Raman detection of single airborne aerosol particles of isovanillin

Single airborne aerosol particle of isovanillin were detected using a compact Raman spectroscopy system. The Raman system consisted of a 10 W, 532-nm cw laser, a 50x aerosol concentrator, an aerosol flow cell, an f/1.0 single-sided collection optics, an f/1.8 Raman spectrometer with a spectral range of 400-1400 cm-1, and a low-noise CCD camera (1340 x 400 pixels; 20 x 20 μm/pixel). The combined collection and detection efficiency of the Raman system was 1.0%. The diameters of eleven particles were determined to be 3.4, 3.1, 3.5, 3.4, 2.3, 3.1, 2.5, 2.6, 2.5, 3.0, and 3.1μm based on the fundamental Raman equation. The accuracy of the particle diameter is estimated to be ±0.1 μm using measured concentration of the atmospheric CO2. © 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). [http://dx.doi.org/10.1063/1.4984226]


INTRODUCTION
2][3][4] However, detection of single aerosol particles via Raman spectroscopy in a flowing system has not been yet reported.In this paper, we describe the first detection of single 3 µm flowing airborne aerosol particles flowing through a Raman system, which is a simplified version of the previously reported system 5 with a 532-nm, 10W cw double-pass laser, 532-nm isolator, and double-sided collection optics.The current system has single-pass laser, no 532-nm isolator, and single-sided collection optics.[8]

EXPERIMENTAL
Isovanillin aerosol particles were generated with a small household blender.Details of the Raman collection system have been given previously. 5The laser power was 10 W in the laser beam waist of 50 µm, which was determined by the power transmitted through a pinhole.The transit time τ of the aerosol particle through the laser beam is equal to the diameter of the laser beam waist divided by the aerosol particle speed v.The value of v was determined to be 2.4 cm/s as the ratio of the flow rate of 10 mL/min or 1/6 mL/s and the input cross section of 0.071 cm 2 corresponding to the 3 mm diameter of the inlet into the flow cell.The value of τ is then equal to 2.1 ms.
Figure 1 shows a schematic of the current Raman system used for the present experiment.
The collection efficiency of the current system is about ½ of that in Ref. 5. The detection sensitivity of the current system is about ¼ of that in Ref. 5. The Raman signal scales as the cube of the particle diameter.Therefore, the detection sensitivity of the current system in terms of the particle diameter is only (1/4) 1/3 = 0.63 of that in Ref.

RESULTS AND DISCUSSION
Figure 2 shows two single isovanillin aerosol single particles, which were observed in a 1340x240-pixel region of interest of the CCD camera in an integration time of 10 s.The integration time of 10 s is the acquisition time of the CCD.However, the Raman signal from each particle is only coming from an exposure time of 2.1 ms.Only two particles were observed in the acquisition time of 10 s using a cw laser and a CCD camera but not a fast camera or a single laser pulse.The input mass concentration of isovanillin, as measured by the TSI (Shoreview, MN 55126, USA) optical particle sizer (OPS) model 3330, into the Raman system was ∼ 36 pg/cm 3 .Raman spectra were collected in the low concentration range of isovanillin in order to avoid the overlap of single particles on the CCD in the high concentration range.Raman modes of isovanillin are observed as bright dots at x-pixel #124 (507 cm -1 ), #875 (1116 cm -1 ), #1058 (1242 cm -1 ), and #1110 (1275 cm -1 ).Raman modes of CO 2 9 are observed as slightly curved lines at x-pixel #1094 (1265 cm -1 ), 1125 (1285 cm -1 ), and #1291 (1388 cm -1 ).Raman modes of H 2 O vapors 10 are also observed as slightly curved lines at x-pixel #55 (443 cm -1 ), #70 (457 cm -1 ), #101 (485 cm -1 ), #123 (506 cm -1 ), #179 (557 cm -1 ), #234 (606 cm -1 ), #288 (653 cm -1 ), and #345 (702 cm -1 ).
Figure 3 shows the Raman spectrum of isovanillin particle #1and atmospheric CO 2 at y-pixels in the #109-123 region centered at pixel #116.The background-corrected Raman signal R s of the 1116 cm -1 mode of isovanillin is 918 photons integrated over the 1110.5-1121.1 cm -1 region.The background-corrected Raman signal of 1285 cm -1 mode of CO 2 is 2090 photons integrated over the 1280.5-1288.9cm -1 region.
The diameter of isovanillin particle can be determined using the Raman signal of the 1116 cm -1 mode and the basic Raman equation Here η c is the collection efficiency; η d is the CCD detection efficiency; T s is the Raman system transmittance; σ R (cm 2 ) is he Raman cross section of the 1116 cm -1 mode of isovanillin; N m is the number of isovanillin molecules (cm -2 ); P L is the laser power incident upon the isovanillin particle; τ is the transit time of the isovanillin particle through the laser beam; hν L is the laser photon energy.Using values of 5.1x10 -2 for η c for f /1.1 single-sided collection, 0.42 (measured) for η d , 0.48 (calculated) for T s , 3.3x10 -28 cm 2 for σ R , 3.18x10 17 d cm -2 for N m (d is the diameter of the isovanillin particle in units of µm), 4.0x10 -3 d 2 W for P L , 2.1x10 -3 s for τ, and 3.73x10 -19 J for hν L , Eq. (1) becomes Using a value of 918 photons for R s , we obtain a value of 3.4 µm for the diameter d of particle #1.
Figure 4 shows the Raman spectrum of particle #2.The value of R s for particle #2 is 738 photons, which yields a value of 3.1 µm for its diameter.
Figure 5 shows the Raman spectrum of particle #3.The value of R s for particle #3 is 1035 photons, which yields a value of 3.5 µm for its diameter.
Figure 6 shows the Raman spectrum of particle #4.The value of Rs for particle #4 is 947 photons, which yields a value of 3.4 µm for its diameter.
Raman spectra of other particles #5-11 are similar to those of Figs.2-5.The diameters of particles #4-11 were determined to be 2.3, 3.1, 2.5, 2.6, 2.5, 3.0, and 3.1 µm, respectively.Table I lists the values of R s for the 1116 cm -1 mode of isovanillin and particle diameter for all the eleven particles reported here.
The accuracy of the particle diameter is estimated to be ±0.1 µm based on the accuracy of the measurement of the CO 2 concentration as discussed below.The accuracy of d is similar in magnitude based on the angle-and size-dependent characteristics of Raman scattering by microspheres. 11he basic Raman equation for the 1285 cm -1 mode of CO 2 is Here σ R (cm 2 ) is the Raman cross section of the 1285 cm -1 mode of CO 2 ; N m (cm -3 ) is the molecular concentration of CO 2 ; L is the length of the laser beam that is used in Fig. 2. Using values of 5.1x10 -2 for η c , 0.42 for η d , 0.48 for T s , 4.5x10 -30 cm 2 for σ R , 187.5 µm for L, 10 W for P L , 10s for τ, and 3.73x10 -19 J for hν L , Eq. ( 3) is written as Using the value of 2090 for R s in Fig. 2, Eq. ( 4) yields a value of 9.0x10 15 cm -3 for N m , which corresponds to 367 ppm compared with the value of 470 ppm measured by a CO 2 probe located outside the Raman system.The Raman concentration of 367 ppm is 22% lower than the value of 470 ppm measured by the CO 2 probe.We assume the accuracy of the CO 2 concentration to be ±22%, which corresponds to an accuracy of ±2.8% in the diameter of the isovanillin aerosol particles because the Raman signal of a particle scales as the diameter of the isovanillin particle.Hence, the accuracy in the diameter of the aerosol particles is estimated to be ±0.1 µm.The estimated accuracy of the particle diameter is independent of the accuracy of the collection efficiency η c , CCD detection efficiency η d , and the Raman system transmittance T s because the same values of η c , η d , and T s are used both in the determination of the particle diameter and the concentration of CO 2 .
The mass concentration of isovanillin may also be determined with the basic Raman Eq. ( 3) as used for CO 2 .Using values of 3.3x10 -28 cm 2 for σ R of isovanillin, and 0.3 cm for L, the volume concentration of isovanillin is given by R s = 2.73 × 10 −10 N m (5)   Using the value of 1656 for R s photons, which is equal to the sum of the Raman signals of 918 and 738 photons for particles #1 and #2, respectively, Eq. ( 5) yields a value of 6.06x10 12 molecules/cm 3 for the concentration of isovanillin in the Raman system.The mass of an isovanillin molecule is 2.53x10 -10 pg.The mass concentration of isovanillin inside the Raman system was 1.53x10 3 pg/cm 3 , which corresponds to the input mass concentration of 31 pg/cm 3 considering the aerosol concentration factor of ∼ 50 for the aerosol concentrator in the Raman system.This value of 31 pg/cm 3 is consistent with an input mass concentration of ∼36 pg/cm 3 measured by the OPS.This implies that the diameters of the isovanillin particles measured by the Raman system are consistent with those measured by the OPS.

SUMMARY
One, two, or three single particles of isovanillin were detected in 10 s by Raman scattering spectroscopy.The diameters of eleven particles were determined to be 3.4, 3.1, 3.5, 3.4, 2.3, 3.1, 2.5, 2.6, 2.5, 3.0, and 3.1µm with an accuracy of ±0.1 µm using the basic Raman equation.Also, the concentration of CO 2 inside the Raman system was determined to be 367 ppm, which is 22% lower than the value 470 ppm measured by a CO 2 probe located outside the Raman system.

TABLE I .
Raman signal and diameter of eleven isovanillin particles.