On Background Observations of the Content of Trace Elements in the Solid Phase of the Snow Cover of Urban Lands

The problem of background observations is seen in the fact that there are no justifications independent of the researcher for choosing suitable observation points. The high pipes of the emission source allow pollutants to be scattered over long distances, creating a wind shadow near it with a low flow of dust and scattered substances. The points directly adjacent to the source of the emission of pollutants sometimes turn out to have a lower content compared to remote ones. Therefore, the distance from the source of the emission is not a guarantee of the absence of anthropogenic influence; i.e., natural conditions for the formation of the microelement composition of snow cover dust. As a result, it turns out that the choice of a priori points remote from the urban area in forests and on agricultural land cannot undoubtedly be considered background. Averaging values at such points can lead to the fact that the content of pollutants in them will be higher than at points located in the zone of direct influence of the anthropogenic factor. The choice of background points is made by the authors of studies without an evidence base, which leads to an underestimation or overestimation of the environmental hazard of industrial emissions. Using the example of Tyumen, an algorithm for selecting background observation points, regardless of the observer, is proposed. The territory of the city is divided into squares, and the location of observation points is made randomly. Next, a chain is used: converting the initial data into relative values, hierarchical cluster analysis, calculating the mean values of the content of trace elements for all clusters and chemical elements, and selecting the cluster with the lowest values (the points of which are taken as the background). The same algorithm can be used for any depositing media: soil, sediments, peat bogs, etc.