Understanding the Formation and Growth of New Atmospheric Particles at the Molecular Level through Laboratory Molecular Beam Experiments

Atmospheric new particle formation (NPF), which exerts comprehensive implications for climate, air quality and human health, has received extensive attention. From molecule to cluster is the initial and most important stage of the nucleation process of atmospheric new particles. However, due to the complexity of the nucleation process and limitations of experimental characterization techniques, there is still a great uncertainty in understanding the nucleation mechanism at the molecular level. Laboratory-based molecular beam methods can experimentally implement the generation and growth of typical atmospheric gas-phase nucleation precursors to nanoscale clusters, characterize the key physical and chemical properties of clusters such as structure and composition, and obtain a series of their physicochemical parameters, including association rate coefficients, electron binding energy, pickup cross section and pickup probability and so on. These parameters can quantitatively illustrate the physicochemical properties of the cluster, and evaluate the effect of different gas phase nucleation precursors on the formation and growth of atmospheric new particles. We review the present literatures on atmospheric cluster formation and reaction employing the experimental method of laboratory molecular beam. The experimental apparatuses were classified and summarized from three aspects of cluster generation, growth and detection processes. Focus of this review is on the properties of nucleation clusters involving different precursor molecules of water, sulfuric acid, nitric acid and NxOy, respectively. We hope this review will provide a deep insight for effects of cluster physicochemical properties on nucleation, and reveal the formation and growth mechanism of atmospheric new particle at the molecular level.

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.

Electrospray Ionization-Based Synthesis and Validation of Amine-Sulfuric Acid Clusters of Relevance to Atmospheric New Particle Formation

Atmospheric new particle formation (NPF) is the process by which atmospheric trace gases such as sulfuric acid, ammonia, and amines cluster and grow into climatically relevant particles. The mechanism by which these particles form and grow has remained unclear, in large part due to difficulties in obtaining molecular-level information about the clusters as they grow. Mass spectrometry-based methods using electrospray ionization (ESI) as a cluster source have shed light on this process, but the produced cluster distributions have not been rigorously validated against experiments performed in atmospheric conditions. Ionic clusters are produced by ESI of solutions containing the amine and bisulfate or by spraying a sulfuric acid solution and introducing trace amounts of amine gas into the ESI environment. The amine content of clusters can be altered by increasing the amount of amine introduced into the ESI environment, and certain cluster compositions can only be made by the vapor exchange method. Both approaches are found to yield clusters with the same structures. Aminium bisulfate cluster distributions produced in a controlled and isolated ESI environment can be optimized to closely resemble those observed by chemical ionization in the CLOUD chamber at CERN. These studies indicate that clusters generated by ESI are also observed in traditional atmospheric measurements, which puts ESI mass spectrometry-based studies on firmer footing and broadens the scope of traditional mass spectrometry experiments that may be applied to NPF.

The Interplay Between Hydrogen Bonding and Coulombic Forces in Determining the Structure of Sulfuric Acid-Amine Clusters

Acid-base cluster chemistry drives atmospheric new particle formation (NPF), but the details of the growth mechanisms are difficult to experimentally probe. Clusters of ammonia, alkylamines, and sulfuric acid, species fundamental to NPF, are probed by infrared spectroscopy. These spectra show that substitution of amines for ammonia, which is linked to accelerated growth, induces profound structural rearrangement in clusters with initial compositions (NH₄⁺) _n+1(HSO₄^-) _n (1 ≤ n ≤ 3). This rearrangement is driven by the loss of N-H hydrogen bond donors, yielding direct bisulfate-bisulfate hydrogen bonds, and its onset with respect to cluster composition indicates that more substituted amines induce rearrangement at smaller sizes. A simple model counting hydrogen bond donors and acceptors explains these observations. The presence of direct hydrogen bonds between formal anions shows that hydrogen bonding can compete with Coulombic forces in determining cluster structure. These results suggest that NPF mechanisms may be highly dependent on amine identity.

Mechanisms of Atmospherically Relevant Cluster Growth

Atmospheric aerosols impact global climate either directly by scattering solar radiation or indirectly by serving as cloud condensation nuclei, which influence cloud albedo and precipitation patterns. Our scientific understanding of these impacts is poor relative to that of, for instance, greenhouse gases, in part because it is difficult to predict particle number concentrations. One important pathway by which particles are added to the atmosphere is new particle formation, where gas phase precursors form molecular clusters that subsequently grow to the climatically relevant size range (50-100 nm diameter). It is predicted that up to 50% of atmospheric particles arise from this process, but the key initial chemical processes are poorly resolved. In general, a combination of inorganic and organic molecules are thought to contribute to new particle formation, but the chemical composition of molecular clusters and pathways by which they grow to larger sizes is unclear. Cluster growth is a key component of new particle formation, as it governs whether molecular clusters will become climatically relevant. This Account discusses our recent work to understand the mechanisms underlying new particle growth. Atmospherically relevant molecular clusters containing the likely key contributors to new particle formation (sulfuric acid, ammonia, amines, and water) were investigated experimentally by Fourier transform mass spectrometry as well as computationally by density functional theory. Our laboratory experiments investigated the molecular composition of charged clusters, the molecular pathways by which these clusters may grow, and the kinetics of base incorporation into them. Computational chemistry allowed confirmation and rationalization of the experimental results for charged clusters and extension of these principles to uncharged and hydrated clusters that are difficult to study by mass spectrometry. This combination of approaches enabled us to establish a framework for cluster growth involving sulfuric acid, ammonia, amines, and water. Charged or uncharged, cluster growth occurs primarily through an ammonium (or aminium) bisulfate coordinate. In these clusters, proton transfer is maximized between acids and bases to produce cations (ammonium, aminium) and anions (bisulfate), whereas additional molecules (water and unneutralized sulfuric acid) remain un-ionized. Experimental measurements suggest the growth of positively charged clusters occurs by successive acidification and neutralization steps. The acidification step is nearly barrierless, whereas the neutralization step exhibits a significant activation barrier in the case of ammonia. Bases are also incorporated into these clusters by displacement of one base for another. Base displacement is barrierless on the cluster surface but not within the cluster core. The favorability of amines relative to ammonia in charged clusters is governed by the trade-off between gas phase basicity and binding energetics. Computational studies indicate that water has a relatively small effect on cluster energetics. In short, amines are effective at assisting the formation and initial growth of clusters but become less important as cluster size increases, especially when hydration is considered. More generally, this work shows how experiment and computation can provide important, complementary information to address problems of environmental interest.

New Particle Formation from Methanesulfonic Acid and Amines/Ammonia as a Function of Temperature

Previous studies have shown that methanesulfonic acid (MSA) reacts with amines and ammonia to form particles, which is expected to be particularly important in coastal and agricultural areas. We present the first systematic study of temperature dependence of particle formation from the reactions of MSA with trimethylamine (TMA), dimethylamine (DMA), methylamine (MA), and ammonia over the range of 21-28 °C and 0.4-5.9 s in a flow reactor under dry conditions and in the presence of 3 × 10¹⁷ cm^-3 water vapor. Overall activation energies (E_overall) for particle formation calculated from the dependence of rates of particle formation on temperature for all of these bases are negative. The negative E_overall is interpreted in terms of reverse reactions that decompose intermediate clusters in competition with the forward reactions that grow the clusters into particles. The average values of E_overall for the formation of detectable particles are: TMA, -(168 ± 19) kcal mol^-1; DMA, -(134 ± 30) kcal mol^-1; MA, -(68 ± 23) kcal mol^-1; NH₃, -(110 ± 16) kcal mol^-1 (±1σ). The strong inverse dependence of particle formation with temperature suggests that particle formation may not decline proportionally with concentrations of MSA and amines if temperature also decreases, for example at higher altitudes or in winter.

Introductory lecture: atmospheric chemistry in the Anthropocene

The term “Anthropocene” was coined by Professor Paul Crutzen in 2000 to describe an unprecedented era in which anthropogenic activities are impacting planet Earth on a global scale. Greatly increased emissions into the atmosphere, reflecting the advent of the Industrial Revolution, have caused significant changes in both the lower and upper atmosphere. Atmospheric reactions of the anthropogenic emissions and of those with biogenic compounds have significant impacts on human health, visibility, climate and weather. Two activities that have had particularly large impacts on the troposphere are fossil fuel combustion and agriculture, both associated with a burgeoning population. Emissions are also changing due to alterations in land use. This paper describes some of the tropospheric chemistry associated with the Anthropocene, with emphasis on areas having large uncertainties. These include heterogeneous chemistry such as those of oxides of nitrogen and the neonicotinoid pesticides, reactions at liquid interfaces, organic oxidations and particle formation, the role of sulfur compounds in the Anthropocene and biogenic–anthropogenic interactions. A clear and quantitative understanding of the connections between emissions, reactions, deposition and atmospheric composition is central to developing appropriate cost-effective strategies for minimizing the impacts of anthropogenic activities. The evolving nature of emissions in the Anthropocene places atmospheric chemistry at the fulcrum of determining human health and welfare in the future.

Aerosol Chemistry Resolved by Mass Spectrometry: Insights into Particle Growth after Ambient New Particle Formation

Atmospheric oxidation of volatile organic compounds (VOCs) yields a large number of different organic molecules which comprise a wide range of volatility. Depending on their volatility, they can be involved in new particle formation and particle growth, thus affecting the number concentration of cloud condensation nuclei in the atmosphere. Here, we identified oxidation products of VOCs in the particle phase during a field study at a rural mountaintop station in central Germany. We used atmospheric pressure chemical ionization mass spectrometry ((-)APCI-MS) and aerosol mass spectrometry for time-resolved measurements of organic species and of the total organic aerosol (OA) mass in the size range of 0.02-2.5 and 0.05-0.6 μm, respectively. The elemental composition of organic molecules was determined by offline analysis of colocated PM 2.5 filter samples using liquid chromatography coupled to electrospray ionization ultrahigh-resolution mass spectrometry. We found extremely low volatile organic compounds, likely from sesquiterpene oxidation, being the predominant signals in the (-)APCI-MS mass spectrum during new particle formation. Low volatile organic compounds started to dominate the spectrum when the newly formed particles were growing to larger diameters. Furthermore, the APCI-MS mass spectra pattern indicated that the average molecular weight of the OA fraction ranged between 270 and 340 amu, being inversely related to OA mass. Our observations can help further the understanding of which biogenic precursors and which chemical processes drive particle growth after atmospheric new-particle formation.

The effect of acid-base clustering and ions on the growth of atmospheric nano-particles

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.

Silicon is a frequent component of atmospheric nanoparticles

Nanoparticles are the largest fraction of aerosol loading by number. Knowledge of the chemical components present in nanoparticulate matter is needed to understand nanoparticle health and climatic impacts. In this work, we present field measurements using the Nano Aerosol Mass Spectrometer (NAMS), which provides quantitative elemental composition of nanoparticles around 20 nm diameter. NAMS measurements indicate that the element silicon (Si) is a frequent component of nanoparticles. Nanoparticulate Si is most abundant in locations heavily impacted by anthropogenic activities. Wind direction correlations suggest the sources of Si are diffuse, and diurnal trends suggest nanoparticulate Si may result from photochemical processing of gas phase Si-containing compounds, such as cyclic siloxanes. Atmospheric modeling of oxidized cyclic siloxanes is consistent with a diffuse photochemical source of aerosol Si. More broadly, these observations indicate a previously overlooked anthropogenic source of nanoaerosol mass. Further investigation is needed to fully resolve its atmospheric role.

Observations on the formation, growth and chemical composition of aerosols in an urban environment

The charge and chemical composition of ambient particles in an urban environment were determined using a neutral particle and air ion spectrometer and an aerodyne compact time-of-flight aerosol mass spectrometer. Particle formation and growth events were observed on 20 of the 36 days of sampling, with eight of these events classified as strong. During these events, peaks in the concentration of intermediate and large ions were followed by peaks in the concentration of ammonium and sulfate, which were not observed in the organic fraction. Comparison of days with and without particle formation events revealed that ammonium and sulfate were the dominant species on particle formation days while high concentrations of biomass burning OA inhibited particle growth. Analyses of the degree of particle neutralization lead us to conclude that an excess of ammonium enabled particle formation and growth. In addition, the large ion concentration increased sharply during particle growth, suggesting that during nucleation the neutral gaseous species ammonia and sulfuric acid react to form ammonium and sulfate ions. Overall, we conclude that the mechanism of particle formation and growth involved ammonia and sulfuric acid, with limited input from organics.

Concluding remarks: challenges for aerosols and climate

We study aerosols for many reasons, including their effects on human health and climate. For climate, it is important to distinguish between the overall radiative effect of aerosols and the radiative forcing, which has been the anthropogenic change (after rapid atmospheric adjustments) since pre-industrial times. The radiative forcing is in principle much harder to observe than the overall effect because one must understand which particles are natural in today's atmosphere and what aerosols were like in the atmosphere before large-scale human influence. Because we cannot go back and measure the past, the only way to calculate radiative forcing may often require modeling detailed aerosol processes. This is a motivation for many of the processes studied at the Faraday Discussion 165. Other processes may need more attention by the aerosol climate community.