Active real-time analyzers vs. passive/diffusive samplers for hydrogen sulfide (H2S) in air: a critical comparison

Hydrogen sulfide (H2S) is a gas pollutant discharged in air from a large number of natural and anthropogenic sources. Its peculiar rotten-egg smell, causing odor nuisance to neighboring communities, is detectable at concentrations between 0.7 and 42 μg/m3 (Schiffman & Williams, 2005). High H2S concentrations could cause eye irritation, damage to the upper respiratory apparatus and loss of smell. The effects of long-term low level (< 2,800 μg/m3) exposures to H2S are still matter of debate (Bates et al., 2013). Hence, the development of techniques for accurate measurements of H2S in air at a wide range of concentrations is a primary issue in environmental monitoring. Two different approaches are currently used: 1) passive samplers and 2) real-time measurements. The latter are generally expensive and require a power supply. On the contrary, passive samplers are low cost and can be deployed in the field with minimal maintenance. Therefore, passive samplers offer an appealing alternative to real-time measurements, especially for regional-scale monitoring. However, the reliability of passive samplers in outdoor applications strongly depends on several environmental factors, such as temperature, humidity and wind speed (Delgado-Saborit & Esteve-Cano, 2006). In this study a comparison between H2S measurements using diffusive radial-type passive samplers (Radiello) and a real-time gas analyzer (Thermo Scientific Model 450i) based on pulsed fluorescence, is presented. The measurements were carried out in areas affected by both anthropogenic and natural sources using both techniques. The results show substantial differences. The passive samplers systematically produce higher H2S concentrations than those of the active analyzer. The relative error was up to > 1,000% for concentrations < 7 μg/m3 and exposure duration ≥ 2 hours. H2S measurements by Radiello were affected by meteo parameters (wind, rain, humidity, temperature). The efficiency of this method was demonstrated to be also strongly dependent on H2S concentrations. In addition, passive samplers give an average concentration value for the exposure period, but are not able to detect short-term H2S increments. These results show that the use of passive samplers for environmental monitoring should thus be limited to preliminary largescale semi-quantitative assessment. A reliable study on the dispersion dynamics of contaminants in air cannot exclude the acquisition of high-frequency data through active analyzers

Measurements of Hg0 (and H2S) at the Solfatara Crater (Southern Italy): Estimating the atmospheric distribution with a real-time approach

Volcanic and geothermal areas are important emitters of natural gas compounds into the atmosphere, which can be of concern when discharging close to densely, populated sites. Mercury has a strong environmental impact, its organic and inorganic complexes being toxic. The dominant form of Hg in the atmosphere is gaseous elemental mercury (GEM), which has high volatility and residence time of 1-2 years. Volcanic degassing accounts for a significant part of the natural mercury emissions. No mercury limits or target values in ambient air are present in the EU legislations, whereas US-EPA and ATSDR impose 300 and 200 ng/m3, respectively, as a limit for chronic exposure. WHO has proposed the annual average value of 1,000 ng/m3 as a guideline for Hg0 in ambient air. The determination of Hg0 concentrations is often performed via passive/diffusive samplers, which provide time-integrated gas concentrations, but not able to assess the highly variable distributions of GEM. Different weather factors and photochemical reactions indeed affect the Hg0 dispersion. In volcanic/geothermal sites, GEM measurements can be associated with H2S, an irritating and suffocating substance and detectable at very low concentrations (7 \u3bcg/m3, ~5 ppb) due to its typical rotten eggs odor. WHO recommends a guideline value of 150 \u3bcg/m3 (~107 ppb) with a 24h averaging time. In April 2014 real-time Hg0 and H2S measurements in air were conducted at the Solfatara Crater, which is nested in the town of Pozzuoli (Southern Italy). The main aims were to (1) test this new methodological approach and (2) investigate the Hg0 the H2S concentrations and their spatial distribution. GEM and H2S continuous measurements were determined with a portable Zeeman atomic absorption spectrometer with high frequency modulation of light polarization (Lumex RA-915M, DL: 2ng/m3) and a pulsed fluorescence gas analyzer (Thermo 450i, DL: 1 ppb), respectively. The GEM and H2S and meteorological data were acquired along previously planned pathways at an average speed <5 km/h. The Hg0 and H2S concentrations were between 12 and 77 ng/m3 and 0.2 and 2400 ppb, respectively. The highest measured concentrations corresponded to the main gas discharging areas, whereas the lowest values were measured outside the crater and in the vegetated areas. The results of this study indicate that this technique approach is highly efficient and effective and provides reliable and reproducible Hg0 and H2S concentrations, which can be used to define the exposure that tourists and inhabitants, living close to volcanic and geothermal areas, may suffer

A new geochemical approach to estimate the distribution of air pollutants from natural and anthropogenic sources: examples from Solfatara Crater (Campi Flegrei, Southern Italy) and Mt. Amiata Volcano (Siena, Central Italy)

Volcanic and geothermal systems significantly contribute to the input of volatile contaminants, such as mercury and hydrogen sulfide, into the atmosphere. Mercury has a strong environmental impact. In the atmosphere the prevalent elemental form is Hg0 (~98 %), whose main physical-chemical features are: high volatility, low solubility and chemical inertness. Hydrogen sulfide (H2S), one of the most abundant gas compounds in volcanic fluids, is highly poisoning and corrosive and unpleasantly smells of rotten eggs. Measurements of Hg0 and H2S concentrations in air are commonly performed by means of passive samplers. However, real-time measurements, coupled with monitoring of local atmospheric conditions, are strongly recommended for a reliable reconstruction of the dispersion dynamics once such contaminants are discharged in air. In this paper, a new real-time measurement method for Hg0 and H2S is presented. A portable Zeeman atomic absorption spectrometer with high frequency modulation of light polarization (Lumex RA-915M) and a pulsed fluorescence gas analyzer (Thermo Scientific Model 450i) were used for Hg0 and H2S measurements, respectively. These instruments were synchronized and set to high-frequency acquisition. Measurements were carried out along transects at an average speed &lt;10 km/h. GPS data and meteorological parameters (wind direction and intensity) were also recorded. The proposed method was applied in two different sites, characterized by natural (Solfatara Crater, Campi Flegrei, Southern Italy) and anthropogenic (Mt. Amiata Volcano, Siena, Central Italy) emissions. With this highly efficient and effective approach, a reliable and reproducible Hg0 and H2S distribution in air was provided, allowing to identify and characterize the gas sources from such different environments. At Solfatara, the distribution of Hg0 and H2S concentrations, the highest values being measured close to the fumarolized areas (&gt;60 ng/m3 and &gt;2,100 μg/m3, respectively), suggests that these gases were discharged from both fumaroles and diffuse degassing from the crater bottom. At Mt. Amiata, the maximum Hg0 and H2S concentrations (&gt;100 ng/m3 and &gt;35 μg/m3, respectively) were recorded close to the geothermal power plants of Piancastagnaio. According to detailed dot-maps constructed on the basis of the measured values, as expected, wind was the main environmental parameter able to control the behavior and the dispersion halo of the Hg0- and H2S-rich plumes emitted from the contaminant sources

Real-time measurements of Hg0 in volcanic, geothermal and anthropogenic systems: a multi-methodological approach using Lumex® instrumentation

Mercury represents a pollutant of global concern and strong environmental impact since is highly toxic. Hg is present in air in the oxidation states of 0 and +2, the former being the dominant species with a residence time of 1-2 years due to its high volatility, relatively low solubility and chemical inertness. Both volcanic/geothermal and anthropogenic systems are crucial contributor to the release of Hg0 in the atmosphere. In this work, a Lumex® (RA-915M) was used to evaluate the environmental impact in air of Hg0 from: i) the abandoned Hg mining site and geothermal areas from Mt. Amiata (Siena, Central Italy) and ii) selected Mediterranean volcanic and geothermal systems. The Lumex® instrumentation, based on atomic absorption spectrometric technique with Zeeman effect, allows to measure Hg0 at high frequency, in real-time and at a wide range of concentrations (from 2 to 50,000 ng/m3). Hg0 measurements were coupled with those of other pollutants, such as CO2 H2S, and SO2. Carbon dioxide was measured using a Multi-GAS instrument manufactured by INGV-Palermo, whereas H2S and SO2 using Thermo Scientific® Model 450i analyzer. GPS and meteorological parameters were continuously recorded, too. The data acquisition was carried out along transects at an approximately constant speed or at selected fixed points. Wherever possible, the analytical data were then converted into a spatial interpolation providing a qualitative model for the areal dispersion of the contaminants. The Lumex® device was also applied to measure Hg0 concentrations in interstitial soil gases collected from a probe inserted into the soil at 70 cm depth, in order to produce Hg0 maximum concentration maps in Hg-polluted areas (e.g. Abbadia San Salvatore Hg mining area, Mt. Amiata). Diffuse Hg0 soil fluxes were measured using a chamber positioned above the soil from which, at periodic time intervals, gases were extracted and injected into the Lumex® device. This instrument was also applied to measure Hg0 concentrations along vertical profiles in thermal wells at Santorini (Greece) and Vulcano (Italy) by using a Rilsan® tube lowered into the wells at pre-defined depths. With this approach, a significant stratification of the air masses in terms of Hg0, strictly dependent on water temperature, air pressure and well depth, was observed. The efficiency of Lumex® for these different approaches demonstrated the reliability of this instrument to produce Hg0 data that can be used to identify gaseous Hg-emitters in natural and anthropogenic environments, especially when coupled with other physical and chemical parameters

Fluid geochemistry of the Los Humeros geothermal field (LHGF - Puebla, Mexico): New constraints for the conceptual model

Geothermal power in Mexico is mainly produced in four geothermal fields operated by the Comision Federal de Electricidad (CFE): Cerro Prieto, Los Azufres, Los Humeros, and Las Tres Virgenes. The Los Humeros Geothermal Field (LHGF) is ranked third in terms of generated capacity, and in the last decade its installed capacity has doubled (up to 95.0 MW). Further increases in the geothermal power generation capacity in Mexico are planned, and thus the LHGF warrants further examination. The development and growth phases of any geothermal project must start from an awareness of the conceptual model of the natural system studied. The recharge mechanism, feeding zones, and fluid flow-path must be identified, along with the estimation of the temperature at the productive level and of phase separation (liquid - steam). To accomplish this, detailed fluid geochemical surveys were carried out in June 2017 and March 2018, in which 57 and 87 samples were collected, respectively, from cold and thermal springs, water wells and maar lakes located around and inside the LHGF. Samples from fumaroles inside the producing area were also collected for the first time, together with fluid from re-injection wells. The presence of a meteoric component, which plays an important role at the regional scale, is confirmed by the chemical and isotope data, and its contribution in terms of recharge may be higher than previously assumed. The Sierra Madre Oriental, on the west side of the LHGF, is characterized by widespread outcrops of limestone belonging to the same geological formation as those at the bottom of the LHGF. The isotope composition (delta D and delta O-18, respectively -77.3 parts per thousand and -10.50 parts per thousand for the hypothetical Infiltration Water IW) is similar to that observed in cold springs located in the Sierra Madre Oriental, and from this the evolution of isotopes in the liquid-rock-steam system during water-rock interaction and phase separation processes can be modelled. Thus, the experimental data obtained for natural gas emissions (fumarolic condensates) and for geothermal fluids can be reproduced. These findings suggest that geothermal fluids in the LHGF are likely to be derived from meteoric water infiltrating (IW) the limestone outcrops of the Sierra Madre Oriental. During their flow-path, the infiltrating waters exchange isotopes at a high temperature with the crustal rocks, which have a much higher O-18/O-16 ratio, resulting in a shift towards higher delta O-18 (-4.35 parts per thousand +/- 1) as the water O exchanges with rock O. The vapor phase can be separated from this deep water (DW) and it is discharged from the fumarolic effluents of Loma Blanca. Single Step Vapor Separation (SSVS) and Continuous Steam Separation processes (CSS) were modelled using stable isotopes of water. The results of geochemical modeling agree with available data for geothermal liquids discharged from several geothermal wells, suggesting that steam separation may be interpreted either as SSVS or CSS. Other processes can affect the chemistry and isotope composition of geothermal fluids (e.g. phase segregation, gas exchange, contributions from magmatic-volcanic deep fluids and re-injection fluids). The proposed conceptual model is consistent with both the geochemical data and the geological setting, and provides a useful point of reference for examining the fluid flow-path and geochemical processes active in the LHGF, at least at a general level.An involvement of magmatic-volcanic deep fluids in the feeding mechanism of the geothermal system cannot be excluded at priori, but the regional meteoric end-member is supported by the data and it seems the most important component