Low-cost gas and particle sensors can enhance the spatial coverage of Air Quality (AQ) monitoring networks in urban settings. While their accuracy is insufficient to replace reference instruments, they may still capture spatial differences among different stations, as well as temporal trends and month-to-month variabilities at a specific location. To assess this, we conducted a 19-month study using two Vaisala AQ Transmitters-Monitors (Model AQT530), collocated with reference-grade instruments, at two AQ stations in Nicosia: an urban traffic and an urban background station. These two stations are ideal for the needs of this study considering that the reference measurements carried out there exhibit statistically significant spatial and temporal differences in pollutant concentrations when analysed over the entire period and on a monthly basis.

The AQT530 air quality monitor employs Low-Cost Sensors (LCSs) for gaseous pollutants (i.e., CO, NO2, NO and O3) and particulate matter (PM). Tests of the performance of the two AQT530 monitors during an initial period when those were collocated at the urban traffic station revealed high unit-to-unit agreements for the CO, NO and PM10, and good to moderate for the NO2, O3 and PM2.5 measurements. The CO and PM10 LCS measurements also effectively captured concentration differences between the two stations when averaged over the entire study period or monthly, with some exceptions for specific months. These LCSs successfully detected spatial concentration differences (i.e., monthly, daily and hourly) as long as those were above a certain threshold. Overall, the CO and PM sensors successfully tracked month-to-month trends over the entire study period, similarly to reference instruments, whereas NO2, NO, and O3 sensors struggled due to environmental sensitivities. Despite this, all sensors identified statistically significant month-to-month variations at the same station, with the PM2.5 measurements showing the strongest agreement with reference data.

Metal nitride and metal oxide nanoparticles (NPs) provide key material components for a number of applications due to their unique properties. Here we demonstrate that spark ablation of metallic electrodes, quenched with a pure N₂ flow at atmospheric pressure, can be used as a reactive generator to synthesize metal nitride, metal oxide or pure metallic NPs depending on the material. The composition of the synthesized NPs was determined through their crystal structure using X-ray diffraction and transmission electron microscopy (TEM). Our results show that the composition of the resulting NPs strongly depends on the electrode material: Ti and Al form mixtures of metal nitride and oxide NPs, whereas Mg and Pd produce respectively only oxide and pure metallic NPs. Repeated XRD measurements of the samples after exposing them to ambient air over periods of several months showed that the stability of TiN was higher compared to that of the AIN NPs, with the first being converted to TiN_yO_x and the latter to γ-Al₂O₃ after 9 months.

Cationic silver hydride clusters (Ag_nH⁺) can be formed by a number of physical and chemical processes, holding great promise for a range of applications including photonics, catalysis, sensing, and biomedicine, among others. Here, we present a comprehensive theoretical investigation of Ag_nH⁺ clusters (n = 1–7) using highly accurate coupled-cluster (CC) theory. Multiple low-lying isomers are identified using CC theory with single and double excitations (CCSD), whereas their relative stabilities are determined with the more accurate CCSD(T) method. The CCSD(T) results predict a pronounced odd–even alternation in relative stabilities, with Ag₂H⁺ being the most stable species, which is consistent with experimental mass spectrometry measurements. Ab initio molecular dynamics simulations show that all low-energy isomers remain structurally rigid at room temperature, whereas bonding analyses (frontier molecular orbitals, natural bond orbital, molecular electrostatic potential, quantum theory of atoms in molecules, non-covalent interaction) indicate strong ionic Ag–H interactions, weak non-covalent Ag–Ag interactions, and significant donor–acceptor stabilization in larger clusters. Electrical mobilities of these clusters, computed by the trajectory method, were labelled on experimental spectra, in order to contribute towards their interpretation. Overall, our results resolve inconsistencies from prior theoretical predictions, provide a rigorous description of cationic silver hydride clusters, and are used to improve the interpretation of earlier observations.

Optical Particle Sizers (OPSs) are widely used for measuring size distributions of particles larger than ca. 0.2 μm. To do so, they use mirrors or lenses to gather light scattered by particles passing through a focused beam, directing it to a photo-detector that produces electric pulses from the scattering events. Considering their ability to provide near real-time measurements with minimal attendance and maintenance, and to expand the networks of Particulate Matter (PM) monitoring, several manufacturers have developed low-cost and compact OPS systems. Despite that low-cost OPSs are already available in the market and employed for monitoring PM concentrations, their reported values typically deviate from those of high-end instruments, warranting further efforts to improve their performance. In this work, we designed and built a custom-made yet inexpensive OPS optical system, and studied its performance using a combination of computational and experimental methods at different flow conditions. Our results demonstrate the importance of the flow field within the OPS optical system, and how this can affect its counting and sizing ability. The overall performance of our OPS optical system is very similar to that of high-end instruments, exhibiting a counting efficiency of 50% for particles having a diameter of 320 nm, and a sizing resolution of below 15% for 500-nm particles, complying with the ISO 21501-1 and 21501-4 standards.

This article describes a single-step method for synthesizing nanostructured materials using evaporation-condensation synthesis and inertial impaction of aerosol nanoparticles. The as-deposited films exhibit anisotropic vertical and horizontal sintering of their palladium nanoparticle building blocks, yielding vertical structures. The electrical conductivity of the films is stable and highly sensitive to the presence of hydrogen in the overlaying gas, at concentrations that range from a few hundreds parts per million to a few percent.

Aircraft emissions of (ultra)fine particles during landing and take-off operations pose increasing human health hazards for airport employees and near-airport communities. Measurements of in-operation aircraft are therefore crucial for characterizing real-world aircraft emissions, and their variability. In this work, we develop an approach that enables the gathering of large quantities of data on real-world aircraft-specific emissions. We use three types of portable PM sensors located ca. 200 m downwind of an operational runway at Amsterdam Airport Schiphol, over different seasons, to characterize the plumes from ca. 500 specific operations covering most aircraft types of the global flying fleet. High concentration peaks (in the order of 10⁶ particles/cm³) of sub-25 nm particles are observed in the near field. While departure plumes exhibit higher particle number concentrations than arrival plumes, the values do not necessarily scale with aircraft size or engine thrust rating. We find large variability among aircraft types and engine models, highlighting the importance of incorporating real-world observations when assessing the impacts of aviation on the atmospheric composition and human health.

We have characterised a new Atmospheric-Pressure-interface Time-of-Flight Mass Spectrometer, equipped with an octapole ion trap for accumulating the sampled ions before orthogonally accelerating them into the mass analyser. The characterisation has been carried out using ion standards produced by electrospray ionisation, that were subsequently mobility-selected by a differential mobility analyser operated at atmospheric pressure. Our results show that the detection sensitivity (or limit of detection) of the mass spectrometer is in the parts per quintillion (i.e., 10−3 parts per quadrillion, ppq; which is ∼ 30 ions cm−3) range with temporal resolutions of 1 s. When increasing the temporal resolution up to 1 min, the detection sensitivity can be reduced to the 10 parts per sextillion (i.e., 10−5 ppq; which is ∼ 0.3 ions cm−3) range, enabling the system to measure gaseous ions of extremely low concentrations. In contrast to other mass spectrometers that employ spectra accumulation to improve the detection sensitivity for atmospheric observations, ion accumulation amplifies the signal without increasing the noise level; something that among others is highly important for probing short-lived ionic clusters during new particle formation events in the atmospheric environment. We also show that the mass spectrometer has a transmission of up to 1 %, and a mass resolution of 23 000 for ionic masses of ca. 600 Da, while it can be used in ways to induce collision dissociation of the sampled ions by tuning the operating conditions of the Atmospheric-Pressure-interface stage.

Inhalation of nanosized metal oxides may occur at the workplace. Thus, information on potential hazardous effects is needed for risk assessment. We report an investigation of the genotoxic potential of different metal oxide nanomaterials. Acellular and intracellular reactive oxygen species (ROS) production were determined for all the studied nanomaterials. Moreover, mice were exposed by intratracheal instillation to copper oxide (CuO) at 2, 6, and 12 μg/mouse, tin oxide (SnO₂) at 54 and 162 μg/mouse, aluminum oxide (Al₂O₃) at 18 and 54 μg/mouse, zinc oxide (ZnO) at 0.7 and 2 μg/mouse, titanium dioxide (TiO₂) and the benchmark carbon black at 162 μg/mouse. The doses were selected based on pilot studies. Post-exposure time points were 1 or 28 days. Genotoxicity, assessed as DNA strand breaks by the comet assay, was measured in lung and liver tissue. The acellular and intracellular ROS measurements were fairly consistent. The CuO and the carbon black bench mark particle were potent ROS generators in both assays, followed by TiO₂. Al₂O₃, ZnO, and SnO₂ generated low levels of ROS. We detected no increased genotoxicity in this study using occupationally relevant dose levels of metal oxide nanomaterials after pulmonary exposure in mice, except for a slight increase in DNA damage in liver tissue at the highest dose of CuO. The present data add to the body of evidence for risk assessment of these metal oxides.

Spark ablation is a highly effective and versatile method for producing nanoparticles from bulk conductive electrode materials. For a number of applications, however, the production throughput of the process needs to be increased with respect to the current state of the art. Here we show that this can be achieved by decreasing the diameter of the employed bulk-material electrodes from ca. 12 to 0.15 mm, corroborating previous observations, and demonstrate that the throughput is associated with the ablation efficiency (i.e., the energy spent to produce nanoparticles per total input energy) that respectively increases by a factor of 10. It is also shown that the commonly used theory for predicting the mass of nanoparticles produced by spark ablation cannot capture this effect, and thus we extend it to account for heat losses that affect the process when electrode diameter reduces below ca. 2 mm. Through this exercise we also show that reduced heat losses associated with thinner electrodes provide an effective recipe to increase the ablation efficiency, also referred as the nanoparticle production yield. The new extended theory for estimating spark ablation nanoparticle mass production throughput is also accompanied by an empirical equation predicting its dependence on electrode diameter.

Portable instruments that can measure the number concentration and size of airborne nanoparticles are very useful for assessing their impacts on human health and climate, mainly because they can enable personal monitoring when carried by individuals, and/or 2- or 3-dimensional mappings when employed onboard mobile platforms. Partector 2 (P2), which is a lightweight and portable instrument manufactured by Naneos Particle Solutions GmbH (Windisch, Switzerland), can determine the concentration (up to 10⁶ #/cm³) and average diameter of aerosol particles having sizes from 10 to 300 nm, making it an excellent candidate for such measurements. Although its performance has been investigated at standard conditions (i.e., ground level pressure and room temperatures), it has not been assessed under reduced pressure and temperature conditions that are typically encountered at higher altitudes; e.g., when employed outdoors in mountainous environments and/or onboard Unmanned Aerial Systems; UASs. Here we assess the counting and sizing capabilities of P2 at temperatures from ca. 22 down to 4 °C, and pressures from 1013 down to 710 hPa that correspond to altitudes from sea level to ca. 3 km. Our results show that the performance of the instrument is not substantially affected when operated at these conditions, remaining within the accuracy thresholds of ±30% reported by the manufacturer. P2, therefore, qualifies for outdoor use at higher altitudes, and can be employed in such environments to determine the number concentration and mean size of sub-300 nm aerosol particles, complementing existing portable optical particle counters that are already employed onboard aerial systems.

Experimental evidence shows that hydroxylated metal ions are often produced during cluster synthesis by atmospheric pressure spark ablation. In this work, we predict the ground state equilibrium structures of AgOkHm± clusters (k and m = 1–4), which are readily produced when spark ablating Ag, using the coupled cluster with singles and doubles (CCSD) method. The stabilization energy of these clusters is calculated with respect to the dissociation channel having the lowest energy, by accounting perturbative triples corrections to the CCSD method. The interatomic interactions in each of the systems have been investigated using the frontier molecular orbital (FMO), natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) methods. Many of the ground states of these ionic clusters are found to be stable, corroborating experimental observations. We find that clusters having singlet spin states are more stable in terms of dissociation than the clusters that have doublet or triplet spin states. Our calculations also indicate a strong affinity of the ionic and neutral Ag atom towards water and hydroxyl radicals or ions. Many 3-center, 4-electron (3c/4e) hyperbonds giving rise to more than one resonance structure are identified primarily for the anionic clusters. The QTAIM analysis shows that the O–H and O–Ag bonds in the clusters of both polarities are respectively covalent and ionic. The FMO analysis indicates that the anionic clusters are more reactive than the cationic ones. Using the cluster structures predicted by the CCSD method, we calculate the collision cross sections of the AgOkHm± family, with k and m ranging from 1 to 4, by the trajectory method. In turn, we predict the electrical mobilities of these clusters when suspended in helium at atmospheric pressure and compare them with experimental measurements.

Sub-10 nm metal-based nanoparticles have garnered immense interest due to their unique properties and versatile applications. In this study, we created sub-10 nm Ag-based particles with a spark discharge generator and explored the parameters impacting their size distribution and charging properties, including carrier gas flow rates, spark discharge voltage, electrode gap distances, and capacitance. Our findings illuminate that there is a comparable influence of different factors on both self-charged and neutral particles. Among the different factors, carrier gas flow rates emerging as a paramount determinant in particle size. While increasing spark discharge voltage and capacitance within the spark circuit increases particle concentrations, the associated changes in particle size prove to be less straightforward. Significant differences between the concentration of positive and negative self-charged particles manifest when the carrier gas flow rate surpasses 5.0 L min⁻¹, with positive particles ranging from 0.8 to 1.2 nm and negative particles spanning 0.8 to 3.0 nm. Self-charged particles close to 1 nm tend to exhibit positive charges, whereas those larger than 2 nm tend to acquire negative charges, which suggests the growth of negative particles is faster than positive ones in the spark chamber. Nevertheless, these disparities between bipolar particles diminish with the increase of residence time, leading to the observation of similar particle size distributions. Positive particles consistently bear a single charge, while some negative particles exceeding 3 nm exhibit multiple charges, primarily under carrier gas flow rates exceeding 7.5 L min⁻¹. This study provides insights into the control of properties of nano-sized metal particles, which are crucial for their practical utilization.

Stone-built cultural heritage faces threats from natural forces and anthropogenic pollutants, including local climate, acid rain, and outdoor conditions like temperature fluctuations and wind exposure, all of which impact their structural integrity and lead to their degradation. The development of a water-based, environmentally-friendly protective coatings that meet a combination of requirements posed by the diversity of the substrates, different environmental conditions, and structures with complex geometries remains a formidable challenge, given the numerous obstacles faced by current conservation strategies. Here we report the structural, electrical, and mechanical characterization, along with performance testing, of a nanostructured hydrophilic and self-healing hybrid coating based on hydroxyapatite (HAp) nanocrystals and polyelectrolyte multilayers (PEM), formed in-situ on Greek marble through a simple spray layer-by-layer surface functionalization technique. The polyelectrolyte-hydroxyapatite multilayer (PHM) structure resembled the design of naturally forming trabecular bone, attained at a short procedural time. It exhibited chemical affinity, aesthetical compatibility and resistance to weathering while offering reversibility. The proposed method is able to generate micron-sized coatings with controlled properties, such as adhesion and self-healing, leading to less weathered surfaces. Our results show that the PHM is a highly effective protective material that can be applied for stone protection and other similar applications.

The Condensation Particle Counter (CPC) is an effective instrument for measuring the number concentration of aerosol particles in different environments. With only a few exceptions, CPCs are bench instruments with limited portability for use in the field, and have a cost that prevents their use in large numbers for distributed air quality measurements. This has motivated the development of compact and cost-effective CPCs that are already available in the market. Here we test the performance of such a CPC, designed and built by National Air Quality Testing Services Ltd (NAQTS), Lancaster, UK, which is a part of the V2000 compact air quality monitor that also includes gas sensors. The tests were carried out using monodisperse particles produced by atomization and electrical mobility classification. We found that the NAQTS V2000 CPC has a 50% detection efficiency for particles having diameter of ca. 14 nm. Our results also show that the coincidence error of the core CPC occurs at concentrations higher than 1 × 104 #/cm−3. Considering that the standalone CPC employs an ejector pump that provides a nominal dilution factor between 20 and 50 at its inlet, the coincidence error threshold of the system when sampling particles from ambient air is 2–5 × 105 #/cm−3. To extend its range for aerosols with particle concentrations by one order of magnitude higher, and thus expand its capabilities for a wide range of applications, we provide a simple correction equation. Overall, our results demonstrate that the NAQTS V2000 CPC is a highly effective instrument, and considering that it is currently the most cost-effective CPC in the market, to the best of our knowledge, makes it a highly attractive solution for air quality monitoring.

Positive and negative ions produced by radioactive sources and corona discharges in gases find a number of applications, including charging aerosol particles prior to their measurement by electrical and/or electrical mobility techniques. The degree to which these ions can charge aerosol particles depends on their mobility and mass; properties that are strongly affected by the composition of the carrier gas and the impurities that it contains. We show that when the purity of the carrier gas is increased, the mobility of both positive and negative ions increases by more than 50%, whereas the respective masses reduce by more than 50%. In most cases, the dominant positive species is N ₄ ⁺, whereas NO ₂ ^- and NO ₃ ^- prevail for the negative polarity. Differences in ion mobility and mass resulting from the two ionization methods (i.e., radioactive source and corona discharges) remain limited. When volatile methyl siloxanes (VMS) are introduced deliberately to the gas, the mobility of the cations decreases by 39% and their mass increases by 385%, while the dominant mobility and mass peaks of the negative ions remains almost unaffected. Interestingly, introduction of VMS also leads to consistent and reproducible positive ion properties across all variations of the experiments, which can be especially relevant for charging aerosol particles in a reproducible manner. Taken together, the new measurements we report in this paper corroborate prior knowledge that the composition and purity of the carrier gas strongly influence the properties of positive and negative ions generated in aerosol neutralizers, and provide new evidence regarding their evolution in the presence of impurities.

Poor air quality in workplaces constitutes a great concern on human health as a good fraction of our time is spent at work. In Greece, very unique workplaces are the street corner kiosks, which are freestanding boxes placed on sidewalks next to city streets and vehicular traffic, where one can find many consumer goods. As such, its employees are exposed to both outdoor and indoor air pollutants. Very few studies have examined the occupational exposure of kiosk workers to air pollutants, and thus the magnitude of this unique indoor and outdoor exposure remains unknown. The objective of this study is to investigate and compare the levels of indoor and outdoor particulate matter (PM₁₀ and PM_2.5), ultrafine particles (UFPs) and black carbon (BC) in different kiosks located in Athens, Greece, in urban-traffic and urban-background environments. Continuous measurements of the above-mentioned pollutants were carried out on a 24-h basis over 7 consecutive days at three kiosks from September to October 2019. Indoor PM₁₀ concentrations in the urban kiosk ranged from 19.0 to 44.0 μg/m³, PM_2.5 values ranged from 14.0 to 33.0 μg/m³, whereas BC concentrations ranged from 1.2 to 7.0 μg/m³ and UFPs from almost 9.5 to 47.0 × 10³ pt/cm³. Outdoor PM₁₀ and PM_2.5 measurements ranged from 29.0 to 59.0 μg/m³ and from 22.0 to 39.0 μg/m³, respectively. BC outdoor concentrations ranged from 1.1 to 2.2 μg/m³. The mean hazard quotient (HQ) for PM₁₀ (4.9) and PM_2.5 (4.7) among all participants was >1. The health risk of exposure to PM₁₀ and PM_2.5 was found to be at moderate hazard levels, although in some cases we observed HQ values higher than 10 due to high PM₁₀ and PM_2.5 concentrations in the kiosks. Overall our study indicates that people working at kiosks can be exposed to very high concentrations on particulate pollution depending on a number of factors including the traffic that strongly depends on location and the time of the day.

The stability of nanoparticle (NP) production by atmospheric-pressure spark ablation was studied and found to depend on the composition of the electrodes and the carrier gas (here N₂ or Ar). For materials that do not react with N₂, such as Pd and Ni, NP production was rather stable regardless of the carrier gas employed. In contrast, for materials that can easily produce nitride species (e.g., Al and Mg), both the concentration and size of the resulting NPs exhibited noticeable fluctuations, when ablating them in N₂, which are more pronounced when the electrical energy input to the system is low. The variation in concentration and particle size is attributed to the formation of a metal-nitride region on the face of the electrodes where the sparks hit, as a result of its reaction with the carrier gas, altering the electrical and thermal conductivity, and consequently the ablatability of the electrode at that region. This explanation was corroborated by offline analysis of the face surface of the electrodes, showing two chemically distinct regions: one with high content of N and one without. In addition, the concentration of the Al and Mg NPs produced in N₂ decreased gradually over time until it reached a plateau after several hours. When using Ar, the fluctuation and decreasing trend in NP production, and consequently the formation of nitride compounds on the face surface of the electrodes, were negligible, providing an effective solution for stable ablation of materials that can easily react with N₂.

The density functional theory (DFT) framework was used to investigate the intermolecular interactions between cyanogen chloride (CNCl) pollutant gas molecule with pristine boron nitride nanotubes (BNNT), Al-doped boron nitride nanotubes (BNAlNT), and carbon boron nitride nanotubes (BC₂NNT). The geometric structures of the resulting systems have been optimized using different methods, including B3LYP-D3(GD3BJ)/6-311G(d), ωB97XD/6-311G(d), and M06-2X/6-311G(d). The computed adsorption energies suggest that the studied nanotubes can enhance adsorption of CNCl, and thus promote its detection when employed as sensing materials. Wave function analysis has been implemented to study the type of intermolecular interactions at ωB97XD/6-311G(d,p) level of theory. Natural bond orbital (NBO) analysis has been used to study the charge transfer and bond order. Quantum theory of atoms in molecules (QTAIM) analysis has also been used to determine the type of interactions between the target gas and the nanotubes. To investigate the weak intermolecular interactions we also carried out non-covalent interaction analysis (NCI). The results also indicate that the CNCl-nanotube systems are created through physisorption as they are dominated by non-covalent interactions. The predicted adsorption energies increase as follows: BNAlNT: −1.175 eV > BC₂NNT: −0.281 eV > BNNT: −0.256 eV; this shows that the aluminum-doped boron nitride nanotube is the best option from promoting adsorption of the target gas among them. The HOMO–LUMO energy gaps were as follows: BNNT: 7.090, BNAlNT: 9.193, and BC₂NNT: 7.027 eV at B3LYP-D3/6-311G(d) level of theory.

We have investigated the potential energy curves (PECs) of the LiN heteronuclear diatomic molecule, including its ionic species LiN⁺ and LiN⁻, using explicitly correlated multi-reference configuration interaction (MRCI-F12) calculations in conjunction with the correlation consistent quintuple-𝜁 basis set. The effect of core–valence correlation, scalar relativistic effects, and the size of the basis sets has been investigated. A comprehensive set of spectroscopic constants determined based on the above-mentioned calculations are also reported for the lowest electronic states and all systems, including dissociation energies, harmonic and anharmonic vibrational frequencies, and rotational constants. Additional parameters, such as the dipole moments, equilibrium spin-orbit constants, excitation energies, and rovibrational energy levels, are also documented. We found that the three triplet states of LiN, namely, X ³∑⁻, A ³Π, and 2 ³∑⁻, exhibit substantial potential wells in the PEC diagrams, while the quintet states are repulsive in nature. The ground state of the anion also shows a deep potential well in the vicinity of its equilibrium geometry. In contrast, the ground and excited states of the cation are very loosely bound. Charge transfer properties of each of these states are also analyzed to obtain an in-depth understanding of the interatomic interactions. We found that the core–valence correlation has a substantial effect on the calculated spectroscopic constants.

Nanoparticles (NPs) mixed at the atomic scale have been synthesized by atmospheric-pressure spark ablation using pairs of Pd and Hf electrodes. Gravimetric analysis of the electrodes showed that the fraction of each material in the resulting mixed NPs can be varied from ca. 15-85 at% to 85-15 at% by employing different combinations of electrode polarities and thicknesses. These results were also qualitatively corroborated by microscopy and elemental analysis of the produced NPs. When using pairs of electrodes having the same diameter, the material from the one at negative polarity was represented at a substantially higher fraction in the mixed NPs regardless of whether a pair of thin or thick electrodes were employed. This can be attributed to the higher ablation rate of the electrodes at the negative polarity, as already known from earlier experiments. When using electrodes of different diameters, the fraction of the element from the thinner electrode was always higher. This is because thinner electrodes are ablated more effectively due to, at least in part, the increased importance of the associated heat losses compared to its thicker counterpart. In those cases, the polarity of the electrodes had a significantly smaller effect. Overall, our results demonstrate, for the first time, that spark ablation can be used to control atomic scale mixing and thus produce alloyed NPs with compositions that can be tuned to a good extent by simply using different combinations of electrode diameters and polarities. This expands the capabilities of the technique for producing mixed nanoparticle building blocks of well-defined composition that are highly desired for a wide range of applications.