The Absorption Spectrum and Absolute Absorption Cross Sections of Acetylperoxy Radicals, CH3C(O)O2 in the near IR

Michael Rolletter,Hendrik Fuchs,Christa Fittschen,Emmanuel Assaf,Mohamed Assali

doi:10.5194/egusphere-egu2020-5733

Abstract

&lt;p&gt;Acetylperoxy radicals (CH&lt;sub&gt;3&lt;/sub&gt;C(O)O&lt;sub&gt;2&lt;/sub&gt;) play an important role in the tropospheric chemistry. They are produced by the photooxidation of most emitted biogenic non-methane hydrocarbons. Recent studies show that the CH&lt;sub&gt;3&lt;/sub&gt;C(O)O&lt;sub&gt;2&lt;/sub&gt; + HO&lt;sub&gt;2&lt;/sub&gt; reaction, which is the most important tropospheric loss reaction of acetylperoxy radicals in regions that are dominated by biogenic emissions (low NO emissions), does not only lead to radical chain terminating products but can also regenerate OH. The competing secondary chemistry, e. g., the CH&lt;sub&gt;3&lt;/sub&gt;C(O)O&lt;sub&gt;2&lt;/sub&gt; self-reaction, complicate kinetic measurements. The detection of acetylperoxy radicals in previous kinetic laboratory studies was mainly done in the UV region. However, the spectral overlap of different peroxy species in this region is prone to systematic errors in the quantitative detection. These complications can be avoided, if acetylperoxy radicals are detected by absorption in the near IR.&lt;/p&gt;&lt;p&gt;In our work, the near infrared CH&lt;sub&gt;3&lt;/sub&gt;C(O)O&lt;sub&gt;2&lt;/sub&gt; spectrum was measured in the spectral ranges from 6094&amp;#160;cm&lt;sup&gt;-1&lt;/sup&gt; to 6180&amp;#160;cm&lt;sup&gt;-1&lt;/sup&gt; and 6420&amp;#160;cm&lt;sup&gt;-1&lt;/sup&gt; to 6600&amp;#160;cm&lt;sup&gt;-1&lt;/sup&gt;. CH&lt;sub&gt;3&lt;/sub&gt;C(O)O&lt;sub&gt;2&lt;/sub&gt; radicals were generated by pulsed photolysis of a acetaldehyde/Cl&lt;sub&gt;2&lt;/sub&gt;/O&lt;sub&gt;2&lt;/sub&gt; mixture at 351&amp;#160;nm and were subsequently detected by time-resolved continuous-wave cavity ring-down spectroscopy (cw-CRDS). Experiments were done at 67 hPa in synthetic air and helium. The absorption cross sections of eight discrete absorption lines were determined relative to the absorption cross section of HO&lt;sub&gt;2&lt;/sub&gt;, which has previously been reported.&lt;/p&gt;

Highlights

In the troposphere, the oxidation of volatile organic compounds (VOCs) is mainly driven by hydroxyl (OH) radicals and leads to the formation of organic peroxy radicals (RO2)
The flow reactor contained two identical continuous wave cavity ring-down spectroscopy absorption paths, which were installed in a small angle with respect to the photolysis path
A series of experiments was conducted to quantify the initial amount of radicals. This was achieved by quantitatively converting all Cl-atoms to HO2 radicals, which can be reliably measured by continuous-wave cavity ring-down spectroscopy (cw-CRDS) on a strong absorption line at 6638.205 cm−1 [23,26–29]

Summary

Introduction

The oxidation of volatile organic compounds (VOCs) is mainly driven by hydroxyl (OH) radicals and leads to the formation of organic peroxy radicals (RO2). The fate of these RO2 radicals depends on the chemical composition of the environment. Subsequent to the reaction with NO, alkoxy radicals react with O2 to form hydroperoxy (HO2) radicals. In clean environments with low NOx (NOx = NO + NO2) concentrations, the dominant loss of RO2 is due to its reaction with HO2 forming hydroperoxides ROOH and terminating the radical reaction chain. RO2 radicals can react either with other RO2 as self- (RO2 + RO2) or cross-reaction (RO2 + R’O2) or with OH radicals (RO2 + OH) [1–3]

Methods

Results

Conclusion