Detecting volcanic sulfur dioxide plumes in the Northern Hemisphere using the Brewer spectrophotometers , other networks , and satellite observations

This study examines the adequacy of the existing Brewer network to supplement other networks from ground and space to detect SO2 plumes of volcanic origin. It was found that large volcanic eruptions of the last decade in the Northern Hemisphere have a positive columnar SO2 signal seen by the Brewer instruments located 45 under the plume. It is shown that a few days after the eruption the Brewer instrument is capable of detecting significant columnar SO2 increases on the average exceeding 2 DU relative to a considered as unperturbed prevolcanic baseline 10-day-period, with a mean close to zero and σ = 0.46, as calculated from the 33 Brewer

Abstract.This study examines the adequacy of the existing Brewer network to supplement other networks from the ground and space to detect SO 2 plumes of volcanic origin.It was found that large volcanic eruptions of the last decade in the Northern Hemisphere have a positive columnar SO 2 signal seen by the Brewer instruments located under the plume.It is shown that a few days after the eruption the Brewer instrument is capable of detecting significant columnar SO 2 increases, exceeding on average 2 DU relative to an unperturbed pre-volcanic 10-day baseline, with a mean close to 0 and σ = 0.46, as calculated from the 32 Brewer stations under study.Intercomparisons with independent measurements from the ground and space as well as theoretical calculations corroborate the capability of the Brewer network to detect volcanic plumes.For instance, the comparison with OMI (Ozone Monitoring Instrument) and GOME-2 (Global Ozone Monitoring Experiment-2) SO 2 space-borne retrievals shows statistically significant agreement between the Brewer network data and the collocated satellite overpasses in the case of the Kasatochi eruption.Unfortunately, due to sparsity of satellite data, the significant positive departures seen in the Brewer and other ground networks following the Eyjafjallajökull, Bárðarbunga and Nabro eruptions could not be statistically confirmed by the data from satellite overpasses.A model exercise from the MACC (Monitoring Atmospheric Composition and Climate) project shows that the large increases in SO 2 over Europe following the Bárðarbunga eruption in Iceland were not caused by local pollution sources or ship emissions but were clearly linked to the volcanic eruption.Sulfur dioxide positive departures in Europe following Bárðarbunga could be traced by other networks from the free troposphere down to the surface (Air-Base (European air quality database) and EARLINET (European Aerosol Research Lidar Network)).We propose that by combining Brewer data with that from other networks and satellites, a useful tool aided by trajectory analyses and modelling could be created which can also be used to forecast high SO 2 values both at ground level and in air flight corridors following future eruptions.

Introduction
Volcanic eruptions are an important source of natural emissions of sulfur dioxide (SO 2 ) into the troposphere and the stratosphere.Ash particles and gases injected into the atmosphere by large volcanic eruptions can affect solar radiation and climate (e.g.Robock, 2000) and air quality (e.g.Schmidt et al., 2015) and may also impact local environments (e.g.Durant et al., 2010).Volcanic emissions (e.g.ash and SO 2 ) can reach different heights in the atmosphere and can be transported in different directions (e.g.Prata et al., 2010).Thomas and Prata (2011) have shown that the eruption can be divided into an initial ash-rich phase, a lower-intensity middle phase and a final phase where considerably greater quantities of both ash and SO 2 are released, which in the case of ash can result in air travel disruptions (e.g.Flentje et al., 2010).These effects make the ash and SO 2 in volcanic plumes important parameters to be studied, monitored and forecasted on small and larger spatial scales.Our study focuses on volcanic columnar SO 2 amounts because of the existence of the fairly continuous set of direct sun measurements with the Brewer network.
Measurements of SO 2 are important for tracking and assessing impacts of emissions from pollution sources and in quantifying natural SO 2 emissions by volcanoes.Pollution sources typically result in a few Dobson unit (DU, 1 DU = 2.69 × 10 26 molec km −2 ) increases in column SO 2 amounts unless observations are made near a source.The Brewer network is useful for plume tracking because it can track SO 2 columnar amounts from a large number of stations and wide geographical extent.The primary application of the groundbased Brewer spectrophotometer is to measure total ozone column by using UV spectrophotometry.Direct sunlight intensities are measured at five wavelengths (between 306 and 320 nm; see also Sect.2.1) to simultaneously calculate ozone and SO 2 column integrals (Kerr et al., 1980).These instruments have been used extensively to monitor stratospheric ozone (e.g.WMO Scientific Assessment of Ozone Depletion reports, 2011, 2014) and have a long history of studying atmospheric SO 2 columns (e.g.De Backer and De Muer, 1991;Bais et al., 1993;Fioletov et al., 1998;Zerefos et al., 2000Zerefos et al., , 2009;;Ialongo et al., 2015).Ground-based measurements of atmospheric SO 2 using the Brewer instrument have played an important role in the development and validation of satellitebased SO 2 measurements (Schaefer et al., 1997;Spinei et al., 2010;Rix et al., 2012;Ialongo et al., 2015) used primarily for detecting and tracking volcanic emissions.Since the Brewer instruments are located at stationary ground-based monitoring sites, a volcanic plume of SO 2 must pass over the site if useful data are to be obtained.Validation of satellite measurements by the Brewer instrument also requires that a satellite overpass is available when the plume is over the ground-based site (Kerr, 2010).
There have been various initiatives during recent years that used satellite measurements of SO 2 to monitor volcanic eruptions in support of aviation safety, e.g.ESA's Support to Aviation Control Service (SACS) (Brenot et al., 2014).These initiatives together with modelling forecasting In the present work we investigate the efficiency of the existing Brewer network in the Northern Hemisphere to detect volcanic SO 2 plumes during the past decade.The main focus is to show the sensitivity of the Brewer network in detecting SO 2 plumes of volcanic origin in synergy with other groundbased observations, satellite data and dynamic transport calculations.The Brewer spectroradiometric measurements are compared to collocated satellite measurements from OMI and GOME-2 as described in the next paragraph.This paper did not include analyses of the SO 2 measurements from IASI and AIRS since both instruments are IR spectroradiometers.We compared Brewer measurements to the OMI and GOME-2 data that are derived using information from differential optical absorption in the UV spectrum, which also forms the basis of the Brewer measurement methodology.In the case of Brewer-IASI or Brewer-AIRS comparison we would also have to consider differences in the spectroscopy and the corresponding retrieval algorithm concepts, which would require further analysis which is beyond the scope of this paper.
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 • N, 168.13 • W; 1073 m above sea level (a.s.l.); 12 July 2008; Prata et al., 2010) followed by the Kasatochi eruption, Alaska (52.17 • N, 175.51 • W; 300 m a.s.l.; 7-8 August 2008; e.g.Kristiansen et al., 2010;Krotkov et al., 2010;Waythomas et al., 2010), which was detected over large areas of the Northern Hemisphere.Okmok and Kasatochi volcanoes in Alaska erupted for a short time span of less than a month, and therefore we decided to study the evolution of the Brewer SO 2 columnar measurements following the latter volcanic eruption (Kasatochi).The third eruption took place at Sarychev in Russia (48.1  Haywood et al., 2010).The evolution of the SO 2 volcanic plume from Sarychev was mostly observed over the North Pacific, North America and North Atlantic (Haywood et al., 2010).There was only one North American Brewer station (Saturna Island) in the path of the plume from Sarychev eruption.The record shows SO 2 columns of 8.6 DU detected on 19 June 2009 and 3.7 DU on 20 June 2009.This volcanic eruption is not investigated any further in this paper.The next eruption on the list, Eyjafjallajökull in Iceland (63.63 • N, 19.62 • W; 1666 m a.s.l.; from 14 April to 23 May 2010), resulted in interruption of the air traffic over NW Europe (e.g.Flemming and Inness, 2013).The fifth eruption Grímsvötn 2011 (64.42 • N, 17.33 • W; 1725 m a.s.l.; 21 May 2011) was studied by Flemming and Inness (2013), and by Moxnes et al. (2014).This eruption provided an interesting example of a clear separation of the volcanic SO 2 plume (transported mostly northwestward), while the fine ash was transported mostly southeastward.Unfortunately the volcanic plume did not overpass any Brewer station, and therefore we do not include any results post Grímsvötn eruption.The sixth eruption recorded features the Nabro in Africa (13.37 • N, 41.70 • E; 2218 m a.s.l.), which occurred on 12-13 June 2011 (e.g.Bourassa et al., 2012;Sawamura et al., 2012;Clarisse et al., 2014).We present here a case study that described the detection of the Nabro volcanic SO 2 plume over ground-based stations.The plume was clearly detected by the Brewer instrument over Izaña (and poorly from space), then over Taiwan by both Brewer and satellite instruments, and finally at Mauna Loa, Hawaii (mostly by the Brewer instrument).The seventh eruption was Tolbachik, Russia (55.83 • N, 160.33 • E; 3.611 m a.s.l.), on 27 November 2012 (e.g.Telling et al., 2015).As in the case of Grímsvötn, the plume has not passed over any Brewer station that was verified by trajectory analysis.The next eruption on the list is the volcanic eruption from Bárðarbunga, Iceland (64.64 • N, 17.56 • W; 2005 m a.s.l.) that was observed between 31 August 2014 and 28 February 2015 (e.g.Schmidt et al., 2015).This last eruption, although not yet rated on the VEI scale, has been extensively studied in view of the observed increased SO 2 concentrations that have been observed all the way through the troposphere and reaching down to the surface in Europe (Ialongo et al., 2015;Schmidt et al., 2015).
The capability of the Brewer network to measure columnar SO 2 amounts above the local air pollution levels is also presented and discussed.The qualitative evidence that the plume can be detected in many single cases by the Brewer network has been quantitatively tested by calculating correlation coefficients with collocated satellite data.Only in the case of the Kasatochi 2008 eruption was it possible to test the sensitivity of SO 2 abundance measured by the Brewer spectrophotometers and from space.Correlations between the Brewer and collocated satellite SO 2 data from the Aura OMI and GOME-2 are presented in Sect. 3 where the correlation coefficients were found to be statistically significant at a confidence level of 99 %.For the other eruptions, unfortunately due to the sparsity of satellite data, no firm conclusions can be drawn, as discussed in Sect.3.
The paper is structured in the following order.Sect. 2 describes the data sources and the methods of analysis of the columnar SO 2 measurements by the Brewer spectrophotometers (hereinafter simply referred to as the "Brewers").Section 3 presents the analysis of the Brewer measurements during four of the volcanic eruptions listed in Table 1, along with satellite data and dynamic volcanic plume transport simulations.The conclusions are provided in Sect. 4.

Ground-based data
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 2 columns (Kerr et al., 1980).About 200 Brewer spectrophotometers around the world con-tribute high-precision ozone data to the global ozone monitoring network (Kumharn et al., 2012).The existing Brewer network also delivers frequent SO 2 columnar measurements as well, which can be used for analyses, but with caution.This is because the signal-to-noise ratio for the SO 2 absorption is usually quite low and therefore well-calibrated instruments are required to monitor nominal SO 2 columnar amounts (Koukouli et al., 2014).Details on the method with which SO 2 is measured by the Brewer spectrophotometer can be found in Kerr et al. (1980, 1985, 1988) and De Backer and De Muer (1991).According to Fioletov et al. (2016), the uncertainty of the Brewer direct sun (DS) SO 2 measurements is about 1 DU and is typically insufficient for air quality applications.A more accurate method (with an uncertainty as low as 0.13 DU) based on Brewer "group-scan" spectral direct sun radiation measurements at 45 wavelengths from 306 to 324 nm was developed (Kerr, 2002) but not implemented for routine operations due to its complexity (Fioletov et al., 2016).Although the Brewer instrument has difficulties in detecting low columnar SO 2 concentrations, in extreme cases, such as volcanic eruptions, the SO 2 levels typically rise well above the instrumental noise and can be identified with the Brewer instrument as shown in this paper and in Fioletov et al. (1998).
Before proceeding to the analysis of Brewer measurements, the methodology to derive columnar SO 2 is first presented.To determine ozone and SO 2 column amounts, the measured raw photon counts at the five operational channels in the Brewer instrument are converted to radiation intensity.The Beer-Lambert absorption law is applied at each wavelength λ, and the measured intensity of direct sunlight is given by the following formula: where I λ is the measured radiation intensity at wavelength λ, I 0λ is the measured extraterrestrial spectrally resolved intensity at λ, β λ is the Rayleigh scattering coefficient at λ, δ λ is the particulate scattering coefficient at λ, α λ is the ozone absorption coefficient (square centimetres per molecule) at λ, O 3 is the total ozone column (molecules per square centimetre), σ λ is the SO 2 absorption coefficient at λ, SO 2 is the column amount of sulfur dioxide, µ R , µ p and µ are the optical path lengths (air masses) corresponding to the effective heights of molecules, particles and ozone respectively.According to the Brewer retrieval algorithm, the following ratios are formed: and where F is the weighted ratio of direct sun measurements at four (or six for double Brewer) spectral channels, F = log I 2 − 0.5 log I 3 − 2.2 log I 4 + 1.7 log I 5 , and F 0 , β, and α are the same linear combinations for log I 0λ , β λ , and α λ .The F is the SO 2 ratio and F = log I 1 − 4.2 log I 4 + 3.2 log I 5 and F 0 , β , α and σ the corresponding linear combinations for log I 0λ , β λ , α λ and σ λ .Both of these functions have weights which eliminate the effects of particulate scattering, while the function F is weighted to remove SO 2 absorption effects as well.The extraterrestrial constants F 0 and F 0 are determined from a long series of intercomparison measurements as well as zero air mass (µ) extrapolations.
The total ozone column is determined by the formula and the SO 2 by the formula where A is the ratio of the SO 2 absorption coefficient to the O 3 absorption coefficient; A = 2.44.From the above-described operational Brewer algorithm it is evident that the estimation of columnar SO 2 is the result of the difference between two columnar terms (O 3 + SO 2 ) and O 3 .Both terms have uncertainties (weighting functions, calibrations, random errors, systematic errors).Systematic negative values could be the result of a systematic offset in the measurements that can be related to the calibration of the instrument (usually optimized only for the ozone measurements).Randomly varying positive and negative values around zero, suggest that the signal of SO 2 is small (and thus the difference of two terms should be close to 0), but since both terms have uncertainties, negative values are possible, indicating that the amount of SO 2 in the atmosphere is below the detection limit of the instrument and could be considered as noise.In this work we have repeated our analysis excluding the negative values, and the results remained the same; i.e. a positive increase after a major volcanic eruption was confirmed as described in the following sections.Finally, we need to point out that perturbations by ash present in the volcanic plumes have been shown not to affect the Brewer SO 2 measurements.This is based on the result of the Pappalardo et al. ( 2013) paper based on EARLINET (European Aerosol Research Lidar Network) observations following the Eyjafjallajökull eruption, in which they found that the Ångström exponent of the volcanic ash optical depth is close to 0. This indicates that the effect of ash in the UV and visible region on the aerosol extinction is almost independent of wavelength.The Brewer SO 2 measurements taken in a narrow wavelength band in the UV are therefore not expected to be influenced by the presence of volcanic ash considering the weights already applied in the operational Brewer algorithm.
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 2 measurements were averaged over a large number of instruments  2. and datasets during periods following volcanic eruptions.Random errors in the measurements of individual Brewer stations are reduced significantly by the averaging processes to calculate regional means.
Daily SO 2 columns at Churchill, Goose, Edmonton, Regina, Saturna Island and Toronto in Canada, T'aipei in Taiwan, and Boulder and Mauna Loa in the US were obtained from the World Ozone and Ultraviolet Radiation Data Centre (WOUDC; http://www.woudc.org/)and the NOAA-EPA Brewer Spectrophotometer UV and Ozone Network (NEUBrew; http://www.esrl.noaa.gov/gmd/grad/neubrew/).The data have been checked for quality assurance/quality control by the individual data providers.It is important to note the participation of most of the European Brewer data providers in a recent EU COST Action (EUBREWNET, http://www.eubrewnet.org/cost1207/)programme.Its focus is to establish a coherent network of European Brewer Spectrophotometer monitoring stations in order to harmonize operations and develop approaches, practices and protocols to achieve consistency in quality control, quality assurance and coordinated operations.
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 3 and SO 2 was less than 2.5 DU, the total columnar ozone was between 250 and 450 DU, and the solar zenith angle was less than 73.5 • .To exclude erratic data of SO 2 from our analysis, values exceeding ±6σ of the mean of all SO 2 individual Brewer measurements were considered erroneous and were not included in the calculations.Therefore, the range of analysed values was limited to a maximum of ±35 DU for an individual measurement (i.e.6σ , with σ being equal to 5.8 as estimated from all available sub-daily SO 2 values).Then we calculated daily SO 2 columns at each station only if at least three individual measurements passed these criteria for each day.Brewers are useful because they provide more than one observation per day.For plumes which change rapidly, more than one observation per day would be useful, especially to complement satellites which typically have just one local overpass.2014) respectively.For the case of Kasatochi, Eyjafjallajökull and Bárðarbunga, we analysed daily SO 2 columns at 30 sites (listed in Table 2), while for the case of Nabro, whose impact was mostly seen over low latitudes in the Northern Hemisphere (e.g.Bourassa et al., 2012), we analysed SO 2 columns at three low-latitude sites in the Northern Hemisphere, namely Izaña, Mauna Loa and T'aipei.
Only for the case of the Bárðarbunga eruption in 2014 were the columnar SO 2 measurements over Europe also compared with surface SO 2 measurements from groundbased European stations.The surface SO 2 data were obtained from the European Environment Agency database (AirBase; http://www.eea.europa.eu/data-and-maps/data/aqereporting-1#tab-european-data) covering the bimonthly period September-October 2014.Only rural background stations, i.e stations in classes 1-2 according to the Joly-Peuch classification methodology for surface sulfur dioxide (Joly and Peuch, 2012), located at a distance of less than 150 km from the nearest Brewer station, were used in the analysis.A total of seven stations in Europe (see Table 3) fulfilled the above-mentioned criteria and were included in the current analysis.Observed data from the AirBase network were available in hourly resolution, from which we calculated daily surface SO 2 values.We note here that SO 2 in the troposphere over western Europe is very low (e.g.Zerefos et al., 2009;Wild, 2012), and therefore plumes from volcanic eruptions are easy to detect against a lower background level.

Satellite data
The columnar SO 2 records from remote-sensing spectrophotometers over Europe, Canada, USA and Taiwan were compared with space-borne measurements from (a) the OMI aboard the EOS (Earth Observing System)-Aura (e.g.Ialongo et al., 2015) satellite and (b) the GOME-2 aboard the MetOp-A (e.g.Rix et al., 2009) satellite.We use MetOp-A instead of MetOp-B because it covers a longer time period.Both OMI and GOME-2 satellite SO 2 data products were downloaded from the Aura Validation Data Center (AVDC) (available from http://avdc.gsfc.nasa.gov/index.php?site=_245276100).GOME-2 level-2 overpass data have been processed with the GOME Data Processor (GDP) version 4.7.We analysed station overpass data for the various midlatitude stations listed in Table 2 and for the low-latitude stations at Mauna Loa, Izaña and T'aipei.The available OMI version 1.2.0 overpass (collection 3) data analysed in this study include pixels within 50 km radius from the nearest Brewer site and is not affected by OMI row anomalies.The available GOME-2 level-2 overpass data include pixels within 100 km radius from the Brewer sites.
For the case of OMI, the SO 2 data are provided from October 2004 to the present.There are four SO 2 products: (1) the planetary boundary layer SO 2 column (PBL), corresponding to a centre of mass altitude (CMA) at 0.9 km; (2) the lower tropospheric SO 2 column (TRL) corresponding to CMA of 2.5 km; (3) the middle-tropospheric SO 2 column (TRM), usually produced by volcanic degassing, corresponding to CMA of 7.5 km; and (4) the upper-tropospheric and stratospheric SO 2 column (STL), usually produced by explosive volcanic eruptions, corresponding to CMA of 17 km.Details on OMI SO 2 columns can be found in various studies (Levelt et al., 2006;Yang et al., 2007;Fioletov et al., 2011Fioletov et al., , 2013;;McLinden et al., 2012;Li et al., 2013;Ialongo et al., 2015).In this study, we made use of the product for the middle-tropospheric SO 2 column (TRM) following the recommendation that the TRM retrievals should be used for volcanic degassing at all altitudes because the PBL retrievals are restricted to optimal viewing conditions and TRL data are overestimated for high-altitude emissions (> 3 km) (Ialongo et al., 2015).Also, we analysed the STL data which are intended for use with explosive volcanic eruptions where the volcanic cloud is placed in the upper troposphere/stratosphere.The standard deviation of TRM retrievals in background areas is reported to be about 0.3 DU in low and midlatitudes and about 0.2 DU for the STL retrievals.This is similar to the standard deviations (indicative of typical uncertainties of the measurements) that we find for the TRM and STL retrievals in the four bimonthly periods under this study.For the best data quality, we used data from the scenes near the centre of the OMI swath (rows 4-54) as recommended by Ialongo et al. (2015), who found that data from the edges of the swath tend to have greater noise.
For GOME-2, we analysed the total SO 2 columns from April 2007 to the present.The standard deviation found in our study for the GOME-2 retrievals is on the order of 0.4 DU.We analysed satellite SO 2 measurements whenever the O 3 column was between 250 and 450 DU and the solar zenith angle was less than 73.5 • .We used SO 2 data defined as having a cloud radiance fraction (across each pixel) of less than 50 %, as they were found to have a smaller standard deviation than all sky data.Moreover, a range of SO 2 values between −35 and 35 DU was used to screen for outliers.In cases of multiple daily data matched to the station overpass, all available measurements within a radius of 50 (100) km from the Brewer site in the case of OMI (GOME-2) are averaged.
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 (σ ) of departures from the unperturbed pre-volcanic period, for the three studied periods of volcanic importance at each station, only if at least 25 daily values were available.The bimonthly averages for each station in the examined periods are presented in Table 4a.Table 4b shows the mean and standard error (σ/ √ N ) of all bimonthly averages in each studied volcanic period.Averaging the departures from the pre-volcanic baseline for all Brewer stations and for all bimonthly periods gives a mean SO 2 columnar departure of 0.10 ± 0.03 DU.This estimate is on the same order of magnitude as the corresponding statistics for OMI (TRM) SO 2 column departures (0.05 ± 0.02 DU), OMI (STL) (0.04 ± 0.01 DU) and that measured by GOME-2 (0.09 ± 0.02 DU).The standard deviation of the bimonthly averages relative to their baselines, which was calculated from a large sample of data, was taken here as an approximation of the typical uncertainties in the columnar SO 2 measurements performed by the group of Brewers, OMI and GOME-2 instruments following volcanic eruptions.

Modelling tools
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 45 × 45 km.The initial and boundary conditions for the × 1 • analysis.A total of 40 000 tracer particles are assumed for each release in FLEX-PART simulations.The use of 1-hourly WRF meteorological fields at a 45 × 45 km spatial resolution allows a more detailed representation of the volcanic plume dispersion but also implies a significant increase in computational time.To overcome this computational time cost, source-receptor relationships between station measurements and volcanic activity are also analysed with the use of HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) model trajectories (Stein et al., 2015) of long-range transport driven by the 3-hourly meteorological dataset Global Data Assimilation System (GDAS) at a resolution of 1 • × 1 • .
3 Results and discussion

The 2014 Bárðarbunga case
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 FLEX-PART 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 2 release rate of 119 kilotons per day as reported by Gíslason et al. (2015) from near the source SO 2 measurements during the first weeks of the eruption.Similar emission rates are also suggested by Schmidt et al. (2015) through comparisons between NAME simulations (UK Met Office's Numerical Atmospheric-dispersion Modelling Environment) and OMI satellite retrievals.The emission height is set to between 0 and 3500 m above ground level, consistent throughout the simulation period.The establishment of an anticyclonic flow over the British Isles on 21 September 2014 (not shown here) resulted in the separation of the volcanic SO 2 field into two distinct plumes (Fig. 2a).On 22 September the primary plume (plume 1) becomes stagnant over the topographic barrier of the Alps (Fig. 2b).The secondary plume is advected southwards by the intense northerly winds over the North Sea.The two plumes overlap at about 09:00-11:00 UTC.Taking a closer look at the surface SO 2 values sampled during this event by surface air quality stations in the Netherlands, several days of enhanced SO 2 were discovered, which indicate an area of stagnation or blocking of the flow.Trajectory calculations performed at the Royal Netherlands Meteorological Institute (KNMI) correspond well to the calculations shown in Fig. 2 but also show that the air parcels stayed over northern Europe for some time after a very fast flow over the North Sea, which agrees with the spikes found in the surface SO 2 records observed over the Netherlands during a period of several days.The high SO 2 concentrations, which were recorded almost simultaneously at stations over Europe at various sites during the period 21-29 September 2014, are thus associated with the activity of Bárðarbunga volcano (Ialongo et. al., 2015; Table A1, see Appendix A).This is also supported by the back trajectory analysis performed with the HYSPLIT dispersion model that is shown in Fig. 3.All back trajectories start at 12:00 UTC on the day of maximum SO 2 observations for each of the Brewer stations and indicate that the arrival of air masses originated from Iceland.
As shown in Fig. 4a, the SO 2 plume was detected by the Brewer instruments located in the passage of the volcanic SO 2 plume and from different ground-based networks.However, no coincident measurements were available from the OMI and GOME-2 overpasses at the time of the high SO 2 ex-   cursions.Also it should be noted here that no enhanced SO 2 columns were detected by the Brewers located outside of the geographical area covered by the volcanic plume (Fig. 4b).In all volcanic cases we have applied a criterion according to which each daily average from either OMI or GOME-2 should be calculated if and only if more than half of the individual overpasses had data on a given day.
The eruption took place at the beginning of September 2014, and several European countries experienced high concentrations of SO 2 at ground level during the rest of September.Figure 5 shows the response of ground-level Air-Base stations under the plume located within 150 km from the nearest Brewer station plotted together with the coincident Brewer SO 2 column measurements.Interestingly, it suggests that the highest amount of the SO 2 column measured by the majority of the Brewers on 21 September 2014 due to the volcano reached the surface with a time lag of about 1 day.The high volcanic concentrations were successfully measured by the ground-based AirBase network.Due to strong European efforts over the last decades to reduce SO 2 emissions, high concentrations of SO 2 are now quite rare in western Europe (e.g.Vestreng et al., 2007) except in the areas affected by industrial or shipping emissions.In situ air quality stations observed high values of SO 2 at the ground level on the coast of France, in the United Kingdom, the Netherlands and Germany between 21 and 25 September 2014.This all points towards a volcanic episode with a large spatial extent.
As can be seen from Fig. 4a, the highest SO 2 column departures from the pre-volcanic baseline were observed from 21 to 22 September 2014.The mean SO 2 column measured by the Brewers under the plume was 2.4 ± 0.8 DU, which was 5 times greater than the mean column of SO 2 measured by the Brewers outside of the plume (−0.1 ± 0.1 DU) by 2.5 DU on average.The error bars show the standard deviation of the daily SO 2 values of all stations during the non-perturbed 10-day period prior to the volcanic eruption.These differences provide rough estimates of the additional SO 2 loading induced by the volcanic eruption over Europe which exceeds 3σ .Comparison between satellite data and Brewer are limited for the purposes of interpretation because satellite measurements are sparse, representing an average SO 2 column over a relatively large satellite pixel, while the Brewer observations are designed to provide a local point measurement.In spite of the sparsity of OMI observations post Bárðarbunga volcanic eruption, satellite data were used for assimilation in the SO 2 analyses and forecasts produced with the MACC (Monitoring Atmospheric Composition and Climate) system (http://atmosphere.copernicus.eu/).This near-real-time forecasting system assimilates satellite observations to constrain modelling forecasts (Inness et al., 2015;Flemming et al., 2015).The OMI instrument aboard the Aura satellite provided information about concentrations of volcanic SO 2 emitted by the Icelandic Bárðarbunga volcano on 20 September; these observations were assimilated in 2014 by the MACC system in cases of volcanic eruptions, i.e. when OMI values exceeded 5 DU.As shown in Fig. 6 (the charts of total column SO 2 are taken from the website http://atmosphere.copernicus.eu/),the subsequent forecasts capture the transport of the plume of volcanic SO 2 southward, while spreading over the continent on 21 and 22 September.The plume stretched all the way from Finland through Poland, Germany and France to southern England.A parallel forecast, for which no OMI data were used (Fig. 6, right), did not show any elevated SO 2 values, confirming that "normal" emissions of SO 2 (including shipping and industrial activities) could not explain the observed situation.
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 155hourly HYSPLIT back trajectories during this period and for the heights of the layer detected by the lidar (∼ 2.5-3.5km) is shown in Fig. 8.The increased wind shear that is evident between these heights does not allow a robust characterization of the air masses.However, the source contribution of about 20 % from Icelandic air masses supports the volcanic origin of the detected plume.During volcanic eruptions, ash and SO 2 may be injected to different altitudes and may follow different trajectories for long-range transport.EARLINET lidars can provide alerts on volcanic ash dispersion over Europe, especially when the systems are employed with depolarization capabilities (e.g.Pappalardo et al., 2013).For the Brewer network capabilities and the Hohenpeißenberg station, Figs.7 and 8 demonstrate that a similar approach can be applied to contribute towards an early warning synergistic tool, as evidenced in the Bárðarbunga case study.The role of the Brewer stations in this system will be the early detection of SO 2 plumes transported over continental areas that would trigger the associated forecasting systems (models and networks).

The 2011 Nabro volcano plume
A major eruption of Mt Nabro, a 2218 m high volcano on the border between Eritrea and Ethiopia (13.37 • N, 41.7 • E), occurred on 12-13 June 2011.The volcanic eruption injected ash, water vapour and an estimated 1.3-2.0Tg of SO 2 into the upper troposphere and lower stratosphere (Fairlie et al., 2014, and references therein).In the first phase of the eruption, the main transport pattern of emitted SO 2 followed the strong anticyclonic circulation over the Middle East and Asia associated with the Asian summer monsoon at that time of year (Clarisse et al., 2014, and references therein).In the first month after the eruption stratospheric aerosols were mainly observed over Asia and the Middle East and by day 60 covered the whole Northern Hemisphere.Estimated aerosol altitudes from various instruments were between 12 and 21 km (Clarisse et al., 2014).By July 2011 Nabro had cumulatively emitted 5 to 10 % of what was released by Mount Pinatubo in 1991 (∼ 20 Tg), ranking it among the largest SO 2 emissions in the tropical stratosphere (up to at least 19 km) since Pinatubo (Krotkov et al., 2011).Sulfur dioxide signals of volcanic origin were detected both by Brewer and satellite measurements over East Asia, where the volcanic SO 2 plume was transported, as demonstrated in Figs. 9 and 10a.Measurements were taken by Brewer in T'aipei, Taiwan, Asia.This is also evident from the back trajectory analysis performed  .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 2 arriving anywhere between 2.5 and 3.5 km over Leipzig originates from the specific direction.
with the HYSPLIT dispersion model for T'aipei (Taiwan) as shown in Fig. 10a.The analysis indicates that the uppertropospheric air masses arriving at T'aipei on 19 June, when the peak in SO 2 is observed, originate from Africa.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 groundbased and satellite observations of the volcanic SO 2 plume.A Brewer located in Tenerife, Spain, detected an increase in the SO 2 column, which was not clearly detected by the OMI and GOME-2 satellite overpasses.
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-lowerstratospheric 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 2 departures from the unperturbed 10-day pre-volcanic baseline measured by the Brewer at Izaña following Nabro.The daily mean SO 2 departures (Fig. 11) show a 0.3 DU increase at the beginning of the event (19 June), reaching 0.6 DU on 29 June when the layer is found at a higher altitude.The signal is not strong and is near the error of 0.5 DU estimated for SO 2 measurement (Stanek, personal communication, 2016), but the observations are consistent (independent of the ozone and air mass), since at Izaña about 100 O 3 /SO 2 measurements per day are performed resulting in reduced standard errors associated with daily means as compared to individual observations.The Langley calibration is tracked between calibrations by measurements of the internal lamp (Langley and lamp are shown in Supplement Fig. S1).The increase in SO 2 due to the passage of the Nabro volcano plume over the Canary Islands is significant using both methods (Fig. 11).In this case the Brewer at Izaña has been able to detect an SO 2 plume at a high altitude from a volcano located 7000 km from the Canary Islands, indicating that the Brewer network is sensitive enough to be incorporated into columnar SO 2 monitoring from volcanic eruptions in worldwide networks.
The case of the 2011 Nabro eruption shows an example of the importance of the Brewer spectrophotometers in measuring and detecting changes in SO 2 amounts in the atmo-sphere due to volcanic eruptions, in cases where signal in the satellite overpasses is low.This is true for the case of Izaña (Tenerife), where it appears that OMI and GOME-2 did not clearly detect increases in SO 2 column of volcanic origin between 19 June and 1 July as was the case with the Brewer instrument (Fig. 11).During some days between 19 June and 1 July, the Brewer SO 2 columns at Izaña rose above the uncertainty of 0.5 DU for the Brewer SO 2 measurements at this station, whereas the satellite SO 2 columns stayed mostly within the uncertainty of 0.4 DU estimated for OMI and GOME-2 satellite retrievals.
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 case of the 2010 Eyjafjallajökull volcanic eruption
The Eyjafjallajökull volcano, Iceland (63.63 • N, 19.6215 • W; 1666 m a.s.l.), erupted explosively on 14 April 2010 and continued to emit ash and gas until 24 May (Flentje et al., 2010;Thomas and Prata, 2011;Stohl et al., 2011;Flemming and Inness, 2013).Despite the relatively modest size of the eruption, the prevailing wind conditions advected the volcanic plume toward the southeast leading to unprecedented disruption to air traffic in western Europe.This caused significant financial losses for the airlines and highlights the importance of efficient volcanic cloud monitoring and forecasting.Results demonstrate that the eruption can be divided into an initial ash-rich phase (14-18 April), a lower-intensity middle phase (19 April until early May) and a final phase (4-24 May), where considerably greater quantities of both ash and SO 2 were released (Thomas and Prata, 2011).
Figure 12 shows the responses of Brewer stations under the volcanic SO 2 plume and the average of Brewer stations outside of the plume together with OMI and GOME-2 satellite observations.We determined 10 stations as being under the plume in 2010 and 10 stations as being outside of the plume based on the analysis of forward and backward trajectories of air masses following the volcanic eruption.The stations determined to be under the plume in 2010 (shown in Fig. 12a) are Sodankylä, Obninsk, Manchester, De Bilt, Uccle, Belsk, Reading, Hohenpeißenberg, Davos and Arosa.The stations determined to be outside of the plume are Vindeln, Oslo, Norrköping, Copenhagen, Hradec Králové, Aosta, Kislovodsk, Rome, Thessaloniki and Athens (Fig. 12b).
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 2 is 5.8 DU at De Bilt and −0.2 DU at Uccle.As seen in the corresponding back trajectories for that day in Fig. 13, air masses originating from Iceland arrive at De Bilt at heights of 6-7 km and have probably transported the volcanic cloud over that station.In contrast similar back trajectories on the same day for the case of Uccle indicate transport of air masses from Iceland but at lower heights (3-4 km) that were probably not affected by the volcanic emissions.In another case of transport on 11 May 2010, Uccle was outside of the plume (see Fig. 13), while the mean daily SO 2 for De Bilt was 0.9 DU.On 18 May 2010, both De Bilt and Uccle stations detected a volcanic cloud with SO 2 daily means of 1.7 and 1.2 DU respectively.To summarize, in spite of the proximity of Uccle and De Bilt, the transport heights and trajectories can have a different result in transporting volcanic gases.
In Table A1 of Appendix A, we present the dates when the examined Brewer stations were either under or outside of the volcanic SO 2 plume.Careful analysis of the trajectories of the volcanic plumes in 2010 and 2014 helped verify these analyses.The distinction between stations outside of the plume and stations under the plume was done as follows: whenever SO 2 at each station measuring exceeded 2 DU (2σ ), back trajectories were calculated and the origin was compared to the location of the volcanic eruption.All these stations have been considered to be under the SO 2 plume.All other stations, for which columnar SO 2 amounts were within 2σ and did not originate from the area of the eruption, were considered to be outside of the volcanic SO 2 plume.
As we can see from Fig. 12, the columnar SO 2 departures at stations located under the passage of the volcanic SO 2 plume exceeded 0.3 DU (reaching 1.5 DU in some cases), whereas at stations located outside of the plume, the columnar SO 2 departures did not exceed 0.3 DU.Moreover, during the explosive phase 2, there were three main periods in which the volcanic aerosol content was observed by EAR-LINET over Europe: 15-26 April, 5-13 May and 17-20 May.These periods were determined from measurements of the integrated backscatter at 532 nm in the volcanic layers (Pappalardo et al., 2013).We estimate high SO 2 columnar departures measured by the Brewers under the plume during 14 April and 23 May 2010 of up to 6.0 DU (e.g.Arosa, 18 May 2016).
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 2 columnar departures measured by the Brewers under the plume in the UK (Manchester and Reading) during these two periods were estimated to 1.1 ± 0.3 and 1.5 ± 0.4 DU respectively.These amounts were higher than the amounts measured outside of the plume (−0.1 ± 0.2 and −0.1 ± 0.1 DU, accordingly) by almost 1.4 DU on average.

An eruption of larger-scale importance -the 2008 Kasatochi case
The eruption of Kasatochi volcano on 7-8 August 2008 injected large amounts of material and SO 2 into the troposphere and lower stratosphere of the northern middle latitudes during a period of low stratospheric aerosol background concentrations.The Kasatochi volcano in the central Aleutian Islands of Alaska (52.17  (Bitar et al., 2010).Aerosols from the volcanic eruption were detected by lidar in Halifax shortly after the eruption (Bitar et al., 2010).The total mass of SO 2 injected into the atmosphere by the eruption is estimated at 1.7 Tg, with about 1 Tg reaching the stratosphere (above 10 km a.s.l.) (Kristiansen et al., 2010).
We have studied the columnar SO 2 amounts following the Kasatochi eruption in August 2008 from ground-based and satellite data.Figure 14 shows the columnar SO 2 departures from the unperturbed 10-day pre-volcanic period over Canada and the USA and Europe during the bimonthly period August-September 2008 as measured by the Brewers in comparison with the satellite observations by OMI and GOME-2.August-September 2008.The statistical tests gave significant results and verified the capability of the Brewers in detecting natural SO 2 emitted by volcanoes when the volcanic plume of SO 2 passes over the ground sites.We note here that there is a general consistency between all three datasets (Brewers, OMI and GOME-2) on the changes in SO 2 column following the Kasatochi volcanic eruption.
Table 5 summarizes the correlation coefficients between the mean columnar SO 2 measured by all Brewers over Europe and provided by the satellite products of OMI and GOME-2 during the globally extended Kasatochi event.The correlation coefficients have high statistical significance explaining more than 70 % of the total variance between the columnar SO 2 measurements from the ground and space in the case of Kasatochi.However, the discrepancies found between satellite and Brewer observations during the other volcanic eruptions could be impacted by the sparsity of coincident measurements and thus cannot confirm or deny Kasatochi case findings at high significance levels.

Conclusions
In this work we provide evidence that the current network of Brewer spectroradiometers is capable of identifying columnar SO 2 plumes of volcanic origin.The study is based on the results from the three largest volcanic eruptions (VEI ≥ 4) in the past decade when elevated SO 2 plumes have passed over Brewer stations in the Northern Hemisphere.The analysis included a fourth eruption, namely Bárðarbunga, because it perturbed the SO 2 regime over large parts of Europe and extended from the free troposphere down to the surface.Back and forward trajectory analysis have been used to aid identifying and selecting measurements taken under and outside of the volcanic SO 2 plume.When the plume was passing over a site, the SO 2 signal was found to be quite high, exceeding 3σ of daily values relative to the average levels taken during the unperturbed measurements over 10 days preceding each eruption.On average, the mean SO 2 columnar amount to be attributed to the volcano is estimated to be on the order of 2 DU as discussed in Sect.3. In addition to the Brewer network, comparisons were made with other instruments (e.g.surface SO 2 sensors) that were located under the volcanic SO 2 plumes.Moreover, satellite measurements of columnar SO 2 from OMI and GOME-2 collocated with the Brewer network were used for comparisons.
From the results discussed in Sect. 3 some general remarks can be put forward concerning SO 2 levels and detection time after the eruption.Starting with the Kasatochi eruption, as it appears from Fig. 14, the plume can be detected 4 days after the eruption over Canada and the US and about 7 days over Europe with an average amplitude on the order of 2 DU compared to the unperturbed 10-day pre-volcanic period (baseline).All estimates are based obviously on measurements taken under the plume.The Kasatochi eruption provided a very good example for a volcanic SO 2 plume to be observed not only by the ground-based instruments but by space-borne ones as well (OMI and GOME-2).Relative to the undisturbed period before Kasatochi, the amplitude of the signal is 2 DU for GOME-2 and 1.5 DU for OMI.The results for the other volcanic eruptions are similar for the Brewer network, but unfortunately because of the sparsity of satellites passing over the Brewer stations, the satellite data concur with those from the Brewers only in Kasatochi.Based on the above discussion, it appears that currently no single network can independently and fully monitor the evolution of volcanic SO 2 plumes.Among a few reasons are a lack of measurements during peak values, complications from meteorological events, ejection heights and exposure conditions.The evidence presented here suggests that a combination of observations from various instruments, aided by chemical transport models and operated in synergy, could address such a complex issue.
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 2 values are detected at a Brewer station above a specific threshold (e.g.3σ of station's daily values) or when a lidar instrument detects highly depolarizing layers that were not advected from a geographical location over a desert.The operational use of such synergistic activity could provide near-real-time and forecasting information on the evolution of volcanic episodes and also develop a comprehensive database of measurements useful to improve model forecasts.This new well-tuned and organized synergistic activity of monitoring networks, observations and modelling from the ground and space could create a promising monitoring tool for volcanic and other extreme emissions, which would form the basis of a new regional SO 2 columnar forecasting facility.A1.Dates at which the Brewers were determined to be under or outside of the volcanic SO 2 plume, based on analysis of back trajectories of the volcanic plumes in 2010 and 2014.The distinction between stations outside of the plume and stations under the plume was done as follows: at each station measuring SO 2 exceeding 2 DU (2σ ), we calculated back trajectories and found that their origin was at the volcanic eruption.All these stations have been considered to be under the SO 2 plume.All other stations, for which columnar SO 2 amounts were within 2σ and did not originate from the area of the eruption, were considered to be outside of the volcanic SO 2 plume.During the Kasatochi eruption all Brewers were considered to be under the volcanic SO 2 plume.

Figure 1 .
Figure 1.All stations with accessible SO 2 column data from Brewers analysed in this study as listed in Table2.

Figure 3 .
Figure 3. HYSPLIT 120 h back trajectories of air masses arriving on the day of maximum SO 2 records for each of the Brewer stations at De Bilt, Hohenpeißenberg, Hradec Králové, Jokioinen, Obninsk and Sodankylä.

Figure 4 .
Figure 4. Mean SO 2 column departures from the unperturbed 10-day pre-volcanic baseline measured by Brewers, OMI (TRM, STL) and GOME-2 during September-October 2014 over Europe following the 2014 Bárðarbunga volcanic eruption for (a) stations under the volcanic SO 2 plume and (b) stations outside of the plume.The error bars for the Brewer observations show the standard deviation of all daily values during the unperturbed 10-day period prior to the volcanic eruption.Brewer stations under the plume are Sodankylä, Vindeln, Jokioinen, Oslo, Norrköping, Copenhagen, Obninsk, Manchester, De Bilt, Reading, Uccle, Hradec Králové, Hohenpeißenberg and Aosta.Stations outside of the plume are Warsaw, Belsk, Davos, Arosa, Kislovodsk, Rome and Athens.Each daily average from either OMI or GOME-2 was calculated if and only if more than half of the individual overpasses had data on a given day.The arrow marks the starting date of the eruption (beginning on 31 August 2014 and continuing to be active throughout the whole bimonthly period and beyond).

Figure 5 .
Figure 5. Mean surface SO 2 measured by AirBase class 1-2 stations located within 150 km from seven nearest Brewer stations in Europe as listed in Table3.The arrow marks the starting date of the eruption (beginning on 31 August 2014 and continuing to be active throughout the whole bimonthly period and beyond).

Figure 6 .
Figure 6.Charts of forecasted total column SO 2 produced within the MACC system for 21 September 2014 with OMI data assimilation (left) and without OMI data assimilation (right).

Figure 7 .
Figure 7. 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.

Figure 8
Figure8.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 2 arriving anywhere between 2.5 and 3.5 km over Leipzig originates from the specific direction.

Figure 11 .
Figure 11.SO 2 column departures from the unperturbed 10-day pre-volcanic baseline measured by the Brewer, OMI (TRM, STL) and GOME-2 over Izaña, Tenerife, during June-July 2011 following the 2011 Nabro volcanic eruption.SO 2 calculations by the Brewer were performed using the Langley calibration and the zero calibration at Izaña (assuming SO 2 = 0 during the days 6 and 7 June 2011).

Figure 12 .
Figure 12.Mean SO 2 column departures from the unperturbed 10-day pre-volcanic baseline measured by Brewers, OMI (TRM, STL) and GOME-2 during April-May 2010 over Europe following the 2010 Eyjafjallajökull volcanic eruption for (a) stations under the volcanic SO 2 plume and (b) stations outside of the plume.The error bars for the Brewer observations show the standard deviation of all daily values during the unperturbed 10-day period prior to the volcanic eruption.Brewer stations under the plume are Sodankylä, Obninsk, Manchester, De Bilt, Uccle, Belsk, Reading, Hohenpeißenberg, Davos and Arosa.Stations outside of the plume are Vindeln, Oslo, Norrköping, Copenhagen, Hradec Králové, Aosta, Kislovodsk, Rome, Thessaloniki and Athens.Each daily average from either OMI or GOME-2 was calculated if and only if more than half of the individual overpasses had data on a given day.The arrow marks the starting date of the eruption (beginning on 14 April until 24 May 2010).

Figure 14 .
Figure 14.Mean SO 2 column departures from the unperturbed 10-day pre-volcanic baseline measured by Brewers, OMI (TRM, STL) and GOME-2 during August-September 2008 over Europe and Canada and the USA following the 2008 Kasatochi volcanic eruption.The error bars for the Brewer observations show the standard deviation of all daily values during the unperturbed 10-day period prior to the volcanic eruption.Stations in Europe include Sodankylä, Vindeln, Jokioinen, Norrköping, Copenhagen, Manchester, De Bilt, Belsk, Reading, Uccle, Hradec Králové, Hohenpeißenberg, Davos, Arosa, Aosta, Kislovodsk, Rome, Thessaloniki and Athens.Stations in Canada and the USA include Churchill, Edmonton, Goose Bay, Regina, Saturna Island, Toronto, Boulder and Mauna Loa.Each daily average from either OMI or GOME-2 was calculated if and only if more than half of the individual overpasses had data on a given day.The arrow marks the date of the eruption (7 August 2008).

Table 1 .
Volcanic eruptions in the past decade considered in this study.

Table 2 .
Stations with accessible SO 2 column data from Brewers analysed in this study.Stations are sorted from high to lower northern latitudes.

Table 3 .
Rural AirBase stations analysed in this study (see text).

Table 4 .
SO 2 column departures at midlatitude stations averaged in bimonthly periods following volcanic eruptions.
SE: standard error.* Missing values are those possessing < 25 days of data in each bimonthly period or no data.