Online analysis of atmospheric particles with a transportable laser mass spectrometer during LACE 98

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D21, 8132, doi: /2001jd000590, 2002 Online analysis of atmospheric particles with a transportable laser mass spectrometer during LACE 98 A. Trimborn, K.-P. Hinz, and B. Spengler Institute of Inorganic and Analytical Chemistry, Justus Liebig University, Giessen, Germany Received 5 March 2001; revised 13 July 2001; accepted 31 July 2001; published 15 November [1] A novel mobile mass spectrometric acquisition and evaluation system for on-line analysis of single airborne particles and characterization of particle populations (aerosols), laser mass analyzer for particles in the airborne state (LAMPAS 2), was employed during the Lindenberg Aerosol Characterization Experiment 1998 in July and August 1998 (LACE 98). The major aim of this experiment was to perform a size-resolved determination of the chemical composition of atmospheric particles within populations in order to evaluate the climate forcing created by anthropogenic aerosol particles. During the field campaign, the LAMPAS 2 instrument continuously analyzed size and composition of individual particles from five size ranges with a time course resolution of 1 hour, recording several ten thousand single-particle spectra. By using a semiautomated data processing unit, the amount of data could be significantly reduced from several hundreds of particle mass spectra to only a few particle classes in every size range. Statistical evaluation of the mass spectral data measured during LACE 98 resulted in the determination of 10 basic classes of particles, yielding detailed information on the aerosol composition. Chemical interpretation of these particle classes and correlation to meteorological data and physicochemical particle properties (optical parameters) were performed. 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 Citation: Trimborn, A., K.-P. Hinz, and B. Spengler, Online analysis of atmospheric particles with a transportable laser mass spectrometer during LACE 98, J. Geophys. Res., 107(D21), 8132, doi: /2001jd000590, Introduction [2] A detailed understanding of atmospheric processes is essential for the prediction of changes in Earth climate. Since many atmospheric processes are influenced by individual particle properties [Crutzen and Arnold, 1986; Solomon, 1990], direct chemical and physical analysis of airborne particles is of major interest. It is well known, for example, that aerosols play an important role in the irradiation budget due to their light scattering abilities [Charlson et al., 1992; Andreae and Crutzen, 1997; Kiehl, 1999]. [3] Size and chemical composition are the two main characteristics of aerosol particles. Conventional sampling and analyzing methods, like filter samplers and impactors, lead to artifact formation and loss of volatile particle components. By employing an impact-free online analysis based on a differentially pumped particle inlet system these limitations are substantially reduced. Bipolar time-of-flight mass spectrometry provides for high-level chemical speciation of the atmospheric particles examined [Hinz et al., 1996; Gard et al., 1997]. Single particle time-of-flight mass spectrometry has been applied successfully during several field campaigns including investigation of the upper troposphere using a Copyright 2002 by the American Geophysical Union /02/2001JD mobile instrument in a research airplane [Murphy et al., 1998], ship-based measurements of marine aerosols in the Indian Ocean [Guazzotti et al., 2000], and measurements of traffic emissions using the laser mass analyzer for particles in the airborne state (LAMPAS 2) instrument [Hinz et al., 1997]. [4] The aim of the Lindenberg Aerosol Characterization Experiment 1998 (LACE 98) was the quantification of radiative effects of the summer aerosol in central Europe. During this field campaign the LAMPAS 2 instrument was used to measure the chemical composition of single particles and particle populations for examination of optical aerosol parameters on the one hand, and for source tracing on the other hand. Optical properties are important parameters for calculation of radiative effects in field measurements or in climate models. [5] Our data handling method [Hinz et al., 1999] allows us to reduce the large data quantities from single particle analysis and to ease the task of defining main particle classes representative for the whole particle population. Using main particle classes for characterization of the aerosol provides for a strikingly simple way of correlating chemical and meteorological data or other aerosol properties, such as light scattering coefficients or particle growth behavior after water uptake. Size-resolved chemical data also possess the potential of being used to synthesize an aerosol of interest for LAC 13-1

2 LAC 13-2 TRIMBORN ET AL.: ONLINE ANALYSIS WITH LAMPAS 2 DURING LACE 98 Figure 1. Setup of the LAMPAS 2 instrument. further examination in the laboratory. Such investigations allow us to determine the refractive index and the single scattering albedo on the basis of the determined chemical composition. In order to synthesize the various types of environmental aerosol particles, however, a precise characterization of their chemical properties and of their relative abundance is crucial. Information on size and composition of individual particles within a given particle population is essential for source identification and source strength determination, and provides for a deeper understanding not only of transportation phenomena but also of chemical processes of atmospheric particles. 2. Instrumentation [6] The aerosol was examined by the transportable online laser mass spectrometer LAMPAS 2 (Figure 1). A detailed description of the instrument is published elsewhere [Trimborn et al., 2000]. Aerosol particles are introduced into the mass spectrometer by separating them from ambient air employing a differentially pumped inlet system. The inlet system was designed to transfer aerosol particles with diameters between 0.2 mm and 10 mm with high efficiency. [7] In the main vacuum chamber, the accelerated particles cross the focus of a continuous laser beam (l = 532 nm), the scattered light of which is detected by a photomultiplier system. The photomultiplier output signal triggers the ionization laser after a certain delay time, characteristic for the particle velocity and thus for the aerodynamic diameter of the particle. The ionization laser pulse hits the particle downstream from the detection region, evaporating and partially ionizing it. Spectra of positive and negative ions are detected simultaneously by a bipolar time-of-flight mass spectrometer and are stored together with particle-size data on a personal computer. An improved version of the size classification method using an active triggering circuit [Prather et al., 1994; Reilly et al., 2000] for ionization of each detected particle was realized recently [Hinz et al., 2000]. [8] With the instrument it is possible to record thousands of spectra during long-term measurements. To evaluate these data an automatic evaluation system is mandatory. The purpose of the evaluation system is to reduce the amount of data without loss of significant information, to classify particles into groups, and to characterize particle classes in terms of chemical composition. For evaluation of the LACE 98 data an in-house data analysis system was employed based on a fuzzy clustering algorithm [Hinz et al., 1999]. The clustering algorithm vectorizes the positive-ion and negative-ion spectra of each particle. The data vectors of the measured particles are written into a classification matrix and main class centers are calculated by the algorithm, displayed as mean mass spectra. This approach has two important advantages over commonly used data analysis systems: First, the chemical compositions of the particle classes are easily characterized and second, the abundance of each particle class within the aerosol is obtained simultaneously. The determined particle classes can easily be compared with known (for example, aged sea salt) or unknown particles types. [9] Once the particle classes within an aerosol are known there is the attractive possibility to synthesize particles of the various classes and with it the entire aerosol in the laboratory. In our case, synthesized aerosol particles were produced from aqueous solutions of components determined from the respective spectra. These solutions were nebulized using an ultrasonic nebulizer, and resulting aerosols were introduced into the LAMPAS 2 instrument. Evaporation of the liquid was facilitated by passing the aerosols through a diffusion dryer. Simultaneously the

3 TRIMBORN ET AL.: ONLINE ANALYSIS WITH LAMPAS 2 DURING LACE 98 LAC 13-3 Figure 2. Spectra pattern of the LACE 98 aerosol. Spectra pattern of (a) exhaust particles, (b) mineral particles with secondary components, (c) aged sea salt particles (pure), (d) aged sea salt particles with organic components, (e) potassium salt particles with carbon.

4 LAC 13-4 TRIMBORN ET AL.: ONLINE ANALYSIS WITH LAMPAS 2 DURING LACE 98 Figure 3. Spectra pattern of the LACE 98 aerosol. Spectra pattern of (a) sodium salt particles with carbon, (b) carbon particles with salt, (c) biogenic carbon particles, (d) carbon particles with secondary components, (e) pure carbon particles.

5 TRIMBORN ET AL.: ONLINE ANALYSIS WITH LAMPAS 2 DURING LACE 98 LAC 13-5 Figure 4. Size resolved aerosol composition of 10 August. aerosols were transferred to a filter sampler for subsequent determination of the refractive indices. 3. Results and Discussion 3.1. Chemical Composition of the Aerosol [10] A size-dependent evaluation of particle spectra detected during LACE 98 was performed in five size ranges. These size ranges have mean aerodynamic diameters of 0.2, 0.5, 0.8, 1.0, and 1.5 mm, respectively. In this paper we present the results obtained on the following selected days during the field experiment (Golden days [Ansmann et al., 2002]): 31 July to 1 August 1998, 5 6 August 1998, and 9 11 August Several hundreds of particle spectra were acquired each day, comprising different size ranges. Evaluation of the spectra recorded on the specified days resulted in 4 to 6 representative particle classes for each day for each size range. Comparing the particle classes evaluated for each day and each size range led to the striking result that the particle populations of all days can be described by a total of just 10 basic chemical particle classes. Sorting the particle classes by the fuzzy clustering algorithm showed that these 10 particle classes describe about 90% of the particle classes evaluated for each day and size range. Only 10% of the particle classes are characterized by low membership coefficient to any of the 10 basic particle classes. Hence, differences in the aerosol composition result from different abundances of the 10 basic particle classes and not from appearance of completely new particles types. Figures 2 and 3 show the representative spectra patterns for the particle classes under investigation. The ten attributed particle classes can be described chemically as follows: [11] The first spectra pattern (Figure 2a) represents sodium/calcium/carbon mixed particles. They are denoted exhaust particles because of their similarity to engine exhaust particles measured in former experiments [Hinz et al., 1997]. Their spectra are characterized by intense signals of sodium and calcium. Additionally, the particles contain nitrate, phosphate, sulfate, and some carbon. [12] Figure 2b shows the pattern of mineral particles ( mineral dust ). This particle class is featured by high amounts of metal and metal oxides, respectively, such as sodium, aluminum, aluminum oxide, potassium, and iron. Additionally the particles contain chloride and phosphate. Such particles also contain secondary components like ammonia, nitrate, sulfate, and a significant amount of water. Signals from lead are observed in most of the particles of this class. [13] The spectra pattern shown in Figure 2c is significant for aged sea salt particles. Particle spectra are dominated by sodium compounds as well as sodium and potassium clusters and magnesium. These particles contain high amounts of nitrate, but less chloride compared to sea salt particles, as a result of atmospheric exchange processes [Harrison and Kitto, 1990; Gard et al., 1998]. [14] The class of aged sea salt particles with organics ( aged sea salt + organics ) is represented by the spectra pattern shown in Figure 2d. Particles of this class are chemically very similar to the class of aged sea salt particles (Figure 2c), with the major difference that they contain a higher amount of organic components. [15] Figure 2e shows the spectra pattern of the class of potassium salt particles containing carbon. This class of particles is characterized by an intense potassium peak, and smaller peaks for sodium, nitrate, phosphate, carbon and lead. [16] The chemical composition of sodium salt particles with carbon (Figure 3a) is similar to the class of potassium salt particles (Figure 2e), with the difference of the amount of sodium being higher and the amount of potassium being considerably lower. [17] The class of carbon particles with salt ( C + salt, Figure 3b) is dominated by carbon signals. Additionally, the

6 LAC 13-6 TRIMBORN ET AL.: ONLINE ANALYSIS WITH LAMPAS 2 DURING LACE 98 Figure 5. Correlation of particle composition and meteorological data. The bars in the diagram show the abundance (%) of the particle classes in two size ranges (d aer <0.8mm and d aer >0.8mm). See color version of this figure at back of this issue. particles contain sodium, potassium, aluminum, aluminum oxide, nitrate, and sulfate. [18] The spectra pattern in Figure 3c describes the class of biogenic carbon particles which are dominated by carbon compounds. Additionally, they contain nitrate and sulfate as well as phosphate and large amounts of potassium, both of them being markers for the biological origin [Andreae, 1983; Artaxo et al., 1994]. [19] Figure 3d shows the spectra pattern of carbon particles with secondary components ( C + sec ). These particles are dominated by carbon and ammonia, nitrate and sulfate representing the portion of the particles being formed by secondary atmospheric processes. Particles of this type contain a significant amount of water. [20] The last particle class (Figure 3e) describes pure carbon particles with no water attached. [21] The discrimination of only 10 basic particle classes comprising all particle size ranges measured during the specified days enabled us to compare the aerosol composition with respect to size dependent differences. Two main size ranges (d aer <0.8mm and d aer >0.8mm) of fine and coarse particles were found, similar to a previous classification [e.g., Heintzenberg, 1989]. Chemical compositions represented by signal patterns and class abundances differed only slightly within each of the two size ranges (Figure 4), whereas huge differences were observed comparing the two size ranges for a single day. It was observed during the complete measuring period that aged sea salt particles are present only in the larger size range, while carbon particles dominate the small particles size range. Mineral dust particles were found in all size ranges as minor components of the aerosol. It is important to mention that particles possessing an aerodynamic diameter d aer = 0.8 mm can be considered as mixtures of particles with d aer <0.8mm and d aer >0.8mm. Dividing the particle sizes into two distinct ranges not only reduces calculation power needed for computer modeling purposes, but also simplifies comparison to other aerosol and meteorological parameters. [22] Additionally, discrimination into a fine and a coarse mode is reasonable from a chemical point of view.

7 TRIMBORN ET AL.: ONLINE ANALYSIS WITH LAMPAS 2 DURING LACE 98 LAC 13-7 Figure 6. Residence times of the air parcel trajectories. The presence of only 10 considerably different particle classes for all days evaluated can be explained assuming that the aerosol measured at Lindenberg was stable with respect to physicochemical properties over a long period of time. It is therefore reasonable to speak of a typical Lindenberg aerosol with only the abundance of single particle classes differing, depending on the meteorological conditions. We can speculate that the stability of the observed chemical particle classes is a result of the extended urban areas and marine surrounding in close vicinity to the measuring point Comparison of Particle Composition and Meteorological Data [23] Figure 5 shows the calculated air parcel trajectories (96 h, 950 hpa, Meteorological Observatory Lindenberg, unpublished data, 1998) and the determined compositions of particle populations from seven evaluated days during LACE 98. Correlation of meteorological data with chemical particle data can easily be performed on the basis of the two distinct particle size ranges fine and coarse, which were defined in section 3.1. [24] Particle populations of all selected days contained aged sea salt particles only in the size range larger than 0.8 mm. The prominence of aged sea salt particles was as expected, resulting from transportation of the respective air parcels across the sea. The abundances of aged sea salt particles in the coarse mode were between 33% of the total number of particles on 1 August and 75% on 31 July. [25] Aerosols with a long passage time over rural areas (11 August and 10 August) were characterized by a high abundance of mineral particles (17 25%), present in both size ranges. Many exhaust and carbon containing particles, on the other hand, were observed on days when the aerosol had traveled over densely populated areas. This was the case, for example, on 31 July when the air parcels had traveled over France and western Germany before arriving at Lindenberg. A similar situation was found on 1 August when the aerosol had traveled over the Netherlands and Northern Germany before reaching the observation point. As a result, between 18% and 35% of the detected particles contained black carbon in the fine particle mode and up to 25% of them were typical exhaust particles. The high portion of carbon-containing particles on 9 August of about 30% in both size ranges is probably the result of transport of air over Berlin immediately before reaching the observation point in Lindenberg. [26] These data show a strong correlation between chemical composition of an aerosol and its travel path, suggesting that the chemical composition of the aerosol is an important tracer for the particle sources. Comparison of the aerosol composition and air parcel trajectories for 10 and 11 August is an instructive example for this dependence. Air parcel trajectories were rather similar for the two days, with the aerosols having comparable dwelling times over land and ocean (Figure 6). [27] The total particle concentration measured on 11 August was up to 3.5 times higher than that measured on 10 August [Neusüß et al., 2002]. A closer look at absolute particle numbers reveals that the number of aged sea salt particles was similar for both days, whereas the absolute number concentration of mineral dust particles for all size ranges was up to 5 times higher on 11 August compared to 10 August. In the small-particle range, higher abundances of carbon particles were observed on 11 August, contributing to the higher total particle numbers. [28] The air parcel back trajectories for 10 and 11 August differed considerably during the last 42 hours before analysis, when the aerosol of 11 August traveled over Poland, Czech Republic, and Germany. During its travelling time the aerosol had crossed rural and industrial areas, leading to a higher abundance of mineral dust and carbon particles. Correlation of aerosol compositions and air parcel trajectories is obviously a valuable tool for

8 LAC 13-8 TRIMBORN ET AL.: ONLINE ANALYSIS WITH LAMPAS 2 DURING LACE 98 Figure 7. Positive-ion and negative-ion spectra of an aged sea salt particle measured during LACE 98 (a) and of a synthesized aged sea salt particle (b). discrimination of long-term transportation effects and localsource effects Evaluation of Optical Properties of the Aerosol [29] The fact that all measured aerosol particles could be described by a total of just 10 basic particle classes allowed us to perform additional laboratory experiments to further evaluate the physical properties of the aerosol. Especially optical properties (e.g., refractive index of particles) are known to be of major relevance for the radiative forcing. In agreement with other research groups chemically analyzing the aerosol during the LACE 98 campaign (P. H. Wieser et al., Chemical characterization of atmospheric aerosol particles by laser microprobe mass analysis (LAMMA): A contribution to LACE 98, submitted to Journal of Geophysical Research) [Ebert et al., 2002; Neusüß et al., 2002] and with published data [Koepke et al., 1997], our set of 10 basic particle classes

9 TRIMBORN ET AL.: ONLINE ANALYSIS WITH LAMPAS 2 DURING LACE 98 LAC 13-9 was reduced to 4 main particle types for comparison. These 4 classes were then correlated to optical parameters determined by the other groups (Table 1) [see Bundke et al., 2002] for an improved climate modeling. For this purpose particle classes with similar major components were put together in one group (Table 1). [30] First attempts to reproduce aerosol particle populations in the laboratory and to resemble the particle spectra patterns observed during LACE 98 are currently underway. Salt solutions of various compositions were already investigated with the aim to determine the refractive index of the main particle types. Figure 7 compares single particle spectra of an aged sea salt particle obtained during the LACE 98 campaign and spectra of a synthesized particle of the determined composition. The solution used for synthesis of the particles as presented in Figure 7 consisted of sodium chloride, sodium sulfate, magnesium chloride and nitric acid in a molar ratio of 1:0.85:2.3:0.11 [Kester et al., 1967]. [31] As listed in Table 2, the synthesized aerosol exhibited signal patterns similar to the natural aerosol. [32] The synthesized aerosol contained water in a comparable amount as the natural aged sea salt particle population. [33] Measurement of the single scattering albedo and refractive index of the synthetic aged sea salt particle aerosol was performed by Ulrich Bundke at the University of Frankfurt, employing the method described by Hänel, [1994]. The refractive index was determined as n = i, in good agreement with values published by Koepke et al. [1997]. Future investigations will be focused on the determination of refractive index and single scattering albedo of the entire aerosol by synthesis of the main types of particles in various abundances. 4. Conclusion [34] The measurements performed during the LACE 98 campaign showed that the aerosol composition at Lindenberg can be described by ten main particle classes. The varying abundances of these particle classes at different measuring days and within different particle size ranges were found to be a direct result of the traveling path of the particles across the continent and the sea. All of the investigated aerosols were influenced by marine aerosol sources, represented by high abundances of aged sea salt particles. Rural and urban influences on the aerosol composition could be discriminated on the basis of varying Table 1. Main Particle Types Determined During LACE 98 and Their Assumed Optical Properties Particle Type Spectra Patterns Optical Property Type 1, exhaust scattering mineral mineral dust Type 2, aged sea salt scattering sea salt aged sea salt + organics Type 3, salt + carbon salt + C (K) salt + C (Na) scattering and absorption Type 4, carbon C + salt biogenic carbon C + sec pure carbon high absorption Table 2. Significant Ion Signals of Aged Sea Salt Particles Their Chemical Interpretation Mass, m/z Positive Ion Spectrum Negative Ion Spectrum 23 Na + 24/25/26 Mg + 35/37 CI 40 NaOH Na 2 NO Na(OH) 2 62 NO 3 63 Na 2 OH + 81 Na 2 Cl Na 2 NO NaN 2 O NaN 2 O NaN 2 O Na 3 SO 4 abundances of carbon containing, exhaust and mineral particles, reflected in the abundances of the main particle classes. A particle class is representative for a certain type of aerosol particles and reflects the particle s origin as well as meteorological and travelling influences. Far and local particle sources could be distinguished as well, by data reduction of the measured aerosol to a small number of particle classes. This feature was demonstrated for two days of the LACE 98 campaign. [35] Synthesis of aerosols in the laboratory is possible by using the determined composition of the main particle types. As a first approach four main types of particles have to be produced and mixed to synthesize the aerosols for all measured days and sizes. For an exact simulation of the Lindenberg aerosol all 10 particle classes have to be synthesized. As an example, aged sea salt particles were successfully synthesized on the basis of the typical signals found in the mass spectra. The synthetic aerosol was then used to determine the single scattering albedo and the refractive index of this aerosol component. The results were in a good agreement with measurements of other groups. Mass spectra of the synthesized particles showed high similarity of those of the native particles of the simulated particle class. [36] The mass spectrometric results achieved during the LACE 98 experiment are available to the other groups of the joint project for comparing, correlating and evaluating their data acquired with other methods and can be used for an improved climate modeling in the future. The performed experiments were very successful, in sum and showed the broad applicability and the high informational contents of the method. Impact-free and time-resolved chemical analysis by online bipolar laser mass spectrometry has proven to be an invaluable tool for airborne-particle analysis, atmospheric research, environmental chemistry, and climate modeling. [37] Acknowledgments. This work was supported by the Bundesministerium für Bildung und Forschung (BMBF), Germany, grant 07AF1141. References Andreae, M. O., Soot carbon and excess fine potassium: Long-range transport of combustion derived aerosols, Science, 220, , Andreae, M. O., and P. J. Crutzen, Biogeochemical sources and role in atmospheric chemistry, Science, 276, , 1997.

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11 TRIMBORN ET AL.: ONLINE ANALYSIS WITH LAMPAS 2 DURING LACE 98 Figure 5. Correlation of particle composition and meteorological data. The bars in the diagram show the abundance (%) of the particle classes in two size ranges (d aer <0.8mmand d aer >0.8mm). LAC 13-6