Environ. Sci. Technol. 1094, 28, 1513-1520
Individual Aerosol Particle Composition Variations in Air Masses Crossing the North Sea Lleve A. De Bock,’ Hans Van Malderen, and Ren6 E. Van Grleken Department of Chemistry, University of Antwerp, Universiteitsplein 1, 8-26 10 Antwerp-Wilrijk, Belgium
Single-particleanalysis on North Sea aerosol and rainwater samples was performed by electron-probe X-ray microanalysis (EPXMA). The analysis was mainly focused on the determination of the inorganic composition of giant particles with diameters above 1pm. Multivariate techniques were used for the reduction of the data set and for source apportion. Based on the relative abundances found by hierarchical cluster analysis according to the Ward error sum method, three to eight different aerosol types were distinguished. Crossing the North Sea, the changes in air mass composition appeared as a decrease in the abundance for the aluminosilicate particles and a relative increase for NaCl and seawater crystallization products. Principal factor analysis revealed four different aerosol sources: aluminosilicates and NaC1, seawater crystallization products as a marine source, and two industrial sources. Relations between the particle composition, origin, and shape were studied by manual EPXMA, and for most of the particle types, a characterization based on their shape was obtained.
Introduction It has been shown that atmospheric deposition represents a major input route to the North Sea for some pollutants, like heavy metals (I). Within the scope of the EUROTRAC Project “Air-Sea Exchange”, aerosol and rainwater samples were collected above the North Sea on board two research vessels continuously positioned downwind from each other. The aim of this project was to study variations in the composition of air masses crossing the North Sea due to air-sea exchange processes in the lower troposphere. The two major exchange processes considered to be responsible for possible changes in air mass composition are dry deposition, such as sedimentation or gravitational fallout and impaction, and wet deposition, such as rainout, snowout, and washout. A decrease in particle concentration could also be the result of vertical dilution of the air masses. Obviously, chemical and physical reactions in the atmosphere as well as parameters like wind speed, relative humidity, and temperature affect these processes. The single-particle analysis in the present study was performed by electron probe X-ray microanalysis (EPXMA), one of the most commonly used nondestructive microanalytical techniques. In spite of its unfavorable detection limits (0.1% 1, automated EPXMA is, in combination with multivariate techniques, a powerful method for the determination of the chemical composition and characterization of a large number of individual particles in a very short time. The determination of the chemical composition could provide assignment to specific sources while the particle group abundance characterizes the
* Address correspondence to this author; e-mail address:
[email protected]. 00 13-938X/94/0928- 15 13$04.50/0
0 1994 American Chemical Society
source strength. Manual EPXMA, on the other hand, offers the possibility of morphological studies and element mapping. Bulk and alternative individual single-particle analyses have been performed on the same samples by energydispersive X-ray fluorescence and electron proton microprobe analyses, respectively (2). The aerosol analysis in this work was mainly focused on aerosol particles with diameters above 1pm, the so-called giant aerosols. Although the number of these giant particles in the lower troposphere is very low compared to the condensation-mode particles, their contribution to the atmospheric deposition is of extreme importance (35). Due to the slow realization of the importance of giant aerosol particles in the atmosphere together with sampling difficulties and measurement errors, many questions still remain unanswered, and further research will be necessary.
Experimental Procedures Sampling Strategy. A sampling campaign was organized for September 15-27,1991. During this campaign, two research vessels, F.S.Alkor and R.V. Belgica, were positioned on a circle, with a 200-km diameter, continuously downwind from each other in the central area of the North Sea (Figure 1). Sampling started on the F.S. Alkor (upwind ship) on the 16th at 1 a.m. and stopped on the 25th at 11a.m. Sampling on the R.V. Belgica (downwind ship) was delayed according to the calculated air mass travel time, based on the actual wind speed aboard the F.S. Alkor and the R.V. Belgica. A wind speed of 10 m/s-l and a distance of 200 km resulted in a transport time of ca 5.5 h by which the downwind ship had to delay its sampling interval. Every 8 h new positions were taken on the circle. When achieved, the ships were holding position within a few miles facing the wind for undisturbed sampling conditions. In this way, the same air mass was sampled with an interval of 200 km. During the travel time to reach the new position, sampling was stopped. The result of this campaign gave an idea about the changes in composition that aerosols and rainwater undergo during air mass travel. Meteorological data were provided by radiosonde inspection aboard the F.S. Alkor every 8 h. The whole sampling campaign was characterized by stormy weather. Due to prevailing southwesterly winds, the United Kingdom was the main source region for anthropogenic and soil-derived components. Aerosols were collected on the main deck using a cascade impactor and filter units, both positioned inside a wind tunnel, for single-particle and bulk analyses, respectively. In a cascade impactor, aerosols are segregated in size based on their inertial characteristics. Microscopeslides covered with special coated Nuclepore filters (COSTAR Europe Ltd. Nuclepore Filtration Products) were used as the impaction surface. The special coating is necessary to reduce effects like “bounce off”and “reentrainment”,which Environ. Sci. Technol., Vol. 28, No. 8,
1994
1613
4" Flgure 1.
North Sea map with indication of the sampling sites on a circle of 200 km, around
influence the collection efficiencyand change the apparent size distribution. The type of cascade impactor applied during this campaign was based on the design of May (6). It offers a very good resolution of particle size due to sharp cutoffs atdifferentstages (20,8,4,2,1, and0.5pm),minimal internal losses, the possibility for quantitative analyses at sampling speeds of 20 L/min, easy handling due to its compactness and fast dismantlement, and stainless steel composition to prevent corrosion. To collect the total airborne particulate matter, filter units were also applied. Because no size segregation is required, the Nuclepore filters were not coated. A representative sampling of atmospheric aerosols is only possible when isokinetic conditions are fulfilled. This means that there cannot be any kind of disturbance of the air stream at the inlet of the impactor, resulting in a nongradual translation of the aerosol particles into the impactor nozzle. The wind tunnel used, designed at the consists of a 1.2-m thin-walled University of Essex (9, tube with a diameter of 0.25 m. In the middle of the tube 1514
Envlron. Scl. Technol., Vol. 28. No. 8, 1994
55.0' N and 4.0' E.
immediately behind a honeycomb structure, which creates a laminar flow, the impactor and filter unit are positioned. At the end, a ventilator sucks air into the tunnel at the same speed as it is sucked into the sampling device by the pump, which guarantees isokinetic sampling. To keep the inlet of the tunnel in the prevailing wind direction, it is provided with a 1-m2wind vane. Due to the suppressing effect of a portable wind tunnel on airstream fluctuations around the impactor and the filter unit, isokinetic sampling is possible, and giant particles can quantitatively be collected also. For the collection of rainwater, a PVC funnel was connected to a low-density polyethylene barrel on the main deck. The barrels were first treated with ultrapure nitric acid during 1 month and washed with Milli-Q water afterwards. After each rain shower, the funnel was closed, and the collected rainwater was frozen to prevent microorganism growth. In the lab, the bottles were thawed, and each time 100 mL of rainwater was filtered on a Nuclepore membrane.
Instrumentation. A set of 75 size-segregated aerosol samples and five rainwater samples was analyzed by EPXMA. The automated analyses of 27 500 particles was performed on a JEOL 733 Superprobe equipped with a Tracor Northern TN-2000 system, using the particle analysis program 733B written in Fortran (8). For every impactor stage, 300 particles were analyzed, and 500 particles were analyzed for each rainwater sample. The analysis was carried out at an acceleration voltage of 25 kV and a beam current of 1 nA. The energy-dispersive X-ray spectra accumulation time was fixed at 20 s to obtain satisfactory signal/noise ratios. In the 733B program, the localization of a particle is performed by successive horizontal line scans with the electron beam, followed by saving the contour pixel of the particle. After saving all the contour pixels, the area, perimeter, and diameter of the particles are calculated, and the X-ray spectrum is accumulated. All the information is stored on disks for off-line data processing on a Unix computer. Reducing the data set was performed by multivariate techniques. To study the relations between the particle composition, origin, and shape and obtain even a possible characterization of particles based on their shape, over 200 giant aerosol particles were manually analyzed by EPXMA using the 733A program. The particles were collected for singleparticle analysis on a Nuclepore filter in the wind tunnel. The only difference with 733B is that the particles are localized manually and the X-ray spectrum accumulation time was 100 s. ZAF corrections were performed on these spectra for the following elements : Na, C1, Mg, Al, Si, P, S, K, Ca, Fe, Ti, Ni, Cu, and Zn, resulting in normalized concentrations for each element expressed in weight percentages. The sensitivity of a conventional EPXMA instrument is poor for Na; even at relatively high concentrations, Na may escape detection. Matrix Correction and Multivariate Techniques. EPXMA is a fast method for element identification. However to obtain quantitative information about the elements present, a very complex procedure is needed, called ZAF correction. Due to electron-sample interactions, processes occur which influence the production and collection of X-rays. The ZAF procedure performs a correction for the atomic number effect (Z),the absorption effect ( A ) ,and the fluorescence effect (F).Z represents the difference in electron scattering and retardation in the sample and the standard. Loss of X-rays due to absorption in the sample is represented by A, and the artificial increase of X-ray intensity of an element due to ionization by X-rays originating from an other element is corrected by F. Without correction, errors in excess of 10% could result. After a ZAF correction, the element concentrations present are normalized and expressed in weight percentages. Reduction of the data set was performed by hierarchical cluster analysis on each of the aerosol and rainwater samples (9), producing a classification into groups of particles with chemically similar composition. A hierarchical cluster analysis starts with n particles or clusters from which the most similar ones are joined successively into new clusters. Different strategies are possible to join two clusters; Ward’s error sum method is the one we applied because it provides a maximum internal homogeneity into the separated groups (10). Principal factor analyses (PFA) with orthogonal varimax rotation was used to discover the interrelation of 13
Table 1. Varimax Rotated Factor Loading Matrix for 75 North Sea Giant Aerosol Sampless variable Na M3 A1 Si
P S
c1 K
factor 1 factor 2 factor 3 factor 4 communality SD -0.34
-0.81 0.82
0.91 0.87
0.16
0.17
0.85
-0.40 -0.23 0.30 -0.33
0.87
0.31 -0.72 0.17
0.80 0.79
0.31 0.25
0.792 0.708 0.851 0.826 0.854 0.872 0.678 0.824 0.747 0.813 0.878 0.933 0.912
0.15 0.18 0.15 0.15 0.15 0.12 0.21 0.15 0.18 0.15 0.12 0.09 0.12
Ca 0.83 0.16 0.30 Ti Fe 0.93 0.13 Ni -0.18 0.12 0.48 0.81 Zn 0.59 0.73 0.18 eigenvalue 5.05 2.83 1.78 1.02 % variance 38.5 21.6 13.6 7.8 explained a Only values greater than three times their standard deviation were reported.
variables (Na, Mg, Al, Si, P, S, C1, K, Ca, Ti, Fe, Ni, and Zn) in our aerosol data set, which led to the identification of different sources of giant aerosols. This multivariate technique splits the data set into subsets of strongly correlated variables. The rotation of these subsets or ‘factors’ provides a better factor loading. Elements were considered to be detected if the X-ray intensities were found above the detection limit in one of the 300 analyzed particles, for each air sample. Elements like Cu, Pb, Mn, Cr, Ba, Pt, and V were occasionally detected in particles. These elements were found in particles which contributed less than 0.5% of the total aerosol abundance, and therefore, they were further excluded from the data matrix. The major problem in PFA is the choice of the number of factorsP that should be taken into account in the model or how many factors are required to estimate the communalities. The value of P was determined by the following criteria (11): the number of factors should be significantly less than the number of variables, a large fraction of both the total variance of the variables and the individual variable variance has to be explained by the factor variation, which means that the communalities should be close to 1 and factors that contribute with a variance less than 1 should be excluded from the model. Considering these criteria, the appropriate number of factors is established. Results and Discussion
Automated EPXMA. Principal Factor Analyses. By performing PFA on the correlation matrix of the data set, four eigenvalues greater than 1were produced (5.05,2.83, 1.78, and 1.02); the fifth value was 0.86. Because no standard procedure exists for the selection of the number of factors, factor matrices with four and five eigenvalues were calculated. After comparing the two varimax rotated matrices, the solution with four factors was preferred, and the varimax rotated factor matrix is represented in Table 1 (the most important values are shown in bold). To facilitate factor interpretation, factor loadings smaller than three times the standard deviation were rejected because they are considered to be not statistically significant (11). The four factors explain together 81.4% of the total variance of the variables, and communality values for most of the variables are high, situated between Environ. Scl. Technol., Vol. 28, No. 8, 1994
1515
0.74 and 0.93, except for Mg, Ca, and C1. The first factor is determined by high loadings of Al, Si, Fe, Ti, Zn and Na and C1. Explaining 63.7% of the total variance this factor provides the two major particle types: marine seasalt particles and aluminosilicates such as fly ash and soil dust with a continental or anthropogenic origin. The inverse correlation between both group concentrations is expressed by the minus sign of the Na and C1loading. The combination of the seven elements in those two element groups is confirmed by an additional factor analysis. Factor two contains mainly Mg, K, S, and Ca which indicates the presence of seawater crystallization products such as different combinations of CaC03, CaS04, KzS04, MgS04, and dolomite. The NaCl crystals, which should represent the main crystallization product [70% of the total dissolved solids inside a seawater droplet (1611, were probably classified in the first factor due to their dominance as compared to the other crystallization salts. Recalculations of the data set by choosing three factors instead of four confirms this. The loadings of Na and C1 increase, but the communalities that describe how well the factor analysis reproduces the reality are lower. The solution with four factors is therefore more appropriate. Factors three and four can be associated mainly with industrial origins because of high loadings for Ni and Zn, inversely correlated with Na, and for Ni and P, respectively. Elements like Ni and V are released by oil combustion processes in power plants due to oil-fired furnaces. Because the low contribution of natural sources to the total emission of Ni and V, respectively 14% and 16% (12, 13), Ni and V are used as indicator elements of oil-fired power plants and oil combustion sources in numerous industries. Iron, steel, and ferro alloy plants are responsible for the release of Zn. The emissions are distributed evenly over Europe, but large emission areas are located in the United Kingdom, Spain, and Italy. The occurrence of P in the presence of Fe can be assigned to anthropogenic emissions because of its use in ferro alloy. Hierarchical Clustering. Particle classification into different groups based on their chemical composition was achieved by hierarchical cluster analysis. Correct interpretation of the cluster analysis results was possible by taking into account the exact ship positions and meteorological data like wind speed, wind direction, and relative humidity during sampling periods. From September 15 to September 27, wind speed fluctuated between 3.4 and 14.5 m/s, and the prevailing wind direction was southwest, characterizing the samples by both continental and marine influences. The results of the hierarchical clustering of 300 particles for each of the five impactor stages collected on September 18 are shown in Tables 2and 3, on the upwind and downwind ship, respectively. The stage number corresponds to the theoretical cutoff diameter of the particles collected on a certain stage. For each particle type the percent of abundance in a group of 300 particles is given as well as its average diameter (pm) and its composition. Clustering results for aerosol and rainwater samples will be discussed separately. Aerosols. Depending on the sample, three to eight different aerosol types were distinguished. Due to stormy weather and high wind speeds, the concentration of seasalt particles is very high in all the impactors, ranging from 10 to 90% of the total aerosol abundance. In a marine atmosphere, sea spray particles are mainly produced by the bubble-bursting mechanism. Due to breaking waves, 1516 Environ. Sci. Technol., Vol. 28, No. 8, 1994
Table 2. Hierarchical Clustering Results of Five Aerosol Samples Taken on the F.S. Alkor (Upwind Ship) on September 18,1991 stage 1, d
> 20 pm
2,8pm,< d < 20pm
3,4 pm < d < 8pm
4 , 2 pm
< d 4 pm
5 , 1 pm < d
< 2pm
av abundance ( % ) diameter (pm) composition 49 24 6.0 6.0 4.0 72 16 3.3 3.0 2.0 75 12 7.0 2.3 1.0 1.0 74 7.7 6.3 3.4 3.0 3.0 70 18 3.0
6.8 5.3 4.0 4.5 6.7 3.1 4.6 1.7 3.6 3.5 1.3 1.2 1.8 1.7 1.2 2.2 2.0 2.1 2.4 1.8 1.5 3.2 2.2 1.7 1.8
NaCl organic AI, Si, Fe Cr-rich
KCI NaCl NaC1, Cas04 CaSOr, C1 Al, Si, Fe Fe, C1 Cl-rich organic NaCl Al, Si, Fe Fe-rich Si, C1 NaCl Na-rich CaSO4, NaCl organic Cas04 NaCl, Al, Si organic C1-rich Si-rich
Table 3. Hierarchical Clustering Results of Five Aerosol Samples Taken on the R.V. Belgica (Downward Ship) on September 18, 1991, from Same Air Mass with Delay of 6 h stage 1, d > 20 pm
2,8pm
