## Materials
### Measurement of Gaseous Mercury  
Gaseous mercury surveys are conducted using portable analytical devices to measure gaseous mercury in the atmosphere. The analytical device measures the Hg concentrations at fixed time intervals and stores the results in an internal data logger. The geographic position is recorded and stored in a GPS device in order to assign coordinates to the geographic position of the recorded data. A LUMEX RA-915 Hg analyzer is a portable multifunctional atomic absorption spectrometer (AAS) with Zeeman background correction, which eliminates the effect of interfering impurities. This equipment has an analytical gaseous Hg range of 1-100,000 ng/m\({}^{3}\) with a detection limit of 0.1 ng/m\({}^{3}\) and an uncertainty of Hg measurement range between 6.3 and 9%. The equipment takes in air flow at a rate of 10 L/min and analyzes the Hg concentration with a frequency of one sample per second; the average value is stored every ten seconds in the internal data logger. The LUMEX RA-915 has been widely used in the scientific literature and by reference organizations to monitor gaseous mercury in different conditions and environments. Together with the Hg analyzer, a GPS Garmin Etrex Touch 35 was used. The GPS was programmed to record positions every second to match the data with the Hg measurements. The LUMEX RA-915 analyzes only gaseous mercury because the Hg particulates are removed by means of a filter at the entrance of the analyzer. Airborne mercury levels in the area have been recorded in previous studies [18]. Furthermore, the monitoring campaigns were designed to be systematic to obtain the most accurate information possible to define the site and design work protocols. As a result, a route was established that included 22 control points located throughout the area where measurements of Hg-gas concentrations were to be performed (Figure 2). There are three levels or floors: points 1 to 12 are on level 0, points 13 to 17 are on level 1, and points 18 to 22 are on level 2. The height difference between the levels is around 10 m. Levels 0 and 2 are where the work was conducted within the SUBproducts4 LIFE project [18; 19]. The measurement is made at each place for a time ranging from 2 to 5 min, depending on the observed fluctuations. In the same way, the route will not run more than one hour to reduce exposure. In this research, a chemical-physical process of a gas which can be influenced by pressure, temperature, or other variables was studied. This made it clear that environmental conditions such as temperature, wind, rain, relative humidity, atmospheric pressure, or solar ultraviolet radiation can influence the emission of gaseous mercury. For example, it can be observed that the wind dilutes the concentration of Hg over the demolition debris. For this reason, measurements were taken in days with low wind speed (average daily speed of 6.5 km/h) and measurements over the debris, 2 to 5 min, were taken when there was no wind. It can also be observed that the emission is lower when it has been raining the previous days due to water evaporation, which can cold the debris and reduce  
Figure 1: Image of the decommissioned mine. its temperature. Similarly, clouds can diminish emissions due to lower solar ultraviolet radiation. To avoid the influence of these variables, the measurements were taken on sunny days without rain, clouds, and wind. On the other hand, the range of variation in the atmospheric pressure at this latitude (between 98.0 kPa with low pressure and 104.0 kPa with high pressure) does not significantly influence emission. Finally, although the relative humidity could have an influence, this variable strongly depends on temperature, which means that the gaseous mercury concentration can be expressed as a function of only the temperature. That is, taking the measurements at moments of atmospheric stability with sun, without rain, and without wind, Hg gas emission depends mainly on the temperature, and other conditions produce only an aleatory dispersion of the emission value.