J. Salm

Physical characterization of aerosol particles during nucleation events

Particle concentrations and size distributions have been measured from different heights inside and above a boreal forest during three BIOFOR campaigns (14 April–22 May 1998, 27 July–21 August 1998 and 20 March–24 April 1999) in Hyytiälä, Finland. Typically, the shape of the background distribution inside the forest exhibited 2 dominant modes: a fine or Aitken mode with a geometric number mean diameter of 44 nm and a mean concentration of 1160 cm−3 and an accumulation mode with mean diameter of 154 nm and a mean concentration of 830 cm−3. A coarse mode was also present, extending up to sizes of 20 μm having a number concentration of 1.2 cm−3, volume mean diameter of 2.0 μm and a geometric standard deviation of 1.9. Aerosol humidity was lower than 50% during the measurements. Particle production was observed on many days, typically occurring in the late morning. Under these periods of new particle production, a nucleation mode was observed to form at diameter of the order of 3 nm and, on most occasions, this mode was observed to grow into Aitken mode sizes over the course of a day. Total concentrations ranged from 410–45 000 cm−3, the highest concentrations occurring on particle production days. A clear gradient was observed between particle concentrations encountered below the forest canopy and those above, with significantly lower concentrations occurring within the canopy. Above the canopy, a slight gradient was observed between 18 m and 67 m, with at maximum 5% higher concentration observed at 67 m during the strongest concentration increases.

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.

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.

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.

Observation of condensation on small air ions in the atmosphere

Air ion mobility spectrum was recorded at a rural site in a period of one year. Examples of spectra are explained on the basis of the hypothesis about the condensation of trace gases on cluster ions.

Derivation of the size spectrum of aerosol particles from a mobility spectrum

Publisher Summary Air–ion mobility spectrum is a traditional subject of atmospheric electric research. The electric mobility is well correlated with the particle size in the range of intermediate and large air ions. Correspondingly, a mobility spectrum can be transformed into a size spectrum and the atmospheric electric measurements can be used as a source of information about the atmospheric aerosol particle size-distribution. The mobility spectra of air ions measured in atmospheric electrical research give essential information about atmospheric aerosols. The air–ions and aerosol particles were simultaneously measured during a monthly period at a rural site, and the aerosol particle size-spectrum was derived from the air ion measurements. The comparison of results shows satisfactory agreement between the measured size spectra at the common size subrange and essential supplementary information about nanometer particles extracted from air ion measurements.

Characterization of atmospheric aerosols according to atmospheric-electric measurements

Atmospheric-electric measurements consist of considerable information about the atmospheric aerosols. The data collected during long time continuous measurements of the atmospheric electric quantities in more than ten stations situated in various countries is stored in the World Data Centre for Atmospheric Electricity (Dolezalek, 1992). The correlation of atmospheric electrical data with aerosol particle concentration has been pointed out by Russian scientists (e.g. Svarts, 1980) but the collected data and the running measurements are insufficiently used in aerosol research until now.

Electrostatic dispersion of charged aerosol particles

Abstract It is proved that a normal mobility distribution is retained in the process of electrostatic dispersion of aerosol particles. At that, the average mobility of the particles and the total electric charge density are related with a functional dependence. A lognormal mobility distribution is not retained.

Influence of Diffusion on the Resolution of a Multi-Channel Electrical Mobility Analyzer

A long multi-channel electrical mobility analyzer (long MCEMA) has been developed and presented for classifying and measuring the size distribution of aerosol particles in the range of 10 to 1000 nm (Intra and Tippayawong, 2009, 2011). In the idealized model, when there are no molecular and no turbulent diffusion, the charged particles in the MCEMA move along precisely determined trajectories, and the apparatus gives the undistorted mobility spectrum. Here we neglect the effect of the finite width of aerosol inlet channel and of the finite width of the electrometer rings. Practically, however, molecular and turbulent diffusion randomly scatters the trajectories of the particles, giving a smoothed mobility spectrum. The degree of smoothing can be characterized by the resolution. The resolution of the MCEMA can be expressed by an analytical equation because molecular diffusion submits itself relatively well to a theoretical description. Because of the complexity of turbulent diffusion, simplifying assumptions were made for this work. The MCEMA resolution values were calculated based on expected turbulence parameter data. An increase in the inner electrode voltage resulted in an increase in the resolution of the mobility spectrometer. Higher electrometer ring numbers were found to have higher resolution values of the long MCEMA than the lower electrometer ring numbers. The resolution R d for the electrometer ring number 22 was found to be as high as 150, 280, and 550 for inner electrode voltages of 1, 2, and 3 kV, respectively. The influence of Brownian diffusion on the resolution of the long MCEMA was significant for particles smaller than 100 nm, corresponding to electrometer ring numbers lower than 10. Approximate calculations show that the mobility resolution decreased considerably even with low turbulence in the classifier.