An automatic system for continuous monitoring of CO₂, H₂S, SO₂and meteorological parameters in the atmosphere of volcanic areas

Abstract

An automatic system for the continuous monitoring of CO₂, H₂S, SO₂ and meteorological parameters in atmosphere has been developed. The system has been tested in the laboratory in order to verify the stability and reliability of each sensor and of the whole system. A field test for a period of one month, at the Solfatara of Pozzuoli has also been carried out. The acquired data during the field test reveal a correlation between the wind speed and the concentrations of CO₂, H₂S, and SO₂ in the atmosphere. With a wind speed of over 4 ms^-1 the concentration of the three gases reached constant background values of 600 ppm for CO₂ and about 2 ppm vol. for H₂S and SO₂. The different ratios of H₂S/SO₂ measured in the fumaroles (~100) and in the atmosphere (1–0.1) clearly indicate that H₂S is oxidized to SO₂ during the transport.

Introduction

Active volcanic areas are generally indicated by different kinds of degassing, such as fumaroles, mofettes and diffuse soil degassing. The intensity of degassing, as well as the composition of the gaseous manifestations, are strictly related to the state of activity of each volcano. In the last years, several studies carried out in different volcanic areas clearly indicate that the presence of gas hazard is quite recurrent and that it is essentially controlled by the strength of the source, the morphology of the area, and meteorological parameters. [1–5]

According to the composition of volcanic gases, the main gaseous constituents that could be responsible for the existence of gas hazard are essentially CO₂, H₂S, and SO₂.

The last two gases are normally present in areas close to fumaroles and the ratio H₂S/SO₂ in fumarolic gases can change in a wide range, being essentially controlled by the temperature and oxygen fugacity values of fumaroles (at higher temperature and fo₂, SO₂ is more abundant while H₂S is the dominant species at lower temperatures and fo₂).[6] The CO₂ is always present in all gaseous manifestations such as fumaroles, mofettes, diffuse soil degassing and it is the most abundant species of the incondensable gases.

It is then necessary to have the possibility of monitoring the content of these three gases in the atmosphere in order to control the gas hazard in a given area. The main objective of this work was to develop an automatic apparatus for the continuous and remote control of the concentrations of the aforementioned gases in the atmosphere, as well as the meteorological parameters (rain, wind speed and direction, atmospheric pressure and temperature).

Automatic station for continuous monitoring of CO₂, SO₂, H₂S and meteorological parameters

Characteristics of the system

The system can essentially be divided into three parts:

1. Data logger (DL)

The DL used has been projected and assembled in the I.N.G.V. electronic laboratory. The main purpose to develop a new data logger was essentially due to the very corrosive environment normally present in volcanic areas.

The DL has 15 channels for analog inputs and each one may be connected to one sensor to read what follows: 0–2 V; 0–20 mA; K V, where K is a fixed constant for all the channels. In this way, the sensors normally used to monitor geochemical parameters (thermocouple, thermoresistance, electrochemical sensors, spectrophotometer IR, etc.) can be connected directly to the analog input of the data logger without any use of other transducers.

The DL is powered by a 12 DCV (direct current voltage) battery, and the battery is recharged by two solar panels. The total current consumption (electronic and sensors) is 600 mA. It takes a few minutes before reading some geochemical parameters; this depends on the activation of external devices like electrovalves, pumps, etc. It should be necessary to pilot it with a small computer, to run all these operations with a normal DL. In our DL, all these operations are temporised, only once at the boot time of the data logger and then, it repeats automatically the prefixed sequence, in all successive readings.

The data logger has 16 channels for digital output, ON/OFF type, and 8 digital input channels. Each one of them can send an alarm message.

In the initial boot of the data logger, for each analog channel input, some parameters are memorised as:

(i) Interval between successive readings.

(ii) Minimum and maximum alarm value of the parameter.

(iii) Pre-set to switch on and off a digital output channel at a prefixed time before the analog reading take place.

(iv) Data logger with a memory for more than 6000 readings.

(v) Maximum power consumption is when the data are discharged via radio.

The acquired data, stored in the DL memory, can be remotely discharged through a radio link. If some parameters reach values outside the fixed maximum and minimum, a window will automatically send an alarm message.

This possibility is a good improvement because it is possible to discharge the data during the day, when the batteries are recharged by the solar panels. This configuration is a good compromise between electric power saving, data archive for research purposes and their immediate disposability for geo-chemical surveillance of volcanic activity.

2. Sensors

Each remote station is equipped with sensors for: CO₂, SO₂, H₂S and meteorological parameters.

The chemical sensors are manufactured by LSI S.p.A. (Laboratori Strumentazione Industriale).

The electronic weather monitor (temperature internal and external, relative humidity, barometric pressure, wind speed and direction, rainfall) is a Wm-918, furnished by the Huger Electronics GmbH.

The block diagram of the developed station is reported in Fig. 1.

Figure 1

Block diagram of the automatic station for the continuous monitoring of CO₂, H₂S, and SO₂.

3. Transmission system

The acquired data can be transmitted through direct computer interfacing using RS232, radio modem and cellular modem connection.

Laboratory test

In order to verify the performance of the automatic system one month of measurement has been spent in the laboratory. The acquired data regarding CO₂, SO₂, and H₂S concentrations in the atmosphere, internal and external values of temperature, relative humidity and atmospheric pressure, have been reported in Fig. 2.

Figure 2

Hourly data from the automatic system for continuous monitoring of CO₂, H₂S and SO₂, and meteorological parameters recorded in the laboratory.

In the same figure a calibration signal of 1000 mV has also been plotted, it has been divided by ten in order to plot all signals in the same figure.

Very constant values of this last parameter, (1000 ± 2 mV) indicate the good performance of the overall electronics. In other words, the potential error introduced by the electronics of the system is ± 0.2% of the signal.

The concentration values of SO₂ and H₂S have been multiplied by ten in order to plot all signals in the same figure. The output of both sensors was stable, exhibiting variations in the range ± 0.1 ppm. The concentration values of CO₂ in Fig. 2 have been divided by ten. All recorded values, corrected for the zero shift, are in the range 350–700 ppm. The peaks have been recorded during daytime when people were working in the lab. Data acquired from 23rd December to the end of the month (holiday period) exhibit CO₂ concentration values in the range 350–450 ppm.

Temperature changes of a few degrees have been observed in both sensors (internal and external to the box of the station). Of course, the internal sensor always showed higher temperature values than the external one. The same pattern can be observed for what concerns internal and external relative humidity. Naturally, in the last case, the range of variations is wider.

Finally, as expected, small changes of a few millibars of pressure have been recorded. All the tested sensors show a very good long-term stability with any drifts of the signal.

Field experiments

As regards the field test, the Solfatara of Pozzuoli has been selected. The choice of this area has been suggested both for logistic problems (easy to reach) and for the presence of detectable amounts of CO₂, H₂S and SO₂ in the atmosphere. Infact, inside the Solfatara crater there are several fumaroles and intense soil degassing. Previous studies on the concentration of CO₂ and H₂S in the atmosphere of this area[7] clearly indicate the presence of a gas hazard. The automatic station has been placed inside the Solfatara on the eastern side at a distance of about 200 m from the 'Bocca Grande' and from the 'Stufe' fumaroles (Fig. 3).

Figure 3

Map showing the location of the monitoring system inside the Solfatara crater of Pozzuoli.

The choice of the location of the monitoring site was a consequence of the location of the main fumaroles that are chiefly located on the eastern side of the Solfatara.

The station worked for about one month (18th June–12th July 1999) without any technical problems. The atmospheric gases were pumped through a Teflon tube with the open external side placed at 1 m from the ground. The pump outlets were connected to the sensors in series.

The selected frequency of measurements was 1 h. The wind speed during the period of observation was in the range 0–5 m s^-1 and the dominant direction was from N–NE (220°).

Data related to CO₂, H₂S, and SO₂ are reported in Fig. 4a, 4b, and 4c, respectively. For the CO₂ content, variations from about 600 to 2000 ppm have been observed. H₂S ranged between 1 and 4 ppm while SO₂ changed between 1 and 9 ppm vol. Naturally, the content of the three gases in the atmosphere depends on several factors:

Figure 4

(a) CO₂ variations in the measuring point of Solfatara of Pozzuoli during the period 18th June–12th July, (b) H₂S variations in the measuring point of Solfatara of Pozzuoli during the period 18th June–12th July, (c) SO₂ variations in the measuring point of Solfatara of Pozzuoli during the period 18th June–12th July.

• (i) strength and location of the source;

• (ii) meteorological parameters and morphology of the area;

• (iii) reactions during the transport.

During the period of observation we assume that the strength and the location of the source have been constant, as well as the morphology of the area. The observed variations could mainly be related to the other factors such as meteorological parameters and chemical reactions during the transport. Among the investigated species CO₂ can be considered, from a chemical point of view, an inert gas while H₂S and SO₂ can initiate chemical reaction during the transport. In this context we will first analyse the behaviour of CO₂ and that of the two other species.

CO₂variations

No clear relationship was found between wind direction and CO₂ concentrations. On the other hand, the station was surrounded by gas emissions in all directions. A clear relationship instead was found between CO₂ and the wind speed (Fig. 5). Since the wind speed was lower than 4 ms^-1, the CO₂ concentrations changed in a wide range with the amplitude of the variations, generally decreasing as the wind speed increased.

Figure 5

Correlation between CO₂ and wind speed.

With the wind speed above 4 ms^-1, the CO₂ concentration reached a value close to 600 ppm. This value seems to be the background value of the CO₂ content in the atmosphere of the Solfatara.

Considering that the wind speed is an extremely variable parameter, we computed for each class of wind speed (0–1, 1–2 ... 4–5 m s^-1) the medium value of CO₂ concentration. And we have associated this value with a medium value of wind speed for each class. The obtained results are reported in Fig. 6 where it is possible to emphasize a good correlation between both examined parameters.

Figure 6

Correlation between CO₂ and wind speed.

The inverse correlation between CO₂ and the wind speed can also be observed using average values calculated for every hour of the day (see Fig. 7). During the night a decrease in wind speed and an increase in CO₂ concentration are often observed (Fig. 7). The same behaviour has been observed in other volcanic areas investigated to evaluate the gas hazard.[8])

Figure 7

Inverse correlation between CO₂ and wind speed.

H₂S and SO₂variations

As shown in Fig. 4b and 4c, H₂S and SO₂ concentrations changed in a wide range.

The frequency of variations was very high while their amplitude, as observed for CO₂, was related to the wind speed. With the wind speed above 4 ms^-1 the concentrations of the two gases reached their background values close to 2 ppm (Fig. 8). No clear correlation was found between H₂S and SO₂ for the whole period of observation. If we consider a twelve-terms moving average of H₂S and SO₂ concentrations (Fig. 9), periods of good correlation (from 19th to 30th June) can be observed. On the contrary, there are periods that exhibit significant changes in the SO₂ content at rather constant H₂S values.

Figure 8

(a) Correlation between H₂S and wind speed, (b) Correlation between SO₂ and wind speed.

Figure 9

Moving average of H₂S (lower part of the picture) and SO₂concentration values.

On the other hand, the abundance of H₂S and SO₂ in the atmosphere is highly dependent on the strength of the source as well as the chemical reactions during the transport. The H₂S content in the source gases (fumaroles) was 0.05% vol. The SO₂ content was below the detection limit of the analysed fumarolic gases (5 ppm by gas chromatography), the H₂S/SO₂ ratio being higher than 100. This ratio is completely different from the ratio measured in the atmosphere that changed between 1 and 0.1, it is 2–3 orders of magnitude lower than that of the source gases. The explanation for this observation must take into account the reaction during the transport.

The most probable reactions that produce SO₂ starting from H₂S are the following:

H₂S + O₃ = SO₂ + H₂O

H₂S + O = OH + HS

H₂S + OH = HS + H₂O

These reactions are completed in the following way:[9]

HS + O₂ = SO + OH

HS + O = SO + H

SO + O = SO₂

SO + O₂ = SO₂ + O

SO + O₃ = SO₂ + O₂

These mechanisms depend on the temperature, humidity, aerosol, particulate, insulation, and are characterized by rather different kinetics. It is not possible to clarify all possible oxidation mechanisms than can take place inside the Solfatara with the acquired data, even though the variability of the H₂S/SO₂ ratio (from 100 in the fumaroles to 1 or 0.1 in the atmosphere), gives evidence of the H₂S oxidation.

Conclusion

The automatic apparatus developed in this study after the laboratory tests was successfully tested inside the Solfatara of Pozzuoli. All sensors gave a very good performance in terms of stability and reliability as well as the electronic aspect. The CO₂, H₂S and SO₂ concentrations changed in a wide range in certain correlation with the wind speed. Above a wind speed of 4 ms^-1 all three monitored species reached a constant value of 600 ppm for CO₂, and about 2 ppm for H₂S and SO₂. These values seem to be the background values inside the Solfatara. The low H₂S/SO₂ ratio (1–0.1) measured in the atmosphere, compared to the same ratio in the fumarolic gases (100) clearly indicates that H₂S undergoes severe oxidations during the transport in the atmosphere. The developed system is able to operate for a long time in severe corrosive conditions, in remote areas with no electric power supply, and is a powerful apparatus to control the gas hazard in volcanic and some industrial areas.

Table 1 Sensor characteristics

Table 2 Specification of the electronic weather station

Table 3 Specific characteristics for the different system