H. E. Manninen

Direct Observations of Atmospheric Aerosol Nucleation

Aerosol Formation Most atmospheric aerosol particles result from a growth process that begins with atmospheric molecules and clusters, progressing to larger and larger sizes as they acquire other molecules, clusters, and particles. The initial steps of this process involve very small entities—with diameters of less than 2 nanometers—which have been difficult to observe. Kulmala et al. (p. 943; see the Perspective by Andreae) developed a sensitive observational protocol that allows these tiny seeds to be detected and counted, and they mapped out the process of aerosol formation in detail. Detailed aerosol measurements provide a consistent framework for the formation of particles from atmospheric gases. [Also see Perspective by Andreae] Atmospheric nucleation is the dominant source of aerosol particles in the global atmosphere and an important player in aerosol climatic effects. The key steps of this process occur in the sub–2-nanometer (nm) size range, in which direct size-segregated observations have not been possible until very recently. Here, we present detailed observations of atmospheric nanoparticles and clusters down to 1-nm mobility diameter. We identified three separate size regimes below 2-nm diameter that build up a physically, chemically, and dynamically consistent framework on atmospheric nucleation—more specifically, aerosol formation via neutral pathways. Our findings emphasize the important role of organic compounds in atmospheric aerosol formation, subsequent aerosol growth, radiative forcing and associated feedbacks between biogenic emissions, clouds, and climate.

Measurement of the nucleation of atmospheric aerosol particles

The formation of new atmospheric aerosol particles and their subsequent growth have been observed frequently at various locations all over the world. The atmospheric nucleation rate (or formation rate) and growth rate (GR) are key parameters to characterize the phenomenon. Recent progress in measurement techniques enables us to measure atmospheric nucleation at the size (mobility diameter) of 1.5 (±0.4) nm. The detection limit has decreased from 3 to 1 nm within the past 10 years. In this protocol, we describe the procedures for identifying new-particle-formation (NPF) events, and for determining the nucleation, formation and growth rates during such events under atmospheric conditions. We describe the present instrumentation, best practices and other tools used to investigate atmospheric nucleation and NPF at a certain mobility diameter (1.5, 2.0 or 3.0 nm). The key instruments comprise devices capable of measuring the number concentration of the formed nanoparticles and their size, such as a suite of modern condensation particle counters (CPCs) and air ion spectrometers, and devices for characterizing the pre-existing particle number concentration distribution, such as a differential mobility particle sizer (DMPS). We also discuss the reliability of the methods used and requirements for proper measurements and data analysis. The time scale for realizing this procedure is 1 year.

Particle Size Magnifier for Nano-CN Detection

A new particle size magnifier (PSM) for detection of nano-CN as small as ∼1 nm in mobility diameter was developed, calibrated and tested in atmospheric measurements. The working principle of a PSM is to mix turbulently cooled sample flow with heated clean air flow saturated by the working fluid. This provides a high saturation ratio for the working fluid and activates the seed particles and grows them by condensation of the working fluid. In order to reach high saturation ratios, and thus to activate nano-CN without homogeneous nucleation, diethylene glycol was chosen as the working fluid. The PSM was able to grow nano-CN to mean diameter of 90 nm, after which an ordinary condensation particle counter was used to count the grown particles (TSI 3010). The stability of the PSM was found to be good making it suitable for stand-alone field measurements. Calibration results show that the detection efficiency of the prototype PSM + TSI 3010 for charged tetra-alkyl ammonium salt molecules having mobility equivalent diameters of 1.05, 1.47, 1.78, and 2.57 nm are 25, 32, 46, and 70%, respectively. The commercial version of the PSM (Airmodus A09) performed even better in the smallest sizes the detection efficiency being 51% for 1.47 nm and 67% for 1.78 nm.

New particle formation in the free troposphere: A question of chemistry and timing

From neutral to new Many of the particles in the troposphere are formed in situ, but what fraction of all tropospheric particles do they constitute and how exactly are they made? Bianchi et al. report results from a high-altitude research station. Roughly half of the particles were newly formed by the condensation of highly oxygenated multifunctional compounds. A combination of laboratory results, field measurements, and model calculations revealed that neutral nucleation is more than 10 times faster than ion-induced nucleation, that particle growth rates are size-dependent, and that new particle formation occurs during a limited time window. Science, this issue p. 1109 New particles form in the free troposphere mainly through condensation of highly oxygenated compounds. New particle formation (NPF) is the source of over half of the atmosphere’s cloud condensation nuclei, thus influencing cloud properties and Earth’s energy balance. Unlike in the planetary boundary layer, few observations of NPF in the free troposphere exist. We provide observational evidence that at high altitudes, NPF occurs mainly through condensation of highly oxygenated molecules (HOMs), in addition to taking place through sulfuric acid–ammonia nucleation. Neutral nucleation is more than 10 times faster than ion-induced nucleation, and growth rates are size-dependent. NPF is restricted to a time window of 1 to 2 days after contact of the air masses with the planetary boundary layer; this is related to the time needed for oxidation of organic compounds to form HOMs. These findings require improved NPF parameterization in atmospheric models.

An Instrumental Comparison of Mobility and Mass Measurements of Atmospheric Small Ions

Ambient, naturally charged small ions (<2000 Da) were measured in Hyytiälä, Finland, with a mass spectrometer (atmospheric pressure interface time-of-flight, APi-TOF) and two mobility spectrometers (air ion spectrometer, AIS, and balanced scanning mobility analyzer, BSMA). To compare these different instrument types, a mass/mobility conversion and instrumental transfer functions are required to convert high-resolution mass spectra measured by the APi-TOF into low-resolution mobility spectra measured by the AIS and BSMA. A modified version of the Stokes-Millikan equation was used to convert between mass and mobility. Comparison of APi-TOF and BSMA results showed good agreement, especially for sizes above 200 Da (Pearsons R = 0.7–0.9). Below this size, agreement was fair, and broadening BSMA transfer functions improved the correlation. To achieve equally good agreement between APi-TOF and AIS, AIS results needed to be shifted by 1–1.5 mobility channels. The most likely cause was incorrect sizing in the AIS. In summary, the mass and mobility spectrometers complement each other, with the APi-TOF giving superior chemical information, limited to relatively small ions (<2.5 nm diameter), whereas the mobility spectrometers are better suited for quantitative concentration measurements up to 40 nm. The BSMA and AIS were used to infer a transmission function for the APi-TOF, making it possible to give quantitative estimates of the concentrations of detected chemical ions.

On Operation of the Ultra-Fine Water-Based CPC TSI 3786 and Comparison with Other TSI Models (TSI 3776, TSI 3772, TSI 3025, TSI 3010, TSI 3007)

In this study we examined performance characteristics of an ultrafine water condensation particle counter (UWCPC, TSI3786). The detection efficiency was investigated using different temperature differences between saturator and growth tube. The cut-sizes D90, D50, D10, and D0 were determined by fitting a two-free-parameter equation to the experimental data. The determined cut-sizes were comparable (± 8%) with other two widely used fitting equations. The cut-sizes were studied changing the growth tube temperature from 65 to 78°C and varying the saturator temperature from 8 to 20°C. For silver particles the smallest detected cut-size D50 was 2.9 nm, and the largest one –4.5 nm, and in default operation conditions it was 3.9 nm. Additionally, the effect of particle chemical composition on the detection efficiency was studied. The cut-sizes D50 were 2 2.9, 2.3, and 1.8 nm for silver, ammonium sulfate, and sodium chloride particles, respectively. A concentration calibration was performed with high particle number concentrations. Within ±10% accuracy the highest reliable measured number concentration was 100000 cm −3 . The determined detection efficiency of the UWCPC was compared with other commercial CPCs (TSI3785, TSI3776, TSI3772, TSI3025, TSI3010, TSI3007) using default operation regimes of the instruments. The results show that the tested UWCPC has a larger cut-size for silver particles than do butanol-based ultrafine CPCs (TSI3776, TSI3025), but smaller cut-size than other tested TSI CPCs. In default operation regime, the tested TSI3776 had the lowest detection limit (D50% = 3.2 nm) of the silver particles, and the corresponding size of TSI3025 was 3.6 nm.

Detection Efficiency of a Water-Based TSI Condensation Particle Counter 3785

In this article we present observations on the detection efficiency of a recently developed TSI 3785 Water Condensation Particle Counter (WCPC). The instrument relies on activation of sampled particles by water condensation. The supersaturation is generated by directing a saturated airflow into a “growth tube,” in which the mass transfer of water vapor is faster than heat transfer. This results in supersaturated conditions with respect to water vapor in the centerline of a “growth tube.” In this study, the cut-off diameter, that is, the size, where 50% of the sampled particles are successfully activated, varied from 4 to 14 nm for silver particles as a function of temperature difference between the saturator and the growth tube. The solubility of the sampled particles to water played an important role in the detection efficiency. Cut-off diameters for ammonium sulphate and sodium chloride particles were 5.1 and 3.6–3.8 nm, respectively at nominal operation conditions. Corresponding cut-off diameter for hydrophobic silver particles was 5.8 nm.

Amazon boundary layer aerosol concentration sustained by vertical transport during rainfall

The nucleation of atmospheric vapours is an important source of new aerosol particles that can subsequently grow to form cloud condensation nuclei in the atmosphere. Most field studies of atmospheric aerosols over continents are influenced by atmospheric vapours of anthropogenic origin (for example, ref. 2) and, in consequence, aerosol processes in pristine, terrestrial environments remain poorly understood. The Amazon rainforest is one of the few continental regions where aerosol particles and their precursors can be studied under near-natural conditions, but the origin of small aerosol particles that grow into cloud condensation nuclei in the Amazon boundary layer remains unclear. Here we present aircraft- and ground-based measurements under clean conditions during the wet season in the central Amazon basin. We find that high concentrations of small aerosol particles (with diameters of less than 50 nanometres) in the lower free troposphere are transported from the free troposphere into the boundary layer during precipitation events by strong convective downdrafts and weaker downward motions in the trailing stratiform region. This rapid vertical transport can help to maintain the population of particles in the pristine Amazon boundary layer, and may therefore influence cloud properties and climate under natural conditions.

Ion – particle interactions during particle formation and growth at a coniferous forest site in central Europe

13 In this work, we examined the interaction of ions and neutral particles during atmospheric 14 new particle formation (NPF) events. The analysis is based on simultaneous field 15 measurements of atmospheric ions and total particles using a neutral cluster and air ion 16 spectrometer (NAIS) across the diameter range 2 25 nm. The “Waldstein” research site is 17 located in a spruce forest in NE Bavaria, Southern Germany, known for enhanced radon 18 concentrations, presumably leading to elevated ionization rates. Our observations show that 19 the occurrence of the ion nucleation mode preceded that of the total particle nucleation mode 20 during all analysed NPF events. The time difference between the appearance of 2 nm ions and 21 2 nm total particles was typically about 20 to 30 minutes. A cross correlation analysis showed 22 a rapid decrease of the time difference between the ion and total modes during the growth 23 process. Eventually, this time delay vanished when both ions and total particles did grow to 24 larger diameters. Considering the growth rates of ions and total particles separately, total 25 particles exhibited enhanced growth rates at diameters below 15 nm. This observation cannot 26 be explained by condensation or coagulation, because these processes would act more 27 efficiently on charged particles compared to neutral particles. To explain our observations, we 28

Counting Efficiency of a TSI Environmental Particle Counter Monitor Model 3783

In this study, we present our investigations on the counting efficiency of a TSI Environmental Particle Counter (EPC) Monitor Model 3783. The operation of the instrument is based on activation of sampled particles by water condensation, thus making it essentially a water-based condensation particle counter (WCPC). The counting efficiency was measured and simulated using different temperature settings on the conditioner and the growth tube, as well as using hygroscopic and water-soluble ammonium sulfate and sodium chloride particles, and insoluble and hydrophobic silver particles. The parameter that best describes the performance of the instrument is the cut-off diameter, or the diameter where 50% of sampled particles are activated and detected. In this study, the cut-off diameter for silver particles varied from 5.2 nm to 7.4 nm as a function of the temperature difference between the conditioner and the growth tube. The chemical composition and, specifically, the water solubility of particle components, had a significant effect on the counting efficiency. Using the standard operation settings of the EPC, the cut-off diameter for silver particles was 6.4 nm, and for ammonium sulfate and sodium chloride particles it was 4.8 nm and 4.3 nm, respectively. The simulations made with Comsol Multiphysics 3.5a simulation software gave the silver-water contact angle in the range of 29–45° depending on the temperature difference between the conditioner and the growth tube, and the effective contact angles of 25° and 21° for ammonium sulfate and sodium chloride particles. Copyright 2013 American Association for Aerosol Research