Measuring ultrafine aerosols by direct photoionization and charge capture in continuous flow

This paper successfully demonstrates the potential of weakly coupled piezoelectric MEMS (Micro-Electro-Mechanical Systems) gravimetric sensors for the detection of ultra-fine particulates. As a proof-of-principle, the detection of diesel soot particles of 100 nanometres or less is demonstrated. A practical monitoring context also exists for diesel soot particles originating from combustion engines, as they are of serious health concern. The MEMS sensors employed in this work operate on the principle of vibration mode-localisation employing an amplitude ratio shift output metric for readout. Notably, gains are observed while comparing parametric sensitivities and the input referred stability for amplitude ratio and resonant frequency variations, demonstrating that the amplitude ratio output metric is particularly suitable for long-term measurements. The soot particle mass directly estimated using coupled MEMS resonators can be correlated to the mass, indirectly estimated using the condensation particle counter used as the reference instrument.

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“…The particle size is varied by changing the flow rate of propane into the flame. Higher flow rates of propane would yield larger particles [40].…”

Section: Construction Of Soot Particle Generatormentioning

confidence: 99%

Weakly Coupled Piezoelectric MEMS Resonators for Aerosol Sensing

Chellasivalingam

Imran

Pandit

et al. 2020

Sensors

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“…(which increases with the particles charging state q), and the material dependent exponent m. For example, the charging probability of NaCl aerosols is substantially lower than for carbonaceous species like soot, which basically enables material specific measurements [9]. The photoelectric charging efficiency is enhanced especially for particles smaller than 50 nm, and it was further shown by Nishida et al [9] that the sensor response of their photoelectric charger is proportional to the particles surface area…”

Section: Theory and Measurement Principlementioning

confidence: 99%

Charging-Based PN Sensing of Automotive Exhaust Particles

Schriefl¹,

Longin²,

Bergmann³

2019

Eurosensors 2018

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Mobile measurement of particle number concentration (PN) in the exhaust of motor vehicles has recently become an integral part of emission legislation. Charge-based sensing techniques for the examination of PN, like Diffusion Charging (DC), represent a promising alternative to condensational particle counters (CPCs) as established PN sensors, because they enable to build robust, compact and energy efficient systems. However, due to the charging process, particle properties like size and morphology have a big impact on the sensor’s PN response. For particles of different size and shape we experimentally investigated those impacts using own-built charging-based sensors. The PN response of the DC sensor showed desired behavior for compact NaCl particles, but less satisfying behavior for combustion aerosol standard (CAST) particles, which is a widely used test aerosol for automotive applications. With a photoelectric charger, the PN response of CAST particles was significantly better.

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“…The electrons released via photoemission from particles in a carrier gas collide with neutral gas molecules, forming gaseous ions. It has been previously shown that under conditions where the recombination rate of ions with charged particles is not dominant, the total photoelectric activity generated over the total irradiation time (t irr ) can be measured by collecting the total current of photoemitted electrons (i e ), or the remaining positively charged particles (i o ) (Nishida et al, 2017(Nishida et al, , 2018. For polydispersed particle size distributions over a range of conditions, the contribution of each photoemission event to the net photoelectric current may be assumed to be independent (Nishida et al, 2019).…”

Section: Theorymentioning

confidence: 99%

“…Jung and Kittelson (2005) measured the size and composition-dependent responses of two unipolar charging-based instruments and one photoelectric instrument as a function of Fuchs surface area, which is directly proportional to the particle mobility diameter and measurement regime. Therefore, the mobility diameter may be used in the photoelectric charging equations by neglecting the effect of particle morphology (Nishida et al, 2018). However, total photoelectric yield, as measured by electrical current in Eqn 3, is proportional to the photoelectrically active surface, which can be conflated with geometric surface area or an equivalent surface area based on mobility measurements.…”

Section: Theorymentioning

confidence: 99%

“…In the free molecular regime, the inverse of particle mobility is largely proportional to the square of mobility diameter, d 2 m . The dependence of photoemission yield on the inverse of particle mobility has been demonstrated in the free molecular and transition regimes (20 < d p < 100 nm) for sintered spheres and fractal-like silver agglomerates (Schmidt-Ott et al, 1990;Schmidt-Ott, 1988;Zhou et al, 2013), recrystallized Ag and Au (Keller et al, 2001), poly-aromatic hydrocarbon (PAH) coated graphite aerosols (measured by diffusion battery) (Niessner, 1986), carbon agglomerates (Burtscher, 1992;Nishida et al, 2018), and diesel soot (Burtscher et al, 1998;Kittelson et al, 2005).…”

Section: Introductionmentioning

confidence: 99%

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Measuring aerosol active surface area by direct ultraviolet photoionization and charge capture in continuous flow

Nishida

Johnson

Boies

et al. 2019

Aerosol Science and Technology

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Direct ultraviolet photoionization electrically charges particles using a mechanism distinct from diffusion charging. The purpose of this study is to evaluate aerosol photoemission theory as a function of aerosol particle size, concentration, material, and morphology. Particles are classified using an aerodynamic aerosol classifier (AAC) and subsequently measured with a scanning mobility particle sizer (SMPS) and photoionization measurement system in parallel. This configuration allows direct comparison of photo-emission from high concentrations of initially neutral, monodisperse aerosols with different morphologies or materials. Under all examined conditions, the overall photoelectric yields of particles of self-similar material (silver and unconditioned soot) and morphology (sintered spheres and agglomerates) are each linearly proportional to the second moment of the mobility-equivalent diameter distribution, even in the transition regime (mobility diameter 30-200 nm), with agglomerate silver particles resulting in 5× higher photoelectric yield than unconditioned soot from a propane flame. It is shown for the first time that the photoelectric yield is significantly higher (2.6×) for fractal-like agglomerate silver particles than sintered, close-packed spherical particles of the same material and mobility-equivalent diameter, which is inferred to be due to the larger material surface area exposed externally to the particle surroundings. It is demonstrated that photoelectric measurements of aerosols reflect the photoelectrically active surface area which depends on the particle morphology and therefore the state of sintering.

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Measuring ultrafine aerosols by direct photoionization and charge capture in continuous flow

Cited by 10 publications

References 39 publications

Weakly Coupled Piezoelectric MEMS Resonators for Aerosol Sensing

Weakly Coupled Piezoelectric MEMS Resonators for Aerosol Sensing

Charging-Based PN Sensing of Automotive Exhaust Particles

Measuring aerosol active surface area by direct ultraviolet photoionization and charge capture in continuous flow

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