Calibration of a photoacoustic spectrometer cell using light 1 absorbing aerosols . A technical note

11 Multi-pass photoacoustic spectrometer (PAS) is an important tool for the direct measurement of 12 light absorption by atmospheric aerosol. And accurate PAS measurements require an accurate 13 calibration of its signal. Ozone is often used for calibrating the PAS by relating the instrument 14 signal to the absorption coefficient measured by an independent method such as cavity ring down 15 spectroscopy (CRD), cavity enhanced spectroscopy (CES) or an ozone monitor. We report here a 16 calibration method that uses measured absorption coefficients of aerosolized, light absorbing 17 organic materials and offer an alternative approach to calibrate the PAS at 404 nm. To 18 implement this method we first determined the complex refractive index of an organic dye using 19 spectroscopic ellipsometry, and then we use this well characterized material as a standard 20 material for PAS calibration. 21


Introduction
Light absorption by atmospheric aerosols still poses one of the greatest uncertainties associated with the effective radiative forcing due to aerosol-radiation interactions (IPCC, 2013).
Absorption of incoming solar radiation exerts positive radiative forcing to the top of the atmosphere due to heat transfer from light absorbing aerosols to their surroundings.It may also lead to stagnation and clouds dissipation.In terms of positive radiative forcing, black carbon (BC) aerosols are commonly considered to be second only to CO 2 (Bond et al., 2013) with strong absorption throughout the solar spectrum.Characterization and quantification of light absorbing organic aerosols, referred to as brown carbon (BrC), have been given increasing attention in the past decade (Andreae and Gelencser, 2006;Bond and Bergstrom, 2006;Alexander et al., 2008;Flores et al., 2012b;Lack et al., 2012a;Flores et al., 2014;Laskin et al., 2015;Moise et al., 2015).Although the atmospheric burden of BrC is estimated to be more than three times that of BC (Feng et al., 2013) its absorption is strongly spectral-dependent with strong absorption in the UV and visible spectrum and weak to non-absorbing in the longer wavelengths (Hoffer et al., 2004;Kirchstetter et al., 2004;Kaskaoutis et al., 2007;Sun et al., 2007;Chen and Bond, 2010;Moosmuller et al., 2011;Lack et al., 2012a).
Light absorption properties of BrC and its mixing with absorbing and non-absorbing aerosol components introduce a need for sensitive and accurate direct measurement of light absorbing aerosols which is still very challenging.For example, filter based techniques such as the Particle Soot Absorption Photometer (PSAP), the Multi Angle Absorption Photometer (MAAP) and the aethalometer, require correction factors which are based on some a-priori information regarding aerosol type and source and have accuracy in the range of 20% to 35% (Bond et al., 1999;Weingartner et al., 2003;Collaud Coen et al., 2010;Muller et al., 2011).
Multi-pass photoacoustic spectrometer (PAS) at several wavelengths throughout the visible spectrum has the potential to produce sensitive direct measurements of light absorption due to BrC aerosols.Coupling PAS instruments to a thermal denuder and cavity ring down spectrometers (CRD-S) to measures the absorption and extinction coefficients simultaneously at temperatures ranging from ambient to over 450 0 C allow attribution of aerosol light absorption to BrC, BC, and BC with enhanced absorption due to less-absorbing coating (Cappa et al., 2012;Lack et al., 2012b).
In a PAS cell, a modulated laser light is absorbed by a sample of particles or gas, generating a modulated acoustic wave with intensity that is proportional to the energy absorbed by the sample.This acoustic wave, which is detected by a sensitive microphone, has a characteristic radial and longitudinal resonance when the light source is modulated at the cavity resonance frequency (F r ).A more detailed description of the PAS method for aerosol light absorption measurement may be found in Arnott et al., (1999) and Nagele and Sigrist, (2000).
While direct, in-situ aerosol absorption measurement using PAS avoids the disadvantages of filter based techniques, an accurate calibration procedure is required to relate the instrument signal to the absorption coefficient (α abs ).One way of achieving this is to directly relate the α abs to the microphone response and the laser power by a theoretical relation (Arnott et al., 1999): where P Laser is absolute laser power in the resonator, A res is resonator cross sectional area, γ is isobaric to isochoric specific heats taken as a constant in dry air, P mic is the microphone signal power, Q is the resonant cavity quality factor and FWHM is the full width half maximum of the acoustic response curve.The F r and the FWHM are sensitive to temperature, pressure and type of carrier gas.
When the laser intensity inside the PAS cell is unknown or when it is not possible to measure, as in the case of an astigmatic cell alignment, the instrument's response needs to be calibrated empirically.This involves comparing the PAS signal to an independent transmittance measurement where scattering is negligible (Arnott et al., 2000;Lack et al., 2006).A common PAS calibration procedure is done by comparing direct measurements of α abs at several concentrations of an absorbing gas by cavity ring down spectrometer (CRD-S) with a parallel measurements by the PAS cell (Lack et al., 2006;Lack et al., 2012b;Lambe et al., 2013).This method is applicable when the Rayleigh scattering by the gas molecules is several orders of magnitude smaller than the absorption and can be neglected.Since gas absorption cross sections can be highly dependent on the wavelength (Vandaele et al., 2002;Bogumil et al., 2003), it is essential to precisely know the wavelength of the light source used in each instrument or to use the same light source in both instruments.At λ = 404 nm wavelength, NO 2(g) is highly absorbing with molecular absorption cross section (σ abs ) of 6.12×10 -19 cm 2 molecule -1 (Bogumil et al., 2003).However, at this wavelength NO 2 has a large quantum yield (Φ) of 0.44 (Troe, 2000) and it readily photolyases.Without accurate determination of the laser's power, it is difficult to quantify the photolysis.At the same wavelength, O 3 has the advantage of being stable with Φ approaching zero (Bauer et al., 2000) and it is easily produced on-site or in the laboratory.The disadvantage of O 3 for PAS calibration at λ=404 nm, is that its σ abs is about 4 orders of magnitude lower than that of NO 2 .Different studies reported a wide range of σ abs for O 3 at λ = 404 nm ranging from 1.5×10 -23 to 6.3×10 -23 cm 2 molecule -1 (Burrows et al., 1999;Voigt et al., 2001;Bogumil et al., 2003;Axson et al., 2011) (Figure 1).For this reason, O 3 calibration requires very high concentration (in the order of 100's to 1000's ppmv) which may cause equipment degradation.An additional concern is that at concentrations in the order of 1000's ppmv, O 3 may change the F r of the PAS cell.The extent of this effect depends on the O 3 concentration and on the instruments' sensitivity to gas composition i.e. the Q, and it can also be easily calculated using a simple thermodynamic model for the speed of sound.In such a case the laser modulation frequency should be adjusted to the new F r value.
An alternative calibration method is to use a standard aerosol with well-known absorption properties.PAS calibration using size selected light absorbing particles requires a standard material with accurate information of its complex refractive index at the instrument's wavelength, which is not widely available.This procedure is also time consuming in comparison to the use of a light absorbing gas and may be more difficult to implement on field and aircraft applications.Lack et al., (2012b), reported the development of the current PAS instrument.They calibrated their PA-CRD-S (PAS coupled to a CRD-S) cells at 405 nm and at 532 nm with O 3 and commented that NO 2 calibration at 405 nm is possible using a photolysis correction factor for the CRD-S measurements.Several other publications used the same instruments as in Lack et al., (2012b) using O 3 calibration procedure (Cappa et al., 2012;Lack et al., 2012a;Lambe et al., 2013).
Using a similar PAS cell as Lack et al., (2012b), we attempted to measure α abs and extinction coefficients (α ext ) of brown carbon (BrC) proxy materials using the PA-CRD-S following calibration of the PAS using O 3 .The results yielded very high α abs values which were not consistent with other measurements.Therefore, we developed a reliable procedure to calibrate the PAS instrument using light absorbing particles produced in the laboratory with a widely available water soluble absorbing materialnigrosin.In this study, we describe the details of this procedure which includes high accuracy measurement of the nigrosin complex refractive index (RI) using spectroscopic ellipsometry.We also show that there are significant differences between the PAS response curve calculated using nigrosin particles and the PAS response curve calculated using O 3 .

Methodology Photoacoustic aerosol spectrometer
The multi-pass astigmatic PAS cell that is used in this work is described in Lack et al., (2012b) and only a brief description is given here.It is composed of dual half-wavelength resonators (11 cm long, 1.9 cm diameter) capped on either end with 1/4 wavelength acoustic notches.The total sample cell volume is 185 cm 3 .While both resonators are open to the sample flow, only one is exposed to the modulated laser light; the other is planned for noise cancellation.
Microphones are placed at the antinode of the sound wave in the center of each resonator and the speaker is placed at the background resonator.The F r of the system is found by producing 1 sec segments of white noise using the speaker located in the reference resonator.Each segment is sampled by the microphones at a 100 kHz rate and the F r is found by performing a Fast Fourier through the PAS cell through two 1 mm thick windows (CVI Laser, Albuquerque, NM, USA), each with a high transmissivity (T > 99.5%) antireflective coating.

Cavity ring down spectrometer
A detailed description of the CRD-S method for aerosol light extinction measurement may be found in Pettersson et al., (2004), Abo Riziq et al., (2007), Smith and Atkinson, (2001), Bluvshtein et al., (2012) and references therein.The CRD-S used in this study differs from the one described in a previous publication (Bluvshtein et al., 2012).Here the laser modulation rate is varied to meet the PAS cell F r .The cavity length and the aerosol filled length were extended to 95 and 80 cm respectively.Additionally, the gas/aerosol inlet to the cavity was moved to the center of the cavity with two outlets at the cavity sides (Figure 3) from which the gas/aerosols are pulled out.In this configuration the uncertainty associated with the ratio of cavity length to aerosol filled length is reduced significantly regardless of flow conditions (i.e.ratio of sample flow to mirror purge flow and cavity inner diameter).Discussion on errors associated with R L uncertainty may be found in Miles et al., (2011) and Toole et al., (2013).

Photoacoustic aerosol spectrometer coupled to a cavity ring down aerosol spectrometer
The photoacoustic aerosol spectrometer coupled to a cavity ring down aerosol spectrometer (PA-CRD-S) described in this section (Figure 3) is composed of a 110 mW 404 nm diode laser (iPulse, Toptica Photonics, Germany) modulated in the measured PAS resonance frequency at 50% duty cycle.The laser beam is split into two separate optical paths (CRD-S and PAS) using a variable polarized beam splitter.The beam splitter is composed of a quarter waveplate (¼λ) and a polarizing beam splitter (PBS).With the current setup, turning the ¼λ between 0 and 90° varies the intensity ratio between the two optical paths from 0:1 to 1:1 CRD-S to PAS optical path, respectively.The beam directed to the PAS is turned and aligned into the PAS cell through a set of two plano-convex lenses (focal lengths of 30 mm and 50 mm) used as a telescope in order to collimate the beam into a diameter of about 1.5 mm.The beam, directed to the CRD-S, passes through another ¼λ plate, which together with the PBS serves as a variable attenuator protecting the laser head from the beam reflected backwards by the CRD-S highly reflective mirror.This back-reflected beam (dashed arrow in Figure 3) is transmitted through the PBS and is detected by a photodiode and is used as an external trigger source for the CRD-S intensity decay measurement.The forward beam is then turned and aligned into the CRD-S cavity by a set of turning mirrors.While the PAS sensitivity depends on the laser power, the CRD-S system requires only the minimal laser power needed by the photodiode.This allowed us to divert approximately 78% of the laser power (about 86 mW) to the PAS cell and thus optimize its sensitivity.

PAS calibration
To calibration the PAS cell, gas flow was pulled and split between the PAS and the CRD-S at a flow ratio of 3:1 to equal the volume ratio of the two instruments.

Measurement of the complex refractive index of nigrosin by Spectroscopic ellipsometry
To infer the complex refractive index (RI) of nigrosin at λ = 404 nm from ellipsometry measurements, five silicon wafers (surface area -4 cm 2 ) with 300 nm of silicon thermal oxide (Virginia Semiconductor, VA, U.S.A) were coated with nigrosine using concentrated aqueous solutions and a spin coater (WS-400A-6NPP/LITE; Laurell Technologies Corporation, PA, U.S.A).The concentration of the nigrosin solution was 1.25 and 1.5 times the room temperature solubility limit of nigrosin (10 gr L -1 ) and was kept at 40°C under constant stirring to maintain solubility.Spin coating was done in two stages, a coating stage and a drying stage.During the coating stage each sample was covered by the nigrosin solution and the spinning was done at 100, 500 or 700 RPM under dry N 2 flow for 14 minutes.The drying stage was performed at 3500 rpm for 1 minute in order to reach complete dryness and to remove liquid droplets adhering to the wafer edges.Dry N 2 flowed from below the wafer stage in an upward direction so it will not affect the liquid spreading on the wafer.
Spectroscopic ellipsometry is a proven method to determine thin film thickness and complex refractive index (m = n + ik) of materials (Fujiwara, 2007).Briefly, ellipsometry uses Spectroscopic ellipsometry measurements were performed on the five film samples (described above) using a J.A. Woollam M-2000 DI ellipsometer in the spectral range of 193 nm to 1700 nm at angles of incidence of 55°, 65°, and 75°.The instrument was pre-calibrated with a calibration wafer to minimize systematic errors that are related to angle, wavelength and delta offsets.In case of light absorbing materials k is often correlated with the sample thickness.To overcome this issue, we employed the interference enhancement technique to improve sensitivity to light absorption as described in Hilfiker et al., (2008).The resulting film optical constants were evaluated by comparison with the optical constants obtained from a simultaneous analysis of all five samples (multi-sample analysis; MSA).With the two methods, we obtained high sensitivity to light absorption.The Kramers-Kronig consistent complex refractive index of the nigrosin films was modeled using five Gaussian oscillators along with a Sellmeier function.The best-fit model was determined by the Levenberg-Marquardt regression algorithm and tested for both statistical errors and model systematic errors.Statistical errors were estimated by the Bootstrap re-sampling method (Rosa, 1988) and the model systematic errors were estimated using the difference between the measured data and the best-fit model generated data.

PAS calibration with measurements of nigrosin aerosol
A nigrosin solution was atomized, and the resulting aerosol dried, size selected (250 nm to 325 nm at 25 nm steps) (Bluvshtein et al., 2012;Flores et al., 2012a) and the absorption signal was measured with the PAS instrument at several number concentrations (counted by a condensation particle counter; CPC).Size selection was performed using an electrostatic classifier (3080L, TSI, MN, U.S.A) equipped with an impactor (nozzle diameter of 457 µm).
Sample flow was set between 1 to 0.7 LPM such that the 50% cut-off diameter of the impactor was 50 nm above the selected size.The impactor was used to reduce multiply charged particles contribution.The signal of the PAS was compared to the aerosol α abs calculated using Mie theory algorithm from the complex RI retrieved from the dry film SE measurements together with the particles number concentration.

Results
Figure 4 shows the result of a single sample and a multi-sample analysis of the spectroscopic ellipsometry complex RI retrieval, at 300 nm to 800 nm range.The single sample analysis shown was performed on the sample with the thickest retrieved nigrosin film (137.2 ± 0.3 nm, coated with 15 gr L -1 nigrosin solution at 100 RPM).The imaginary part from the SE analysis is in good agreement with the imaginary part calculated from aqueous solution UV-Vis absorption measurement (Sun et al., 2007).The density of nigrosin for this calculation was taken as 1.6 gr cm -3 (Moteki et al., 2010).The complex RI of nigrosin at λ = 404 nm was determined by the spectroscopic ellipsometry analysis to be m = 1.624 (±0.008) + i 0.154 (±0.008).The summed precision and accuracy of the retrieved complex RI are about 0.5% for n and 5% for k. Figure 4 also shows previously published complex RI values for nigrosin retrieved at 532 nm and 355 nm wavelengths using CRD-S (Lack et al., 2006;Dinar et al., 2008;Lang-Yona et al., 2009;Bluvshtein et al., 2012;Flores et al., 2012a).Such wide spread of complex RI values emphasizes the need for a more accurate measurement for future use of nigrosin as a standard material, and the limitations of the CRD method, that can benefit from a new well-established standard.
To further verify the validity of the new calibration approach we have used Pahokee peat fulvic acid (PPFA) and Suwannee river fulvic acid (SRFA) which are often used as a proxy material for atmospheric brown carbon due to their complex organic composition and their UV-Vis absorption spectrum.In an accompanying paper Bluvshtein et al., showed that the mass absorption cross section (MAC) of PPFA and SRFA, calculated from UV-Vis aqueous solution absorption spectrum, is within the value range of the MAC calculated for ambient water soluble organic aerosol collected during a biomass burning event.
Size selected PPFA and SRFA particles were measured with the PA-CRD-S and the complex RI was retrieved from the CRD-S measurements using a Mie theory algorithm taking into account the multiply charged particles (MCP) contribution (Flores et al., 2012a;Washenfelder et al., 2013;Bluvshtein et al., 2016).The imaginary part of the complex RI was also calculated from UV-Vis aqueous solution absorption measurement using material density estimation of 1.1 to 1.3 gr cm -3 .Our best estimation of the complex RI of PPFA and SRFA at λ = 404 nm are m = 1.699 (±0.012) + i 0.036 (±0.010) and m = 1.685 (±0.020) + i 0.013 (±0.010) respectively.This information together with the measured particle number concentration and MCP contribution was used to calculate α abs using Mie theory.Calculated α abs of PPFA and SRFA are plotted against the measured PAS signal in Figure 5.In addition, Figure 5 shows an O 3 calibration curve with a slope of 4.975×10 -7 cm -1 V -1 and a nigrosin calibration curve with a slope of 2.533×10 -7 cm -1 V -1 .Figure 5 clearly demonstrates that the PAS response curves calculated for the three types of organic aerosols agree with each other, while the slope of the response curve produced with O 3 over-estimates the instrument's response by a factor of about two.This implies that measurements of aerosols α abs at λ = 404 using PAS calibrated with O 3 may be significantly over estimated.
With a parallel flow configuration, higher loss of O 3(g) molecules to the PAS (aluminum) walls in comparison to the CRD-S walls (stainless steel and lower surface to volume ratio) would result in an underestimation of the PAS response and an overestimation of the calibration slope.
A similar artifact could result from reaction of the O 3(g) with residual aerosol material on the CRD-S walls, producing ultra-fine light scattering particles.These particles, if produced would increase the CRD-S α ext and cause an overestimation of the calibration slope.Repetitions of the calibration procedure in tandem flow configuration, linearity and repeatability of the calibration curve and the stability of the CRD-S signal ruled-out these affects as possible causes for the overestimation of the PAS response due to the O 3(g) calibration procedure.
Additionally, we did not find any literature information regarding significant energy relaxation processes following UV-Vis light absorption by O 3(g) which do not involve thermal conversion.

Conclusion
In this study we demonstrate a new calibration for PAP instrument using nigroisn, a widely available water-soluble absorbing material.We have derived the complex refractive index of nigrosin throughout the UV and visible range using spectroscopy ellipsometry and suggest that it can now be used as a standard material to calibrate PAS instruments at the UV-Vis-NIR wavelength range for measurements of light absorbing aerosols.Nigrosin can also be used to validate other chosen PAS calibration procedures.Our measurements also imply that calibration of PAS with O 3 at 404 nm may lead to over-estimation of light absorption by aerosol.
Figure2were F r is the frequency at the peak of the fitted Lorentzian curve.Typical Fr and Q values for our instrument are in the range of1360-1385 Hz and 40-50 (unitless), respectively, over the pressure (97-101 kPa), relative humidity (RH; 0 -11% RH) and temperature (20 to 24 °C) ranges, and with two carrier gases (N 2 or synthetic air).The instrument described inLack et al., (2012b) produced an F r in the range of 1320 to 1360 Hz, Q in the range of 50 to 90 over pressure and temperature ranges of 20 to 90 kPa and 12 to 23 o C when dried air was used as carrier gas.The astigmatic optical configuration consists of two high reflectivity mirrors (ARW Optical, Wilmington, NC, USA; dielectric coating R > 99.5%), 1.5" diameter, spaced 35 cm apart.The laser side mirror has a cylindrical radius of curvature of 43 cm and a 2 mm hole drilled in the center.The back mirror has a cylindrical radius of curvature of 47 cm, and is rotated 90° to the radius of curvature of the laser side mirror.Astigmatic alignment is achieved by aligning the laser through the 2 mm hole drilled in the center of the first mirror and on to an off-center target on the second mirror.Each following reflection is also directed to an off-center target on the opposite mirror.Each mirror was placed on kinematic mirror mounts for easy alignment (KM200, with an AD2-1.5 adaptor; Thorlabs, U.S.A).The PAS cell is mounted within the path of the laser multi-pass and is covered by 50 mm thick acoustic foam.The laser light passes Atmos.Meas.Tech.Discuss., doi:10.5194/amt-2016-323,2016   Manuscript under review for journal Atmos.Meas.Tech.Published: 8 November 2016 c Author(s) 2016.CC-BY 3.0 License.
O 3 was generated by a constant flow of high purity (99.999%) O 2 through a UV lamp O 3 generator (model 300, Jelight Company, Inc. CA, U.S.A) for up to 800 ppm O 3 and through a corona discharge ozone generator (model L21, Pacific Ozone, CA, U.S.A) for up to 4000 ppm O 3 .The O 3 out flow was mixed with dry N 2 to a 90% N 2 10% O 2 /O 3 mixture.The O 3 concentrations were varied by adjusting the height of the cover glass of the UV lamp O 3 generator and by adjusting the voltage gauge of the corona discharge ozone generator.At each gas concentration, data were acquired at a rate of 1 Hz and averaged over intervals of two minutes.
Atmos.Meas.Tech.Discuss., doi:10.5194/amt-2016-323,2016   Manuscript under review for journal Atmos.Meas.Tech.Published: 8 November 2016 c Author(s) 2016.CC-BY 3.0 License.polarized light to characterize thin film and bulk materials.A change in the electric field amplitude and phase for p-and s-polarizations is measured after reflecting light from the surface.Thin film thickness and optical constants (n and k) are derived from the measurement.

Figure 2 :
Figure 2: Fast Fourier transform (FFT) resultant power spectra with different carrier gas composition and O 3 concentration.O 3 was measured downstream to the PAS using the CRD-S assuming O 3 σ abs of 1.5x10 -23 cm 2 molecule -1 from Axson et al., (2011).

Figure 3 :
Figure 3: Schematic of the photo-acoustic spectrometer (PAS) coupled to a cavity ring down (CRD) spectrometer (PA-CRD-S).Abbreviations: PBS, polarizing beam splitter; PD, photodiode; PMT, photomultiplier tube.Small black arrows indicate the entrance of the purge flows, and the thinker black arrows the direction of the aerosol flow (Bluvshtein et al., 2016).

Figure 4 :
Figure 4: Complex RI results of spectroscopic ellipsometry measurements of nigrosin coating on Si oxide.Also shown are results of imaginary part calculated from aqueous UV-Vis measurements based on Sun et al., (2007) with density value of 1.6 gr cm -3 (Moteki et al., 2010).

Figure 5 :
Figure 5: PAS O 3 calibration curve and regression (gray), nigrosin calibration curve and regression based on SE analysis (black circles and line) and PPFA and SRFA based on complex RI retrieval from CRD-S measurements and aqueous UV-Vis measurements.