Characterization of light-absorbing aerosols from a laboratory combustion source with two different photoacoustic techniques

Figures & data

Figure 1. A schematic plot of the experimental setup and measurement instruments.

Table 1. Specifications of Multi-PAS III and RGB-DPAS photoacoustic instruments.

Physical Properties	Photoacoustic cell
	Multi-PAS III	RGB-DPAS
Cell type	Cylindrical	Cylindrical
Cell length (cm)	11	10
Resonance frequency (Hz)	∼1370	∼1670
Quality factor	∼30	∼18
Laser type	Diode laser modules	Diode-pumped solid-state lasers
Laser power after amplitude modulation (mW)	80 (532 nm), 100 (422 and 781 nm)	33 (473 nm), 84 (532 nm), 69 (671 nm)
Beam diameter at the aperture of 1/e^2 (mm)	2	∼1.5
Microphone	Optoacoustic microphone	Si-based MEMS (Micro-Electro-Mechanical System) microphone array
Data processing	Fast Fourier-Transform	Fast Fourier-Transform

Figure 2. Comparison of the AAE values obtained from the Multi-PAS III and normalized equivalence ratio. Symbols are colored by Multi-PAS III measured absorption coefficient at 532 nm (b_{abs,532,Multi-PAS III}). Error bars represent measurement uncertainties (see Uncertainty Analysis in SI).

Figure 3. Comparison of the measured absorption coefficients from the Multi-PAS III and single-pass RGB-DPAS. Black dotted line is 1:1 reference line. The Purple dotted line is the linear fitting (b_{abs,532,RGB-DPAS} = 1.005 b_{abs,532,Multi-PAS III}, R² = 0.9437). Symbols are colored by Multi-PAS III measured AAE (AAE_{Multi-PAS III}). Error bars represent measurement uncertainties (see Uncertainty Analysis in SI).

Figure 4. Comparison of the AAE values obtained from the Multi-PAS III and single-pass RGB-DPAS. Black dotted line is 1:1 reference line. The Purple dotted line is the linear fitting (AAE_RGB-DPAS = 1.021 AAE_{Multi-PAS III}, R² = 0.9171). Symbols are colored by Multi-PAS III measured absorption coefficient at 532 nm (b_{abs,532,Multi-PAS III}). Error bars represent measurement uncertainty (see Uncertainty Analysis in SI).

Figure 5. Comparison between measured and predicted the absorption coefficients at (a) 422 nm and (b_{abs,422,Multi-PAS III}) and (b) 781 nm (b_{abs,781,Multi-PAS III}) for Multi-PAS III, and (c) 473 nm (b_{abs,473,RGB-DPAS}) and (d) 671 nm (b_{abs,671,RGB-DPAS}) for RGB-DPAS. Predicted absorption coefficients at each wavelength for Multi-PAS III or RGB-DPAS are calculated using obtained the absorption coefficients at 532 nm on the instrument and the AAE values of the other instrument (e.g., b_{abs,Multi-PAS III} (λ) = b_{abs,532,Multi-PAS} (532/λ)^AAERGB-DPAS). Black dotted line is 1:1 reference line. The Purple dotted line is the linear fitting [(a) b_{abs,Multi-PAS III,422,predicted} = 0.9838 b_{abs,Multi-PAS} III,422, R² = 0.8493; (b) b_{abs,Multi-PAS III,781,predicted} = 0.8761 b_{abs,Multi-Pas III,781}, R² = 0.9580; (c) b_{abs,RGB-DPAS,473,predicted} = 0.9722 b_{abs,RGB-DPAS,473}, R² = 0.8923; (d) b_{abs,RGB-DPAS,671,predicted} = 0.9547 b_{abs,RGB-DPAS,671}, R² = 0.9921]. Symbols are colored by Multi-PAS III measured AAE (AAE_{Multi-PAS III}). Error bars represent measurement uncertainties (see Uncertainty Analysis in SI).

Figure 6. Calibration of the Multi-PAS III instrument with NO₂ and air-borne Cab-O-Jet aerosols. Y axis represents the obtained normalized signal responses (microphone signal/photodiode signal). X axis represents the expected light absorption coefficient.

Figure 7. Comparison of carbonaceous aerosol AAE values retrieved from the Multi-PAS III measurements based on NO₂ calibration and those based on Cab-O-Jet calibration. Symbols are colored by b_abs,532 values measured by the Multi-PAS III based on Cab-O-Jet calibration. Error bars represent measurement uncertainties (see Uncertainty Analysis in SI).