Can Particle Size Magnifiers detect HOMs with carbon numbers between C10 and C30?
Authors: Scholz Wiebke , Rörup Birte , Leiminger Markus , Steiner Gerhard , Lehtipalo Katrianne , Kangasluoma Juha , Hansel Armin 
Published: March 2020

&lt;p&gt;&lt;span&gt;The Particle Size Magnifier (PSM, Airmodus) [1] is able to size sub 3 nm particles in mobility diameter. A PSM&amp;#8216;s sensitivity and cutoff size &lt;/span&gt;&lt;span&gt;(50% detection efficiency)&lt;/span&gt;&lt;span&gt; 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 o&lt;/span&gt;&lt;span&gt;f 1.3 nm &lt;/span&gt;&lt;span&gt;in mobility diameter&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;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. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;In this study we aim to understand the PSM&amp;#8216;s response to purely biogenic particles as well as to highly oxidized molecules (HOMs) with carbon backbones of different sizes (C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;10&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;-, C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;15&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;-, C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;20&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;- and C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;30&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;-HOMs). &lt;/span&gt;&lt;span&gt;We used a flow reactor with a short reaction time of about 9s that allows for undisturbed radical &amp;#8211; radical reactions due to negligible wall contacts to quantitatively generate HOMs by ozonolysis of different precursors. Reactants were ozone and either alpha-Pinene (C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;10&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;H&lt;/span&gt;&lt;sub&gt;&lt;span&gt;16&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;) or beta-Caryophyllene (C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;15&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;H&lt;/span&gt;&lt;sub&gt;&lt;span&gt;24&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;) with and without an OH-scavenger.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;A recent study [3] demonstrated, that the ozonolysis of alpha-Pinene produces covalently bound C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;20&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;-HOMs from self- and cross-reactions of two C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;10&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;-peroxy radicals. The C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;30&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;-HOMs are formed equivalently from C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;15&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;-radicals of beta-Caryophyllene oxidation. This mechanism shows, why the C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;20&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;- and C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;30&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;-HOMs increase quadratically, in contrast to the C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;10&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;- and C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;15&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;-HOMs, that increase linearly with respective reacted precursor concentrations. Making use of this principle, we are able to show, that already &lt;/span&gt;&lt;span&gt;C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;20&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;-HOMs are detected by the PSM, but with a much smaller detection efficiency than the C&lt;/span&gt;&lt;sub&gt;&lt;span&gt;30&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;-HOMs, that have an ion mobility diameter of approximately 1.6&lt;/span&gt;&lt;span&gt; nm&lt;/span&gt;&lt;span&gt;. 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.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;span&gt;[1] J. Vanhanen, J. Mikkil&amp;#228;, K. Lehtipalo, M. Sipil&amp;#228;, H. E. Manninen, E. Siivola, T. Pet&amp;#228;j&amp;#228; &amp; M. Kulmala (2011) Particle Size Magnifier for Nano-CN Detection, Aerosol Science and Technology, 45:4, 533-542, DOI: 10.1080702786826.2010.547889&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;[2] J. Kangasluoma, M. Attoui, H. Junninen, K. Lehtipalo, A. Samodurov, F. Korhonen, N. Sarnela, A. Schmidt-Ott, D. Worsnop, M. Kulmala, T. Pet&amp;#228;j&amp;#228; (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&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;[3] T. Berndt&lt;/span&gt;&lt;span&gt;, B. &lt;/span&gt;&lt;span&gt;Mentler, W. Scholz, L. Fischer, H. Herrmann, M. Kulmala, A. Hansel (2018) Accretion Product Formation from Ozonolysis and OH Radical Reaction of &lt;/span&gt;&amp;#945;&lt;span&gt;-Pinene: Mechanistic Insight and the Influence of Isoprene and Ethylene, &lt;/span&gt;&lt;span&gt;Environ. &lt;/span&gt;Sci. Technol. 2018, 52, 19, 11069-11077, DOI: https://doi.org/10.1021/acs.est.8b02210&lt;/p&gt;