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Transformation of low-volatility gaseous precursors to new particles affects aerosol number concentration, cloud formation and hence the climate. The clustering of acid and base molecules is a major mechanism driving fast nucleation and initial growth of new particles in the atmosphere. However, the acid-base cluster composition, measured using state-of-the-art mass spectrometers, cannot explain the measured high formation rate of new particles. Here we present strong evidence for the existence of base molecules such as amines in the smallest atmospheric sulfuric acid clusters prior to their detection by mass spectrometers. We demonstrate that forming (H2SO4)1(amine)1 is the rate-limiting step in atmospheric H2SO4-amine nucleation and the uptake of (H2SO4)1(amine)1 is a major pathway for the initial growth of H2SO4 clusters. The proposed mechanism is very consistent with measured new particle formation in urban Beijing, in which dimethylamine is the key base for H2SO4 nucleation while other bases such as ammonia may contribute to the growth of larger clusters. Our findings further underline the fact that strong amines, even at low concentrations and when undetected in the smallest clusters, can be crucial to particle formation in the planetary boundary layer.

The Particle Size Magnifier is widely used for measuring nano-sized particles. Here we calibrated the newly developed Particle Size Magnifier version 2.0 (PSM 2.0). We used 1–10 nm particles with different compositions, including metal particles, organic particles generated in the laboratory, and atmospheric particles collected in Helsinki and Hyytiälä. A noticeable difference among the calibration curves was observed. Atmospheric particles from Hyytiälä required higher diethylene glycol (DEG) supersaturation to be activated compared to metal particles (standard calibration particles) and other types of particles. This suggests that chemical composition differences introduce measurement uncertainties and highlights the importance of in situ calibration. The size resolution of PSM 2.0 was characterized using metal particles. The maximum size resolution was observed at 2–3 nm. PSM 2.0 was then operated in Hyytiälä for ambient particle measurements in parallel with a Half Mini differential mobility particle sizer (DMPS). During new particle formation (NPF) events, comparable total particle concentrations were observed between the Half Mini DMPS and PSM 2.0 based on Hyytiälä atmospheric particle calibration. Meanwhile, applying the calibration with metal particles to atmospheric measurements caused an overestimation of 3–10 nm particles. In terms of the particle size distributions, similar patterns were observed between the DMPS and PSM when using the calibration of Hyytiälä atmospheric particles. In summary, PSM 2.0 is a powerful instrument for measuring sub-10 nm particles and can achieve more precise particle size distribution measurements with proper calibration. [ABSTRACT FROM AUTHOR]

Secondary organic aerosols (SOAs) significantly impact Earth's climate and human health. Although the oxidation of volatile organic compounds (VOCs) has been recognized as the major contributor to the atmospheric SOA budget, the mechanisms by which this process produces SOA-forming highly oxygenated organic molecules (HOMs) remain unclear. A major challenge is navigating the complex chemical landscape of these transformations, which traditional hypothesis-driven methods fail to thoroughly investigate. Here, we explore the oxidation of α-pinene, a critical atmospheric biogenic VOC, using a novel reaction discovery approach based on molecular dynamics and state-of-the-art enhanced sampling techniques. Our approach successfully identifies all established reaction pathways of α-pinene ozonolysis, as well as discovers multiple novel species and pathways without relying on a priori chemical knowledge. In particular, we unveil a key branching point that leads to the rapid formation of alkoxy radicals, whose high and diverse reactivity help to explain hitherto unexplained oxidation pathways suggested by mass spectral peaks observed in α-pinene ozonolysis experiments. This branching point is likely prevalent across a variety of atmospheric VOCs and could be crucial in establishing the missing link to SOA-forming HOMs. The ozonolysis of α-pinene is important in atmospheric secondary organic aerosols formation. Here, the authors use a molecular dynamics guided reaction discovery approach and identify endoperoxide-to-alkoxy isomerization as a key branching point in the mechanism. [ABSTRACT FROM AUTHOR]

The Italian Po Valley is one of the most polluted regions in Europe. During winter, meteorological conditions favor long and dense fogs, which strongly affect visibility and human health. In spring, the frequency of nighttime fogs reduces while daytime new particle formation events become more common. This transition is likely caused by a reduction in particulate matter (PM2.5), leading to a decrease in the relevant condensation sink. The physics and chemistry of fog and aerosol have been studied at the San Pietro Capofiume site since the 1980s, but the detailed processes driving the observed trends are not fully understood. Hence, during winter and spring 2021/22, the Fog and Aerosol Interaction Research Italy (FAIRARI) campaign was carried out, using a wide spectrum of approaches, including in situ measurements, outdoor chamber experiments, and remote sensing. Atmospheric constituents and their properties were measured ranging from gas molecules and molecular clusters to fog droplets. One unique aspect of this study is the direct measurement of the aerosol composition inside and outside of fog, showing a slightly greater dominance of organic compounds in the interstitial compared to the droplet phase. Satellite observations of fog provided a spatial context and agreed well with in situ measurements of droplet size. They were complemented with in situ chamber experiments, providing insights into oxidative processes and revealing a large secondary organic aerosol-forming potential of ambient air upon chemical aging. The oxidative potential of aerosol and fog water inferred the impact of aerosol–fog interactions on particle toxicity. SIGNIFICANCE STATEMENT: Aerosol and fog influence our climate and can have adverse effects on human health and visibility. Through anthropogenic emissions, such as from industry or transport, humans can influence the physical and chemical properties of fog and aerosol. The FAIRARI campaign investigated the formation of aerosols and aerosol–fog interactions in the Italian Po Valley, one of the most polluted regions in Europe. State-of-the-art experimental methods revealed the details of aerosols and fogs from the molecular level to the droplet scale. We observed a clear transition from winter to spring, with fog-dominated periods followed by an intensified secondary aerosol formation period. Organic and nitrate compounds dominated the composition of aerosol and fog residuals. The results will contribute to various aerosol–fog interaction modeling activities. [ABSTRACT FROM AUTHOR]

A dilute plasma is continuously maintained in the troposphere by ionising particle radiation from galactic cosmic rays and radon decay. Small ions in the 1–2 nm size range play an important role in atmospheric processes such as ion-induced nucleation of aerosol particles. Consequently there is a need for precise and robust instruments to measure small ions both for atmospheric observations and for laboratory experiments that simulate the atmosphere. Here we describe the design and performance of the Cluster Ion Counter (CIC, Airel OÜ), which simultaneously measures the number concentrations of positively- and negatively-charged ions and particles below 5 nm mobility diameter, with low noise and fast time response. The detection efficiency is above 80 % for ions and charged particles between 1.2 and 2.0 nm, and above 90 % between 2.0 and 3.0 nm. The ion concentrations measured by the CIC agree well with reference instruments. The noise level (1 σ of background measurements) is typically between 20 and 30 ions cm-3 at 1 Hz sampling rate and an air flow rate of 7 l min-1 per analyzer. The noise level improves when higher flow rates and longer sampling periods are used. The CIC responds rapidly with 1 s time resolution to pulses of ionisation produced in the CLOUD chamber by a CERN particle beam. [ABSTRACT FROM AUTHOR]

Chemical ionisation inlets are fundamental instrument components in chemical ionisation mass spectrometry (CIMS). However, the sample gas and reagent ion trajectories are often understood only in a general and qualitative manner. Here, we evaluate two atmospheric pressure interface chemical ionisation inlets (MION2 and Eisele type inlet) with 3D computational fluid dynamics physicochemical models regarding the reagent ion and sample gas trajectories and estimate their efficiencies of reagent ion production, reagent ion delivery from the ion source volume into the ion–molecule mixing region, and the interaction between reagent ions and target molecules. The models are validated by laboratory measurements and quantitatively reproduce observed sensitivities to tuning parameters, including ion currents and changes in mass spectra. The study elucidates how the different transport and chemical reactions proceed within the studied inlets, where space charge can already be relevant at ion concentrations as low as 10 7 cm-3 , and compares the two investigated inlet models. The models provide insights into how to operate the inlets and will help in the development of future inlets that further enhance the capability of CIMS. [ABSTRACT FROM AUTHOR]

About half of present-day cloud condensation nuclei originate from atmospheric nucleation, frequently appearing as a burst of new particles near midday. Atmospheric observations show that the growth rate of new particles often accelerates when the diameter of the particles is between one and ten nanometres. In this critical size range, new particles are most likely to be lost by coagulation with pre-existing particles, thereby failing to form new cloud condensation nuclei that are typically 50 to 100 nanometres across. Sulfuric acid vapour is often involved in nucleation but is too scarce to explain most subsequent growth, leaving organic vapours as the most plausible alternative, at least in the planetary boundary layer. Although recent studies predict that low-volatility organic vapours contribute during initial growth, direct evidence has been lacking. The accelerating growth may result from increased photolytic production of condensable organic species in the afternoon, and the presence of a possible Kelvin (curvature) effect, which inhibits organic vapour condensation on the smallest particles (the nano-Köhler theory), has so far remained ambiguous. Here we present experiments performed in a large chamber under atmospheric conditions that investigate the role of organic vapours in the initial growth of nucleated organic particles in the absence of inorganic acids and bases such as sulfuric acid or ammonia and amines, respectively. Using data from the same set of experiments, it has been shown that organic vapours alone can drive nucleation. We focus on the growth of nucleated particles and find that the organic vapours that drive initial growth have extremely low volatilities (saturation concentration less than 10(-4.5) micrograms per cubic metre). As the particles increase in size and the Kelvin barrier falls, subsequent growth is primarily due to more abundant organic vapours of slightly higher volatility (saturation concentrations of 10(-4.5) to 10(-0.5) micrograms per cubic metre). We present a particle growth model that quantitatively reproduces our measurements. Furthermore, we implement a parameterization of the first steps of growth in a global aerosol model and find that concentrations of atmospheric cloud concentration nuclei can change substantially in response, that is, by up to 50 per cent in comparison with previously assumed growth rate parameterizations.

The growth of freshly formed aerosol particles can be the bottleneck in their survival to cloud condensation nuclei. It is therefore crucial to understand how particles grow in the atmosphere. Insufficient experimental data has impeded a profound understanding of nano-particle growth under atmospheric conditions. Here we study nano-particle growth in the CLOUD (Cosmics Leaving OUtdoors Droplets) chamber, starting from the formation of molecular clusters. We present measured growth rates at sub-3 nm sizes with different atmospherically relevant concentrations of sulphuric acid, water, ammonia and dimethylamine. We find that atmospheric ions and small acid-base clusters, which are not generally accounted for in the measurement of sulphuric acid vapour, can participate in the growth process, leading to enhanced growth rates. The availability of compounds capable of stabilizing sulphuric acid clusters governs the magnitude of these effects and thus the exact growth mechanism. We bring these observations into a coherent framework and discuss their significance in the atmosphere.

Nitrous acid (HONO) is an important precursor of OH radicals which affects not only the sinks of primary air pollutants but also the formation of secondary air pollutants, but its source closure in the atmosphere is still controversial due to a lack of experiment validation. In this study, the HONO budget in Beijing has been analyzed and validated through the coronavirus disease (COVID-19) lockdown event, which resulted in a significant reduction in air pollutant emissions, providing a rare opportunity to understand the HONO budget in the atmosphere. We measured HONO and related pollutants from 1 January to 6 March 2020, which covered the Chinese New Year (CNY) and the COVID-19 lockdown. The average concentration of HONO decreased from 0.97 ± 0.74 ppb before CNY to 0.53 ± 0.44 ppb during the COVID-19 lockdown, accompanied by a sharp drop in NO x and the greatest drop in NO (around 87 %). HONO budget analysis suggests that vehicle emissions were the most important source of HONO during the nighttime (53 ± 17 %) before CNY, well supported by the decline in their contribution to HONO during the COVID-19 lockdown. We found that the heterogeneous conversion of NO 2 on ground surfaces was an important nighttime source of HONO (31 ± 5 %), while that on aerosol surfaces was a minor source (2 ± 1 %). Nitrate photolysis became the most important daytime source during the COVID-19 lockdown compared with that before CNY, resulting from the combined effect of the increase in nitrate and the decrease in NO. Our results indicate that reducing vehicle emissions should be an effective measure for alleviating HONO in Beijing. [ABSTRACT FROM AUTHOR]

Aerosol particles undergo continuous changes in their chemical composition and physical properties throughout their lifecycles, leading to diverse climate and health impacts. In particular, organic nanoparticle's surface equilibrium vapor pressure stands as a critical factor for gas–particle partitioning and is pivotal for understanding the evolution of aerosol properties. Herein, we present measurements of evaporation kinetics and surface equilibrium vapor pressures of a wide array of laboratory-generated organic nanoparticles, employing the dynamic-aerosol-size electrical mobility spectrometer (DEMS) methodology, a recent advancement in aerosol process characterization. The DEMS methodology is founded on the principle that the local velocity of a size-changing nanoparticle within a flow field has a one-to-one correspondence with its local size. Consequently, this approach can facilitate the in situ probing of rapid aerosol size-changing processes by analyzing the trajectories of size-changing nanoparticles within the classification region of a differential mobility analyzer (DMA). We employ the DEMS with a tandem DMA setup, where a heated sheath flow in the second DMA initiates particle evaporation in its classification region. Through analysis of the DEMS response and the underlying mechanism governing the evaporation process, we reconstruct temporal radius profiles of evaporating nanoparticles and derive their surface equilibrium vapor pressures across various temperatures. Our results demonstrate a good agreement between the vapor pressures deduced from DEMS measurements and those documented in literature. We discuss the measurable vapor pressure range achievable with DEMS and elucidate associated uncertainties. Furthermore, we outline prospective directions for refining this methodology and anticipate its potential to contribute to the characterization of aerosol-related kinetic processes with currently unknown mechanisms. [ABSTRACT FROM AUTHOR]

Isolation of extracellular vesicles (EV) has been developing rapidly in parallel with the interest in EVs. However, commonly utilized protocols may not suit more challenging samplematrixes and could potentially yield suboptimal results. Knowing and assessing the pitfalls of isolation procedure to be used, should be involved to some extent for EV analytics. EVs in cow milk are of great interest due to their abundancy and large-scale availability as well as their cross-species bioavailability and possible use as drug carriers. However, the characteristics of milk EVs overlap with those of other milk components. This makes it difficult to isolate and study EVs individually. There exists also a lack of consensus for isolation methods. In this study, we demonstrated the differences between various differential centrifugation-based approaches for isolation of large quantities of EVs from cow milk. Samples were further purified with gradient centrifugation and size exclusion chromatography (SEC) and differenceswere analyzed. Quality measurements were conducted onmultiple independent platforms. Particle analysis, electron microscopy and RNA analysis were used, to comprehensively characterize the isolated samples and to identify the limitations and possible sources of contamination in the EV isolation protocols. Vesicle concentration to protein ratio and RNA to protein ratios were observed to increase as samples were purified, suggesting co-isolation with major milk proteins in direct differential centrifugation protocols. We demonstrated a novel size assessment of vesicles using a particle mobility analyzer that matched the sizing using electron microscopy in contrast to commonly utilized nanoparticle tracking analysis. Based on the standards of the International Society for Extracellular Vesicles and the quick checklist of EVTrack. org for EV isolation, we emphasize the need for complete characterization and validation of the isolation protocol with all EV-related work to ensure the accuracy of results and allow further analytics and experiments. [ABSTRACT FROM AUTHOR]

Particulate matter has major climate and health impacts, and it is therefore of utmost importance to be able to measure the composition of these particles to gain insights into their sources and characteristics. Many methods, both offline and online, have been employed over the years to achieve this goal. One of the most recent developments is the Vaporization Inlet for Aerosols (VIA) coupled to a nitrate Chemical Ionization Mass Spectrometer (NO 3 -CIMS), but a thorough understanding of the VIA–NO 3 -CIMS system remains incomplete. In this work, we ran a series of tests to assess the impacts from different systems and sampling parameters on the detection efficiency of highly oxygenated organic molecules (HOMs) in the VIA–NO 3 -CIMS system. Firstly, we found that the current VIA system (which includes an activated carbon denuder and a vaporization tube) efficiently transmits particles (> 90 % for particles larger than 50 nm) while also removing gaseous compounds (> 97% for tested volatile organic compounds – VOCs). One of the main differences between the VIA and traditional thermal desorption (TD) techniques is the very short residence time in the heating region, on the order of 0.1 s. We found that this short residence time, and the corresponding short contact with heated surfaces, is likely one of the main reasons why relatively reactive or weakly bound peroxides, for example, were observable using the VIA. However, the VIA also requires much higher temperatures in order to fully evaporate the aerosol components. For example, the evaporation temperature of ammonium sulfate particles using the VIA was found to be about 100–150 °C higher than in typical TD systems. We also observed that the evaporation of particles with larger sizes occurred at slightly higher temperatures compared to smaller particles. Another major aspect that we investigated was the gas-phase wall losses of evaporated molecules. With a more optimized interface between the VIA and the NO 3 -CIMS, we were able to greatly decrease wall losses and thus improve the sensitivity compared to our earlier VIA work. This interface included a dedicated sheath flow unit to cool the heated sample and provide the NO 3 -CIMS with the needed high flow (10 L min -1). Our results indicate that most organic molecules observable by the NO 3 -CIMS can evaporate and be transported efficiently in the VIA system, but upon contact with the hot walls of the VIA, the molecules are instantaneously lost. This loss potentially leads to fragmentation products that are not observable by the NO 3 -CIMS. Thermograms, obtained by scanning the VIA temperature, were found to be very valuable for both quantification purposes and for estimating the volatility of the evaporating compounds. We developed a simple one-dimensional model to account for the evaporation of particles and the temperature-dependent wall losses of the evaporated molecules, and we thereby estimate the concentration of HOMs in secondary organic aerosol (SOA) particles. Overall, our results provide much-needed insights into the key processes underlying the VIA–NO 3 -CIMS method. Although there are still some limitations that could be addressed through hardware improvements, the VIA–NO 3 -CIMS system is a very promising and useful system for fast online measurements of HOMs in the particle phase. [ABSTRACT FROM AUTHOR]