This study examines the adequacy of the existing Brewer
network to supplement other networks from the ground and space to detect
SO
Volcanic eruptions are an important source of natural emissions of sulfur
dioxide (SO
Volcanic eruptions in the past decade considered in this study.
Measurements of SO
There have been various initiatives during recent years that used satellite
measurements of SO
In the present work we investigate the efficiency of the existing Brewer
network in the Northern Hemisphere to detect volcanic SO
Table 1 lists in chronological order all major volcanic eruptions in the Northern Hemisphere between 2005 and 2015 with a volcanic explosivity scale index (VEI) of at least 4 (Newhall and Self, 1982; Robock, 2000; Zerefos et al., 2014). The study also provides a separate analysis for the Bárðarbunga eruption, which although not rated 4 has been already studied with the Brewer instrument at Sodankylä by Ialongo et al. (2015).
As seen from Table 1, chronologically, the first case was the volcanic
eruption at Mount Okmok, Alaska (53.43
The capability of the Brewer network to measure columnar SO
The paper is structured in the following order. Sect. 2 describes the data
sources and the methods of analysis of the columnar SO
Sulfur dioxide in the atmosphere can be measured from ground-based
instruments and by instrumentation onboard the spacecraft and can be estimated
with the help of models. The Brewer is an automated, diffraction-grating
spectrophotometer that provides observations of the sun's intensity in the
near-UV range. The spectrophotometer measures the intensity of radiation in
the ultraviolet absorption spectrum of ozone at five wavelengths (306.3,
310.1, 313.5, 316.8 and 320.1 nm) with a resolution of 0.6 nm. These data
are used to derive the total ozone column (Kerr et al., 1980). Because sulfur
dioxide has strong and variable absorption in this spectral region, the
Brewer spectrophotometer has additionally been proposed to derive SO
Before proceeding to the analysis of Brewer measurements, the methodology to
derive columnar SO
According to the Brewer retrieval algorithm, the following ratios are
formed:
The total ozone column is determined by the formula
From the above-described operational Brewer algorithm it is evident that the
estimation of columnar SO
All stations with accessible SO
In this study we analysed 23 stations located in Europe, 6 Brewer
stations in Canada, 2 in the USA and 1 in Taiwan; their geographical
positions are shown in Fig. 1. SO
Daily SO
Stations with accessible SO
In our analysis only DS measurements satisfying the following
criteria have been used: a Brewer DS measurement was included if and only if
for every measurement cycle of five sets of measurements (from which also total
columnar ozone is derived) the standard deviation of O
Daily sulfur dioxide (SO
Rural AirBase stations analysed in this study (see text).
Only for the case of the Bárðarbunga eruption in 2014 were the columnar
SO
The columnar SO
SO
SE: standard error.
For the case of OMI, the SO
Integrated column of SO
For GOME-2, we analysed the total SO
Finally, both for the satellite and the Brewer data, we have considered that
during a 10-day period prior to any eruption both the surface and the
satellite datasets represent a baseline reference from which subsequent
departures after the eruption should be tested as to their significance.
Therefore, we calculated averages and standard deviations (
Dispersion of volcanic emissions is simulated with the Lagrangian transport
model FLEXPART (FLEXible PARTicle dispersion model; Stohl et al., 2005; Brioude et al., 2013). The model is
driven by hourly meteorological fields from the Weather Research and
Forecasting (WRF) atmospheric model (Skamarock et al., 2008) at a horizontal
resolution of
HYSPLIT 120 h back trajectories of air masses arriving on the
day of maximum SO
Mean SO
Bárðarbunga was continuously active during September–October 2014, but it was only during 18–26 September when meteorological conditions
favoured transport towards Europe as shown by back trajectory analyses. A
detailed description of the transport of Bárðarbunga plumes towards
the station of Hohenpeißenberg is provided using the FLEXPART Lagrangian
particle dispersion model offline coupled with the WRF_ARW atmospheric
model. The simulation period is 18–26 September 2014. We assume a constant
SO
The high SO
As shown in Fig. 4a, the SO
Mean surface SO
The eruption took place at the beginning of September 2014, and several
European countries experienced high concentrations of SO
As can be seen from Fig. 4a, the highest SO
Charts of forecasted total column SO
Finally, it should be mentioned here that the thin aerosol layer that was detected by the PollyXT lidar (Engelmann et al., 2016) over Leipzig at
around 2–3 km on 23 and 24 of September 2014 was mostly associated with
volcanic ash advection (Fig. 7). A corresponding cluster analysis of all
155-hourly HYSPLIT back trajectories during this period and for the heights
of the layer detected by the lidar (
Range-corrected signal at 1064 nm from the PollyXT lidar in Leipzig on 23 (up) and 24 September 2014 (down). The red rectangle indicates the location of the volcanic ash layer.
Cluster analysis of the HYSPLIT back trajectories that arrive
every hour (from 23 September 12:00 UTC up to 24 September 18:00 UTC) at
2.5–3.5 km height over Leipzig. A 54 % cluster percentage means that there
is 54 % chance that the SO
A major eruption of Mt Nabro, a 2218 m high volcano on the border between
Eritrea and Ethiopia (13.37
The Nabro volcanic plume was mainly transported to East Asia and was
detected by various satellite instruments which provide better spatial
coverage than the Brewers. A special case study focuses on discrepancies
found between ground-based and satellite observations of the volcanic
SO
SO
HYSPLIT back trajectories of air masses
SO
More specifically, Fig. 10b shows back trajectories from Izaña (Tenerife)
during 19–29 June 2011 at 15, 17.5 and 20 km heights. It appears that the
upper-tropospheric–lower-stratospheric air masses arriving at Tenerife during
19–29 June originated from Nabro. In June 2011 the Nabro volcano ash plume
was detected by the Micropulse Lidar (MPL) located at Santa Cruz de Tenerife
(Canary Islands, Spain). The volcanic plume height ranged from 12 km on
19 June to 21 km on 29 June (Sawamura et al., 2012). Figure 11 shows the
columnar SO
Mean SO
In this case the Brewer at Izaña has been able to detect an SO
The case of the 2011 Nabro eruption shows an example of the importance of the
Brewer spectrophotometers in measuring and detecting changes in SO
These findings can provide clues to the detection limits of such events from a well-calibrated Brewer network and a space-borne instrument. They need further clarification with more Brewers and a larger number of cases.
The Eyjafjallajökull volcano, Iceland (63.63
Figure 12 shows the responses of Brewer stations under the volcanic SO
HYSPLIT 120 h back trajectories of air masses arriving at De Bilt (left column) and Uccle (right column) on 2 May 2010 (first row), 11 May 2010 (second row) and 18 May 2010 (third row).
We should note here that volcanic clouds can be rather narrow plumes with
diameters on the order of a few tens of kilometres (e.g. Stohl et al., 2011;
Webley et al., 2012; Thorsteinsson et al., 2012; Kristiansen et al., 2012;
Kokkalis et al., 2013), and thus it is possible that a volcanic layer detected at a specific station is not observed by neighbouring stations. The
measurements at Uccle and De Bilt that are located at a horizontal distance
of 150 km are different during the Eyjafjallajökull episode and provide
a very good example. On 2 May 2010 the mean daily SO
In Table A1 of Appendix A, we present the dates when the examined Brewer
stations were either under or outside of the volcanic SO
Mean SO
As we can see from Fig. 12, the columnar SO
We note here that the ash cloud caused further disruptions to air
transportation on 4–5 May and 16–17 May 2010, particularly over Ireland and
the UK. The average SO
Correlation coefficients between the mean columnar SO
Bold: all the above correlations are significant at confidence level 95 %
or greater (
The eruption of Kasatochi volcano on 7–8 August 2008 injected large amounts
of material and SO
We have studied the columnar SO
The SO
The high amounts of SO
The Brewer data have been correlated with those from OMI and GOME-2. The
Pearson's correlation coefficients between the three datasets were all highly
statistically significant (
Table 5 summarizes the correlation coefficients between the mean columnar
SO
In this work we provide evidence that the current network of Brewer
spectroradiometers is capable of identifying columnar SO
From the results discussed in Sect. 3 some general remarks can be put
forward concerning SO
The combination of the observation discussed above and modelling tools can
assist in detecting existing volcanic plumes but also in forecasting their
evolution, which can have importance not only for air traffic warnings but
also for air pollution in the lower layers of the atmosphere. Therefore, an
automated source–receptor modelling tool could be proposed as follows: a
modelling system based on FLEXPART and HYSPLIT backward-trajectory
simulations could be automatically triggered whenever high SO
SO
Dates at which the Brewers were determined to be under or outside of
the volcanic SO
The authors would like to particularly thank Andreas Engel and two anonymous reviewers for their valuable comments. This research was supported by the Copernicus Atmosphere Monitoring Service (CAMS), the Mariolopoulos-Kanaginis Foundation for the Environmental Sciences and the project of EUMETSAT, O3M SAF. We acknowledge the COST Action ES1207 “A European Brewer Network (EUBREWNET)”, the WMO World Ozone and Ultraviolet Radiation Data Centre (WOUDC), the NOAA-EPA Brewer Spectrophotometer UV and Ozone Network (NEUBrew), the NASA GSFC Aura Validation Data Center (AVDC) and the EEA European air quality database (AirBase).
This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 654109. Edited by: A. Engel Reviewed by: two anonymous referees