Corresponding author: F. Navas-Guzmán, Department of Applied Physics, University of Granada, Granada, 18071, Spain. (email@example.com)
 A fraction of the volcanic plume that originated from the Eyjafjallajökull volcanic eruption on Iceland in 2010 reached the southern Iberian Peninsula in May 2010. The plume was monitored and characterized in terms of optical and microphysical properties with a combination of Raman lidar and star- and Sun-photometers. Our observations showed that the plume arriving at the Iberian Peninsula was mainly composed of sulphate and sulphuric-acid particles. To our knowledge, this is the first study of optical properties and inverted microphysical properties of volcanic sulphate particles in the lower troposphere/boundary layer based on multiwavelength Raman lidar measurements. A remarkable increase in the particle number concentration in the accumulation mode was determined from the inversion of the aerosol optical properties. The large Ångström exponents and low linear particle depolarization ratios (4–7%) indicated the presence of small and spherical particles. The particle effective radii ranged between 0.30 and 0.55 µm. In situ instrumentation confirmed an increase of sulphate particles at ground level during this period.
 The Eyjafjallajökull volcano in southern Iceland (63.63°N, 19.61°W) started to erupt in March 2010. Seismic activity started at the end of 2009 and gradually increased in intensity until 20 March 2010. The first phase was characterized by an effusive eruption that produced lava flows on the ground and only minor emissions into the atmosphere. On 14 April an explosive eruption started a period with major activity that lasted until 22 May 2010 [Emeis et al., 2011]. After 14 April the volcanic plume drifted from Iceland to central Europe due to strong westerly winds, causing the closure of large parts of the European airspace, and it affected hundreds of thousands of travelers with the subsequent economic impact. The synoptic conditions did not favor the arrival of the volcanic plume over the Iberian Peninsula until 5 May [Revuelta et al., 2012; Sicard et al., 2012].
 Volcanic eruptions emit ash, water vapor, and other gases into the atmosphere. Furthermore, water vapor and sulphur dioxide injected into the stratosphere during particularly explosive events can produce global veils of sulphuric acid droplets that affect the Earth's climate [Robock, 2000]. In the troposphere, sulphur dioxide is converted to sulphate particles within a few days [Graf et al., 1997]. On the other hand, volcanic ash is composed of nonspherical mineral particles with a large range of sizes [Gislason et al., 2010]. In the troposphere these particles can cause hazard to humans and machinery on the ground. Moreover, volcanic ash particles may heavily affect aircraft [Miller and Casadevall, 2000] because, like in this case, they can be highly abrasive and can have melting temperatures below that within aircraft engines. Therefore, the detection and tracking of ash plumes, e.g., by satellites and airborne- or ground-based remote sensing for aviation hazard mitigation is indispensable for Volcanic Ash Advisory Centers [Carn et al., 2009; Flentje et al., 2010].
 The lidar technique is one of the most important remote optical methods in atmospheric aerosol studies. In fact, lidar systems have been used by the scientific community to observe major volcanic eruptions reaching the stratosphere [Jäger, 1992; Winker and Osborn, 1992; Chazette et al., 1995; Osborn et al., 1995; Wandinger et al., 1995; DiGirolamo et al., 1996; Mattis et al., 2010].
 The Eyjafjallajökull volcanic ash particles have been characterized with lidar systems at different locations in Europe [Ansmann et al., 2010, 2011; Gasteiger et al., 2011; Groß et al., 2011; Marenco and Hogan, 2011; Mona et al., 2011; Royer et al., 2011; Chazette et al., 2012; Pappalardo et al., 2012; Papayannis et al., 2012; Sicard et al., 2012]. A unique combination of Raman lidar, Sun- and star-photometer and in situ instrumentation is used in this work to characterize the optical and microphysical properties of the volcanic plume that was observed over Granada (Spain) from 7 to 8 May 2010. Our study reveals that mainly sulphate and sulphuric-acid particles were at least episodically present over southwest Europe. Previous studies of volcanic plumes [Jäger, 1992; Winker et al., 1992; Chazette et al., 1995; Osborn et al., 1995; Wandinger et al., 1995; Mattis et al., 2010] dealt with the characterization of sulphuric-acid droplets in the lower stratosphere and upper troposphere only. Additional importance of our study therefore arises from the fact that, to our knowledge, this is the first time that an optical and microphysical characterization of these particles has been performed in the lower troposphere with a multiwavelength Raman lidar.
 This paper is organized as follows: In section 2, the instrumentation and the methodology are briefly outlined. The characterization of the volcanic aerosols is presented in terms of their optical and microphysical properties in section 3. Finally, a summary is presented in Sect. 4.
2 Measurements and Methodology
 Data from remote and in situ measurements were collected at the Andalusian Center for Environmental Research(CEAMA) located in the city of Granada (Spain, 37.16°N, 3.6°W, 680 m above sea level (asl)). Granada is a nonindustrialized medium-sized city, with a population of 300,000 that increases up to 600,000 if we include the metropolitan area. The city is situated in a natural basin surrounded by mountains with elevations between 1000 and 3500 m asl. The study area is also at a short distance, about 200 km away from the African continent and approximately 50 km away from the western Mediterranean basin. Granada is affected by two major aerosol types: natural dust, mainly from North Africa, and anthropogenic pollutants from Europe. Moreover, the Mediterranean basin can represent an additional source of atmospheric aerosol due to its complex meteorology that favors the aging of polluted air masses and induces a high level of airborne particles [Alados-Arboledas et al., 2011].
 Aerosol vertical profiling was performed by means of a Raman lidar model LR331D400 (Raymetrics S.A., Greece). The system is configured in a monostatic biaxial alignment pointing vertically to the zenith. A Nd:YAG laser emits pulses at 1064 nm (110 mJ), 532 nm (65 mJ) and 355 nm (60 mJ) simultaneously, firing laser shots with a repetition rate of 10 Hz. A 0.4 m diameter Cassegrain telescope collects radiation backscattered by atmospheric molecules and particles. The receiving subsystem also includes a wavelength separation unit with dichroic mirrors, interferential filters, and a polarization cube. Detection is carried out in seven channels corresponding to elastic wavelengths at 1064, 532 (parallel- and perpendicular-polarized), and 355 nm, and to inelastic wavelengths at 607 nm (nitrogen Raman shifted signal excited by radiation at 532 nm), 387 (nitrogen Raman-shifted signal excited by radiation at 355 nm), and 408 nm (water vapor Raman-shifted signal excited by radiation at 355 nm). The instrument is operating with a spatial vertical resolution of 7.5 m. Due to the instrument setup, the incomplete overlap between the laser beam and the receiver field of view limits the lowest observations. Correction of the overlap effect is performed by applying the procedure suggested by Wandinger and Ansmann . The Raman lidar was incorporated to EARLINET (European Aerosol Research Lidar NETwork) [Bösenberg et al., 2001] in April 2005. It has taken part of the EARLINET Advanced Sustainable Observation System project and currently is involved in the Aerosols, Clouds, and Trace gases Research InfraStructure Network European project. Further details in relation to this instrument can be found in Guerrero-Rascado et al. [2008, 2009].
 This lidar system provides vertical profiles of the particle backscatter and extinction coefficients at 355, 532 nm at nighttime (by using the Raman lidar method [Ansmann et al., 1992]) and the corresponding extinction-to-backscatter ratios (lidar ratios). At daytime, the elastic backscatter lidar method [Fernald, 1984] is predominantly used to determine particle backscatter coefficients at 355, 532, and 1064 nm. Moreover, backscatter- and extinction-related Ångström exponents (β-AE, α-AE) can be determined [Ansmann et al., 2003].
 Linear particle depolarization ratios (δp) were calculated from the cross-polarized signals with respect to the state of polarization of the light emitted at 532 nm [Cairo et al., 1999; Freudenthaler et al., 2009; Bravo-Aranda et al., 2012]. Using the depolarization information it was possible to obtain the contributions of nonash (fine fraction) and ash particles (coarse fraction) to total particle backscatter and extinction coefficients [Ansmann et al., 2011]. The method uses lidar observations of particle backscatter coefficient and δp at 532 nm in order to separate the backscatter contributions of the weakly light depolarizing nonash aerosol (1%) from the strongly light depolarizing ash particles (35–37%). This separation method is based on the work outlined in detail by Tesche et al.  and refined recently for ash and fine-mode particles by Ansmann et al. . The separation technique assumes a simple, externally mixed two-component aerosol. Thus, the potential impact of particle aging on the particle depolarization ratio is ignored (e.g., partly coating of ash particles with sulfate and subsequent changes in the shape characteristics) [Ansmann et al., 2011]. Several works have shown that coated effect are negligible over different kinds of particles (Saharan dust, volcanic ash particles) [Schumann et al., 2011; Seifert et al., 2011; Tesche et al., 2009] and these particles retained their irregular nonspherical shape. Therefore, coating effect on the depolarization measurement seems to be small [Ansmann et al., 2011]. Uncertainties in the separation of the backscatter coefficients of spherical and nonspherical particles are around 15–20% [Ansmann et al., 2011].
 Optical data were used to retrieve microphysical properties by means of an inversion algorithm [Müller et al., 2001; Ansmann and Müller, 2005a]. This method uses a combination of three backscatter and two extinction coefficients as input information [Müller et al., 2001; Veselovskii et al., 2002]. Effective radius, volume and surface area concentration, complex refractive index, and single-scattering albedo (ω0) are derived with this inversion scheme. In the worst case we found uncertainties of <30% for mean effective radius, up to 50% for mean values of volume and surface area concentration, and ±0.05 for the real part of the complex refractive index. The imaginary part is found to its correct order of magnitude, if it is less than 0.01i. For larger values of the imaginary part, the uncertainty is reduced to less than 50%. The ω0 is retrieved to an accuracy of ±0.05 for measurement errors of ~10%. Details on error analysis are given by Müller et al. [1999, 2001] and Veselovskii et al. [2002, 2004].
 Daytime, column-integrated characterization of the atmospheric aerosol has been done by means of a Sun-photometer CE-318-4 included in the AERONET (Aerosol RObotic NETwork, http://aeronet.gsfc.nasa.gov/). A complete description can be found in Holben et al. . This instrument acquires direct Sun irradiance and sky radiance measurements. The direct Sun irradiance measurements allow the retrieval of aerosol optical depth (AOD) at six narrowband filters centered at 340, 380, 440, 500, 675, 870, and 1020 nm (nominal wavelengths) and to obtain the precipitable water vapor (W) by an additional filter centered at 936 nm. The estimated uncertainty in computed AOD, due primarily to calibration uncertainty, is around 0.010–0.020 for field instruments (which is spectrally dependent, with the higher errors in the UV) [Eck et al., 1999]. The Sun-photometer radiance measurements in conjunction with the direct Sun measured AOD are used to retrieve optical equivalent, column-integrated aerosol size distributions and refractive indices. The retrieval of the particle volume size distribution was demostrated to be adequate in practically all situations [e.g. AOD(440) > 0.05]. Errors in aerosol size distribution retrievals were dependent on particle size, aerosol type, and actual values of the size distribution. For particles in the size range 0.1 < r < 7 µm, retrieval errors are around 10–35%, while for sizes lower than 0.1 µm and higher than 7 µm, retrieval errors rise up to 80–100% [Dubovik et al., 2000]. Using microphysical information the spectral dependence of ω0 is also calculated [Dubovik and King, 2000; Dubovik et al., 2006].
 During nighttime, column-integrated characterization of the atmospheric aerosol was done by means of a star-photometer. The star-photometer EXCALIBUR (iTec. Astronómica S.L., Spain) allows measurements of direct flux from a given star. This instrument consists basically of a Schmid-Cassegrain telescope and a CCD camera as detector. It has a filter wheel with narrowband filters at 380, 440, 500, 670, 880, 940, and 1020 nm. A detailed description including the data analysis techniques is given by Pérez-Ramírezet al. . The calibration of the star-photometer EXCALIBUR is performed at the high mountain site of Calar Alto (37.2°N, 2.5°W, 2168 m asl), once a year [Perez-Ramirez et al., 2011]. AODs at the selected spectral channels is computed following the methods described in the works of Alados-Arboledas et al.  and Pérez-Ramírez et al. . Uncertainties in AOD(λ) are 0.02 for λ < 800 nm and 0.01 for λ > 800 nm [Perez-Ramirez et al., 2011].
 An integrating nephelometer (TSI, model 3563) was used to measure aerosol scattering coefficient (σsca) at 450, 550, and 700 nm. The instrument draws the ambient air through at a flow rate of 30 L min–1, illuminates the sample with a halogen lamp, and measures scattered light at the aforementioned wavelengths. The scattered light is integrated over an angular range of 7–170 from the forward direction. Using the backscatter shutter, this range can be adjusted to either 7–170° or 90–170° to give total scatter and backscatter signals. Pressure and temperature are measured in the scattering chamber and used to calculate scattering by air molecules, which is then subtracted from total scattering to determine scattering by aerosol particles. Calibration of the nephelometer was carried out every three months using CO2 and filtered air. Due to Nephelometer design limitations, measurements do not cover the full (0°–180º) angular range, and scattering data need correction [Anderson and Ogren, 1998; Quirantes et al., 2008]. In this study, nonidealities due to truncation errors were corrected using the method described by Anderson and Ogren  that account for the particle-size dependence of the truncation error through the measured wavelength dependence of light scattering.
 Particle size distributions and aerosol concentrations were measured by means of an Aerodynamic Aerosol Sizer (APS-3321, TSI). This instrument is an optical particle counter design to measure in real time aerosol number density and particle aerodynamic diameter in the range 0.50–20 µm using 52 nominal-sized bins. The minimum and maximum number densities that this instrument can measure are 0.001 and 10,000 particles/cm3, respectively. For solid particles, counting efficiencies range from 85% to 99% [Volckens and Peters, 2005]. The APS was operated at a flow rate of 5 L min−1 and an averaging time of 5 min.
 Aerosol absorption coefficients are determined by means of a Multi-Angle Absorption Photometer (MAAP). This instrument simultaneously measures radiation transmitted through and scattered back from particles deposited on a filter, and uses radiative transfer calculations to determine the aerosol absorption coefficient to correct for errors that occur in other conventional instruments [Petzold and Schonlinner, 2004]. The MAAP draws the ambient air at constant flow rate of 16.7 l min–1 and provides 1 min values. In this study, ω0 have been calculated using the scattering and the absorption coefficients from nephelometer and MAAP measurements, respectively [Titos et al., 2012].
 Moreover, chemical analysis was performed with a high-volume sampler (flow rate 30 m3 h–1 for sampling PM10 (CAV-A/MSb) using quartz fiber filters. The sampling period was 24 h starting at 07:00 UTC. More details about the sampling procedure and the chemical analysis can be found in Titos et al. .
3 Results and Discussion
 The volcanic plume arrived over the Iberian Peninsula for the first time on 5 May 2010. During this time, a high-pressure system located south of Iceland and west of Ireland, and a low-pressure system over southern France caused the flow of the volcanic plume toward the Iberian Peninsula [Revuelta et al., 2012; Sicard et al., 2012]. The Lagrangian particle dispersion model FLEXPART was used to simulate the dispersion of volcanic aerosols [Stohl et al., 1998, 2005] and indicated the presence of volcanic ash in the south of the Iberian Peninsula during the period from 5 to 13 May [Toledano et al., 2012]. Volcanic particles were observed with different instrumentation at Granada (Spain) during this period. The weather was quite unstable during these days, characterized by the presence of clouds and rain and therefore Raman lidar observations were available only sporadically. In this paper, we focus on optical and microphysical properties of the volcanic plume in the lower troposphere retrieved during the night from 7 to 8 May when weather conditions allowed this complex characterization.
3.1 Sun- and Star-Photometer Observations
 Before we discuss the lidar retrievals obtained during this event, we begin with an overview of the situation as observed with photometers. Figure 1 shows the evolution of AOD at 440 and 436 nm measured by the Sun- and star-photometer, respectively. The same figure shows the Ångström exponent (AE) values calculated in the wavelength ranges 440–870 nm and 440–880 nm for the Sun- and star-photometer, respectively. There was an evident increase of AOD after 09:00 UTC on 7 May 2010. Nighttime values were about 0.42–0.45. The AE was in the range of 1.2–1.3, indicating a predominance of fine-mode particles. After sunrise on 8 May, AODs decreased significantly while the AE presented values larger than 0.9 during all the day. The gap in the Sun-photometer data from 9 to 15 UTC was due to the presence of clouds.
 Column-integrated volume size distributions retrieved using the AERONET inversion algorithm show a clear increase in the fine-mode volume concentration between the morning of 7 May and the morning of 8 May. However, the coarse-mode volume concentration shows only a slight increase, indicating that the change in the presence of large particles is negligible during this period (Figure 2a). Similar behavior in the size distribution was observed at ground level in Madrid during this volcanic event. This increase in the aerosol fine-mode was in coincidence with an increase in ambient sulphate concentration [Revuelta et al., 2012]. According to 5 day backward trajectories (Figure 2b, HYSPLIT model [Draxler and Rolph, 2003]) and available information on the chemical composition of the particles at ground level at our station, we hypothesize that this increase in the fine-mode is caused by sulphates and sulphuric-acid particles associated with the volcanic plume.
3.2 Raman Lidar Measurements
 Figure 3 provides an overview of the lidar observations performed during the night from 7 to 8 May in Granada. A multilayered structure is evident. A decoupled lofted aerosol layer, which originated in the volcano area according to backward trajectory analyses [Sicard et al., 2012], was observed around 3 km asl. This layer descended during the night without being mixed with the aerosol in the planetary boundary layer (PBL). Another aerosol layer underneath subsided and was mixed into the residual aerosol layer during the night. Patches of low clouds were present during some time intervals perturbing the lidar observations at heights above 2 km asl.
 Extensive and intensive vertically resolved aerosol properties changed during the night indicating a temporal evolution of the vertical distribution of the aerosol load and a possible change in chemical composition. Figure 4 shows aerosol optical properties at the beginning and the end of the nighttime lidar observations. During the first measurement period (22:30–23:20 UTC on 7 May) the backscatter and extinction profiles show the presence of aerosol particles up to 3.5 km height asl, with a decoupled layer between 2.7 and 3.5 km asl. Values of approximately 1·10–5 m–1 sr–1 and 3.8·10–4 m–1 for the backscatter and extinction coefficients, respectively, were retrieved in the center of this layer. Lidar ratios are very similar at both wavelengths (355 and 532 nm), reaching values around 50 sr in the center of the layer. These values are in agreement with values observed for stratospheric particles during other volcanic eruptions [Mattis et al., 2010]. Moreover, similar values were also found in the troposphere at other stations during this volcanic event. However, the low δp (5.0 ± 0.1 %), which is much lower than the one observed at other EARLINET stations in central, south, and southeastern Europe during the volcanic outbreak [Ansmann et al., 2010; Emeis et al., 2011; Mona et al., 2011; Papayannis et al., 2012] suggests a dominance of spherical particles at our station. Using the depolarization information it was possible to obtain the contributions of nonash (fine fraction) and ash particles (coarse fraction) to total particle backscatter and extinction coefficients [Ansmann et al., 2011]. These results indicated that almost 82% of the particles in this plume correspond to the fine-mode aerosol fraction. The α-AE in the range from 355 to 532 nm reached values around 0.7 ± 0.1 whereas larger values around 1.1 ± 0.2 were observed for β-AE at 355–532 nm. The β-AE at 532–1064 nm in this layer is 2.1 ± 0.1 indicating that the backscatter coefficient changes more sensitively at longer wavelengths. A similar behavior has been observed in other stations located in the Iberian Peninsula (Évora, Portugal) [Sicard et al., 2012]. All these results support the results obtained with the CIMEL Sun photometer and suggest that the volcanic aerosol plume arriving at our station was mainly composed of fine-mode particles, namely volcanic sulphuric acid droplets and sulphates.
 Figure 3 shows that the lofted layer subsided during the night, although it remained decoupled from the aerosol layers in the PBL. The values of backscatter and extinction coefficients observed for this layer at the end of the night (Figure 4, bottom) are similar to those registered during the previous period. A similar behavior was observed for the lidar ratios and the α-AE (355–532 nm). However, some changes were observed in the β-AEs. The values of β-AE (355–532, 532–1064) showed a slight decrease (0.7 ± 0.1 and 1.7 ± 0.3, respectively), which suggests an increase in the particle size.
 According to the large AEs and the low δp of this aerosol layer, which indicate a predominance of spherical particles, it was feasible to retrieve microphysical aerosol properties using the retrieval scheme proposed by Ansmann and Müller [2005b]. Wandinger et al.  for example showed that trustworthy values of effective radius can be derived even if δp are as large as 7%. Particle effective radius was retrieved for this lofted aerosol layer in the two intervals marked in Figure 4. Effective radius was 0.32 ± 0.14 µm at the beginning of the observational period and 0.55 ± 0.13 µm at the end. This result is in agreement with the changes detected in β-AE. These findings together with the negligible changes in δp (from 6 to 7%) suggest that the increase in the size of the dominant particles was not a result of increasing contribution by coarse-mode particles of volcanic origin but likely an increase of the size of particles in the accumulation mode. These results are confirmed by the CIMEL columnar retrieval that evidences a broadening of the particle size distribution toward large particles in the accumulation mode (Figure 2a). An unusually large accumulation mode was also observed during the low-explosive phase of this volcanic event at the Mace Head Atmospheric Research Station, Ireland (53.3°N, 54.2°W) [O'Dowd et al., 2011]. In this phase, a very important increase in non-sea-salt sulphate mass was observed by in situ instrumentation at this station. Similar results were also obtained from AERONET measurements in 1993 when Mt. Pinatubo volcanic sulphate aerosols were present in the stratosphere. Size distribution retrievals showed an unexpected middle mode peak at ~0.6 µm radius [Eck et al., 2010]. All these observations are in agreement with our results and support that mainly volcanic sulphate particles arrived at our station.
 The effective radii determined in this work for the lofted aerosol layer are in the range of values observed with lidar for stratospheric aerosol layers after the eruptions of Mt. Pinatubo [Chazette et al., 1995; Wandinger et al., 1995] and El Chichon [Chazette et al., 1995].
 A second lofted aerosol layer has been studied. This layer presents a stronger subsidence than the previous layer descending from 2.0 to 1.5 km asl during the observation period (Figure 3). Associated to this subsidence process, the layer became narrower and the peak values of the backscatter and extinction coefficients increased (Figure 4). Some additional features differentiate this layer from the upper one. For the period from 22:30 to 23:20 UTC, β-AE (355–532 nm) and α-AE are 1.7 ± 0.1 and 0.8 ± 0.1, respectively, which are larger than the values we found for the upper layer. However, an opposite tendency was observed for β-AE (at 532–1064 nm) that reached a value of 1.4 ±0.2 in the lower layer. Lower δp (around 4%) than those measured in the upper layer were also detected, suggesting an even lower contribution of nonspherical particles. There were some changes in the mentioned properties during the night, which was likely associated to the subsidence of the aerosol layer. We find a negligible decrease in α-AE and an evident decrease in β-AE (at 355–532 nm and 532–1064 nm), especially for the latter pair of wavelengths. In spite of these changes, the lidar ratios were rather constant during the night (around 55 sr at 355 nm and 75 sr at 532 nm). The effective radius at the beginning of the night (0.30 ± 0.11 µm) was slightly lower than the value retrieved for the upper layer. It showed an increase (up to 0.39 ± 0.10 µm) during the night, which was clearly lower than the increase occurring in the upper layer. Table 1 presents the mean optical and microphysical properties for both layers retrieved in the two time intervals.
Table 1. Optical and Microphysical Properties for Two Volcanic Layers Retrieved During in Two Time Intervals During the Night From 7 to 8 May
0.30 ± 0.11
1.7 ± 0.1
1.4 ± 0.2
55 ± 2
0.39 ± 0.10
1.4 ± 0.1
0.7 ± 0.1
54 ± 5
0.32 ± 0.14
1.1 ± 0.2
2.2 ± 0.1
48 ± 14
0.55 ± 0.13
0.7 ± 0.1
1.7 ± 0.3
47 ± 7
3.3 Optical and Chemical Analysis From In Situ Instrumentation
 An unexpected increase of scattering coefficient at 450, 550, and 700 nm was measured in the surface boundary layer during the night of 7–8 May with an integrating nephelometer (Figure 5, bottom). Large scattering-related AE (~2) was observed for these measurements. According to the ω0 retrievals (Figure 5, bottom), this aerosol was less absorbent than the typical aerosol in our urban station [Lyamani et al., 2010]. An increase in fine-mode particles (Figure 5, top) was also observed with an aerodynamic aerosol sizer.
 On the other hand, chemical analysis was performed with a high-volume sampler (flow rate 30 m3 h–1 for sampling PM10 (CAV-A/MSb) using quartz fiber filters. The sampling period was 24 h starting at 7:00 GMT. The PM10 fraction showed a significant increase of nonmarine sulphate particles (SO42–) during the period from 6 to 8 May. The concentration of sulphate particles changed from 2.9 µg/m3 on 6 May to values as large as 4.8 µg/m3 on 8 May. This increase was combined with a decrease of nitrates and organic and elemental carbon, thus suggesting the nonanthropogenic origin of these sulphates (Figure 6a). The observed value of sulphate concentration on 8 May (Sunday) is significantly higher than the mean value (3 µg/m3) obtained at our station in the period 2006 to 2010 from available chemical analysis during weekends. It is interesting to note that the increase of the sulphate particle concentration was also observed in the remote regional background European Monitoring and Evaluation Programme (EMEO) station Víznar (37°14′N, 03°28′W and 1260 m asl), which is located 6 km northeast of the city of Granada. The sulphate concentration for this background station ranged from 0.34 µg/m3 on 5 May to as high as 1.22 µg/m3 on 8 May. Low values were observed again on the following days (0.29 µg/m3 on 9 May).
 This last information from in situ measurements offers additional insight into the nature of the lofted aerosol layers. In principle the slight differences in their optical and microphysical properties and the corresponding evolution can be explained by the fact that the upper layer was likely composed of a mixture of sulphuric acid droplets and sulphates. However, in the lower layer the sulphuric acid droplets were neutralized when the layer descended and reacted with anthropogenic particles contained in the residual nocturnal layer. As a result of this reaction the composition of this layer was mainly of sulphates. This could explain the differences in the increase of effective radius during the night. On the one hand, the effective radius of the particles in the upper layer was almost two times larger than the one determined for the lower layer. This could have been caused by the relative humidity. Forecast of relative humidity on the basis of model analysis data issued by the NOAA shows values close to 80% in the upper level. The absence of sulphuric acid droplets could be the reason for the smaller increase of the effective radius of around 50% in the lower aerosol layer [Biskos et al., 2009].
3.4 Aerosol Classification
 Scatter plots of intensive aerosol properties are presented in Figure 7. Intensive properties are appropriate to distinguish different types of aerosols [Groß et al., 2011]. Figure 7a shows effective radius versus the lidar ratio at 355 nm retrieved for different layers. A total of 18 inversions of optical data into microphysical particle properties were performed successfully during the selected period. Up to now, just a few studies relating microphysical and optical properties of volcanic sulphuric droplet particles have been performed. For comparison reasons, our results are presented together with the values for fresh smoke particles measured at Granada [Alados-Arboledas et al., 2011] and stratospheric volcanic particles observed after the Mt. Pinatubo [Chazette et al., 1995; Wandinger et al., 1995] and El Chichon [Chazette et al., 1995] eruptions. We can see that effective radii of the volcanic particles observed at our station present a large portion of values than overlap with those observed for stratospheric particles (Mt. Pinatubo and El Chichon). The highest effective radii observed in Granada, which exceed the range observed for stratospheric particles, could be due to a greater presence of sulphates (larger particles) in the troposphere, while the presence of sulphuric acid droplets (smaller particles) is more important in the stratosphere. In this plot, we also observe that fresh smoke particles present effective radii values lower than tropospheric volcanic particles measured in our station. A large variability of the lidar ratio at 355 nm is observed for the volcanic particles observed at Granada, ranging from 34 to 67 sr. The lidar ratios of the sulphate particles measured at the Granada station are larger than the lidar ratios of the Mt. Pinatubo sulphate particles. The larger values could be explained by the fact that the sulphate particles measured at Granada are mixed with anthropogenic particles. This hypothesis is supported by the fact that larger values of lidar ratio are observed below and above the volcanic layers (Figure 4), indicating the presence of anthropogenic particles at those altitudes.
 Figure 7b presents the scatter plot δp versus lidar ratio at 355 nm for our measurements of the volcanic event and the volcanic ash observed over central Europe. For comparison purposes, we also show results of measurements of marine aerosols, Saharan dust and biomass burning aerosols observed during the SAMUM field experiments in 2006 and 2008 [Groß et al., 2011]. The volcanic sulphate particles are clearly distinguishable from other aerosol types; particularly remarkable are the large differences between the ash particles observed in central Europe and the sulphate particles measured in our station. The sulphate particles show much lower δp values indicating a more spherical particle shape compared to the other aerosol types. However, the lidar ratio is not so useful to differentiate the sulphates from the other types of particles. The values of the volcanic sulphate particles observed at Granada overlap with the values measured for the other aerosols displayed in Figure 7.
 We present a study of optical and microphysical properties of sulphate and sulphuric-acid particles that originated from the Eyjafjallajökull volcanic eruption in Iceland in 2010. This is the first time that both optical and microphysical characterizations of these particles have been performed in the lower troposphere by a multiwavelength Raman lidar. The observations were complemented with star- and Sun-photometer operated at the lidar site. On 7 May, an increase in the AOD at 440 nm was observed, reaching maximum values (around 0.45) during nighttime. The corresponding AE was in the range of 1.2–1.3, indicating a predominance of the fine-mode particles. An increase in the accumulation mode was also observed between the morning of 7 May and the morning of 8 May. Lidar observations allowed us to characterize two volcanic layers during the night from 7 to 8 May 2010. An upper lofted layer subsided during the night without being mixed with the aerosol in the PBL, while a lower layer presented a stronger subsidence and reached the residual nocturnal layer. No significant changes in the lidar ratios were observed during the night for both layers. The values were around 55 and 75 sr at 355 and 532 nm, respectively, for the lower layer, and around 55 sr at both wavelengths for the upper lofted layer. However, a decrease of the β-AE in both layers was evident. The particle effective radius for the upper lofted layer changed from 0.32 ± 0.14 µm at the beginning of the night to 0.55 ± 0.13 µm at the end of the night. For the lower layer the values changed from 0.30 ± 0.11 µm to 0.39 ± 0.10 µm. The increase of effective radius during the night could be due to hygroscopic growth because relative humidity was forecasted to be comparably high (close to 80 %) in both layers. The larger increase in the upper lofted layer could be explained by the fact that this layer contained a higher concentration of sulphuric acid droplets, which have stronger hygroscopic growth, whereas the lower layer may have contained a larger fraction of sulphate particles. The increase of sulphates at ground level was observed by in situ instruments and they were characterized as small and less absorbent particles. The volcanic layers had a rather low δp value (4–7 %), which indicates the presence of spherical particles. These values are clearly lower than the depolarization ratios observed for other types of aerosols.
 This work was supported by the Andalusia Regional Government through projects P08-RNM-3568 and P10-RNM-6299, by the Spanish Ministry of Science and Innovation through projects CGL2011-16124-E, CGL2010-18782, CSD2007-00067 and ChArMEx-SP2 (CGL2011-13580-E/CL); and FEDER funds under the Complementary Action CGL2011-13580-E/CLI; and by EU through ACTRIS project (EU INFRA-2010-1.1.16-262254) This work was also funded by the Korea Meteorological Administration Research and Development Program under grant CATER 2009-3112 and CATER 2012-7080. The authors express gratitude to the NOAA Air Resources Laboratory (ARL) and Naval Research Laboratory for the HYSPLIT transport and dispersion model and the NAAPS aerosol maps.