Peroxy radical measurements by photoacoustic spectroscopy coupled to chemical amplification

<p>Peroxy radicals (HO<sub>2</sub>+RO<sub>2</sub>) are crucial intermediates in many key atmospheric processes and contribute to the formation of major air pollutants, such as ozone and secondary organic aerosols<sup>1</sup>. Due to their high reactivity and their extremely low concentrations (typically <100 pptv), in-situ real time and interference-free measurements of peroxy radicals remain challenging. In the present work, photoacoustic spectroscopy (PAS)<sup>2</sup> is applied, for the first time to our best knowledge, to the measurements of peroxy radicals with the help of the well established chemical amplification approach. Peroxy radical chemical amplification (PERCA)<sup>3</sup> is based on chemical conversion of peroxy randicals into NO<sub>2</sub> and followed by chemical amplification to achieve the necessary measurement sensitivity for the measurement of atmospheric peroxy radical concentration. The resulting NO<sub>2</sub> concentration is measured by PAS to infer the total concentration of peroxy radicals. The performance of the developed PERCA-PAS approach was demonstrated with a reference ECHAMP chemical amplification system using cavity attenuated phase shift spectroscopy (CAPS) for NO<sub>2</sub> monitoring. The determined amplification gains (referred to as chain length, CL) of the ECHAMP system using PAS are well consistent with the values determined using CAPS. A 1-σ limit of detection of ~12 pptv for peroxy radicals was achieved in an integration time of 90 s at a relative humidity of about 9.8%. The detection limit of the current ECHAMP-PAS system can be further improved by using higher laser power and increasing the number of microphones in the photoacoustic spectrophone, which would allow reaching sub-pptv detection limits for the measurements of peroxy radicals in the atmosphere.</p><p>This work provides a promising technique to develop novel compact and very cost-effective (compared to all methods currently used) sensors, which will allow readily developing network measurements and investigation of the spatial distribution of peroxy radicals in the atmosphere.</p><p><strong>Acknowledgments. </strong>This work is supported by the French national research agency (ANR) under MABCaM and LABEX-CaPPA contracts, the European Funds for Regional Economic Development through the CaPPA project, the CPER-CLIMIBIO program, the LEFE/CHAT INSU program. It is also supported by the National Natural Science Foundation of China (22073013), Natural Science Foundation of Chongqing (cstc2018jcyjAX0050) and Fundamental Research Funds for the Central Universities (2020CDJXZ002).</p><p><strong>Reference</strong></p><p>[1] J. J. Orlando, G. S. Tyndall, Laboratory studies of organic peroxy radical chemistry: an overview with emphasis on recent issues of atmospheric significance, Chem. Soc. Rev. <strong>41</strong>(2012) 6294-6317.</p><p>[2] W. Chen et al., Photonic Sensing of reactive atmospheric species, in Encyclopedia of Analytical Chemistry © 2017 John Wiley & Sons, Ltd. DOI: 10.1002/9780470027318.a9432.</p><p>[3] C. Cantrell, D. Stedman, A possible technique for the measurement of atmospheric peroxy radicals, Geophys. Res. Lett. <strong>9</strong> (1982) 846-849.</p>

Peroxy radical measurements by ethane – nitric oxide chemical amplification and laser-induced fluorescence during the IRRONIC field campaign in a forest in Indiana

Peroxy radicals were measured in a mixed deciduous forest atmosphere in Bloomington, Indiana, USA, during the Indiana Radical, Reactivity and Ozone Production Intercomparison (IRRONIC) during the summer of 2015. Total peroxy radicals ([XO2]≡[HO2]+Σ[RO2]) were measured by a newly developed technique involving chemical amplification using nitric oxide (NO) and ethane (C2H6) followed by NO2 detection by cavity-attenuated phase-shift spectroscopy (hereinafter referred to as ECHAMP – Ethane CHemical AMPlifier). The sum of hydroperoxy radicals (HO2) and a portion of organic peroxy radicals ([HO2*]=[HO2]+Σαi[RiO2], 0<α<1) was measured by the Indiana University (IU) laser-induced fluorescence–fluorescence assay by gas expansion instrument (LIF-FAGE). Additional collocated measurements include concentrations of NO, NO2, O3, and a wide range of volatile organic compounds (VOCs) and meteorological parameters. XO2 concentrations measured by ECHAMP peaked between 13:00 and 16:00 local time (LT), with campaign average concentrations of 41±15 ppt (1σ) at 14:00 LT. Daytime concentrations of isoprene averaged 3.6±1.9 ppb (1σ), whereas average concentrations of NOx ([NO] + [NO2]) and toluene were 1.2 and 0.1 ppb, respectively, indicating a low impact from anthropogenic emissions at this site. We compared ambient measurements from both instruments and conducted a calibration source comparison. For the calibration comparison, the ECHAMP instrument, which is primarily calibrated with an acetone photolysis method, sampled the output of the LIF-FAGE calibration source which is based on the water vapor photolysis method and, for these comparisons, generated a 50 %–50 % mixture of HO2 and either butane or isoprene-derived RO2. A bivariate fit of the data yields the relation [XO2]ECHAMP=(0.88±0.02;[HO2]+[RO2])IU_cal+(6.6±4.5) ppt. This level of agreement is within the combined analytical uncertainties for the two instruments' calibration methods. A linear fit of the daytime (09:00–22:00 LT) 30 min averaged [XO2] ambient data with the 1 min averaged [HO2*] data (one point per 30 min) yields the relation [XO2]=(1.08±0.05)[HO2*]-(1.4±0.3). Day-to-day variability in the [XO2]/[HO2*] ratio was observed. The lowest [XO2]/[HO2*] ratios between 13:00 and 16:00 LT were 0.8 on 13 and 18 July, whereas the highest ratios of 1.1 to 1.3 were observed on 24 and 25 July – the same 2 d on which the highest concentrations of isoprene and ozone were observed. Although the exact composition of the peroxy radicals during IRRONIC is not known, zero-dimensional photochemical modeling of the IRRONIC dataset using two versions of the Regional Atmospheric Chemistry Mechanism (RACM2 and RACM2-LIM1) and the Master Chemical Mechanism (MCM 3.2 and MCM 3.3.1) all predict afternoon [XO2]/[HO2*] ratios of between 1.2 and 1.5. Differences between the observed ambient [XO2]/[HO2*] ratio and that predicted with the 0-D modeling can be attributed to deficiencies in the model, errors in one of the two measurement techniques, or both. Time periods in which the ambient ratio was less than 1 are definitely caused by measurement errors (including calibration differences), as such ratios are not physically meaningful. Although these comparison results are encouraging and demonstrate the viability in using the new ECHAMP technique for field measurements of peroxy radicals, further research investigating the overall accuracy of the measurements and possible interferences from both methods is warranted.