Volcanic ash over Scandinavia originating from the

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, D09201, doi:10.1029/2011JD017090, 2012

Volcanic ash over Scandinavia originating from the Grímsvötn eruptions in May 2011 M. Tesche,1 P. Glantz,1 C. Johansson,1,2 M. Norman,2 A. Hiebsch,3 A. Ansmann,3 D. Althausen,3 R. Engelmann,3 and P. Seifert3 Received 1 November 2011; revised 16 March 2012; accepted 17 March 2012; published 2 May 2012.

[1] A volcanic ash plume that originated from the eruptions of Iceland’s Grímsvötn volcano in May 2011 was observed over the Nordic countries using a combination of satellite observations and ground-based measurements. The dispersion of the plume was investigated using London VAAC ash forecasts and MODIS observations. Hourly PM10 concentrations at air quality monitoring stations in the southern parts of Norway, Sweden, and Finland exceeded 100 mg/m3 for several hours. The FLEXPART dispersion model has been used to confirm the Icelandic origin of the sampled air masses. Column-integrated quantities from a Sun photometer and vertical profiles from a Raman lidar were used to estimate the ash concentration within an elevated layer over Stockholm. A lofted layer with an optical thickness of 0.3 at 532 nm passed Stockholm in the morning hours of 25 May 2011. Considering a realistic range of coarse-mode fractions and specific ash extinctions from the literature, an estimated range of maximum ash mass concentration of 150–340 mg/m3 was derived from the lidar measurements at an altitude of 2.8 km. The lower estimate of the lidar-derived ash mass concentrations within the planetary boundary layer was found to be in good agreement with surface observations of PM10. Citation: Tesche, M., P. Glantz, C. Johansson, M. Norman, A. Hiebsch, A. Ansmann, D. Althausen, R. Engelmann, and P. Seifert (2012), Volcanic ash over Scandinavia originating from the Grímsvötn eruptions in May 2011, J. Geophys. Res., 117, D09201, doi:10.1029/2011JD017090.

1. Introduction [2] The eruption of the Eyjafjallajökull volcano in southern Iceland in March 2010 [Gudmundsson et al., 2010] produced an ash plume that disrupted air traffic throughout most parts of Europe [Ansmann et al., 2010; Schumann et al., 2011] and caused significant financial losses. Only one year after this event, the eruption of the Grímsvötn volcano (64.42 N, 17.33 W, 1725 m summit height) threatened to repeat this scenario [Kerminen et al., 2011]. Grímsvötn is the most active of Iceland’s volcanoes with on average one eruption every ten years. The latest eruption occurred at the same scene as the last one in November 2004 [Witham et al., 2007] (also Icelandic Met Office, Update on volcanic activity in Grímsvötn, 2011, available at http://en.vedur.is/earthquakesand-volcanism/articles/nr/2180). The Icelandic Met Office reported the start of the eruption of the Grímsvötn volcano at about 1730 UTC on 21 May 2011 and set the official end to 0700 UTC on 28 May 2011. The initial plume reached a height of some 20 km. In the later days the maximum plume height descended to values of 10 km on 23 May 2011 and 1

Department of Applied Environmental Science, Stockholm University, Stockholm, Sweden. 2 SLB-Analys, City of Stockholm Environment and Health Administration, Stockholm, Sweden. 3 Leibniz Institute for Tropospheric Research, Leipzig, Germany. Copyright 2012 by the American Geophysical Union. 0148-0227/12/2011JD017090

below 5 km after 24 May 2011. In contrast, the ash plume of the Eyjafjallajökull eruption in 2010 only reached a maximum plume height of 8 km [Stohl et al., 2011]. Compared to the phreatomagmatic eruption of Eyjafjallajökull in 2010 [Gislason et al., 2011], the eruption of the Grímsvötn volcano was less explosive and resulted in coarser ash particles which were considered less likely to remain airborne long enough to interfere with European airspace (Icelandic Met Office, online publication, 2011). Because of these circumstances and the prevailing meteorological conditions, the Grímsvötn eruption was expected to have a rather small effect on European air traffic. Initially after the eruption, the ash plume remained close to Iceland. Satellite observations show that an effective separation of ash and sulfate occurred during this time [Kerminen et al., 2011]. Based on the meteorological situation and the forecast of the London Volcanic Ash Advisory Centre (VAAC, http://www.metoffice. gov.uk/aviation/vaac/) at UK Met Office’s headquarter in Exeter, which is responsible for monitoring and forecasting the movement and dispersion of volcanic ash originating from volcanoes in the north-eastern part of the north Atlantic Ocean, ash from the Grímsvötn volcano was expected to be transported southwards only at heights below 5–8 km. The bigger part of the plume was forecast to be transported at larger heights and in northerly direction to Greenland or toward the northern Atlantic Ocean. Together with this beneficial transport pattern, cloudy conditions over most of northern and central Europe might have caused a significant removal (wet deposition) of ash particles. As a consequence,

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TESCHE ET AL.: GRÍMSVÖTN VOLCANIC ASH OVER SCANDINAVIA

air traffic interruptions were restricted to airports in Greenland, Iceland, Ireland, the northern part of Great Britain (starting at 24 May 2011), and northern Germany (at 25 May 2011). [3] The purpose of this paper is to describe the history of the ash transport using model calculations, remote-sensing observations, and ground-based measurements of particle mass (PM) concentrations, and to show the impact of the eruption on surface PM concentrations in different parts of the Nordic countries. PM levels in the days after the Grímsvötn eruption are compared with normally observed concentrations in order to judge the potential health risks associated with such events. We consider our documentation of the impact of the Grímsvötn eruption as complementary to the observations at Helsinki presented by Kerminen et al. [2011]. In section 2 we begin with an overview of the tools and instruments applied in this study. Section 3 provides a review of the meteorological conditions during the investigated time period from 21 to 27 May 2011 and presents the forecast of the dispersion of the ash plume as provided by the London VAAC. The observations are presented in section 4 and the paper concludes with a summary and discussion in section 5.

2. Tools and Instrumentation [4] To study the distribution of the Grímsvötn ash cloud over northern Europe in general and Stockholm in particular we utilized spaceborne observations performed with the MODerate resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite, ground-based in situ measurements at stations throughout Scandinavia, and Sun photometer (SPM) and lidar measurements in Stockholm, Sweden. In this section we will provide an overview of the data sources, instruments, and the respective data analysis. 2.1. VAAC Ash Forecast [5] Initially after the eruption, forecasts of ash mass concentrations over Europe were provided by the London VAAC. These forecasts are based on calculations using the Numerical Atmospheric dispersion Modeling Environment (NAME) model of the UK Met Office. VAAC ash forecasts are provided for three layers that extend from the surface to flight level 200 (FL200, standard atmospheric pressure altitude in feet divided by 100, 6.1 km height), from FL200 to FL350 (6.1–10.7 km height), and from FL350 to FL550 (10.7–16.8 km height). Areas of different peak ash concentrations are provided for each of these layers: 200–2000 mg/m3 (low ash contamination), 2000–4000 mg/m3 (medium ash contamination), and >4000 mg/m3 (high ash contamination) [International Civil Aviation Organization, 2011; Witham et al., 2012]. These forecasts are used to issue ash warnings and to recommend closures of air space when ash concentrations reach levels that might be harmful to aircraft jet engines. Detailed descriptions of NAME are provided by Jones et al. [2007] and Witham et al. [2007]. Comprehensive validations of NAME ash predictions for the Eyjafjallajökull plume are presented by Dacre et al. [2011] and Devenish et al. [2012]. In this paper we apply the operational VAAC ash forecasts for the lowermost layer as an indicator for the temporal development and dispersion of the Grímsvötn ash plume. The respective forecasts were publicly available from the London VAAC and as Google Earth visualization

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(http://ogleearth.com/2011/05/grimsvotns-ash-cloud-visualizedin-google-earth/). 2.2. AOT Retrieved From MODIS Observations [6] We used the algorithm described by von HoyningenHuene et al. [2003, 2006, 2011] to obtain aerosol optical thickness (AOT) from MODIS level 1 calibrated top of atmosphere (TOA) radiance with 1 1 km2 resolution at ground. The method has been adapted to the MODIS observation geometry and performs best over dark surfaces. We performed a pixel by pixel comparison (averaged to the MODIS Collection 5 resolution of 10 10 km2) of the results of our retrieval to the operational MODIS output over land surfaces in eastern Europe (P. Glantz and M. Tesche, Assessment of diverse algorithms used for MODIS aerosol retrievals over land surfaces in Europe, submitted to Atmospheric Measurement Techniques, 2012) and generally obtained values that are well within the expected uncertainty of one standard deviation of the MODIS retrieval. Further applications of the retrieval to MODIS measurements also over the ocean are presented by Glantz et al. [2009a, 2009b]. In the present study results of AOT for the wavelength of 488 nm are used. Note that the applied look-up tables that describe the relationship between aerosol reflection and AOT have been established for spherical aerosols and low and medium aerosol loadings, while volcanic ash is associated with non-spherical particles. Though it is justified to assume that AOT is somewhat underestimated in our retrieval, the spatial distribution of the ash plume is captured properly. [7] We included a cloud screening routine that considers clouds to be bright and homogeneous with an increased TOA reflectance for cloudy pixels. The TOA normalized aerosol radiance sTOA is obtained by subtracting the Rayleigh path reflectance and surface scattering from the total radiance. A pixel is sorted as a thick cloud, if sTOA > 0.3 [Kokhanovsky et al., 2006]. In a second step, the ratio of the standard deviation and the mean of sTOA at 443 nm is obtained for 9 adjacent pixels and used to characterize the spacial variability. If the ratio is larger than 0.03, the central pixel is identified as cloudy. Erroneous screening can occur at cloud edges, when single pixels contain both clouds and aerosols. However, we found the cloud screening to work appropriately in the area of interest in the considered scenes. 2.3. Ground-Based in Situ Particle Sampling [8] In this paper we utilized in situ measurements of PM performed at the locations shown in Figure 1. PM10 in Oslo and at the sites in Sweden was monitored by using Tapered Element Oscillating Microbalance (TEOM 1400a) [Patashnick and Rupprecht, 1991]. The TEOM operates at a temperature of 50 C to avoid condensing water. To account for the loss of volatile particle material, all TEOM data were corrected following Areskough [2007]. In short, built-in TEOM corrections were removed, mass concentrations were expressed at ambient pressure and temperature and, finally, concentrations were multiplied by 1.19 and an offset of 1.15 was added. For the site in Oslo TEOM data are multiplied by 1.1 [Areskough, 2007]. At the measurement stations in Stockholm (roof site at Torkel Knutssonsgatan and street site at Hornsgatan, both described in detail by Gidhagen et al. [2004]) the instruments are equipped with both PM10 and PM2.5 inlets. Two electrical ball-valves are

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Figure 1. Measurement sites included in this study: Birkenes (Bi), Oslo (Os), Gothenburg (Go), Vavihill (Va), Karlshamn (Kh), Karlskrona (Kk), Linköping (Li), Hälleforsnäs (Ha), Stockholm (St), Norr Malma (Nm), Sundsvall (Su), Umeå (Um), Malmberget (Ma), Turku (Tu), Tampere (Ta), Helsinki (He), Kuopio (Ku), and Virolahti (Vi). Black and white stars indicate whether an increase in PM10 was observed or not. used to automatically switch between the two inlets. Hourly mean values are derived for both PM10 and PM2.5. PM2.5 was also measured in Oslo and Gothenburg. [9] The particle size distribution was obtained with an optical particle counter (OPC, GRIMM 1109) at Hornsgatan, Stockholm. The OPC was operated at ambient conditions. It measures the number of particles in 31 different size intervals between 0.25 and 32 mm. Note that we did not include corrections for effects of refractive index and particle shape. This is likely to result in an undersizing [Liu and Daum, 2000] and particle size distributions should be considered as approximate with an uncertainty of at least a factor of 2 [Johnson et al., 2012]. The number size distribution can be converted to mass size distribution by using an ash density of 2.6 g/cm3 (see Ansmann et al. [2011] and Schumann et al. [2011] for a discussion of this value). [10] A GRIMM 190 OPC measured PM1, PM2.5, and PM10 at Birkenes, Norway. The instrument was only recently deployed at the measurement site and quality assurance of the collected data have not been performed, yet (M. Fiebig, personal communication, 2011). Therefore measurements of this instrument will be discussed only qualitatively. Hourly mean values of PM10 and PM2.5 at Turku, Helsinki, Tampere, Kuopio, and Virolahti in southern Finland were obtained through the Finnish air quality portal www.airquality.fi. 2.4. Lidar [11] The compact Raman lidar Polly of the Leibniz Institute for Tropospheric Research (IfT), Leipzig, was stationed at Stockholm University (18.09 E, 59.37 N, 40 m asl) in August 2010. The instrument emits light pulses at 532 nm wavelength into the atmosphere. Backscattered light (elastic scattering by particles and air molecules at 532 nm and Raman scattering by nitrogen molecules at 607 nm) is measured with a two-channel receiver. These signals allow for a retrieval of 532–nm particle extinction and backscatter

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coefficients independent of each other during nighttime [Ansmann and Müller, 2005], from which the extinction-tobackscatter (lidar) ratio S can be determined. Raman signals for an independent solution of the lidar equation are too weak to be detected under bright daytime conditions. Therefore, profiles of the particle backscatter coefficient are calculated from elastic backscatter signals according to Klett’s method [Klett, 1981; Fernald, 1984] during daytime. A detailed description of the instrument is provided by Tesche et al. [2007] and Althausen et al. [2009]. Radiosondes launched daily at 0000 and 1200 UTC at Visby, about 190 km south of Stockholm, were used for the analysis of the lidar measurements. Klett’s method was used to obtain the aerosol profiles presented in this study. [12] The relative errors of the derived aerosol properties are mainly determined by signal noise. Temporal averaging and vertical smoothing is used to reduce this effect. Systematic uncertainties in the backscatter retrieval (e.g., correction of Rayleigh scattering and air density, signal calibration in the clean free troposphere, choice of the lidar ratio in case of applying Klett’s method) are estimated to be approximately 10%. [13] Ash mass concentration profiles can be calculated from lidar measurements of the extinction coefficient a as M¼

fc a fc bS ¼ Kext Kext

ð1Þ

by using the specific extinction Kext for volcanic ash and the coarse-mode fraction fc under the assumption that the coarse mode only incorporates ash particles [Ansmann et al., 2011; Marenco and Hogan, 2011]. Values of Kext were obtained from lidar and airborne in situ observations of the Eyjafjallajökull ash plume in 2010. Some current estimates are 0.44–0.99 m2/g (mean of 0.64 m2/g) [Ansmann et al., 2011], 0.49–1.23 m2/g (mean of 0.78 m2/g) [Gasteiger et al., 2011], 0.6–0.9 m2/g [Marenco and Hogan, 2011], and 0.45–1.06 m2/g (mean of 0.6 m2/g) [Johnson et al., 2012]. Note that the density of ash is included in the specific ash extinction and that the first two studies assumed a value of 2600 kg/m3 while the other two assumed a value of 2300 kg/m3. For a detailed discussion of the presented values we refer to the respective papers. The ash-related coarsemode fraction can be derived from polarization-sensitive lidar measurements in case of an external mixture of two aerosol types with known depolarization ratios [Tesche et al., 2009; Ansmann et al., 2011; Marenco and Hogan, 2011]. Linear particle depolarization ratios of 0.34–0.38 at both 355 and 532 nm were measured within the Eyjafjallajökull ash plume shortly after the eruption [Ansmann et al., 2011; Groß et al., 2012; Marenco and Hogan, 2011]. Such high values represent a dominance of non-spherical particles and are believed to represent undiluted ash conditions, i.e., conditions under which volcanic ash is not mixed with a fine aerosol component. We assume that these values are valid for Grímsvötn ash as well. 2.5. Sun Photometry [14] SPM measurements in Stockholm were performed with a SP1A manufactured by Dr. Schulz & Partner GmbH, Buckow, Germany. The instrument is located next to the Polly lidar since January 2011 and records AOTs at eight

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wavelengths from 380–1044 nm with a temporal resolution of 60 s. It was last calibrated in March 2010 at Mount Izaña, Canary Islands, Spain, and compared to the AErosol RObotic NETwork (AERONET) [Holben et al., 1998] instrument at Leipzig before being installed at Stockholm. AOT uncertainties are of the order of 0.015 (402–1044 nm) and 0.03 (380 nm) around noon and about a factor of 2 lower at low sun elevations. Cloud screening is performed in conjunction to the cloud-sensitive lidar observations. Ångström exponents [Ångström, 1964] were calculated from the spectral AOT measurements. This parameter is defined as the slope of the logarithm of AOT versus the logarithm of wavelength. It is commonly used to characterize the wavelength dependence of AOT and to provide some basic information on the aerosol size distribution [Eck et al., 1999]. 2.6. FLEXPART Dispersion Modeling [15] To identify the origin of the aerosols observed over Stockholm the Lagrangian particle dispersion model FLEXPART, maintained by the Norwegian Institute for Air Research, was used [Stohl et al., 1998, 2005]. Lagrangian particle models compute trajectories of a large number of so-called particles (not necessarily representing real particles, but infinitesimally small air parcels) to describe the transport and diffusion of tracers in the atmosphere [Stohl et al., 2005]. FLEXPART can simulate long-range and mesoscale transport, diffusion, dry and wet deposition, and radioactive decay of tracers released from point, line, area, or volume sources [Stohl et al., 2005]. [16] FLEXPART is run at IfT Leipzig with archived meteorological data from the NCEP FNL (National Centers for Environmental Prediction final analysis) Global Tropospheric Analyses with a temporal resolution of 6 hours (00, 06, 12, and 18 UTC) and a horizontal resolution of 1 1 . Particles are transported both by the resolved winds and by parameterized sub-grid motions. Dry and wet deposition as well as gravitational settling of particulate matter was set to the values used in the study by Stohl et al. [2011] for the dispersion of ash from the Eyjafjallajökull eruption in 2010. We refer to this reference for further information on ash dispersion modeling with FLEXPART. For our estimation of the origin of the observed aerosol layers we used one species with a particle diameter of 6.5 mm which can be considered as the most typical size of ash particles (N. Kristiansen, personal communication, 2011).

3. Overview of the Grímsvötn Eruption in May 2011 [17] In contrast to the time period after the eruption of the Eyjafjallajökull volcano in April 2010, when the ash plume was transported almost directly (i.e., southeasterly) toward central Europe [Dacre et al., 2011; Devenish et al., 2012; Heinold et al., 2012], a high pressure system prevailed over the British Isles between 21 and 23 May 2011. Therefore, ash from the Grímsvötn eruption initially stayed close to Iceland and was only transported some degrees southward—at least in the lower half of the troposphere. At larger heights, ash was transported northward to Greenland and Spitsbergen [Kerminen et al., 2011]. There is another difference between the plumes of Eyjafjallajökull and Grímsvötn. For the first case, ash and SO2 were transported together in mixed

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layers [Ansmann et al., 2011; Marenco and Hogan, 2011; Schumann et al., 2011]. In the case of Grímsvötn on the other hand, an effective separation of the two eruption products was observed from satellite measurements with SO2 traveling mainly toward Greenland and North America [Kerminen et al., 2011]. Starting on 24 May 2011, a strong trough over Scandinavia with a surface low over southern Finland caused an eastward transport of the ash from Iceland over the northern part of Great Britain to southern Scandinavia. Many clouds prevailed over the transport area due to the close connection to the upper-level trough. [18] Figure 2 shows the spatial distribution of the ash plume as predicted by the London VAAC for the layer between the surface and FL200 (the layer which represents the lower half of the troposphere and is most likely to represent the conditions of relevance for our study) in intervals of six hours between 24 and 27 May 2011. On 24 May 2011, the ash concentration to the south and north of Iceland was very high. An ash plume was slowly moving south toward Ireland and Great Britain while the one moving northward reached Greenland in the morning of 24 May 2011. Strong eastward transport of ash only occurred north of 70 N. As the ash plume south of Iceland was transported further eastward, peak ash concentrations were predicted to decrease (lowering the risk to air traffic). The ash plume was forecast to arrive over Scotland between 0000 and 0600 UTC on 24 May 2011 and over Norway between 1200 and 1800 UTC on 24 May 2011. First traces of the ash cloud were forecast to arrive over the region of Stockholm between 0000 and 0600 UTC on 25 May 2011 and to be gone at 1200 UTC the same day. For the same time period, much higher ash concentrations were predicted for the region south of Stockholm extending all the way to Denmark. In the evening of 25 May 2011, the ash plume was predicted to be over southern Finland. After 25 May 2011, the plume was expected to be quite diluted with relatively low ash concentrations over most of the north Atlantic and northern Europe. The last ash forecast was issued at 1800 UTC on 27 May 2011.

4. Observations [19] This section describes the eastward transport of the Grímsvötn ash plume as observed with MODIS and at in situ stations scattered throughout southern Scandinavia. The second part of this section is focused on a detailed discussion of the observations at Stockholm. 4.1. MODIS [20] The best way to comprehensively observe the dispersion of a volcanic ash plume is provided by satellite observations. However, during the days in the aftermath of the Grímsvötn eruption on 21 May 2011, clouds were present over most of northern and central Europe. [21] Satellite observations of the Grímsvötn ash plume were performed with the Spinning Enhanced Visible and Infrared Imager (SEVIRI) aboard the geostationary Meteosat Second Generation (MSG) satellite (http://fred.nilu.no/sat/, http://savaa.nilu.no/Grimsvotn/tabid/4563/Default.aspx) and the Ozone Monitoring Instrument (OMI) aboard the Aura satellite [Kerminen et al., 2011]. These observations show a spatial distribution of the ash plume that generally agrees with the prediction of the London VAAC.

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Figure 2. London VAAC forecast of the dispersion of the Grímsvötn ash plume between the surface and FL200. The different colors indicate peak ash concentrations of 200–2000 mg/m3 (yellow), 2000–4000 mg/m3 (orange), and >4000 mg/m3 (red). The black lines mark the area of ash warning as issued by the London VAAC. The stars mark the locations of measurement sites considered in this study (see Figure 1). [22] Figure 3 shows the location of the Grímsvötn ash plume on 22, 23, and 24 May 2011 as observed with MODIS aboard the Aqua satellite during scenes that were not fully disturbed by clouds. Figure 3 indicates the transport of the ash plume from a region south of Iceland (22 and 23 May 2011) over the North Sea to the western coast of Norway (24 May 2011). For 22 May 2011, 488-nm AOTs of approximately 0.56 0.14 and 0.97 0.19 were obtained south of Iceland for the marine areas associated with lower and higher AOT enhancement, respectively. In the surrounding, unaffected marine areas AOT did not exceed 0.2 with the majority of values being lower than 0.1. Such numbers are typical for the region between Iceland and Norway during unperturbed aerosol conditions. Figure 3 shows that similar differences in AOT, between areas influenced by the ash plume and pure marine aerosols, are also present on 22 and 23 May 2011. The images also show areas at cloud edges for which the present cloud screening encounters difficulties. Questionable areas are the borders between cloud and aerosol fields over Iceland and further southeast of the ash plume in Figure 3a, the patchy aerosol fields southwest of the ash

plume in Figure 3b, and the large areas of estimated AOT values west and east of the ash plume in Figure 3c. All these areas show scattered clouds in the RGB picture. 4.2. In Situ Measurements [23] The overpass of the ash plume could be observed at air quality monitoring stations in the southern parts of Norway, Sweden, and Finland. Time series of PM10, PM2.5, and the difference between PM10 and PM2.5 are shown in Figure 4. The colored bars in Figure 4c refer to the time periods during which PM10 mass concentrations exceeded values of 40 mg/m3. We chose this value (which is well above background levels) to get a lower estimate of the time period during which ash from the Grímsvötn volcano was present at the respective measurement site. Ash at a ground station was first detected at Birkenes (gray bar in Figure 4c, concentration plot not shown) shortly before noon on 24 May 2011. Two maxima were detected at 1200 and 1800 UTC, respectively, and values decreased to normal at about 2000 UTC (M. Fiebig, personal communication, 2011). At Gothenburg (black lines and bar), volcanic ash was first detected at

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Figure 3. Maps of 488-nm AOT as inferred from measurements of MODIS aboard Aqua at (a) 1315 UTC on 22 May 2011, (b) 1355 UTC on 23 May 2011, and (c) 1300 UTC on 24 May 2011. Warm and cold colors refer to high and low AOT, respectively. MODIS data are available at http:// ladsweb.nascom.nasa.gov/data/. 1200 UTC on 24 May 2011 and PM10 reached a maximum value of 167 mg/m3. The mass concentration of particles with diameters between 2.5 and 10.0 mm reached values of 150 mg/m3, i.e., the high concentrations were caused by coarse particles. Under the specific circumstances and considering the findings of FLEXPART dispersion modeling, we are confident that coarse particles are the equivalent of volcanic ash during the period under investigation. The

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values observed at Gothenburg represent the highest numbers observed at the considered stations during the volcanic episode in May 2011. Slightly lower mass concentrations were observed at Linköping (1500 UTC on 24 May 2011 to 0100 UTC on 25 May 2011, concentration plot not shown in Figure 4) and Oslo (1600–2200 UTC on 24 May 2011). [24] At the roof site Torkel Knutssonsgatan in Stockholm PM10 mass concentration exceeded 40 mg/m3 during the evening of 24 May 2011 and reached a maximum of 145 mg/m3 at 0000 UTC on 25 May 2011. At the same time PM2.5 increased from below 5 mg/m3 to almost 30 mg/m3 and the concentration of coarse particles reaches values of 120 mg/m3. Note that measurements of urban background aerosol mass concentrations at roof level between 2005 and 2011 in Stockholm revealed that less than 2% of all measured values of PM10 and PM2.5 are above 40 mg/m3 and 30 mg/m3, respectively. However, PM10 concentrations at Stockholm’s Hornsgatan street site are usually quite high in springtime due to large concentrations of road dust, i.e., about 15% of measurements in April and May show PM10 above 100 mg/m3 [Norman and Johansson, 2006]. The period of increased PM10 in Stockholm last until 0600 UTC on 25 May 2011. A detailed discussion of the particle size distribution during this time range follows later in the text together with the discussion of lidar and SPM measurements in Stockholm. In accordance to the eastward movement of the ash plume, the measurement stations at Hälleforsnäs (100 km west of Stockholm) and Norr Malma (75 km northeast of Stockholm) observed ash at the surface for a similar time period but 3 h earlier and 1 h later than in Stockholm, respectively. No time series of PM concentrations are shown for these two stations in Figure 4 due to their similarities to the measurements in Stockholm. [25] After the ash plume passed the Baltic Sea in the early morning of 25 May 2011 it was also detected at the southern Finish stations Turku (0330–1500 UTC on 25 May 2011, PM10 of 80 mg/m3, PM10–PM2.5 of 60 mg/m3), Helsinki (0445–1330 UTC, 110/80 mg/m3), and Virolahti (0800– 1615 UTC, 80/70 mg/m3). An increase in PM10 to maximum values of 80 mg/m3 was also observed at 1200 UTC on 25 May 2011 at Tampere (not shown). [26] For the entire ash period, no increase in PM10 at the surface was observed at Vavihill, Karlshamn, and Karlskrona in southern Sweden, at Sundsvall, Umeå, and Malmberget in northern Sweden, and at Kuopio in eastern Finland. These sites are marked as white stars in Figure 1. Figure 2 shows that ash was forecast to pass these sites. Note that surface measurements cannot be used to validate a forecast of peak ash concentrations within an atmospheric layer from the surface to 6.1 km height. Only observations that cover the entire column (SPM, satellite) or result in height profiles (lidar, aircraft measurements) provide the necessary data for a thorough validation of dispersion modeling [Dacre et al., 2011; Devenish et al., 2012; Heinold et al., 2012]. 4.3. Observations at Stockholm [27] Figure 5 gives a detailed view of the observations performed at Stockholm with SPM (column-integrated values), lidar (height-resolved observations), and in situ particle samplers (surface values) within the time period between 1800 UTC on 24 May 2011 and 1200 UTC on 25 May 2011. SPM observations can only be performed during daytime.

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Figure 4. Aerosol mass concentrations of (a) PM10, (b) PM2.5, and (c) PM10–PM2.5 as measured at in situ stations in Gothenburg (black), Oslo (red), Stockholm (roof site at Torkel Knutssonsgatan, dark blue), Turku (magenta), Helsinki (light blue), and Virolahti (green). The colored bars in Figure 4c refer to the time period during which PM10 mass concentrations exceeded 40 mg/m3 at the respective measurement site. The white lines in the colored bars give the time of highest PM10 mass concentration. In addition to the six stations whose PM time series are shown, colored bars are also given for Birkenes (gray), Linköping (orange), Hälleforsnäs (dark green), and Norr Malma (light magenta) to illustrate the eastward movement of the ash plume. See Figure 1 for the locations of the measurement stations.

The instrument only captures the last traces of the ash plume over Stockholm in terms of a decrease in AOT from 0.6 to 0.1 and an increase in Ångström exponents (AE) from 0.1 to 0.6. Small Ångström exponents between 0 and 0.4 can be expected when large particles like mineral dust or volcanic ash are present in the atmosphere [Eck et al., 1999; Ansmann et al., 2011]. In the end of the shown period, the AOT is at an equally low level as observed before the arrival of the ash plume. Note that typical AOTs and PM10 concentrations at Stockholm are