Welcome to the database of the Network for Observation of Volcanic and Atmospheric Change (NOVAC).

Technical information

NOVAC Instrument

Instrument type

Scanning Mini-DOAS/NOVAC Mark I (ScanDOAS)

Spectrometer specifications

Crossed Czerny-Turner spectrometer; 2400 g/mm holographic grating; 280-420 nm; 0.6 nm FWHM res.; 12 bit ADC; CCD detector

Fore-optics specifications

Single or dual beam, mono lens refractor, f/4 telescope; HOYA U330 filter (cutoff > 360 nm); 600 μm mode-mixer optical fibre effective FOV of 8 mrad per beam; dual beam separation of 80 mrad; 1.8° step scanning head

Control unit specifications

Linux microcomputer and home-built electronics for measurement control and data storage and transmission

Other specifications

GPS antenna for logging time and position; operation at ambient T; 12 V power, usually from solar panels; power consumption < 10 W, sleep-mode timer switch.

Measurement Description

A standard scan measurement is composed of 51 radiance spectra taken across the scanning plane at steps of 3.6°. Two different scan modes are used: "flat" and "conical". In the flat mode the scan is made from horizon to horizon along a vertical surface passing zenith. In the conical mode the scan is made over a conical surface with opening angle of 60°, hereby a more efficient scan geometry is obtained, providing results under larger deviation in wind direction. Each scan sequence starts with optimization of the exposure time for a spectrum taken in the position closest to zenith (limited to 1000 ms). This exposure time is kept constant for all measurements in the same scan. Usually 15 spectra are averaged at each position to improve S/N. An additional dark-current/offset spectrum is measured on each scan, with the fore-optics facing the shadowed nadir position of the scanner. Data is saved in-situ and then transmitted to the observatory for real-time evaluation. The instrument is usually put into sleep-mode during night-time to save power.

Algorithm Description (Version 1)

Retrieval of relative SO2 slant column densities (SCD)

Each spectrum is checked for quality and corrected for dark-current and 0th-order offset. A shift- correction against a solar Fraunhofer (Kurucz) spectrum is then performed. A DOAS non-linear fitting of the ratio of each spectrum respect to the reference spectrum taken closest to zenith position is applied, including absorption cross sections of SO2 (Vandaele_1994_296K or Bogumil_2003_293K), O3 (Voigt_2001_223K) and a modeled Ring-effect pseudo absorber. A 5th-order polynomial is also included to account for broad-band spectral effects. The standard fitting window is between 309.6 and 324.6 nm. Each cross-section is prepared by convolution with a ILS taken from measurement of the 302.15 nm emission line of Hg taken at room temperature.

Retrieval of SO2 vertical column densities (VCD)

The background SO2 column is approximated as the average of the 20% lower SCD with acceptable fit error and then subtracted from all SCD in the scan. These SCD are then used to estimate the angular centre-of-mass of the plume, used for plume location. The vertical column densities are calculated using a geometrical approximation of the AMF.

Retrieval of SO2 flux and plume parameters

The average VCD between two consecutive scans is multiplied by the horizontal extent of each segment at the distance of the plume. The sum of all segments is multiplied by the normal component of plume velocity at the centre of mass of the plume. Plume altitude and direction can be estimated from triangulation of two scans. Plume speed is usually obtained from a meteorological model. In this version, analyzed wind data from ECMWF ERA-interim database was used, with a resolution of 0.125×0.125°, 6 h and up to 60 vertical levels from ground up to 0.1 hPa. Data is interpolated to the location of the volcanic vent and time of measurement for each flux calculation.


The daily average (arithmetic mean) and standard deviation (s.d.) of the reported variables were calculated when > 5 valid measurements were found on the day. A SO2 flux measurement is considered valid if it has a nearly complete coverage of the plume (completeness > 0.8, a minimum number of valid SCD in the scan and other conditions. See References for more details.


Arellano, S., Galle, B., the NOVAC collaboration, Synoptic analysis of a decade of daily measurements of SO2 emission in the troposphere from volcanoes of the network for observation of Volcanic and Atmospheric Change, Submitted.

Galle, B., Johansson, M., Rivera, C., Zhang, Y., Kihlman, M., Kern, C., Lehmann, T., Platt, U., Arellano, S. and Hidalgo S. (2010), Network for Observation of Volcanic and Atmospheric Change (NOVAC) — A global network for volcanic gas monitoring: Network layout and instrument description, J. Geophys. Res., 115, D05304, doi:10.1029/2009JD011823.

Galle, B., Oppenheimer, C., Geyer, A., McGonigle, A., Edmonds, M., and Horrocks, L. (2003), A miniaturised ultraviolet spectrometer for remote sensing of SO2 fluxes: a new tool for volcano surveillance, J. Volcanol. Geotherm. Res., 119: 1–4, p. 241-254, doi:10.1016/S0377-0273(02)00356-6.

Edmonds, M., Herd, R., Galle, B., Oppenheimer, C., (2003), Automated, high time-resolution measurements of SO2 flux at Soufrière Hills Volcano, Montserrat, Bull. Volcanol., 65: 578, doi:10.1007/s00445-003-0286-x.

Johansson, M., Galle, B., Zhang, Y., Rivera, C., Deliang, C. and Wyser K., (2009), The dual-beam mini-DOAS technique—measurements of volcanic gas emission, plume height and plume speed with a single instrument, Bull. Volcanol., 71: 747, doi:10.1007/s00445-008-0260-8.

Dee, D. P., Uppala, S. M., Simmons, A. J., Berrisford, P., Poli, P., Kobayashi, S., Andrae, U., Balmaseda, M. A., Balsamo, G., Bauer, P., Bechtold, P., Beljaars, A. C. M., van de Berg, L., Bidlot, J., Bormann, N., Delsol, C., Dragani, R., Fuentes, M., Geer, A. J., Haimberger, L., Healy, S. B., Hersbach, H., Hólm, E. V., Isaksen, L., Kållberg, P., Köhler, M., Matricardi, M., McNally, A. P., Monge-Sanz, B. M., Morcrette, J.-J., Park, B.-K., Peubey, C., de Rosnay, P., Tavolato, C., Thépaut, J.-N. and Vitart, F. (2011), ERA-Interim reanalysis: configuration and performance of the data assimilation system, Q.J.R. Meteorol. Soc., 137: 553–597, doi:10.1002/qj.828.