## 2 Materials and Methods
### Geochemical Indices  
The effectiveness of indices often used by other researchers for general assessment of urban geochemical state and search for potential pollution hotspots and sources will be analyzed. Only indices for calculation of which the natural levels of analytes were taken were examined. The general term \"geochemical indices\" (gI) was used for them. The indices based on regulatory levels (which are set by legal advices) is not discussed in this study. To avoid the confusion between indices based on _Clarke values_ (\(Cv\)) and those based on _native background values_ (\(Bn\)), different letters were used after apostrophe in the abbreviation of each gl: (a) in the first case, the letter was \"\(C\)\" and the source of \(Cv\) (for unification) was [PERSON] and Gao [32]; (b) in the second case, the letter was \"\(N\)\". The following \(gl\) was used in this study. The abbreviations in the formulas below are the following: C--the content of selected element in selected site, \(Cv\) and \(Bn\) are explained above, \(C_{AI}\) is the content of Al in site, \(Cv_{AI}\) is _Clarke value_ of Al, and \(Bn_{AI}\) is the native background value of Al. 1. Two alternatives of _geoaccumulation index Igeo: Igeo'C_ when \(Cv\) is used in the denominator (Equation (1)) and _Igeo'N_ when \(Bn\) is in the denominator (Equation (2)). \[Igeo=\log_{2}C/(1.5*Cv) \tag{1}\]  
\[Igeo=\log_{2}C/(1.5*Bn) \tag{2}\]  
The geoaccumulation index (usual abbreviation _Igeo_) was introduced by [PERSON] for classification of river sediments according to contamination level [33]. Presently _Igeo_ is widely used by researchers for classification of contamination not only of sediments or soil, but also of dust, e.g., [34; 35; 36; 37] and others. They apply the same rule to distinguish dust contamination level, i.e., seven Igeo-classes earlier used for evaluation of river sediment contamination [38]: 0 (_Igeo \(<\) 0_)--uncontaminated, 1 (_Igeo_: 0-1)--uncontaminated/moderately contaminated, 2 (_Igeo_: 1-2)--moderately contaminated, 3 (_Igeo_: 2-3)--moderately/strongly contaminated, 4 (_Igeo_: 3-4)--strongly contaminated, 5 (_Igeo_: 4-5)--strongly/extremely contaminated, 6 (_Igeo_: 5)--extremely contaminated. 2. Two variants of _normalized enrichment factor_\(nEF\) based on Al as reference element: \(nEF\)_/Al'C_ when \(Cv\) is used in the denominator (Equation (3)) and \(nEF\)_/Al'N_ when \(Bn\) is in the denominator (Equation (4)). \[nEF^{\prime}Al^{\prime}C=(C/C_{AI})/(Cv/Cv_{AI}) \tag{3}\]  
\[nEF^{\prime}Al^{\prime}N=(C/C_{AI})/(Bn/Bn_{AI}) \tag{4}\]  
The term \"_normalized enrichment factor_\" (with abbreviation \(F_{xm}\)) was firstly proposed by [PERSON] [39]. Presently _normalized enrichment factor_ is widely used and usually entitled \"_enrichment factor_\" without the word \"_normalized_\" and with abbreviation \(EF\). It was used for dust studies by some researchers, e.g., [36; 40; 41; 42] and others. Now this index is sometimes used for dust contamination classification, e.g., [43; 44; 45], to distinguish five so-called contamination classes (SCC): <2--minimal; 2-5--moderate; 5-20--significant; 20-40--very high; >40--extreme. However, more often the researchers use \(nEF\) index to distinguish the presumable origin of analytes in dust, i.e., natural or anthropogenic. Most commonly the researchers citing previous publications accepted \(nEF<1\) to indicate significant terrigenous (crustal) origin, while \(nEF\geq 10\)--significant anthropogenic contributions [45; 46; 47]. 3. Index of potential atmogenic enrichment of toposil \(aE\)_N_ (Equation (5)). \[aE^{\prime}N=C/Bn \tag{5}\]  
The analogous ratio of aerosol concentration to soil concentration was firstly used in 1971 by [PERSON] and was entitled as \"_enrichment factor_\" (abbreviation \(F_{x}\)) [39].