Condensation particle counters for sub- 3 nm aerosol particles

Essay: Condensation particle counters for sub- 3 nm aerosol particles.

Aerosol Measurement Techniques.

In this essay, the measurement of sub-3 nm particles with Condensation Particle Counters (CPCs) is discussed. The main focus revolves around the nature of fine particles, CPCs working principles, operating guidelines, increasing detection limits and future prospects of measuring particle number concentration of sub-3 nm particles.

The discussed range of aerosol particles is commonly present in the atmosphere. Sources of sub-3 nm particles include atmospheric aerosols, industrial processes, traffic, engines, 3D printers, and various natural and anthropogenic activities. Atmospheric aerosols represent a significant source, and their presence is notable in various geographic locations, including the boreal forest, the Amazon rainforest, urban centers, mountainous regions, and even in the pristine air of Antarctica (Lehtipalo, K. et al., 2021). Thus, in real-life conditions they are almost omnipresent.

The concern about fine particle measurement is rising, since fine particles can induce inflammation in the respiratory system, triggering immune responses and subsequently cause chronic inflammation. Fine particles are small enough to be inhaled deeply into the lungs and penetrate into alveoli, bypassing the body's natural defense mechanisms in the upper respiratory tract. It is known that prolonged exposure to fine particulate matter has been linked to the development of chronic respiratory and cardiovascular diseases. The small size of these particles also allows them to enter the bloodstream, leading to cardiovascular issues such as heart attacks, strokes, and other cardiovascular diseases.

A Condensation Particle Counter (CPC) is a device, which operates on a certain technique, where nanoparticles are detected through the condensation of a selected vapor onto particles. Due to condensation they grow to a minimum diameter of around 300 nm, allowing them to scatter light from a laser source (Kangasluoma, J. & Attoui, M., 2019). The counting of scattered light pulses from individual particles is accomplished with a photodetector.

Usually the CPCs have 2 operating modes: single particle counting mode for low concentrations,  when each particle is counted depending on whether the scattered light exceeds a threshold value or not, and the photometric mode, when the total scattered light is detected and converted in particle number concentration (Giechaskiel, B., Melas, A., and Mamakos, A., 2023). Overall, CPCs are widely used due to their high sensitivity and extremely low background noise. Designing instruments for the detection of sub-3 nm particles using the CPC technique entails careful consideration of various aspects.

Still, measuring small particles presents a significant technical challenge, primarily because of substantial diffusion losses during particle transport and their limited activation efficiency when exposed to conventional supersaturated working fluids in CPCs. The groundbreaking achievement of developing the first CPC capable of detecting particles as small as 3 nm was accomplished by Stolzenburg and McMurry in 1991 (Liu, Y. et al., 2021).

Before working with CPC, it needs thorough drying and draining before the actual measurement, especially if it has been tilted or moved. Drying involves draining the instrument and running it in DRY mode for a minimum of 2–4 hours. Regular draining is advised to prevent water accumulation (Lehtipalo, K. et al, 2021).

In addition, the preparation procedure includes calibration. The CPC is usually calibrated using various test aerosols, including tetraheptyl ammonium bromide, ammonium sulfate, sodium chloride, tungsten oxide, sucrose, candle flame products, and limonene ozonolysis products. Each substance serves as a representative test particle for evaluating the CPC's response and ensuring its reliability across a spectrum of particle types and sizes.

The lowest detection limits of a CPC are characterized by the smallest detectable particle size, denoted as d50. The concentration calibration is done by comparing the CPC's measurement against a reference CPC or faraday cup electrometer (FCE) for size-selected aerosol particles. The expected concentration range is typically falling between  particles per cubic centimeter ). In other words, d50 represents the lowest diameter at which the CPC still counts half of the particles, characterizing the instrument's detection efficiency. If the composition of the sampled particles is completely unknown, the obtained particle concentrations at the size range of the d50 can have significant uncertainties (Kangasluoma J. et al., 2017).[pic 1][pic 2]

The cut-off curve represents the detection efficiency as a function of particle size, ideally showing a sharp or step-function, indicating that the CPC can count all particles larger than a specific size and none smaller. However, practical considerations, including variations in supersaturation profiles and particle composition, can influence the shape of the cut-off curve, making it less steep in reality (Lehtipalo, K. et al, 2021).

One crucial factor is ensuring a sufficiently high fluid supersaturation. This is essential for activating condensational growth (also referred as «activation») when particles are exposed to the supersaturated fluid. The Kelvin equation, describing the saturation vapor pressure over curved surfaces, provides a fundamental model for predicting the required supersaturation. Fletcher, in 1958, further expanded on this by constructing a heterogeneous nucleation theory for a nucleus growing on a curved particle surface (Kangasluoma, J. & Attoui, M. 2019).

The nature of the working liquid impacts the performance of the CPC (Attoui, M. & Kangasluoma, J. 2019). Different working liquids affect the cut-off size, with variations reported, especially for water-based and diethylene glycol (DEG) CPCs. Hygroscopic particles, when water is used as a working fluid, may have a lower detection limit than hydrophobic particles. The most common working fluid for CPCs in butanol, however DEG is also used for sub-2 nm particle detection s due to its low vapor pressure and high surface tension (Liu, Y. et al.,2021). The light toxicity of the working fluids is considered as disadvantage, because these organic liquids can introduce contaminant molecules to vapor phase (Kangasluoma, J. et al., 2017).