Number size distributions and concentrations of marine aerosols: Observations during a cruise between the English Channel and the coast of Antarctica

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D24, 4753, doi: /2002jd002533, 2002 Number size distributions and concentrations of marine aerosols: Observations during a cruise between the English Channel and the coast of Antarctica Ismo K. Koponen Department of Physical Sciences, University of Helsinki, Helsinki, Finland Aki Virkkula, Risto Hillamo, and Veli-Matti Kerminen Finnish Meteorological Institute, Helsinki, Finland Markku Kulmala Department of Physical Sciences, University of Helsinki, Helsinki, Finland Received 15 May 2002; revised 12 July 2002; accepted 17 July 2002; published 19 December [1] Submicron aerosol number size distributions were measured on board the research ship Akademic Fedorov during a cruise from the English Channel to the coast of Antarctica. The observed sized spectra were fitted with three lognormal modes (the nucleation, Aitken and accumulation modes), and the data were classified according to calculated air mass back trajectories. The total particle number concentrations were mostly <1000 cm 3 in marine air masses and between about 1000 and cm 3 in continentally influenced air masses. Most of the eastern midlatitude Atlantic was affected by European pollution with high concentrations of both nucleation and Aitken mode particles. Another pollution peak, caused probably by biomass burning in Africa, was seen at about 10 N. Marine air masses showed distinctive latitudinal changes. Over the tropical Atlantic no nucleation mode could be seen, and the total particle number concentration remained usually below 500 cm 3. In the midlatitude and high-latitude Atlantic several episodes of elevated particle concentrations (<1000 cm 3 ) caused by either the nucleation or Aitken mode were observed. The geometric mean diameter of the accumulation mode decreased gradually toward the higher southern latitudes with a more rapid decline close to Antarctica. Our observations indicate further that there is no simple and universal relationship between the total particle number concentration, the submicron aerosol volume (or mass), and the number of particles able to act as cloud condensation nuclei over the southern hemispheric oceans. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0394 Atmospheric Composition and Structure: Instruments and techniques; 9325 Information Related to Geographic Region: Atlantic Ocean; KEYWORDS: marine aerosols, particle size distributions, ultrafine particles Citation: Koponen, I. K., A. Virkkula, R. Hillamo, V.-M. Kerminen, and M. Kulmala, Number size distributions and concentrations of marine aerosols: Observations during a cruise between the English Channel and the coast of Antarctica, J. Geophys. Res., 107(D24), 4753, doi: /2002jd002533, Introduction [2] Marine areas provide an ideal environment to investigate the formation and fate of natural sea-salt and sulfur aerosols, as well as their interaction with anthropogenic pollution. The most important properties of marine aerosols are their number size distribution and chemical composition. Detailed information on these aerosol properties is needed in order to quantify the climatic effects of natural or anthropogenic aerosols, or when trying to close the global Copyright 2002 by the American Geophysical Union /02/2002JD budgets of sulfur, chlorine and many other important trace compounds present in the atmosphere. [3] Several reviews on marine aerosols have been published [Junge, 1972; Fitzgerald, 1991; O Dowd et al., 1997; Heintzenberg et al., 2000]. Although the chemical composition of the marine aerosol and its variability has not been fully characterized, the global coverage of the aerosol chemical data can be considered better than that of the corresponding physical data (size distribution). The great majority of aerosol size distribution measurements has been conducted either over the Pacific Ocean [Hoppel and Frick, 1990; Covert et al., 1996b; Porter and Clarke, 1997] or over the northern or tropical Atlantic [Hoppel et al., 1986; Jensen et al., 1996; Clarke et al., 1996; Raes et al., 1997; AAC 6-1

2 AAC 6-2 KOPONEN ET AL.: DISTRIBUTIONS AND CONCENTRATIONS OF MARINE AEROSOLS Figure 1. The route of Akademik Fedorov. Bates et al., 2001]. Very few size distribution measurements have been made over the Southern Ocean [Bates et al., 1998; Brechtel et al., 1998; Covert et al., 1998], even though this area constitutes a significant fraction of the global oceans. [4] In this paper, an analysis of submicron aerosol size distributions from shipboard measurements along a transect from 60N to 70S across the Atlantic and Southern Ocean is presented. Our principal aims are (1) to provide a comprehensive data set on modal aerosol size distribution parameters, and (2) to investigate how these parameters are affected by the latitude and the degree of continental influence. The measurements rely on a novel system based on recent advances in both the instrumentation and the calibration methods. 2. Measurement Setup and Instrumentation [5] The measurement campaign was performed on board the Russian research vessel Akademik Fedorov, the route of which is shown in Figure 1. The principal aim of the ship was to transport scientific instruments and supplement logistical cargo for several stations and expeditions in Antarctica. Therefore the route was chosen independently of this study. The measurements were started on November 8, 1999 at the time of passing the English Channel ( N; W). The ship arrived at Cape Town on November 26 and continued towards Antarctica on December 2. During its stay in Cape Town, no measurements were made. The measurements were finished on December 8 ( S; E) before the ship reached the coast of Antarctica. [6] A schematic picture on the measurement setup can be seen in Figure 2. The sample air was taken from the clean side of the ship. The inlets were placed 1.5 meters above the bridge, and the sample air was led through stainless steel and conductive flexible tubing having inner diameters of 10 mm and 25 mm, respectively. The total lengths of the sampling lines were around 10 meters. Losses of particles inside the sampling lines were determined both theoretically and experimentally and taken in to account in the data analysis. [7] The total aerosol number concentration was measured using two condensation particle counters (CPC): a TSI model 3025 instrument which measures particles >3 nm in diameter [Stolzenburg and McMurry, 1991] and a TSI model 3010 instrument which measures particles >10 nm in diameter [Quant et al., 1992]. There were technical problems with the model 3025, so most of the total concentration data concerning the measurements before Cape Town is from the model The main purpose of the total aerosol concentration measurements was to control the quality of particle size spectrum measurements. [8] The aerosol number size distribution was measured over the size range nm using a twin differential mobility particle sizer (DMPS). The twin DMPS system consists of two Vienna type DMA s (lengths 11 and 28 cm; Figure 2. Instrumentation for aerosol measurement instrumentation in the Akademik Fedorov. Notation: VI, Virtual Impactor; SDI, Small Deposit area Impactor; Neph, nephelometer; CPC, Condensation Particle Counter; Eta, Aethalometer; LPC, Laser Particle Counter; and DMPS, Differential Mobility Particle Sizer.

3 KOPONEN ET AL.: DISTRIBUTIONS AND CONCENTRATIONS OF MARINE AEROSOLS AAC 6-3 see Winklmayr et al. [1991]), which are used for electrical mobility diameter size classification of the particles and of two CPC s (a TSI model 3025 and 3010) measuring the particle number concentration after classification. The sheath air volume flow rates of the DMA s were 5.4 and 17.5 lmin 1 covering the subranges 3 15 nm and nm (particle diameter). The measurement cycle for the whole size range was 10 minutes. The sheath flows of DMAs are maintained utilizing a closed sheath air loop using critical orifices [Jokinen and Mäkelä, 1997]. The relative humidities of the sheath flows were kept below 20%. All CPC s and DMA s were calibrated before the campaign. The calibration method has been described in detail by Aalto et al. [2001]. In the data analysis, observed pressure and temperature were used to calculate the kernel matrix for the DMPS system. The data were inverted by using a MATLAB function nnls (Non-negative leastsquares) [Math Works, Inc., 1998]. Fittings to measured DMPS size distributions in order to extract the individual modes and the corresponding modal parameters (the aerosol number concentration (N), geometric mean diameter (GMD), and geometric standard deviation (s g )) were made according to the method presented by Mäkelä et al.[2000]. All the data are presented in UTC time. [9] The meteorological parameters, including the temperature, relative humidity, pressure and wind speed and its direction, were measured with a Milos 200 weather station permanently installed in Akademik Fedorov. The air mass backward trajectories were calculated using the NOAA HYSPLIT4 model (Draxler and Hess [1997, 1998]; HYS- PLIT). The meteorological input data used by the model were obtained from the NOAA FNL archives, and the vertical motion was calculated using the omega fields. a) Concentration [cm 3 ] N 43N 32N 20N 8N 3S 9S 18S 27S Day of year 3. General Character of the Aerosol [10] During the cruise, a total of 3000 aerosol size distribution spectra over the particle diameter range nm were measured. All the spectra were analyzed to obtain the modal parameters and to check for a potential contamination by the ship. There are several good indicators for the identification of the undesired contamination caused by the ship activities. Here the concentrations of aerosol black carbon (given by aethalometer), ozone and, most importantly, of total aerosol number concentration were used for identifying the contaminated periods. Almost 50% of the data were contaminated, either by direct emissions from the chimney of ship or by other activities on board the ship. All the data suffered from contamination were discarded and will not be considered further in this paper. Note that although a quite high percentage of the particle number data was rejected due the contamination, the particles coming from the ship activities are usually small (below 100 nm) and therefore the corresponding particle volume (mass) may have remained unaffected. Thus, the number of contaminated periods of the samples conducted for chemical analysis is significantly smaller. The data were classified according to both latitude and air mass origin. In this respect, a distinction was made between the data collected during the first part of the cruise from the English Channel to Cape Town, and the data collected during the second part of the cruise from Cape Town to Figure 3. Temporal and latitudinal distribution of the measured total particle number concentration. Figure 3a represents the cruise from English Channel to Cape Town, and Figure 3b represents the cruise from Cape Town to the coast of Antarctica. Antarctica. Polluted and the marine air masses were considered separately. The latter category included all air masses that had not been in contact with the European or African continent for the 3 days prior to reaching the ship Aerosol Number Concentration [11] The total number concentration of aerosol particles is depicted in Figure 3 as a function of latitude and time. During the first four days of the cruise (the latitudes >32 N), the measured air came directly from Europe and the particle concentrations were >2000 cm 3. Somewhat less polluted air was encountered between 32 N and 15 N, the concentrations being mostly between about 1000 and 2000 cm 3. A peak in the particle number concentration was seen close to the African coast at around 10 N. This peak was very likely due to biomass burning, since the concentrations of both black carbon and non-sea-salt potassium were quite large in these air masses. [12] When passing the equator, the particle number concentrations decreased considerably to values between 300

4 AAC 6-4 KOPONEN ET AL.: DISTRIBUTIONS AND CONCENTRATIONS OF MARINE AEROSOLS and 600 cm 3. The main reason for these low particle concentrations was that the measured air masses were almost entirely marine, in contrast to air masses encountered over the north Atlantic. The distance between the ship and the continental areas was also at its greatest during this part of the cruise. [13] After Cape Town several episodes of elevated particle concentrations close to or above 1000 cm 3 were observed. Before roughly 55 S, these episodes were mainly due to high Aitken mode particle concentrations, whereas closer to Antarctica the role of the nucleation mode became more important (see sections 4.1 and 4.2). A closer analysis of this feature, which seems to be specific for midlatitude and high-latitude oceanic areas in the southern hemisphere, will be presented in section Aerosol Number Size Distributions [14] All the measured particle number size distributions could be fitted quite well using two or three lognormal modes: an accumulation mode (particle mean diameter >100 nm), an Aitken mode ( nm) and a nucleation mode (<20 nm). Both the accumulation and the Aitken mode could be detected in all air masses, regardless of their origin. The nucleation mode was present only occasionally and its magnitude relative to the two other modes varied considerably. [15] Examples of aerosol number size spectra measured during polluted air masses are shown in Figure 4a. Two of the spectra (A and B) represent air originating from Europe and one (the spectrum C) air moving along the coast of Africa. In all these spectra, the total particle number concentration is high ( cm 3 ), and the Aitken and the accumulation mode are strongly overlapping. Both these features are common to air masses affected by fresh anthropogenic pollution [e.g., Shi et al., 1999; Väkevä et al., 2000]. [16] Examples of typical marine particle number spectra are shown in Figure 4b. The spectrum D is bimodal with no nucleation mode, and it has a total particle number concentration of about 300 cm 3. This kind of a spectrum is typical for remote marine boundary layer in both tropics and midlatitudes [Hoppel and Frick, 1990; Covert et al., 1996b; Jensen et al., 1996; Raes et al., 1997; Brechtel et al., 1998]. The clear separation between the Aitken and the accumulation mode in the spectrum D can be ascribed to cloud processing, as suggested by Hoppel et al. [1986] and confirmed later by several other investigators. The spectrum E represents a case where the measured air comes from the European continent but has spent at least 40 hours over the ocean. The total particle number concentration in this spectrum is much lower (1000 cm 3 ) than in the spectra displayed by Figure 4a. The most likely reason for this is that an originally very polluted air has been diluted into the free marine troposphere by entrainment, as demonstrated by Van Dingenen et al. [2000] for polluted marine boundary layers using a simple model approach. It is also probable that the spectrum E has undergone some cloud processing, since it had already developed a clear bimodal character. Finally, the spectrum F represents a case with a prominent nucleation mode (600 cm 3 ). These kind of distributions were relatively common when approaching Antarctica. [17] Figures 5 and 6 show the particle number concentration and the geometric mean diameter, respectively, of the Figure 4. Examples of measured particle number size distributions in (a) continental and (b) marine air masses. The different spectra are measured at latitudes 35 N (the spectrum A), 7 N (B), 7 S N (C), 7 S (D), 25 N (E), and 55 N (F). three submicron modes as a function of latitude and time. A statistical summary of the modal parameters in different air mass types is given in Table 1. It can be seen that the high particle concentrations observed in polluted air masses in the northern hemisphere were caused by either the Aitken or the nucleation mode. The exception are the latitudes around 10 N when polluted air from Africa was encountered. In these air masses a very prominent accumulation mode was present. The nucleation mode was practically absent from marine air masses encountered over the tropical or northern Atlantic, but become relatively more frequent south of about 55 S. [18] The mean diameter of the accumulation mode varied between about 100 and 200 nm, being clearly the largest (average 175 nm) for the marine air measured during the first part of the cruise. Much lower mean diameters (average 123 nm) were seen during the second part of the cruise, and diameters as low as 100 nm were quite common close to Antarctica. The mean diameter of the Aitken mode was more stable, being on average 52 nm for the continental air and around 40 nm or slightly below for the marine air. The mean diameter of the nucleation mode had two frequency

5 KOPONEN ET AL.: DISTRIBUTIONS AND CONCENTRATIONS OF MARINE AEROSOLS AAC 6-5 Figure 5. Temporal and latitudinal distribution of the particle number concentration in the three submicron modes observed. Figures 5a and 5b are as in Figure 3. maxima: one around 10 nm and another between about 15 and 20 nm. 4. Discussion 4.1. Summary of Observed Modal Features [19] In the polluted air masses measured in this study, the particle number concentration averaged about 1500 cm 3 for the Aitken mode and about 500 cm 3 for the accumulation mode. These numbers are very similar to those reported by Van Dingenen et al. [1995] for polluted air in the eastbound leg of their traverse across the north Atlantic. The modal mean diameters averaged 52 nm for the Aitken mode and 154 nm for the accumulation mode, which are clearly lower than the respective values of 78 and 190 nm reported by Van Dingenen et al. [1995]. No apparent reason

6 AAC 6-6 KOPONEN ET AL.: DISTRIBUTIONS AND CONCENTRATIONS OF MARINE AEROSOLS Figure 6. Temporal and latitudinal distribution of the geometric mean diameter of the three submicron modes observed. Figures 6a and 6b are as in Figure 3. for this difference can be pointed out, even though one possibility might be more pronounced SO 2 -to-sulfate conversion in the air masses measured by Van Dingenen et al. [1995]. Indirect support for this possibility can be obtained when comparing the measured non-sea-salt-sulfate concentrations: these were mostly <1.5 mgm 3 for the polluted samples collected during our cruise, and about 2 mgm 3 or larger for the corresponding samples by Van Dingenen et al. [1995]. [20] The very high number concentrations between about 2000 and cm 3 during the first four days of our cruise are rather high when compared with other marine data. The main reason for this feature is the prominent role of the nucleation mode, explaining most of the peaks in these air masses. On average, the nucleation mode contributed about a third of the total particle number concentration in the polluted air masses (Figure 5, Table 1). [21] In the marine air masses measured, the particle number concentration averaged about 250 cm 3 for the Aitken mode, and about 140 cm 3 for the accumulation mode during the first part of the cruise and 200 cm 3 during the second part. These values are broadly consistent with those presented by Heintzenberg et al. [2000] in their compilation. The average modal mean diameter of the Aitken mode was about the same for the air masses collected during the first and the second part of the cruise, whereas that of the accumulation mode decreased from 176 nm to 123 nm. Again, these features are in a qualitative agreement with those reported by Heintzenberg et al. [2000]. [22] No nucleation mode was seen in marine air masses measured north of 40 S. This is consistent with other observations, demonstrating that the nucleation mode is extremely rare in clean marine air in both tropics and the northern midlatitudes [Clarke et al., 1996; Covert et al., 1996a; Raes et al., 1997; Bates et al., 1998]. When approaching Antarctica, the nucleation mode started to occur more frequently with peak concentrations close to 1000 nuclei cm 3. Similar events of elevated nuclei concentrations (up to 4000 nuclei cm 3 ) have been observed by Davison et al. [1996] in their measurements over the Weddell Sea, as well as by Gras [1993] and Ito [1993] in their measurements at coastal Antarctica Features Specific to Marine Air Masses [23] As already discussed, clear changes in measured aerosol characteristics for marine air masses were observed between latitudes north of Cape Town (the tropical and midlatitude northern Atlantic) and more southerly latitudes (the midlatitude and high-latitude southern Atlantic). In the following we consider two features related to these changes, namely (1) the strong reduction in the modal mean diameter of the accumulation mode toward the south, and (2) the several occurrences of elevated particle number concentrations in southern air masses. [24] The mean diameter of the accumulation mode decreased gradually from typical values close to 200 nm in the tropical Atlantic to values mostly <150 nm in the high-latitude southern Atlantic, with a more rapid decline to about 100 nm after 65 S when approaching Antarctica (Figure 6). Simultaneously, the accumulation mode particle number concentration displayed practically no latitudinal change. A direct consequence of these observations is that ratio between the accumulation mode particle number con- Table 1. Nine Size Distribution Parameters Obtained From the Fitting Procedure a Continental Air Marine Air A Marine Air B Nucleation Mode D p, nm 21.9 (11.6) (8.0) N, cm (1560) (169) s 1.48 (0.7) (0.7) Aitken Mode D p, nm 51.9 (15.4) 39.7 (11.3) 37.9 (8.5) N, cm (2450) 252 (942) (200) s 1.59 (0.2) 1.58 (0.3) 1.5 (0.3) Accumulation Mode D p, nm (34.4) (33.9) (54) N, cm (1385) 138 (525) (24.3) s 1.53 (0.2) 1.35 (0.2) 1.5 (0.3) a The standard devation of each quantity is given in parantheses. In the table, Marine A stands for data measured before Cape Town, and Marine B for data measured after Cape Town.

7 KOPONEN ET AL.: DISTRIBUTIONS AND CONCENTRATIONS OF MARINE AEROSOLS AAC 6-7 centration NACC and the submicron aerosol volume concentration VSM increased substantially (by a factor 2 5) toward the south. This differs from polluted air masses (excluding fresh pollution) over the eastern North Atlantic, where an almost constant value for the ratio NACC/VSM has been observed [Hegg and Russell, 2000; Van Dingenen et al., 2000]. The strong non-linearity between NACC and VSM observed here indicates that there is no simple relation between submicron aerosol volume (or sulfate mass) concentration and the number concentration of cloud condensation nuclei. This complicates significantly the estimation of both direct and indirect aerosol climatic forcing in clean marine areas in the southern hemisphere. [25] The episodes of particle number concentrations close to or above 1000 were mostly due to an elevated number of Aitken mode particles in latitudes between about 30 S and 55 S. Covert et al. [1996a], based on their measurements over Pacific Ocean, presented a conceptual model for the cycling of marine aerosol in the southern hemisphere. In their model it was proposed that free troposphere-to-boundary layer flux is the main source of Aitken mode particles in the southern midlatitudes. A similar flux might also explain our observations in the southern Atlantic. [26] Near Antarctica, elevated particle number concentrations were mainly due to the nucleation mode. A closer look at the measured size spectra reveals that the nucleation mode appeared typically between 5 and 15 nm of particle diameter, and practically never below 5 nm. Once present, the nucleation mode fluctuated with respect to its number concentration, while little temporal change with respect to its modal mean diameter could be observed. Taken together, these features indicate that the nuclei were not formed in the marine boundary layer close to the measurement site but were transported there from somewhere else. One possible origin for these nuclei could be the Antarctic free troposphere, as suggested by Ito [1993] based on measurements in the Syowa station (69 S). The suppressed growth of the observed nucleation mode suggests further that these particles are not an important source of new Aitken mode particles, or of cloud condensation nuclei (CCN), within the marine boundary layer of the high-latitude southern Atlantic. This is consistent with the observations of Davison et al. [1996] concerning the bursts of small nuclei over the Weddell Sea near Antarctica. 5. Summary and Conclusions [27] During a cruise from English Channel to the coast of Antarctica in November and December 1999, aerosol number size distributions were measured over the particle diameter range nm on board the research ship Akademic Fedorov. After discarding all the data contaminated by the ship, over 1500 aerosol number size spectra remained. These spectra were fitted with two or three lognormal modes (nucleation, Aitken, and accumulation mode), and finally classified according to the air mass origin as either polluted or marine. The latter group contained all air masses not been in contact with European or African continent for the last 3 days before reaching the ship. [28] Several observations common to many earlier studies were made. Among these were the high particle number concentrations exceeding 1000 cm 3 over most of the midlatitude eastern Atlantic due to European pollution, another peak in aerosol concentrations at around 10 N caused by African pollution, and much lower aerosol concentrations in marine air masses. The high concentrations in the northern midlatitudes were caused mainly by Aitken or nucleation mode particles, whereas close to 10 N also the accumulation mode particle number concentration was very high. [29] In marine air masses, several important features were observed. First, no nucleation mode could be detected over the tropical or the northern Atlantic. As a result, the total particle number concentrations were mostly below 500 cm 3 in non-polluted air at these latitudes. Second, several episodes of elevated particle concentrations close to or above 1000 cm 3 were observed over the midlatitude and high-latitude southern Atlantic. These episodes were mainly due to high Aitken mode particle concentrations in the midlatitudes, whereas closer to Antarctica the role of the nucleation mode became more important. A likely, but by no means definite, explanation for these episodes was the free troposphere-to-marine boundary layer transportation, as has been suggested earlier by some other investigators. Finally, the geometric mean diameter of the accumulation mode decreased gradually toward the higher southern latitudes with a more rapid decline close to Antarctica. Simultaneously, no significant changes in the particle number concentration of the accumulation mode with latitude were observed. [30] Overall, our observations indicate strongly that there is no simple and universal relation between the total particle number concentration, the submicron aerosol volume (or mass) concentration, and the number of particles able to act as cloud condensation nuclei over the southern hemispheric oceans. In order to estimate the aerosol climatic forcing in these areas, detailed information on the aerosol number size distribution is needed. An important challenge is to quantify the processes responsible for the formation of the Aitken and the accumulation mode, and the processes determining their magnitude and mean diameter in the marine boundary layer and above. [31] Acknowledgments. We wish to thank Pasi Aalto for technical support and the crew of Akademik Fedorov for their co-operation during the journey. This work was funded by the Academy of Finland and was part of FINNARP Antarctic Expedition. References Aalto, P., et al., Physical characterization of aerosol particles during nucleation events, Tellus, Ser. B, 53, , Bates, T. S., V. N. Kapustin, P. K. Quinn, D. S. Covert, D. J. Coffman, C. Mari, P. A. Durkee, W. J. De Bruyn, and E. S. Satzman, Processes controlling the size distribution of aerosol particles in the lower marine boundary layer during the First Aerosol Characterization Experiment (ACE 1), J. Geophys. Res., 103, 16,369 16,383, Bates, T. S., P. K. Quinn, D. J. Coffman, J. E. Johnson, T. Miller, D. S. 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Bates, A simple nonlinear relationship between aerosol accumulation mode number and submicron volume, explaining their observed ratio in the clean and polluted marine boundary layer, Tellus, Ser. B, 52, , Winklmayr, W., G. P. Reischl, A. O. Lindner, and A. Berner, A new electromobility spectrometer for the measurement of aerosol size distributions in the size range from 1 to 1000 nm, J. Aerosol Sci., 22, , I. K. Koponen and M. Kulmala, Department of Physical Sciences, University of Helsinki, P. O. Box 64, Gustaf Hällströmin katu 2, FIN Finland. (ismo.k.koponen@helsinki.fi) R. Hillamo, V.-M. Kerminen, and A. Virkkula, Finnish Meteorological Institute, Sahaajankatu 20E, FIN Helsinki, Finland.