4.1. Particle Size
 Figure 4 shows that the anthropogenic particles observed on 25 August 2003 were much smaller than the biomass-burning particles. The smoke particles were also considerably larger than European haze particles observed in the free troposphere with six-wavelength lidar during the Second Aerosol Characterization Experiment (ACE 2) [Müller et al., 2002]. Effective radii of aged particles that were generated by anthropogenic sources in the European continent were 0.15 ± 0.06 μm. The forest fire particles were also larger than mixtures of anthropogenic and biomass-burning particles found in free-tropospheric plumes during the Indian Ocean Experiment (INDOEX) [Müller et al., 2003]. In that case, particle sizes were 0.2 ± 0.08 μm. Lidar observations of an aged biomass-burning plume that had originated from western Canada in 1998 and which was observed over Germany during the Lindenberg Aerosol Characterization Experiment (LACE) 98 showed effective radii from 0.11 ± 0.03 μm to values as large as 0.27 ± 0.04 μm; see Tables 1 and 2 of Wandinger et al.  and Tables 3 and 4 of Veselovskii et al. . In situ observations carried out by aircraft in the same plume showed particle effective radii as large as 0.25 ± 0.07 μm [Fiebig et al., 2002; Wandinger et al., 2002]. The Ångström exponent was as large as 0.06 which puts it in the range of values found in this study. Dual-wavelength Raman lidar observations of Siberian forest fire smoke were carried out at Tokyo on 21 May 2003 [Murayama et al., 2004]. Effective radii were smaller than what was found at the Leipzig site on 29 May 2003. Numbers varied around 0.21 μm, which is equivalent to an Ångström exponent on the order of 1.35.
 Effective radii of the particles are considerably larger than what is usually observed near source regions of forest fires (e.g., Table 2 of Fiebig et al. [2003, and references therein]). Because of the unknown burning conditions that prevailed at the source of the Siberian and Canadian fires, and insufficient information regarding the physical, chemical, and meteorological processes that occurred along the transport path of the plumes to our site, we can merely speculate on possible reasons that generated these large particles.
 In situ observations of particles in South America during the Smoke, Clouds and Radiation–Brazil (SCAR-B) field project indicated that the kind of fires, i.e., flaming versus smoldering combustion, has a significant influence on the size of the particles [Reid and Hobbs, 1998; Reid et al., 1998]. Burning of biomass at higher temperatures, i.e., flaming fires, generates smaller particles than smoldering fires; see, e.g., Figure 12 of Reid et al. . In forest fires, the flaming stage is followed by a longer period of smoldering fires [Ward et al., 1996; Ferek et al., 1998]. The large smoke particles we observed may thus have originated from the smoldering phase of the fires.
 The kind of burnt material as well as the combustion efficiency also influence the size of the particles. For that reason, considerably larger particles than what usually are found in, e.g., South America and southern Africa (see below) may have already been generated in the source regions in Siberia and Canada. Fire radiative power determined from measurements with the MODIS sensor aboard the Aqua and Terra satellites indicates that vegetation fires burn less intense in Russia compared to Canadian fires [Wooster and Zhang, 2004]. The lower fire intensity is probably due to the fact that the burning of surface fuels is predominant over the more intense burning of tree crowns [Furyaev, 1996; Conard and Ivanova, 1997].
 There is strong evidence from observations of biomass-burning plumes in different regions of the world that particles grow in size during the aging of the plumes, e.g., Table 2 of Fiebig et al. . Processes that lead to the increase of particle size are gas-to-particle conversion of inorganic and organic vapors [Reid and Hobbs, 1998; Reid et al., 1998], condensation of large organic molecules from their gas phase in the first few hours of aging [Reid and Hobbs, 1998; Pósfai et al., 2004], particle growth due to coagulation [Westphal and Toon, 1991; Fiebig et al., 2003], and photochemical and cloud-processing mechanisms. Figure 4 shows that relative humidity generally was <50% in the haze plumes observed on 29 May, 26 June, and 10 July 2003. In many cases it varied between 10% and 20%. Thus hygroscopic growth [Hobbs et al., 1997] seems to be a minor factor responsible for the rather large particle size.
 Ångström exponents on the order of 1.8 for rather fresh forest fire smoke (distances up to 600 km from the sources) were determined from Sun photometer observations carried out in north central Canada during the Boreal Ecosystem-Atmosphere Study (BOREAS) [Markham at al., 1997], and during long-term observations carried out in the same area from 1994 to 1999 [Holben et al., 2001]. Observations of smoke from intense forest fires in Canada in 1998 were carried out with Sun photometer located in different spots downwind of the fire events [O'Neill et al., 2002]. The authors noted a decrease of Ångström exponent with distance to fire source. According to regression analysis the Ångström exponent decreased from 1.9 at 30 km distance to 1.4 at 2000 km distance. It has to be kept in mind that local sources of anthropogenic pollution in the planetary boundary layer contributed to the overall signal. Effective radii were retrieved from the optical measurements. Regression analysis showed that particle size slightly increased from 0.13 μm at 30 km distance to 0.15 μm at 2000 km distance. Such a distance is equivalent to a transport time of smoke of up to 5 days. The cases presented here describe even longer transport times. Fiebig et al.  presented model calculations of the growth of particles, which were observed with airborne in situ instruments during LACE 98. The authors showed that particle coagulation processes can sufficiently well explain the large effective radii observed, if transport over a period of 6 days was assumed. An increase of particle size of up to 50% was found from the scenarios considered in their study.
 The evolution of a massive haze plume that originated from strong forest fires in central Quebec (Canada), and which then spread across northeastern parts of the United States, was analyzed with lidar, Sun photometer, and airborne in situ instrumentation [Colarco et al., 2004]. Mean Ångström exponents obtained with aircraft in the plume up to 1000 km downwind of the source varied from 0.83 to 1.23 [Colarco et al., 2004; Taubman et al., 2004] for the wavelength range from 450 to 700 nm, and thus point to relatively large particles. The measurements were carried out at ≤20% relative humidity. Taubman et al.  found the peak of the size distribution at particle diameters from 0.3 to 0.6 μm, which points to much larger particles than what is usually found for anthropogenic pollution. Average particle size may have been even larger, as the authors could not rule out loss effects of the aircraft inlet used in their study. Model simulations showed that particle coagulation processes occurring along the way from the source to the observation sites could explain the low Ångström exponents [Colarco et al., 2004]. Sun photometer observations of that event resulted in values of ∼1.24 (440–670 nm), which is at the upper end of numbers obtained from the aircraft observations [Eck et al., 2003]. Anthropogenic pollution in the planetary boundary layer rather likely caused these higher values.
 Particle size measurements were carried out with a balloon-borne optical particle counter in combination with lidar observations in the framework of the Mildura Aerosol Tropospheric Experiment (MATE 98) in southern Australia [Rosen et al., 2000]. At times lidar detected free tropospheric layers at heights up to 13 km. Effective radius was 0.18 μm for one of the observed free-tropospheric particle size distributions. The measurement was characterized by advection of haze plumes with the prevailing westerly winds from southern Africa and/or South America. In view of the transport times the particles probably were rather aged.
 Liousse et al.  found a significant increase of particle size with age of plumes that originated from savanna fires in Ivory coast in Africa. A decrease of the Ångström exponent (450–550 nm wavelength), which is, as mentioned, equivalent to an increase of particle size was observed for fires in tropical forest and cerrado during SCAR-B [Reid et al., 1998]. In situ observations showed that Ångström exponents were on the order of 2.2 ± 0.2 for fresh smoke and 1.2 ± 0.2 for aged smoke. The numbers for aged smoke are at the upper end of values reported here.
 Ångström exponents of particle plumes generated by strong fires events in Mexico in 2002 were 1.28–1.58 for the wavelength range from 440 to 670 nm [Kreidenweis et al., 2001]. Observations were carried out with Sun photometer. These numbers also are in the upper range of values presented here, and describe plumes from 2 days up to 10 days of age. Care has to be taken in this comparison, as the column mean values most certainly do not describe pure biomass-burning particles. The plumes were transported from Mexico to the United States, and thus most likely were affected by anthropogenic pollution. In contrast, the free-tropospheric particle layers we observed were rather likely much less influenced by anthropogenic pollution in the planetary boundary layer.
 Table 4 summarizes the results for particle effective radius and Ångström exponents of particles generated by fires in boreal regions. There is almost a factor 3 difference between particles detected near the sources [see O'Neill et al., 2002] and particles we detected far away from the fire events. In addition there seems to be a systematic shift of particle size toward larger values with increasing distance from the source of boreal fires. It has to be emphasized once more that the fires described here and the fires discussed in the cited literature certainly had different properties. Thus definite conclusions regarding the increase of particle size with duration of transport have to be treated with caution, and are subject to future studies.
4.2. Single-Scattering Albedo
 The mean single-scattering albedo was >0.9 at 532 nm in the forest fire plumes, and indicates moderately absorbing particles. The single-scattering albedo of the Siberian biomass-burning plume that was observed with Raman lidar over Tokyo on 21 May 2003 was similar to the value for the Siberian smoke observed over Leipzig on 29 May 2003 [Murayama et al., 2004]. Wandinger et al.  and Fiebig et al.  found a considerably lower value of 0.78–0.83 at visible wavelengths for an aged Canadian biomass-burning plume. We can merely speculate on the reasons why those particles were much more absorbing than the ones discussed here.
 Black carbon, which is the main source for absorbing aerosols is emitted primarily in the hot flaming stage of vegetation fires. As mentioned before, the flaming stage of forest fires is followed by a smoldering phase. During that time more organic carbon than black carbon is emitted [Ward et al., 1996; Ferek et al., 1998]. Temperatures of the fires in the source regions in 1998 may have been higher than in the present case, and thus responsible for higher-absorbing particles. Fromm et al.  noted increased values for particle optical depth in the lower stratosphere in August 1998. Strong fire activity may have partly been responsible for the increased aerosol load, and thus also for a comparably high amount of absorbing material. The amount of low-absorbing organic vapors on existing soot particles may have been lower. Fiebig et al.  noted that a small amount of highly absorbing iron oxide may have been present in the particles observed during LACE 98.
 Aircraft observations of smoke from the boreal fires that occurred in central Quebec in summer 2002 showed single-scattering albedos of 0.93 ± 0.02 at 550-nm wavelength approximately 1000 km downwind of the source region [Colarco et al., 2004; Taubman et al., 2004]. Dubovik et al.  reported a value of around 0.94 at 440–670 nm wavelength for North American boreal forest fires. This number was derived from Sun photometer observations. No information on the age of the particle plumes is available. O'Neill et al.  showed that single-scattering albedo at 500 nm was >0.9 at distances around 2000 km downwind of Canadian forest fires. Ferrare et al.  estimated an increase of single-scattering albedo with distance (up to 3000 km downwind) from boreal forest fires on the basis of satellite observations.
 On average, lower single-scattering albedos than reported here were obtained for fresh smoke (less than 5 min old) from boreal fires in North America. Numbers varied from 0.8 to 0.95 in the green spectrum of light [Radke et al., 1988; Hobbs et al., 1997]. These numbers are 0.05–0.1 lower than those for young flaming or smoldering fires (less than 4 min old) in the tropical forests of Amazonia observed during SCAR-B [Reid and Hobbs, 1998].
 A rather high single-scattering albedo of around 0.97 at 670 nm was reported for aged biomass-burning plumes advected from Mexico to the United States [Kreidenweis et al., 2001]. These column-averaged values were explained by water uptake of the particles, and a lower content of absorbing carbon compared to African or South American biomass-burning particles. Our measurements were characterized by extremely low relative humidities. As mentioned before, the measurements of the Mexican haze layers may have been affected by anthropogenic pollution in the boundary layer.
 Single-scattering albedo measured in tropical regions tends to be lower than what we found in our observations. Measurements of biomass-burning aerosols with AERONET Sun photometer in a savanna region in south central Africa in 1997 yielded a single-scattering albedo of 0.82–0.85 at 550 nm wavelength [Eck et al., 2001]. In situ observations of tropical forest fire smoke carried out in the Amazon region during the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA-EUSTACH) in September-October 1999 [Guyon et al., 2003] showed single-scattering albedos of 0.9 ± 0.03 at 545 nm for ambient relative humidities <80%.
 Haze from tropical fires in South America and South Africa was characterized by aircraft during the Transport and Atmospheric Chemistry near the Equator–Atlantic (TRACE-A) experiment. Column mean values for the single-scattering were <0.9 at 500 nm during all research flights [Anderson et al., 1996]. This number refers to dry as well as to humidity-corrected particle conditions. There was no significant difference of single-scattering albedo measured during flights over the continents, i.e., close to the source regions, and in the outflow areas along the coastlines of the two continents. However, the aircraft did not follow any air mass over large distances, so that possible aging effects could not be identified.
 Aircraft measurements of haze observed during intense fires in Indonesia in 1997 showed a mean single-scattering albedo of 0.93–0.95 at 532 nm, which puts them in the range of numbers reported here. Smoldering peat fires rather likely were the reason for the observed high single-scattering albedos. In contrast, flaming fires in northern parts of Australia showed considerably lower single-scattering albedos of 0.85 [Gras et al., 1999].
 Measurements indicate that particle single-scattering albedo increases with age of the pollution plume. Magi et al.  found a mean value of 0.81 ± 0.02 at 550 nm in various areas of southern Africa during the dry biomass-burning season. The aircraft-borne in situ measurements represent the properties in the planetary boundary layer up to 4-km height, and were corrected for relative humidity. Background conditions which most likely were determined by much more aged biomass-burning aerosol showed a considerably larger mean value of 0.89 ± 0.03.
 Abel et al.  observed agricultural fires in southern Africa, and found an increase of single-scattering albedo from 0.84 at 550 nm wavelength at the source to 0.9 after a transport time of 5 hours. Changes due to hygroscopic growth could be neglected as the mean relative humidity was approximately 22% within the smoke plumes [Magi and Hobbs, 2003]. Aircraft-based in situ observations were carried out by Reid et al.  during forest fires and cerrado fires in Brazil during SCAR-B. The authors found an increase of single-scattering albedo by 0.06 after a transport time of 2–4 days. The change of single-scattering albedo was likely caused by an increase of scattering material due to the condensation of volatile organics during the particle aging process [Reid et al., 1998]. Condensation of, e.g., organic carbon away from the fire source can produce significant amounts of new particles [Hobbs et al., 2003; Pósfai et al., 2003].
 Table 4 summarizes the results for single-scattering albedo of particles generated by fires in boreal regions. In contrast to the result obtained for particle size, i.e., that there seems to be a systematic shift toward larger particle size with increasing distance from fire source, single-scattering albedo does not seem to undergo any systematic change with travel time.
4.3. Complex Refractive Index
 Table 3 shows that the mean value of the real part of the complex refractive index varied from 1.37 to 1.6, and the mean imaginary part was always <0.01i. The real parts on average are lower than the values derived from multiwavelength lidar observations of the aged Canadian biomass-burning plume observed during LACE 98. Values were estimated to be 1.56–1.77 at 532 nm [Wandinger et al., 2002]. Imaginary parts in that case varied between 0.04i and 0.07i, which is roughly an order of magnitude higher than what was found in the present study.
 Westphal and Toon  report complex refractive indices on the order of 1.45–1.55 for the real part, and 0.01i for the imaginary part at 500 nm wavelength for biomass burning in Canadian forests in July 1982. No information on relative humidity was reported. Dubovik et al.  found 1.5 ± 0.04 for the real part, and 0.009i ± 0.003i for the imaginary part from Sun photometer observations. Taubman et al.  estimated a real part of 1.58 for Canadian fires in 2002, which is about 0.1 larger than what we found on average. Aged biomass-burning plumes advected from Mexico to North America resulted in real parts of 1.41–1.45 at 670 nm [Kreidenweis et al., 2001], which is rather close to our numbers. As mentioned before, high relative humidities and contamination by anthropogenic pollution could be responsible for the low real parts. Imaginary parts were not reported. However, on the basis of the large single-scattering albedo and the comparably large size of the particles it has to be assumed that the imaginary part was well below 0.01i.
 Most of the information on complex refractive indices is available for biomass-burning particles in tropical areas of South America and Africa. In general, numbers are larger that what we found in our measurements. A mean refractive index of 1.54–0.018i was derived for aged regional haze observed with aircraft during the Southern African Regional Science Initiative (SAFARI 2000) [Haywood et al., 2003a]. Similar values were found from Sun photometer observations by AERONET in the same area [Haywood et al., 2003b]. Observations during the period of intense biomass burning in South America showed a value of 1.41 ± 0.05 for the real part and 0.013i ± 0.005i for the imaginary part. Measurements were carried out at 545 nm wavelength, and hold for relative humidities from 48 to 80% [Guyon et al., 2003]. Again the imaginary part is much higher than what is reported here. Sun photometer observations carried out by AERONET in Amazonian forests in Brazil (1993–1994) and in Bolivia (1990–1999) showed column mean values of 1.47 ± 0.03 for the real part and 0.0093i ± 0.003i for the imaginary part [Dubovik et al., 2002]. The numbers hold for the wavelength range from 440 to 1020 nm. Biomass burning in South American cerrado in Brazil resulted in values of 1.52 ± 0.01 for the real part and 0.015i ± 0.004i for the imaginary part. Previous observations in these areas with AERONET radiometers during SCAR-B resulted in slightly larger values of 1.53 ± 0.04 for the real part at 440 and 670 nm wavelength. Haze that resulted from intense forest fires in Southeast Asia in 1997 was characterized by real parts <1.55 in the wavelength range from 560 to 870 nm [von Hoyningen-Huene et al., 1999]. Table 4 summarizes once more the results for the imaginary part of particles generated by fires in boreal regions. As in the case of single-scattering albedo it is not possible to derive any conclusion regarding the effect of transport time on the change of the absorption behavior.