Formation and growth of sub-3-nm aerosol particles in experimental chambers

Temperature, humidity, and ionisation effect of iodine oxoacid nucleation

Iodine oxoacids are recognised for their significant contribution to the formation of new particles in marine and polar atmospheres. Nevertheless, to incorporate the iodine oxoacid nucleation mechanism into global simulations, it is essential to comprehend how this mechanism varies under various atmospheric conditions. In this study, we combined measurements from the CLOUD (Cosmic Leaving OUtdoor Droplets) chamber at CERN and simulations with a kinetic model to investigate the impact of temperature, ionisation, and humidity on iodine oxoacid nucleation. Our findings reveal that ion-induced particle formation rates remain largely unaffected by changes in temperature. However, neutral particle formation rates experience a significant increase when the temperature drops from +10 °C to -10 °C. Running the kinetic model with varying ionisation rates demonstrates that the particle formation rate only increases with a higher ionisation rate when the iodic acid concentration exceeds 1.5 × 107 cm-3, a concentration rarely reached in pristine marine atmospheres. Consequently, our simulations suggest that, despite higher ionisation rates, the charged cluster nucleation pathway of iodic acid is unlikely to be enhanced in the upper troposphere by higher ionisation rates. Instead, the neutral nucleation channel is likely to be the dominant channel in that region. Notably, the iodine oxoacid nucleation mechanism remains unaffected by changes in relative humidity from 2% to 80%. However, under unrealistically dry conditions (below 0.008% RH at +10 °C), iodine oxides (I2O4 and I2O5) significantly enhance formation rates. Therefore, we conclude that iodine oxoacid nucleation is the dominant nucleation mechanism for iodine nucleation in the marine and polar boundary layer atmosphere.

Trimethylamine from Subtropical Forests Rival Total Farmland Emissions in China

Many types of living plants release gaseous trimethylamine (TMA), making it a potentially important contributor to new particle formation (NPF) in remote areas. However, a panoramic view of the importance of forest biogenic TMA at the regional scale is lacking. Here, we pioneered nationwide mobile measurements of TMA across a transect of contiguous farmland in eastern China and a transect of subtropical forests in southern China. In contrast to the farmland route, TMA concentrations measured during the subtropical forest route correlated significantly with isoprene, suggesting potential TMA emissions from leaves. Our high time-resolved concentrations obtained from a weak photo-oxidizing atmosphere reflected freshly emitted TMA, indicating the highest emission intensity from irrigated dryland (set as the baseline of 10), followed by paddy field (7.1), subtropical evergreen forests (5.9), and subtropical broadleaf and mixed forests (4.3). Extrapolating their proportions roughly to China, subtropical forests alone, which constitute half of the total forest area, account for nearly 70% of the TMA emissions from the nation's total farmland. Our estimates, despite the uncertainties, take the first step toward large-scale assessment of forest biogenic amines, highlighting the need for observational and modeling studies to consider this hitherto overlooked source of TMA.

Nitrate Radicals Suppress Biogenic New Particle Formation from Monoterpene Oxidation

Highly oxygenated organic molecules (HOMs) are a major source of new particles that affect the Earth's climate. HOM production from the oxidation of volatile organic compounds (VOCs) occurs during both the day and night and can lead to new particle formation (NPF). However, NPF involving organic vapors has been reported much more often during the daytime than during nighttime. Here, we show that the nitrate radicals (NO3), which arise predominantly at night, inhibit NPF during the oxidation of monoterpenes based on three lines of observational evidence: NPF experiments in the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN (European Organization for Nuclear Research), radical chemistry experiments using an oxidation flow reactor, and field observations in a wetland that occasionally exhibits nocturnal NPF. Nitrooxy-peroxy radicals formed from NO3 chemistry suppress the production of ultralow-volatility organic compounds (ULVOCs) responsible for biogenic NPF, which are covalently bound peroxy radical (RO2) dimer association products. The ULVOC yield of α-pinene in the presence of NO3 is one-fifth of that resulting from ozone chemistry alone. Even trace amounts of NO3 radicals, at sub-parts per trillion level, suppress the NPF rate by a factor of 4. Ambient observations further confirm that when NO3 chemistry is involved, monoterpene NPF is completely turned off. Our results explain the frequent absence of nocturnal biogenic NPF in monoterpene (α-pinene)-rich environments.

Role of sesquiterpenes in biogenic new particle formation

Biogenic vapors form new particles in the atmosphere, affecting global climate. The contributions of monoterpenes and isoprene to new particle formation (NPF) have been extensively studied. However, sesquiterpenes have received little attention despite a potentially important role due to their high molecular weight. Via chamber experiments performed under atmospheric conditions, we report biogenic NPF resulting from the oxidation of pure mixtures of β-caryophyllene, α-pinene, and isoprene, which produces oxygenated compounds over a wide range of volatilities. We find that a class of vapors termed ultralow-volatility organic compounds (ULVOCs) are highly efficient nucleators and quantitatively determine NPF efficiency. When compared with a mixture of isoprene and monoterpene alone, adding only 2% sesquiterpene increases the ULVOC yield and doubles the formation rate. Thus, sesquiterpene emissions need to be included in assessments of global aerosol concentrations in pristine climates where biogenic NPF is expected to be a major source of cloud condensation nuclei.

The synergistic role of sulfuric acid, ammonia and organics in particle formation over an agricultural land

Agriculture provides people with food, but poses environmental challenges. Via comprehensive observations on an agricultural land at Qvidja in Southern Finland, we were able to show that soil-emitted compounds (mainly ammonia and amines), together with available sulfuric acid, form new aerosol particles which then grow to climate-relevant sizes by the condensation of extremely low volatile organic compounds originating from a side production of photosynthesis (compounds emitted by ground and surrounding vegetation). We found that intensive local clustering events, with particle formation rates at 3 nm about 5-10 times higher than typical rates in boreal forest environments, occur on around 30% of all days. The requirements for these clustering events to occur were found to be clear sky, a low wind speed to accumulate the emissions from local agricultural land, particularly ammonia, the presence of low volatile organic compounds, and sufficient gaseous sulfuric acid. The local clustering will then contribute to regional new particle formation. Since the agricultural land is much more effective per surface area than the boreal forest in producing aerosol particles, these findings provide insight into the participation of agricultural lands in climatic cooling, counteracting the climatic warming effects of farming.

Aerosol Alteration of Behavioral Response to Pheromone in Bombyx mori

Because of the complexity to study them, aerosols have been neglected in nearly all studies on olfaction, especially studies dealing with odor capture. However, aerosols are present in large quantities in the atmosphere and have the physico-chemical ability to interact with odor molecules, in particular the many pheromones with low volatility. We submitted male moths of Bombyx mori to bombykol puffs, the main fatty alcohol component of its sex pheromone, depending on whether the air is free of aerosols, charged with ambient concentration aerosols or supplemented with aqueous aerosols and recorded their arousal behavior. Aerosols and pheromone do interact consistently over all experiments and moths react better in low aerosol-concentration conditions. We propose four hypotheses for explaining this impediment, the two most likely resorting to competition between odor molecules and aerosols for the olfactory pores and postulate a reversal to a positive impact of aerosols on communication, depending on the particular physico-chemical properties of the multiphasic interaction. Studying the partitioning between gas and particulate phases in the transport and reception of odors is key for advancing the chemico-physical understanding of olfaction.

Measurement of atmospheric nanoparticles: Bridging the gap between gas-phase molecules and larger particles

Atmospheric nanoparticles are crucial components contributing to fine particulate matter (PM_2.5), and therefore have significant effects on visibility, climate, and human health. Due to the unique role of atmospheric nanoparticles during the evolution process from gas-phase molecules to larger particles, a number of sophisticated experimental techniques have been developed and employed for online monitoring and characterization of the physical and chemical properties of atmospheric nanoparticles, helping us to better understand the formation and growth of new particles. In this paper, we firstly review these state-of-the-art techniques for investigating the formation and growth of atmospheric nanoparticles (e.g., the gas-phase precursor species, molecular clusters, physicochemical properties, and chemical composition). Secondly, we present findings from recent field studies on the formation and growth of atmospheric nanoparticles, utilizing several advanced techniques. Furthermore, perspectives are proposed for technique development and improvements in measuring atmospheric nanoparticles.

Estimation of the nucleation barrier in a multicomponent system with intermolecular potential

The formation of a "critical nucleus" prior to phase change is a crucial step for new particle formation (NPF) in the atmosphere. However, the nucleation occurring below ∼1 nm is hard to observe directly. As an effective alternative, theoretical nucleation models have been widely studied. An energy barrier is involved in the nucleation and is the fundamental factor for the nucleation model. Typical atmospheric nucleation agents such as H₂SO₄, H₂O and NH₃ are dipole molecules, whose intermolecular interactions are non-ignorable. Herein, a dipole-dipole potential model is adopted to determine the interaction between molecules instead of the traditional hard sphere model, and graph theory is used to describe the structure of the cluster and the cluster-molecule interaction. The nucleation barriers (ΔE_b) of H₂SO₄-H₂SO₄, H₂SO₄-H₂O, H₂SO₄-NH₃ and H₂SO₄-H₂O-NH₃ are derived and compared to each other. In the presence of H₂O and NH₃, the ΔE_b value is decreased by 17-28% compared to that in the pure H₂SO₄ nucleation system. NH₃ is identified to be a key factor for ternary nucleation based on an orthogonal test. Atmospheric concentrations of H₂SO₄, H₂O and NH₃ are considered to investigate the influence of [H₂O + NH₃]/[H₂SO₄] on ΔE_b and the related effective collision coefficient (α). The α value in the ternary nucleation system reaches the range of (2.5-25) × 10^-5, which is 3-4 orders of magnitude higher than that in the pure H₂SO₄ system. Due to a significant enhancement of α, NH₃ and H₂O should be focused on in future aerosol particle estimation and control.

The contribution of new particle formation and subsequent growth to haze formation

We investigated the contribution of atmospheric new particle formation (NPF) and subsequent growth of the newly formed particles, characterized by high concentrations of fine particulate matter (PM_2.5). In addition to having adverse effects on visibility and human health, these haze particles may act as cloud condensation nuclei, having potentially large influences on clouds and precipitation. Using atmospheric observations performed in 2019 in Beijing, a polluted megacity in China, we showed that the variability of growth rates (GR) of particles originating from NPF depend only weakly on low-volatile vapor - highly oxidated organic molecules (HOMs) and sulphuric acid - concentrations and have no apparent connection with the strength of NPF or the level of background pollution. We then constrained aerosol dynamic model simulations with these observations. We showed that under conditions typical for the Beijing atmosphere, NPF is capable of contributing with more than 100 μg m^-3 to the PM_2.5 mass concentration and simultaneously >10³ cm^-3 to the haze particle (diameter > 100 nm) number concentration. Our simulations reveal that the PM_2.5 mass concentration originating from NPF, strength of NPF, particle growth rate and pre-existing background particle population are all connected with each other. Concerning the PM pollution control, our results indicate that reducing primary particle emissions might not result in an effective enough decrease in total PM_2.5 mass concentrations until a reduction in emissions of precursor compounds for NPF and subsequent particle growth is imposed.

Synergistic HNO3-H2SO4-NH3 upper tropospheric particle formation

New particle formation in the upper free troposphere is a major global source of cloud condensation nuclei (CCN)^1-4. However, the precursor vapours that drive the process are not well understood. With experiments performed under upper tropospheric conditions in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates that are orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high concentrations of ammonia and ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region^5,6. Once particles have formed, co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid growth to CCN sizes with only trace sulfate. Moreover, our measurements show that these CCN are also highly efficient ice nucleating particles-comparable to desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid, multi-acid HNO₃-H₂SO₄-NH₃ nucleation in the upper troposphere and producing ice nucleating particles that spread across the mid-latitude Northern Hemisphere.

Real-time monitoring of aerosol particle formation from sulfuric acid vapor at elevated concentrations and temperatures

In the present study, time-resolved aerosol particle formation from sulfuric acid vapor is examined with special attention to the stabilization of molecular clusters in the early phase of unary nucleation. An important factor governing this process is the amount of condensable acid vapor. Here it is produced from fast gas-phase reactions in a batch-type reaction cell for which we introduce modifications enabling real-time monitoring. The key component for size- and time-resolved detection of ultrafine particles is a new 1 nm-SMPS. With this new tool at hand, the effect of varying the precursor concentration over two orders of magnitude is investigated. We demonstrate the ability to tune between different growth scenarios as indicated by the size-resolved particle traces which exhibit a transition from sigmoidal over quasi-stationary to peak-like shape. The second key parameter relevant for nucleation studies is the temperature-dependent cluster evaporation. Due to a temperature rise during the mixing stage of the experiment, evaporation is strongly promoted in the early phase. Therefore, the present study extends the T-range used in, e.g., smog chambers. We investigate this temperature effect in a kinetic simulation and can successfully combine simulated and measured data for validating theoretical evaporation rates obtained from DLPNO-CCSD(T₀)-calculations.

What controls the observed size-dependency of the growth rates of sub-10 nm atmospheric particles?

The formation and growth of atmospheric particles involving sulfuric acid and organic vapors is estimated to have significant climate effects. To accurately represent this process in large-scale models, the correct interpretation of the observations on particle growth, especially below 10 nm, is essential. Here, we disentangle the factors governing the growth of sub-10 nm particles in the presence of sulfuric acid and organic vapors, using molecular-resolution cluster population simulations and chamber experiments. We find that observed particle growth rates are determined by the combined effects of (1) the concentrations and evaporation rates of the condensing vapors, (2) particle population dynamics, and (3) stochastic fluctuations, characteristic to initial nucleation. This leads to a different size-dependency of growth rate in the presence of sulfuric acid and/or organic vapors at different concentrations. Specifically, the activation type behavior, resulting in growth rate increasing with the particle size, is observed only at certain vapor concentrations. In our model simulations, cluster–cluster collisions enhance growth rate at high vapor concentrations and their importance is dictated by the cluster evaporation rates, which demonstrates the need for accurate evaporation rate data. Finally, we show that at sizes below ∼2.5–3.5 nm, stochastic effects can importantly contribute to particle population growth. Overall, our results suggest that interpreting particle growth observations with approaches neglecting population dynamics and stochastics, such as with single particle growth models, can lead to the wrong conclusions on the properties of condensing vapors and particle growth mechanisms.

A combination of cluster population simulations and chamber experiments was used to disentangle the factors governing the observed growth rates of atmospheric particles.

Direct measurement of curvature-dependent surface tension of an alcohol nanomeniscus

Surface tension is a key parameter for understanding nucleation in the very initial stage of phase transformation. Although surface tension has been predicted to vary with the curvature of the liquid-vapor interface, particularly at the large curvature of, e.g., the subnanometric critical nucleus, experimental study still remains challenging due to inaccessibility to such a small cluster. Here, by directly measuring the critical size of a single capillary-condensed nanomeniscus using atomic force microscopy, we address the curvature dependence of surface tension of alcohols and observe that the surface tension is doubled for ethanol and n-propanol with a radius-of-curvature of ∼-0.46 nm. We also find that the interface of larger negative (positive) curvature exhibits larger (smaller) surface tension, which evidently governs nucleation at the ∼1 nm scale and below, indicating more facilitated nucleation than normally expected. Such well characterized curvature effects contribute to better understanding and accurate analysis of nucleation occurring in various fields including materials science and atmospheric science.

Formation and growth of sub-3 nm particles in megacities: impact of background aerosols

New particle formation (NPF) occurs frequently in various atmospheric environments and contributes majorly to the aerosol number budget. In megacities, the high concentrations of gaseous precursors and background aerosols add complexity to this process. Based on long-term measurements (373 days) in urban Beijing, we examine the formation and growth of sub-3 nm particles under the effects of background aerosols, as indicated by the condensation sink (CS) or the Fuchs surface area. The median CS and the median PM_2.5 mass concentration for the days with NPF events were 0.03 s^-1 and 34 μg m^-3, respectively. The high loss rates of both molecular clusters and sub-3 nm particles to background aerosols reduce their atmospheric residence time and suppress their survival. As the key clusters for H₂SO₄-base nucleation, sulfuric acid dimer and trimer concentrations in Beijing decrease significantly when CS increases and the scavenging becomes stronger. The occurrence of NPF events and the formation of sub-3 nm particles in Beijing is governed by CS. 95% of the observed NPF days occurred with CS values below 0.03 s^-1. During NPF events, high concentrations of sub-3 nm particles were formed and they mostly ranged from 10³ to 10⁵ cm^-3 with a median value of 6.2 × 10³ cm^-3. Driven by the fast H₂SO₄-base nucleation, the daily maximum formation rate of 1.5 nm particles in Beijing has a mean value of 77 cm^-3 s^-1 and is much higher than that in clean environments. However, the mean growth rate of sub-3 nm particles in Beijing was only 2.6 nm h^-1, not significantly different from that in clean environments. The relatively low growth rate and the high level of scavenging by background aerosols result in low survival of newly formed particles. The analyses also reinforce prior results on the need to correct conventional methods to adequately quantify the formation and growth rates when analyzing data from megacities with strong coagulation scavenging due to background aerosols. The conventional balance formula underestimates the formation rate of 1.5 nm particles, while the conventional appearance time method overestimates the growth rate of sub-3 nm particles. These findings highlight the governing role of background aerosols in urban NPF.

Molecular Specificity and Proton Transfer Mechanisms in Aerosol Prenucleation Clusters Relevant to New Particle Formation

Atmospheric aerosol particles influence the Earth's radiative energy balance and cloud properties, thus impacting the air quality, human health, and Earth's climate change. Because of the important scientific and overarching practical implications of aerosols, the past two decades have seen extensive research efforts, with emphasis on the chemical compositions and underlying mechanisms of aerosol formation. It has been recognized that new particle formation (NPF) contributes up to 50% of atmospheric aerosols. Nowadays, the general consensus is that NPF proceeds via two distinct stages: the nucleation from gaseous precursors to form critical nuclei of sub-1-2 nm size, and the subsequent growth into large particles. However, a fundamental understanding of both the NPF process and molecular-level characterization of the critical size aerosol clusters is still largely missing, hampering the efforts in developing reliable and predictive aerosol nucleation and climate models.Both field measurements and laboratory experiments have gathered convincing evidence about the importance of volatile organic compounds (VOCs) in enhancing the nucleation and growth of aerosol particles. Numerous and abundant small clusters composed of sulfuric acid or bisulfate ion and organic molecules have been shown to exist in ∼2 nm sized aerosol particles. In particular, kinetic studies indicated the formation of clusters with one H₂SO₄ and one or two organics being the rate-limiting step.This Account discusses our effort in developing an integrated approach, which involves the laboratory cluster synthesis via electrospray ionization, size and composition analysis via mass spectrometry, photoelectron spectroscopic characterization, and quantum mechanics based theoretical modeling, to investigate the structures, energetics, and thermodynamics of the aerosol prenucleation clusters relevant to NPF. We have been focusing on the clusters formed between H₂SO₄ or HSO₄^- and the organics from oxidation of both biogenic and anthropogenic emissions. We illustrated the significant thermodynamic advantage by involving organic acids in the formation and growth of aerosol clusters. We revealed that the functional groups in the organics play critical roles in promoting NPF process. The enhanced roles were quantified explicitly for specific functional groups, establishing a Molecular Scale that ranks highly hierarchic intermolecular interactions critical to aerosol formation. The different cluster formation pathways, probably mimicking the various polluted industrial environments, that involve cis-pinonic and cis-pinic acids were unveiled as well. Furthermore, one intriguing fundamental phenomenon on the unusual protonation pattern, which violates the gas-phase acidity (proton affinity) prediction, was discovered to be common in sulfuric acid-organic clusters. The mechanism underlying the phenomenon has been rationalized by employing the temperature-dependent experiments of sulfuric acid-formate/halide model clusters, which could explain the high stability of the sulfuric acid containing aerosol clusters. Our work provides critical molecular-level information to shed light on the initial steps of nucleation of common atmospheric precursors and benchmarks critical data for large-scale theoretical modeling to further address problems of environmental interest.