Measured and Modeled Light Scattering Values for Dry and Hydrated Laboratory Aerosols*

Transcription

1 VOLUME 21 JOURL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY JULY 2004 Measured and Modeled Light Scattering Values for Dry and Hydrated Laboratory Aerosols* PIR KUS University of Illinois at Urbana Champaign, Urbana, Illinois CHRISTIAN M. CARRICO Colorado State University, Fort Collins, Colorado MARK J. ROOD University of Illinois at Urbana Champaign, Urbana, Illinois ALLEN WILLIAMS Illinois State Water Survey, Champaign, Illinois (Manuscript received 31 December 2003, in final form 5 February 2004) ABSTRACT Closure experiments were completed to compare measured and modeled aerosol optical properties and their dependence on controlled relative humidity (RH) and wavelength of light. NaCl, (NH 4 ) 2 SO 4, and NH 4 NO 3 aerosol particles with approximate geometric mass mean diameters of 0.2 m and geometric standard deviations of 1.7 were tested as part of this study. High evaporative losses (up to 40%) were observed for NH 4 NO 3 aerosol at this particle size range due to heating, and the results from these tests have been excluded from the closure analysis. Aerosol optical properties were measured with a RH-scanning nephelometry system (humidograph) and modeled with a Mie Lorentz light scattering model. Particle size distributions were measured with a scanning differential mobility analyzer. Closure between the measured and modeled values of the total light scattering coefficient ( sp ), backscatter ratio (b), and Ångström exponent (å) for dry (low RH) aerosols was achieved within 0.0% 5%, 4% 15%, and 3% 17%, respectively. The values of f (RH), hemispheric b, and å at 80% RH agreed within 2% 27%, 1% 27%, and 1% 28%, respectively. Correcting for nephelometer nonidealities, including a heating artifact, improved the agreement between the measured and predicted sp values at RH 80% from 35% to 13% for the TSI nephelometer at the maximum heating condition, and from 18% to 11% for the Radiance Research, Inc. (RR), nephelometer. Accurate quantification of the closure for these optical properties is important when establishing visibility standards, and assessing the progress toward meeting those standards, as well as reducing the uncertainties in estimating radiative forcing due to those aerosols. 1. Introduction Investigation of the effects of aerosols on atmospheric radiative transfer and hence visibility and climate relies on a range of techniques for measuring and modeling aerosol optical properties. The integrating nephelometer, an instrument that can measure total and back light scattering coefficients by aerosol particles ( sp and bsp, respectively), has been developed, commercialized, and * Illinois State Water Survey Manuscript Number J-482. Corresponding author address: Prof. Mark. J. Rood, Civil and Environmental Engineering, University of Illinois at Urbana Champaign, 205 N. Mathews St., Urbana, IL mrood@uiuc.edu used for visual air quality studies over many decades (Charlson et al. 1969; Heintzenberg and Charlson 1996). Nephelometers have been characterized during laboratory and ambient aerosol experiments at low relative humidity (RH) conditions [e.g., RH 40%; Bodhaine et al. (1991); Anderson et al. (1996); Anderson and Ogren (1998)]. The intent of this study is to investigate the performance of nephelometers in measuring an aerosol s optical properties as a function of RH. Accurate quantification of the optical properties of aerosols is important when establishing visibility standards and assessing the progress toward meeting those standards (Watson 2002), as well as reducing the uncertainties in estimating radiative forcing due to those aerosols (Quinn et al. 1996). Measurements by integrating nephelometers provide 2004 American Meteorological Society 981

2 982 JOURL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 21 extensive and intensive aerosol optical properties that are important in determining the impacts of aerosols on visibility and climate (Charlson et al. 1992). The optical extensive property investigated here is sp and the optical intensive parameters of interest are the hygroscopic growth factor [ f (RH), the ratio of sp at a given RH to sp for a low RH condition], the hemispheric backscatter fraction (b, the ratio of bsp to sp ), and the Ångström exponent (å; Ångström 1964). The Ångström exponent characterizes the dependence of sp on wavelength ( ) of scattered light and is defined as log[ sp ( 1 )/ sp ( 2 )]/log( 1 / 2 ). Relative humidity is one of the most important meteorological influences on aerosol particle size and light scattering as determined from modeling studies (Orr et al. 1958; Hegg et al. 1993; Nemesure et al. 1995), experimental laboratory investigations (Tang 1980, 1996, 1997), and experimental ambient aerosol studies (Covert et al. 1979; Charlson et al. 1984). As a result, investigations of aerosol optical properties using controlled RH nephelometry (humidography) have been pursued for at least three decades (Covert et al. 1972). Experiments have been implemented in many forms and with many RH control strategies ranging from passive systems with one dry RH measurement and one ambient RH measurement (Li-Jones et al. 1998; Gassó et al. 2000; Bergin et al. 2001), dilution/mixing systems for RH control (Covert et al. 1972; Rood et al. 1985), and using a combination of water vapor addition and/or thermal control (Carrico et al. 1998, 2003; Kotchenruther et al. 1999; ten Brink et al. 2000; Day et al. 2000). Also, these experiments have investigated diverse aerosol populations including background marine aerosols (Carrico et al. 1998), biomass smoke (Kotchenruther and Hobbs 1998), class 1 National Park sites (Malm and Day 2001), mineral dust (Li-Jones et al. 1998; Carrico et al. 2003), and polluted aerosols in urban (Rood et al. 1987) and rural continental regions (Koloutsou-Vakakis et al. 2001). Despite the numerous field applications of humidography, limited closure studies are available regarding these measurements, particularly for polydisperse and hydrated aerosols that typically exist in the lower troposphere. An important concern is sample losses and the extent to which the measurement itself can alter the aerosol s properties [e.g., through sample heating and volatility losses; Willeke and Baron (1990)]. For example, previous studies have shown that sample heating within a nephelometer with a continuous light source is of several degrees Celsius and as large as 4 7 C depending on experimental conditions (e.g., aerosol sample flow rate, inlet temperature, lamp power) (Anderson et al. 1996; Molenar 1997; Heintzenberg and Erfurt 2000). Studies by Dougle et al. (1998), Bergin et al. (1997), and ten Brink et al. (2000) examined the losses of NH 4 NO 3 in a nephelometer for a range of temperatures, residence times, and aerosol size distributions while heating the aerosol. Bergin et al. observed a 10% decrease in sp due to NH 4 NO 3 particle loss because of the aerosol particles volatility at 27 C and a 40% decrease at 47 C for the typical mean residence time of 4.8 s. The particles tested had a geometric number median diameter of 0.2 m and geometric standard deviation of 1.6. Closure studies compare agreement between different methods of measurements and modeling of aerosol properties (Quinn et al. 1996). Measured and modeled values of sp and bsp for monodisperse aerosol at low RH provided closure within 10% (Anderson et al. 1996). Studies of dry polydisperse ambient marine aerosol have demonstrated closure within 3% 19% for sp and from 8% to 40% for bsp (Quinn et al. 1996). This study evaluates integrating nephelometer measurements of sp, bsp, f (RH), b, and å for dry and hydrated polydisperse laboratory aerosols with increasing and decreasing RH conditions and examines the closure between these measured and modeled optical properties for dry and hydrated polydisperse laboratory aerosols (Fig. 1). The aerosol particles consist of single component dry or aqueous solutions that contain sodium chloride (NaCl), ammonium sulfate [(NH 4 ) 2 SO 4 ], or ammonium nitrate (NH 4 NO 3 ) solute, which are three common components of ambient aerosols. These results are also important in that they provide a quantitative analysis of the effects of heating within the nephelometer and recommendations for correcting for this heating, especially when studying aerosols at ambient conditions. 2. Experimental approach a. Measured physical and optical aerosol properties The experimental setup used to measure light scattering coefficients and particle size consisted of an aerosol generator, measurement of dry particle size distributions, and measurement of the temperature, RH, pressure, and optical properties of the aerosol (Fig. 2). Solutions were prepared by mixing reagent-grade solute with deionized, distilled, and filtered water (NOpure; Barnstead International, Dubuque, Iowa, model D4741). A polydisperse aerosol was then generated with an atomizer, dried with filtered dry air in a mixing chamber, and then passed through an impactor that allows particles with an aerodynamic diameter (D p,a ) 1 m to remain in the gas stream (Fig. 2). The aerosol flow was then split with 1 alpm (actual liters per minute) flowing to the aerosol particle sizing system and 22 alpm flowing through an RH-scanning nephelometry system (humidograph), as described below. The aerosol size distribution was measured with a scanning differential mobility analyzer (SDMA) that included an electrostatic classifier (TSI Inc., Shoreview, Minnesota, model 3936 differential mobility analyzer) interfaced to a condensation particle counter (CPC; TSI, model 3010-S). Appropriate operating conditions for the

3 JULY 2004 KUS ET AL. 983 FIG. 1. Schematic describing the method used to evaluate closure between the measured and modeled light scattering. SDMA were tested by comparing measured and vendor reported particle sizes for polystyrene latex (PSL) spheres (Duke Scientific Corporation, Palo Alto, California). These PSL particles were generated in the same manner as the inorganic salt particles. The aerosol flow through the humidograph first passed through a low-rh reference nephelometer (TSI, model 3563) that measured sp and bsp at dry conditions (RH 10%). Subsequently, RH was then controlled by adding water vapor with a humidifier using a water permeable membrane and by cooling with a custom thermoelectric cooler (Melcor Inc., Trenton, New Jersey). The humidifier consisted of a water-jacketed encasing of a 1.59 cm (5/8 in.) inner-diameter water vapor permeable membrane composed of Goretex that was custom manufactured by W. L. Gore and Associates, Inc., Newark, Delaware (Koloutsou-Vakakis et al. 2001). The aerosol sample stream then split with FIG. 2. Experimental apparatus used in this study.

4 984 JOURL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 21 approximately 11 alpm of aerosol flowing through two RH-scanning nephelometers in parallel. One nephelometer had a continuous light source (TSI, model 3563) and the other nephelometer had a flash lamp light source [Radiance Research Inc. (RR), Seattle, Washington, model M903]. The TSI nephelometers measured sp and bsp at 450, 550, and 700 nm and used a quartz halogen continuous light source, while the RR nephelometer measured sp at 530 nm with a quartz halogen flash lamp as its light source. A complete experiment consisted of increasing and then decreasing RH scans between 35% RH 85% in the RH-scanning nephelometers during a 45-min period. Increasing RH scans began from a sample condition with RH 35% in all of the nephelometers. The (NH 4 ) 2 SO 4 and NaCl aerosols are dry at this condition since negligible growth was observed until the deliquescent relative humidity (DRH) of these salts (Tang 1996). Decreasing RH scans, on the other hand, began from a hydrated aerosol condition at RH 85% (Carrico et al. 2003). Dry filtered air and CO 2 were used separately to calibrate the light scattering measurements of both nephelometers, and those measured scattering values agreed within 5% with published values (Anderson et al. 1996). All light scattering measurements were corrected to most accurately describe the true optical properties with corrections for known angular nonidealities, namely, truncation and non-lambertian error examined for the TSI nephelometer (Anderson et al. 1996; Anderson and Ogren 1998). The same angular nonidealities were assumed for the RR nephelometer. To facilitate the comparison of light scattering values for the two different nephelometers, the results from the RR nephelometer were adjusted to 550 nm using å values calculated for wavelengths of 450 and 550 nm from the TSI nephelometer. Corrections for nephelometer nonidealities were applied as a function of RH (Carrico et al. 2000). An adjustment of the optical properties to 25 C and 1013 mb was performed using the nephelometers sample temperature (T) and pressure values to compare the modeled results to the measured results. The pressure sensor in the TSI nephelometer is located just after the inlet of the instrument and before the location of the continuous light source. The sample temperature sensor is located just after the continuous light source. The RH of the sample volume for the RR nephelometer was calculated and based on the dry-bulb temperature as measured with a type K thermocouple located 2.5 cm inside of the nephelometer s outlet and the dewpoint temperature determined for the RH-scanning TSI nephelometer. The total pressure of the RR nephelometer s sample volume was assumed equal to the total pressure of the TSI nephelometer. Experimental f (RH) measurements were then fit to functions of the forms given below for the increasing branch of a deliquescent curve [Eq. (1)] and for the decreasing branch of a deliquescent curve [Eq. (2); Kotchenruther et al. (1999)] to facilitate comparisons of the experimental and modeled results: [ ] [ ] g RH 1 RH d [ ] b RH 1 RH d f (RH) 1 a 1 arctan 1 10 c 1 arctan 1 10 and (1) g RH f (RH) c 1. (2) 100 The RH was measured with four capacitive-type RH sensors [two Vaisala, Helsinki, Finland, model HMP- 233 RH sensors and two TSI sample RH sensors (collocated with the TSI outlet temperature sensor) located within the TSI nephelometers]. Dewpoint temperatures were measured with three chilled mirror type sensors (General Eastern Instruments, Wilmington, Massachusetts, models Hygro M1 and M4). Dry-bulb temperatures were measured with thermocouples (OMEGA Engineering, Inc., Stamford, Connecticut, type K), the Vaisala temperature sensors (Vaisala, model HMP-233), and with thermistors (defined as an outlet sample temperature sensor in the TSI-3563 manual) internal to the TSI nephelometers. The detailed locations of the TSI temperature, pressure, and RH sensors can be found in the online manual of the instrument ( com/particle/products/nephelometers/neph.htm). The capacitive-type temperature sensors were calibrated in an isothermal gas flow (T 22 C) at RH 25% and RH 85% using a third Vaisala RH sensor (Vaisala, model HMP-233) that was recently factory calibrated and used as a transfer standard. Agreement between the two dewpoint sensors and the capacitive-type detectors at 22 C was within 2%. The RH sensor in the RR nephelometer was not altered from its factory calibration and was not used in this study as its reading was 10% lower than the other sensors. Intercomparison of the calibrated temperature sensors was in agreement

5 JULY 2004 KUS ET AL. 985 TABLE 1. Manufacturer-specified uncertainties ( ) for temperature and RH measurements for sensors used in this experiment. Sensor T ( C) RH (%) Vaisala HMP-233 Omega type K thermocouple General Eastern dewpoint hygrometer M1 TSI nephelometer RR nephelometer b 0.5 d a 2 c 5 2 a For RH 90%. Given as 1% when calibrated against a high quality certified humidity standard. b Uncertainty in measuring dewpoint temperature. c RH uncertainty at RH 80% found from dewpoint uncertainty and thermocouple dry-bulb temperature uncertainty. d The uncertainty in the temperature measurements in the TSI nephelometer is specified as 0.3 C in Heintzenberg and Erfurt (2000), 0.5 C in Anderson et al. (1996), and as 2 C from the manufacturer (M. Havlicek, TSI Inc. 2003, personal communication). We adopt the middle value here of 0.5 C. to within 0.2 C. The manufacturers specified uncertainties ( ) in these RH and temperature measurements are given in Table 1. The RH was measured and derived with two different instruments [Vaisala RH sensor (RH2, Fig. 2) and two collocated dewpoint hygrometers (DP2, Fig. 2)] at the inlet position of the RH-scanning TSI nephelometer. The RH can then be described most accurately by taking the average of all of the RH measurements representing the same physical position during periods of constant low and high RH conditions. Therefore the RH from the Vaisala RH sensor was calibrated based on the regression line between Vaisala RH versus this averaged RH. The RH of the sample volume for the RH-scanning TSI nephelometer was determined by its internal sample RH sensor (RH3, Fig. 2). The sample RH for the RH-scanning RR nephelometer, however, was determined from the calculated dewpoint temperature [using the inlet Vaisala dry-bulb temperature and RH sensors (T2 and RH2, respectively, Fig. 2)] and the dry-bulb temperature for its sample volume (T4, Fig. 2). b. Description of experiments Experiments were performed under conditions to produce light scattering values that are comparable to those at ambient conditions, and with select TSI nephelometer lamp power settings and ventilation conditions to investigate the effects of instrumental heating on measured optical properties. Tests included initial conditions with a cold start, where the nephelometers were off for 1 h, and initial conditions with a steady state condition, where instruments were powered on for 1 h. Experiments with maximum instrumental heating occurred at full lamp power (75 W) with no additional ventilation other than that provided by the instrument and removal of the TSI nephelometer s enclosures. The maximum heating case is nearly a worst case scenario with about a 4.5 C warming for the TSI nephelometer, though greater heating may occur if the instrument has its cover in place and is operated at a lower sample flow rate. Intermediate instrumental-heating experiments occurred similarly with lowered lamp power (50 W) provided to the RH-scanning TSI nephelometer. The minimum instrumental heating occurred with further reduced lamp power (25 W) and with additional ventilation using external forced and induced draft fans ventilating the nephelometers. The fans were located external to but surrounding the light scattering chamber of the nephelometer. c. Modeled aerosol optical parameters and evaluation of closure A computer program based on the Mie Lorentz light scattering (BHMIE) code of Bohren and Huffman (1983) was used to calculate aerosol optical properties for the conditions of the experiment. Particles were assumed to be homogeneous spheres of uniform density for dry and hydrated aerosol particles. Aerosol chemical properties including the refractive index and density as a function of RH for the Mie light scattering model were taken from Tang (1996) and Tang and Munkelwitz (1994). Diameter growth factors (D p,wet /D p,dry ) for each particle size bin across each size distribution were determined from the mean diameter for each channel of the SDMA. Scattering coefficients as a function of RH were calculated by assuming that all of the particles in each size bin grow equally and aerosol particle number concentration is conserved (Fig. 1). 3. Results and discussion a. Aerosol particle size distributions Measured geometric mean particle diameters (D p,g ) of the PSL spheres were 54 and 304 nm, and were measured with the SDMA to within 1% of the diameters reported by the vendor of the spheres. The measured dry particle size distributions from the laboratory-generated inorganic aerosols had approximately lognormal distributions with D p,g and g based on masses of approximately 0.2 m and 1.7, respectively (Fig. 3). Measurement of one dry particle size distribution took 1 2 min while an increasing or decreasing RH scan took 15 min. Therefore, generation of a stable test aerosol was essential. The stability of the output from the aerosol generator was checked by monitoring the change in dry sp during a complete RH scan. The changes in sp were within 1% during an RH scan, indicative of stable aerosol generation. b. Effects of sample heating on measurements of aerosol optical properties Dry-bulb temperature values at startup for all of the tests were within 0.2 C of one another. As previously mentioned, the locations of these temperature sensors

6 986 JOURL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 21 FIG. 3. Typical dry particle size distributions for (NH 4 ) 2 SO 4, NH 4 NO 3, and NaCl. Geometric mass means (D p,g ) and geometric standard deviations ( g ) are provided above. are described schematically in Fig. 2. Steady-state temperature profiles were then recorded for an aerosol passing through the humidograph at three lamp power settings as described in Fig. 4. The maximum rise in the sample temperature of 4.5 C is for the 75-W lamp power setting and occurs between the inlet of the RH-scanning TSI nephelometer (T2, Fig. 2) and its sample temperature within the nephelometer (T3, Fig. 2). This increase in temperature was achieved within 35 min of a cold start. A more modest heating of 2.0 C was observed in these nephelometers when the lamp power is 25 W. The RR nephelometer also shows a temperature increase of 2 C from a cold start though the exact profile is uncertain due to the precision of the temperature measurement ( 0.5 C as it reports only integer values). Heating in the RR nephelometer is due to a combination of factors including its light source and possibly heating due to heat transfer from the surroundings as a result of the close proximity of having all three instruments located in the same sampling container. The agreement between three nephelometers for low RH conditions (RH 35% for all three nephelometers) is observed to be within 5% for the nonvolatile NaC1 and (NH 4 ) 2 SO 4 for the minimum, intermediate, and maximum heatings. The minimum, intermediate, and maximum heating cases represent temperature rises of 2, 3, and 4 C and measured sample temperatures of 23,25, and 29 C, respectively. However, substantial decreases in sp values for volatile NH 4 NO 3 aerosols were observed when comparing measured sp by the RH-scanning and low RH reference TSI nephelometers. A 10% 15% reduction in sp values for the minimum heating case and a 35% 45% reduction in sp values for the maximum heating case were observed with NH 4 NO 3 aerosols. The larger reduction in sp for NH 4 NO 3 when compared to NaCl and (NH 4 ) 2 SO 4 are attributed to the evaporation of the NH 4 NO 3 particles. Tests with NH 4 NO 3 therefore have been excluded from the closure analysis. Although these losses were likely FIG. 4. Temperature profile in the humidograph system for minimum, moderate, and maximum heating cases during the steady-state tests. Locations of temperature sensors are described in Fig. 2. exacerbated due to the small sizes of the generated aerosol, the need to minimize the heating with regard to the volatilization losses of such semivolatile components of the aerosol is noted. Ratios of dry sp values from the low RH reference nephelometer to the RH-scanning nephelometers range from 0.90 to 1.04 for NaCl and (NH 4 ) 2 SO 4 aerosol. Ratios of these sp values were used in the following analyses to normalize f (RH) to account for small calibration and particle loss differences between the reference and scanning RH nephelometer so that it is unity at the low RH condition. Results for bsp values were also normalized in the same manner as their corresponding bsp values, except for the NaCl aerosol tests. The NaCl tests had low signal-to-noise ratios for the bsp values due to the low backscattering values caused by the low NaCl mass concentrations used during the tests. The difference between bsp values for the low RH reference and RH-scanning nephelometers was used as an offset to allow f (RH) for bsp values to begin at one. All the correction factors [i.e., for standard temperature and pressure (STP), angular nonidealities, normalization, and wavelength adjustment for the RR nephelometer] applied to the measured scattering coefficients at 550 nm were tabulated in Table 2. c. Closure between modeled and measured total scattering coefficients Measured and corresponding predicted sp values at 550 nm are compared in Fig. 5 for dry and hydrated NaCl aerosols during the minimum and maximum heating cases. The minimum heating cases described in Fig. 5a report sp values, obtained directly from the nephelometers without any corrections, versus RH measured immediately upstream of the nephelometers. Figure 5b

7 JULY 2004 KUS ET AL. 987 TABLE 2. Correction factors that are used to correct the total scattering coefficients at 550 nm for the TSI and RR nephelometers for selected tests. Corrections for angular nonidealities for the RR nephelometer are assumed equal to the same factors for the TSI nephelometer. Test conditions (NH 4 ) 2 SO 4 TSI- 25W-CS (NH 4 ) 2 SO 4 TSI 74W-SS NaCl TSI 25W-CS NaCl TSI 75W-SS Correction factors for TSI nephelometer Angular nonidealities STP Normalization* Low RH High RH Correction factors for RR nephelometer Wavelength adjustment STP Low RH High RH Normalization TSI, RH-scanning continuous light source nephelometer; RR, RH-scanning flash lamp light source nephelometer; CS, cold start tests; SS, steady-state tests; 25 and 75 W are the power settings for the TSI nephelometer; and STP, standard temperature and pressure. * Normalization is the multiplication of the sp values by the ratio of sp from the reference nephelometer to sp from the RH-scanning nephelometer during the low RH sample period. reports corrected sp values (i.e., nephelometer nonidealities, standard temperature and pressure, and the wavelength correction for RR nephelometer measurements) versus internal sample RH (i.e., accounting for instrumental heating). Figures 5c and 5d are the same as Figs. 5a and 5b, respectively, except they apply to the maximum heating condition. Predicted and measured scattering coefficients reveal very good agreement (i.e., within 2%) for initially dry polydisperse NaCl particles at RH DRH for both the TSI and RR nephelometers. However, discrepancies between measured and modeled sp values are apparent for hydrated aero- FIG. 5. Total light scattering coefficients at 550 nm as a function of RH for NaCl before and after corrections: (a) before any corrections to sp vs RH inlet (Vaisala measured RH just before the nephelometer) for minimum heating, (b) corrected sp vs actual sample RH (RH as experienced by the aerosols in the scattering volume of the instrument) for minimum heating, and (c), and (d) as is (a), (b) except for maximum heating cases, respectively.

8 988 JOURL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 21 FIG. 6. For minimum heating, controlled RH measurements of hygroscopic growth in light scattering factor [ f (RH) sp (RH)/ sp (dry)] at 550 nm for (a) NaCl and (b) (NH 4 ) 2 SO 4. sols when the sample heating is not considered and the inlet RH (or ambient RH for field testing conditions) is used to represent the RH within the sample volume (Figs. 5a and 5c). Discrepancies become even more apparent for the hydrated aerosol at the maximum heating case. For example, the modeled sp value at a sample RH of 80% is 130 Mm 1 (inverse megameters), while sp values measured at an inlet RH of 80% by the RR and TSI nephelometers are 107 and 86 Mm 1, respectively. The discrepancies resulting in f (RH 80%) 9.5 (predicted), 7.8 (measured by RR nephelometer), and 6.2 (measured by TSI nephelometer), causing the differences between modeled and experimental results to range from 18% to 35%. Implementation of corrections due to nephelometer nonidealities (Anderson and Ogren 1998) and heating within the nephelometers reduced the differences between the modeled and experimental results to 10% over the full range of increasing and decreasing RH conditions tested here. Light scattering values obtained during increasing RH conditions as described in Figs. 5b and 5d (open dark and gray triangles for the TSI and RR nephelometers, respectively) also display the effect of heating at RH values just below the DRH of the aerosol. Aerosols can deliquesce immediately upstream of the increasing RH nephelometer as the RH of the nephelometer s sample volume approaches the DRH of the aerosol. Such a response occurs as a result of the reduction in RH due to heating within the nephelometer. Since sample heating lowers the RH in the nephelometer s sample volume, particles that deliquesce immediately upstream of the sample volume move down the metastable branch of the aerosol s hysteresis loop to an extent determined by the magnitude of the heating in the nephelometer. Therefore, these increasing RH points fall on the upper branch of the hysteresis loop as seen in the Figs. 5b and 5d. Heating within the nephelometers also causes an artifact when measuring DRH values. The aerosol DRH value is identified as the large step change in sp values for a small change in RH during increasing RH conditions. This rapid change in sp occurs at a sample volume RH well below the previously reported DRH value of 75% for NaCl. For example, the measured DRH is 62% for NaCl during the maximum heating case when using the RH-scanning TSI nephelometer and its sample RH. Since sample heating lowers the RH in the sample volume, particles that deliquesce immediately upstream of the sample volume move down the metastable branch of the aerosol s hysteresis loop to an extent determined by the magnitude of the heating in the nephelometer. Thus, the step change in sp caused by deliquescence as reported by the nephelometer represents the transition from measuring sp for the complete population of dry particles to measuring sp of metastable droplets on the upper branch of their hysteresis loop at a sample RH the aerosol s actual DRH. Intermediate sp values that occur between the beginning and end of the step change (i.e., reported DRH value) result from the existence of a mixed population of dry particles and hydrated metastable droplets. Therefore, the lower branch of the hysteresis loops in Figs. 5b and 5d are plotted versus the inlet RH until the end of the step change in the light scattering values. The sample RH is then used for the light scattering values above this step change in addition to light scattering values along the upper branch of the hysteresis loop. The RH corresponding to the top of this step change in the light scattering values during increasing RH conditions is 75% (Fig. 6) for the NaCl particles. In practice as measured with the humidograph, the deliquescence step change occurs over an RH range of 3% 4% and is a result of the RH gradients within the instrumentation (e.g., particles near the walls of the humidifier may experience higher RH conditions than along the centerline of the humidifier). Measured DRHs and crystallization RHs (CRHs) for the two salts tested here are compared to previously reported values in Table 3. The DRH and CRH values for (NH 4 ) 2 SO 4 and NaCl are in

9 JULY 2004 KUS ET AL. 989 TABLE 3. Experimental deliquescence and crystallization relative humidities for the (NH 4 ) 2 SO 4 and NaCl. Inorganic salt (NH 4 ) 2 SO 4 NaCl Measured by this study DRH CRH N Previously reported DRH CRH Reported by Tang (1996) ten Brink et al. (2000) Tang (1996) ten Brink et al. (2000) not available. N number of tests. agreement with previously reported values. These results underscore the need to characterize both the inlet RH (for the correct measurement of DRH) and the sample RH internal to the nephelometer (for the correct correspondence between the measured scattering values and the RH of the nephelometer s sample volume) when heating occurs within the nephelometer. Another reason to limit the sample heating is to minimize the volatility of the aerosol. The magnitude of the sample heating also limits the maximum sample RH for which measurements are possible without causing condensation upstream of the instrument, which is apparent in Fig. 5d where the maximum sample RH was 74% when the inlet RH was 91%. The results for measured and modeled sp values for (NH 4 ) 2 SO 4, which were obtained before the tests with NaCl, show poorer agreement when compared to the NaCl tests. Measured results for absolute sp values for (NH 4 ) 2 SO 4 were consistently 40% 3% high compared to the modeled values due to the use of an aerosol dilution system. Therefore, the absolute sp values for those tests are excluded from this analysis. This is a result of undercounting of particles by the CPC due to a problem with the aerosol dilution system that was located upstream of the CPC, resulting in an inaccurately measured dilution factor for the (NH 4 ) 2 SO 4 aerosol. This is seen in a constant percentage difference between the measured and modeled sp for low and high RH and for all tests with (NH 4 ) 2 SO 4, which is not subject to the volatility losses that were previously discussed for NH 4 NO 3. Improved agreement between the measured and predicted sp values was observed with the subsequent tests using NaCl after rearrangement of the dilution system s plumbing (Table 4). Also, as will be discussed below, measured and modeled intensive properties showed similar agreement for both salts. d. Measured and modeled intensive optical properties The final values for sp, as described above, were then used to calculate f (RH), b, and å as a function of RH for NaCl and (NH 4 ) 2 SO 4 as illustrated in Figs. 6 and 7 for steady-state conditions with minimum heating. The measured and modeled f (RH) values demonstrate agreement within 7% 12% over the entire hysteresis loop for NaCl and (NH 4 ) 2 SO 4. Also of note are the much greater f (RH) values for NaCl when compared to the corresponding values for (NH 4 ) 2 SO 4, even though both salts have similar dry size distributions. These results are consistent with previous results comparing the hygroscopic properties of NaCl and (NH 4 ) 2 SO 4 (Tang 1996; Tang and Munkelwitz 1994). The largest dis- TABLE 4. Comparison of measured and modeled optical parameters for the tests discussed here. Percent difference of measured value from modeled value for TSI Percent difference for RR Test number Test conditions Low RH sp b å RH 80% f(rh) b å Low RH RH 80% sp f(rh) A1 A2 A3 A4 A5 C1 C2 C3 C4 C5 C6 (NH 4 ) 2 SO 4 TSI 25W CS (NH 4 ) 2 SO 4 TSI 25W SS (NH 4 ) 2 SO 4 TSI 50W SS* (NH 4 ) 2 SO 4 TSI 75W SS* (NH 4 ) 2 SO 4 TSI 75W SS* NaCl TS1 10 W CS NaCl TS1 25 W CS NaCl TS1 25 W SS NaCl TS1 50 W SS* NaCl TS1 75 W CS* NaCl TS1 75 W SS* TS1, RH-scanning continuous light source nephelometer; RR, RH-scanning flash lamp light source nephelometer; CS, cold start tests; SS, steady-state tests; and 25W, 50W, and 75W are the power settings for the TSI nephelometer. * In these tests, 80% RH along the metastable branch was not achieved due to high heating. The value from the extrapolated fit at 80% was used instead.

10 990 JOURL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 21 FIG. 7. For minimum heating, controlled RH measurements of hemispheric backscatter ratio (b) at 550 nm and Ångström exponent (å) for a nm wavelength pair for (a) NaCl and (b) (NH 4 ) 2 SO 4. Vertical error bars represent the standard deviation of the averaged parameter. crepancies with the theory for f (RH) values exist at high RH values (RH 80%). This may be due to a number of factors ranging from increased particle loss for larger more hygroscopic particles (i.e., NaCl), increased particle loss due to aerosol flowing through the smaller RR nephelometer, and larger uncertainties in measurement of RH at high RH conditions. Fitted equations used for increasing and decreasing RH portions of the deliquescent type [i.e., NaCl and (NH 4 ) 2 SO 4 ] aerosols agreed well with the experimental data. The goodness of the fit was evaluated based on the root-mean square of the standard error (rmse), which is the square root of the sum of the square of the differences between the measured and fitted points divided by the degrees of freedom. A value closer to 0 means a better fit. The rmse values ranged from 0.05 to 0.76, having higher error in runs with lower lamp power where the noise is higher. Another parameter that was considered for the quality of the fit is the confidence width on the calculated f (RH 90%) value. The maximum achievable RH in the nephelometer is reduced in the TSI nephelometer when operating with a high lamp power setting and necessitates the use of the fitted equation to obtain f (RH) values beyond the measured RH values. The confidence width on the f (RH) value at 90% (i.e., RH that is out of the measured RH range) was calculated at the 95% confidence level. The predicted value is actually within the lower (extrapolated value confidence width) and upper (extrapolated value confidence width) limits of the confidence interval. For example, an f (RH) value of 2 with a confidence width of 0.3 indicates that the f (RH) value can be between 1.7 and 2.3 with a 95% confidence level. A smaller confidence width suggests higher confidence in the extrapolated portion of the fit. Confidence intervals for f (RH 90%) ranged from 0.2 for runs with lower lamp power to 3.1 for the runs with higher lamp power and the most hygroscopic reagent NaCl. It should be noted that the extrapolation of the f (RH) values to RH of 90% was done to assess the quality of the fit. The confidence interval at RH 90% represents simply the statistical uncertainty for the fit, not the ability of the nephelometer to make measurements at RH 90%. Optical parameters b and å demonstrate similarly good agreement between measured and modeled values for the entire hysteresis loop for NaCl and (NH 4 ) 2 SO 4 for the select minimum heating runs (Fig. 7). The agreement between the measured and predicted b and å values ranged from 4% to 15% at RH 35% and 2% to 11% at RH 80% (Fig. 7). At low RH conditions, the agreement for b is better when the lamp power is 75 W compared to the reduced lamp power since the signalto-noise ratios for bsp values with reduced lamp power were affected significantly with lamp power settings between 25 and 75 W. As RH increases and particles grow, the hemispheric backscatter fraction decreases as larger particles preferentially scatter light in the forward direction and the wavelength dependence of sp decreases (Bohren and Huffman 1983). Both parameters are observed to have a nearly linear dependence on the controlled RH conditions when existing as an hydrated aerosol along the upper branch of the hysteresis loop as observed for ambient aerosol as well (Carrico et al. 2000, 2003). e. Sensitivity analysis of measured and derived optical aerosol properties to temperature and RH The use of a sample temperature or RH internal to the nephelometer is dependent on how representative the measurement is of the sample volume as well as the accuracy of the measurement. For example, the sample temperature measurement in the RR nephelometer is recorded as an integer resulting in an uncertainty of 0.5 C while the uncertainty of the RH measurement internal to the TSI nephelometer is reported by the manufacturer as 5% (Table 1). The effects on the optical properties of the hydrated aerosol from an error in the measured sample temperature (0.2, 0.5, and 1.0 C) and the resulting error in RH (1%, 2%, and 5% RH) are tabulated in Table 5 assuming a base case sample tem-

11 JULY 2004 KUS ET AL. 991 TABLE 5. Resulting uncertainties in aerosol optical parameters at RH 80% for given temperature/relative humidity uncertainties using modeled optical properties for salts in this study. Temperature uncertainty RH uncertainty (at RH 80%) T 0.2 C RH 1% T 0.5 C RH 2% T 1.0 C RH 5% f(rh 80%) b å NaCl (NH 4 ) 2 SO 4 NaCl (NH 4 ) 2 SO 4 NaCl (NH 4 ) 2 SO 4 5%; 3% 3% 1% 1% 1% 1% 13%; 9% 10%; 8% 2% 3% 2% 2% 26%; 15% 19%; 15% 5% 8%; 7% 4%; 5% 3%; 4% perature of 25 C and RH 80%. Resulting relative uncertainties for NaCl aerosol and T 1.0 C ( RH 5%) are 26% and 15% for f (RH 80%), 5% and 5% for b, and 4% and 5% for å.similar results in terms of the percentage uncertainties in the optical parameters are observed for (NH 4 ) 2 SO 4 (Table 5). The effects on b and å are less than 10% for a temperature uncertainty of 1.0 C. Another effect on f (RH) is the 2% uncertainty in measuring RH, which results in an error of 13% to 9% for f (RH 80%). In addition to temperature and RH uncertainties, heating in the sample volume of the instrument results in artifacts such as volatile particle loss and inaccurate estimation of DRH when attempting to measure aerosols at ambient conditions. Furthermore, use of a high-accuracy RH measurement device external to the nephelometer is dependent on simultaneously measuring temperature at both locations where RH and light scattering measurements are made in order to correct the RH measurements as discussed above. If the sample RH was not characterized accurately, especially for ambient samples at high RH (e.g., 85%), the resulting measured and derived parameters will diverge significantly from the actual values. Values of f (RH), b, and å as a function of inlet RH (RH inlet ) and temperature differentials ( Ts) between the inlet and sample temperature of 2.0, 3.0, and 4.0 C are shown in Figs. 8a d. These correspond to RH differences of approximately 11%, 16%, and 21% RH for a base case of T 25 C and RH 85%. These calculations used the modeled optical parameters for (NH 4 ) 2 SO 4, assuming a dry size distribution with a geometric mean volume diameter of 0.3 m and geometric standard deviation of 1.7, which is representative of the ambient aerosol. Heating of the aerosol by 4 C by the instrument s light source leads to a 40% underestimation in f (RH 85%) and an increase in b and å by 18% and 14% at the same RH, respectively. Due to the exponential nature of the growth curve, the difference in the optical properties as a function of RH undoubtedly increases for RH 85%. Heating of the ambient aerosol sample, without adequate corrections, will result in underestimation of f (RH) but an overestimation of b, which are important parameters in quantifying the radiative effects of ambient aerosols. f. Discussion and recommendations It should be emphasized that the results of this experiment apply strictly to submicron single chemical component aerosols that have no imaginary component of the refractive index in the visible spectrum. These results demonstrate the importance of isothermal sampling when measuring the optical properties of hydrated aerosols. For example, reducing lamp power for the TSI nephelometer and increasing ventilation decreased the sample heating from 4.5 to 1.3 C, which reduced the error in measuring f (RH 80%) from 21% to 7%. Alternately, the use of a lower-wattage light source that maintains the same spectral characteristics may assist in reducing the instrumental noise, particularly in the shorter wavelengths, which are most affected by lowering the lamp s power. More extensive modifications of the TSI nephelometer have been described to further reduce the sample heating to 0.5 C without lowering the lamp power and thus producing a negligible reduction in the instrument s sensitivity while completing measurements for haze and fog studies (Heintzenberg and Erfurt 2000). Though the internal instrumental heating of the RR nephelometer is likely small, a more extensive characterization of its nonideal properties (e.g., truncation error, non-lambertian light source, spectral output of light source, particle losses) is clearly needed to better analyze the uncertainties when measuring the optical properties of aerosol. This closure study is constrained by the accuracy of the sample RH and temperature sensors and how well the sensors characterize the nephelometer s sample volume. Use of higher accuracy temperature and RH measurement devices internal to the sample volume of the nephelometer will assist in reducing the uncertainties when measuring the optical properties of aerosol particles. 4. Summary and conclusions In situ optical properties of dry and hydrated NaCl and (NH 4 ) 2 SO 4 aerosols were characterized in the laboratory by comparing measured and modeled (Mie Lorentz theory) total and backscattering coefficients ( sp and bsp, respectively) while using two different commercially available nephelometers. Closure was then

12 992 JOURL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 21 FIG. 8. Influence of increasing dry-bulb temperature within the nephelometer s scattering volume compared to ambient conditions (RH inlet ) when considering the sample s (a) RH, (b) hygroscopic growth factor [ f (RH)], (c) hemispheric backscatter fraction (b), and (d) Ångström exponent (å) for the (NH 4 ) 2 SO 4 aerosol. The aerosol has a dry size distribution with a geometric volume mean diameter 0.3 m and a geometric standard deviation 1.7. evaluated for sp, the hygroscopic growth response [ f (RH)], hemispheric backscatter fraction (b), and the Ångström exponent (å). Measured sp values obtained by the TSI and RR nephelometers were in agreement to within 5% for low RH (RH 35%) conditions with NaCl and (NH 4 ) 2 SO 4 aerosols. Closure for measured and modeled sp values at the low RH condition for NaCl was achieved within 2% and 5% for the TSI and RR nephelometers, respectively. The scattering coefficients at higher RH values showed larger discrepancies from the modeled values when the inlet RH (or ambient RH in the case of the ambient measurements) was used as the sample RH within the nephelometer. Correcting for the nephelometer nonidealities and the heating improved the agreement between the measured and predicted sp values at RH 80% from 35% to 13% for the TSI nephelometer at the maximum heating condition, and from 18% to 11% for the RR nephelometer. Each correction step has its own uncertainty, but comprehensive uncertainty analyses for the measured and predicted optical properties were not carried out in this study. Obtaining closure for the absolute value of sp when using (NH 4 ) 2 SO 4 aerosols was hindered due to uncertainties in the total particle concentration resulting from a dilution system upstream of the particle sizing system. However, comparable agreement among all tests between measured and modeled results was observed with the intensive properties f (RH 80%), b, and å. The hygroscopic growth properties of the aerosol were fitted to three equations according to the growth type (monotonic or deliquescent) and the direction of the RH scan to be able to evaluate closure. The equations described the hygroscopic properties very well with a low root-meansquare error, of 0.2 on average. The capability of the fit equation to predict f (RH) values outside the measured RH range was assessed by a 95% confidence width on the predicted f (RH 90%). The confidence intervals were within 10% of the f (RH 90%) value for the minimum heating cases. At RH 80%, the percent differences between the measured and modeled f (RH 80%), b, and å values for all three salts after correction for nephelometer nonidealities and heating ranged from 2% to 27%, 1% to 27%, and 1% to 28%, respectively. Deliquescent and crystallization relative humidity

13 JULY 2004 KUS ET AL. 993 (DRH and CRH, respectively) values were also measured and compared to previously reported values for NaCl and (NH 4 ) 2 SO 4 aerosols. The DRH and CRH values for (NH 4 ) 2 SO 4 and NaCl are in agreement with previously reported values. Determining DRH values accurately for ambient or test aerosols when heating occurs will require RH measurements before the sample enters the nephelometers and within the sample volume of the nephelometer since particles can form hydrated solution droplets that do not crystallize as the aerosol approaches its DRH value upstream of the nephelometer. The experiment demonstrated the substantial influences of instrumental heating both with regard to loss of volatile species as well as ambiguities in the sample RH, thus underscoring the desirability of eliminating sample heating. Heating in the sampling volume of the nephelometer also reduces the achievable maximum RH (i.e., inside the nephelometer). Heating of 2.0 and 4.0 C would cause 85% RH of the ambient sample to decrease to 74% and 64%, respectively. These results underscore the limitations of using the TSI nephelometer for high RH or near ambient conditions without taking additional measures to limit its heating. The sensitivity analysis with respect to the magnitude of the heating that a sample has experienced was performed. It revealed that heating of the aerosol by 4 C by the instrument s light source leads to a 40% underestimation in f (RH 85%). Care must be taken when interpreting controlled RH measurements, as heating causes an artifact in measuring the optical properties of aerosol particles. Ideally, measurements of inlet RH and sample RH should be made when considering controlled RH measurements from a nonisothermal nephelometer. The closure experiment revealed the importance of keeping the nephelometry system isothermal for measuring particle scattering coefficients as a function of RH or as particles exist in the ambient environment. In the case of a nonisothermal measurement, the RH should be adequately measured in the nephelometer s sample volume and correction to the ambient measurement should be performed if at all possible. In addition, accurate changes in an aerosol s light scattering properties can be obtained during decreasing and increasing RH conditions. The f (RH) values can differ significantly depending on the composition and RH history of the ambient particles, and the aerosol s type of hygroscopic growth. Acknowledgments. The authors acknowledge Prof. Susan Larson for use of aerosol sizing instrumentation and Drs. Tad Anderson, David Covert, John Ogren, Mike Bergin, Jost Heintzenberg, and Doug Orsini for helpful discussions related to these experiments. 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