Design and Application of a Pulsed Cavity Ring- Down Aerosol Extinction Spectrometer for Field Measurements

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1 Aerosol Science and Technology ISSN: (Print) (Online) Journal homepage: Design and Application of a Pulsed Cavity Ring- Down Aerosol Extinction Spectrometer for Field Measurements Tahllee Baynard, Edward R. Lovejoy, Anders Pettersson, Steven S. Brown, Daniel Lack, Hans Osthoff, Paola Massoli, Steve Ciciora, William P. Dube & A. R. Ravishankara To cite this article: Tahllee Baynard, Edward R. Lovejoy, Anders Pettersson, Steven S. Brown, Daniel Lack, Hans Osthoff, Paola Massoli, Steve Ciciora, William P. Dube & A. R. Ravishankara (2007) Design and Application of a Pulsed Cavity Ring-Down Aerosol Extinction Spectrometer for Field Measurements, Aerosol Science and Technology, 41:4, , DOI: / To link to this article: Published online: 06 Mar Submit your article to this journal Article views: 886 Citing articles: 67 View citing articles Full Terms & Conditions of access and use can be found at

2 Aerosol Science and Technology, 41: , 2007 Copyright c American Association for Aerosol Research ISSN: print / online DOI: / Design and Application of a Pulsed Cavity Ring-Down Aerosol Extinction Spectrometer for Field Measurements Tahllee Baynard, 1,2 Edward R. Lovejoy, 1 Anders Pettersson, 1,2 Steven S. Brown, 1 Daniel Lack, 1,2 Hans Osthoff, 1,2 Paola Massoli, 1,2 Steve Ciciora, 1 William P. Dube, 1,2 and A. R. Ravishankara 1 1 NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, CO 2 Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO This paper describes the design and application of a pulsed cavity ring-down aerosol extinction spectrometer (CRD-AES) for insitu atmospheric measurement of the aerosol extinction coefficient and its relative humidity dependence. This CRD-AES measures the aerosol extinction coefficient (σ ep ) at 355 nm, 532 nm, 683 nm, and 1064 nm with a minimal size dependent bias for particles with diameter less than 10 μm. The σ ep at 532 nm is measured with an accuracy of 1% when extinction is 10 Mm 1. The precision is limited by statistical fluctuations within the small optical volume and the time resolution of extinction at 2% uncertainty for various air mass types is evaluated. The CRD-AES is configured with two separate cavity ring-down cells for measurement of the extinction coefficient at 532 nm. This allows the determination of the RH dependence of extinction at 532 nm through independent RH control of the sample for each measurement. Gas phase absorption and minimization of potential interferences is also considered. 1. INTRODUCTION Atmospheric particles directly influence the earth s radiative balance by scattering and absorbing radiation (Chylek and Coakley, 1974). The sum of the aerosol scattering coefficient, σ sp, and absorption coefficient, σ ap, is the aerosol extinction coefficient, σ ep = σ sp + σ ap. The partitioning of extinction between scattering and absorption is given by the single scattering albedo, ω, which is defined as the dimensionless ratio of the scattering coefficient to the extinction coefficient, ω = σ sp /σ ep. The current uncertainty in radiative forcing by aerosol is almost equal to the magnitude of the aerosol forcing and is limited by the ability to model the atmospheric aerosol loading and optical properties under ambient conditions (Penner et al. 2001). Received 9 September 2006; accepted 16 January This work was funded by NOAA s Climate Program. The authors thank Richard McLaughlin and Mark Paris for design and machining support. Address correspondence to Tahllee Baynard, NOAA, 325 S. Broadway, Boulder, CO 80305, USA. Tahllee.Baynard@noaa.gov The uncertainty in aerosol radiative forcing needs to be reduced by a factor of 3 to 10 to support regulatory decisions related to global climate change (Schwartz, 2004). The large spatial and temporal variability of aerosol contribute significantly to the uncertainty in the aerosol radiative forcing. In-situ measurements are essential for relating aerosol optical properties to chemical emissions and processing and for validating remote sensing retrievals, atmospheric models, and emission inventories (Seinfeld et al. 2004; Bates et al. 2006). This work was motivated by the need for improved accuracy and time resolution in measuring aerosol optical properties, especially extinction and absorption at atmospherically relevant particle concentrations. Typically, extinction is determined with the combination of a scattering measurement using nephelometry and an absorption measurement based on the attenuation of light by an aerosol loaded filter (e.g., with the PSAP and Aethelometer R ). With the development of highly precise and accurate extinction measurements the uncertainties and biases of these techniques can be evaluated (Bond et al. 1999; Saathoff et al. 2003; Weingartner et al. 2003; Virkkula et al. 2005a). A comparison of techniques to measure aerosol optical properties suggests that the various techniques agree within the limits of the propagated uncertainties (Reid et al. 1998; Russell et al. 2002). However, the propagated uncertainties are large. Recent developments of aerosol cavity ring-down systems (Smith and Atkinson 2001; Bulatov et al. 2002; Atkinson 2003; Strawa et al. 2003; Pettersson et al. 2004; Moosmuller et al. 2005) and optical extinction cells (Weiss and Hobbs 1992; Virkkula et al. 2005b) to measure aerosol extinction coupled with photoacoustic systems (Arnott et al. 1999; Arnott et al. 2000; Lack et al. 2006) and the multi-angle absorption photometer (Petzold et al. 2005) to measure aerosol absorption promise to reduce the uncertainties in the aerosol optical properties. Aerosol extinction is dependent on relative humidity (RH) and it is important to include the influence of RH when calculating direct aerosol radiative forcing. The RH dependence of scattering is typically measured using a pair of nephelometers, one operated at a low reference RH (typically 40%) and the 447

3 448 T. BAYNARD ET AL. other at elevated humidities (typically 80 85% RH) (Charlson et al. 1984). The scattering ratio between the high RH and reference RH, f σ (sp) (RH,RH ref ) is given by Equation (1) f σ (sp) (RH, RH ref ) = σ sp(rh) σ sp (Ref), [1] and is typically used as the scale factor for the dry mass scattering efficiency. A similar quantity is defined for the RH dependence of extinction, f σ (ep) (RH, RH ref ) = σ ep(rh) σ ep (Ref). [2] The RH dependence of scattering, f σ (sp) (RH,RH ref ), and extinction, f σ (ep) (RH,RH ref ), are not necessarily equivalent because extinction includes a contribution from absorption. f σ (ep) (RH,RH ref ), in combination with f σ (sp) (RH,RH ref ), can be used to determine the RH dependence of the single scattering albedo ω and of absorption, if the uncertainty in f σ (ep) (RH,RH ref ) and f σ (sp) (RH,RH ref ) are small (e.g., 2%). Assuming that absorption is independent of RH, f σ (sp) (RH,RH ref ) has been used to estimate the RH dependence of extinction and the single scattering albedo (Carrico et al. 2003; Nessler et al. 2005). The RH dependence for extinction, f σ (ep) (RH,RH ref ), has been investigated with lidar measurements in the vicinity of clouds, (Feingold and Morley 2003, Pahlow et al. 2004) but not by in-situ measurements. This paper includes (1) the details of the cavity ring-down aerosol extinction spectrometer (CRD-AES) field instrument, (2) special considerations for in-situ aerosol extinction measurements in the atmosphere, (3) detailed analysis of CRD assumptions, (4) CRD-AES comparisons of identical CRD-AESs; three wavelength nephelometer; and Mie scattering calculations, (5) applications of CRD-AES capabilities to reduce the overall uncertainty of aerosol optical properties. This instrument has been used for atmospheric and laboratory measurements. Data from the New England Air Quality Study Intercontinental Transport and Chemical Transformation 2004 (NEAQS-ITCT 2004) study measured onboard the NOAA RV Ronald H. Brown in the Gulf of Maine from July 5 to August 13, 2004 have been used to highlight some of these considerations and capabilities. 2. MEASUREMENT TECHNIQUE The CRD technique (Busch and Busch 1999) has traditionally been used for spectroscopic measurements of gas phase molecules (O Keefe and Deacon 1988; Scherer et al. 1997); and more recently for atmospheric measurements of aerosol extinction (Smith and Atkinson 2001; Atkinson 2003; Strawa et al. 2003; Pettersson et al. 2004; Moosmuller et al. 2005). The pulsed CRD technique for aerosol extinction measurements has recently been validated by comparison of measured optical cross-section to calculated optical cross-section for size selected particles of known refractive index (Bulatov et al. 2002; Pettersson et al. 2004). The fundamental differences between pulsed and continuous-wave (CW) cavity ring-down techniques are captured in a review by Brown (2003). In a pulsed cavity ring-down experiment a laser pulse is coupled into an optical cavity defined by a pair of high reflectivity mirrors. Extinction is determined from the temporal decay of the light intensity measured outside one end of the cavity. The light intensity decays exponentially in time, with a time constant τ, due to losses within the cavity and at the mirrors. The light intensity transmitted through the mirrors as a function of time is described by ( I = I 0 exp t ). [3] τ Extinction due to scattering and absorption by gases and particles reduces the lifetime of the light within the cavity. The extinction coefficient, σ e,isgivenby σ e = R ( L 1 c τ 1 ), [4] τ 0 where R L is the ratio of optical cavity length to sample length, c is the speed of light, τ is the ring-down time constant with sample present, and τ 0 is the ring-down time constant in the absence of sample. Figure 1 shows a general layout of a cavity-ring down cell used for aerosol measurements. The sample length ( A) is the distance between the apertures separating the purge volume and sample volume. The cavity length (B) is the distance between the mirror surfaces. The uncertainty in measured extinction is determined by the ability to measure the ring-down time constants (τ and τ 0 ) and R L. The relationship between ring-down time constants with and without sample provides an absolute measurement of extinction Instrument Description and Design Considerations A diagram of the field CRD-AES is shown in Figure 2. The field instrument was composed of two volumes that incorporated five independent optical cavities. The CRD-AES used a pulsed low divergence (<1 mrad) Nd:YAG laser with outputs at 1064 nm, 532 nm, and 355 nm, and all pulse durations were <10 ns. The excess energy at 532 nm was Raman shifted (H 2 Stokes line) to produce 683 nm light. The Raman cell was 77 cm long and contained H 2 at 150 psi. The main measurement cell (6 cm. diameter), contained four independent parallel optical cavities with wavelengths of 1064 nm, 532 nm, 355 nm, and 683 nm equally spaced on a 3.6 cm diameter circle. This configuration allowed the aerosol extinction to be measured simultaneously at 4 wavelengths in essentially the same volume at the same RH. Laser pulses of approximately 0.5 mj were coupled into the cavities without mode matching. The cavity mirrors had 6 m radius of curvature and coatings optimized for the individual wavelengths. The reflectivities of the mirrors were %,

4 CRD-AES: FIELD INSTRUMENT 449 FIG. 1. Generalized pulsed cavity ring-down aerosol extinction spectrometer (CRD-AES). A = sample length, B = optical length, R L = B/A. The optical cavity lengths were 76.2 cm for the ref CRD cell, 87.4 cm for, 532 nm, 1064 nm, and 355 nm in the main cell, and 93.2 cm for 683 nm of the main CRD cell. The ratio of optical cavity length to sample lengths were 1.52, 1.33, 1.33, 1.48, 1.59 for 532 nm reference, 532 nm main, 1064 nm, 355 nm, and 683 nm, respectively. except for the 355 nm mirrors which had reflectivities 99.91%. A small purge flow of ultra-high purity zero air or nitrogen at 0.1% of the total sample flow isolated the mirror surface from the sample flow, maintained the required reflective quality of the mirrors by reducing contamination of the mirror surfaces, and defined the boundary between sample volume and purge volume. The sample volume and purge volume are separated by an aperture with an inner diameter of 0.25 inches. Typical values of τ 0 (filtered ambient air at 1 atm) were 123 μs, 75 μs, 3.9 μs, and 82 μs, for 1064 nm, 532 nm, 355 nm, and 683 nm, respectively. An external cell at 532 nm with 1 m radius of curvature mirrors and τ 0 of 133 μs was operated with independent temperature control, allowing the RH dependence of light extinction at 532 nm to be measured. During NEAQS-ITCT 2004 the external cell was held at constant temperature (36 C) and, consequently, the RH varied between 20 and 35%. This configuration allowed the measurement of the RH dependence of extinction, FIG. 2. Diagram of field cavity ring-down aerosol extinction instrument with five optical cavities including the capability to determine f σ (ep) (RH,RH ref and automated τ 0 measurements.

5 450 T. BAYNARD ET AL. f σ (ep) (RH,RH ref ), at 532 nm by taking the ratio of the extinction coefficient in the main cell to the reference cell. The RH control of the inlet and main cell was accomplished by active temperature control with a potential range from 10 C to 60 C. However, high temperatures were avoided to minimize evaporation of semi-volatile species. For typical operation the main cell was temperature controlled (<30 C) to the desired RH (e.g., 60 85%). The ability to scan the RH was limited by the thermal mass of the instrument and the necessity to minimize thermal gradients within the sample volume. Thermal gradients deteriorated the quality of the single-exponential decays and compromised the accuracy of the extinction coefficient due to RH gradients. In this configuration, where the sample is neither exposed to low RH to force efflorescence or elevated RH to force deliquescence, the measured RH dependence is related to the ambient hygroscopic state. Alternatively, a humidification stage within the flow system to ensure the particles had experienced a high RH for deliquescence, without subsequent efflorescence, could have been used in conjunction with temperature control to determine the RH dependence of the humidified state. The optical cavity lengths, B in Figure 1, were 76.2 cm for the reference cell, 87.4 cm for, 532 nm, 1064 nm, and 355 nm in the main cell, and 93.2 cm for 683 nm. The ratio of optical cavity length to sample lengths, R L (=B/A) in Figure 1 were 1.52, 1.33, 1.33, 1.48, and 1.59 for 532 nm reference, 532 nm main, 1064 nm, 355 nm, and 683 nm, respectively. R L was based on the physical dimensions. The light intensities transmitted through the output mirrors were measured with photomultiplier tubes coupled to the cavity with optical fibers. Optical band-pass filters centered at the desired wavelength (λ ± 10 nm) and positioned in front of the photomultiplier window minimized artifacts in the exponential decay due to stray light and incomplete separation of laser wavelengths (e.g., 1064 nm and 532 nm). The signals were digitized with 14 bit resolution at 25 MS/s for measurement at 355 nm and 16 bit resolution at 1 MS/s for measurements at 532 nm, 1064 nm, and 683 nm. The flow through the main cell, diameter 6.0 cm, was 30 slpm (liters per minute at 1 atm and 273 K) and through the external reference cell, diameter 1.73 cm, was 3 slpm. Residence times of 4 seconds and 3 seconds for the main cell and external cell, respectively, were determined from the extinction temporal profile after switching from unfiltered to filtered sample. The flow through the systems was laminar with Reynolds numbers less than 700. An isokinetic pickoff in the center of the flow at the exit of the main cell was used to provide the sample flow for the reference cell. A pickoff efficiency of ± 0.005, based on extinction, was determined for sub-1 μm ammonium sulfate particles. To determine τ 0, the sample flow was diverted through two parallel glass fiber filters with organic binder. The filters effectively removed the particles (manufacture specification: 99.98% at 0.3 μm) at 30 slpm and were a minimal loss (<2%) for absorbing gases (NO 2 and O 3 ) that can interfere with the aerosol extinction measurements at 355 nm and 532 nm (see Section 2.2). To test the system performance during field measurements and to track the gas phase absorption, τ was measured periodically when the sample volume was filled with 1 atm of synthetic zero air, i.e., τ zero-air. In regular field operation this option was used once every two hours for 30 seconds to track any changes in system performance. The drift in τ zero-air was <1% for the 7 week deployment without mirror cleaning or alignment adjustment. Using a single measurement of τ zero-air for the entire cruise would contribute an overall uncertainty to the measured gas phase extinction at 532 nm of only 0.6 Mm 1. The data processing accounted for dilution by purge flows and Rayleigh scattering corrections. The purge flow for this instrument configuration ranged from 0.1 to 1% of the total sample flow. A pressure drop (0.007 atm) across the filter decreased Rayleigh scattering and artificially enhanced τ 0 by 0.05 to 0.3%. τ 0 was corrected based on the pressure drop and the Rayleigh scattering cross-section of air at each of the wavelengths (Penndorf 1957). It was assumed that the scattering efficiency of gas phase water was the same as air. This leads to negligible errors (<0.08%) in the τ 0 correction due to low mixing ratios of water and similar scattering efficiencies for air and water (Sutton and Driscoll 2004). The extinction coefficient is a function of the cavity ringdown time, and is independent of the laser pulse energy. Potentially, the measured extinction could be biased by non-single exponential decays associated with poor cavity alignment, mechanical vibrations, poor mirror quality, non-linear PMT response, thermal gradients, and electrical noise affecting the PMT background. Figure 3 shows a representative decay of the PMT signal and the residual of the single exponential fit for the external 532 nm channel. In this case the ring-down time constant was ± 0.06 μs. The residual is the difference between the photon intensity and the single exponential fit normalized to the photon intensity. In this case there was no systematic curvature to the residual and the error in the fit was 0.3% (1 std. deviation) Gas Phase Absorption In-situ aerosol optical property instruments must account for the gas absorption and minimize its interference. Comparison of the ring-down time constants during the NEAQS-ITCT 2004 for synthetic zero air and filtered ambient air indicated that the gas phase extinction at 1064 nm and 683 nm was less than the detection limit of this instrument (i.e., <0.37 and 0.43 Mm 1, respectively, for 1 sec, see Section 2.4). At 532 nm and 355 nm NO 2 and O 3 are the most important atmospheric gas phase absorbers. The absorption cross-sections for NO 2 are cm 2 /molec (0.35 Mm 1 /ppb at 1 atm and 273 K) and cm 2 /molec (1.19 Mm 1 /ppb at 1 atm and 273 K) for 355 nm and 532 nm, respectively (Harwood and Jones 1994). NO 2 has highly structured absorption throughout the visible spectrum with cross-sections and

6 CRD-AES: FIELD INSTRUMENT 451 FIG. 3. Analysis of photon intensity decay for CRD-AES. Upper figure is the PMT profile (average of 100 laser shots). The lower figure is the fractional residual of the single exponential decay. The fractional residual was determined from the difference between the photon intensity decay and the fit of the decay divided by the photon intensity decay. atmospheric concentrations that are sufficient to make the NO 2 absorption comparable to aerosol absorption in the wavelength range of 350 nm to 550 nm. The absorption cross-sections for O 3 are cm 2 /molec ( Mm 1 /ppb) and cm 2 /molec ( Mm 1 /ppb) for 355 nm and 532 nm, respectively (Burkholder and Talukdar 1994). Hence, for typical atmospheric ozone concentrations (<100 ppb), the gas phase absorption of ozone (<1Mm 1 ) is generally significantly smaller than the aerosol extinction. The gas phase contribution to extinction was measured by periodically passing the sample through a filter that removed the aerosol, but did not remove NO 2 and O 3. The filtered ring down time was used as τ 0 for the aerosol extinction measurement in order to eliminate the gas phase contribution from the aerosol extinction. Alternatively, the gas phase absorption could be measured continuously with a dedicated aerosol filtered CRD channel. The periodic filtering approach was used effectively for the ground and ship based field measurements where the variation of the gas phase extinction was slow relative to the filtering period. During the NEAQS-ITCT 2004 campaign a separate analysis of gas phase absorbers at 532 nm using cavity ring-down spectroscopy was performed. NO 2 accounted for the majority of the gas phase absorption in the high-nox environment off the New England coast (Osthoff et al. 2006). A typical measurement sequence of the CRD-AES extinction at 532 nm during NEAQS-ITCT 2004 is shown in Figure 4. Figure 4a includes the ring-down time constants τ zero-air (synthetic air), τ 0 (filtered), and τ (unfiltered). The measurement cycle was 2 minutes of ambient unfiltered air sample τ followed by 20 seconds of filtered zero τ 0, with appropriate delays for switching (approximately 4 seconds). At 10 minutes into the sample period there is a significant decreases in τ 0 which lasts for 25 minutes. Figure 4b shows the light extinction coefficient at 532 nm due to aerosol and gas absorption. If all the gas absorption in this example is

7 452 T. BAYNARD ET AL. FIG. 4. Example data from NEAQS-ITCT 2004 showing measurement cycles used to distinguish between aerosol light extinction and gas phase absorption, (a) shows the ring-down time constants τ zero-air, τ 0 (filtered), and τ (unfiltered). The decrease in to between 10 and 35 minutes is due to gas phase absorption by NO 2 and O 3. (b) shows the partitioning of extinction between aerosol and gas. The time resolution of the aerosol extinction is 5 sec. The gas phase extinction is based on interpolation between τ 0 measurements every 120 sec. The peak in the gas phase absorption is equivalent to 14 ppb of NO 2. (c) shows the ratio of aerosol extinction and total extinction (aerosol + NO 2 + O 3 ). attributed to NO 2, the concentration in the sample volume at the peak of absorption was 14 ± 2 ppb. The independently measured levels of NO 2 during this period were 2.5 ppbv for the background and 16 ppbv for the peak mixing ratio in the plume, consistent with the conclusion from these measurements. Figure 4c shows the ratio of light extinction by aerosol to aerosol plus gas phase absorption. The gas phase absorption, if it is not accounted for properly, can cause a significant bias in reported aerosol optical properties. At the peak of gas phase absorption only 80% of light extinction is due to aerosol. If particle absorption was determined using the difference between extinction and scattering without accounting for the gas phase absorption, the particle absorption would have been over estimated by 360%. The correction of the measured aerosol extinction for the gas phase absorption interference requires accurate knowledge of the filter transmission for the gas phase absorbers. The transmission of NO 2 through commercially available filters and conductive silicone tubing were measured as a function of flow rate and NO 2 concentration. The transmission efficiencies were dependent on residence time and to a lesser extent on concentration. At 30 lpm the glass fiber filters with organic binder had NO 2 losses of <2% over the range of 10 to 100 Mm 1 of extinction due to NO 2. The NO 2 losses on fresh filters decreased with time, e.g., from initial values of 25% to steady state values of 2% after a few minutes. Upon subsequent exposure the initial losses were consistent at the previous steady state value. The uncertainty in the measured aerosol extinction is dependent on the ratio of aerosol extinction to gas extinction. For typical field conditions, the losses of the gas phase absorber during filtration caused minimal uncertainty in the aerosol extinction coefficient (<0.2%) because the aerosol extinction generally overwhelmed the gas phase absorption, and the losses of absorbing gases were small during filtration Forward Scattering and Coarse Particles The scattering phase function is dependent on particle size. Figure 5 shows the size dependent fraction of total scattering for various forward scattering angles calculated with Mie scattering theory for spherical monodisperse particles with a refractive index of n = 1.53 and wavelength of 532 nm (e.g., ammonium

8 CRD-AES: FIELD INSTRUMENT 453 sulfate (Toon and Pollack 1976)). It is estimated that the upper limit for the angle of acceptance at the mirror surface that could re-couple the scattered photon to the CRD cavity is 0.09, based on the CRD-AES geometry and estimated optical volume (see section 2.4). The narrow acceptance angle for coupling photons to the CRD cavity allows extinction to be measured without significant size dependent biases (i.e., bias <0.1% for particles smaller than 10 μm). Figure 5 also shows the scattering contribution from 0 to 7 that causes the truncation error of the TSI Integrating Nephelometer Even though there is significant forward scattering between 0 and 7, Anderson and Ogren (1998) successfully developed a truncation correction based on the angstrom exponent for the TSI nephelometer that is applicable for the fine particle mode (i.e., <1 μm). Gravitational settling and inertial deposition are minimal (<1%) with the current CRD-AES design for particles below 8 μm and a flow rate of 30 slpm. This analysis indicates that the aerosol extinction coefficient for the sub 10 μm fraction can be determined using the CRD-AES. However, inlet and sampling limitations need to be addressed for the specific installation, especially in the case of measurements from aircraft Measurement Limits The accuracy of the CRD aerosol extinction measurement is limited by the mirror reflectivity, optical volume, PMT response, and A/D conversion sample rate. The CRD-AES sensitivity limit is given by σ min = R L 2δ(τ0 ), [5] c Rt where δ(τ 0 ) is the standard deviation of τ 0 based on a single laser shot, R is the repetition rate and t is the sampling time (Pettersson et al. 2004). σ min at 1 second resolution for typical field operating conditions for all channels are given in Table 1. These values include the reduction in sensitivity due to optical instability from mechanical vibrations, flow noise, and thermal gradients. The differences in the detection limit at these wavelengths are related to hardware. The measurement at 532 nm has a better detection limit because the superior quality of the high reflectivity mirrors, photon source, and PMT. The 683 nm output of the Raman cell had sufficient intensity fluctuations to degrade the quality of the ring-down time constant analysis (i.e., δ(τ 0 )). The 355 nm measurement was limited by the quality of the high reflectivity mirrors (i.e., τ 0 ). The measurement at 1064 nm was limited by the PMT which affected δ(τ 0 ). Equation (5) is only valid when the standard deviation of τ 0 is dominated by instrumental fluctuations. In the case of aerosols, statistical fluctuations in the number of particles within τ 2 0 FIG. 5. The size dependence of the fraction of total scattering within degrees (upper limit for CRD-AES), 0 1 degrees, and 0 7 degrees (TSI Nephelometer) for monodisperse particles with refractive index of i and 532 nm. Based on an estimated acceptance angle of 0.09 degrees for re-coupling of forward scattering photons, the error related to forward scattering for the CRD-AES measurement of extinction is <0.1% for particles 10 μm.

9 454 T. BAYNARD ET AL. TABLE 1 Detection limit for 1 second average using Equation 5 for typical atmospheric measurement conditions τ 0, δ (τ 0 ), σ min (1 sec), λ channel μs μs R L Mm nm (main) nm (main) nm (external) nm (main) nm (main) τ 0 = rind-down time constant, δ(τ 0 ) = standard deviation of τ 0 based on individual laser shots, R L = ratio of optical to sample lengths, and σ min = detection limit for 1 second resolution. the optical volume can contribute significantly to the variation in the ring-down time constant. For typical atmospheric conditions, uncertainty related to counting statistics can limit the time resolution. Pettersson et al. (2004) showed that the relative uncertainty imposed by statistical fluctuations in monodisperse particle number is given by σ stat σ = NV NV 1 1 =, [6] Rt NVRt where N is the aerosol number density and V is the effective laser beam volume. Combining the instrument and statistical fluctuations gives, σ 2 σ = NVRt + σ min 2. [7] In the limit of small NV (=average number of particles in the laser volume), the statistical fluctuations in the number of particles in the laser volume dominate the uncertainty. For a cavity length of 92 cm and R L of 1.42 Pettersson et al. (2004) derived optical volumes of cm 3 and cm 3 for 1 meter and 6 meter radius of curvature systems, respectively, by analyzing the variability of σ as a function of N for size selected aerosol. Based on Equation (7) and generalized particle size distributions, Pettersson et al. (2004) predicted the field performance of a pulsed cavity ring-down system with improper statistical weighting of the particle size distribution. Equation (8) is appropriate for estimates of polydisperse particle distributions ( σ T = 1 ( ) ) 2 1/2 σi σ i [8] σ T σ T σ i=0 T where σ T is the total extinction of the aerosol distribution, σ i is the extinction coefficient for diameter increment i, and σ i is the uncertainty in the extinction coefficient for particles in diameter increment i. σ i σ T is the statistical weighting term for the increment. Equation (8) gives results that are consistent with standard deviations observed in field measurements (e.g., std 7% for 1 second averages with σ ep 5Mm 1 ). Table 2 summarizes calculated standard deviations for three representative atmospheric aerosol size distributions TABLE 2 Calculated extinction and CRD-AES measurement errors for three different representative atmospheric size distributions. This is an update of Table 2 from Pettersson et al. (2004) Case 1 Case 2 Case 3 Conditions Marine boundary layer (MBL) Pacific free troposphere (FT) Polluted urban boundary Date 17 May May May 2002 Location Eastern Pacific near central Eastern Pacific near central Downtown Los Angeles California coast California coast Altitude (m) Wind speed (m/s) Temp ( C) RH (%) Particle Number Density (cm 3 ) Calculated Extinction (Mm 1 ) Time Resolution σ ep (Mm 1 ) σ ep (Mm 1 ) σ ep (Mm 1 ) σ ep (Mm 1 ) σ ep (Mm 1 ) σ ep (Mm 1 ) (seconds) (Total) (sub-1 μm) (Total) (sub-1 μm) (Total) (sub-1 μm)

10 CRD-AES: FIELD INSTRUMENT 455 TABLE 3 Time resolution for 2% uncertainty in extinction Marine boundary layer (MBL) Pacific free troposphere (FT) Polluted urban boundary σ ep 532 nm 10.4 Mm Mm Mm 1 σ ep 532 nm (2%) 1 sec 6 sec 0.3 sec σ ep 355 nm 16.3 Mm Mm Mm 1 σ ep 355 nm (2%) 60 sec 600 sec 1 sec σ ep 683 mn 7.4 Mm Mm 1 38 Mm 1 σ ep 683 nm (2%) 5 sec 30 sec 5 sec σ ep 1064 mn 4.6 Mm Mm Mm 1 σ ep 1064 nm (2%) 10 sec 50 sec 20 sec frhext (532 nm) 2 sec 12.5 sec 0.6 sec as a function of measurement time resolution. Values for V (0.031 cm 3 ), τ (74 μs), and δ(τ 0 ) (0.6 μs) are the same as those used in the calculation by Pettersson et al. (2004) to provide a correction to the previously reported values. The previous analysis over predicted the statistical fluctuations because the statistical fluctuation for a particular particle size was not normalized to the extinction coefficient of the entire distribution. The three cases are typical of the marine boundary layer (Case 1), free troposphere (Case 2), and polluted urban boundary layer (Case 3) (Neuman et al. 2003; Brock et al. 2004) where the relative uncertainty is determined using Equations (7) and (8). Table 3 reports the time required for each channel to achieve 2% relative uncertainty. The time resolution required to achieve 2% precision in f σ (ep) (RH,RH ref ) is also included. The variability in the measurement capabilities as a function of wavelength is attributed to the wavelength dependence of the mirror quality, Rayleigh scattering, PMT response, and aerosol extinction. The distribution of the relative uncertainty as a function of particle size is shown in Figure 6 and is a corrected version of Figure 5 case 1 from Pettersson et al. (2004). The current error treatment predicts lower overall uncertainty and less contribution to the uncertainty from larger particles than reported by Pettersson et al. (2004). FIG. 6. Representative marine boundary layer aerosol size and extinction distributions, and associated relative accumulated error in CRD-AES extinction. The total sub-10 μm aerosol extinction is 10.4 Mm 1 at 532 nm and the uncertainty for 1 sec averages is 2.1%.

11 456 T. BAYNARD ET AL. The maximum measurable extinction was limited by the signal processing electronics. The A/D conversion was performed at 1 MS/sec or 25 MS/sec providing a measurement of the laser intensity every 1 μs or 0.04 μs, respectively. The PMT signal was amplified by a 10 MHz amplifier. The upper limit of the extinction coefficient using this technique is determined by the time resolution of the photon intensity decay. As the extinction coefficient increases the number of points defining the photon intensity decay decreases. At the limit of 10 points to define the photon intensity decay and the weakest photon intensity with a S/N 10, the maximum measurable extinction coefficient for 1 MS/sec is approximately 1700 Mm 1. In the case of 25 MS/sec the sample resolution is limited by the 10 MHz amplifier. At τ = 1 μs the light intensity decay was defined by 100 measurements and the extinction coefficient was 3200 Mm 1. This upper limit appears to exceed the required level for atmospheric measurements, but the capability to measure high extinction values becomes important when measurements are made at elevated RH due to the significant enhancement RH Dependence The ability of the CRD-AES to measure the RH dependence of extinction was demonstrated in Garland et al. (2006) by comparing the measured f ext (80 ± 3% RH, dry) to calculated f ext (80% RH, dry) for size selected ammonium sulfate. The calculated values were based on the water uptake measurements by Tang (1996). The reported f ext (80% RH, dry) for size selected particles with an effective mean diameter (Baynard et al. 2006) of 300 nm were 3.1 ± 0.3 and 3.0 for the measured and calculated values, respectively. The difference between the measured and calculated values is within the propagated uncertainty. The accuracy of the RH dependence of extinction was limited by the RH uncertainty rather than inherent limitations of the CRD technique. CRD-AES was also used to measure the deliquescence and efflorescence point of laboratory generated particles. For these experiments the aerosol was generated by atomizing a 1 wt% aqueous solution followed by drying to <10% RH by passing the sample through a horizontal flow tube half filled with 13 molecular sieve (residence time = 3 min). Water vapor was then added to the sample by passing the aerosol flow through a temperature controlled permeable membrane. The amount of water was controlled to prevent deliquescence during aerosol preparation. Following sample preparation the water content was held constant and temperature control was used to vary the RH. A temperature control manifold connecting CRD 1 and CRD 2 was used to control the phase (aqueous or solid) of the aerosol entering CRD 2 and measure the critical RH. The RH of CRD 1 and CRD 2 were held constant as the RH of the temperature control manifold was varied. For the efflorescence measurements the RH of CRD 1 was held above the deliquescence point and CRD 2 was held below the deliquescence point (e.g., ammonium sulfate: CRD 1 85% RH, CRD 2 60% RH). The RH of CRD 1 ensured that the sample entering the temperature control manifold was aqueous. The extinction coefficient measured at CRD 2 depended on the phase. Therefore, a discontinuity in the ratio of extinction coefficients as a function of the RH of the temperature control manifold occurred at the efflorescence point. A similar configuration was used to determine the deliquescence point, but CRD 1 and CRD 2 were held below the deliquescence point and the RH of the temperature control manifold was raised to force deliquescence (e.g., ammonium sulfate: CRD 1 and 2 60% RH). The measured deliquescence and efflorescence points for ammonium sulfate at 22 C were 78 ± 2% RH and 37 ± 2% RH, respectively. These values are consistent with previously reported values of 79% to 81% RH for deliquescence and 34% to 48% RH for efflorescence (e.g., Tang 1980; Richardson and Spann 1984; Cohen et al. 1987a, 1987b; Tang and Munkelwitz 1994) Absolute Uncertainty of CRD-AES Measurements In the following analysis, the absolute uncertainty of the aerosol extinction coefficient using the CRD technique is determined by propagating the uncertainties associated with determination of τ, R L, mixing of purge flow, aerosol transmission efficiency, gas phase interference, RH, and evaporation of volatile compounds. The absolute uncertainty was estimated using relative uncertainties in σ ep for each parameter in brackets of σ ep (τandτ 0 ) = 0.3%, σ ep (R L ) = 0.3%, σ ep (purge flow (mixing)) = 0.2%, and σ ep (gas interference) = 0.2%, combined in quadrature. The uncertainty in the extinction at ambient RH or as a function of RH was estimated by considering the uncertainties in the measured RH. The uncertainty associated with evaporation of semi-volatile compounds was not included in this analysis because it is composition and temperature specific. The water uptake property of (NH 4 ) 2 SO 4 was used in the uncertainty analysis because of the large contribution of (NH 4 ) 2 SO 4 to the aerosol mass loading (Penner et al. 1994). Uncertainties of ±1% RH for RH between 0 and 85% and ±2% RH for RH above 85% were used, giving the following uncertainties in the aerosol extinction coefficient: σ ep ( dry ) < 1%, σ ep (85 ± 2% RH) = 5.1%, σ ep (95 ± 2% RH) = 12.1%. 3. COMPARISONS AND APPLICATIONS The uncertainty and instrument-to-instrument variability of the CRD-AES technique was investigated with a series of experiments using laboratory generated aerosol. These experiments used non-absorbing particles to eliminate uncertainty related to the absorption component of extinction. The following results are related to CRD-AES instrument comparison, Mie scattering comparison, and CRD-AES and nephelometer comparison.

12 CRD-AES: FIELD INSTRUMENT 457 FIG. 7. Direct comparison of 1 second average extinction for two 532 nm CRD-AES systems using dry size selected ammonium sulfate particles. The main and external 532 nm CRD cells were compared by simultaneously measuring the extinction of aerosol sampled from a common mixing volume. Uncharged polydisperse dry ammonium sulfate particles, generated by atomizing a 1% wt (NH 4 ) 2 SO 4 solution and dried using a 13 molecular sieve with a residence time of 3 minutes, were diluted to a total flow of 32 lpm and flowed through a 30 L mixing volume. The two CRD cells sampled the aerosol in parallel, main flow = 30 lpm and external flow = 1.5 lpm, and the isokinetic pickoff employed during the RH dependent measurements was not used. For 60 second averages, the extinction coefficients determined from the two CRD-AESs agreed within 0.5%. Two CRD-AESs identical to the external 532 nm CRD-AES were setup for comparison using size-selected (NH 4 ) 2 SO 4. The particles were generated as described above, but instead of diluting the aerosol sample, the particles were size selected using a Differential Mobility Analyzer (DMA) (Pettersson et al. 2004). Figure 7 compares 1 second averages of σ ep (λ = 532 nm) for size selected ammonium sulfate particles using two single channel CRD-AES systems in series. The data includes measurements for a wide range of particle sizes ( nm) and number densities (10 10,000 particles/cc). The standard deviations for 1 second (shown in Figure 7) and 10 second averages were in agreement with predicted variation addressed in the previous section. For 1 second data the standard deviation of extinction ranged from 3% to 18% depending on number density. Comparison of the extinction coefficients from the two CRD-AES gives a slope of ± The absolute accuracy of the CRD-AES technique was checked by comparing measured extinction coefficients of polystyrene spheres with predictions of Mie theory. Shown in Figure 8 are Q ext (ratios of the optical cross-sections to geometric cross-sections) for monodisperse polystyrene spheres of diameter 299 nm, 453 nm, 599 nm, and 800 nm at wavelengths of 1064 nm, 532 nm, and 355 nm. Q ext was determined using the extinction coefficient from the CRD-AES and particle number density from a TSI 3022 condensation particle counter. The solid line was calculated using Mie scattering theory (Bohren and Huffman 1983). Indices of refraction of 1.628, 1.598, and (Nikolov and Ivanov 2000) were used for wavelengths of 355 nm, 532 nm, and 1064 nm, respectively. The measurement error bars were propagated from uncertainties in aerosol size (monodisperse particles Duke Scientific no. 3300A, 3450A, 3600A, and 3800A), particle number

13 458 T. BAYNARD ET AL. FIG. 8. Comparison of measured (points) and calculated (lines) Qext (ratio of optical cross-section to geometric cross-section) for polystyrene spheres at 355 nm, 532 nm, and 1064 nm. density (TSI 3022), and the extinction coefficient (CRD-AES). The agreement between measurement and theory averaged about 5% for these wavelengths, with better agreement at larger sizes. The CRD-AES extinction measurement and the TSI nephelometer scattering measurement were compared by using nonabsorbing (NH 4 ) 2 SO 4. In this case, the extinction coefficient and scattering coefficient are equivalent (σ ep = σ sp ). Ammonium sulfate aerosol were generated from an atomized 1 wt% (NH 4 ) 2 SO 4 solution and dried to < 10% RH by passing over 13 molecular sieve. The particles were diluted with filtered room air. The small contribution of gas phase absorption (<0.5 Mm 1 due to NO 2 and O 3 in the dilution air) was accounted for in τ 0 as described in section 2.3. The size distribution was dominated by small particles (e.g., mean diameter = 150 nm), but a D aero < 1 μm Berner-Type impactor was used to remove particles larger than 1 μm. The CRD-AES and nephelometer were run in series, with the CRD-AES first, followed by the nephelometer. The flow rate through both instruments was 30 vlpm. The TSI scattering data were corrected for the truncation error using C = Å (Anderson and Ogren 1998), where C is a multiplicative correction to the scattering coefficient and Å is the angstrom exponent defined in Equation (9) σ 550 nm ) Å = log ( σ 450 nm ( ). [9] log For the specific size distribution studied, the measured nephelometer Å = 1.49, giving a truncation correction factor of The nephelometer scattering coefficient measured at 550 ± 50 nm was corrected to the CRD-AES wavelength (532 nm) by using Å and Equations (9) and (10) ( σ 532 nm = 10 Å log σ 550 nm ) [10] giving an extinction coefficient ratio of Figure 9 shows the comparison for σ ep measured with the CRD-AES (λ = 532 nm) and σ sp measured with the TSI 3563 nephelometer (λ = 550 nm converted to 532 nm) for laboratory generated (NH 4 ) 2 SO 4 particles (approximate log normal size distribution of D p = 125 nm and ln(σ g ) = 0.6). The analysis spans 0 to 2000 Mm 1 and is based on 1000 measurements with 5 second time resolution. The stated uncertainties are <1%

14 CRD-AES: FIELD INSTRUMENT 459 FIG. 9. CRD-AES and TSI Nephelometer comparison for ammonium sulfate particles (non-absorbing aerosol, σ ep = σ sp ) at 5 second resolution. The TSI 3563 integrating nephelometer 550 nm measurement corrected for angular non-idealities (C = Å) and converted to 532 nm applying the angstrom exponent (Å = 1.49). The agreement (slope = 0.979) is within the combined uncertainties of the nephelometer (4 to 7%) (Anderson and Ogren, 1998) and CRD-AES (1%). for the CRD-AES system and 4 7% for the TSI nephelometer. The linear regression of the CRD-AES and TSI nephelometer measurements at 532 nm gives a slope of ± and R 2 of , which is within the quoted instrumental uncertainties (Anderson and Ogren 1998). Using the described CRD-AES capabilities the extinction coefficient can be applied in combination with other techniques to reduce the uncertainty of aerosol optical properties. A similar analysis of propagated uncertainties has been performed (Strawa et al. 2003), but did not include the uncertainty in the scattering response of filter based absorption measurements (Bond et al. 1999). In Figure 10a the propagated uncertainty of absorption for various instrument combinations are presented. For measurement of absorption the Photoacoustic Spectrometer (PAS) has a reported uncertainty of 5% (Lack et al. 2006) and the Radiance Research Particle Soot Absorption Photometer (PSAP) has a reported uncertainty of 20% plus 2% of the scattering coefficient (Bond et al. 1999). The PSAP and Aethelometer R are limited at high single scattering albedo values by the scattering response (Bond et al. 1999; Arnott et al. 2005). Both of these filter based techniques provide comparable uncertainties in the presented analysis. The uncertainty for absorption derived from the difference technique (CRD-AES extinction minus nephelometer scattering) has been propagated using nephelometer uncertainties of 7% (Anderson and Ogren 1998) and 2%, following normalization to the CRD-AES using non-absorbing particles. The lower scattering uncertainty is based on comparison of the nephelometer with the CRD-AES for non-absorbing aerosol. These uncertainty values are limited to dry sub-1 μm particles. In this analysis an aerosol extinction coefficient of 25 Mm 1 and a detection limit of 0.1 Mm 1 for all instruments were assumed. The necessary time resolution required to achieve this detection limit is instrument dependent. When the single scattering albedo approaches one, the uncertainty in PAS absorption is significantly smaller than that derived from the filter and difference techniques. However, the difference technique (CRD-AES minus scattering) is useful for technique validation, calibration of other absorption measurement techniques, and atmospheric measurements with high quality RH control and moderate absorption levels (ω 0.95). In Figure 10b the uncertainties in single scattering albedo ω are compared. The uncertainty in ω for the CRD-AES and PAS

15 460 T. BAYNARD ET AL. FIG. 10. Relative uncertainty in (a) absorption and (b) single scattering albedo as a function of single scattering albedo using CRD-AES, TSI Nephelometer, PSAP, and PA. The propagated uncertainty is based on CRD-AES (1%), TSI nephelometer (7% and 2%), PSAP (20% plus 2% of scattering) and PAS (5%). The uncertainty is propagated based on 25 Mm 1 of extinction and a detection limit for all instruments of 0.1 Mm 1. combination was calculated using and ( ) ( ) ω ω (1 ω) = ω ω ω ( ) ( σap ) ω 2 = + ω σ ap ( σep σ ep [11] ) 2. [12] where ω is the co-albedo, the ratio of absorption to extinction, equal to 1 ω. Because of the co-variance between scattering and extinction when using the combination of scattering and absorption to determine ω, the uncertainty for this case was calculated using specific conditions (i.e., σ ep = 25 Mm 1 and detection limit of 0.1 Mm 1 ), and included the uncertainty of the scattering correction for the filter based absorption measurement. The combination of CRD-AES and PAS yields the smallest uncertainty in the single scattering albedo and co-albedo. The CRD- AES and nephelometer combination also provides an excellent determination of the single scattering albedo if the uncertainty in the nephelometer can be reduced to 2%. 4. CONCLUSIONS We have presented the capabilities, validation, and comparison of a new field cavity ring down aerosol spectrometer that measures directly the aerosol extinction coefficient. The current CRD-AES measures extinction at 355 nm, 532 nm, 683 nm, and 1064 nm. Interference caused by gas phase absorption is minimized by making frequent or continuous measurements of the gas phase absorption. Based on statistical fluctuations of particle number density within the optical volume the CRD-AES measurement of aerosol extinction has an uncertainty of has a σ ep = 1% at a time resolution of 1 sec to 60 sec for typical atmospheric size distributions. The RH dependence of extinction, f σ (ep) (RH,RH ref ), is determined with a precision of ± 2% on a fast time scale using two CRD-AES in series. The combination of the excellent time resolution and the f σ (ep) (RH,RH ref ) capabilities of the CRD-AES allow investigation of the variability of optical and chemical properties of atmospheric aerosol. The stability, accuracy, time resolution, and ability to address gas phase interference on a short time scale allow the CRD-AES to be deployed on a variety of platforms. Absolute measurements of extinction with the CRD-AES in combination with existing instruments reduce the uncertainty of aerosol optical properties, especially for the coarse particle mode. The CRD- AES has the potential to significantly reduce aerosol optical