Measurements of non-volatile fractions of pollution aerosols with an eight-tube volatility tandem differential mobility analyzer (VTDMA-8)

Aerosol Science 35 (2004) 185 – 203 www.elsevier.com/locate/jaerosci

Measurements of non-volatile fractions of pollution aerosols with an eight-tube volatility tandem di'erential mobility analyzer (VTDMA-8) S. Philippin, A. Wiedensohler∗ , F. Stratmann Institute for Tropospheric Research, Permoserstrae 15, Leipzig 04318, Germany Received 12 July 2003; accepted 31 July 2003

Abstract A volatility tandem di'erential mobility analyzer (VTDMA) was developed that o'ers high size resolution with high temporal resolution: the eight heating columns of the VTDMA-8 allow rapid measurement of the non-volatile component of aerosol particles within a narrow particle size range in about 15 min. The non-volatile composition of these quasi-monodisperse particles is described in terms of number fraction
N and volume fraction
V of the non-volatile material. Laboratory studies were made to determine the transfer e6ciency and the volatile behavior of sulfate particles in the VTDMA-8. Its applicability to atmospheric aerosols was con8rmed during three 8eld measurements. Since the distribution of carbonaceous material depends on the distance to combustion sources and varies with region (i.e., urban or rural) and with time, we made continuous surface measurements in air masses polluted to di'erent levels: (a) in Falkenberg, a moderately polluted, rural region in north-eastern Germany, (b) in the city of Leipzig, a strongly polluted environment, and (c) directly at a combustion source in the exhaust of a diesel-fueled vehicle. The results show that the refractory material contained in 150 nm atmospheric particles (at Falkenberg and Leipzig) ranged between 8% and 25% in volume and represents an external mixture of particles containing various amounts of non-volatile material. However, the vehicle-emitted 50 nm particles at a single combustion source were internally mixed, and that non-volatile fraction ranged between 75% and 98% in volume, depending on the operating conditions of the vehicle (speed, engine revolution). The results show that throughout all measurements the total number of particles remained unchanged after evaporation of the volatile material. Our results demonstrate that the VTDMA-8 can rapidly measure the non-volatile fractions of size-segregated pollution aerosols and that it is applicable to study the mixing state of aerosols and the e'ect of carbonaceous material on aerosol aging processes. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Volatility; TDMA; Non-volatile aerosol

∗

Corresponding author. Tel.: +49-341-2352467; fax: +49-341-2352361. E-mail address: [email protected] (A. Wiedensohler).

0021-8502/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaerosci.2003.07.004

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1. Introduction Carbonaceous particles comprise a signi8cant fraction of atmospheric aerosols, particularly in the submicrometer size regime (Heintzenberg & Covert, 1984; Gray, Cass, Huntzicker, Heyerdahl, & Rau, 1986; Hitzenberger & Puxbaum, 1993; Venkataraman, Lyons, & Friedlander, 1994). However, the wide variety of carbonaceous particles and their structural complexity make measurements dif8cult, so that the composition and structure of the carbon component of atmospheric aerosol particles are still not well understood. Carbonaceous constituents of atmospheric aerosol particles are emitted as primary combustion products in the form of organic and elemental carbon. Elemental carbon represents the non-volatile component of carbonaceous material that is thermally refractory up to temperatures of 650 –700◦ C. The non-volatile carbon particles increase the surface area available for condensation of condensable vapors and might, therefore, suppress new particle formation. Non-volatile carbon has been shown to catalyze the oxidation of SO2 , especially in the presence of water (Chang, Toossi, & Novakav, 1981; Smith et al., 1989) and to increase the hygroscopicity when coated with organic compounds (Andrews & Larson, 1993). Measurement and quanti8cation of carbonaceous material in the submicrometer particle size range are di6cult due to the small size and corresponding small mass of these particles. Only airborne volatility techniques can resolve their size-segregated physico-chemical composition with the necessary high temporal resolution. The most common airborne volatility methods to determine aerosol composition are based on the principle of passing the aerosol through a conditioning tube where the speci8c chemical components are evaporated at prede8ned temperatures (Clarke, Ahlquist, & Covert, 1987; Pinnick, Jennings, & Fernandez, 1987; Clarke, 1991; Burtscher et al., 2001). To determine the corresponding refractory fraction, the residual size distribution is measured with either an optical particle counter or a scanning mobility particle sizer, and is then compared to the initial distribution. To separate the volatile from the non-volatile compounds (e.g., elemental carbon, crustal material, sea salt), most of these measurements are made at temperatures below 300◦ C. To infer the fraction of elemental carbon, measurements are made between 800 –1000◦ C, however, they require complex instrumentation and are limited to highly polluted environments (Jennings & O’Dowd, 1990; Jennings, O’Dowd, Cooke, Sheridan, & Cachier, 1994; Smith & O’Dowd, 1996). In situ volatility measurement techniques provide information on the carbon-component of aerosols with high temporal resolution. Examining only the change in particle size of an initially polydisperse size distribution, however, does not provide information on the mixing state of the chemical compounds before conditioning, and most importantly, does not permit measured changes in particle number concentrations to be assigned to individual, narrow size intervals. Therefore, residual particles cannot be traced back to their original size and it remains unclear how individual particle sizes are transformed during the volatilization process. To overcome this uncertainty, a volatility tandem di'erential mobility analyzer (VTDMA) (Rader & McMurry, 1986; Covert & Heintzenberg, 1993) was developed to investigate the behavior of quasi-monodisperse aerosol particles during thermal conditioning. The early VTDMA was used to quantify the predominate sulfuric acid fraction of aerosol particles in the remote marine boundary layer at temperatures up to 150◦ C (Orsini, Wiedensohler, Stratmann, & Covert, 1999). Based on the principles of operation of this previous VTDMA, in this study we developed a volatility analyzer capable of evaporating volatile aerosol particles at temperatures above up to 300◦ C and used it to investigate the non-volatile carbonaceous fraction of particles in air masses with various degrees of pollution.

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The VTDMA that we developed selects monodisperse submicrometer particles from a polydisperse aerosol population. In this VTDMA, the volatile compounds are evaporated by heating the aerosol to temperatures between 25◦ C and 300◦ C. To permit rapid selection of the desired evaporation temperature, the heating unit is equipped with eight symmetric columns kept at di'erent temperatures. We therefore call this instrument VTDMA-8. The eight columns allow the heating temperature to be rapidly changed so that a higher temporal resolution can be achieved compared to the previous VTDMA. The change in particle size is recorded by measuring the number size distribution for each column after evaporation of the corresponding volatile fraction (refractory number size distribution). The results can be quanti8ed in terms of non-volatile number and volume fractions with respect to the initial quasi-monodisperse particle size distribution. Therefore, the VTDMA-8 delivers actual, size-segregated information. In this study, 8rst the VTDMA-8 was calibrated to interpret the measurement results. In the calibration, laboratory studies were made to determine the transfer e6ciency and the characteristic temperature at which the major aerosol compounds (sulfate, ammonium) completely evaporated in the VTDMA-8. Second, the VTDMA-8 was used during several 8eld measurements to examine the non-volatile nature of aerosols in environments polluted to di'erent levels. The pollution level of an air mass is governed by the number of nearby combustion sources and their distance from the measurement point. Combustion sources produce large amounts of non-volatile material such as elemental carbon. A link between the pollutant levels and the non-volatile aerosol fractions should therefore exist. The goal of 8eld measurements was to investigate the non-volatile composition of aerosols containing di'erent amounts of carbonaceous material and to give evidence of their mixing state. We focused our investigations on a de8ned, narrow particle size range, particularly for particle size less than 150 nm where previous methods could not yield size-speci8c information. 2. Method 2.1. Instrument The VTDMA-8 is an in situ technique that evaporates the volatile components of aerosol particles within a narrow particle size range and determines the size distribution of the resulting residual particles. The narrow size range is de8ned by the transfer function of the di'erential mobility analyzer (DMA) (Knutson & Whitby, 1975). The VTDMA-8 consists of three sections (cf. Fig. 1): particle selection, particle conditioning, and measurement of residual particle number size distribution. Our measurements focused on a particle diameter of 150 nm, which represents particles in the small, submicrometer size range (accumulation mode aerosols). All aerosol lines in the VTDMA-8 are stainless steel with inner and outer diameters of 4 and 6:35 mm, respectively. To reduce particle losses, the tube lengths and number of bends were also minimized. The Low through all sampling lines is maintained laminar, with a Reynolds number, Re ≈ 350. During continuous operation, the generally steady Low rates are controlled once a day to ensure Low equilibrium in the DMA. The sheath air is dried and passed through a citric-acid 8lter to remove traces of ammonium entering the system that would otherwise neutralize the sulfuric acid on the particles. The relative humidity, RH, of both the sheath and excess air are continuously monitored with RH-sensors and maintained between 0% and 5%. The particles exiting the DMA are, therefore, considered dry.

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1. Particle Selection Atmospheric Aerosol

Bipolar Charger

dN/dlogDp

DMA -1-

CPC -1-

Dp

2. Particle Conditioning

Heating Section

T1

T7

T2

T8

3. Measurement of Residual Particle Number Size Distribution DMA -2-

dN/dlogDp

CPC -2-

Dp

Fig. 1. Schematic of the VTDMA-8.

In the 8rst part of the VTDMA-8, DMA-1 selects particles within a narrow size range from a polydisperse aerosol (cf. Fig. 1(1)). DMA-1 is operated at an aerosol and sheath air Low rate of 2 and 20 1 min−1 , respectively, creating an aerosol-to-sheath-air ratio of 0.1. In this case, the transfer function is ideal and we consider the particles quasi-monodisperse. The Low of the quasi-monodisperse aerosol is split for simultaneous measurement of the number concentration with a condensation particle counter (CPC-1, TSI Model 3010) and for conditioning the remainder of the Low in a heating unit. In the second part of the VTDMA-8 (cf. Fig. 1(2)) the aerosol is heated to prede8ned temperatures in one of the eight parallel heating columns. Each column consists of an inner, 6.35-mm-diameter stainless steel tube for the aerosol Low, and an outer, 25.4-mm-diameter stainless steel tube. A

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glass-silk heating wire is coiled around the outer tube to allow fast heating of the aerosol. The space between the inner and outer tubes is 8lled with sand to act as an insulation bu'er between the heating coil and the aerosol tube. Sand responds slowly to temperature changes and ensures a stable and homogeneous temperature 8eld along the walls of the heating column. For additional insulation, the tubes and heating wire are surrounded by glass wool and quartz 8ber Leece. The residence time of the particles in the heating column is 0:3 s (length = 395 mm). The temperature of the heating columns, Ti , can be independently set to 25 ¡ Ti ¡ 300◦ C. The ambient temperature (i.e., Ti = 25◦ C) column is used as a reference for unheated aerosol. Operation of Ti ¿ 300◦ C requires the use of non-oxidative carrier gases, such as helium or nitrogen, to avoid creating measurement artifacts due to charring of organic matter in the particles. To continuously measure the aerosol temperature, a thermocouple is located at the center of the aerosol tube 36 mm from the end of each heating column. The temperature inside each aerosol tube is computer controlled at ± 2% of the set-point temperature. Using multiple, parallel heating columns permits instantaneous switching between di'erent conditioning temperatures and is achieved by using a computer-controlled valve assembly. In the third part of the VTDMA-8 (cf. Fig. 1(3), the change in size of the particles exiting the heating column is measured with a second DMA-CPC combination. The second DMA (DMA-2) is operated in a scanning mode for a range of particle diameters between 10 and 200 nm. To improve the size resolution of the VTDMA-8 without reducing its temporal resolution (high size resolution decreases the temporal resolution and vice versa), DMA-2 is operated at an aerosol and sheath air Low rate of 1 and 5 1 min−1 , respectively, creating an aerosol-to-sheath-air ratio of 0.2. To optimize the VTDMA-8 performance (i.e., size and temporal resolution), the selected number of size bins and scan range of DMA-2 can be adjusted to the individual measurement conditions (e.g., initial particle diameter, conditioning temperature, and residual number size distribution). For two volatility scans at two di'erent temperatures (e.g., Ti = 25◦ C and 300◦ C), the time resolution of the VTDMA-8 is about 15 min. Data inversion is required to determine the inlet particle size distribution from the measured size distribution. Such data inversion must account for the DMA transfer function. This is achieved by using an advanced data inversion algorithm developed by Voutilainen, Stratmann, and Kaipio (2000). The inverted results de8ning the residual particles are parameterized in terms of number and volume concentration. These so-called non-volatile number and volume fractions,
N and
V , can be derived by dividing the total number, Ntotal , and volume, Vtotal , of the particles measured after heating to Ti , by the Ntotal and Vtotal for the unheated aerosol (i.e., Ti = 25◦ C) as follows:
N; i = Ntotal (Ti◦ C )=NtotalTi◦ C (T25◦ C );


V; i = Vtotal (Ti◦ C )=Vtotal (Ti◦ C )(T25◦ C ):

2.2. Calibration The VTDMA-8 was calibrated to determine its transport e6ciency and its response to an aerosol of known composition. The transport e6ciency is a'ected by particle di'usional and thermophoretic losses in the sampling lines. Penetration measurements were performed using sodium chloride (NaCl) because NaCl is easy to generate and evaporates only at Ti ¿ 600◦ C and is, therefore, una'ected up to Ti =300◦ C. Particle di'usion and thermophoresis are size- and temperature dependent, and therefore the penetration measurements were carried out for a range of particle diameters, Dp (10 ¡ Dp ¡ 150 nm)

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S. Philippin et al. / Aerosol Science 35 (2004) 185 – 203 1.00 25 °C 120 °C 180 °C 300 °C

Transport Efficiency

0.75

0.50

0.25

0.00 0

25

50

75

100

125

150

Particle diameter Dp [nm]

Fig. 2. Transport e6ciency of the VTDMA-8 as a function of particle diameter Dp and conditioning temperature Ti . The lines denote the exponential 8t of the measured (symbols) and standard deviation (error bars).

and temperatures, Ti (25 ¡ Ti ¡ 300◦ C) (cf. Fig. 2). In the VTDMA-8, decreased with decreasing Dp and increasing Ti . For Dp = 150 nm, = 96% for Ti = 25◦ C, whereas = 81% for Ti = 300◦ C. For 10 . Dp . 20 nm (25 ¡ Ti ¡ 300◦ C), = 50%, which we therefore de8ne as the lower limit of the VTDMA-8. Size distributions measured with the VTDMA-8 should be corrected with these measured . Furthermore, we investigated the response of the VTDMA-8 to an aerosol of known composition. Considering sulfate as the dominant submicrometer aerosol component), we determined the temperature at which sulfuric acid (H2 SO4 ) and its neutralized products, ammonium sulfate ((NH4 )2 SO4 ) and ammonium bisulfate (NH4 HSO4 ), were completely evaporated in the VTDMA-8 (i.e., the so-called volatilization temperature Ti ). Particulate nitrate (often in the form of ammonium nitrate (NH4 NO3 )) was not tested because it is more volatile than sulfate (O’Dowd and Smith, 1993). The decrease of
V with temperature is described for Dp = 150 nm:
V (H2 SO4 ) = 50% at Ti ∼ = 65◦ C, whereas
V (NH4 HSO4 ) and
V (NH4 )2 SO4 = 50% at Ti ∼ = 106 and 107◦ C, respectively. At Ti = 120◦ C, still 6% of H2 SO4 is available. Its volatilization temperature is estimated to be around 130◦ C. This volatilization temperature is slightly higher than the reported temperatures of 110 –120◦ C, possibly due to a slight contamination of the reactive H2 SO4 solution. On the other hand, NH4 HSO4 and (NH4 )2 SO4 evaporated at higher temperatures in the VTDMA-8: at Ti = 140◦ C;
V (NH4 HSO4 ) and
V ((NH4 )2 SO4 ) ¡ 1%. Both sulfate compounds are completely vanished at Ti = 180◦ C. Our measurements for the quasi-monodisperse particles indicate lower volatilization temperatures of NH4 HSO4 and (NH4 )2 SO4 than those found in previous studies for polydisperse aerosol particles, with reported volatilization temperatures of about 200 –210◦ C (Clarke et al., 1987; Pinnick et al., 1987). The elevated volatilization temperatures in latter studies were probably necessary due to the relatively short residence time (∼ 1 s) of the polydisperse particles in the heating section (110 ¡ ODp ¡ 2400 nm for 145 ¡ DP p ¡ 1800 nm, with DP p denoting the mean diameter of the analyzed particle size range ODp (Jennings & O’Dowd, 1990)). In order to completely volatilize polydisperse NH4 HSO4 =(NH4 )2 SO4 particles at Ti ∼ = 180◦ C, residence times of at least 20 –30 s would be required (Burtscher et al., 2001). In the VTDMA-8, the residence time

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is comparatively short (0:3 s); however, only a narrow particle size range (quasi-monodisperse particles: 5 ¡ ODp ¡ 20 nm for 50 ¡ Dp ¡ 150 nm) is passed through the heating section. Therefore, due to the smaller particle mass evaporated in the VTDMA-8 compared to other volatility systems, the temperature required for complete volatilization is decreased. The results of these measurements allow to assume that at Ti = 180◦ C and for Dp 6 150 nm, H2 SO4 and its neutralized species (NH4 )2 SO4 and NH4 HSO4 , are completely evaporated in the VTDMA-8. Thus, we consider the residual species for Ti ¿ 280◦ C refractory (this allows for additional evaporation of potential volatile (organic carbon) species for 180 ¡ Ti ¡ 280◦ C, with 300◦ C being the maximum temperature feasible in the VTDMA-8). Because H2 SO4 , NH4 HSO4 , (NH4 )2 SO4 , organic carbon (OC), and elemental carbon (EC) are the main components of submicrometer aerosol particles (Burtscher et al., 2001), we assume the refractory material to consist mostly of non-volatile, elemental carbon, and possibly a fraction of less volatile, organic compounds. 2.3. Measurement The VTDMA-8 was tested during three 8eld measurements at sites of di'erent pollution levels: (1) To examine aerosols in a moderately polluted environment, we made measurements in Falkenberg, a rural region in eastern Germany, close to the Polish border. The air masses reaching the measurement site were inLuenced by a number of large cities (e.g., Berlin) about 50 –100 km away. Volatility size distributions were recorded continuously during a 4-week period between 13 July and 14 August, 1998. (2) To study aerosol particles in a strongly polluted, urban environment, we used the VTDMA-8 during winter in the city of Leipzig, Germany. Urban regions are generally characterized by high particle number concentrations due to the close proximity to pollutant sources (e.g., vehicle tra6c, heat and power generation, industrial emissions). Continuous volatility measurements were carried out during a 1-week period between 10 and 16, February 1999 at the Institute for Tropospheric Research (IfT), about four kilometers northwest of the city center. The IfT is located near busy roads, which are particularly busy during weekdays. (3) To examine particles directly at a combustion source, we performed measurements in the exhaust of a diesel-fueled vehicle, known to contain large amounts of non-volatile carbon (Morawska, Bo8nger, Kocis, & Nwankwoala, 1998). The exhaust pipe of the vehicle was connected to a standard dilution tunnel with a Lexible tube, which was equipped with a constant-volume sampler (CVS). The CVS regulated the amount of dilution air to keep the emitted volume Low constant for the sampling probe. The distance between the automobile exhaust pipe and the sampling port of the VTDMA-8 was approximately 14 m, corresponding to a residence time of the particles in the sampling lines of about 5 s. The volatility measurements were carried out using a chassis dynamometer at driving speeds of 50, 70, and 100 km=h and at VTDMA-8 conditioning temperatures of 25◦ C and 250◦ C. To demonstrate the advantages of using the VTDMA-8 to investigate chemical composition compared to conventional methods, we collected samples using two cascade impactors during the measurements in the moderately polluted and strongly polluted case (cases 1 and 2, respectively) for o'-line, chemical analysis. The cascade impactors had 50% cut sizes of 0.05, 0.14, 0.42, 1.2, 3.5, and 10 m (cf. Table 1). The material collected with one impactor was analyzed for ionic compounds by using ion chromatography, and the material collected with the other impactor was analyzed for OC and EC by using a thermal desorption/oxidation method. Both OC and EC were bulk-analyzed by heating the samples to speci8c temperatures to di'erentiate the carbonaceous compounds. Heating

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S. Philippin et al. / Aerosol Science 35 (2004) 185 – 203

Table 1 Impactor stages and corresponding aerodynamic diameters of particles sampled for o'-line chemical analysis Impactor stage

1

2

3

4

5

Aerodynamic particle size range (nm)

50 –140

140 – 420

420 –1200

1200 –3500

3500 –10000

was conducted at 590◦ C under a nitrogen atmosphere to evaporate the organic material, and further heating at 650◦ C under an oxygen atmosphere was conducted to separate the elemental, non-volatile carbon (NeusQuR et al., 1999). The analyzed OC and EC are given as mass fractions
M (= mass of each compound as part of the total weighed and RH-corrected mass (Philippin, 2000)), i.e.,
M; OC and SM; EC , respectively. Moreover, to improve the comparison of the VTDMA-8 results at Ti = 280◦ C with the chemical analysis results in the strongly polluted environment (case 2) during thermodesorption/oxidation of the chemical impactor samples, an additional heating stage was used at a temperature of 280◦ C in an O2 atmosphere to evaporate the volatile, OC fraction. Thus, OC is composed of the more volatile OC (OCI ) and the less volatile OC (OCII ). It follows, that the residual mass fraction of the chemical sample at Ti = 280◦ C is the sum of SM; EC and
M; OCII . We refer to this residual mass fraction,
M; EC+OCII , as the VTDMA-equivalent portion that is volatile at 280◦ C. For the comparison, the measurements periods during each case study were divided into subperiods, which correspond to the chemical sampling intervals that were simultaneously made. In the moderately polluted environment (case 1), we divided the 4-week measurement period into 26 subperiods. For each of the individual subperiods, which varied from 8 to 24 h, the measured number size distributions at 25◦ C and 280◦ C were averaged and the corresponding
N and
V were calculated. In the strongly polluted environment (case 2), we divided the 1-week measurement period into 11 subperiods. The corresponding chemical samples included 8 h of daytime and 15 –16 h of nighttime, including morning and evening rush hours. 3. Application to Atmospheric Aerosols 3.1. Aerosol volatility in a moderately polluted environment Fig. 3a shows a typical number size distribution of 150 nm particles recorded at Falkenberg on 7–8 August, 1998, measured at Ti = 25◦ C; 80◦ C; 180◦ C, and 280◦ C for a 24-h time period. The size distributions shown in Fig. 3a represent an average of 19 individual scans during which the absolute number of 150 nm particles, N150 nm , ranged between 41 and 94 cm−3 , with a mean value of 64 cm−3 . To retrieve non-volatile fractions independent of temporal variations, the volatility scans were normalized with respect to the current concentration of 150 nm particles measured at Ti =25◦ C. The loss of particle volume in the heating section on VTDMA-8 is evident as a shift of the initial (i.e., for Ti = 25◦ C) size distribution towards smaller sizes. The shift was monomodal up to a temperature of 80◦ C, where Dp decreased about 6% and
V (80◦ ) = 86 ± 0:7%. At 180◦ C and at 280◦ C,
V (180◦ )=17±0:5% and
V (280◦ )=11±0:4%, however, the shape of the size distributions

S. Philippin et al. / Aerosol Science 35 (2004) 185 – 203 (a)

(b)

1.0

1.0

strongly polluted

moderately polluted

0.8

0.8

25 °C ambient 80 °C 180 °C 280 °C

0.6

0.6

0.4

0.4

0.2

0.2

0.0

dN/dlogDp, norm

dN/dlogDp , norm

193

0.0 100

20

Dp [nm]

100

20

Dp [nm]

Fig. 3. Average, normalized number size distributions of 150 nm particles conditioned to 25◦ C (ambient), 80◦ C, 180◦ C, and 280◦ C (a) in a moderately polluted environment between Julian Day 218.3 and 219.3 and (b) in a strongly polluted environment between Julian Day 41.76 and 41.78.

was multimodal, with one mode near the initial Dp exiting DMA-1, and another, broader mode at smaller Dp . The mode near the initial Dp did not change signi8cantly between 180◦ C and 280◦ C, and represents those particles that are predominantly non-volatile. These particles are considered quasi-non-volatile, because Dp decreased only slightly, namely, about 1% at Ti = 25◦ C and about 2–3% between Ti = 25◦ C and 280◦ C. Therefore, these particles are close to, but not completely non-volatile. The number and volume fraction of these quasi-non-volatile particles, ’N; qnv and ’V; qnv , are 6.3% and 5.9% for Ti = 180◦ C and 4.9 and 4.7% for Ti = 280◦ C, respectively. Fig. 3a shows the average of the data taken over a 24-h time period, whereas Fig. 4 shows the individual data for Ti = 25◦ C and 280◦ C in a three-dimensional plot. For Ti = 25◦ C, the mode of the size distribution was at about 148 ± 0:5 nm and the number concentration for individual scans varied from 880 to 2200 cm−3 . For Ti = 280◦ C on the other hand, the distribution was bimodal, with modes located at 143 and 42 nm, the number concentration for individual scans varied from 40 to 120 cm−3 and from 130 to 560 cm−3 , respectively. The smaller mode (at 143 nm) represents the fraction of quasi-non-volatile (’qnv ) material. The mean Dp of the 8rst mode remains relatively constant and ranged between 139 and 148 nm, whereas the mean Dp of the second mode ranged between 34 and 49 nm, and shows a more irregular structure than the 8rst mode. The corresponding non-volatile number fraction
N and volume fraction
V of the size distributions at Ti = 280◦ C are given in Table 2. At Ti = 280◦ C,
N = 98 ± 2:3%, averaged over all scans, indicating that all particles included a non-volatile core. Of these, 5% belong to the quasi-non-volatile mode, ’N; qnv .
V = 11 ± 2:3%, indicating that most of the volume of the 150 nm particles consisted of volatile compounds. About half of
V corresponds to the quasi-non-volatile mode, ’V; qnv , which is about 5% of the initial particle volume. The impactor measurements of the corresponding chemical sample yielded
M; OC = 15:1% and
M; EC = 22:2%, for particles with an aerodynamic diameter between 50 and 140 nm, and
M; OC = 10:8% and
M; EC = 15:2% for particles between 140 and

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25

500 1500 400 1200 300 900 dN/dlogDp -3 200 600 [cm ] 100 300 0

°C gDpdN/lo

gDpdN/lo

.PA 1 n o S le tic r a icA r e h p s o tm lB s o r e e g h C r la o ip P -1 A M pC D -1 D .e t2 r la c d io n g C P a tiH c e S g n o1 T 2T T 7 8 T .o f3 s la c r b m tie u n d z S D P M o R NC P 2 p D

-

219.2

20

219.0 218.8

Dp [nm]

500 500 400 400 p 300 300 dN/dlogD -3 200 200 [cm ] 100 0

218.6

100

150 218.4 200

280

°C

219.2

20

219.0 218.8

Dp [nm]

218.6

100

Julian Day

150 218.4 200

Fig. 4. Number size distributions of ambient (25◦ C) and conditioned (280◦ C) 150 nm particles during 24 h period between Julian Day 218.3 and 219.3. The initial particle diameter Dp at 150 nm is also shown for reference.

Table 2 Calculated fractions of non-volatile aerosol compounds from volatility size distributions of 150 nm particles during a 24-h period (cf. Fig. 6), representing the number fraction
N , volume fraction
V , as well as the number and volume fraction of the 8rst signi8cant mode near the original particle size (’N; qnv and ’V; qnv , respectively), denoting quasi-non-volatile cores Sample

Julian day


N


V

’N; qnv

’V; qnv

Sample

Julian day


N


V

’N; qnv

’V; qnv

1 2 3 4 5 6 7 8 9 10 11

218.35 218.40 218.46 218.51 218.56 218.61 218.67 218.72 218.77 218.82 218.88

96.4 94.5 101.8 95.5 93.7 98.9 97.0 95.0 96.7 97.0 98.8

11.9 13.0 12.0 12.0 11.6 12.7 16.2 15.6 12.6 10.0 9.6

4.5 5.3 4.7 5.1 5.1 5.6 8.2 8.0 5.1 3.9 3.7

4.2 5.0 4.3 4.9 4.9 5.3 8.0 7.6 4.9 3.8 3.6

12 13 14 5 16 17 18 19

218.93 218.98 219.04 219.09 219.14 219.19 219.25 219.30

98.1 101.1 98.7 99.1 98.9 100.9 98.9 100.0

9.8 11.0 8.6 7.7 7.8 8.9 10.3 11.1

4.6 6.0 4.4 3.5 3.7 4.6 5.7 6.6

4.4 5.9 4.1 3.2 3.5 4.4 5.6 6.3

98 ± 2:3

11 ± 2:3

5 ± 1:3

5 ± 1:3

mean

420 nm. However, due to the relatively small amount of carbonaceous mass in the particles, the time and the size resolution of the chemical method used to identify the non-volatile aerosol fraction was poor. Compared to results from conventional methods for measuring chemical composition, for which only one representative value over the 24-h time period was available, the results of above

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example demonstrates the advantage of using the VTDMA-8 to measure the non-volatile fraction of submicrometer aerosol particles with a considerably higher temporal resolution. Generally, conventional chemical o'-line methods only allow temporal resolution on the order of hours or days, whereas the VTDMA-8 has a temporal resolution of about 15 min. The following comparison between the results of the VTDMA-8 and the impactor measurements is only qualitative because the impactor-based chemical method where particles are sampled onto 8lter substrates, infers mass fractions for bulk particle size ranges, whereas the VTDMA-8 measures volume fractions for quasi-monodisperse particles. To relate the VTDMA-8 results with the chemical analysis results, the particle density must be determined, because for chemical analysis the √ particle size is correlated to an aerodynamic diameter. With Dp; aero = Dp; geo , the geometric diameter of 150 nm corresponds to an aerodynamic diameter of 140 nm for an assumed particle density of 0:87 g cm−3 (140 nm separates the impactor stages 50 –140 and 140 –420 nm). The density of black carbon (elemental carbon, graphitic carbon and other optically absorbing carbon compounds) is believed to be smaller than 2 g cm−3 (Wagner, 1981) and often a value of 1 g cm−3 is used if the black carbon consists of chains of individual spherules (Schultz, 1993; Hitzenberger et al., 1999). Establishing a representative density is not straightforward if the carbonaceous material consists of spheres or agglomerates (Lahaye & Prado, 1981; Ishiguro, Takatori, & Akihama, 1997). Particle densities below 1 g cm−3 are assumed if non-spherical particles are in the form of a Lu'y material (Zier & Goetz, 1982; Samson, Mulholland, & Gentry, 1987). To simplify the comparison, we assumed a density of 1 g cm−3 . In this case, the geometric diameter of 150 nm selected by VTDMA-8 corresponds to the impactor stage two (size range 140 –420 nm). Moreover, because the Falkenberg samples contained little EC (i.e., a few tens of nanograms per cubic meter), the EC of the smallest impactor size range of 50 –140 nm is usually close to the detection limit of the thermodesorption method used so that the values are too uncertain to be compared to the results of the VTDMA-8. Fig. 5 shows the results of the of the VTDMA-8 and the chemical analysis for the 26 subperiods. For the entire measurement period, 8 ¡
V ¡ 21% with a mean value of 11 ± 2:9%, and 93 ¡
N ¡ 102% with a mean value of 98 ± 2:1%. The signi8cance of
N ≈ 100% is that it represents the fraction of particles containing non-volatile cores. On average, ’N; qnv = 6:1 ± 1:9% and 4 ¡ ’N; qnv ¡ 10%, representing the quasi-non-volatile particle fraction (i.e., the particles belong to the smallest mode shown in Fig. 4), making up about 51% of
V by volume. Over the 4-week measurement period, the impactor measurements yielded 2:8 ¡
M; EC ¡ 24% with a mean value of 14±6:4% for impactor stage one and 8 ¡
M; EC ¡ 19% with a mean value of 12±2:9% for impactor stage two. The total carbon mass fraction
M; TC is given by the sum of
M; EC and
M; OC , thus,
M; TC = 27% and
M; TC = 23% for impactor stages one and two, respectively. However, the chemical analysis contains large uncertainties because the particle concentration and mass were small, and therefore close to the detection limit of the chemical-analysis instruments. 3.2. Aerosol volatility in a strongly polluted environment A typical set of volatility data measured at the IfT is shown in Fig. 3b, recorded during an early evening rush hour on 11 February, 1999. Compared to the data for the moderately polluted environment shown in Fig. 3a, the number size distribution at 80◦ C, shifted to a smaller particle size, is accompanied by a broadening of the mode.
V (80◦ ) = 78%, i.e., about 22% of the initial particle volume evaporated (compared to 14% for the moderately polluted case), possibly due to

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ΦN

ΦV

Φ M,EC

Φ N, Φ V, Φ M

1.0 0.8 0.6 0.4 0.2 0.0

2

4

6

8

10 12 14 16 18 20 22 24 26 Averaging Interval

Fig. 5. Non-volatile number fraction
N (hatched) and volume fraction
V (black) derived from averaged 150 nm volatility distributions of the 26 time periods (each encompassing between 8 and 24 h) in the moderately polluted case.
M; EC denotes the chemically derived mass fraction of EC of impactor stage 2.

a larger fraction of more volatile nitrates (NO− 3 ) present in urban aerosols (case 2) than rural aerosols (case 1). The presence of more NO− 3 in case 2 than case 1 was veri8ed by concurrent chemical measurements: the NO− concentration for submicrometer aerosol particles (aerodynamic 3 Dp ¡ 1:2 m) for cases 1 and 2 averaged 0:2 ± 0:4 g m−3 and 9:6 ± 2:8 g m−3 , respectively. At 180◦ C, a multi-modal distribution developed, showing a dominant mode near the original particle size of 150 nm, with
V (180◦ ) = 37%. The volatility distribution at 280◦ C is similar to the distribution at 180◦ C,
V (280◦ ) = 27%. The 8rst signi8cant mode, centered at 145 nm, changed little between 180◦ C and 280◦ C : ’N; qnv =26% and ’V; qnv =24% for Ti =180◦ C, and ’N; qnv =23% and ’V; qnv =20% for Ti = 280◦ C, respectively. This indicates that the non-volatile volume fraction was larger in the strongly polluted than in the moderately polluted atmosphere. For both atmospheres, the
N remained near 100% for all Ti . All distributions showed a mode near the initial particle diameter, indicating those particles that were mainly composed of non-volatile material. Fig. 6 shows an overview of the volatility analysis of 150 nm particles in the strongly polluted environment. The entire measurement period was divided into six time intervals which correlate with signi8cant changes of N150 nm . The weather conditions during the initial measurement Period I and during Period II were sunny with low winds (average wind speeds of 1:7 ± 0:6 and 1:1 ± 0:6 ms−1 , respectively) and temperatures down to −10◦ C (average temperatures of −4:3 ± 2:4◦ C and −6:2 ± 2:8◦ C, respectively). N150 nm = 140 ± 25 cm−3 during Period I and N150 nm = 214 ± 88 cm−3 during Period II (cf. Fig. 6). The sharp decrease of N150 nm during Periods III and IV (N150 nm = 59 ± 25 cm−3 and 31 ± 11 cm−3 , respectively) was probably due to reduced tra6c emissions on the weekend (Julian days 43– 44), but also coincided with increased winds (average of 3:1 ± 0:8 ms−1 ), frontal activity, and slight precipitation in the form of snow (total of 0:055 mm). N150 nm increased in Period V (N150 nm = 74 ± 22 cm−3 ) with the return of weekday tra6c and decreased during

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197

Fig. 6. Particle number concentration N150 nm of 150 nm particles (circles) measured with the VTDMA-8 and derived number fraction
N (hatched) and volume fraction
V (black) for urban particles of the di'erent time intervals which correlate with signi8cant changes in the absolute number of N150 nm .

Period VI (N150 nm = 19 ± 10 cm−3 ) due to strong winds (5:8 ± 0:7 ms−1 ) and heavy precipitation (total of 1:4 mm). The volatility size distributions at 25◦ C and 280◦ C were averaged for each period. All measured volatility size distributions at 280◦ C show typical multi-modal structure (cf. Fig. 3b) with a distinct non-volatile mode near 145 nm. Fig. 6 shows
N and
V for these measurements. The average
N for the measurement period was 100 ± 0:9%, indicating that all of these particles contained a fraction of non-volatile material. ’N; qnv = 13:1%, representing the corresponding quasinon-volatile particle fraction. In general,
V varied from 12% to 24%, with a mean value of 18 ± 4:6%. The occurrence of low non-volatile volume fractions coincided with the weekend period:
V = 12% during Period III and
V = 16% during Period IV. To examine the characteristics of the non-volatile material, the VTDMA-8 results over the entire measurement period at the IfT were compared to the concurrent chemical measurements. Fig. 7 shows the results for both the VTDMA-8 and the thermodesorption/oxidation analysis for the 11 subperiods.
N varies between 98% and 111% and is on average 102±3:6%, with 8 ¡ ’N; qnv ¡ 21% and ’N; qnv = 14 ± 4:1%.
V ranged between 12% and 25% and had a mean non-volatile volume of 18 ± 4:0%. There are no di'erences in mass fractions between the day samples and night samples, because the night samples were extended to include morning and evening rush hours. From the chemical analysis, 3:6 ¡
M; EC ¡ 14% with a mean value of 8 ± 3:5% and 4:6 ¡
M; EC+OCII ¡ 16% with a mean value of 11 ± 3:9% for impactor stage 2. On average,
M; OCII = 3:9 ± 2:4%, representing about 19% of the total organic mass. In less polluted areas than urban environments where the organic aerosol fraction is smaller due to lower vehicle emissions, the
M; OC is, therefore, even smaller. The data shown in Fig. 7 indicate that
M; EC+OCII for each time period correlates with
V with a 8tting parameter, R2 = 0:57, suggesting that the residual mass analyzed by the VTDMA-8 contained mainly non-volatile carbonaceous material, i.e., EC. However, because of the di6culty of comparing
V and
M , the indication that the non-volatile
V measured with the VTDMA-8

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ΦN

ΦV........ ΦM, EC+OC ........ ΦM,EC II

ΦV , ΦN , ΦM

1.0 0.8 0.6 0.4 0.2 0.0 1

2

3

4

5

6

7

8

9

10

11

Averaging Interval

Fig. 7. Non-volatile number fractions SN and volume fractions SV derived from averaged 150 nm volatility distributions corresponding to the chemical sampling intervals in the strongly polluted case (odd numbers refer to the night samples, and even numbers to the day samples). The dotted line denotes the mean value of
N over all intervals (=1.02).
M; EC denotes the chemically derived mass fraction of EC (of impactor stage 2).
M; EC+OCII refers to the carbon mass fraction that is non-volatile at 280◦ C; and includes EC as well as less volatile organic carbon (OCII ).

contains only a minor fraction of less volatile OC is only qualitative. Therefore, we assume that the non-volatile residues in the VTDMA-8 contained a small fraction of low-volatility OC. 3.3. Aerosol volatility of particles at a combustion source In contrast to the volatility measurements of atmospheric aerosol particles, the measurements of the diesel exhaust were con8ned to a particle diameter of 50 nm, because this particle size was located near the maximum of the number size distributions of particles emitted by diesel-fueled vehicles (LyyrQanen, Jokiniemi, Kauppinen, & Joutsensaari, 1999; Philippin, 2000). Nevertheless, the residual particles were always larger than the lower VTDMA-8 detection limit of 10 –20 nm. Fig. 8 shows the measured number size distributions. For all vehicle speeds, the volatility size distributions show a similar shift at 250◦ C of the initial distribution towards smaller particle diameters with a slight broadening of the peak. Compared to the multimodal number size distributions typically seen for the atmospheric particles (cf. Fig. 3a and b), the structure of the particle size distribution of the automotive emissions did not change above 140◦ C, namely, it remained monomodal. Heating to 250◦ C yielded
N = 102 ± 8:1%, therefore,
N is constant for particles emitted by diesel-fueled vehicles.
V = 88 ± 11:9%, averaged over all speeds, and with increasing speed from 50 to 100 km=h,
V increased from 75% to 98%. An increase in the fraction of non-volatile particle material is expected with increasing engine speeds because the air–fuel mixture becomes richer. In a rich air–fuel mixture, an insu6cient amount of oxygen is present during the combustion process, leading to incomplete combustion of carbon-containing fuels, which favors the formation of soot. Since diesel particles contain a large fraction of EC (Kleeman, Schauer, & Cass, 1999), the non-volatile volume fractions correspond to the fraction of non-volatile carbon. The structure of the non-volatile

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199

dN/dlogDp, norm

1.0

25 °C 250 °C

0.8 0.6 0.4 0.2

ΦN : ΦV :

97 % 75 %

ΦN : ΦV :

98 % 90 %

50 km/h

0.0 dN/dlogDp, norm

1.0 0.8 0.6 0.4 0.2

70 km/h

0.0 dN/dlogDp, norm

1.0 0.8 ΦN : 111 % ΦV : 98 %

0.6 0.4 0.2 0.0

100 km/h 30

40

50

60

70

Dp [nm]

Fig. 8. Normalized number size distributions of 50 nm particles at 25◦ C and 250◦ C emitted from a diesel-fueled vehicle at di'erent driving speeds. The non-volatile number fraction
N and volume fraction
V are those derived from the VTDMA-8 measurements.

particle number size distribution suggests that the emitted particles represent internally mixed, 50 nm particles. 4. Results and discussion The VTDMA-8 measurements reveal clear di'erences in the amount of non-volatile material present in aerosol particles originating from environments polluted to di'erent levels. The results show that the fraction of non-volatile material increased with increasing proximity to combustion sources. The general structure of the volatility size distributions of 150 nm particles in moderately and strongly polluted environments are similar (cf. Fig. 9a and b), both exhibiting a multimodal structure of non-volatile material after the predominant volatile material (i.e., nitrate and sulfate compounds) was evaporated in the heating column of the VTDMA-8. In the moderately polluted environments, N150 nm = 50 ± 68 cm−3 ,
V = 11 ± 2:9% and 8 ¡
V ¡ 21%. In the strongly polluted environment N150 nm = 89 ± 84 cm−3 ,
V = 18 ± 4:0%, and 12 ¡
V ¡ 25% (cf. Table 3). Both multimodal volatility size distributions of these atmospheric measurements show a noticeable peak of quasi-non-volatile material centered near the initially selected particle size of 150 nm, and the peak was less pronounced in the moderately than in the strongly polluted air: ’N; qnv = 6% in case 1 versus 14% in case 2, respectively. The corresponding size distributions of the refractory material suggest that, at a distance to the combustion sources, these 150 nm particles represent an external mixture of particles with non-volatile cores of various size, representing those particles that are quasi-non-volatile, i.e., contain primarily non-volatile material with a thin volatile coating.

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S. Philippin et al. / Aerosol Science 35 (2004) 185 – 203 (b)

(a)

dN/dlogDp, norm

1.0

ambient

(c)

1.0

ambient

1.0

0.8

0.8

0.8

0.6

0.6

0.6

0.4

0.4

0.4

0.2

conditioned (280 °C)

0.2

0.0 10

100

Dp [nm]

10

conditioned (250 °C)

0.2

conditioned (280 °C)

0.0

ambient

100

0.0 30

Dp [nm]

40

50

60

Dp [nm]

Fig. 9. Normalized volatility size distributions of 25◦ C ambient (dotted) and 250◦ C=280◦ C conditioned (solid) aerosol particles in di'erent environments, representing (a) 150 nm particles in a moderately polluted air mass, (b) 150 nm particles in a strongly polluted air mass, and (c) vehicle-emitted, 50 nm diesel particles at the source. Data in a and b represent averaged results over all spectra measured.

Table 3 Summary of averaged non-volatile number fractions
N , quasi-non-volatile number fractions ’N; qnv , and non-volatile volume fractions
V for the di'erently polluted case studies Condition

Particle diameter (nm)


N (%)

’N; qnv (%)


V (%) (avg, min 7 max)

Moderately polluted Strongly polluted At combustion source

150 150 50

98 ± 2:1 102 ± 3:6 102 ± 8:1

6:1 ± 1:9 14 ± 4:1 -

11 ± 2:9 (7:7=21) 18 ± 4:0 (12=25) 88 ± 12 (75=98)

In contrast to the atmospheric particles, diesel-fueled, 50 nm particles contained a large fraction of non-volatile material,
V averaging 88 ± 12% and N150 nm = 700 ± 2600 cm−3 . Furthermore, the size distribution of the non-volatile cores was monomodal. Thus, the refractory material was evenly distributed within this size interval, representing internally mixed particles. Each combustion source produces di'erent internally mixed particles, and, when combined with aging processes such as coagulation and condensation, can transform an aerosol size distribution such that the size distribution of its non-volatile cores is multimodal, similar to the multimodal distributions observed for the atmospheric particles (cf. Fig. 3a and b). In our atmospheric and exhaust measurements, the total number of the 150 and 50 nm particles, respectively, did not decrease after evaporation of all of the volatile material. This suggests that all particles in polluted environments contain a fraction of non-volatile material, i.e., the refractory core. This agrees with the 8ndings of Clarke, Uehara, and Porter (1997) who found evidence of

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refractory cores in pollution aerosols during an aircraft study over the Atlantic. This is important for studying the mechanisms of secondary-particle formation in polluted air masses, because it indicates that non-volatile (e.g., soot) particles might serve as seed nuclei and might catalytically promote subsequent condensation processes at the expense of new particle formation. In contrast to conventional chemical analysis methods, the VTDMA-8 volatility technique has high temporal resolution, which is achieved by limiting the measurements to representative particle diameters. We cannot, however, infer from the VTDMA-8 measurements that the non-volatile material is exclusively non-volatile carbon. However, based on the qualitative agreement with the results of other chemical analysis, only a minor fraction of the derived volume fraction consists of non-volatile, OC compounds. The fraction of non-carbonaceous particle components such as sea-salt, crustal material, or Ly ash is considered a minor constituent in this small, submicrometer particle size range. The chemical analysis made at an equivalent volatilization temperature of 280◦ C (cf. Fig. 7) suggests that
M; OCII ¡ 20% in strongly polluted environments, where the organic carbonaceous contribution is already large. The high non-volatile fraction detected with the VTDMA-8 in vehicle-emitted particles supports this assumption. Therefore, the results of our experimental study validates the VTDMA-8 as a suitable tool for detecting non-volatile aerosol fractions within a de8ned particle size range, in strong as well as in mildly polluted environments. By extending the measurements to several discrete particle diameters, the VTDMA-8 measurements permit determination of the non-volatile composition and mixing state of the original aerosol population.

5. Conclusions We developed a volatility tandem di'erential mobility analyzer (VTDMA) that uses eight heating tubes for rapid measurement of the non-volatile component of aerosol particles. The VTDMA-8 o'ers high size resolution with improved temporal resolution over other methods; measurements can be made in about 15 min, compared with hours or days for conventional chemical methods, in a narrow particle size range. The VTDMA-8 was continuously used over extended time periods (days to weeks) to analyze the non-volatile particle composition near 50 and 150 nm. Our measurements and analysis with the VTDMA-8 show that the fraction of non-volatile material increases with increasing proximity to combustion sources. High amounts of non-volatile material were found at the source, averaging 88% by volume for 50 nm particles. In atmospheric particles not directly at the source, the non-volatile portion decreased signi8cantly to between 8% and 25% by volume. However, aerosols in a strongly polluted environment still show an appreciable non-volatile volume fraction of about 18% in 150 nm particles. In aerosols in a moderately polluted atmosphere, the non-volatile volume fraction was about 11%. The non-volatile material inferred from the VTDMA-8 measurements is assumed to be non-volatile carbon with a fraction of low volatility, namely, organic carbon that is not volatile at the maximum volatilization temperature of 280◦ C used in the VTDMA-8. Therefore, the refractory core characterizes pollution aerosols. Most importantly, the measured data demonstrate that all of the aerosol particles examined here contained a refractory core that varied in size. The VTDMA-8 is therefore a valuable tool for investigating the size-dependent mixing state of aerosol particles and for studying the e'ect of non-volatile carbonaceous material on thee processes a'ecting particle formation and aging.

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