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New particle formation (NPF) is a major source of aerosol particles and cloud condensation nuclei in the troposphere, playing an important role in both air quality and climate. Frequent NPF events have been observed in heavily polluted urban environments, contributing to the aerosol number concentration by a significant amount. The Po Valley region in northern Italy has been characterized as a hotspot for high aerosol loadings and frequent NPF events in southern Europe. However, the mechanisms of NPF and growth in this region are not completely understood. In this study, we conducted a continuous 2-month measurement campaign with state-of-the-art instruments to elucidate the NPF and growth mechanisms in northern Italy. Our results demonstrate that frequent NPF events (66 % of all days during the measurement campaign) are primarily driven by abundant sulfuric acid (8.5×106 cm -3) and basic molecules in this area. In contrast, oxygenated organic molecules from the atmospheric oxidation of volatile organic compounds (VOCs) appear to play a minor role in the initial cluster formation but contribute significantly to the consecutive growth process. Regarding alkaline molecules, amines are insufficient to stabilize all sulfuric acid clusters in the Po Valley. Ion cluster measurements and kinetic models suggest that ammonia (10 ppb) must therefore also play a role in the nucleation process. Generally, the high formation rates of sub-2 nm particles (87 cm -3 s -1) and nucleation-mode growth rates (5.1 nm h -1) as well as the relatively low condensational sink (8.9×10-3 s -1) will result in a high survival probability for newly formed particles, making NPF crucial for the springtime aerosol number budget. Our results also indicate that reducing key pollutants, such as SO 2 , amine and NH 3 , could help to substantially decrease the particle number concentrations in the Po Valley region. [ABSTRACT FROM AUTHOR]

Observations of atmospheric sub‐3 nm particles are essential for understanding the initial stages of new particle formation (NPF) and the origin of aerosol particles. In this study, 3 years (2018–2020) of measurements of sub‐3 nm particles were conducted in the Yangtze River Delta (YRD) of eastern China. High concentrations of sub‐3 nm particles were observed, with number concentration in the range from 103 to 106 cm−3. During the daytime, the sub‐3 nm particle concentration was found to peak at around the noon, indicating strong photochemical nucleation processes. The formation rates of sub‐3 nm particles were high during the NPF event days, with an average value of 86 cm−3 s−1, and this rate was related to the sulfuric acid (SA) concentration. The particle growth rates below 3 nm were about 1–2 nm h−1, much lower than the growth rates of larger particles. At nighttime, sub‐3 nm particle concentrations remained relatively high (3 × 103 to 1.2 × 104 cm−3) and were related to the NOx concentration, suggesting traffic emission to be a significant source. The sub‐3 nm proxy was developed to estimate the contributions of NPF and traffic emission. During the daytime, 74.8% and 12.4% of the sub‐3 nm particles were estimated to originate from SA‐driven NPF and traffic emissions, respectively. However, other sources were estimated to contribute 61.8% of sub‐3 nm particles at nighttime, suggesting the formation mechanisms of sub‐3 nm particles are still unclear in this environment. Our study sheds more light on the characteristics and sources of sub‐3 nm particles in polluted environments. Plain Language Summary: Sub‐3 nm particles play a vital role in the initial stage of new particle formation (NPF) as well as haze formation. We performed long‐term measurements of the sub‐3 nm particles and supporting atmospheric parameters from 2018 to 2020 in the Yangtze River Delta of eastern China. High number concentrations of sub‐3 nm particles were observed, with obvious diurnal and seasonal variations. NPF and traffic emissions were found to be two major sources of sub‐3 nm particles. The formation rate of sub‐3 nm particles was high and related to the sulfuric acid (SA) concentration during the NPF events. Sub‐3 nm particle concentrations remained relatively high at nighttime and were related to the NOx concentration. By developing the proxy of sub‐3 nm particle concentration, we estimated SA‐driven NPF accounted for over 70% of the sub‐3 nm particles during the daytime. At nighttime, over 30% of the sub‐3 nm particles were emitted from traffic sources, while about 60% remain unknown. This study sheds more light on the characteristics and sources of sub‐3 nm particles in polluted environments. Key Points: High number concentration of sub‐3 nm particles was observed in eastern China, with obvious diurnal and seasonal variationsSulfuric acid‐driven new particle formation is the dominant source of sub‐3 nm particles during the daytimeTraffic emission contributes over 30% of sub‐3 nm particles, while the origin of 62% of sub‐3 nm particles remains unclear during the night [ABSTRACT FROM AUTHOR]

New particle formation (NPF) is a major source of aerosol particles and cloud condensation nuclei in the troposphere, playing an important role in both air quality and climate. Frequent NPF events have been observed in heavily polluted urban environments, contributing to the aerosol number concentration by a significant amount. The Po Valley region in northern Italy has been characterized as a hotspot for high aerosol loadings and frequent NPF events in Southern Europe. However, the mechanisms of NPF and growth in this region are not completely understood. In this study, we conducted a continuous 2-month measurement campaign with state-of-the-art instruments to elucidate the NPF and the growth mechanisms in Northern Italy. Our results show that abundant sulfuric acid, ammonia and amines from agricultural activities may be the dominant components driving the frequent NPF events (66 % of all days during the measurement campaign) in this area. In contrast, organics seem to have a smaller role in cluster formation but contribute to the consecutive growth process. According to ion cluster measurements and kinetic model results, dimethylamine is not sufficient to stabilize all of the sulfuric acid during springtime in the Po Valley, suggesting that other amines and ammonia can also be involved. Generally, the high formation rates of sub-2 nm particles (87 cm−3 s−1) and nucleation mode growth rates (5.1 nm h−1) together with the relatively low condensational sink (8.9 × 10−3 s−1) will result in a high survival probability of newly formed particles, making NPF crucial for the springtime aerosol number budget in the Po Valley region. [ABSTRACT FROM AUTHOR]

During summer, the Southern Ocean is largely unaffected by anthropogenic emissions, which makes this region an ideal place to investigate marine natural aerosol sources and processes. A better understanding of natural aerosol is key to constrain the preindustrial aerosol state and reduce the aerosol radiative forcing uncertainty in global climate models. We report the concentrations of gaseous sulfuric acid, iodic acid, and methanesulfonic acid (MSA) together with a characterization of new particle formation (NPF) events over a large stretch of the Southern Ocean. Measurements were conducted on board the Russian icebreaker Akademik Tryoshnikov from January to March 2017. Iodic acid is characterized by a particular diurnal cycle with reduced concentration around noon, suggesting a lower formation yield when solar irradiance is higher. Gaseous MSA does not have a diurnal cycle and measured concentrations in gas and condensed phase are compatible with this species being primarily produced via heterogeneous oxidation of dimethyl sulfide and subsequent partitioning into the gas phase. We also found that NPF in the boundary layer is mainly driven by sulfuric acid but it occurred very rarely over the vast geographical area probed and did not contribute to the cloud condensation nuclei budget in a directly observable manner. Despite the near absence of NPF events in the boundary layer, Aitken mode particles were frequently measured, supporting the hypothesis of a free tropospheric source. Iodic acid and MSA were not found to participate in nucleation, however, MSA may contribute to aerosol growth via heterogeneous formation in the aqueous phase. Key Points: Production of methanesulfonic acid occurred predominantly in the condensed phase, followed by enhanced volatilization at lower relative humidityThe observed new particle formation events were probably driven by sulfuric acid and the environmental conditions (mainly temperature and condensation sink)Boundary layer new particle formation over the Southern Ocean in summer occurred very sporadically and does not contribute to the cloud condensation nuclei budget [ABSTRACT FROM AUTHOR]

New particle formation (NPF) is a significant source of atmospheric particles, affecting climate and air quality. Understanding the mechanisms involved in urban aerosols is important to develop effective mitigation strategies. However, NPF rates reported in the polluted boundary layer span more than 4 orders of magnitude, and the reasons behind this variability are the subject of intense scientific debate. Multiple atmospheric vapours have been postulated to participate in NPF, including sulfuric acid, ammonia, amines and organics, but their relative roles remain unclear. We investigated NPF in the CLOUD chamber using mixtures of anthropogenic vapours that simulate polluted boundary layer conditions. We demonstrate that NPF in polluted environments is largely driven by the formation of sulfuric acid–base clusters, stabilized by the presence of amines, high ammonia concentrations and lower temperatures. Aromatic oxidation products, despite their extremely low volatility, play a minor role in NPF in the chosen urban environment but can be important for particle growth and hence for the survival of newly formed particles. Our measurements quantitatively account for NPF in highly diverse urban environments and explain its large observed variability. Such quantitative information obtained under controlled laboratory conditions will help the interpretation of future ambient observations of NPF rates in polluted atmospheres. [ABSTRACT FROM AUTHOR]

In the present-day atmosphere, sulfuric acid is the most important vapour for aerosol particle formation and initial growth. However, the growth rates of nanoparticles (<10 nm) from sulfuric acid remain poorly measured. Therefore, the effect of stabilizing bases, the contribution of ions and the impact of attractive forces on molecular collisions are under debate. Here, we present precise growth rate measurements of uncharged sulfuric acid particles from 1.8 to 10 nm, performed under atmospheric conditions in the CERN (European Organization for Nuclear Research) CLOUD chamber. Our results show that the evaporation of sulfuric acid particles above 2 nm is negligible, and growth proceeds kinetically even at low ammonia concentrations. The experimental growth rates exceed the hard-sphere kinetic limit for the condensation of sulfuric acid. We demonstrate that this results from van der Waals forces between the vapour molecules and particles and disentangle it from charge–dipole interactions. The magnitude of the enhancement depends on the assumed particle hydration and collision kinetics but is increasingly important at smaller sizes, resulting in a steep rise in the observed growth rates with decreasing size. Including the experimental results in a global model, we find that the enhanced growth rate of sulfuric acid particles increases the predicted particle number concentrations in the upper free troposphere by more than 50 %. [ABSTRACT FROM AUTHOR]

New particle formation (NPF) represents the first step in the complex processes leading to formation of cloud condensation nuclei. Newly formed nanoparticles affect human health, air quality, weather, and climate. This review provides a brief history, synthesizes recent significant progresses, and outlines the challenges and future directions for research relevant to NPF. New developments include the emergence of state‐of‐the‐art instruments that measure prenucleation clusters and newly nucleated nanoparticles down to about 1 nm; systematic laboratory studies of multicomponent nucleation systems, including collaborative experiments conducted in the Cosmics Leaving Outdoor Droplets chamber at CERN; observations of NPF in different types of forests, extremely polluted urban locations, coastal sites, polar regions, and high‐elevation sites; and improved nucleation theories and parameterizations to account for NPF in atmospheric models. The challenges include the lack of understanding of the fundamental chemical mechanisms responsible for aerosol nucleation and growth under diverse environments, the effects of SO2 and NOx on NPF, and the contribution of anthropogenic organic compounds to NPF. It is also critical to develop instruments that can detect chemical composition of particles from 3 to 20 nm and improve parameterizations to represent NPF over a wide range of atmospheric conditions of chemical precursor, temperature, and humidity. Plain Language Summary: In the atmosphere, invisible to the human eye, there are many microscopic particles, or "nanoparticles," that affect human health, air quality, and climate. We do not fully understand the chemical processes that allow these fine particles to form and be suspended in the air nor how they influence heat flow in Earth's atmosphere. Laboratory experiments, field observations, and modeling simulations have all shown different results for how these particles behave. These inconsistencies make it difficult to accurately represent the processes of new particle formation in regional and global atmospheric models. Scientists still need to develop instruments that can measure the smallest range of nanoparticles and to find ways to describe particle formation that allow for differences in temperature, humidity, and level of pollution. Key Points: Recent progresses include development of instruments and observations of multicomponent nucleation and NPF at various locationsThere is a lack of understanding of fundamental chemical mechanisms responsible for NPF in diverse environments, especially in megacitiesThere is a need to develop NPF parameterizations to represent a wider range of pollutants, temperature and humidity [ABSTRACT FROM AUTHOR]

Atmospheric oxidation is an important phenomenon which produces large quantities of low-volatility compounds such as sulfuric acid and oxidized organic compounds. Such species may be involved in the nucleation of particles and enhance their subsequent growth to reach the size of cloud condensation nuclei (CCN). In this study, we investigate α-pinene, the most abundant monoterpene globally, and its oxidation products formed through ozonolysis in the Cosmic Leaving OUtdoor Droplets (CLOUD) chamber at CERN (the European Organization for Nuclear Research). By scavenging hydroxyl radicals (OH) with hydrogen (H2), we were able to investigate the formation of highly oxygenated molecules (HOMs) purely driven by ozonolysis and study the oxidation of sulfur dioxide (SO2) driven by stabilized Criegee intermediates (sCIs). We measured the concentrations of HOM and sulfuric acid with a chemical ionization atmospheric-pressure interface time-of-flight (CI-APi-TOF) mass spectrometer and compared the measured concentrations with simulated concentrations calculated with a kinetic model. We found molar yields in the range of 3.5-6.5% for HOM formation and 22-32% for the formation of stabilized Criegee intermediates by fitting our model to the measured sulfuric acid concentrations. The simulated time evolution of the ozonolysis products was in good agreement with measured concentrations except that in some of the experiments sulfuric acid formation was faster than simulated. In those experiments the simulated and measured concentrations met when the concentration reached a plateau but the plateau was reached 20-50 min later in the simulations. The results shown here are consistent with the recently published yields for HOM formation from different laboratory experiments. Together with the sCI yields, these results help us to understand atmospheric oxidation processes better and make the reaction parameters more comprehensive for broader use. [ABSTRACT FROM AUTHOR]

A recent CLOUD (Cosmics Leaving OUtdoor Droplets) chamber study showed that sulfuric acid and dimethylamine produce new aerosols very efficiently and yield particle formation rates that are compatible with boundary layer observations. These previously published new particle formation (NPF) rates are reanalyzed in the present study with an advanced method. The results show that the NPF rates at 1.7 nm are more than a factor of 10 faster than previously published due to earlier approximations in correcting particle measurements made at a larger detection threshold. The revised NPF rates agree almost perfectly with calculated rates from a kinetic aerosol model at different sizes (1.7 and 4.3 nm mobility diameter). In addition, modeled and measured size distributions show good agreement over a wide range of sizes (up to ca. 30 nm). Furthermore, the aerosol model is modified such that evaporation rates for some clusters can be taken into account; these evaporation rates were previously published from a flow tube study. Using this model, the findings from the present study and the flow tube experiment can be brought into good agreement for the high base-to-acid ratios ( ~100) relevant for this study. This confirms that nucleation proceeds at rates that are compatible with collision-controlled (a.k.a. kinetically controlled) NPF for the conditions during the CLOUD7 experiment (278 K, 38% relative humidity, sulfuric acid concentration between 1×106 and 3×107 cm-3, and dimethylamine mixing ratio of ~40 pptv, i.e., 1×109 cm-3/. [ABSTRACT FROM AUTHOR]

New particle formation driven by acid-base chemistry was initiated in the CLOUD chamber at CERN by introducing atmospherically relevant levels of gas-phase sulfuric acid and dimethylamine (DMA). Ammonia was also present in the chamber as a gas-phase contaminant from earlier experiments. The composition of particles with volume median diameters (VMDs) as small as 10 nm was measured by the Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS). Particulate ammonium-to-dimethylaminium ratios were higher than the gas-phase ammonia-to-DMA ratios, suggesting preferential uptake of ammonia over DMA for the collected 10-30 nm VMD particles. This behavior is not consistent with present nanoparticle physicochemical models, which predict a higher dimethylaminium fraction when NH3 and DMA are present at similar gas-phase concentrations. Despite the presence in the gas phase of at least 100 times higher base concentrations than sulfuric acid, the recently formed particles always had measured base : acid ratios lower than 1 : 1. The lowest base fractions were found in particles below 15 nm VMD, with a strong size-dependent composition gradient. The reasons for the very acidic composition remain uncertain, but a plausible explanation is that the particles did not reach thermodynamic equilibrium with respect to the bases due to rapid heterogeneous conversion of SO2 to sulfate. These results indicate that sulfuric acid does not require stabilization by ammonium or dimethylaminium as acid-base pairs in particles as small as 10 nm. [ABSTRACT FROM AUTHOR]

The concentrations of neutral and charged sub-3nm clusters and their connection to new particle formation (NPF) were investigated during the PEGASOS campaign (7 June-9 July 2012) at the San Pietro Capofiume measurement station in the Po Valley, Italy. Continuous high concentrations of sub-3nm clusters were detected during the measurement period, although the condensation sink was relatively high (median value 1.1 ? 10-2 s-1). The median cluster concentrations were 2140 and 7980 cm-3 in the size bins of 1.5-1.8 and 1.8-3 nm, and the majority of them were electrically neutral. NPF events were observed during the measurement period frequently, on 86% of the days. The median growth rates of clusters during the events were 4.3, 6.0 and 7.2 nm h-1 in the size ranges of 1.5-3, 3-7 and 7-20 nm. The median formation rate of 1.6 nm clusters was high, 45 cm-3 s-1, and it exceeded the median formation rate of 2 nm clusters by 1 order of magnitude. The ion-induced nucleation fraction was low; the median values were 0.7% at 1.6 nm and 3.0% at 2 nm. On NPF event days the neutral cluster concentration had a maximum around 09:00 (local winter time), which was absent on a non-event day. The increase in the cluster concentrations in the morning coincided with the increase in the boundary layer height. At the same time radiation, temperature and SO2 concentration increased, and RH and condensation sink decreased. The concentrations of neutral and charged clusters were observed to have a positive correlation with sulfuric acid proxy, indicating the significance of sulfuric acid for the cluster formation in San Pietro Capofiume. The condensation sink had a negative correlation with the concentration of charged clusters but no clear relation to the neutral cluster concentration. This finding, together with back-trajectory analysis, suggests that the precursor vapors of the clusters and background aerosol particles, acting as their sink, have possibly originated from the same sources, including e.g., power plants and industrial areas in the Po Valley. [ABSTRACT FROM AUTHOR]

Atmospheric aerosols formed by nucleation of vapors affect radiative forcing and therefore climate. However, the underlying mechanisms of nucleation remain unclear, particularly the involvement of organic compounds. Here, we present high-resolution mass spectra of ion clusters observed during new particle formation experiments performed at the Cosmics Leaving Outdoor Droplets chamber at the European Organization for Nuclear Research. The experiments involved sulfuric acid vapor and different stabilizing species, including ammonia and dimethylamine, as well as oxidation products of pinanediol, a surrogate for organic vapors formed from monoterpenes. A striking resemblance is revealed between the mass spectra from the chamber experiments with oxidized organics and ambient data obtained during new particle formation events at the Hyytiälä boreal forest research station. We observe that large oxidized organic compounds, arising from the oxidation of monoterpenes, cluster directly with single sulfuric acid molecules and then form growing clusters of one to three sulfuric acid molecules plus one to four oxidized organics. Most of these organic compounds retain 10 carbon atoms, and some of them are remarkably highly oxidized (oxygen-to-carbon ratios up to 1.2). The average degree of oxygenation of the organic compounds decreases while the clusters are growing. Our measurements therefore connect oxidized organics directly, and in detail, with the very first steps of new particle formation and their growth between 1 and 2 nm in a controlled environment. Thus, they confirm that oxidized organics are involved in both the formation and growth of particles under ambient conditions. [ABSTRACT FROM AUTHOR]

The article focuses on a study which investigates in the real atmosphere using an extensive set of state-of-the-art ion and mass spectrometers deployed in a boreal forest environment. Topics discussed include fraction of aerosols, which affect air quality and climate, is formed from gaseous precursors, highly oxygenated organic molecules (HOMs) are essential to grow the newly formed particles and evidence to initiate ion-induced nucleation in chamber experiments in the absence of sulfuric acid.

In this paper we introduce in-situ observations of trace gases, aerosol particles and their precursors in the vicinity of an oil refinery and industrial area in Kilpilahti, Southern Finland. We conducted a one-month measurement campaign near the oil refinery during summertime when the sulphur dioxide concentrations at the site are typically the highest. The source areas around the measurement location were divided into three sectors: oil refinery area, industrial area and non-industrial area. The atmospheric concentrations of aerosols and trace gases showed a large temporal variability, when exposed to the different source areas. The median sulphur dioxide concentrations for the oil refinery, industrial and non-industrial area were 1.88 ppbv, 0.75 ppbv and 0.38 ppbv, respectively, and the corresponding sulphuric acid concentrations were 11.5 × 10 6 molecules/cm 3 , 4.4 × 10 6 molecules/cm 3 and 1.3 × 10 6 molecules/cm 3 . The observed concentrations were similar to what have been measured in urban or industrial sites. The ratio between sulphuric acid and sulphur dioxide was the highest when the air mass was coming from the oil refinery. The correlation between the sulphuric acid and 1–2 nm particle concentrations was significant, but the composition of the particles remained unknown as no neutral sulphuric acid clusters were detected with the mass spectrometer. Only a few new particle formation events were observed during the measurement period, and during these events a large fraction of the particle growth could be explained by sulphuric acid condensation. [ABSTRACT FROM AUTHOR]