retrieval of absolute so2 column amounts from scattered-light spectra: implications for the evaluation of data from automated doas networks
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2016
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Abstract
Scanning spectrometer networks using scattered solar radiation in the
ultraviolet spectral region have become an increasingly important tool for
monitoring volcanic sulfur dioxide (SO2) emissions. Often measured
spectra are evaluated using the differential optical absorption spectroscopy
(DOAS) technique. In order to obtain absolute column densities (CDs), the
DOAS evaluation requires a Fraunhofer reference spectrum (FRS) that is free
of absorption structures of the trace gas of interest. For measurements at
volcanoes such a FRS can be readily obtained if the scan (i.e. series of
measurements at different elevation angles) includes viewing directions where
the plume is not seen. In this case, it is possible to use these viewing
directions (e.g. zenith) as FRS. Possible contaminations of the FRS by the
plume can then be corrected by calculating and subtracting an SO2
offset (e.g. the lowest SO2 CD) from all viewing directions of the
respective scan. This procedure is followed in the standard evaluations of
data from the Network for Observation of Volcanic and Atmospheric Change
(NOVAC). While this procedure is very efficient in removing Fraunhofer
structures and instrumental effects it has the disadvantage that one can
never be sure that there is no SO2 from the plume in the FRS.
Therefore, using a modelled FRS (based on a high-resolution solar atlas) has a great advantage. We followed this approach and investigated an SO2
retrieval algorithm using a modelled FRS. In this paper, we present
results from two volcanoes that are monitored by NOVAC stations and which
frequently emit large volcanic plumes: Nevado del Ruiz (Colombia) recorded
between January 2010 and June 2012 and from Tungurahua (Ecuador) recorded
between January 2009 and December 2011. Instrumental effects were identified
with help of a principal component analysis (PCA) of the residual structures
of the DOAS evaluation. The SO2 retrieval performed extraordinarily
well with an SO2 DOAS retrieval error of
1 − 2 × 1016 [molecules cm−2]. Compared to a standard evaluation,
we found systematic differences of the differential slant column density
(dSCD) of only up to ≈ 15 % when looking at the variation of the
SO2 within one scan. The major advantage of our new retrieval is that
it yields absolute SO2 CDs and that it does not require complicated
instrumental calibration in the field (e.g. by employing calibration cells
or broadband light sources), since the method exploits the information
available in the measurements.
We compared our method to an evaluation that is similar to the NOVAC approach, where a spectrum that is recorded directly before the scan is used as an FRS and an SO2 CD offset is subtracted from all retrieved dSCD in the scan to correct for possible SO2 contamination of the FRS. The investigation showed that 21.4 % of the scans (containing significant amounts of SO2) at Nevado del Ruiz and 7 % of the scans at Tungurahua showed much larger SO2 CDs when evaluated using modelled FRS (more than a factor of 2). For standard evaluations the overall distribution of the SO2 CDs in a scan can in some cases indicate whether the plume affects all viewing directions and thus these scans need to be discarded for NOVAC emission rate evaluation. However, there are other cases where this is not possible and thus the reported SO2 emission rates would be underestimated. The new method can be used to identify these cases and thus it can considerably improve SO2 emission budgets.
We compared our method to an evaluation that is similar to the NOVAC approach, where a spectrum that is recorded directly before the scan is used as an FRS and an SO2 CD offset is subtracted from all retrieved dSCD in the scan to correct for possible SO2 contamination of the FRS. The investigation showed that 21.4 % of the scans (containing significant amounts of SO2) at Nevado del Ruiz and 7 % of the scans at Tungurahua showed much larger SO2 CDs when evaluated using modelled FRS (more than a factor of 2). For standard evaluations the overall distribution of the SO2 CDs in a scan can in some cases indicate whether the plume affects all viewing directions and thus these scans need to be discarded for NOVAC emission rate evaluation. However, there are other cases where this is not possible and thus the reported SO2 emission rates would be underestimated. The new method can be used to identify these cases and thus it can considerably improve SO2 emission budgets.
| Reference Key |
lbcke2016atmosphericretrieval
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|---|---|
| Authors | ;P. Lübcke;J. Lampel;S. Arellano;N. Bobrowski;F. Dinger;B. Galle;G. Garzón;S. Hidalgo;Z. Chacón Ortiz;L. Vogel;S. Warnach;U. Platt |
| Journal | bioorganic & medicinal chemistry |
| Year | 2016 |
| DOI |
10.5194/amt-9-5677-2016
|
| URL | |
| Keywords |
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