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Environ. Sci. Technol. 2006, 40, 1223-1230

CCN Activation of Pure and Coated Carbon Black Particles U. DUSEK,† G. P. REISCHL, AND R. HITZENBERGER* University of Vienna, Institute for Experimental Physics, Boltzmanngasse 5, 1090 Wien, Austria

The CCN (cloud condensation nucleus) activation of pure and coated carbon black particles was investigated using the University of Vienna cloud condensation nuclei counter (Giebl, H.; Berner, A.; Reischl, G.; Puxbaum, H.; KasperGiebl, A.; Hitzenberger, R. J. Aerosol Sci. 2002, 33, 16231634). The particles were produced by nebulizing an aqueous suspension of carbon black in a Collison atomizer. The activation of pure carbon black particles was found to require higher supersaturations than predicted by calculations representing the particles as insoluble, wettable spheres with mobility equivalent diameter. To test whether this effect is an artifact due to heating of the lightabsorbing carbon black particles in the laser beam, experiments at different laser powers were conducted. No systematic dependence of the activation of pure carbon black particles on laser power was observed. The observations could be modeled using spherical particles and an effective contact angle of 4-6° of water at their surface. The addition of a small amount of NaCl to the carbon black particles (by adding 5% by mass NaCl to the carbon black suspension) greatly enhanced their CCN efficiency. The measured CCN efficiencies were consistent with Ko¨ hler theory for particles consisting of insoluble and hygroscopic material. However, coating the carbon black particles with hexadecanol (a typical film-forming compound with one hydrophobic and one hydrophilic end) efficiently suppressed the CCN activation of the carbon black particles.

1. Introduction Globally, large amounts of soot particles are released into the atmosphere by fossil fuel combustion. Black carbon emissions were earlier estimated (2) between 1.3 and 14 TgC/ year. Later estimates range from 6.4 TgC/year (3) to 8.0 TgC/ year (4). The latest value is 3.0 TgC/year (5). In the upper troposphere/lower stratosphere (UTLS) region, combustion particles released by air traffic can influence the formation and lifecycle (6) of cirrus clouds. Microphysical (7) and radiative (8) properties of cirrus clouds can also be affected. Water insoluble carbonaceous particles emitted from aircraft engines play an important role in contrail formation (9) and have a high potential to initiate cirrus cloud formation via heterogeneous freezing processes (9, 10). Therefore, the ability of combustion particles to nucleate cloud droplets is of substantial interest although radiative forcing by anthropogenically influenced cirrus clouds is still an open question (11). * Corresponding author phone: 43 1 4277-51111; fax: 43 1 42779511; e-mail: [email protected]. † Present address: Biogeochemistry Department, Max Planck Institute for Chemistry, P.O. Box 3060, Mainz, 55020, Germany. 10.1021/es0503478 CCC: $33.50 Published on Web 01/05/2006

 2006 American Chemical Society

Combustion particles emitted from aircraft engines consist of soot associated with substantial amounts of organic carbon and trace amounts of hygroscopic material (e.g., 12). An earlier study conducted in the framework of the EU project PARTEMIS (overview, 13) on aircraft engine exhaust showed that both the hygroscopic growth under subsaturated conditions (14) and the CCN activation of the exhaust particles under supersaturated conditions (15) depended on the fuel sulfur content. The measured activation ratio (i.e., the CCN concentration divided by the total particle concentration), however, could neither be explained by Kelvin theory (for insoluble particles) nor Ko¨hler theory (for insoluble particles coated with a hygroscopic substance). Kelvin theory gave severely underestimated activation ratios except in the case of lowest fuel sulfur content (410 ppm by mass), while the Ko¨hler model always overestimated the activation ratio severely. The best fit was obtained using the semiempirical model (14), which assumed insoluble particles coated with varying amounts of sulfuric acid (as a function of fuel sulfur content), but even here the model overestimated the activation ratio appreciably. One possible reason could be an effect of organic material present on the particles (16). According to Ko¨hler theory, the presence of inorganic substances on a combustion particle can be expected to facilitate cloud droplet nucleation. Whether a coating of organic material increases or decreases the critical supersaturation for a particle of a given size and composition, however, depends on this material’s affinity for water. Organic compounds can be hygroscopic, hydrophobic, surface active, or slightly soluble (e.g., 17, 18). For a soot particle coated with organic material, the properties of this coating will determine whether the particle is activated at lower or higher critical supersaturation than predicted by Kelvin theory for insoluble particles. Laboratory studies have shown that the CCN activation of strongly soluble organics agrees well with predictions by Ko¨hler theory, whereas the activation of slightly soluble organics also depends on their contact angle with water and on the humidity history before activation (e.g., 1, 19-21). It was also found (22) that a coating of surface active compounds on soot particles increases their hygroscopicity. Some other studies show that even thick films of hydrophobic coating do not reduce or delay the CCN activation of inorganic salt particles significantly (23, 24). However, to date there are no studies that demonstrate the effect of hydrophobic coatings on the activation of insoluble particles. To improve our understanding of the activation behavior of combustion particles, we conducted systematic studies of the CCN activation of laboratory generated soot particles coated with an inorganic salt (NaCl) or with a hydrophobic organic substance (hexadecanol), which were produced by nebulizing liquid suspensions of carbon black. Although both substances are not expected to be present on real world combustion particles in appreciable amounts, they can serve as proxies for the soluble and film-forming material, respectively, influencing CCN activation.

2. Theoretical Background 2.1. Activation of Insoluble Particles: The Kelvin Effect. In the simplest case, insoluble particles can be activated to grow to cloud droplets if they are completely wettable. At the critical supersaturation (ScK with K for Kelvin) a spherical particle with (critical) diameter dc is able to sustain a film of liquid water and undergoes nonequilibrium growth to a large cloud droplet. Due to the increase of vapor pressure of water above VOL. 40, NO. 4, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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a curved surface ScK decreases with increasing particle diameter (dc) as

[ () ]

(1)

4σwMw RTFw

(2)

ScK ) exp

A - 1 × 100% dc

with

A)

A depends on temperature T and the surface tension σw of the water film on the solid soot core. The constants Mw and Fw denote the molecular mass and the density of water, and R the universal gas constant. For irregularly shaped, insoluble particles (such as the pure carbon black particles used in this study) there exists no simple theory of activation, since the curvature is not constant along the particle surface and the particles cannot unambiguously be described by a single particle diameter. Several different definitions of equivalent particle diameters are used to describe such particles (e.g., volume equivalent, electrical mobility equivalent, aerodynamic equivalent diameters), though none of these has a direct relationship to the actual curvature of the particle surface. Since the shape factor of the investigated soot particles is not known, we use the electrical mobility equivalent diameter as a proxy for particle size. 2.2. Effect of Contact Angle. If a particle’s surface is not completely wettable, the water molecules cannot form a contiguous film but nucleate as one or more droplet embryos with a certain contact angle on the surface. An embryo droplet necessarily has a higher curvature than the particle itself, and the critical supersaturation is therefore higher than for a completely wettable particle. Due to this effect, the critical supersaturation Sca (with a for contact angle) increases with the contact angle θ (e.g., 25, pp 302 ff). Rewriting eq 9-25 in this book (originally given by ref 26) to express S as a function of θ, Sca can be approximated for small contact angles as

[ (

Sca ) exp

) ]

1 - cos(θ) A + - 1 × 100% dc (0.662 + 0.022 ln(d/2))1/2 (3)

Equation 3 is similar to eq 1, except for the additional term in the exponent that describes the increase of the critical supersaturation due to the nonzero contact angle. 2.3. Effect of Particle Heating Caused by Light Absorption. Pure soot particles are strongly light absorbing. In our CCNC, an 11 mW laser beam traverses the chamber at the height of the maximum supersaturation (see below), so particle heating by light absorption has to be considered. If the soot particles reach thermal equilibrium in the beam before water starts to condense on them, their temperature will be slightly higher than that of their surroundings. With the use of eqs 1 and 4 given by ref 27, the temperature of a 100 nm diameter soot particle will be about 0.07 K higher, the temperature of a 400 nm soot particles about 0.26 K higher. The effect of this temperature increase on the critical supersaturation was also described by ref 27. For lightabsorbing, insoluble particles in thermal equilibrium with a light source the critical supersaturation Sch (with h for heated) can be written as

[ ( ) ]

Sch ) exp

A+γ - 1 × 100% dc

(4)

with the so-called heating parameter γ which is given by 1224

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γ)

∆HFwΦ 8πk′airTσw

(5)

Here ∆H denotes the latent heat of condensation at temperature T, Fw the density of water, Φ the energy uptake per particle in W, k′air the thermal conductivity of air corrected for the noncontinuum effect (see ref 27 for further details), and σw the surface tension of water. 2.4. Activation of Coated Particles: Modified Ko1 hler Curves. Insoluble particles with a coating of soluble material are also able to act as cloud droplets. The effect of a spherical insoluble core (with diameter du) on the critical supersaturation Scs (with s for shell) is described by a modified Ko¨hler equation (25).

[ (

Scs ) exp

) ]

6imsσw A - 1 × 100% (6) dc πF M (d 3 - d 3) w s c u

where ms and Ms are the mass and the molar mass of the soluble material and i denotes the van’t Hoff factor. All other symbols were defined in eq 2. We used a van’t Hoff factor of 2 for NaCl.

3. Experimental Methods For this study, pure and coated carbon black particles are generated using a Collison atomizer. Their CCN activation is subsequently investigated using the University of Vienna CCNC, which operates on the basis of a static thermal diffusion chamber (1). Details of particle generation and the experimental setup are described in section 3.1. Issues relevant to the calibration of the CCNC are described in section 3.2. 3.1. Measurement Principle. The pure carbon black particles used in this study are generated by atomizing a liquid suspension. First, powdered soot (carbon black, Elftex 125, Cabot Corp.) is weighed and then heated at 400 °C for several hours to remove adsorbed semivolatile material, which might be soluble. The soot powder is then immediately transferred into a mixture of 20% reagent grade 2-propanol and 80% ultrapure water, and the mixture is placed in an ultrasonic bath for 5 min to generate a liquid suspension of carbon black particles. This suspension is dispersed using a Collison atomizer, which is operated with pressurized air at a flow rate of 1 L/min and produces liquid droplets containing one or more soot cores. The output of the atomizer is mixed with 18 L/min of dry, particle-free air to evaporate the water from the soot cores. During the evaporation process the primary soot particles are compressed to tight, almost spherical clusters due to the surface tension of the evaporating droplet. The relative humidity of the combined output flow is below 10%. The dried particles are passed through a neutralizer (85Kr, ca. 1 mCi at the present time) to ensure charge equilibrium. A Vienna-type differential mobility spectrometer (DMPS; details given by ref 1; sheath and excess air flow rates both 13.23 L/min, flow rates of sample and classified aerosol streams both 1.4 L/min to suit the requirements of the CNC; inlet impactor with a cut diameter of 1 µm) operated in a closed loop arrangement (28, 29) is used to select a fraction of carbon black particles within a narrow electrical mobility range. These particles are then passed to the CCNC, which counts the activated droplets at a defined supersaturation (S). A condensation nuclei counter (TSI, 3760) located in the bypass flow of the CCNC determines the total particle concentration N prior to each CCN measurement. In principle the particles exiting the DMPS contain a fraction of doubly charged particles which have a larger diameter than singly charged particles and are activated at lower S in the CCNC. The particle diameters selected in this study are larger than

the mode of a typical soot size distribution (∼160 nm) and therefore lie on the descending slope of the size distribution. In this case, the concentration of doubly charged particles is generally less than 10% of the concentration of singly charged particles and can be neglected, which is also seen from the activation curves which do not have a “shoulder” in the range of low S. The static thermal diffusion chamber used to measure the CCN concentrations is described in detail by ref 1. It consists of a sealed chamber formed by two circular horizontal aluminum plates of 10 cm in diameter, separated by a glass ring of 1.2 cm in height. A laser beam generated by an 11 mW laser diode crosses the chamber at the height of the maximum supersaturation. The light scattered by the growing droplets is recorded by a CCD camera directed at a right angle toward the laser beam. The sensitive volume for counting the droplets is defined by both the width of the laser beam and the viewing angle of the camera. In a typical measurement cycle the chamber is first flushed, then sealed, and the supersaturation profile develops very quickly, which can be seen by the appearance of droplets in the laser beam as early as 1 s after closing the valves. Images of the droplets formed by CCN activation are recorded by the CCD camera at a rate of approximately 10 frames per second for 45 s. Usually, the droplet concentration reaches a maximum within 5 s and then decreases as the droplets sediment out of the sample volume and fewer particles sediment into it from above. In the usual CCNC calibration (see the overview over current CCNCs; 30), this maximum droplet number Cd is divided by the calibrated sensitive volume to obtain the number concentration of CCN (NCCN). For a typical measurement point we average NCCN obtained during 10-25 subsequent measurement cycles at the same supersaturation to reduce the random error by a factor of 3-5. As the CN counter determines the total number concentration of particles (N) prior to the start of each CCN cycle, the activation ratio can be calculated, which is defined as NCCN divided by N. 3.2. Calibration of the CCNC. 3.2.1. Sensing Volume. For the determination of the CCN concentration from the counted droplets, the sensing volume must be known. This volume is defined both by the (cylindrical) laser beam, the part of the image selected by the image analysis program for droplet counting, and the focus of the CCD camera. As the original width of the laser beam changes by passing through the glass sidewall of the chamber, its actual width inside the chamber is difficult to determine. Therefore the sensing volume must be empirically calibrated. The easiest way is to set the supersaturation in the chamber high enough so that all particles of a given size class (selected by the DMPS system) are certainly activated and perform a comparison between the counted droplets and the particle concentration N in this size class (given by the CN counter). In this case, N corresponds to the concentration of CCN (NCCN). If all these particles are activated, the effective sensing volume Veff is given by Veff ) Cd/N, where Cd is the maximum number of droplets in the sensing volume during a measurement cycle. As mentioned above, Cd is usually determined from the picture with the maximum number of droplets (e.g., 1, 31). This common approach, however, is not suited when the pictures are taken at high frequency and/or the droplet number is relatively low, as is shown by the following analysis. We determined the effective sensing volume Veff using both carbon black particles with diameter from between 200 and 326 nm and NaCl particles with diameters of 37 nm. The supersaturation in the chamber was set to 3% for the measurements with soot particles and 2% for the measurements with NaCl particles. The resulting Veff shows a strong dependence on particle number concentration N (Figure 1a). Veff decreases with increasing N independently of particle

FIGURE 1. Effective sensing volume Veff as a function of particle concentration: (a) shows all data points derived from the measurements; (b) compares the effective sensing volume based on the maximum droplet counts (open squares) and on average droplet counts (filled diamonds). In the latter case, the sensing volume becomes nearly independent of particle number concentration showing only a slight decrease which can be explained by coincidence effects (solid line). The dashed line shows a simple statistical model estimate of the effect of counting statistical outliers. size and composition. The dependence of Veff on N is especially strong at low particle concentrations, which rules out droplet coincidence as an explanation. The strong decrease of Veff with increasing N can be explained by a statistical artifact resulting in an overestimation of the droplet concentration at low droplet numbers. This bias is caused by the convention that only the single picture with the highest droplet number is taken as representative for the CCN concentration during one measurement cycle. Due to the small sensing volume (∼15 mm3) and the uncertainty in the image analysis, the number of visible droplets in each picture taken near the maximum droplet concentration is subject to statistical fluctuations. Repeated measurements of the maximum droplet number Cd should follow a Poisson distribution with a mean of Cd and a standard deviation of (xCd. If pictures are taken at high frequencies, there is a high probability that an estimate of Cd based on the picture containing the maximum droplet number does not lie close to the mean, but far in the tail of the Poisson distribution, i.e., that Cd is overestimated. The overestimate is on average more drastic at low droplet concentrations, where the standard deviation of the Poisson distribution is large compared to Cd. To avoid this problem the data evaluation routine of the CCNC was modified. In the newer version the value of the CCN concentration for each measurement cycle, Cd, is approximated by the average number Cd of droplets counted in a total of nine images centered at the image with the maximum particle concentration, instead of this image alone. VOL. 40, NO. 4, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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The time span during which the nine images are taken is approximately half a second. The results of the two methods are compared in Figure 1b. For the same measurement cycles Veff was calculated first based on only one image with the maximum droplet number (open squares) and subsequently based on the average droplet number of nine images (full diamonds). The strong increase of Veff at low particle concentration disappears with the averaging process. Veff still decreases slightly with increasing N, even when it is derived based on average droplet numbers. An estimate of the decrease in Veff due to droplet coincidence in the laser beam is shown as the solid line. This line agrees with the Veff obtained from the averaging process, which indicates that droplet coincidence could explain the residual slight decrease in Veff. The dashed line shows a theoretical estimate of the effect of the statistical overestimation on Veff. For this estimate, we assume that the “true” maximum droplet number Cd is overestimated on average by x times the standard deviation. The droplet number Cdm in the image with the most droplets is then given by

Cdm ) Cd + xxCd

(7)

The dashed line represents Veff, which was calculated using Cd from eq 7 under the assumption that Cd is given by the solid line and x ) 1.7. This estimate approximates the measured data quite well and can be used to correct CCN data based on Cdm instead of Cd. Some of the measurements presented in the following sections were made using a droplet number based on the evaluation of a single image, and we therefore only know Cdm. (This includes a part of the pure soot measurements at 11 mW laser power and the NaCl calibration measurements.) As the data evaluation was performed online, the original images are no longer available for reevaluation and averaging. However, the data can be corrected with the help of eq 7. For CCN concentrations based on a single image the measured maximum droplet number Cdm is known. The “true” maximum droplet number Cd can then be estimated by solving eq 7. All results presented in the following sections have either been corrected in this way or are based on averaging nine subsequent images. 3.2.2. Calibration of the Supersaturation. Contrary to the usual procedure applied in CCNCs, the nominal supersaturation Snom at the half-height of the chamber is not calculated from the readings of the temperature sensors located in the upper and lower plates of the CCN chamber but is calculated using a heat transport model (1), which takes into account both these temperatures as well as the heat transfer through the materials between the sensors and the evaporating water surfaces. This estimated Snom is validated using NaCl particles of different sizes as supersaturation sensors. The nominal supersaturation at which 50% of the NaCl particles are activated (Snom50%) is calibrated against their critical supersaturation (Sc) given by Ko¨hler theory (Sc, 25), which is considered representative of the actual maximum supersaturation in the chamber. Figure 2a shows the fractions of activated NaCl particles as a function of Snom. A cumulative normal distribution (i.e., error function, erf) is fitted to the data to determine Snom50% quantitatively. The resulting best fits are shown as solid lines. Snom50% is defined for each curve as the point where the error function reaches its half-height (erf(Snom50%) ) erf(∞)/2). In Figure 2a, the Snom50% are indicated by crosses. The vertical lines indicate Sc, which are larger than Snom50%, suggesting that the actual supersaturation in the chamber is slightly higher than the one calculated with the heat transfer model. Figure 2b shows a comparison of nominal supersaturations (Snom50% for NaCl particles of several sizes) with the actual supersaturations, given by the respective Sc. The 1226

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FIGURE 2. (a) Typical activation curve of NaCl particles of different sizes. The supersaturations at 50% activation are indicated by crosses with the uncertainties indicated by horizontal error bars. The vertical lines indicate the critical supersaturation according to Ko1 hler theory. (b) Comparison of the actual supersaturation (from Ko1 hler theory) to the nominal supersaturation (from the heat transport model); data obtained for NaCl particles of different sizes. horizontal error bars correspond to 95% confidence intervals for Snom50% and are based on confidence intervals for the fitted parameters of the error function. The calculations underestimate the supersaturation in the chamber by approximately 20%. Nominal and actual supersaturations are well correlated (R2 ) 0.98). The calibration experiment shows that the actual S in the chamber is reproducible and closely related to Snom50%. A linear regression through the points in Figure 2b yields the calibration equation S ) Sc (%) ) 1.25Snom50% (%) + 0.05. This equation is used to calculate the actual supersaturation in the chamber (S) as a function of Snom for the following experiments.

4. Activation of Pure Carbon Black Particles The dependence of the activation ratio on supersaturation was measured for carbon black particles with mobility equivalent diameters between 150 and 400 nm. The upper limit of the size range is given by the size distribution of dry carbon black particles produced by the Collison atomizer, while the lower limit is given by the maximum S in the diffusion chamber (approximately 5%). Figure 3 shows examples of CCN spectra for carbon black particles with diameters of 180, 255, and 374 nm. The supersaturation at which 50% of the particles are activated (S50%) is estimated as described in the previous section and indicated by a dashed gray line. The activation ratio of carbon black particles increases more gradually with S than the activation ratio of NaCl particles (Figure 2a), probably because the shape and the hydrophilic/hydrophobic properties are more variable among individual carbon black particles than among NaCl particles.

FIGURE 3. Activation of pure soot particles of different sizes: (a) 374 nm, (b) 255 nm, and (c) 180 nm mobility equivalent diameter. The experimentally determined S of 50% activation (S exp) is indicated by the dashed vertical line, while the critical supersaturation according to Kelvin theory (ScK) for a particle of the respective size is indicated by a solid vertical line. 4.1. Comparison with the Kelvin Equation. Since the carbon black particles we used contained no soluble material as shown by ion chromatographic analysis of carbon black samples (H. Urban, personal communication), the most basic model for their activation behavior is given by the Kelvin equation, which is valid for solid, spherical, wettable particles. The critical supersaturation ScK predicted for the particles using the Kelvin eq 1 are indicated in Figure 3 by solid vertical lines. A comparison of the predicted ScK with the actual activation curves shows that the carbon black particles are activated at significantly higher S than expected from the Kelvin equation. Since the soot clusters are composed of individual small spherules, it would be possible in principle that the curvature of the individual spherules is determining the critical supersaturation. In this case, the diameter of the individual spherule should be used in the Kelvin equation, rather than an equivalent diameter for the cluster. However, in this case the critical supersaturation would be independent of the cluster diameter and much higher than in the present measurements (e.g., for spherule diameters between 30 and 50 nm Sc would lie between 7% and 4.5%). A possible reason that the curvature of individual spherules does not determine the critical supersaturation might be that clusters of small soot particles have negative curvature at the contact points, where water would condense already at lower S or even in subsaturated conditions. There are several other possible reasons why Kelvin theory might not be a good model for carbon black particles. Despite the production of carbon black particles from a liquid suspension they are not necessarily perfectly spherical, possibly not completely wettable, and they are light absorbing. For nonspherical particles, the curvature of the particle surface which determines the critical supersaturation is not necessarily a simple function of the particle diameter. The actual effective curvature might be higher or lower which

could lead to a broadening of the CCN spectrum but also to a bias in the critical supersaturation. Moreover, not completely wettable or light-absorbing particles would both be activated at S higher than predicted by Kelvin theory, which could explain the discrepancies between the data and the simple Kelvin model. 4.2. Discussion. To investigate the latter two effects, the difference between the experimentally determined S50% and Kelvin ScK is plotted in Figure 4 as a function of particle diameter for all experiments using pure carbon black particles. Most experiments were performed at a laser power of 11.5 mW (squares) but some at reduced laser powers of 7 mW (circles) and 4.5 mW (triangles). The difference between S50% and ScK does not increase significantly with the laser power. This is a strong indication that particle heating in the laser beam is not the cause of the elevated S50% of carbon black particles. This conclusion is corroborated by a comparison with the calculated increase in supersaturation due to particle heating (Sch, eq 4, black line; for the calculations, a refractive index of carbon black of 1.96-0.66i was assumed; 32) and due to a finite contact angle (Sca, eq 6, dashed lines). The difference between Sch and ScK increases strongly with particle diameter, which disagrees with the measurements. However, the observations could be explained by a fairly limited range of contact angles between 4 and 6°. In summary, Figure 4 illustrates that a small but nonzero contact angle of water on the particles’ surfaces is a more likely explanation for the observed deviation of carbon black particle activation from Kelvin theory than light absorption by the carbon black particles in the laser beam. The activation of the compacted soot particles we investigated can well be parametrized by their mobility equivalent diameter and a finite contact angle. This derived contact angle is a convenient model to describe the activation of irregularly shaped particles and therefore somewhat dependent on the choice of the equivalent diameter and not necessarily characteristic of the VOL. 40, NO. 4, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Difference between the experimentally determined supersaturation at 50% activation (S50%) and Kelvin theory (ScK) as a function of particle size. The symbols represent measurements made at different laser power (11.3 mW, squares; 7 mW, circles; 4.5 mW, triangles). The dashed lines show the difference between the critical supersaturation of a particle with a finite contact angle with water (Sca) and Kelvin theory (ScK). The solid black line shows the difference between the critical supersaturation taking into account possible heating of the particle in the laser beam (Sch) and Kelvin theory (ScK). actual contact angle between the soot material and water. For example one might use the volume equivalent diameter of a compact cluster of four spheres as a model to describe the soot particles instead of the mobility equivalent diameter. Assuming a dynamic shape factor of 1.17 for such a cluster (33, p 52), this would decrease the derived contact angle to values around 3°. However, to reconcile the measured and calculated critical supersaturations without invoking a contact angle at all, an average shape factor of around 1.7 would be necessary. This would for example correspond to a straight chain, with an axis ratio of 10:1 (33, p 52), which is not a very realistic model for soot particles produced from a suspension. Compared to the study by ref 34 the contact angles we derived for the carbon black particles in this study are much lower than for their spark discharge soot produced in a pure argon carrier gas (θ ) 37-50°). On the other hand, they are larger than the contact angles around θ ) 0°, which were found for soot particles produced in white gas flames, which contain trace amounts of soluble material (35). The reason for these differences might be the different production mechanisms. In our study, the soot particles were produced from a liquid suspension of carbon black in ultrapure water mixed with 2-propanol. The fact that the particles are activated at higher S than suggested by Kelvin theory suggests that the water and 2-propanol of the suspension are largely evaporated, although trace amounts of both could still be adsorbed. Moreover, it is usually found that evaporation of droplets produced by nebulizing even deionized, purified water leaves residual nanosized particles. This effect is partly due to remaining impurities in the water but could also be attributed to leaching of material from container walls (36). In our case, these unknown residual compounds in the deionized water could form a film on or an internal mixture with the carbon black particles that might reduce the contact angle of water with pure soot, but likely not to 0°. The fact that the CCN activation of carbon black particles does not reflect the expected increase in ScK due to light absorption in the laser beam is probably a consequence of the design of the chamber. The laser beam, which crosses the chamber in the region near the peak supersaturation, is relatively narrow (diameter ca. 1 mm). Only the dry carbon black particles present in the beam at the start of the 1228

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measurement cycle are heated. Since these heated particles have critical supersaturations that are 1-4% higher than the Kelvin supersaturation (Figure 3), they generally stay unactivated at chamber supersaturations below 3% for particle diameters considered in this study. The particles right above the laser beam, however, are activated without influence of heating. The droplets formed on these particles sediment eventually into the sensing volume, where they are counted as CCN. Since the supersaturation profile changes relatively slowly near its maximum in the middle of the chamber, these particles are activated at only slightly lower S than expected. The peak in the CCN concentration might therefore occur somewhat later in the measurement cycle, but the number of CCN is probably not significantly reduced. We therefore conclude that it is possible to measure the CCN concentration of strongly absorbing particles without the influence of particle heating in the laser beam in our CCNC. In similar static diffusion chambers operating with wider laser beams (e.g., 37), however, the effect of particle heating on the measured CCN concentration could be larger.

5. Activation of Carbon Black Particles Coated with NaCl and Hexadecanol For the generation of coated carbon black particles, NaCl or hexdecanol is added to the carbon black suspension at a ratio of 95% carbon black to 5% coating material. The solution is dispersed in the Collison atomizer, and the droplets are neutralized and dried. After drying off, NaCl can crystallize on and among the carbon black clusters. In this case, we use the term “coating” to indicate that the NaCl crystals are located on the outside of the soot cores (as opposed to a homogeneous mixture) and not in the sense of a shell of equal thickness. Again monodisperse particles are selected using the DMPS system. The particles are now composed of carbon black and coating material, but not necessarily at the 95% to 5% ratio. Since neither the soot cores nor the droplets produced by the Collison atomizer are monodisperse, a core of, e.g., 80 nm enclosed in a 1.5 µm droplet will have more solution around it (and therefore more NaCl after drying) than a core of 500 nm enclosed in a 1 µm droplet. The mass fraction of coating material in the mixed particles can be estimated. If an insoluble core (s) is enclosed in a solution droplet (d), then the volume of the remaining solution around the core is Vsol ) Vd - Vs. After drying, the mass of coating material is given as

(

mc ) cVsol ) c

)

π 3 ms d 6 d Fs

(8)

where ms is the mass of the carbon black core, c is the concentration of coating material in the carbon black suspension in g/L, and Fs and Fc are the densities of carbon black and the coating material. If we know both the dry diameter (dp) of a mixed particle and the diameter of the droplet from which it originated (dd), assume volume additivity (i.e., Vs ) Vp - Vc) and sphericity of the dry particle, ms is given as

π ms ) Fs 6

c 3 d Fc d c 1Fc

dp3 -

(9)

The mass fraction fc of coating material in the mixed, dry particle

fc )

mc mc + ms

(10)

FIGURE 5. Theoretical estimate of the frequency of occurrence of soot particles with a given mass fraction of NaCl. can therefore be expressed as a function of the selected diameter dp, the concentration of the salt solution c, and the droplet diameter dd. If the size distribution of the droplets produced by the Collison atomizer is known, the probability distribution of finding a coating material mass fraction fc in a particle of dry size dp can be estimated from equations 8-10. Figure 5 shows an example of the frequency of occurrence of different values of fc in a mixed particle of 200 nm dry size. The calculation is based on the nominal droplet size distribution of the Collison atomizer with a mean diameter of 1 µm and a standard deviation of 1.7 (Reischl, personal communication). Possible compositions span a large range from particles consisting of pure coating material to particles coated only with trace amounts of the material. This simplified model likely overestimates the spread of coating fractions, since soot cores below 80 nm and above 600 nm are rare in our measurements but could be present in the model. The most frequent particle composition lies between 1% and 2% coating material by mass for 200 nm particles and 0.5% and 1% for 288 nm particles, which is somewhat less than in the bulk solution. Figure 6 shows a comparison of the activation curves for coated particles (squares) with two different curves measured for pure carbon black (filled and open triangles). The addition of roughly 2% NaCl decreases the critical supersaturations of the carbon black particles to values that commonly occur in the atmosphere. On the other hand a coating of roughly 1% by mass by the hydrophobic hexadecanol (corresponding to a hexadecanol layer of ca. 1 nm thickness; particle sphericity assumed) hinders the particle activation, as can be seen in Figure 6b (circles). The carbon black particles coated by hexadecanol activate only gradually and at much higher supersaturations than pure carbon black particles. The activation of particles coated with NaCl can be compared to Ko¨hler theory for a mixture of insoluble and hygroscopic material using the information about the relative abundance of particles with different mass fractions of coating material (presented in Figure 5). To each value of fs corresponds a critical supersaturation Sc(fs). The activation ratio at supersaturation S is the integration of the probability distribution shown in Figure 5 over all fs where Sc(fs) < S. These theoretical activation ratios are also shown in Figure 6 as solid lines. They agree quite well with the measurements for activation ratios smaller than 0.6, corresponding to NaCl mass fractions larger than approximately 1%. For smaller NaCl mass fractions the measured activation ratios are lower than theoretical predictions. The slope of the activation curve decreases and becomes similar to that obtained for pure

FIGURE 6. Measured activation curves of soot particles coated with NaCl (squares) and hexadecanol (circles) are compared to the activation of pure soot particles (triangles): (a) particle size 201 nm; two different data sets for soot particles are given; (b) particle size 288 nm. The black lines show the best fit error functions for the soot curves. The calculated activation curves based on the modified Ko1 hler equation for particles with insoluble cores (eq 6) and with mass fractions of other material given by eq 8-10 (shown as gray lines) agree well with the measurements. carbon black particles. In this regime the hydrophobic surface of the carbon black particles might start to influence the water uptake, and the simple Ko¨hler model of an insoluble wettable sphere coated with hygroscopic material no longer applies. In conclusion, we investigated the CCN activation of pure and coated carbon black particles using the University of Vienna CCNC. We found this counter suitable to study the CCN activation of strongly light-absorbing particles, because particle heating in the chamber is avoided by using a very narrow laser beam. Uncoated carbon black particles are activated at a higher supersaturation than predicted by Kelvin theory, which can be parametrized using a small but nonzero contact angle (4-6°) of water on the soot surface. Pure carbon black particles with diameters between 200 and 300 nm have critical supersaturations between 1% and 2%. Coating these particles with trace amounts of NaCl (5% by mass of NaCl in the suspension) decreases the critical supersaturation to values of atmospheric relevance (∼0.3%). The measured CCN efficiencies compare well with calculations based on Ko¨hler theory for particles consisting of insoluble and hygroscopic material. Coating the soot particles with hexadecanol (a typical film-forming compound with one hydrophobic and one hydrophilic end) efficiently suppresses their CCN activation. While previous studies (24, 25) showed that a VOL. 40, NO. 4, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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coating of hydrophobic material does not hinder the CCN activation of hygroscopic particles, we found that there is a clear effect on insoluble particles.

Acknowledgments The CCNC was developed with the financial support of the Austrian Science Fund (FWF), Grant P 131 43-CHE. We thank the Institute for Chemical Technology and Analytics (CTA) of the University of Technology of Vienna for the chemical analysis of the carbon black samples. We thank H. Giebl and P. Ctyroky for valuable help with the experimental setup and operation of the CCNC.

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Received for review February 21, 2005. Revised manuscript received November 3, 2005. Accepted November 28, 2005. ES0503478