Development and evaluation of a countercurrent parallel-plate membrane diffusion denuder for the removal of gas-phase compounds from vehicular emissions

Abstract

Diffusion denuders have been commonly used to remove trace gases from an aerosol (mixture of gases and particles), while allowing the particles to remain suspended in air. We present the design and evaluation of a high-flow (16.7 L min⁻¹) countercurrent parallel-plate membrane diffusion denuder that has high removal efficiencies for both non-reactive gases such as carbon monoxide (89%), as well as volatile organic compounds (80–85%) from an automobile exhaust. Particle losses were approximately 15% for particles around 100 nm in diameter. This denuder is suitable for toxicological tests involving both human and animal exposures to combustion aerosols. The denuder may also be used for other applications, for example, to reduce the effect of gas-phase sampling artifacts on particle composition.

Keywords::

• Denuder
• diffusion
• vehicular
• removal efficiency

Acknowledgements

We thank Dr. Stephen Rudnick for reading the manuscript and providing helpful suggestions.

Declaration of interest

This work was supported by the US Environmental Protection Agency; the PM Center at the Harvard School of Public Health (grant EPA R-832416) and the Harvard NIEHS Center for Environmental Health (grant P30ES000002).

Appendix

First, for the NaCl aerosol, we calculate the corresponding aerodynamic diameters, da, from the electrical mobility diameters, de, that are given by the TSI SMPS software output, using equation (5) (Hinds, 1999) below:

5

where, ρ_p: density of a NaCl particle (ρ_p = 2.2 g cm⁻³); ρ₀ : unit density of a spherical particle; χ: the dynamic shape factor (NaCl cube shape χ = 1.08), and; C_c: slip correction factor.

Based on the size of the particles, equation (6) or equation (7) is used to determine the C_c value.

For particles between 0.1 and 1 μm in diameter, equation (6) (Hinds, 1999) is used:

6

where, λ: 0.066 μm, the mean free path for gas molecules in air at 1 atm and 20°C.

For particles less than 0.1 μm in diameter, equation (7) (Hinds, 1999) is used:

7

Solver in Microsoft Excel was used to determine d_a.

Subsequently, for the NaCl aerosol, we calculate the corresponding actual diameter using equation (8):

8

Where, d_p: actual particle diameter.