## Appendix A  
There are several main physical processes that are involved in changes in SI concentration. Firstly, SI are constantly created, in pairs, by ionizing radiation that exists in the environment. SI are continuously created when neutral air molecules are irradiated by cosmic rays or radiations from natural radioactive materials in soil and air. Neutral air molecules may be ionized into positive ions and free electrons that are attached to other hydrous or oxygen molecules in air, and in order to become stable these ions are adsorbed to neutral molecules forming cluster ions.  
UV radiation is mainly responsible for ionization of molecules and atoms in high atmosphere, but it is exhausted at higher levels and doesn't arrive to lower troposphere [28]. Several natural sources of ionizing radiation are responsible for air-ion production in the lower troposphere, namely radioactive gases (particularly \({}^{222}\)Rn and its progenies), radioactive substances at ground level (e.g., natural \(\alpha\) and \(\beta\) emitters in the air and soil) and cosmic rays. These three contributions are comparable, approximately 20% of the total surface ionization rate is due to ionization from cosmic rays, and remaining 80% arises from natural \(\alpha\) and \(\beta\) emitters in the air and soil.  
Air-ion pair generation near the ground varies mostly with the concentration of \({}^{222}\)Rn and its progenies. The half-life of \({}^{222}\)Rn is approximately 3.8 days and the decay product is an alpha particle with energy 5.49 MeV. The decay of \({}^{222}\)Rn generates a large number of nitrogen and oxygen molecular ions (order of magnitude \(\sim\)10\({}^{5}\)) per each \(\alpha\)-particle. As a consequence, the near-ground ionization rate caused by background ionization, is about 10 ion pairs/cm\({}^{3}\)s in continental areas [28]. Within microseconds of the ionization process, primary ions evolve through the process of hydration to form small cluster ions, also known as small air ions or nano-air ions. This class of air ions can survive much longer, up to 100 s, depending predominantly on air pollution and air density [22].  
While small air ions technically are particulate matter belonging to ultrafine particulate fraction (albeit only a few nanometers in diameter), it is worth pointing out the following. Small air ions are electrically charged clusters consisting of several molecules in which ordinarily neutral atmospheric molecules/atoms have gained or lost electrons. While particulate matter can also be charged, it is composed of a much larger number of molecules and is thus up to several orders of magnitude larger in diameter compared to the small ion clusters. Since cluster ions readily attach to particles, it is know that their concentration decreases sharply within few tens of meters from the road [29], this making indoor ion sources most significant, which is important to keep in mind for indoor air monitoring.  
SI are also continually being destroyed in a process of recombination, producing neutral molecular clusters. In addition to the process of recombination, SI can attach to PM. Because of this, a change in PM concentration directly results in a change in SI concentration. A significant portion of PM in the urban environment is a result of human activities, where smaller particles are typically associated with the process of combustion occurring in vehicles, industrial activities, biomass burning, and similar, while larger particles are typically due to construction and demolition activities, entrainment of outdoor dust and similar. The SI balance equation can be used to quantitatively describe the above-mentioned processes.