Aerosol Physics

Mobility model of light air ions

Air molecules are constantly ionized due to cosmic rays and radioactive radiation. The formed ions react with different neutral molecules present in air that change the chemical composition of ions in time. The settlement of ions on aerosol particles and recombination of differently charged ions diminishes the number of molecular ions in the atmosphere. As a result of these processes slowly changing in time approximately stationary distribution of ion by their mobility, i.e., by their velocity in the electric field of unit strength. There are long tradition in the measurements of mobility spectra of air ions in the University of Tartu going back to the beginning of last century. Permanent measurements started in Tahkuse air monitoring station in 1984. Along with mobility measurements there are taking place also the mass-spectrometric measurements of air ions in the Järvselja monitoring station starting from the spring of 2018. The accuracy of ion mass measurements enables to determine the atomic composition of air ions, but in case of masses up to about 300 Da event their molecular content. Compared to mass-spectrometers mobility spectrometers are much cheaper and the amount of measurement data collected by them are remarkably larger with history of collected data extending deeper in the past. The interpretation of mobility spectra is made essentially easier if we can relate the mobility of ions by their atomic content and tt is desirable that the corresponding calculation algorithm be a little time consuming. Based on the literature data, a mathematical model has been developed that allows to calculate the ion mobility for light ions (with mobility ⪆ 0.5 cm2V-1s-1) based on their atomic composition at a given temperature and pressure. The model contains parameters that characterize the size of different atoms. However, one parameter implicitly takes into account that as the size of the ion increases, the collisions with the gas molecules change from ionic to elastic. Parameter values were fitted to slightly less than 3,000 experimental mobility values. In 87% of cases, the absolute value of the difference between the model and the experimental values is less than 5%, in 11% of cases between 5 and 10%, and in 2% of cases more than 10%.

Ion kinetic model

Air consists of various gases and particles. Particle growth is one of the processes that determine the particle formation and evolution, therefore a reasonable classification of the particles is the classification by size. The electrically charged particles (air ions) can be arranged into the following size groups: cluster ions (also called small ions), intermediate ions, light and heavy large ions (Hõrrak et al., 2000). The characteristic size of cluster (small) ions is up to about a nanometer. Another possibility is to classify the particles (including ions) according to the nature of the processes that determine the particle evolution. In this case, cluster ions are the air ions whose transformations are mainly determined by ion-molecular chemical reactions. The transformation (evolution) of other air constituents is determined by several processes of other kind.

Ionizing factors affecting air constituents (eg radioactivity, electrical discharges in the air) create so-called primary ions, within ambient air the main primary ions are O2– and N2+ . It can be said that the region of cluster ions starts from these primary ions, since the next transformations are mainly determined by ion-molecular reactions with the ions. Examples of corresponding chemical reaction systems are depicted in Figures "Sample evolution flowchart of small positive/negative air ions". After a few seconds, the transformed ions generally become members of a new class, such as the class of intermediate ions or the class of small aerosol particles.

The models of the evolution of cluster ions mainly deal with the processes within this time period of a few seconds. Examples of the studies carried out in our laboratory are (Parts and Luts, 2004 and 2007; Luts et al., 2006; Tamme et al., 2018), front pages of these studies are in figures „Applications of the cluster ion models (1˗˗4)“. The evolution of intermediate ions and/or aerosol particles is described by other methods, this topic is also known as the evolution of aerosol particles.

Mass-mobility-size interconversion model

Main focus is on understanding the initial steps of air ion formation from their creation by ionising radiation to forming nanoparticles of a few nanometers. To assist the interpretation of air ion measurements from different instrumentations (mobility- and mass-based), soon will be start a new Marie-Curie Global Fellowship project (MaSMob-Lion: Mass-Mobility-Size for light ions/clusters, Pi: Xuemeng Chen), with an aim to present a mass-mobility-size interconversion model that is valid in the free molecular regime extending to sizes relevant for studying atmospheric new particle formation.