10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Temperature and UV Light Affect HOM Chemistry and Biogenic New Particle Formation
MARIO SIMON, Lubna Dada, Martin Heinritzi, Lukas Fischer, Dominik Stolzenburg, Xucheng He, Chao Yan, Andrea C. Wagner, Andreas Kürten, Jasper Kirkby, Joachim Curtius, Goethe University Frankfurt
Abstract Number: 934 Working Group: Aerosol Chemistry
Abstract New particle formation (NPF) is an important source of global aerosol loadings and has been reported under a variety of conditions ranging from marine, remote or urban areas, as well as at ground level, free and upper troposphere. Recently it has been shown that the highly oxygenated molecules (HOMs) formed by the oxidation of monoterpenes (C10H16) contribute to NPF and are even able to form particles at atmospherically relevant concentrations without further assistance of other low volatile compounds (e.g sulfuric acid) (Kirkby et al., 2016; Tröstl et al., 2016).
These HOMs form mostly via an initial oxidant (e.g. O3, OH or NO3) attack on the double bound of the monoterpene, which leads to peroxy radicals (RO2). The highly reactive RO2 radicals undergo rapid autoxidation steps due to subsequent intramolecular H-shifts and form progressively higher oxygenated RO2 radicals (Crounse et al., 2013). The termination products of these reactions are highly oxygenated monomers and even covalently bound dimers which are thought to be responsible for nucleation due to their extremely low volatility (Ehn et al., 2014). The oxidation pathway of HOMs are very sensitive to different environmental conditions and therefore resulting in strong variation of pure organic NPF and a large uncertainty in global aerosol models.
In this study, we show data from the Cosmics Leaving OUtdoor Droplets (CLOUD) experiment at CERN to investigate HOMs formation from the most abundant monoterpene α-pinene and NPF under a large variety of conditions. A nitrate based chemical ionization atmospheric pressure interface Time-of-Flight mass spectrometer (CI-API-TOF) was used to measure neutral HOMs and characterize their chemical composition. Furthermore, a newly developed proton transfer reaction mass spectrometer (PTR-MS) instrument was used to quantify also the low to semi-volatile organic compounds (LVOC and SVOC) to understand their interactions and to complete the picture of their chemistry and formation rate. The nucleation rates of these comprehensive experiments were determined using several particle counters including Particle Size Magnifiers (PSM), Condensation Particle Counter (CPC) as well as a nano-Scanning Mobility Particle Sizer (nanoSMPS).
We studied the effect of temperature on HOM chemistry and NPF at four different temperatures (-50, -25, +5 and +25°C) and various concentrations to simulate a wide range of atmospheric latitudes and altitudes where NPF can occur. Therefore, we measured the impact to the autoxidation rate as well as the RO2 radical termination and the closed shell product distribution of HOMs. Additionally, we investigated the effect of UV light on the HOM chemistry. Furthermore, we discuss the question in how far the formation rate of new particles is affected by temperature and UV changes.
Our results improve the understanding of pure biogenic nucleation as we extend the range of conditions studied. This knowledge also helps to improve the predictions of global aerosol models for very clean environments as well as the pristine pre-industrial climate.