Absorption/transmission measurements of PSAP particle-laden filters from the Biomass Burning Observation Project (BBOP) field campaign

Figures & data

Figure 1. (a) Flight path over Memphis, TN for Filter No. 1 (heaviest loading). The “S” represents initiation of the measurement. (b) Variation of the PSAP absorption coefficient with time (decimal value).

Figure 2. Schematic of the filter transmissivity experimental arrangement.

Figure 3. Schematic of the filter absorptivity experimental arrangement.

Table 1. Various relationship criteria that must be satisfied to assure proper determination of the particle absorption coefficient.

For changing filter case	For changing particle loading
p – isolated particle	l – lower loading
s – blank filter	h – higher loading
ps– particle-laden filter
|dT/dt|ps>|dT/dt|s	|dT/dt|ps,h>|dT/dt|ps,l>|dT/dt|s
τp>τs>τps	τs>τps,l>τps,h
ρs>ρps> (ρp≈ 0)	ρs>ρps,l>ρps,h
βps>βp>βs	βps,h>βps,l>βs
αps>αp>αs	αps,h>αps,l>αs

dT/dt: decay temperature-time derivative; t: transmissivity; ρ: reflectivity; β: absorptivity; α: absorption coefficient.

Figure 4. Magnified images of different PSAP filters under a microscope: (a) blank filter (M = 10) and (b) exposed filter (M = 10). M = microscope magnification. Insert is photograph of a particle-laden PSAP filter. Magnified images of a (c) larger pendent particle hanging from filter fibers (M = 20) and (d) smaller particles loading the filter fiber surface (M = 100).

Figure 5. Field-emission scanning electron microscope image of representative particles (for all images: electron beam energy: 10 keV, current: 130 pA, pixel size [nm]: HFW/1024 horizontal pixels [HFW: horizontal field width], secondary-electron mode, and Everhart–Thornley detector). The particle images are arranged in order of decreasing magnification. The magnification (M), and HFW for each respective image are: (a) M: 120 023 x, HFW: 1.07 μm; (b) M: 99 958 x, HFW: 1.28 μm; (c) 50,000 x, HFW: 2.56 μm; (d) M: 20,000 x, HFW: 6.40 μm. Spectra (e) is of a sample particle (see (d)). Sample sits on a germanium substrate. The map was obtained by rastering at 100 µs per pixel for 100×112 pixels per frame and averaging 300 frames. The electron beam energy was set at 20 keV and the current at 0.62 nA.

Table 2. LDTR results for Filter No. 1. Values for the input are the average of two repeated measurements. The subscript H.O.T. refers to higher order terms (for reflections off of the filter internal surfaces) for the transmissivity, τ (Bohren and Huffman 1983).(a) Input parameters.

Case	Blank filter	Particle-laden filter
Plas,abs [mW]	56.50	30.15
m [mg]	2.040 ± 0.001	2.060 ± 0.001
cp(To) [J·g^−1·K^−1]	0.770 ± 0.001	0.785 ± 0.001
dT/dt [K·s^−1]	−1.98 ± 0.03	−3.06 ± 0.04
A/V [m^−1]	1.023 × 10^−4	1.023 × 10^−4
To [K]	296.8 ± 2.4	297.3 ± 2.4
Tmax [K]	316.3 ± 2.4	325.6 ± 2.4
τ* [s]	9.85 ± 0.01	9.26 ± 0.01
Pinc,ext [mW]	180 ± 1	180 ± 1
Ptra,ext [mW]	0.603 ± 0.010	0.537 ± 0.010

(b) Optical properties and relationship criteria.

Case	Blank filter	Particle-laden filter	Isolated particles
τ	0.00335 ± 0.00001	0.00298 ± 0.00001	0.8909 ± 0.0001
β	0.0550 ± 0.0006	0.1640 ± 0.0018	0.1091 ± 0.0019
ρ	0.9417 ± 0.0006	0.8330 ± 0.0018	0.0000 ± 0.0019
(1− ρ)^2	0.0034	0.00279	0.9999
ρ(1− ρ)	0.0549	0.1391	0.0000
τ <(1− ρ)^2 [β > ρ(1 − ρ)]	0.000054	0.0249	0.1091
First H.O.T. < 1 ϵH.O.T. [Mm^−1]	8.59×10^−1 37.7 ± 0.001	1.01×10^−2 229.5 ± 0.001	—
ϵ[Mm^−1] ϵ' [Mm^−1]	1.652 ± 0.001	228.8 ± 0.1 215.3 ± 0.1	11.81 ± 0.01
αR = ϵps/αp	—	19.36

(c) Addition relationship criteria.

τp>τs	τs>τps	ρs>ρps	ρps>ρp	βps>βp	βp>βs	αps>αp	αp>αs
8.876×10^−1	4.0×10^−4	1.084×10^−1	8.33×10^−1	5.50×10^−2	5.41×10^−2	2.17×10^−4	1.02×10^−5

Table 3. PSAP results for Filter No. 1 (heaviest loading). The given transmissivity, τ, is the value at the cumulated time, t_c [s]. The area of the exposed portion for each filter was estimated to be (1.964 ± 0.014) × 10⁻⁵ m².(a) Input parameters.

τ462	τ523	τ648	Qavg [L·min^−1]	tc = Σ(Δtj) [s]	V = Σ(ΔVj) [m^3]
7.42 × 10^−1	7.85 × 10^−1	8.38 × 10^−1	1.61	7.26 × 10^3	1.92 × 10^−1

(b) Optical properties.

ln(II/It)462	α,462 [m^−1]	ln( II/It)523	α,523 [m^−1]	ln( II/It)648	α,648 [m^−1]
2.984 × 10^−1	3.052 × 10^−5	2.421 × 10^−1	2.476 × 10^−5	1.767 × 10^−1	1.808 × 10^−5

(c) Fitting parameters for τ_p = a₁ + a₂ e^−a₃λ.

Variable	PSAP			LDTR
λ	462 nm	523 nm	648 nm	1064 nm
τp	0.742	0.785	0.838	0.891
A*	0.2984	0.2421	0.1767	0.1155
a1	0.8978
a2	−1.69946
a3	0.00518

Figure 6. Variation of the absorption coefficient with wavelength for the different PSAP filters (Table 4). All four data points for each filter are fit to an exponential decaying function of the form: α= a₁ + a₂e^−a₃λ, see Table 5.

Table 4. Estimation of the particle absorption coefficient, absorption Angström exponent, and enhancement factor from the PSAP and LDTR measurements.

		Particle absorption coefficient [Mm^−1]
		PSAP^‡			LDTR*	Absorption Angström exponent	Enhancement factor LDTR*
Filter No.	Observed loading	λ = 462 nm	λ = 523 nm	λ = 648 nm	λ = 1064 nm	AAE462/1064/AAE462/648	αR = ϵps/αp
1	Heaviest	30.53	24.76	18.08	11.81 ± 0.01	1.138/1.549	19.23
2	Lighter	13.89	12.35	10.03	6.53 ± 0.01	0.905/0.962	20.07
3	Lighter	12.19	10.46	8.11	5.62 ± 0.01	0.928/1.205	23.55
4	Lighter	15.87	11.77	7.55	4.91 ± 0.01	1.406/2.196	21.77
5	Lighter	9.67	8.22	6.33	4.74 ± 0.01	0.855/1.252	25.45
6	Lighter	10.70	8.19	5.57	3.99 ± 0.01	1.182/1.930	24.50
7	Lightest	5.11	4.49	3.55	2.41/2.38 ± 0.01^†	0.901/1.077	27.23/62.01^†
8	Lightest	5.11	4.19	3.04	1.95/1.95 ± 0.01^†	1.155/1.535	30.38/46.46^†

* Based on the PSAP characteristic length.

^‡ The results reported from the PSAP are the corrected values, as stated in Bond et al. (1999). Estimation of the uncertainty is not available because the transmissivity uncertainty was not provided with the data set. An estimation from the literature (NOAA 2016) is given as about 0.15.

^† Satisfying the criterion for [β_ps > β_s > β_p]/for [β_ps > β_p > β_s].

Table 5. Values of the fitting parameters for the exponential decaying function in Figure 6.

Filter Number	a1	a2	a3
1	1.1174 × 10^−5	2.55821 × 10^−4	0.00559
2	5.33981 × 10^−6	3.81024 × 10^−5	0.00324
3	5.23111 × 10^−6	6.20096 × 10^−5	0.00473
4	4.82094 × 10^−6	3.619 × 10^−4	0.00755
5	4.58941 × 10^−6	7.12791 × 10^−5	0.00571
6	3.9472 × 10^−6	2.3007 × 10^−4	0.00764
7	2.15829 × 10^−6	1.90158 × 10^−5	0.00402
8	1.83572 × 10^−6	3.92953 × 10^−5	0.00538

Bohren, C. F., and Huffman, D. R. (1983). Absorption and Scattering of Light by Small Particles, Wiley, New York, pp. 28–41, 77–81.

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Bond, T. C., Anderson, T. L., and Campbell, D. (1999). Calibration and Intercomparison of Filter-Based Measurements of Visible Light Absorption by Aerosols. Aerosol Sci. Technol., 30(6):582–600.

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NOAA. (2016). Description of Particle Soot Absorption Photometer (PSAP). National Oceanic and Atmospheric Administration, Earth System Research laboratory, Global Monitoring Division, Available at http://www.esrl.noaa.gov/gmd/aero/instrumentation/psap.html.

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