds is widespread, and likely to be a global phenomenon. This paper summarises the Christiaan Huygens medal lecture given at the 2021 European Geoscience Union meeting.

The electrostatic force between conductive spheres is always attractive at small separations, irrespective of their mean charge, when the charge on the spheres is constant. In many situations, the charge may not be fixed, such as for water drops in the natural atmosphere which vary in size and charge. We show that the attractive force between charged conductive spheres increases with increasing charge variance. The importance of this unrecognized electrostatic effect between water drops is evaluated for its potential to enhance rain formation.

Understanding rainfall in arid and water-scarce regions is central to the efficient use of water resources in agriculture, irrigation, and domestic food security. This work presents a new dataset with which to study precipitation processes in arid regions, utilising two years (2018–2020) of ceilometer observations made at Al Ain International Airport in the desert region of Al Ain, United Arab Emirates (UAE), where the annual rainfall is 76 mm. Ceilometer data provide a novel method by which to study both the evolution of water droplets from the cloud base down to the surface and the local circumstances required for rain to successfully reach the surface. In this work, we explore how successful precipitation depends on the initial size of the droplets and the thermodynamic profile below the cloud. For 64 of the 105 rain events, the droplet diameters ranged from 0.60 to 3.75 mm, with a mean of 1.84 mm. We find that smaller droplets, higher cloud bases, reduced cloud depths, and colder cloud bases all act to prevent successful precipitation, instead yielding virga (28 out of the 105 rain generating events). We identify how these multiple regional factors combine—specifically, we identify clouds deeper than 2.9 km, droplet diameters greater than 2 mm, and a midpoint below-cloud RH profile greater than 50%—to give successful rainfall, which may ultimately lead to more efficient rainfall enhancing measures, such as cloud seeding.

How can we compare the temperature of one summer’s day to another, and how much can we trust the measurements of record air temperatures such as those of July 2019? The basics of air temperature measurement are simple enough – put a thermometer in the shade and keep air moving past it. However, the flurry around summer temperature records suggests that the details of how to do this aren’t so widely appreciated. For example, how many times have you heard a radio phone-in programme asking listeners for car or garden temperature readings to compare, or a tennis commentator mentioning the temperature on centre court at Wimbledon? For a thermometer anywhere in direct sunlight, sheltered from the wind, its temperature is just that of a hot thing in the Sun. It’s highly unlikely to be a reliable air temperature. Only by using well calibrated sensors with standardised exposures can we expect measured temperatures to be both representative and consistent, spatially and temporally.

A tethered-balloon system (TBS) has been developed and is being operated by Sandia National Laboratories (SNL) on behalf of the U.S. Department of Energy's (DOE) Atmospheric Radiation Measurement (ARM) User Facility in order to collect in situ atmospheric measurements within mixed-phase Arctic clouds. Periodic tethered-balloon flights have been conducted since 2015 within restricted airspace at ARM's Advanced Mobile Facility 3 (AMF3) in Oliktok Point, Alaska, as part of the AALCO (Aerial Assessment of Liquid in Clouds at Oliktok), ERASMUS (Evaluation of Routine Atmospheric Sounding Measurements using Unmanned Systems), and POPEYE (Profiling at Oliktok Point to Enhance YOPP Experiments) field campaigns. The tethered-balloon system uses helium-filled 34 m3 helikites and 79 and 104 m3 aerostats to suspend instrumentation that is used to measure aerosol particle size distributions, temperature, horizontal wind, pressure, relative humidity, turbulence, and cloud particle properties and to calibrate ground-based remote sensing instruments. Supercooled liquid water content (SLWC) sondes using the vibrating-wire principle, developed by Anasphere Inc., were operated at Oliktok Point at multiple altitudes on the TBS within mixed-phase clouds for over 200 h. Sonde-collected SLWC data were compared with liquid water content derived from a microwave radiometer, Ka-band ARM zenith radar, and ceilometer at the AMF3, as well as liquid water content derived from AMF3 radiosonde flights. The in situ data collected by the Anasphere sensors were also compared with data collected simultaneously by an alternative SLWC sensor developed at the University of Reading, UK; both vibrating-wire instruments were typically observed to shed their ice quickly upon exiting the cloud or reaching maximum ice loading. Temperature sensing measurements distributed with fiber optic tethered balloons were also compared with AMF3 radiosonde temperature measurements. Combined, the results indicate that TBS-distributed temperature sensing and supercooled liquid water measurements are in reasonably good agreement with remote sensing and radiosonde-based measurements of both properties. From these measurements and sensor evaluations, tethered-balloon flights are shown to offer an effective method of collecting data to inform and constrain numerical models, calibrate and validate remote sensing instruments, and characterize the flight environment of unmanned aircraft, circumventing the difficulties of in-cloud unmanned aircraft flights such as limited flight time and in-flight icing.

Charging of upper and lower horizontal boundaries of extensive layer clouds results from current flow in the global electric circuit. Layer-cloud charge accumulation has previously been considered a solely electrostatic phenomenon, but it does not occur in isolation from meteorological processes, which can transport charge. Thin layer clouds provide special circumstances for investigating this dynamical charge transport, as disruption at the cloud-top may reach the cloud base, observable from the surface. Here, a thin (~300 m) persistent layer-cloud with base at 300 m and strong wind shear at cloud-top was observed to generate strongly correlated fluctuations in cloud base height, optical thickness and surface electric Potential Gradient (PG) beneath. PG changes are identified to precede the cloud base fluctuations by 2 minutes, consistent with shear-induced cloud-top electrical changes followed by cloud base changes. These observations demonstrate, for the first time, dynamically driven modification of charge within a layer-cloud. Even in weakly charged layer-clouds, redistribution of charge will modify local electric fields within the cloud and the collisional behaviour of interacting charged cloud droplets. Local field intensification may also explain previously observed electrostatic discharges in warm clouds.

A 5-year Clean Air Action Plan was implemented in 2013 to reduce air pollutant emissions and improve ambient air quality in Beijing. Assessment of this action plan is an essential part of the decision-making process to review its efficacy and to develop new policies. Both statistical and chemical transport modelling have been previously applied to assess the efficacy of this action plan. However, inherent uncertainties in these methods mean that new and independent methods are required to support the assessment process. Here, we applied a machine-learning-based random forest technique to quantify the effectiveness of Beijing's action plan by decoupling the impact of meteorology on ambient air quality. Our results demonstrate that meteorological conditions have an important impact on the year-to-year variations in ambient air quality. Further analyses show that the PM2.5 mass concentration would have broken the target of the plan (2017 annual PM2.5 µg m−3) were it not for the meteorological conditions in winter 2017 favouring the dispersion of air pollutants. However, over the whole period (2013–2017), the primary emission controls required by the action plan have led to significant reductions in PM2.5, PM10, NO2, SO2, and CO from 2013 to 2017 of approximately 34 %, 24 %, 17 %, 68 %, and 33 %, respectively, after meteorological correction. The marked decrease in PM2.5 and SO2 is largely attributable to a reduction in coal combustion. Our results indicate that the action plan has been highly effective in reducing the primary pollution emissions and improving air quality in Beijing. The action plan offers a successful example for developing air quality policies in other regions of China and other developing countries.

Organic matter is a major component of PM2.5 in megacities. In order to understand the detailed characteristics of organic compounds (≥ C6) at a molecular level on non-haze and haze days, we determined more than 300 organic compounds in the PM2.5 from an urban area of Beijing collected in November–December 2016 using two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC × GC-ToF-MS). The identified organic compounds have been classified into groups, and quantitative methods were used to calculate their concentrations. Primary emission sources make significant contributions to the atmospheric organic compounds, and six groups (including n-alkanes, polycyclic aromatic hydrocarbons – PAHs, levoglucosan, branched alkanes, n-alkenes and alkyl-benzenes) account for 66 % of total identified organic compound mass. In addition, PAHs and oxygenated PAHs (O-PAHs) were abundant amongst the atmospheric organic compounds on both haze and non-haze days. The most abundant hydrocarbon groups were observed with a carbon atom range of C19–C28. In addition, the total concentration of unidentified compounds present in the chromatogram was estimated in the present study. The total identified compounds account for approximately 47 % of total organic compounds (≥ C6) in the chromatogram on both the non-haze and haze days. The total mass concentrations of organic compounds (≥ C6) in the chromatogram were 4.0 and 7.4 µg m−3 on the non-haze and haze days, respectively, accounting for 26.4 % and 18.5 % of organic matter, respectively, on those days estimated from the total organic carbon concentration. Ratios of individual compound concentrations between haze and non-haze days do not give a clear indication of the degree of oxidation, but the overall distribution of organic compounds in the chromatogram provides strong evidence that the organic aerosol is less GC volatile and hence more highly oxidized on haze days.

Beijing has suffered from heavy local emissions as well as regional transport of air pollutants, resulting in severe atmospheric fine-particle (PM2.5) pollution. This study developed a combined method to investigate source types of PM2.5 and its source regions during winter 2016 in Beijing, which include the receptor model (positive matrix factorization, PMF), footprint and an air quality model. The PMF model was performed with high-time-resolution measurements of trace elements, water soluble ions, organic carbon and elemental carbon using online instruments during the wintertime campaign of the Air Pollution and Human Health in a Chinese Megacity – Beijing (APHH-Beijing) program in 2016. Source types and their contributions estimated by PMF model using online measurements were linked with source regions identified by the footprint model, and the regional transport contribution was estimated by an air quality model (the Nested Air Quality Prediction Model System, NAQPMS) to analyze the specific sources and source regions during haze episodes. Our results show that secondary and biomass-burning sources were dominated by regional transport, while the coal combustion source increased with local contribution, suggesting that strict control strategies for local coal combustion in Beijing and a reduction of biomass-burning and gaseous precursor emissions in surrounding areas were essential to improve air quality in Beijing. The combination of PMF with footprint results revealed that secondary sources were mainly associated with southern footprints (53 %). The northern footprint was characterized by a high dust source contribution (11 %), while industrial sources increased with the eastern footprint (10 %). The results demonstrated the power of combining receptor model-based source apportionment with other models in understanding the formation of haze episodes and identifying specific sources from different source regions affecting air quality in Beijing.

Gravity waves are an important part of the momentum budget of the atmosphere. Despite this, parameterizations of gravity wave spectra in atmospheric models are poorly constrained. Gravity waves are formed by jet streams, flow over topography and convection, all of which produce pressure perturbations as they propagate over the Earth’s surface, detectable by microbarometer arrays used for sensing infrasound. In this study, observations of gravity waves between 2007 and 2011 at an infrasound station in the Ivory Coast, West Africa are combined with meteorological data to calculate parameters such as intrinsic phase speed and wavenumber. Through spectral analysis, the seasonal and daily variations in all gravity wave parameters are examined. The gravity wave back azimuth varies with the migration of the Inter-Tropical Convergence Zone, a region of intense convection, supporting previous studies. Daily variations in gravity wave arrivals at the station can be linked to two distinct convective cycles over the land and ocean. This was achieved by combining the gravity wave parameters with lightning strikes detected by the Met Office’s Arrival Time Difference lightning detection system. Noise generated by turbulence in the middle of the day was found to attenuate smaller pressure amplitude gravity waves, artificially amplifying the daily variations in some gravity wave parameters. Detection of daily and seasonal variations in gravity wave parameters has the potential be used to improve the representation of gravity wave spectra in atmospheric models.

Some air pollution datasets contain multiple variables with a range of measurement units, and combined analysis using positive matrix factorization (PMF) can be problematic but can offer benefits through the greater information content. In this work, a novel method is devised and the source apportionment of a mixed unit dataset (PM10 mass and number size distribution, NSD) is achieved using a novel two-step approach to PMF. In the first step the PM10 data are PMF-analysed using a source apportionment approach in order to provide a solution which best describes the environment and conditions considered. The time series G values (and errors) of the PM10 solution are then taken forward into the second step, where they are combined with the NSD data and analysed in a second PMF analysis. This results in NSD data associated with the apportioned PM10 factors. We exemplify this approach using data reported in the study of Beddows et al. (2015), producing one solution which unifies the two separate solutions for PM10 and NSD data datasets together. We also show how regression of the NSD size bins and the G time series can be used to elaborate the solution by identifying NSD factors (such as nucleation) not influencing the PM10 mass.

Three groups of aliphatic carbonyl compounds, the n-alkanals (C8–C20), n-alkan-2-ones (C8–C26), and n-alkan-3-ones (C8–C19), were measured in both particulate and vapour phases in air samples collected in London from January to April 2017. Four sites were sampled including two rooftop background sites, one ground-level urban background site, and a street canyon location on Marylebone Road in central London. The n-alkanals showed the highest concentrations, followed by the n-alkan-2-ones and the n-alkan-3-ones, the latter having appreciably lower concentrations. It seems likely that all compound groups have both primary and secondary sources and these are considered in light of published laboratory work on the oxidation products of high-molecular-weight n-alkanes. All compound groups show a relatively low correlation with black carbon and NOx in the background air of London, but in street canyon air heavily impacted by vehicle emissions, stronger correlations emerge, especially for the n-alkanals. It appears that vehicle exhaust is likely to be a major contributor for concentrations of the n-alkanals, whereas it is a much smaller contributor to the n-alkan-2-ones and n-alkan-3-ones. Other primary sources such as cooking or wood burning may be contributors for the ketones but were not directly evaluated. It seems likely that there is also a significant contribution from the photo-oxidation of n-alkanes and this would be consistent with the much higher abundance of n-alkan-2-ones relative to n-alkan-3-ones if the formation mechanism were through the oxidation of condensed-phase alkanes. Vapour–particle partitioning fitted the Pankow model well for the n-alkan-2-ones but less well for the other compound groups, although somewhat stronger relationships were seen at the Marylebone Road site than at the background sites. The former observation gives support to the n-alkane-2-ones being a predominantly secondary product, whereas primary sources of the other groups are more prominent.

We demonstrate a hermetically sealed packaging system for integrated photonic devices at cryogenic temperatures with plug-and-play functionality. This approach provides the ability to encapsulate a controlled amount of gas into the optical package allowing helium to be used as a heat-exchange gas to thermalize photonic devices, or condensed into a superfluid covering the device. This packaging system was tested using a silicon-on-insulator slot waveguide resonator which fills with superfluid $^4$He below the transition temperature. To optimize the fiber-to-chip optical integration 690 tests were performed by thermally cycling optical fibers bonded to various common photonic chip substrates (silicon, silicon oxide and HSQ) with a range of glues (NOA 61, NOA 68, NOA 88, NOA 86H and superglue). This showed that NOA 86H (a UV curing optical adhesive with a latent heat catalyst) provided the best performance under cryogenic conditions for all the substrates tested. The technique is relevant to superfluid optomechanics experiments, as well as quantum photonics and quantum optomechanics applications., Comment: Walter W. Wasserman and Raymond A. Harrison contributed equally to this work. 9 pages, 4 figures

Measurement and source apportionment of atmospheric pollutants are crucial for the assessment of air quality and the implementation of policies for their improvement. In most cases, such measurements use expensive regulatory-grade instruments, which makes it difficult to achieve wide spatial coverage. Low-cost sensors may provide a more affordable alternative, but their capability and reliability in separating distinct sources of particles have not been tested extensively yet. The present study examines the ability of a low-cost optical particle counter (OPC) to identify the sources of particles and conditions that affect particle concentrations at an urban background site in Birmingham, UK. To help evaluate the results, the same analysis is performed on data from a regulatory-grade instrument (SMPS, scanning mobility particle sizer) and compared to the outcomes from the OPC analysis. The analysis of the low-cost sensor data manages to separate periods and atmospheric conditions according to the level of pollution at the site. It also successfully identifies a number of sources for the observed particles, which were also identified using the regulatory-grade instruments. The low-cost sensor, due to the particle size range measured (0.35 to 40 µm), performed rather well in differentiating sources of particles with sizes greater than 1 µm, though its ability to distinguish their diurnal variation, as well as to separate sources of smaller particles, at the site was limited. The current level of source identification demonstrated makes the technique useful for background site studies, where larger particles with smaller temporal variations are of significant importance. This study highlights the current capability of low-cost sensors in source identification and differentiation using clustering approaches. Future directions towards particulate matter source apportionment using low-cost OPCs are highlighted.

Summary • The question ‘are fragmented wild populations of raspberry adaptively differentiated from each other and from cultivated forms of the same species?’ is addressed here. • Plants collected from the coast, where commercial raspberries are grown, northwards by 49 km to an altitude of 600 m in Tayside, Scotland, were cultured in two common environments. Twenty phenotypic traits were recorded over 2 yr, on vegetative primocanes and then the single dominant floricane retained for year 2. A novel approach is presented for selecting traits that best discriminate between individuals using principal coordinate analysis. Phenotypic variation among accessions was then quantified using principal coordinate analysis followed by principal component analysis. • A consistent north–south trend was found. Plants from northern sites were shorter, bushier with less lateral growth and fewer flowers per lateral on the dominant fruiting cane. Plants from southern sites produced few, tall primocanes with greater cane diameters, lateral growth and flowering. The results were consistent across test environments. • The results confirm substantial, adaptive differentiation between populations and suggest a limited effect of cultivation on wild forms.

The occurrence and characteristics of rainfall events in arid and water scarce regions are of great interest to many, as it is vital to understand the efficient use of this finite resource, for example in terms of water management, agriculture, irrigation, and domestic food security. Fundamental to this is understanding the numerous environmental aspects that affect the generation and persistence of rain. These include the presence of cloud droplets, activation and growth processes, temperature and relative humidity of the within and below cloud regions, and the cloud base height. Not only must what causes rainfall to be initiated be understood, but also the conditions that allow that rain to reach the surface. This work examines the conditions required for a successful rain event (i.e. one in which rainfall reaches the ground) to occur in the arid desert region of Al Ain, in the United Arab Emirates (UAE) (annual rainfall 76mm). The high surface temperatures and dry air mean that rain events at Al Ain commonly occur as virga, as the rain droplets cannot survive evaporation under the local atmospheric conditions. Here we examine individual rainfall events using backscatter data from a laser ceilometer, in conjunction with C-band radar data, to further understand the processes required for successful rain generation. During the 2 year period of study, there was a total of 57.5 hours of rain (rainfall 0.5% of the time), with a total of 105 rainfall events. Here we examine the effect on rainfall of (a) the initial size of the droplets falling from the cloud base, which must be large enough to survive the fall to the surface; and (b) the effect of the below cloud thermodynamic profile on the evaporation of the droplet (which also depends on the height of the cloud base). Preliminary conclusions find that smaller droplets, higher cloud bases, smaller cloud depths, and lower cloud base temperatures all favour the occurrence of a rain event terminating as virga before it reaches the surface. Understanding the details of why many potential rainfall events don’t reach the surface can ultimately lead to the more efficient implementation of rainfall enhancing measures such as cloud seeding.

Electric charge is always present in the lower atmosphere. If droplets or aerosols become charged, their behavior changes, influencing collision, evaporation, and deposition. Artificial charge release is an unexplored potential geoengineering technique for modifying fogs, clouds, and rainfall. Central to evaluating these processes experimentally in the atmosphere is establishing an effective method for charge delivery. A small charge-delivering remotely piloted aircraft has been specially developed for this, which is electrically propelled. It carries controllable bipolar charge emitters (nominal emission current ±5 μA) beneath each wing, with optical cloud and meteorological sensors integrated into the airframe. Meteorological and droplet measurements are demonstrated to 2 km altitude by comparison with a radiosonde, including within cloud, and successful charge emission aloft verified by using programmed flight paths above an upward-facing surface electric field mill. This technological approach is readily scalable to provide nonpolluting fleets of charge-releasing aircraft, identifying and targeting droplet regions with their own sensors. Beyond geoengineering, agricultural, and biological aerosol applications, safe ionic propulsion of future electric aircraft also requires detailed investigation of charge effects on natural atmospheric droplet systems.

New particle formation (NPF) events occur almost everywhere in the world and can play an important role as a particle source. The frequency and characteristics of NPF events vary spatially, and this variability is yet to be fully understood. In the present study, long-term particle size distribution datasets (minimum of 3 years) from 13 sites of various land uses and climates from across Europe were studied, and NPF events, deriving from secondary formation and not traffic-related nucleation, were extracted and analysed. The frequency of NPF events was consistently found to be higher at rural background sites, while the growth and formation rates of newly formed particles were higher at roadsides (though in many cases differences between the sites were small), underlining the importance of the abundance of condensable compounds of anthropogenic origin found there. The growth rate was higher in summer at all rural background sites studied. The urban background sites presented the highest uncertainty due to greater variability compared to the other two types of site. The origin of incoming air masses and the specific conditions associated with them greatly affect the characteristics of NPF events. In general, cleaner air masses present higher probability for NPF events, while the more polluted ones show higher growth rates. However, different patterns of NPF events were found, even at sites in close proximity (, This research has been supported by the National Centre for Atmospheric Science funded by the U.K. Natural Environment Research Council (R8/H12/83/011)., With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI).

We present an analysis of coronal mass ejections (CMEs) observed by the Heliospheric Imagers (HIs) onboard NASA’s Solar Terrestrial Relations Observatory (STEREO) spacecraft. Between August 2008 and April 2014 we identify 273 CMEs that are observed simultaneously, by the HIs on both spacecraft. For each CME, we track the observed leading edge, as a function of time, from both vantage points, and apply the Stereoscopic Self-Similar Expansion (SSSE) technique to infer their propagation throughout the inner heliosphere. The technique is unable to accurately locate CMEs when their observed leading edge passes between the spacecraft; however, we are able to successfully apply the technique to 151, most of which occur once the spacecraft-separation angle exceeds $180^{\circ }$ 180 ∘ , during solar maximum. We find that using a small half-width to fit the CME can result in inferred acceleration to unphysically high velocities and that using a larger half-width can fail to accurately locate the CMEs close to the Sun because the method does not account for CME over-expansion in this region. Observed velocities from SSSE are found to agree well with single-spacecraft (SSEF) analysis techniques applied to the same events. CME propagation directions derived from SSSE and SSEF analysis agree poorly because of known limitations present in the latter.

Establishing clear evidence of solar-induced lower atmosphere effects is hampered by the small 11-year solar cycle responses, typically swamped by meteorological variability. Strong 27-day cyclic changes are exploited here instead. During the 2007/8 minimum in solar activity, regular 27-day lighthouse-like sweeps of energetic particles crossed the heliosphere and Earth, followed by a burst of solar ultraviolet radiation. Averaging the atmospheric responses at UK sites reveals immediate cooling in the troposphere after the peak energetic particle flux, followed by warming in the stratosphere. Regionally, this is accompanied by zonal wind changes, and temperature changes beneath cloud at the same time. Of two possible rapid distinct routes of solar influence—photochemical (through ozone) and atmospheric electrical (through low level clouds)—the ozone route does not provide a phase-locked response but the electrical route is supported by observed phase-locked thickening of low level clouds. These findings have potential value to weather forecasting.

The scientific career of Michael Rycroft (born in 1938) spans the space age, during which significant changes have occurred in how scientists work, experiment, and interact. Here, as part of his 80th birthday celebrations, we review his career to date in terms of the social and structural changes in collaborative international science. His contributions to research, teaching, and management across solar–terrestrial and ionospheric physics as well as atmospheric and space science are also discussed.

Rainfall is hypothesized to be influenced by droplet charge, which is related to the global circuit current flowing through clouds. This is tested through examining a major global circuit current increase following the release of artificial radioactivity. Significant changes occurred in daily rainfall distribution in the Shetland Islands, away from pollution. Daily rainfall changed by 24%, and local clouds optically thickened, within the nuclear weapons test period. This supports expectations of electrically induced microphysical changes in liquid water clouds from additional ionization.

Particle number size distributions have been measured simultaneously by scanning mobility particle sizers (SMPSs) at five sites in central London for a 1 month campaign in January–February 2017. These measurements were accompanied by condensation particle counters (CPCs) to measure total particle number count at four of the sites and Aethalometers measuring black carbon (BC) at five sites. The spatial distribution and inter-relationships of the particle size distribution and SMPS total number counts with CPC total number counts and black carbon measurements have been analysed in detail as well as variations in the size distributions. One site (Marylebone Road) was in a street canyon with heavy traffic, one site (Westminster University) was on a rooftop adjacent to the Marylebone Road sampler, and a further sampler was located at Regent's University within a major park to the north of Marylebone Road. A fourth sampler was located nearby at 160 m above ground level on the BT tower and a fifth sampler was located 4 km to the west of the main sampling region at North Kensington. Consistent with earlier studies it was found that the mode in the size distribution had shifted to smaller sizes at the Regent's University (park) site, the mean particle shrinkage rate being 0.04 nm s−1 with slightly lower values at low wind speeds and some larger values at higher wind speeds. There was evidence of complete evaporation of the semi-volatile nucleation mode under certain conditions at the elevated BT Tower site. While the SMPS total count and black carbon showed typical traffic-dominated diurnal profiles, the CPC count data typically peaked during night-time as did CPC∕SMPS and CPC∕BC ratios. This is thought to be due to the presence of high concentrations of small particles (2.5–15 nm diameter) probably arising from condensational growth from traffic emissions during the cooler night-time conditions. Such behaviour was most marked at the Regent's University and Westminster University sites and less so at Marylebone Road, while at the elevated BT Tower site the ratio of particle number (CPC) to black carbon peaked during the morning rush hour and not at night-time, unlike the other sites. An elevation in nucleation mode particles associated with winds from the west and WSW sector was concluded to result from emissions from London Heathrow Airport, despite a distance of 22 km from the central London sites.

Understanding the evolution of the solar wind is fundamental to advancing our knowledge of energy and mass transport in the solar system, rendering it crucial to space weather and its prediction. The advent of truly wide-angle heliospheric imaging has revolutionised the study of Coronal Mass Ejections (CMEs) by enabling their direct and continuous observation out to 1 AU and beyond. A catalogue of CMEs has been compiled using data from the Heliospheric Imagers (HIs) on board the two STEREO spacecraft, which began as part of the FP7 HELCATS project. The mission was launched in 2006 and continues to provide data, therefore spanning 13 years, over which more than two-thousand CMEs have been observed using HI. To these CMEs, we apply geometric models that make use of both single-spacecraft and stereoscopic observations in order to determine their kinematic properties. These include CME speed, acceleration, propagation direction and launch time. The resulting kinematic properties and their statistics are discussed in the context of existing CME catalogues produced from coronagraph observations. This is done with emphasis on how the different models we apply influence our results and how these differences evolve over the solar cycle and as the angular separation of the STEREO spacecraft increases throughout the mission.

Cloud processes leading to rainfall generation are suspected to be influenced by droplet charge. Droplet charging is abundant, and even in layer clouds, charging of droplets readily occurs at the horizontal cloud-air boundary. Droplet charging in such circumstances is proportional to the vertical current driven through the cloud by the global electric circuit. Small global circuit variations from natural influences, such as solar modulation of cosmic rays can be used to investigate this, but an alternative is presented by artificial introduction of ionisation. The atmospheric nuclear weapons test period, which reached its peak 1962-1964, caused exceptional anthropogenic disturbance to the global circuit, through the increased ionisation from steady sedimentation of stratospheric radioactive debris.Measurements of the vertical current Jz made at Kew Observatory near London (51°28′N, 0°19′W) were several times greater than normal during 1962-1964, as a result of the widespread extra ionisation in the lower atmosphere. At Lerwick, Shetland (60°09′N, 1°08′W) where deposition of radioactive material occurred, the atmospheric electrical parameters were strongly affected by the enhanced ionisation. To investigate tropospheric ionisation effects on local cloud processes, rainfall days at Lerwick in 1962-64 have been analysed by considering reduced and enhanced ionisation periods. During the enhanced ionisation, the Lerwick rainfall distribution shifted towards heavier rainfall and is significantly different from the rainfall distribution for reduced ionisation days; the Lerwick cloud was also significantly optically thicker during the enhanced ionisation. This contrasts with other years of the Kew record, when Jz was relatively undisturbed. Whilst the ionisation conditions of 1962-64 were exceptional, controlled methods of enhancing tropospheric ionisation by non-radioactive means - such as corona emission - may nevertheless be promising for local rainfall modification, or even geoengineering of cloud properties.