JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D15, 4248, /2001JD001110, 2002

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D15, 4248, /2001JD001110, 2002 European pollution outbreaks during ACE 2: Microphysical particle properties and single-scattering albedo inferred from multiwavelength lidar observations Detlef Müller and Albert Ansmann Institute for Tropospheric Research, Leipzig, Germany Frank Wagner 1 Meteorological Institute, University of Munich, Munich, Germany Kathleen Franke and Dietrich Althausen Institute for Tropospheric Research, Leipzig, Germany Received 27 June 2001; revised 14 November 2001; accepted 30 November 2001; published 6 August [1] We present vertically resolved physical particle properties and the single-scattering albedo at 532 nm of pollution plumes advected from the European continent out over the Atlantic Ocean. The parameters follow from the inversion of optical data, which were obtained from six-wavelength aerosol lidar observations near Sagres (37 N, 9 W), Portugal, in the framework of Aerosol Characterization Experiment 2 (ACE 2) (June July 1997). Particle effective radii were 0.15 ± 0.06 mm, volume concentrations ranged from 6 to 27 mm 3 cm 3, and surface area concentrations were 80 to 1200 mm 2 cm 3. The particles in general showed negligible absorption, with the mean imaginary part of the wavelengthindependent complex refractive index being at 0.009i ± 0.010i. The mean real part was 1.56 ± A mean value of 0.95 ± 0.06 was obtained for the single-scattering albedo at 532 nm. The numbers indicate that the major contributor to the observed pollution was nonabsorbing ammonium-sulfate-like material. A very small fraction was contributed by absorbing sootlike material. Correlation analysis showed that effective radius was well correlated with the Ångström exponents of the underlying extinction spectra. Furthermore, correlations of the single-scattering albedo with the particle extinction-to-backscatter ratio and with the imaginary part of the complex refractive index were found. This result shows that the first two parameters contain information about the chemical state of the observed particles. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere constituent transport and chemistry; 1630 Global Change: Impact phenomena; KEYWORDS: ACE 2, European pollution, inversion, lidar, microphysical particle properties, single-scattering albedo 1. Introduction [2] Ansmann et al. [2001] and Ansmann et al. [2002] reported on six-wavelength lidar observations of European pollution outbreaks during the Aerosol Characterization Experiment 2 (ACE 2; 16 June to 25 July 1997) [Raes et al., 2000; Russell and Heintzenberg, 2000]. The field site was in Sagres, Portugal (37 N, 9 W), at the southwestern tip of the European continent. [3] Ansmann et al. [2001] presented general geometrical characteristics and backscatter and extinction properties of the pollution plumes. Ansmann et al. [2002] gave a detailed characterization in terms of spectrally resolved backscatter and extinction spectra, including Ångström exponents. In 1 Also at Ground Truth Center Oberbayern, Germering, Germany. Copyright 2002 by the American Geophysical Union /02/2001JD this paper we report on the physical particle properties, which were inferred from these optical data sets by means of an inversion scheme. It is for the first time that an extensive parameter set of physical particle properties and the single-scattering albedo on a vertical scale is presented for European pollution plumes. The importance of such parameter sets for the assessment of climate forcing by anthropogenically produced particles is given by Intergovernmental Panel on Climate Change (IPCC) [2001]. [4] The following section briefly describes the inversion scheme. Section 3 presents the measurements. In section 4 we discuss the findings. Section 5 closes with a summary. 2. Methodology 2.1. Inversion Scheme [5] The physical particle parameters are derived with an inversion scheme, which has been specifically designed for AAC 3-1

2 AAC 3-2 MÜLLER ET AL.: EUROPEAN POLLUTION OUTBREAKS DURING ACE 2 the processing of the optical data provided by the sixwavelength aerosol lidar [Althausen et al., 2000; Ansmann et al., 2002]. A detailed description of the inversion algorithm can be found elsewhere [Müller et al., 1999a, 1999b, 2000]. The algorithm numerically solves the mathematical equations, which relate the optical data to the underlying physical quantities: g i ðlþ ¼ Z 1 0 K i ðr; m; l; s Þ 3 4r vr ðþdr i ¼ a; b: ð1þ [6] The parameter g b (l) denotes the backscatter coefficient at wavelength l; g a (l) is the corresponding extinction coefficient. The term v(r) describes the volume concentration of particles per radius interval dr. K b (r, m, l, s) and K a (r, m, l, s) are the kernel efficiencies of backscatter and extinction for single particles, respectively, and may be calculated from Mie-scattering theory [Bohren and Huffman, 1983]. The efficiencies depend on the radius r of the particles, on their complex refractive index m, on the wavelength l of the interacting light, and on the shape s of the particles. Because of the relatively small mean size of the particles considered in this study, scattering effects by nonspherical particles may be neglected [Mishchenko et al., 1997]. [7] Input parameters in the inversion are backscatter coefficients at 355, 400, 532, 710, 800, and 1064 nm, as well as extinction coefficients at 355 and 532 nm. Equation (1) thus describes a system of eight integral equations, which are solved numerically for the investigated volume concentration distribution. The applied technique is Tikhonov s regularization method with constraints [Tikhonov and Arsenin, 1977]. Regularization, which is done with generalized cross-validation [Craven and Wahba, 1979; Golub et al., 1979], is necessary in order to stabilize the numerical solution process. Otherwise, marginal uncertainties in the input data would cause the mathematical system to give physically senseless solutions. The applied contraints are positivity and smoothness of the derived volume concentration distributions. [8] In the first step the volume concentration distribution in equation (1) is approximated by a linear combination of eight weighted base functions, which have a triangular shape on a semilogarithmic scale. These base functions are located next to each other on the radius grid. In this way they define a so-called inversion window. The weight factors then are determined for each of the base functions. The width and position of the base functions are varied. In this way, 50 different inversion windows within the size range from 0.01 to 10 mm are tested for the reconstruction of the particle size distribution. In addition, 420 different wavelength- and sizeindependent complex refractive indices, which are given by the kernel functions in equation (1), are tested for the reconstruction. The real part takes values of 1.33, 1.35 to 1.8 in steps of The imaginary part takes values of 0, to 0.01 in steps of , 0.01 to 0.1 in steps of 0.01, and 0.1 to 0.7 in steps of 0.1. In summary, 21,000 inversions for each optical data set are done. [9] The volume concentration distributions that comply with the constraints define the preliminary solution space. In general, this solution space consists of only a few hundreds out of the 21,000 possible solutions. In combination with the respective complex refractive indices the distributions are then used for Mie-scattering calculations at the wavelengths of the aerosol lidar. The final solution space then is determined from the comparison of the backcalculated optical data to the input, experimental data. Depending on data quality, a deviation of 10 20% between the two data sets with respect to the backscatter and the extinction coefficients and approximately 10% with respect to the particle lidar ratios at 355 and 532 nm is taken as the limit for acceptable solutions. This final solution space in general consists of less than 100 out of the initial 21,000 solutions. The vertical profiles of the volume concentration distributions then allow one to calculate the respective profiles for effective radius, volume, and surface area concentration. Number concentration in general gives no reasonable results if mean particle size drops below approximately 0.15 mm. As this range was found in the present study no number concentrations will be given here. Mean values of the complex refractive index for the wavelength range from to mm implicitly follow from the accepted particle concentration distributions. Finally, the volume concentration distribution and the complex refractive index are used for the calculation of the profile of the singlescattering albedo Data Processing [10] A detailed description of the retrieval of the optical particle parameters has been given by Ansmann et al. [2002]. For the inversion of the optical data the respective profiles were split into layers of variable depth from 400 to 700 m. For each of these layers the individual data points of each profile were averaged. More than 30 different backscatter and extinction spectra, representing aerosol conditions on the three different pollution periods, were inverted. [11] Because of unfavorable measurement conditions and/ or bad data quality it was not possible to retrieve backscatter profiles at 355 and/or 1064 nm in each case presented below. Furthermore, the extinction channel at 355 nm was not available during ACE 2. Ansmann et al. [2002] presented Ångström exponents for the backscatter spectra in the wavelength range from 400 to 532 nm and from 532 to 800 nm. On the basis of these Ångström exponents the missing backscatter coefficients at 355 nm were obtained by respective extrapolation of the Ångström exponents in the wavelength range from 400 to 532 nm with the use of the well-known Ångström power law relation. The missing backscatter coefficients at 1064 nm were obtained by respective extrapolation of the Ångström exponents in the wavelength range from 532 to 800 nm. In combination with the Ångström exponents for the backscatter coefficients and model assumptions on the particle backscatter-to-extinction ratio for marine and continental particles in the measurement wavelength range [Ackermann, 1998], respective exponents for the extinction coefficients in the short- and long-wavelength range were derived [Ansmann et al., 2002]. The extinction coefficients at 355 nm then were generated by extrapolation as in the case of the backscatter coefficients. 3. Measurements 3.1. Case Study From 19 July 1997 [12] In the period from 16 June to 24 July 1997, four pollution outbreaks from the European continent were

3 MÜLLER ET AL.: EUROPEAN POLLUTION OUTBREAKS DURING ACE 2 AAC 3-3 Figure 1. Measurement from 0358 to 0445 UTC on 19 July (a) Profile of the backscatter coefficient at 532 nm (thick solid line), the Ångström exponents on the basis of the extinction coefficients for the wavelength range from 400 to 532 (open circles) and for the wavelength range from 532 to 800 nm (solid circles), and the particle lidar ratio at 532 nm (crosses). (b) Effective radius (solid circles) and single-scattering albedo (open circles). (c) Volume (solid circles) and surface area (open circles) concentration. (d) Real part (solid circles) and imaginary part (open circles) of the complex refractive index. Each plot also contains the profile of the backscatter coefficient. Vertical error bars denote the height ranges across which the optical data were averaged for the inversion. Accordingly, these error bars denote the height uncertainty for the derived parameters. Horizontal error bars denote statistical uncertainty. If no error bar is shown, only one solution was found from the inversion. Arrows for the imaginary part indicate that the lower end of the error bar is at 0. Read 1E 4 as observed [Ansmann et al., 2001]. Figure 1 shows the measurement example from 0358 to 0445 UTC on 19 July The optical properties have already been discussed in detail by Ansmann et al. [2002]. The observation was taken at the peak of the strongest pollution outbreak, which lasted from 17 July until 21 July 1997 [Ansmann et al., 2002]. According to backward trajectories, which are shown in Figure 2, the air masses were advected from western and central parts of the European continent at heights above 500 m the past 3 days prior to the lidar observations. The pollution plume reached from approximately 500-m to 3500-m height according to the profile of the particle backscatter coefficient in Figure 1. The relative humidity was less than 60% for heights below 1900 m. It steadily increased between and 2800-m height and took maximum values between 90% and 95% at 2850-m height [Ansmann et al., 2001]. Figure 1a shows the Ångström exponents for the wavelength range from 400 to 532 nm and from 532 to 800 nm and the lidar ratio at 532 nm for five height layers. The physical parameters are shown in Figures 1b 1d. [13] Figure 1b presents the effective radius for the respective height layers. It shows rather constant values around 0.25 mm from to 1500-m height and a sharp drop to approximately 0.1 mm at 2000-m height. After that, it again increases with height to approximately 0.15 mm. As pointed out by Ansmann et al. [2002], the height range of the sharp decrease of the effective radius coincided with a change in air mass. [14] The observed change in effective radius is quite well correlated with the change of the Ångström exponents shown in Figure 1a. At approximately 2000-m height the Ångström exponent in the long-wavelength range rapidly increases by a factor of approximately 2, indicating a considerable reduction of the concentration of large particles. At the same time the lidar ratio also sharply increases from values less than 50 sr to approximately 77 sr. The effective radius drops by a factor of more than 2 at this height compared with the values below 2000-m height. Above 2000-m height the effective radius slightly increases again. The Ångström exponent in the long-wavelength range varies only by about 15% for heights above 2000 m. However, the Ångström exponent in the short-wavelength range is reduced by 45% at heights above 3000 m compared to the values from 2000 to 3000 m. This change indicates a shift of smaller particles to the larger size range. [15] Figure 1c shows a large variability of volume and surface area concentration. Volume concentration increased from approximately 12 mm 3 cm 3 to 24 mm 3 cm 3 within the strong backscatter peak between and 3000-m height. After that, it decreased to approximately 7.5 mm 3 cm 3 in the secondary peak at 3200 m. Surface area concentration increased from 154 mm 2 cm 3 at the bottom of the pollution layer to 920 mm 2 cm 3 at 2000-m height and then decreased to 140 mm 2 cm 3. [16] The single-scattering albedo at 532 nm, which is shown in Figure 1b, indicates the presence of slightly to moderately polluted particles. Except for the thin particle layer around 2000-m height, values above 0.95 were found. The steep increase of the lidar ratio around 2000-m height is correlated with a significant reduction of the single-scattering albedo to 0.8. [17] The assumption that the lidar ratio as well as the single-scattering albedo are correlated with the absorption properties of the particles and thus with their chemical composition is further supported by an analysis of the complex refractive index. The imaginary part, which is shown in Figure 1d, is below 0.01i throughout most of the layer and reaches a relatively high value of 0.04i only

4 AAC 3-4 MÜLLER ET AL.: EUROPEAN POLLUTION OUTBREAKS DURING ACE 2 Figure 2. Three-day backward trajectories with arrival time at 0000 UTC on 19 July Numbers indicate arrival height above lidar site. Trajectories with arrival time at 1200 UTC showed no significant differences. The trajectories were provided by the Divisão de Previsão Numerica, Instituto de Meteorologia, Processamento e Arquivo, Lisbon, Portugal. within the thin layer at 2000-m height. Next to marine particles, ammonium-sulfate-like material possesses a negligible imaginary part [d Almeida et al., 1991; Seinfeld and Pandis, 1991; Tang and Munkelwitz, 1994; Hess et al., 1998]. This observation also holds for soil-derived dustlike particles. Imaginary parts in the range from 0.004i to 0.008i in the visible wavelength regions have been found from highly different locations on Earth [Grams et al., 1974; Patterson et al., 1977; Carlson and Benjamin, 1980; Patterson and Marshall, 1982; von Hoyningen-Huene et al., 1999]. The fact that in particular within the shallow layer at 2000 m a comparably large imaginary part was found suggests that some of the observed material consisted of absorbing sootlike particles. [18] The real part varies between 1.48 and Particles of marine origin in general have real parts below 1.5 [d Almeida et al., 1991; Seinfeld and Pandis, 1991; Tang and Munkelwitz, 1994; Hess et al., 1998]. The slightly larger values again point toward ammonium-sulfate-like material. Here as well, dustlike material may have contributed to the observed particle properties [Patterson et al., 1977; Patterson and Marshall, 1982] Time Series [19] Figure 3 presents the results for effective radius, single-scattering albedo, and complex refractive index for all evening observations of the major pollution outbreak from 17 to 20 July The air masses were advected from northeasterly and central parts of the European continent. The optical depth of the plumes ranged from 0.1 to 0.25 at 532 nm. The geometrical depth of the pollution layers was approximately 3000 m. Results for effective radius, surface area, and volume concentration from all ACE 2 measurements are summarized in Table 1. The optical properties were discussed by Ansmann et al. [2002]. [20] The values for the morning measurements on 20 July 1997 are lower than what has been been reported previously [Müller et al., 2000]. Further, the column-averaged complex refractive index decreased from i [Müller et al., 2000] to i. The single-scattering albedo increased from 0.84 ± 0.04 [Müller et al., 2000] to 0.89 ± The change in parameters is caused by the refinement of the retrieval method for the extinction coefficients [Ansmann et al., 2002]. As a result we obtained a significantly larger extinction coefficient at 355 nm and thus a considerably steeper spectrum for the extinction coefficients than the one used by Müller et al. [2000]. [21] During the major pollution period there was a notable shift of the effective radius to very low values of approximately 0.1 mm on the morning of 18 July 1997 and the evening of 20 July Large values above 0.2 mm were only found on 19 July Above heights of approximately 1500 m the Ångström exponent in the short-wavelength range was below 1.5 from 17 July to 19 July It showed a significant shift to larger values of almost 2 on 20 July The mean volume concentration gradually decreased from approximately 20 mm 3 cm 3 on 17 July 1997 to approximately 10 mm 3 cm 3 on 20 July 1997 (compare Table 1). On the vertical scale the peak variability of a factor of almost 3 was reached on the morning of 19 July 1997 (compare Figure 1). However, also in the other cases more than 30% change on the vertical scale was observed. Column-averaged surface area concentration was approximately 390 ± 140 mm 2 cm 3 on 17 July 1997 and reached its maximum value of 640 ± 350 mm 2 cm 3 on the morning of 18 July Surface area concentration remained at high levels and peaked once more at 552 ± 354 mm 2 cm 3 on the evening of 20 July As mentioned before, that day showed extremely low effective radii. On the vertical scale a change of surface area concentration by % was observed. A maximum change by a factor of 6 within the plume occurred on the morning of 19 July [22] For completeness, Figure 4 presents two more outbreaks, which were observed on 23 June and 12 July 1997.

5 MÜLLER ET AL.: EUROPEAN POLLUTION OUTBREAKS DURING ACE 2 AAC 3-5 which was approximately 1500-m deep, was 0.05 at 532 nm. Backward trajectories showed the advection of air from the Atlantic Ocean. On the day prior to the observation the air crossed western parts of the European peninsula, where it may have acquired particles from anthropogenic activity in the heavily industrialized area around Lisbon, which is approximately 200 km to the north of Sagres. On 12 July 1997 the air had been advected from northeasterly regions of the European continent. [24] The effective radius on 23 June 1997 was mm. A comparison to the Ångström exponents presented in Figure 8 of Ansmann et al. [2002] shows a rather low exponent in the short-wavelength range below approximately 1000-m height and thus also points to the rather large effective radius. During the remaining periods on 12 July 1997 and July 1997 the effective radius in general was below 0.2 mm. Accordingly, Ångström exponents were well above 1 [Ansmann et al., 2002]. With respect to the pollution plume a column-averaged volume concentration of approximately 8.7 ± 0.2 mm 3 cm 3 was found on 23 June Considerably larger values above 10 mm 3 cm 3 within the center of the respective plumes were found on 12 July 1997 and during the strongest periods from 17 to 20 July Figure 3. Time series of the effective radius, the singlescattering albedo, and the complex refractive index for the measurements in the evening of (a) 17 July 1997, (b) 18 July 1997, (c) 19 July 1997, and (d) 20 July The meaning of the symbols is the same as in Figure 1. The optical properties have been discussed by Ansmann et al. [2002]. [23] A comparably weak pollution outbreak was observed on 23 June The optical depth of the pollution layer, Table 1. Physical Particle Parameters and Respective Standard Deviations for Pollution Outbreaks Observed During ACE 2 a Date Height Range, m r eff, mm v, s, mm 3 cm 3 mm 2 cm 3 23 June 830 ± ± ± ± 0 23 June 1245 ± ± ± ± July 1410 ± ± ± ± July 2070 ± ± ± ± July 1260 ± ± ± ± July 1740 ± ± ± ± July 2490 ± ± ± ± July (m) 1140 ± ± ± ± July (m) 1590 ± ± ± ± July (m) 2250 ± ± ± ± July (m) 2970 ± ± ± ± July (e) 800 ± ± ± ± 0 18 July (e) 1245 ± ± ± ± July (e) 1735 ± ± ± ± July (e) 2230 ± ± ± ± July (e) 2750 ± ± ± ± July (m) 1035 ± ± ± ± July (m) 1630 ± ± ± ± July (m) 2100 ± ± ± ± July (m) 2695 ± ± ± ± July (m) 3240 ± ± ± ± July (e) 1425 ± ± ± ± July (e) 1890 ± ± ± ± July (e) 2590 ± ± ± ± 0 20 July (m) 775 ± ± ± ± July (m) 1395 ± ± ± ± July (m) 1970 ± ± ± ± July (m) 2750 ± ± ± ± July (e) 800 ± ± ± ± July (e) 1260 ± ± ± ± 0 20 July (e) 1760 ± ± ± ± July (e) 2245 ± ± ± ± July (e) 2845 ± ± ± ± 1 a Here, r eff denotes the effective radius, v denotes the volume concentration, and s denotes the surface area concentration. Dates labeled (m) are the results for the morning measurements, and those labeled (e) are the results for the evening measurements.

6 AAC 3-6 MÜLLER ET AL.: EUROPEAN POLLUTION OUTBREAKS DURING ACE 2 Figure 4. Time series of the physical parameters presented in Figure 3, but for the measurements on the evening of (a) 23 June 1997 and (b) 12 July The meaning of the symbols is the same as in Figure 1. The imaginary part at 830-m height on 23 June 1997 is 0. The imaginary part at 2070-m height on 12 July 1997 is ± observed pollution consisted of nonabsorbing material, i.e., a mixture of ammonium-sulfate-like material with some contribution from soil-derived material. The real part of the complex refractive index, however, tends to be larger than what was previously reported for particles having this chemical composition. In particular, at the end of the strongest pollution period on 20 July 1997 the real part shifted above 1.6 at all height levels. Backward trajectories show that approximately 1 2 days prior to the end of the observations the respective air masses had been around 500-m height for about 24 hours. As a consequence an enhanced share of crustal material, which possesses a rather large real part of the complex refractive index [Ishida and Price, 1996], may have been transported into the pollution plume. It is worth noting that a significant increase of the real part for dust particles compared to the one from a mixture of marine and aged-pollution particles was also found during aircraft-based observations performed in the framework of ACE 2 in the Canary Islands [Schmeling et al., 2000]. [28] Another reason for the rather large real parts may be related to the inversion procedure itself. Simulations with synthetic data have shown that in some cases the real part of the complex refractive index may be slightly overestimated [Müller et al., 1999b]. So far, this effect has not been shown to exceed the larger uncertainty given by the measurement errors, if large particles are present. However, if particle effective radii are in the range of approximately 0.1 mm, the inversion intrinsically starts to become unstable. In this case, simulations have shown that in particular the real part of the complex refractive index tends toward larger values, leading to an overestimation by approximately In this context it has to be pointed out that as a consequence also the imaginary part may be slightly overestimated. The reason for this behavior is that for individual solutions of the [25] With respect to surface area concentration the temporal changes were even larger, spanning a factor of 6. Again, on 23 June 1997 the column-averaged surface area concentration of approximately 110 mm 2 cm 3 was comparably low. Considerably higher values were reached after that. [26] The single-scattering albedo showed maximum values around 1 during the slightly polluted period on 23 June 1997, probably caused by considerable contribution of marine particles, and on 12 July During the remaining period the average value dropped to slightly lower values but in general remained above 0.9. The large values indicate the rather low imaginary part of the complex refractive index, found throughout the campaign (compare Figure 3 and Figure 4). The increasing state of pollution is reflected in larger values above 0.002i found for the pollution period from 17 to 20 July Maximum values were 0.04i on the morning of 19 July 1997 (compare Figure 1b). In particular, the real part of the complex refractive index, which ranged between 1.45 and 1.67, showed a notably shift toward larger values by the end of the campaign. Accordingly, the lidar ratio, which was above 40 sr on July 1997, showed a significant shift toward sr on 20 July [27] The large single-scattering albedos and average lidar ratios below 60 sr, which were found throughout the campaign, suggest that as in the case of 19 July 1997 most of the Figure 5. (a) Effective radius versus Ångström exponent for the short-wavelength range from 400 to 532 nm (open circles) and for the long-wavelength range from 532 to 800 nm (open triangles) for the pollution cases observed during ACE 2, and for polluted conditions in the short-wavelength range observed during INDOEX (solid circles). Parameters from linear regression analysis according to the formula y = a + bx are given for the ACE 2 cases [Ansmann et al., 2002] and INDOEX. The correlation coefficients are given in the text. (b) Effective radius versus particle lidar ratio at 532 nm for polluted conditions during ACE 2 (open circles) and during INDOEX (solid circles).

7 MÜLLER ET AL.: EUROPEAN POLLUTION OUTBREAKS DURING ACE 2 AAC 3-7 Figure 6. Single-scattering albedo at 532 nm versus (a) particle lidar ratio at 532 nm and (b) real part and (c) imaginary part of the complex refractive index. The meaning of the symbols and the regression parameters is the same as in Figure 5. The correlation coefficients are given in the text. Also shown are the results for the clean marine case from 20 June 1997 (solid squares) [Müller et al., 2000]. solution space for the complex refractive index, larger real parts are connected to larger imaginary parts (compare, e.g., Figure 4 of Müller et al. [2000]) Correlation Analysis [29] Figures 5 7 show results of a correlation analysis for the physical particle parameters including several optical parameters presented by Ansmann et al. [2002]. A simple linear regression fit was applied to each of the parameter sets, respectively. The correlation parameters are given in the respective legend of each figure. Also shown are results from the inversion of particle optical properties observed on 20 June 1997 under clean marine conditions [Müller et al., 2000]. Furthermore, a few results of six-wavelength lidar measurements performed during the Indian Ocean Experiment (INDOEX) [Müller et al., 2001a, 2001b] are included. INDOEX aimed at the characterization of pollution advected from the Indo-Asian continent out over the Indian Ocean. The parameters considered in Figures 5 7 represent polluted conditions observed on 18 February and 25 March 1999 and 22 March [30] Figure 5a shows that effective radius increases with decreasing Ångström exponents in both wavelength ranges. A correlation coefficient of approximately 0.6 was found in both cases. The strong correlation is explained by the fact that the spectral slope of the extinction coefficient, which determines the Ångström exponents, is mostly influenced by particle size. This finding is further supported if the results for INDOEX are considered. A correlation coefficient of almost 0.8 was found for the short-wavelength range. In addition, the clean marine case from 20 June 1997 possessed a comparably low Ångström exponent of 0.29 in the short-wavelength range. This value was related to a very large effective radius of approximately 0.64 mm [Müller et al., 2000]. [31] Figure 5b shows a wide scatter of effective radii from approximately 0.1 to 0.25 mm for the range of lidar ratios found for ACE 2. This result may be compared to Figure 12 of Ansmann et al. [2002]. In that case an uncorrelated behavior between the lidar ratio and the Ångström exponents was found. [32] With respect to the pollution plumes the largest effective radius was 0.31 mm, found on 23 June The lidar ratio for the mixture of anthropogenic and marine particles was 44 sr. The large effective radius of 0.64 mm, which was found for the clean marine case from 20 June 1997, was connected to a lidar ratio below 30 sr [Müller et al., 2000]. The overall scatter in the relationship of effective radius to lidar ratio is larger than for the correlation with the Ångström exponents and may be explained by the fact that the particle lidar ratio is also controlled by the particle complex refractive index. [33] According to Figure 6a the single-scattering albedo mostly was between 0.9 and 1.0 for the pollution plumes observed during ACE 2. For comparison the clean marine case from 20 June 1997 also showed a high single-scattering albedo of 0.98 [Müller et al., 2000]. The single-scattering albedo mostly was below 0.9 for the INDOEX observations. In contrast to the results from INDOEX, for which a correlation coefficient of almost 0.9 was found, the ACE 2 data showed no clear relationship between the lidar ratio and the single-scattering albedo. Figure 7. Particle lidar ratio at 532 nm versus (a) real part and (b) imaginary part of the complex refractive index. The meaning of the symbols is the same as in Figure 6.

8 AAC 3-8 MÜLLER ET AL.: EUROPEAN POLLUTION OUTBREAKS DURING ACE 2 [34] The relationship of the single-scattering albedo to the complex refractive index is given in Figures 6b and 6c. In the case of the clean marine case from 20 June 1997 a range of values has been reported by Müller et al. [2000]. For the presentation in Figures 6b and 6c we calculated the respective mean values. [35] Figure 6b shows that only for single-scattering albedos close to 1 the complete range of real parts was found. In contrast, decreasing single-scattering albedos successively reduced the range of real parts to values above 1.6. However, as mentioned before, the rather small effective radii that were observed during ACE 2 may have caused a systematic overestimation of the real part by approximately [36] Figure 6c shows a very strong correlation of the single-scattering albedo with the imaginary part; that is, the lower the single-scattering albedo, the higher the imaginary part. The correlation coefficient is approximately 1 for the ACE 2 as well as for the INDOEX results. The correlation is not affected by a possible systematic overestimation of the imaginary part as the result of the respective overestimation of the real part. Rather, there would be a parallel shift of the correlation function along the axis of the imaginary part toward lower values. [37] Figure 7 presents the correlation between lidar ratio and complex refractive index. Figure 7a shows a decrease of the real part with increasing lidar ratio for the ACE 2 data. A moderate correlation coefficient of approximately 0.5 was found. The INDOEX results showed a strongly variable lidar ratio for real parts in the range from 1.6 to 1.7. As in the case of the relationship of the single-scattering albedo to the lidar ratio (compare Figure 6a) there seems to be a different dependence for ACE 2 and INDOEX. The additional offset of the data point from the clean marine case also suggests that different aerosol types are responsible for these differences. Figure 7b shows a large scatter of lidar ratios for imaginary parts below approximately 0.015i, which represents the upper limit of most of the ACE 2 values. A low correlation coefficient of 0.41 was found. As in the case of the single-scattering albedo, we note an increasing correlation of lidar ratio and imaginary part, if the results from INDOEX are included. A very high correlation of almost 0.9 was found for the INDOEX results. Large imaginary parts in general only occur for large lidar ratios. In summary, the results for the dependence from the imaginary part once more emphasize that singlescattering albedo and lidar ratio contain information about the chemical properties of the observed particles. Table 2. Mean Values of the Physical Particle Parameters and the Respective Standard Deviations on the Basis of the Pollution Plumes Observed During ACE 2 a Parameter Value r eff, mm 0.15 ± 0.06 v, mm 3 cm ± 5.3 s, mm 2 cm ± 270 m real 1.56 ± 0.07 m imag ± 0.01 ssa 0.95 ± 0.06 a Here, r eff denotes the effective radius, v denotes the volume concentration, and s denotes the surface area concentration, while m real is the real part, m imag is the imaginary part of the complex refractive index, and ssa denotes the single-scattering albedo at 532 nm. 4. Discussion [38] The results will now be put into the context of other observations made during ACE 2. For this purpose, Table 2 presents the mean values of the derived particle parameters on the basis of all observations made during ACE 2. Further comparison will be given to results from the Tropospheric Aerosol Radiative Forcing Observation Experiment (TAR- FOX), which took place in the North Atlantic east of the United States [Russell et al., 1999a], the Indian Ocean Experiment (INDOEX), which took place in the Indian Ocean, to the west and south of India [Ramanathan et al., 2001], and the Smoke, Clouds, and Radiation-Brazil experiment (SCAR-B), which took place in the Amazon region [Kaufman et al., 1998]. [39] As outlined by Ansmann et al. [2001], the observed pollution plumes in general were confined to altitudes above the marine boundary layer. In situ observations were made at 900-m height atop of Mount Foia, which is approximately 50 km to the northeast of Sagres [Henning, 1998]. Particle size distributions were obtained from routine measurements with a twin differential mobility particle sizer, which covered particle diameters in the range from 3 to 800 nm. The measurements were done at less than 40% relative humidity. Respective analysis of backward trajectories and meteorological data showed that in the period from 17 to 20 July 1997 the in situ platform was well within the continental polluted air mass [Henning, 1998]. Plots of the particle volume concentration versus time for 20 July 1997 show that during the 24-hour measurement, volume concentrations were approximately 10 mm 3 cm 3 or higher. The peak value of 18 mm 3 cm 3 was reached on the night of 20 July to 21 July In this context it has to be observed that surface and hillside effects, for example, raising of dust, may have biased the in situ values. The numbers match very well the inversion results (compare Table 1). A mean volume concentration of approximately 10 mm 3 cm 3 was found for the column from 800- to m height on the morning of 20 July A slightly larger value was found for the evening measurement. It is worth noting that ambient relative humidity was less than 40% in this height range. Therefore differences in the in situ results because of relative humidity effects should be negligible. [40] The in situ data further show that the maximum volume concentration was reached for a particle radius of mm. The peak of the number concentration was in the range of mm in particle radius. Another peak was detected at approximately 0.01 mm, but with number concentrations of more than 1 order of magnitude lower. Effective radius was not provided by the author. For a first approximation, calculations with simple monomodal logarithmic-normal distributions were done. Input parameters were varied such that maximum number and volume concentrations were reached within the above given size ranges of number and volume concentration. In that case, effective radii turned out to be in the range of mm. For comparison the inversion result for the relevant height range was 0.11 mm, which is within the limits of the in situ observations.

9 MÜLLER ET AL.: EUROPEAN POLLUTION OUTBREAKS DURING ACE 2 AAC 3-9 [41] Only for 6 July 1997, aircraft observations provide additional information on the properties of pollution plumes along the continental rim of Europe [Flamant et al., 2000]. A direct comparison to the results presented here is not possible because of failure of one of the lasers of the lidar on that day [Ansmann et al., 2002]. The aircraft carried one lidar and in situ instruments. The discussion concerning the optical particle properties derived in this study is given by Ansmann et al. [2001]. The in situ observations revealed a multimodal particle size distribution. Within the center of the plume the majority of particles were found to have particle radii well below 0.15 mm. This observation also is consistent with the low mean value of 0.15 ± 0.06 mm, which was found in this study. Further conclusion from the aircraft observations was that the observed particles were rather low absorbing. This finding also is in agreement with the low mean value of 0.009i ± 0.010i derived from the lidar observations and the observations atop Mount Foia. [42] Further ACE 2 platforms at the Sagres field site were restricted to surface observations [Carrico et al., 2000]. Consequently, only a polluted marine boundary layer, which most likely was predominantly affected by local pollution sources, was observed. Shipborne observations near Sagres also were restricted to the surface [Novakov et al., 2000a, 2000b; Quinn et al., 2000]. Nevertheless, we compare our measurements with these surface observations. Airborne observations of polluted conditions were done near the Canary islands [Collins et al., 2000; Öström and Noone, 2000; Schmeling et al., 2000], thus making a comparison rather difficult because of particle-aging effects along the track from Europe to the Canary field site and possible mixture with marine particles and desert dust from the Sahara. In all cases of in situ observations, particles were investigated in their dry state, i.e., depending on measurement technique, in the range from 30 to 55% relative humidity, thus requiring corrections to ambient conditions. [43] A mean value of 0.95 ± 0.06 for the single-scattering albedo was found from the multiwavelength lidar observations. Ambient relative humidities ranged from 30 to 85% [Ansmann et al., 2001]. On the basis of nephelometer and aethalometer measurements, Carrico et al. [2000] found single-scattering albedos of 0.93 ± 0.05 at 550 nm for clean periods and 0.94 ± 0.03 for polluted periods at the surface. These numbers refer to particles with a maximum of 10 mm in diameter and a relative humidity of 27%. Transformation to a relative humidity of 82% insignificantly increased both single-scattering albedos to approximately [44] Similar high single-scattering albedos at 550 nm have been reported for continentally influenced air masses from ship-based in situ observations in the ACE 2 area [Quinn et al., 2000]. The measurements were done at a relative humidity of 55%. If air masses originated from western Europe and the Iberian Peninsula, single-scattering albedos were 0.95 ± Lower values of 0.90 ± 0.03 were found at the surface if the air flow was from the Mediterranean area. According to Quinn et al. [2000], the ACE 2 continental values ranged from 0.81 to 0.99, which is in agreement with the variability reported here. For comparison, 0.98 ± 0.01 was found for marine air masses. On the basis of chemical analysis from filter measurements aboard a ship, Novakov et al. [2000a] conclude that contribution by carbon-like material was rather low for the continental air masses. However, as pointed out, these values do not reflect the characteristics within the plumes aloft. [45] Air masses that had originated from the Iberian Peninsula and the British Isles were observed by aircraft in the area around the Canary Islands. In situ observations within the marine anthropogenically polluted boundary layer were reported for 2 days, i.e., 8 July 1997 [Öström and Noone, 2000] and 18 July 1997 [Schmeling et al., 2000]. In general, the transport time of air masses from the western parts of the European continent to the Canaries was on the order of 2 3 days. Measurements were made below 1000-m height. Wavelength-dependent complex refractive indices were calculated on the basis of chemical analysis of particles collected on board and of complex refractive indices of the identified components found in literature [Schmeling et al., 2000]. [46] The real part of the complex refractive index in the study by Schmeling et al. [2000] turned out to be a little less than 1.4 for the visible wavelength range. As pointed out in section 2 the inversion provides a mean wavelength-independent value. The values found from the lidar observations consistently are larger, ranging from approximately 1.37 to almost Table 2 shows that the mean of these values is 1.56 ± As mentioned in section 3.2 an overestimation of for the real part cannot be completely ruled out in the inversion if particle radii are in the range of 0.1 mm or below. On the other hand, aging and mixing effects with marine particles may have led to the considerably lower values several thousand kilometers off the European continent. Table 2 shows that the mean imaginary part in this study is 0.009i ± 0.010i. The results based on the in situ observations also showed negligible absorption with values below 0.005i [Schmeling et al., 2000]. [47] On the basis of the complex refractive indices and the measured particle size distributions, single-scattering albedos were calculated for internal and external mixtures [Schmeling et al., 2000]. Numbers were close to 1. Measurements with nephelometer and particle soot photometer were used to derive single-scattering albedos in their dry state [Öström and Noone, 2000]. Taking account of hygroscopic growth on the basis of measured growth factors, the authors come up with values of at 90% relative humidity in the wavelength range around 550 nm. This range of values is close to the findings presented here (compare Table 2). [48] We found good agreement of the results reported here to aircraft observations made during TARFOX. This experiment provided detailed information on the chemical and physical state of pollution from highly industrialized regions of the North American continent. Given the fact that European industry is quite similar, one should also expect similar composition of the particles for the European pollution outflow. [49] Effective radii reported here are only slightly smaller than those found during TARFOX. With respect to the complex refractive index, real parts of in the visible wavelength range were reported [Russell et al., 1999b]. As with other ACE 2 results these values also are lower than the ones found in this study. Imaginary parts were lower than 0.017i, which is well within the range of imaginary parts provided in this study. On the basis of aircraft observations within the pollution plumes advected

10 AAC 3-10 MÜLLER ET AL.: EUROPEAN POLLUTION OUTBREAKS DURING ACE 2 out over the ocean, single-scattering albedo values of at 550 nm were found [Russell et al., 1999b]. These values are in the lower range of numbers reported here and indicate a slightly larger contribution of absorbing sootlike material to the outflow from the North American continent. [50] There are considerable differences with results from INDOEX and SCAR-B. In these cases the particles resulted from less sophisticated industrial and/or combustion processes that tend to provide a higher carbon content, for example, biomass burning. Average volume concentrations of 21 ± 8 mm 3 cm 3 and surface area concentrations of 420 ± 160 mm 2 cm 3 of pollution plumes observed during INDOEX are only a little larger than for ACE 2. Complex refractive indices turned out to be much larger. For the cases studied so far, refractive indices were in the real part and above 0.02i in the imaginary part. Consequently, the single-scattering albedo ranged from 0.79 to 0.93 [Satheesh et al., 1999; Müller et al., 2001b], which is significantly lower than the values reported for ACE 2. The lidar ratios showed maximum values of 110 sr at 532 nm. The majority of values were in the range from 50 to 80 sr. For comparison, ACE 2 showed maximum values of 80 sr, with the majority being around sr. The numbers for INDOEX suggest a considerable contribution of absorbing material, stemming from diesel and coal combustion and biomass burning [Venkataraman et al., 1999; Novakov et al., 2000b; Franke et al., 2001; Lelieveld et al., 2001; Müller et al., 2001b]. [51] SCAR-B finally was designed for a detailed characterization of aerosols from intense biomass burning in the Amazon region. There as well, values of 0.01i 0.04i for the imaginary part of the complex refactive index [Li and Mao, 1990; Lenoble, 1991] were larger than for ACE 2. Real parts were [Yamasoe et al., 1998; Remer et al., 1998] and thus higher than for TARFOX but within the range of our ACE 2 observations. Single-scattering albedos for SCAR-B were in the range of at 550 nm [Anderson et al., 1996; Reid and Hobbs, 1998; Reid et al., 1998, 1999], which, again, is significantly lower than the values found for ACE Summary [52] On the basis of vertically resolved optical particle parameters we presented detailed results of vertically resolved physical particle properties and the single-scattering albedo for several periods of European pollution outflow observed during ACE 2. The lidar site at the southwestern tip of Portugal also allowed us to observe the conditions during the advection of clean marine air masses from the Atlantic Ocean, thus providing the offset to the continental polluted conditions. [53] The majority of pollution was located in lofted plumes above 500-m height and thus well above the marine boundary layer. As expected, particle effective radii on the order of 0.15 ± 0.06 mm were in the range characteristic for urban-industrial emissions. A high variability of mean particle size, ranging from approximately 0.1 to 0.3 mm, was observed between different pollution outbreaks. In some cases, strong variability was also given within the vertical scale. Accordingly, particle surface area and volume concentration showed a high variability. Mean wavelengthindependent values of the complex refractive index were 1.56 ± 0.07 in the real and 0.009i ± 0.010i in the imaginary part. These values are within the range expected for a mixture of nonabsorbing ammonium-sulfate-like material and a small share of absorbing sootlike material. Accordingly, the single-scattering albedo at 532 nm was 0.95 ± 0.06, also indicating the low contribution by absorbing material. [54] There was a strong correlation of effective radius with the Ångström exponents in the wavelength range from 400 to 532 nm and from 532 to 800 nm. The mean particle size was rather independent of the particle extinction-to-backscatter (lidar) ratio. In particular, there was a strong correlation of decreasing single-scattering albedo with increasing imaginary part of the complex refractive index, and of decreasing single-scattering albedo with increasing lidar ratio. The fact that the imaginary part of the complex refractive index also increased with increasing lidar ratio indicates that the singlescattering albedo as well as the lidar ratio contain information on the chemical state of the particles. [55] The similarity of the derived parameters to those found during TARFOX shows that, for example, industrial combustion processes in Europe do not differ very much from those in North America. Considerable offset was found from the parameters determined from INDOEX and SCAR-B. Carbon-producing processes, for example, coal and diesel burning and/or biomass burning in the first case and biomass burning in the second case, caused larger complex refractive indices and accordingly lower singlescattering albedos. [56] There still exists a lot of uncertainty about the influence of vertically varying particle properties on regional and global climate, cloud formation, and air chemistry. This series of papers about the results of multiwavelength lidar observations of pollution plumes during ACE 2 [Ansmann et al., 2001, 2002] shows the exceptional potential of multiwavelength lidar observations for climate research. Such instruments provide detailed, vertically resolved, and statistically sound information on optical and physical particle properties measured under ambient atmospheric conditions. [57] Acknowledgments. We are grateful to Ana Maria Silva, University of Evora, Portugal, for a perfect organization at the Sagres site and the Estação Radio Naval de Sagres for kindly providing us with a wellprepared field site. References Ackermann, J., The extinction-to-backscatter ratio of tropospheric aerosol: A numerical study, J. Atmos. Oceanic Technol., 15, , Althausen, D., D. Müller, A. Ansmann, U. Wandinger, H. Hube, E. Clauder, and S. Zörner, Scanning six-wavelength eleven-channel aerosol lidar, J. Atmos. Oceanic Technol., 17, , Anderson, B. E., et al., Aerosols from biomass burning over the tropical South Atlantic region: Distributions and impacts, J. Geophys. Res., 101, 24,117 24,137, Ansmann, A., F. Wagner, D. Althausen, D. Müller, A. Herber, and U. Wandinger, European pollution outbreaks during ACE 2: Lofted aerosol plumes observed with Raman lidar at the Portuguese coast, J. Geophys. Res., 106, 20,725 20,734, Ansmann, A., F. Wagner, D. Müller, D. Althausen, A. 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