Hannes Tammet

Size and mobility of nanometer particles, clusters and ions

Abstract The macroscopic model of a particle as a sphere with an exactly determined surface is not adequate in the nanometer size range. Two various parameters are used to describe the size of a particle. The difference between the collision radius and the mass radius of a particle is estimated to be 0.115 nm fitting a new semiempirical model to the experimental data. Transition from the elastic collisions specific for molecules to the inelastic collisions specific for macroscopic particles is described using the Einstein factor of the “melting” of the particle internal energy levels. Dipol polarization interaction is included into the model using the (∞ − 4) potential. The model is approaching the Chapman-Enskog equation in the free molecule limit and the Millikan equation in the macroscopic limit. An algorithm is presented to calculate the particle mobility and diffusion coefficient according to the parameters of ambient gas and the particle.

Electrical aerosol spectrometer of Tartu University

The electrical aerosol spectrometer (EAS) of the parallel measuring principle at Tartu University is an efficient instrument for rapid measurement of the unstable size spectrum of aerosol particles. The measuring range from 10 nm to 10 μm is achieved by simultaneously using a pair of differential mobility analyzers with two different particle chargers. The particle spectrum is calculated and measurement errors are estimated in real time by using a least-squares method. Experimental calibration ensures reliability of measurement. The instrument is well suited for continuous monitoring of atmospheric aerosol.

Bursts of intermediate ions in atmospheric air

The mobility spectrum of air ions has been measured at Tahkuse Observatory in Estonia for several years. The average concentration of intermediate ions with mobilities of 0.05-0.5 cm 2 V -1 s -1 in atmospheric air is about 50 cm -3 , On the level of this low background, high concentration bursts of intermediate air ions occur occasionally. A burst can be followed by subsequent evolution of intermediate ions into larger ones. To explain the bursts of intermediate air ions, two hypotheses can be advanced: (1) A burst of neutral particles occurs due to homogeneous nucleation, and the particles are charged by the attachment of cluster ions. (2) The cluster ions grow by ion-induced nucleation in proper environmental conditions.

Statistical characterization of air ion mobility spectra at Tahkuse Observatory: Classification of air ions

A database of 8615 hourly averaged air ion mobility spectra in the range of 0.00041–3.2 cm2 V−1 s−1 was measured at Tahkuse Observatory, Estonia, during 14 months in 1993–1994. The average mobility spectrum over the whole period shows distinct peaks of small and large ions. Intermediate ions with mobilities of 0.034–0.5 cm2 V−1 s−1 are of low concentration of about 50 cm−3 in the average spectrum. They experience occasional bursts of up to about 900 cm−3 during 6–10 hours at daytime. The number of burst events recorded during 14 months was 101, with maximum frequency in spring and minimum frequency in winter. Physically, large and intermediate ions can be called aerosol ions, and small ions can be called cluster ions. The principal component analysis was applied to detect the structure of an air ion mobility spectrum. As a result, the mobility spectrum in the range of 0.00041–3.2 cm2 V−1 s−1 (diameters of 0.36–79 nm) was divided into five classes: small cluster, big cluster, intermediate, light large, and heavy large ions. The boundaries between the classes are 1.3 cm2 V−1 s−1 (diameter of 0.85 nm), 0.5 cm2 V−1 s−1 (1.6 nm), 0.034 cm2 V−1 s−1 (7.4 nm), and 0.0042 cm2 V−1 s−1 (22 nm). The five principal components that are closely correlated with the respective ion classes explain 92% of total variance. The classification of aerosol ions is in accord with the three-modal structure of the size spectrum of submicron aerosol particles.

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.

Variation of fair weather atmospheric electricity at Marsta Observatory, Sweden, 1993-1998

Abstract A modified atmospheric electrical station of the Kasemir–Dolezalek construction is continuously operating in the Marsta Observatory (59°56′N,17°35′W) located in rural area 10 km north of Uppsala, Sweden. The routinely recorded parameters are the electric field, positive and negative polar conductivities of air, and space charge density. The effect of possible local anthropogenic air pollution on the fair weather atmospheric electric measurements at Marsta is estimated according to Sheftel et al., 1994a (J. Geophys. Res. 99, 10,793) by comparing the Sunday and weekday values of air conductivity. The effect of local air pollution appears essentially less than at other evaluated continental atmospheric electric stations. The natural periodic variations of fair weather electric field and vertical air-earth current averaged over many years at Marsta are compared with the periodic variations of electric field measured during the Carnegie expeditions over the oceans where the global component of variations dominates over the local component. The diurnal variations of electric field and vertical current at Marsta are well correlated with the Carnegie curve during winter and ill correlated during summer. The correlation coefficient reaches 98% for the winter measurements of vertical air-earth current. In addition, a test has been carried out for a hypothesis that numerical reduction of the data according to the local temperature and wind variation could suppress the local component of fair weather atmospheric electric variations and thus help to study the global component of variation. The hypothesis proved to be inadequate. The reduction suppresses the annual variation, but the shape of the diurnal variation remains the same and the correlation with the Carnegie curve is even worse than in the case of unreduced measurements. The Marsta Observatory is recommended as a basis station for long-term routine atmospheric electric measurements to gather data for the study of climate variation because of the large weight of global component in the variation of fair weather air-earth vertical current and electric field.

Mobility spectrum of air ions at Tahkuse Observatory

Mobility spectra of air ions have been measured at a rural site in Estonia during several periods. The annual average mobility spectrum of natural small air ions is presented. The concentrations of two groups of air ions with mobilities 0.32–0.5 cm2/(V s) and 0.5–2.5 cm2/(V s) are not correlated; this fact indicates the different nature of the ions of the two groups. The air ions with mobilities 0.5–2.5 cm2/(V s) are interpreted as cluster ions and the air ions with mobilities 0.32–0.5 cm2/(V s) as charged aerosol particles that can be created in the process of ion-induced nucleation. A half-year average mobility spectrum of the large ions with mobilities 3.2×10−4–1.5×10−1 cm2/(V s) is presented. The spectrum is well interpreted on the basis of the average size distribution of aerosol particles and on the theory of diffusion charging of the particles.

Reduction of air ion mobility to standard conditions

The Langevin rule of the reduction of air ion mobility is adequate in case of zero-size ions. An alternative is the Stokes-Millikan equation that is adequate in the limit of macroscopic charged particles. The temperature variation of air ion mobility predicted by the Stokes-Millikan equation radically contradicts the Langevin rule. The temperature and pressure variation of air ion mobility is examined by using a new semiempirical model that describes the transition from the kinetic theory to the Stokes-Millikan equation. The model is valid in full mobility range. It allows to calculate at first the size of an ion according to the measured mobility and then the standard mobility according to the size. The ascent of the temperature-mobility curve on a logarithmic chart approaches the Langevin value of 1 only at very high mobilities not found in the atmosphere. The value of the ascent is 0.6 in the case of small ions of the mobility of 1.5 cm V - s - which brings about a considerable error when using the Langevin rule. It is recommended to store the natural values of the mobility in databases together with the values of temperature and pressure and to definitely indicate the method when the reduced mobilities are presented in publications.

Observations on nocturnal growth of atmospheric clusters

In this paper, we summarize recent observations of nighttime nucleation events observed during 4 yr, from 2003 to 2006, at the SMEAR II station in Hyytiäläa, southern Finland. Formation of new atmospheric aerosol particles has been frequently observed all around the world in daytime, but similar observations in nighttime are rare. The recently developed ion spectrometers enabled us to measure charged aerosol particles and ion clusters to diameters < 1 nm and are efficient tools for evaluating cluster dynamics during nighttime. We observed clear growth of cluster ions during approximately 60 nights per yr. The newly formed intermediate ions usually persisted for several hours with typical concentrations of 100–200 cm-3. The evolution of nighttime growth events is different compared with daytime events. The mechanism behind nighttime events is still unclear, but the behaviour can be described by the hypothesis of activation of clusters.

Air ion measurements as a source of information about atmospheric aerosols

The mobility spectra of air ions recorded in the course of routine atmospheric electric measurements contain information about atmospheric aerosols. The mobility spectrum of air ions is correlated with the size spectrum of aerosol particles. Two procedures of conversion (and conversion errors) are considered in this paper assuming the steady state of charge distribution. The first procedure uses the fraction model of the aerosol particle size distribution and algebraic solution of the conversion problem. The second procedure uses the parametric KL model of the particle size distribution and the least square fitting of the mobility measurements. The procedures were tested using simultaneous side-by-side measurements of air ion mobilities and aerosol particle size distributions at a rural site during a monthly period. The comparison of results shows a promising agreement between the measured and calculated size spectra in the common size range. A supplementary information about nanometer particles was obtained from air ion measurements.