Urmas Hõrrak

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.

Comparative study of nucleation mode aerosol particles and intermediate air ions formation events at three sites

[1] Quantitative characteristics of the formation and growth events of nucleation mode aerosol particles occurred at three stations (Hyytiala and Varrio in Finland and Tahkuse in Estonia) have been compared. Aerosol size distributions were measured on all locations and, in addition, air ion mobility distribution at Tahkuse. Among 157 measuring days, nucleation events were identified on 62, 50, and 45 days at Hyytiala, Varrio, and Tahkuse, respectively. Eighteen days were found when nucleation events occurred on all three locations at the same day. The concentration of condensable vapor, its source strength, the growth rate, the total condensation sink, the formation rate of 3 nm particles, the formation rate, and concentration of 1 nm particles were estimated using measured aerosol number size distributions. The estimated growth rates were in the range of 2-9 nm hr -1 . The growth rates found from the measurements of air ion mobility distributions were comparable with that of nanometer particles. The condensable vapor concentrations were 1-12 x 10 7 cm -3 , which is smaller at Hyytiala than at Tahkuse and Varrio. At Tahkuse, source rates 6-55 x 10 4 cm -3 s -1 were estimated, whereas at Hyytiala and Varrio the rates were 2-14 x 10 4 cm -3 s -1 and 1-20 x 10 4 cm -3 s -1 , respectively. The condensation sink values were usually substantially higher at Tahkuse than on the other locations. Tahkuse size distribution data showed 5-10% underestimates in the total condensation sink considering particles in the diameter range <500 nm. At Tahkuse the charged fraction of nucleation mode particles estimated through comparison of aerosol size distribution and air ion mobility distribution turned to be much higher than steady state charging probability of a single charge on the particle. Analysis of air mass trajectories and meteorological data indicated that the nucleation events are synoptic-scale phenomenon occurring in horizontal extent more than 1000 km.

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.

Overview of the biosphere–aerosol–cloud–climate interactions (BACCI) studies

Here we present research methods and results obtained by the Nordic Centre of Excellence Biosphere–Aerosol–Cloud– Climate Interactions (BACCI) between 1 January 2003 and 31 December 2007. The centre formed an integrated attempt to understand multiple, but interlinked, biosphere–atmosphere interactions applying inter and multidisciplinary approaches in a coherent manner. The main objective was to study the life cycle of aerosol particles and their importance on climate change. The foundation in BACCI was a thorough understanding of physical, meteorological, chemical and ecophysiological processes, providing a unique possibility to study biosphere–aerosol–cloud–climate interactions.

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.

SMEAR Estonia: Perspectives of a large-scale forest ecosystem – atmosphere research infrastructure

Abstract Establishment of the SMEAR Estonia at a hemiboreal mixed deciduous broad-leaved-evergreen needle-leaved forest at Järvselja, South-Eastern Estonia, has strongly enhanced the possibilities for national and international cooperation in the fields of forest ecosystem – atmosphere research and impacts of climatic changes on forest ecosystems, atmospheric trace gases, aerosols and air ions. The station provides a multitude of comprehensive continuously measured data covering key climatic and atmospheric characteristics (state and dynamics of solar radiation, trace gases, aerosols and air ions, meteorological parameters) and forest ecosystem traits (net primary productivity, individual tree growth, gas-exchange characteristics, soil variables). The station follows a multidisciplinary and multiscale approach covering processes in spatial dimensions ranging from nanometres to several hundred square kilometres, being thus able to significantly contribute to worldwide measurement networks and the SMEAR network. Here we present an overview of the station, its data produced and we envision future developments towards sustainable research and development of the large-scale scientific infrastructure SMEAR Estonia.