Can Particle Size Magnifiers detect HOMs with carbon numbers between C10 and C30?

The Particle Size Magnifier (PSM, Airmodus) [1] is able to size sub 3 nm particles in mobility diameter. A PSM‘s sensitivity and cutoff size (50% detection efficiency) is usually calibrated only with particles out of metal oxides, e.g. tungsten oxide [2], or with different salt particles [1]. The PSM used in this experiment has a cutoff size for ammonium sulfate particles of 1.3 nm in mobility diameter. Ternary nucleation in the atmospheric boundary layer, however, involves organic molecules. It is therefore questionable, if the inorganic calibration curves of the PSM can be applied to these particles.

In this study we aim to understand the PSM‘s response to purely biogenic particles as well as to highly oxidized molecules (HOMs) with carbon backbones of different sizes (C₁₀-, C₁₅-, C₂₀- and C₃₀-HOMs). We used a flow reactor with a short reaction time of about 9s that allows for undisturbed radical – radical reactions due to negligible wall contacts to quantitatively generate HOMs by ozonolysis of different precursors. Reactants were ozone and either alpha-Pinene (C₁₀H₁₆) or beta-Caryophyllene (C₁₅H₂₄) with and without an OH-scavenger.

A recent study [3] demonstrated, that the ozonolysis of alpha-Pinene produces covalently bound C₂₀-HOMs from self- and cross-reactions of two C₁₀-peroxy radicals. The C₃₀-HOMs are formed equivalently from C₁₅-radicals of beta-Caryophyllene oxidation. This mechanism shows, why the C₂₀- and C₃₀-HOMs increase quadratically, in contrast to the C₁₀- and C₁₅-HOMs, that increase linearly with respective reacted precursor concentrations. Making use of this principle, we are able to show, that already C₂₀-HOMs are detected by the PSM, but with a much smaller detection efficiency than the C₃₀-HOMs, that have an ion mobility diameter of approximately 1.6 nm. Our size-dependent calibration gave a steep sensitivity increase around the particle size of about 1.8 nm mobility diameter for organic particles, showing, that organics are far more difficult to detect than ammonium sulfate particles.

 

[1] J. Vanhanen, J. Mikkilä, K. Lehtipalo, M. Sipilä, H. E. Manninen, E. Siivola, T. Petäjä & M. Kulmala (2011) Particle Size Magnifier for Nano-CN Detection, Aerosol Science and Technology, 45:4, 533-542, DOI: 10.1080702786826.2010.547889

[2] J. Kangasluoma, M. Attoui, H. Junninen, K. Lehtipalo, A. Samodurov, F. Korhonen, N. Sarnela, A. Schmidt-Ott, D. Worsnop, M. Kulmala, T. Petäjä (2015) Sizing of neutral sub 3nm tungsten oxide clusters using Airmodus Particle Size Magnifier, Journal of Aerosol Science, 87, 53-62, DOI: https://doi.org/10.1016/j.jaerosci.2015.05.007

[3] T. Berndt, B. Mentler, W. Scholz, L. Fischer, H. Herrmann, M. Kulmala, A. Hansel (2018) Accretion Product Formation from Ozonolysis and OH Radical Reaction of α-Pinene: Mechanistic Insight and the Influence of Isoprene and Ethylene, Environ. Sci. Technol. 2018, 52, 19, 11069-11077, DOI: https://doi.org/10.1021/acs.est.8b02210