The principle of non-dispersive absorption spectroscopy itself has already been developed in the 1930's (Lehrer and Luft, 1938) but has mainly been applied in the infra red (NDIR). Its cost-effective and compact application for SO₂ has been nearly impossible until the recent developments in the field of ultra violet (UV) light emitting diodes (LEDs). NDUV makes use of the fact that different gas species absorb light at different wavelengths. Therefore, guiding light through a sample gas and measuring its attenuation at suitable wavelengths, provides information on the gas composition. The absorption efficiency of a gas species at wavelength λ is typically described by its “absorption cross-section” σ(λ), which is well-known for most atmospheric constituents and can be found in data bases like by Keller-Rudek et al. (2013). The characteristic absorption properties of each gas originate from its molecular-atomic structure and the resulting allowed quantum state transitions. In many applications the spectral absorption patterns of different gases overlap and spectrally resolved (“dispersive”) measurements of the attenuation have to be performed to separate the contribution of different absorbers. On the other hand, in a few cases the gas of interest is the only—or at least the dominant—absorber in a distinct wavelength range. Then, a single (“non-dispersive”) measurement of the light's attenuation in this range is sufficient to determine its concentration. For SO₂ in volcanic plumes, this is the case in the ultra violet (UV) at wavelengths around 285 nm (see Figure 1A).

Figure 1B shows the basic setup of the PITSA prototype. Gas is pumped through an aerosol filter into a sample cell of length L = 27 cm, where it is exposed to the parallelized beam of a UV-LED (UV-TOP 280) with a well-known emission spectrum in the desired wavelength range around 285 nm (see Figure 1A). The aerosol filter is a 200 nm pore filter and prevents scattering and absorption of light on particles (see section 3.1.5 for details), assuring that SO₂ absorption is the dominant light attenuating process in the gas. A detector (silicon photodiode with appropriate amplifier and readout electronics, see section 2.3 below) is located behind the cell, measuring the spectrally integrated intensity

The integration limits λ₁ ≈ 200 nm and λ₂ ≈ 1, 100 nm are confined by the sensitivity range of the photodiode but generously bracket the wavelength interval where the emission spectrum of the LED I₀(λ) has non-zero intensity. A solenoid valve allows to occasionally pump the sample air through an SO₂ scrubber (an off the shelf gas mask cartridge), which removes the SO₂ from the sample cell such that the reference intensity

(which is unaffected by SO₂ absorption) can be recorded with the same detector. In the following this procedure will be referred to as “zero point measurement.” By comparing the two intensities I₀ and I, the SO₂ concentration in the cell is calculated (see below).

For a setup with a single detector behind a measurement cell, LED intensity fluctuations and drifts were observed to be the limiting factor for the instrument's detection limit, as they cannot be distinguished from actual changes in the SO₂ concentration in the cell. To overcome this, in the PITSA a part of the light is measured over a “monitoring channel” with no variable absorber in the light path. It allows to monitor the LED intensity and to correct for its variations, which improves the detection limit by a factor of ≈ 5. If not stated otherwise, this correction has been applied to data shown in this article.

From the two intensities I₀ and I the spectrally integrated “optical density”

of the sample gas can be calculated. τ is a convenient measure for the light attenuation, since it is in first approximation proportional to the SO₂ number concentration (see Equation 6). It can therefore be regarded as a kind of uncalibrated instrument signal.

The PITSA setup is exceptional in a way, that it is possible to theoretically derive the relationship between τ and the actual SO₂ concentration to very high precision, hence, a calibration curve can be calculated without feeding test gases to the instrument. The light intensity after passing the cell at a distinct wavelength λ, is given according to the Beer-Lambert-Law (Swinehart, 1962) to:

Here, I₀(λ) is the intensity of the incident UV radiation, which is multiplied by an attenuation factor, that depends on the wavelength dependent absorption cross section σ(λ), the SO₂ number concentration c (molecules per unit volume) and the absorption path length L. Equivalent to the definition before (Equation 3), the optical depth at a distinct wavelength λ is given by:

To obtain the relation between c and the actual instrument response τ (calculated from the integrated intensities I and I₀, as defined in Equations 1 and 2, Equation 5) has to be integrated. Since there are no analytical expressions for either σ(λ) nor I₀(λ), the integrals have to be calculated numerically. However, for small optical depths, (Equation 5) can be linearized, to obtain an approximate analytic expression (detailed derivation in Supplementary Material):

Here, σ_eff is the “effective absorption cross section,” defined as

which is the average value of σ(λ), weighted with the normalized LED emission spectrum I₀(λ)/I₀, both shown in Figure 1A. In practice, it is sufficient to perform the integration over the non-zero region of I₀(λ). For the results presented below, we chose 250 and 330 nm as the lower and upper integration limit respectively.

Finally, the SO₂ number concentration c can be converted to a volume mixing ratio [in the following referred to as “VMR,” given in parts per million (ppm) or parts per billion (ppb)], by applying the ideal gas law, yielding:

with k_B, T, and p being the Boltzmann constant, temperature, and pressure, respectively. To assure an accurate conversion in the PITSA, T, and p are continuously monitored by corresponding sensors on the detector circuit board in the optical setup. For theoretical calculations and error estimations within this article normal temperature and pressure (NTP) conditions are assumed, which are T = 293.15 K and p = 1013.25 hPa. For other conditions, VMRs have to be scaled according to Equation (8).

For a quantitative solution of the sensor response, the SO₂ absorption cross section σ(λ) is taken from Vandaele et al. (2009). Further, the shape of the LED spectrum I₀(λ)/I₀ is required. It was observed to be very stable over time and a wide range of conditions (see section 2.2 and Supplementary Material), it is therefore sufficient to determine it once with a scientific grade spectrometer. Insertion of these values into Equation (6) and the numerical integration of Equation (5), respectively, yields the two response curves shown in Figure 2. Equation (7) yields σeff≈7.6·10-19cm2, meaning that intensity variations < 10⁻⁴ have to be resolved to achieve detection limits < 0.2 ppm in the SO₂ VMR. The linear approximation is sufficient (deviation < 1%) for VMRs up to ≈ 700 ppm, which is suitable for most volcanic applications. For higher VMRs (up to 10000ppm), a higher order analytical solution of Equation (5) or the numerical approach should be applied. As discussed in section 3.1.2, the calculated responses are sufficient to single digit percent accuracies in the measurement. With offset drifts being corrected through the automated zero point measurements with SO₂ scrubbed air, the instrument features an inherent and stable calibration. In fact, all data shown in this article were evaluated without ever applying an experimental SO₂ gas calibration to the instrument.

The optical cell basically consists of a glass tube (27 cm length, 11 mm diameter, 26 ml volume), with air in- and outlets and UV transmissive fused silica windows at each end. A lens in front of the UV-LED parallelizes the light before a beam splitter and a mirror distribute it to the measurement cell and the monitoring channel, respectively. Lenses in front of each detector channel focus the light again (compare Figure 1B). To minimize the impact of mechanical strain (vibration, shock, …) on the measurement (see section 3.1.6), the whole optical setup is mounted on a common aluminum bracket, which is in turn placed on damping feet, for its mechanical decoupling from the housing and less sensitive instrument components. The optical bench is equipped with heating resistors (switchable between 4 and 8 W heating power, to obtain a typical temperature rise above ambient of 10 and 20 K, respectively), which can be activated to prevent condensation (see section 3.1.4). An image of the optical bench can be found in the Supplementary Material. Teflon and Tygon (type 2375) tubings were used to guide the air through the setup. The pump is a diaphragm pump of type TM22-A12 from Topsflow. An off-the-shelf gas mask cartridge (type Eurfilter A2-B2-E2-K2-P3 R from Panarea) served as SO₂ scrubber and Millex-FG filters from Merck Millipore with pore size of 220 nm were used for particle filtering at the inlet.

The LT3092 current source from Linear Technologies (an integrated circuit of few millimeter outline) was used to generate a stable supply current of 7 mA for the UV-LED from the battery voltage, resulting in an optical output power around 0.3 mW. Higher currents up to 40 mA can be applied to the LED to achieve optical output powers >1.5 mW, but were not used in the PITSA, as they decrease the LED's lifetime.

A single detector channel consists of following components: A silicon photodiode (type PC10-2-TO5 by Pacific Silicon Sensor Inc.) produces a photo current (≈ 0.7 μA at negligible absorption in the cell) proportional to the number of photons hitting the diode's active area. A transimpedance amplifier implemented by using a chopper operational amplifier (AD8552 by Analog Devices) with a 3 MΩ feedback resistor converts the photo current to a voltage of ≈ 2 V. A 22-Bit analog to digital converter (MCP3551) converts the analog signal to a digital value. The minimum integration time of the detector is 75 ms, which is the conversion time of the analog to digital converter. A sketch of the detector circuitry can be found in the Supplementary Material.

Detector unit and solenoid valve are controlled by a microcontroller (Arduino Mega 2560 R3) with GPS-capability, which also performs data processing and logging. The recorded data is written to a memory card. For external logging, e.g., in multi sensor systems, data are simultaneously sent to the instrument's USB port and an analog output. The whole setup is integrated into a polycarbonate housing (see Figure 3) together with a 120 Wh rechargeable battery (HE-12V-10Ah-LiMn by Hellpower Industries). Remaining free space in the housing allowed to add a small commercial CO₂ detector (Senseair K30-FR) to the setup, which — in combination with the PITSA — allows to determine (approximate) CO₂/SO₂ ratios. A small LCD screen at the outside of the box (see Figure 3) shows preliminary real-time data. With a power consumption < 5 W (heating disabled), the instrument runs autonomously without any further peripheral equipment for about 24 h. The footprint of the housing (without scrubber) are 40 x 20 x 13 cm, the total weight is ≈ 8 kg.

The total accuracy of the PITSA is restricted by a series of error sources, which are summarized in Table 1 and discussed in detail in the following subsections. Since the impact of some error sources depends on the measurement parameters and conditions, the total accuracy has to be estimated individually for a given application from information in this chapter.

The response time of the instrument is limited by the flushing time of the sample cell. Figure 9 shows response of the PITSA while switching from plume air with 15 ppm SO₂ VMR to scrubbed reference air (gray area), which causes a nearly step-shaped decrease in SO₂ at the sample cell inlet. From the response the flushing time can be determined to t₉₀ ≈ 1.3 s, which corresponds to the time interval in which the VMR dropped to 10% of its initial value. Dashed lines show calculated responses for the given cell volume and air flow, assuming laminar flow and turbulent mixing, respectively. As expected, reality is a mixture of both. These short response times are a major improvement compared to conventional field sensors, as they allow to detect short time variations in volcanic gas compositions which are a recent field of research (e.g., Pering et al., 2014). Furthermore, gas compositions are often derived by evaluating data from multiple collocated instruments against each other. In such setups fast responding sensors are desirable, since long response times and different response behaviors of the individual instruments can cause severe complications and artifacts during data evaluation (Roberts et al., 2012).

The measurement range is limited, mainly because for high SO₂ VMRs, the light intensity reaching the detector becomes too low and the contribution of electronic noise exceeds the sensitivity error specified in section 3.1.2. When applying the numerical solution of Equation 5 (which takes non-linearities of the instrument response into account), the PITSA provides reasonable results up to VMRs of about 1% (10000 ppm) at any data rate, which is sufficient for most volcanic applications. If desired, the measurement range can be shifted to higher VMRs by reduction of the instrument sensitivity, which can either be achieved by using shorter absorption paths or by changing the LED peak emission wavelength (see Figure 1A).

The most critical expandable parts are the scrubber, the aerosol filter, the UV-LED and the pump. For the scrubber and the particle filter, lifetimes are strongly dependent on the measurement conditions. The capacity of the gas mask cartridge (Panarea Eurfilter A2B2E2K2P3) is specified to 3 l (ca. 8 g) of pure SO₂. Unless this limit is exceeded, a leakage < 0.5% is guaranteed by the supplier to meet regulatory requirements. Assuming an average SO₂ VMR of 10 ppm, a PITSA airflow of 1.5 l/min and typical reference measurement intervals (5 s duration, performed in 300 s intervals), a life-time of > 20 years is obtained. However, this simple scaling of the scrubber capacity neglects the impact of other gases and may not apply, since the life-time of an unused cartridge in its sealed packaging is specified to 6 years only. Our observations are not sufficient for a clearer statement: for a new scrubber the leakage was found to be < 0.2%, which is in agreement with the specifications. Further, we derived an upper limit of 2% leakage (limited by the uncertainty of reference instrumentation), after 0.8 ml (2 mg) of SO₂ had been supplied to a single scrubber over a period of 4 months during cloud chamber experiments (see section 4.4). Further investigations are necessary in this domain. The particle filter lasts for several hundred hours of operation in quiescent degassing plumes at low background aerosol, whereas hourly exchange can be necessary when ash containing plumes are sampled (Marco Luzzio, personal communication). On the lifetime of the UV-LED, there is only sparse information given by the supplier. During the atmospheric simulation chamber (see section 4.4) experiments in the laboratory, a linear decrease in optical output power due to ageing of 40% over 1, 000 h of operation was observed, at a supply current of 7 mA and instrument temperatures of (40 ± 3)°C (heating enabled). Even though the exact scaling of lifetime with current and temperature is not known, it is likely that lifetimes of 5, 000 h can be exceeded at few mA and typical environmental temperatures. The typical lifetime of higher grade miniature pumps are of the order of 10000 h.

This section is intended to give a rough impression on the costs of the used components. We regard this as an important aspect to consider, when it comes to the application of instruments in harsh meteorologic conditions and acidic environments as they are encountered during volcanic plume in-situ measurements. A fully functional setup like the PITSA instrument can be realized at material costs of about 2,500 €. At this price, for instance electrochemical sensors are already available with all necessary peripherals assembled and ready to use. Considering development and construction costs, the costs for the PITSA are estimated to about 10,000 €. However, it shall be noted that a part of the cost charges off, since the optical setup has a longer lifetime than electrochemical cells.

On the 23^rd of September 2015, measurements at the rim of the North East Crater of Mt. Etna (Italy) were performed in parallel to a freshly calibrated electrochemical sensor (CiTiceL 3MST/F), as it is usually applied in Multi-GAS sensor systems. Figure 10 on the left shows an excerpt of the total 40 min time series. The light blue curve shows the original PITSA signal (t_int = 0.5 s), which reveals variation in SO₂ on very short time-scales, which are not seen by the electrochemical sensor, due to its slower response. For better comparability, the PITSA's temporal resolution was degraded by convoluting the signal with an exponential decay function (decay time of t_dec = 10 s), which mimics the pulse response of a sensor with t₉₀ = t_dec/log(2) ≈ 14 s. This yields the dark blue dashed curve, which is in very good agreement with the electrochemical sensor data. The scatter plot on the right of Figure 10 confirms the agreement. It shows 10 s average values over the whole 40 min time series.

During a measurement campaign on White Island (New Zealand) in cooperation with the Institute of Geological and Nuclear Science (GNS), airborne measurements ≈ 4 km downwind from the crater were performed with the PITSA on a small propeller aircraft on 8^th of December 2015. At such distances from the source, plumes are already well diluted and in the case of White Island SO₂ VMRs rarely exceed 1 ppm. The flight path is shown on the left in Figure 11A. A series of 17 plume transects were flown perpendicular to the wind direction. Altitude was increased successively in steps of ≈ 60 m, to obtain cross-sectional 2D distributions of the measured gases. Among others, a very sensitive electrochemical sensor (Interscan 4000, measurement range of 2–2,000 ppb) was installed in the aircraft. In this particular case, the PITSA's automated zero point measurements were disabled, to avoid potential loss of valuable measurement time during the short plume crossings. Instead, gas free regions were identified during post processing and used to calculate a baseline (see Figure 11B). Compared to zero point measurements with a scrubber it has the advantage that undesired offsets caused by O₃ cross interference (see section 3.1.3) and water vapor influence (see section 3.1.4) are removed to a large extent. The baseline was determined by fitting a 4^th order polynomial to the presumably gas free data. Figure 11C shows the baseline corrected PITSA measurements in comparison to the Interscan data. Similar as for the PITSA a baseline was also subtracted from the Interscan data (offset of ≈ 45 ppb). Cross sensitivities of the Interscan to hydrogen sulphide (H₂S) were avoided by scrubbing H₂S from the sampled air. The plot is equivalent to Figure 10. For the smoothing (see section 4.1), this time t_dec was chosen to only 2.5 s. Again, the PITSA is the faster instrument, but also exhibits larger noise. The slightly negative VMRs between the transects coincide with the curvature of the flight path and may indicate an influence of the force acting on the instrument, leading e.g., to slight bending of the optical setup and causing relative changes in the light throughput of ≈ 10⁻⁵. Effects of O₃ depletion and water vapor enhancement in the plume seem to be too small to be observed, such that in this particular case detection limits of ≈ 20 ppb were achieved. The asymmetric peak shapes in the Interscan data (independent of flight direction) suggest that the Interscan suffers an asymmetric response behavior, reacting much faster to SO₂ increases than to declines. This explains the change in instrument agreement, which gets significantly worse when leaving the plume. With the fast response a highly resolved 2D SO₂ plume cross section could be recorded, which is shown in Figure 11D. Wind speed and direction were obtained using the wind circle method (Doukas, 2002). Integration over plume cross section and multiplying by the wind speed yielded a total SO₂ emission rate of 280 ± 85t d⁻¹. This is in agreement with results from simultaneously performed remote sensing measurements (320 ± 160 t d⁻¹, derived with a zenith DOAS instrument, also installed on the plane) and reported literature values (Werner et al., 2008).

On the 3^rd of December 2015, the plume of a fumarole on the White Island crater floor ('Fumarole Zero' with a degassing temperature of 170°C) was sampled. To reduce the impact of condensation and the risk of damage inside the PITSA, the sample air was drawn at 5 m distance from the vent, the sampling time was limited to 10 s and the instrument's internal heater was used (see section 2.3). Figure 12 shows the recorded time series. Here, also data of the integrated CO₂ sensor are shown (see section 2.3). Measurements outside the plume just before entering and after leaving were assumed as background and were subtracted. Observed peak values in CO₂ and SO₂ were 9, 800 and 1, 400 ppm, respectively. The data in the gray shaded area yield a CO₂/SO₂ ratio of 6.8. This is in good agreement with ratios measured during sampling at the crater rim on the same day (6 ± 2) and as measured by GNS on the gas flight 5 days later (8 ± 4).

From November 2017 to June 2018 the PITSA was applied in Bayreuth (Germany), during atmospheric simulation chamber experiments (similarly as described in detail by Buxmann et al., 2012) for the investigation of reactive halogen chemistry in volcanic plumes. The chamber consists of a 4 m³ PTFE bag with a diameter of 1.4 m and 2.5 m height, and with a sun simulator made of 7 x 1200 W Osram HMI lamps. Beside the PITSA, several other instruments were connected to the chamber to monitor the conditions and gas concentrations inside, among others a fluorescence SO₂ instrument by Enivronnement S.A., type AF 20M. Figure 13 shows times series of the PITSA and the AF 20M (1 min average values) during an SO₂ injection event in the chamber. The AF 20M and the PITSA both showed offsets of −680 and −370 ppb, respectively, before SO₂ was injected. These offsets were subtracted.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.