Humidified single-scattering albedometer (H-CAPS-PM_SSA): Design, data analysis, and validation

Figures & data

Table 1. Summary of studies relevant to absorbing aerosols, aerosol single scattering albedo, and relative humidity (RH) dependence.

Study	Technique	Measured parameters	Wavelength(s) (nm)	Measured RH dependence?
Garland et al. (2007)	Cavity Ring Down Spectrometer (CRDS)	σep	532	Yes
Baynard et al. (2007)	CRDS	σep	355, 532, 683, 1064	Yes
Langridge et al. (2011)	CRDS	σep	405, 532, 662	Yes
Zhao et al (2017)	Broadband Cavity Enhanced Aerosol Extinction Spectrometry	σep	461	Yes
Massoli et al. (2009)	CRDS + Photoacoustic Spectrometer	σep	355, 532, 1064	Yes
Carrico et al. (2003)	Particle Soot Absorption Photometer + Nephelometer	σap	565	No
		σsp	450, 550, 700	Yes
		SSA	550	In part
Zhou et al. (2020)	Cavity Enhanced Albedometer	σep, σsp, SSA	532	Yes
Brem et al. (2012)	Short-Path Extinction Cell Nephelometer	σep, σsp	467, 530, 660	Yes
Michel Flores et al. (2012)	CRDS	σep	355, 532	Yes
This study	Cavity Attenuated Phase Shift (CAPS)	σep, σsp, SSA	450	Yes

Figure 1. Experimental flow diagram showing RH controlled cavity attenuated phase shift instrument (H-CAPS-PM_SSA). Lower detail shows the interior of the humidifier consisting of a liquid water jacket surrounding the interior aerosol sample flow. Water jacket and aerosol flow are separated by a semi-rigid water-vapor permeable membrane.

Table 2. Typical profile of RH and temperature through H-CAPS-PM_SSA system (arithmetic means and standard deviations of 5-s data averaged over 42 RH scans, n = 22,735).

	T1 (°C)	T2 (°C)	T3 (°C)	T4 (°C)	ΔT3–4 (°C)	ΔRH3–4 (%)
	Inlet	Humidifier	CAPS inlet	CAPS outlet	Across cell	Across cell
Low RH pathway
Mean	22.81	25.54	24.58	24.64	0.06	0.85
StDev	0.26	0.14	0.22	0.19	0.10	0.96
High RH pathway
Mean	22.77	25.60	24.53	24.67	0.14	–1.11
StDev	0.23	0.14	0.21	0.18	0.11	0.58

Figure 2. RH during an RH scan upward as measured immediately upstream and downstream of the CAPS optical cell.

Table 3. Comparison of light extinction, scattering and absorption coefficients (σ_ep, σ_sp, and σ_ap, respectively) at λ = 450 nm of filtered air at low and high RH (n = 297, 5-s data).

	RH < 20%
(1/Mm)	σep	σsp	σap
Mean	–0.06	0.52	–0.58
StDev	0.91	0.83	1.16
	RH = 80%
Mean	1.01	0.50	0.50
StDev	1.84	0.81	1.90

Figure 3. (a) PSL truncation-corrected measured particle loss as a function of diameter (n = 8 from 50 nm < D_p < 1000 nm). Agreement is expressed as ratios of wet to dry extinction and scattering coefficients (with PSL ratio expected as 1.0 due to negligible water uptake). (b) PSL truncation corrected single scattering albedo (SSA) as a function of PSL D_p. Means and standard deviations of ratios of data averaged over ∼5 min (n > = 30) and then ratioed during stable aerosol generation periods. The magnitude of the extinction coefficient for each size is shown as gray bars along the bottom axis where small signals at the extremes of D_p contributed to high variability.

Figure 4. Truncation error effects for CAPS light scattering illustrated by the decrease in uncorrected measured single scattering albedo (SSA) with electrical mobility D_p for a purely scattering ammonium sulfate aerosol.

Figure 5. Repeated measurements of light extinction and scattering with deliquesced ammonium sulfate during eight increasing RH scans (∼20% < RH < 85%) in comparison with simulated Mie calculations. The min and max represent calculations with the uncertainties in input parameters (size, concentration, refractive index, diameter growth factors, and RH).

Figure 6. Comparison of f(RH) for light scattering and extinction for pure ammonium sulfate (dry conditions RH < 40%).

Table 4. Best fit values for optical κ_ep and κ_sp (averaged for RH > 70%) for pure ammonium sulfate as a function of dry (RH < 40%) size (absorption is negligible).

Dp (nm)		κep	κsp
100	Mean	1.34	1.33
	StDev	0.05	0.05
200	Mean	0.87	0.86
	StDev	0.02	0.02
300	Mean	0.50	0.51
	StDev	0.02	0.02

Table 5. Comparison of values of f(RH)_ep for ammonium sulfate.

Ammonium sulfate study	Technique	Dry (RH < 40%) size	f(RH = 80)ep
Zhou et al. (2020)	Broadband Cavity Enhanced Aerosol Extinction Spectrometry 532 nm	200 nm	3.17 ± 0.05
		300 nm	2.83 ± 0.10
Brem et al. (2012)	CRDS Nephelometer 467 nm	Dp < 500 nm polydisperse	∼3.7
Garland et al. (2007)	CRDS (σep only) 532 nm	200	4.1 ± 0.5
		300	3.0 ± 0.4
Michel Flores et al. (2012)	CRDS (σep only) 532 nm	200	3.6 ± 0.43
		300	3.13 ± 0.44
This study	Cavity Attenuated Phase Shift 450 nm	100	6.43 ± 0.52
		200	4.40 ± 0.21
		300	3.08 ± 0.20

Absorption is negligible and thus f(RH)_sp is equivalent to f(RH)_ep.

Figure 7. For dry nigrosin (RH < 40%), the ratio of measured and empirically corrected light absorption for two instruments (CAPS-PM_SSA/PASS-3 that uses the ammonium sulfate SSA = 1 calibration via Figure 4). The PASS-3 σ_ap at 450 nm was calculated from 405 nm PASS-3 σ_ap and AAE values. The open circles and closed circles show observations made over six months apart after a few updates by different operators to demonstrate stability.

Figure 8. Nigrosin (a) σ_ep, (b) σ_sp, (c) σ_ap including data from a multi-λ PASS for comparison, and (d) single scattering albedo (SSA) and RH (right y-axis) time series measured with the H-CAPS-PM_SSA for twelve RH scans, compared with Mie simulations.

Figure 9. Comparison of f(RH) (where “dry” RH < 40%) for pure nigrosin measured with H-CAPS-PM_SSA and corrected for truncation, for (a) σ_ep, (b) σ_sp, (c) σ_ap, (d) SSA(RH), and (e) f(RH)_ap versus f(RH)_sp. Representative best fit optical κ-lines are shown for dry D_p = 255 nm. Representative error bars show the standard deviation of the given f(RH) parameter and ±2% RH sensor accuracy.

Table 6. Best fit values κ (RH > 80%) for optical parameters for pure nigrosin as a function of dry size.

Dp (nm)		κep	κsp	κap
100	Mean	0.029	0.074	0.006
	StDev	0.008	0.015	0.006
110	Mean	0.039	0.069	0.011
	StDev	0.001	0.000	0.001
200	Mean	0.039	0.061	0.011
	StDev	0.003	0.004	0.002
255	Mean	0.048	0.065	0.023
	StDev	0.001	0.001	0.000
300	Mean	0.041	0.056	0.020
	StDev	0.003	0.004	0.004
340	Mean	0.060	0.075	0.035
	StDev	0.002	0.003	0.000
400	Mean	0.066	0.083	0.042
	StDev	0.005	0.005	0.004

Since κ_ep is a composite rather than a sum of κ_sp and κ_ap, it is intermediate such that κ_sp > κ_ep > κ_ap.

Table 7. Comparison of values of f(RH) for light absorption for nigrosin.

Nigrosin study	Technique	Dry size	f(RH = 80) extinction	f(RH = 80) scattering	f(RH = 80) absorption
Zhou et al. (2020)	Cavity Enhanced Aerosol Extinction Spectrometry 532 nm	200 nm	1.22 ± 0.00	1.34 ± 0.00	1.12 ± 0.01
		300 nm	1.26 ± 0.01	1.34 ± 0.02	1.18 ± 0.01
Brem et al. (2012)	CRDS & Nephelometer 467 nm	Dp < 500 nm polydisperse	∼1.26	∼1.33	∼1.2
Michel Flores et al. (2012)	CRDS (σep only) 532 nm	200	1.18 ± 0.06	NA	NA
		300	1.19 ± 0.04
This study	Cavity Attenuated Phase Shift 450 nm	110	1.15 ± 0.02	1.27 ± 0.04	1.05 ± 0
		255	1.18 ± 0.03	1.25 ± 0.05	1.09 ± 0.01
		340	1.23 ± 0.03	1.28 ± 0.04	1.14 ± 0.02

Dry conditions are defined similarly in the studies as RH < 40%. Uncertainties shown are estimated as described in the text and shown in Figure 9.

Figure 10. Hygroscopic growth curves f(RH)_ep and f(RH)_sp for polydispersed pure levoglucosan aerosol generated in the laboratory. σ_ap is negligible for this species. Means and standard deviations over ±2% RH bins are shown at RH = 80%, 85%, and 90%. Dry condition is RH <40%, and best-fit κ_ep curve is shown.

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