A New Volatility Tandem Differential Mobility Analyzer to Measure the Volatile Sulfuric Acid Aerosol Fraction

Transcription

1 760 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 16 A New Volatility Tandem Differential Mobility Analyzer to Measure the Volatile Sulfuric Acid Aerosol Fraction D. A. ORSINI, A.WIEDENSOHLER, AND F. STRATMANN Institute for Tropospheric Research, Leipzig, Germany D. S. COVERT Department of Atmospheric Science, University of Washington, Seattle, Washington (Manuscript received 26 January 1998, in final form 20 July 1998) ABSTRACT A volatility tandem differential mobility analyzer (VTDMA) was developed with the intention to measure the fraction of sulfuric acid in marine fine aerosols (D p 150 nm). This work focused on the design and calibration of an aerosol conditioner for the standard tandem differential mobility analyzer that heated a selected aerosol sample in a controlled manner while it minimized sample losses due to thermophoresis and diffusion. In designing an aerosol heater, the behavior of a monodisperse aerosol was modeled in a heated flow tube with a temperaturecontrolled wall under laminar flow conditions. This allowed for initial estimations of aerosol heating rates, heater dimensions, and aerosol deposition losses within the flow tube. The evaporation rate of sulfuric acid particles was also predicted and used to determine the minimum length of the aerosol heater needed to completely evaporate a sulfuric acid aerosol. Finally, the VTDMA was calibrated and tested for its ability to detect a sulfuric acid mass fraction in a monodisperse aerosol. 1. Introduction There has been an increasing need for size-resolved measurements of the chemical behavior of atmospheric fine aerosols. Until now, modeling work has concentrated on predicting new aerosol particle production based on the binary homogeneous nucleation rates of the H 2 SO 4 H 2 O system (Raes et al. 1992; Hegg et al. 1992; Easter and Peters 1994). This work has been based on the assumption that the primary source of submicrometer aerosol mass is the condensation of gas-phase sulfuric acid (H 2 SO 4 ) and methanesulfonic acid (MSA) in the presence of water vapor. Sulfuric acid and MSA vapors are believed to condense either directly from the gas phase (homogeneous nucleation) or onto preexisting aerosol particles (heterogeneous condensation). Large uncertainties in these theoretical predictions exist, however, mainly because of the lack of chemical measurements of fine atmospheric aerosols (D p 200 nm). The reason for the lack of measurements is due mostly to the inability of standard measurement techniques to study such small particles. Impactor methods, for example, rely on collected aerosol mass, which is analyzed later in this paper for its chemical composition. To ac- Corresponding author address: Dr. A. Wiedensohler, Institute for Tropospheric Research, Permoser Str. 15, D Leipzig, Germany. quire sufficient sample mass with this technique, sampling times are often very long, and lower cutoffs are usually greater than 150 nm. Both the size resolution and time identity of the impactor samples are usually very poor. Previous studies of fine aerosol chemical behavior that have been slightly more successful in resolving the size-resolved chemical nature of the aerosol have been mostly confined to measurements performed on the entire aerosol population. Measurements of aerosol volatility, for example, have been made on aerosol size distributions in both clean marine and continental sites to help understand the aerosol chemical behavior. Clarke (1992) performed an experiment in which he measured the volatility behavior of the aerosol number size distribution after thermal conditioning. The measurement system consisted of a heater upstream of an optical particle counter (OPC), which measured the number size distribution in the diameter size range m. Clarke s results showed that heating of the equatorial marine aerosol size distribution revealed volatile components at temperatures associated with sulfuric acid volatility at 150 C and ammonium sulfate/ bisulfate at 300 C. Aged continental size distributions that were heated to 140 C showed very little mass loss in both accumulation- (100 nm D p 1000 nm) and coarse-mode diameters (D p 1000 nm). Temperatures of 300 C, however, removed a major fraction of the accumulation-mode mass and was speculated as being 1999 American Meteorological Society

2 JUNE 1999 ORSINI ET AL. 761 due to the presence of ammonium sulfates. A volatility measurement system used by Covert et al. (1992) was taken into clean arctic marine conditions. The system consisted of a differential mobility analyzer (DMA) in line with a heater and an OPC. The DMA was used to select a narrow bandwidth aerosol followed by heating of the particles for 10 s. The OPC measured the conditioned size distribution of the aerosol after exiting the heater. Measurements were concentrated on the 200-nm aerosol diameter size and were heated to a maximum temperature of 250 C. Results revealed the different compounds with varying degrees of volatility ranging from 10% to 90% by mass. Volatile properties of marine aerosols were also measured by Jennings et al. (1990), whereby concentrations of selected size intervals of the aerosol distribution were heated before being remeasured. Significant increases in volatile mass were observed for diameter sizes less than 150 nm and heating temperatures less than 400 C, hinting toward the dominance of ammonium sulfate. In a recent study by Weber et al. (1997), ultrafine particle production was measured and correlated with H 2 SO 4 precursor gases. Particle production was seen to occur at H 2 SO 4 values well below predicted concentration levels of H 2 O and H 2 SO 4 required for binary nucleation, indicating that other substances, such as ammonia, are participating in the process. The intention of this work was the construction of a system that could provide more insight into the volatile chemical behavior of fine aerosols. A tandem differential mobility analyzer (TDMA) system was constructed specifically to measure the size-resolved volatile behavior of aerosols in the size range D p 150 nm. A volatility TDMA (VTDMA) was therefore developed that could alter a selected aerosol due to the presence of a sulfuric acid component. It is known that sulfuric acid volatilizes at a rather low temperature ( 110 C). The goal, therefore, was to develop an aerosol heater that could heat an aerosol sample and volatilize the sulfuric acid fraction. Incorporation of the aerosol heater into the TDMA would allow for size-resolved in situ field measurements of the sulfuric acid aerosol fraction. This new system would ultimately identify the fraction of sulfuric acid as a function of particle size in atmospheric aerosols. 2. Review of the TDMA system The TDMA, shown in Fig. 1, uses two DMAs in a series to study aerosol properties as a function of the selected particle size. The system can be broken down into three main steps (Rader and McMurry 1986). Step 1, aerosol selection, consists of the polydisperse aerosol source upstream of the first DMA (DMA 1). The polydisperse source can be either an atmospheric sample or a laboratory-generated aerosol of interest. In this step, DMA 1 is set to a voltage V 1 to select a narrow mobility bandwidth aerosol from the polydisperse aerosol source. The monodisperse sample aerosol flow Q s, which exits DMA 1, is split into two paths: one directs half of the sample to step 2 and the other directs the other half to the condensation particle counter 1 (CPC 1). CPC 1 is used to continuously monitor the concentration of the DMA 1 selected aerosol (i.e., the concentration of the aerosol directed further on to step 2). In step 2, a defined conditioning process acts upon the selected aerosol by altering its mobility distribution. Such conditioning processes could be aerosol humidification or heating. In the case of aerosol humidification, the input mobility distribution selected by DMA 1 is sent through a conditioner, exposing the aerosol to an increased relative humidity. The mobility distribution could grow or broaden depending on the hygroscopic nature of the aerosol. In the case of an aerosol heater as the conditioning process, the mobility distribution could be altered if the aerosol sample is volatile at the given temperature conditions. After conditioning, the sample is directed to step 3, at which point the second DMA (DMA 2), in series with CPC 2, scans a defined particle mobility range to determine the new conditioned mobility distribution. The TDMA focuses specifically on comparing conditioner input and output aerosol distributions as a function of the controlled conditioning parameters. By measuring changes in the mobility distribution as a function of the conditioning, the properties of the selected aerosol can be studied and, therefore, may provide insight into the particle chemical composition. In this work, a TDMA was developed to measure the sulfuric acid fraction existing in fine marine aerosols. The work was based on the need to acquire more information regarding the size fractionated distribution of sulfuric acid in the fine aerosol size range (D p 1000 nm). To measure the sulfuric acid fraction, a conditioning step was needed for the TDMA that altered the selected mobility distribution because of the presence of H 2 SO 4. The idea for a conditioner stemmed from the volatility properties of H 2 SO 4, specifically its phase change from a liquid to a gas at approximately 140 C. Furthermore, H 2 SO 4 is believed to react with atmospheric NH 3, resulting in the products ammonium bisulfate (NH 4 HSO 4 ) and ammonium sulfate [(NH 4 ) 2 SO 4 ]. These species have high volatility temperatures, above 150 C, therefore making it possible to differentiate free sulfuric acid from its evolving ammonia products. Since the TDMA framework has become relatively standard, this paper focuses mainly on the development and calibration of an aerosol conditioner intended to volatilize a sulfuric acid aerosol fraction. Although the design of the aerosol heater was kept relatively simple, more effort was put forth toward understanding the behavior of an aerosol with a volatile mass fraction that passes through a laminar flow heater. The design of the aerosol heater was seen to be divided into three main parts: modeling, construction, and calibration.

3 762 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 16 FIG. 1. Schematic view of the TDMA. 3. Design of an aerosol heater The goal was to construct an aerosol heater that would volatilize the sulfuric acid fraction of a monodisperse aerosol in the diameter size range D p 150 nm. This required that the aerosol heater 1) heat and hold the sample aerosol at a raised temperature long enough to evaporate the sulfuric acid component and 2) be designed to minimize particle losses due to thermophoresis and diffusion as well as to avoid recondensation of evaporated mass to the remaining aerosol. a. Maximizing the aerosol transport efficiency A major goal in the construction of the aerosol heater was a design that minimized aerosol losses due to thermophoresis and diffusion. Maximum aerosol transport was desired because small particles (D p 100 nm) are strongly affected by diffusion, leading to many depositional losses. Since the number concentration of ultrafine particles in the remote atmosphere with diameters D p 20 nm is often very low, it is desirable to maximize transport efficiency of the aerosol through the conditioner to maintain reasonable detection efficiency in the fine aerosol size range. Therefore, to minimize particle deposition, the heater design was restricted to satisfy conditions of laminar sample flow. For the case of aerosol flow through a smooth tube, a Reynolds number Re 2300 is required to satisfy conditions for creating laminar flow. In this flow regime, a gradual aerosol concentration profile is formed over the cross section from the wall boundary to the centerline that transports the aerosol down the tube. Because of Brownian motion, the particles diffuse to the walls, developing a gradual concentration profile as the aerosol flows down the tube. For the Reynolds number case Re 2300 the flow profile becomes turbulent, and Brownian diffusion is accompanied by turbulent eddies near the wall boundary that more effectively transport the particles to the walls, resulting in a steeper aerosol concentration profile in the flow tube and therefore more depositional losses. Hence, aerosol heater dimensions and sample flow rate were restricted to satisfying the laminar flow condition of Reynolds numbers Vd/ Here is the air density, V the flow velocity, d the tube diameter, and the air viscosity.

4 JUNE 1999 ORSINI ET AL. 763 FIG. 2. Modeled 2D aerosol temperature profile for an entering 25 C aerosol heated in a laminar flow tube. While upholding the laminar flow conditions, the next step in the design of the heater was to provide controlled heating of the aerosol. This was approached in two steps. First, the aerosol heating rate was predicted in order to calculate the time required for the aerosol to attain the desired evaporation temperature within the flow tube. Second, the effective residence time at which the aerosol is held at the desired evaporation temperature must be at least the minimum time required to evaporate a sulfuric acid aerosol. This determined the dimensions of the flow tube. b. Aerosol heating rate in a laminar flow tube To make aerosol heating-rate predictions, we used Computational Fluid Dynamics (CFD) modeling code (Patankar 1980; Stratmann et al. 1994), which simulates the particle dynamics of an aerosol in a laminar flow tube under the influence of a temperature-controlled wall. The CFD code predicts the thermophoretic and diffusive behavior of an aerosol as it is heated in a laminar flow tube. The model requires the following input parameters. 1) A flow tube defined by inner radius R c, length L c, and wall temperature T c. The heater wall temperature here is assumed to remain at a constant temperature and act as an infinite heat source. 2) An input aerosol sample flow defined by an entering temperature T 1, flow rate Q s, and selected particle size D p1. For the given set of input parameters, the code predicts the aerosol 2D temperature profile, the aerosol 2D concentration profile, and the total aerosol losses through the heating unit due to thermophoresis and diffusion. The input parameters were selected as L c R c 40.0 cm 0.2 cm, and a constant wall temperature was T c 120 C. In addition, the input aerosol was described with an initial temperature T 1 25 C, flow rate Q s 1 lpm, and D p1 15 nm. With these dimensions, the aerosol velocity on the centerline within the flow tube is 1.33 m s 1, which gives a Reynolds number of approximately 400, which is well within the laminar flow regime. The velocity translates to a total residence time in the flow tube of t 3s. The modeled predictions of the aerosol 2D temperature profile are shown in Fig. 2. A 25 C uniform aerosol concentration profile enters from the left into the laminar flow tube that has a constant wall temperature of 120 C. After entering the heater, the aerosol is heated diffusionally from the wall boundary to the centerline. Between 8 and 9 cm from the entrance, the aerosol is uniformly heated to the desired wall temperature of 120 C. The average rate of aerosol heating on the centerline is 13.3 C cm 1. This gives an entrance heating

5 764 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 16 FIG. 3. Effect of thermophoretic focusing on the aerosol concentration profile in a heated laminar flow tube. time t h 9cm 1/(133 cm s 1 ) 0.07 s. This rapid heating rate occurring at the tube entrance reduces the time of nonuniform heating across the cross section of the laminar aerosol flow. The time in which the aerosol is actually held at the raised temperature (the effective residence time t e ) is determined as t e t T t h, where t T is the total residence time in the aerosol heater. For a length of 40 cm, the effective residence time t e was calculated as 0.23 s. The modeled temperature field is also a controlling factor of the particle dynamics; specifically, it determines the manner in which the entering aerosol concentration profile is altered by thermophoresis and diffusion. With the modeled temperature field, the relative aerosol concentration profile was predicted with the CFD code and is plotted in Fig. 3. As seen in this illustration, thermophoresis will act to focus the aerosol to the centerline. It is acting in the manner opposite to diffusion, which tends to broaden the profile. For the case of 15-nm particles, the effect of thermophoresis clearly dominates the diffusional effect, as seen by the focused aerosol concentration. The slight rebroadening of the aerosol is due to diffusion and is observed only after the aerosol has attained the desired temperature of 120 C. In the case of 15-nm diameter particles, the losses through the heated flow tube are rather minimal due to the thermophoretic focusing. Between entrance and exit, the aerosol penetration was predicted to be 92%. c. Evaporation rate of sulfuric acid particle From the CFD predictions, a residence time of 0.23 s was calculated for the aforementioned flow tube geometry. The remaining question is whether 0.23 s is long enough to evaporate a sulfuric acid aerosol. The evaporation rate of single sulfuric acid aerosol particles as a function of time and temperature must be calculated. The predicted rate of sulfuric acid particle evaporation in the aerosol heater is described here in a short thermodynamic discussion. In general, for an evaporating particle of size D p,a mass balance can be written as d Dp 3 2 l DpJ, (1) dt 6 where D p is the particle diameter, l is the particle density, and J is the mass flux [mass/area(time)] away from the surface of the particle. To evaluate the rate of particle evaporation, an expression for J is needed that describes the evaporation rate of sulfuric acid based on its thermodynamic properties. This term is dependent on particle size, surface energy, vapor pressure, diffusivity, temperature, and the surrounding gas conditions controlling the transport of gas from the particle surface. It is expressed as

6 JUNE 1999 ORSINI ET AL. 765 TABLE 1. Sulfuric acid properties at 25 C. Properties in transition regime Value Source Diffusivity, D ij (in cm 2 s 1 ) Vapor pressure, p o (in dyn cm 2 ) Surface tension, (in dyn cm 1 ) Density, 1 (in g L 1 ) Wilke and Lee (1955) Kulmala and Laaksonen (1990) Sabinina and Terpugow (1935) CRC Press (1991) 2D M ij i 4 Mi J J p exp p c o. (2) DRT p D p lrt Maxwell s Kelvin s term term Maxwell s term, on the left side, is the modified term derived from Maxwell s equation that describes the evaporation rate (mass flux) of a sphere in the continuum regime. Here D ij is the diffusivity of the gas species i, M i is the gas molecular weight, and R and T are the gas constant and temperature, respectively. Variable is the ratio J/J c, which describes the mass flux ratio between the continuum and transition regimes. The continuum regime is defined for particle sizes larger than the molecular mean free path and is described by large values of the Knudsen number Kn k 2 /D p. In this work, we are studying particles in the size range nm, which is approximately on the same order of the mean free path of an evaporating molecule. In the transition regime case in which Kn 1, the Maxwell term is multiplied by to correct for noncontinuum effects. The right-hand term of Eq. (2), Kelvin s term, accounts for the increased vapor pressure over the curved surfaces of small particles. Here is the surface tension, l is the particle density, and p o is the vapor pressure over a flat surface. The last term p on the right-hand side is the partial pressure of the evaporating gas species away from the particle. By substituting Eq. (2) into Eq. (1), the evaporation rate for a single particle is written as [ ] ddp 4DijMi 4 Mi po exp p. (3) dt D RT D RT l p l In this aerosol heater case, we assume that the partial pressure of the evaporating species far away from the particle is zero, and that because of low particle concentrations after the first DMA, vapor buildup in the surrounding gas is negligible. This implies that p 0. For a sulfuric acid particle with an initial diameter size D p1 entering a heated flow tube at time t 1, integration of Eq. (3) over the particle residence time in the heater will yield the predicted final particle size D p1 at time t 1. The thermodynamic parameters of sulfuric acid at 25 C are listed in Table 1 along with the respective sources. The larger the sulfuric acid particle, the longer it will take to completely volatilize. In this work, D p 150 nm was expected to be the largest selected particle size and, therefore, will require the longest effective residence time t e in the aerosol heater to volatilize. Figure 4 shows the prediction of final sulfuric acid particle size D p1 as a function of heater temperature and effective residence time. The complete evaporation time, given by the curve intersection with the x axis, is represented by D p1 0. For the 150-nm sulfuric acid particles held at 80 C, for example, the heater must hold the aerosol for a minimum effective time of 0.45 s before all of the mass has evaporated. For the previously modeled 120 C case, the minimum residence time requirement is less than 0.02 s, which is far below the t e 0.23-s residence time calculated for the 40-cm-long heater. d. The aerosol heater construction FIG. 4. Prediction of final sulfuric acid particle size D p1 as a function of heater temperature and effective residence time. We wanted to keep the construction of the aerosol heater as simple as possible and at the same time satisfy the requirements layed out in the preceding discussion. Illustrated in Fig. 5, the construction used standard quarter-inch stainless steel tubing to facilitate DMA connections and to maintain uniform pathways. The heater consisted of a stainless steel sample line that was 0.4 cm in diameter on the inside and 40 cm long, and was housed in a second hollow concentric steel tube that was 2 cm in diameter. The housing was made in order to create a constant wall temperature similar to an infinite heat source. This was created by filling the volume surrounding the sample line with heated oil that was continuously circulated from a temperature-controlled oil bath. This method of heating maintained the desired

7 766 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 16 FIG. 5. Schematic view of the VTDMA conditioning step: laminar flow heater and bypass system. constant wall temperature and removed uncertainties of hot spots or unknown temperature gradients that could lead to nonuniform heating of the aerosol sample. The laminar aerosol sample flow of 1 lpm that entered the flow tube was therefore heated radially and diffusionally from the wall boundary to the centerline. Thermocouples located in the aerosol sample inlet and outlet flow, as well as in the surrounding oil and oil bath, continuously monitored the temperature of the system. As a complete conditioning unit, the heater was placed in parallel with a room temperature bypass. The incoming sample flow Q s was directed through either the 25 C bypass or the 120 C aerosol heater, depending on the position of the two magnet valves that ran in parallel. Switching between bypass and heater paths allowed for respective conditioner input and output aerosol distributions to be passed on to the DMA 2 scanning step. Experimental testing and calibration of the aforementioned conditioning step is reserved for a later discussion. 4. The VTDMA system With respect to the system illustration shown in Fig. 1, the standard operation of the system with the integrated aerosol heater is described as follows. In normal operation, 2 lpm of the polydisperse aerosol first pass through the Kr-85 bipolar charger. The charged aerosol is directed further on to DMA 1 (Hauke type), which FIG. 6. Results of the NaCl penetration test showing the efficiency of aerosol transport through the conditioning step in dependence on the selected particle size and heater temperature. Error bars result from an averaging of five test runs.

8 JUNE 1999 ORSINI ET AL. 767 TABLE 2. Coefficients of fit according to Eq. (4), which parameterizes the aerosol penetration. Temperature a b K 25 C 70 C 100 C 120 C 150 C P 50 (in nm) is operated with a sheath aerosol airflow rate (Q s1 /Q a ) of 20/2 lpm and less than 5% relative humidity. After size selection, the monodisperse aerosol flow is split into two paths: 1 lpm of the sample is directed to CPC 1 (TSI CPC-3010) in order to continuously monitor the concentration of the selected DMA 1 aerosol sample flow. The remaining 1 lpm of sample flow first encounters one of two magnet valves operated in parallel. Depending on the position of the two valves, the sample is directed either through the bypass line or through the aerosol heater held at 120 C. The exiting size distribution is further directed to DMA 2, which is driven with a sheath aerosol airflow rate of 10/1 and less than 5% relative humidity. Finally, depending on the positions of the magnet valves, DMA 2, in series with CPC 2, scans the respective conditioner input and output aerosol distributions. a. Correcting for particle loss through the aerosol conditioner The main reason why the aerosol penetration through the VTDMA conditioning step must be calibrated is so that the measured mobility distributions can be corrected for the size-dependent diffusional and thermophoretic losses. The particle losses, which were previously modeled with the CFD code, were calculated for the aerosol heater only. The majority of particle losses, however, occur after the aerosol exits the heater and before it enters DMA 2. In this path, the aerosol cools to room temperature, and thermophoresis acts in the same direction as diffusion, bringing the aerosol to the tube walls. To measure total particle losses through the conditioner, the test must be performed including the cooling path up to the DMA 2 sample entrance. The interpretation of TDMA data is a matter of comparing the selected unconditioned mobility distribution with the conditioned distribution. Both distributions correspond to the respective 25 C bypass and heater paths of the aerosol conditioner. Ideally, the differences between these two mobility distributions are due to an evaporated mass fraction. The conditioned distribution, however, is additionally transformed from effects of thermophoresis and diffusion, which are both temperature and particle size dependent. For conditioning to be attributable solely to a volatilized component, the conditioning step FIG. 7. Volatility scans of 55-nm H 2 SO 4 aerosol heated stepwise to 150 C. needs to be calibrated so that the size-dependent correction can be applied. To make these tests, a laboratory-generated test aerosol was needed that remained nonvolatile in the volatility temperature range of sulfuric acid. A nonvolatile test aerosol would allow for measured differences across the conditioning step due only to thermophoresis and/ or diffusion. The VTDMA system was set up to make the aerosol penetration tests, with the exception of the polydisperse source, which was replaced by an atomizer that contained an NaCl solution. The NaCl aerosol was chosen because of its stable, nonvolatile behavior and high volatility temperature of approximately 600 C. After dispersion, the polydisperse NaCl aerosol is further mixed with sheath air for drying and dilution. To make the proper test of particle loss through the conditioning step, particle concentrations were measured with CPC 2 immediately before the aerosol entered DMA 2. This accounted for the complete losses from cooling as well as for the possible particle deposition within magnet valves along the way. Since diffusion and thermophoresis are both temperature and particle size dependent, aerosol penetration tests needed to be carried out varying both parameters. For each DMA 1 selected particle size D p1 chosen within the size range nm, the range of temperatures between 25 and 150 C was scanned. Comparison of concentration readings from CPCs 1 and 2 gave the penetration P(T, D p ) N 1/N 1, where N 1 and N 1 are the selected and conditioned total particle numbers, respectively. The results from this test are shown in Fig. 6. The trends in this figure, illustrating the combined thermophoretic and diffusional losses in the conditioning step, are significantly more than was predicted from the modeling of section 3c. For the 15-nm selected size, less than 50% of the aerosol entering the conditioner reached DMA 2 at 120 C. This was compared to the predicted penetration efficiency of 92%. Once again, this calibration was performed including the aerosol

9 768 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 16 FIG. 8. Inversion of the volatility scans of 55-nm H 2 SO 4 aerosol in Fig. 7. FIG. 9. Extent of sulfuric acid particle volatilization in aerosol heater as a function of initial particle size and heater temperature. cooldown and shows the increased loss after the heated sample exits the aerosol heater. To use these corrections during normal system operation, a parameterized curve fit through the measurement points was needed, as plotted in Fig. 6. The parameterization for each curve is described by the following equation: P 1 1/exp[(D p1 a)/(k D p1 /b)], (4) where D p1 is the selected NaCl size, and a, b, and K are constants of best fit. The fitting results in addition to the 50% penetration diameter P 50, are summarized in Table 2. With these corrections for aerosol penetration, it became possible to compare the aerosol entrance and exit mobility distributions based only on a volatilized sulfuric acid fraction. This further allowed for testing of the system s ability to volatilize a pure sulfuric acid test aerosol and to determine if vapor recondensation could occur. Recall that in the theoretical prediction of the sulfuric acid evaporation rate it was assumed that the partial pressure of the evaporating species surrounding the particle was negligible. This is likely not the case. from a generated 55-nm monodisperse sulfuric acid test aerosol heated stepwise to 150 C. The measured points shown here are raw concentration data measured by CPC 2 after DMA 2. The 25 C curve, located at the far right in Fig. 7, is the H 2 SO 4 monodisperse distribution selected by DMA 1 to enter the aerosol heater (i.e., the conditioner is in the bypass mode). Upon heating, evaporation of sulfuric acid shifts the size distribution to smaller sizes. In order to properly compare the heater input (selected) and output (conditioned) sulfuric acid aerosol as a function of heater temperature, the conditioned mobility distribution was needed before it entered DMA 2. This was acquired by inverting the CPC 2 measurement data using the TDMA inversion program from Stratmann et al. (1997). To obtain the distributions exiting the aerosol heater, the inverted data were loss corrected from the results of Fig. 6. The inverted, corrected distributions n (D p ) are presented in Fig. 8 and represent the size distributions exiting the b. Volatility tests of H 2 SO 4 aerosols in the VTDMA The VTDMA system was run with H 2 SO 4 test aerosols to prove its ability to evaporate a pure sulfuric acid aerosol. These results were used to determine the temperature at which to run the aerosol heater, as predicted by the evaporation theory described earlier. In addition, tests for possible recondensation effects due to vapor buildup of the H 2 SO 4 evaporated gas were also performed. This allowed for generation of the polydisperse H 2 SO 4 aerosol from a prepared chemical solution. The goal of the test was to determine the extent to which the aerosol heater could volatilize a sulfuric acid aerosol as a function of selected particle size D p1 and heater temperature T c. Presented in Fig. 7 is the scanned result FIG. 10. Volatile behavior of (a) ammonium bisulfate and (b) ammonium sulfate.

10 JUNE 1999 ORSINI ET AL. 769 FIG. 11. Schematic view of generator for producing a sulfuric acid coated NaCl test aerosol. aerosol heater. The quantitative results, integral number, mode diameter, and volatile volume were acquired for the distributions by best fitting normal-gauss modes into the solutions. For a nonvolatile aerosol, the integrals measured before and after the aerosol heater should agree. Large disagreement suggests that particle removal through the heater has occurred because of an evaporated aerosol fraction. The aforementioned test was extended to particle diameters of 15, 30, and 150 nm for the same range of heating temperatures. The summarized fit results are shown in Fig. 9. Here we have plotted the fitting results of the H 2 SO 4 particle diameter D p1 exiting the aerosol heater as a function of conditioning temperature T c and initial size D p1. Each curve with its respective error is an average representation of three runs. A few features here deserve comment. First, the general trend of earlier volatilization for smaller particles is observed accompanied by an increased rate with decreasing particle size. This is due to the increased vapor pressure over the curved surfaces of small particles, that is, the Kelvin effect. Second, for the largest 150-nm-diameter size, complete aerosol volatilization occurs at approximately 110 C. This is a much slower evaporation rate than predicted by the aforementioned evaporation theory. For the calculated residence time of 0.23 s, it was predicted that an initial 150-nm particle would completely volatilize already at about 90 C (360 K). For 110 C, the residence time needed for complete volatilization is less than 0.10 s. The reasons for the slower evaporation rates in the heater are unclear. In the theoretical prediction, the H 2 SO 4 vapor pressure buildup due to evaporation may not be negligible at distances away from the particle. Chemical impurities such as those seen in the small nonvolatile aerosol fraction, which we attributed to the distilled water used in the chemical solutions or to contamination that occurred when we prepared the solutions, might also contribute to the slower evaporation rate. The test results shown in Fig. 9 were used to determine the temperature at which to hold the aerosol heater. To keep losses due to thermophoresis at a minimum, a temperature was chosen high enough to ensure complete sulfuric acid volatilization without unnecessary losses due to thermophoresis. Since the largest 150-nm sulfuric acid particle evaporated at about 110 C, 120 C was chosen as the aerosol heater operation temperature. c. Volatility of (NH 4 ) 2 SO 4 and NH 4 HSO 4 aerosol particles In the atmosphere, the presence of ammonia gas will likely neutralize a sulfuric acid aerosol, converting it to the neutralized forms ammonium sulfate ((NH 4 ) 2 SO 4 ) and ammonium bisulfate (NH 4 HSO 4 ). To measure the aerosol sulfuric acid fraction, the experiment must then be able to differentiate between the volatilities of these products. The volatility behavior of these two species within the volatile temperature range of sulfuric acid was measured, and the results are shown in Fig. 10. The entire temperature range was scanned between 25 and 150 C for the selected diameters of 15, 35, 50, and 150 nm. Shown here are, respectively, the beginning and endpoint 25 and 150 C DMA 2 scans for the 50-nm selected size. No significant size shift was observed. d. Recondensation of evaporating species There is one remaining question. Will vapor from the evaporating sulfuric acid fraction recondense to the residual particles as the aerosol returns to room temper-

11 770 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 16 TABLE 3. Fitted-mode properties of the selected and coated distributions. Scanned Fitted mode properties Singlet Doublet Totals NaCl bypass Number fraction fitted ( 6%) Number fraction corrected NaCl core volume (in m 3 ) * 0.098* 0.097* H 2 SO 4 coated Coated volume (in m 3 ) Calculated H 2 SO 4 volume Growth factor Z */Z* * Predicted values based on calculated doublet fraction. p1 p ature? In this case, predictions of sulfuric acid volume fractions will be false. The following section describes the test aimed at answering that question. The test involved generation of an aerosol with a defined volume fraction of sulfuric acid. Directing this aerosol through the VTDMA would reveal whether the known H 2 SO 4 volume volatilized completely. Recondensation could also be checked by varying the aerosol concentration that would add more vapor to the surrounding gas. The setup used for aerosol generation is shown in Fig. 11. The system replaced the aerosol-selection step (step 1) of the TDMA and was designed to generate an NaCl aerosol coated with a known thickness of sulfuric acid. The setup was operated as follows. DMA 1 first selected the desired monodisperse aerosol sample from the atomized polydisperse NaCl aerosol. For this test, both the atomizer and DMA 1 were driven with 99.99% nitrogen-pure, particle-free air. The reason for this is explained herein. The aerosol sample exiting DMA 1 was then directed either through a bypass line or through an oven, depending on the position of a manually operated three-way valve. The oven held a quartz-glass tube within which a small ceramic boat was filled with liquid sulfuric acid (Merck, Pro-analyst 99.8%). The temperature of the oven was held at approximately 90 C in order to fill the quartz tube with sulfuric acid vapor. If the aerosol sample exiting DMA 1 was directed through the quartz tube, then condensation of H 2 SO 4 gas resulted in NaCl particles coated with a layer of sulfuric acid. The degree of H 2 SO 4 gas condensation onto the NaCl aerosol particles was a sensitive function of oven temperature and aerosol residence time. If the coated test aerosol was directed further on to the VTDMA bypass, then DMA 2 scanned the NaCl coated aerosol size. If the NaCl aerosol exiting the bypass of Fig. 11 was directed to DMA 2, then the uncoated NaCl sample could be scanned. This technique allowed for a sample consisting of an NaCl aerosol coated with a known thickness of sulfuric acid. Finally, for different particle sizes and degrees of coating, the test aerosol could be sent through the VTDMA aerosol heater to test for volatilization and possible recondensation. The reasons for using the nitrogen sheath air gas was to avoid possible ammonia or organic contamination from compressor air supplies and to produce an aerosol sample with less than 1% relative humidity. The low relative humidity sample ensured that the NaCl did not dissolve in the H 2 SO 4 to form Na 2 SO 4. [This is due to the reaction rate of NaCl H 2 SO 4 Na 2 HSO 4 HCl being a sensitive function of the water present and of temperature.] Figure 12 shows a 35-nm NaCl aerosol both before and after being sent through the coating process. The error shown is the related Poisson counting error. The aerosol growth was seen as the center mobility shift from the selected Zp* 1.0 to Zp* 0.78 (growth factor 1.08). A smaller peak of larger mobility par- FIG. 12. Volatility scans of 35-nm NaCl uncoated and coated size distributions before heating. FIG. 13. Inversion of the volatility scans of the H 2 SO 4 -coated NaCl aerosol in Fig. 12 before heating.

12 JUNE 1999 ORSINI ET AL. 771 FIG. 14. Volatility scans of 35-nm NaCl uncoated and coated size distributions after heating. FIG. 15. Inversion of the volatility scans of the H 2 SO 4 -coated NaCl aerosol in Fig. 14 after heating. ticles centered at Zp* 0.38 was also seen lying far to the left of the median polydisperse generated size. This is likely attributed to the doubly charged particle fraction arising from the aerosol neutralization process. For a given diameter, there exists a smaller fraction of particles acquiring two elementary charges. Therefore, when selecting a mobility size in a DMA, there exists a fraction of larger doubly charged particles with the same mobility as the singlets. According to the doublet charging distribution of Fuchs (Wiedensohler 1988), the percent of doublets for the 35-nm size is 0.23%. The doublet fraction in this experiment, however, is more significant because the DMA 1 size selection is much smaller than the mean polydisperse atomized size: that is, the doublet size fraction is larger than the singlet size fraction. In this case, 5.5% of the selected particles were calculated as doublets. The inversion method previously used was employed to parameterize the data to give the interpolated and fitted singlet and doublet predicted modes of the NaCl distributions. The inverted curves are shown in Fig. 13, along with the respective parameterization listed in Table 3. (The doublets originated from Zp* 0.5.) The result of the coated aerosol sent through the aerosol heater is shown in Fig. 14. Plotted are the original NaCl selected and the 120 C heated distributions. As seen in the diagram, the conditioned sample nearly returns to its original uncoated state, with an 18% broadening in the dispersion of the respective fitted modes. Applying once again the fit algorithm reveals the conditioned mobility distribution. The two selected and heated inverted distributions are shown in Fig. 15 for comparison. The volatilized volumes of the respective doublet and singlet modes are listed in Table 3. According to the parameterization, the distribution returned to a center mobility of Zp* Comparison of input and output fits yielded a volatilized volume of m 3, with 36% of the volatilized volume originating from the larger particles. Comparing heater input and output volumes shows that 97% of the condensed volume was removed, with any discrepancy most likely due to the broadened distribution after heating. The aforementioned test was further extended to a range of aerosol concentrations. More particles would result in a larger vapor contribution to the surrounding gas. No changes were observed, however, in the extent of evaporation. 5. Summary A volatility TDMA was designed with the intention to volatilize the sulfuric acid aerosol fraction in the fine diameter size range D p 150 nm. The aerosol heater was constructed with the intentions of optimizing (a) the aerosol penetration efficiency with (b) sufficient residence time for complete H 2 SO 4 volatilization. To maximize the penetration, laminar flow conditions were satisfied. With respect to residence times, theoretical predictions of H 2 SO 4 aerosol evaporation rates were calculated as a function of initial particle diameter and heater temperature. Completion of the VTDMA system was attained with a final calibration for diffusion and thermophoretic losses with an NaCl aerosol. Fitted curves to the calibration data provided the back corrections to be used in normal system operation. Finally, particle recondensation was tested with an H 2 SO 4 -coated NaCl aerosol. Results showed no recondensation for the 120 C conditioning temperature. REFERENCES Clarke, A. D., 1992: Atmospheric nuclei in the remote free-troposphere. J. Atmos. Chem., 14, Covert, D. S., V. N. Kapustin, P. K. Quinn, and T. S. Bates, 1992: New particle formation in the marine boundary layer. J. Geophys. Res, 97 (D18), CRC Press, 1991: Handbook of Chemistry and Physics. 72d ed. CRC Press, Inc. Easter, R. C., and L. K. Peters, 1994: Binary homogeneous nucleation:

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