Single Charging of Nanoparticles by UV Photoionization at High Flow Rates

Figures & data

TABLE 1 Model parameters used in the simulations

Ion mobility	Z i	1.4· 10^−4 cm^2· V^−1· s^−1
Ion mass	m i	101 amu
Particle dielectric constant	ϵ r	10
Particle work function	Φ ∞	5.0 eV
Photon energy	hν	7.2 eV
Fowler law power	m	2

FIG. 1 Set-up for charging of monodisperse aerosols with the UV photoionizer.

FIG. 2 Set-up for charging of polydisperse aerosols with the UV photoionizer and with the ⁸⁵Kr ionizer.

FIG. 3 Charging efficiency of the UV photoionizer versus particle diameter under different irradiation conditions: t _r = 0.16 s (q = 30 l· min⁻¹), i = 0.5, 0.25, and 0.1 mA; and t _r = 0.054 s (q = 90 l· min⁻¹), i = 0.5 mA. Experimental values (symbols) and theoretical values (lines) calculated with Equations (5a)–(5d) for the indicated values of K.

FIG. 4 The model parameter K vs. the intensity of the current supplied to the UV lamps. Experimental values (symbols) corresponding to t _r = 0.16 s (q = 30 l· min⁻¹), i = 0.5, 0.25, and 0.1 mA; and fitting polynomial (line).

FIG. 5 Particle mobility distributions measured at the outlet of the UV photocharger in experiments with monodisperse aerosols of the indicated particle diameters d _g . Irradiation conditions were t _r = 0.16 s (q = 30 l· min⁻¹), i = 0.5, 0.25, and 0.1 mA.

FIG. 6 Particle charge distributions of monodisperse aerosols calculated with the analytical solution for pure photocharging, Equations (5a)–(5d), for t _r = 0.16 s (q = 30 l· min⁻¹) and K = 6.5· 10³⁵, 3.75· 10³⁵ and 1.25· 10³⁵ J⁻¹· m⁻²· s⁻¹ (i = 0.5, 0.25, and 0.1 mA).

FIG. 7 Particle mobility distributions measured at the outlet of the UV photocharger in experiments with monodisperse aerosols of the indicated particle diameters d _g . Irradiation conditions were t _r = 0.054 s (q = 90 l· min⁻¹) and i = 0.5 mA.

FIG. 8 Particle charge distributions of monodisperse aerosols calculated with the analytical solution for pure photocharging, Equations (5a)–(5d), for t _r = 0.054 s (q = 90 l· min⁻¹) and K = 5.5· 10³⁵ J⁻¹· m⁻²· s⁻¹ (i = 0.5 mA).

TABLE 2 Description and conditions of the experiments with polydisperse aerosols

		Aerosol flow	Number concentration	Geometric mean	Geometric standard
Test	Description	rate q (l· min^−1)	N p (cm^−3)	diameter d g (nm)	deviation σ g
1	Reference case	30	2.1· 10^7	7.8	1.45
2	Effect of particle concentration	30	3.8· 10^7	9.4	1.51
3	Effect of irradiation time	90	3.2· 10^7	10.2	1.62

FIG. 9 Particle size distributions measured at the outlet of the HF-DMA using the UV photoionizer to charge polydisperse aerosols with particle concentrations N _p of 2.1· 10⁷ cm⁻³ (top) and 3.8· 10⁷ cm⁻³ (bottom) at a flow rate q of 30 l· min⁻¹. Irradiation conditions were t _r = 0.054 s in both experiments, i = 0.5 and 0.1 mA (top), i = 0.1 and 0.04 mA (bottom).

FIG. 10 Particle size distributions of monodisperse aerosols measured at the outlet of the HF-DMA using the ⁸⁵Kr ionizer (top) and the UV photoionizer (bottom) to charge polydisperse aerosols with a particle concentration N _p of 3.2· 10⁷ cm⁻³ at a flow rate q of 90 l· min⁻¹. A geometric standard deviation σ _g ∼ 1.05 was obtained for particles of all sizes with the two aerosol chargers.

FIG. 11 Comparison of particle number concentrations of monodisperse aerosols measured at the outlet of the HF-DMA using the ⁸⁵Kr ionizer and the UV photoionizer to charge polydisperse aerosols at flow rates q of 30 l· min⁻¹ (top) and 90 l· min⁻¹ (bottom).

FIG. 12 Comparison of the fraction of charged particles (top) and the particle charge distribution (bottom) at the outlet of the UV photoionizer calculated with the analytical solution for pure photocharging, Equations (5a)–(5d), for the experiments with monodisperse aerosols and with the numerical model including diffusion charging, Equations (1a)–(1c), for an experiment with a polydisperse aerosol. Irradiation conditions were t _r = 0.054 s (q = 90 l· min⁻¹) and i = 0.5 mA; ion and particle losses were not considered.

FIG. 13 Fractions of singly charged particles (top) and ratio of singly negatively to singly positively charged particles (bottom) in the aerosol at the outlet of the UV photoionizer, at the inlet of the HF-DMA, and at the aerosol slit calculated with the numerical model including diffusion charging, Equations (1a)–(1c), and experimental values of N _{− 1}/N_{+ 1} (open circles). Irradiation conditions were t _r = 0.054 s (q = 90 l· min⁻¹) and i = 0.5 mA; ion and particle losses were not considered in the calculation.

FIG. 14 Fractions of singly charged nanoparticles f _{± 1} (top) and ratio of singly negatively to singly positively charged particles N _{− 1}/N _{+ 1} (bottom) in the aerosol at the outlet of the UV photoionizer, at the inlet of the HF-DMA, and at the aerosol slit calculated with the numerical model including diffusion charging, Equations (1a)–(1c), and experimental values of N _{− 1}/N _{+ 1} (open circles). Irradiation conditions were t _r = 0.054 s (q = 90 l· min⁻¹) and i = 0.5 mA; an ion depletion rate w _i,UV of 77 s⁻¹ was assumed in the calculation.