Abstract
Abstract. Field campaigns have been carried out with the FAGE (fluorescence assay by gas expansion) technique in remote biogenic environments in the last decade to quantify the in situ concentrations of OH, the main oxidant in the atmosphere. These data have revealed concentrations of OH radicals up to a factor of 10 higher than predicted by models, whereby the disagreement increases with decreasing NO concentration. This was interpreted as a major lack in our understanding of the chemistry of biogenic VOCs (volatile organic compounds), particularly isoprene, which are dominant in remote pristine conditions. But interferences in these measurements of unknown origin have also been discovered for some FAGE instruments: using a pre-injector, all ambient OH is removed by fast reaction before entering the FAGE cell, and any remaining OH signal can be attributed to an interference. This technique is now systematically used for FAGE measurements, allowing the reliable quantification of ambient OH concentrations along with the signal due to interference OH. However, the disagreement between modelled and measured high OH concentrations of earlier field campaigns as well as the origin of the now-quantifiable background OH is still not understood. We present in this paper the compelling idea that this interference, and thus the disagreement between model and measurement in earlier field campaigns, might be at least partially due to the unexpected decomposition of a new class of molecule, ROOOH, within the FAGE instruments. This idea is based on experiments, obtained with the FAGE set-up of the University of Lille, and supported by a modelling study. Even though the occurrence of this interference will be highly dependent on the design and measurement conditions of different FAGE instruments, including ROOOH in atmospheric chemistry models might reflect a missing piece of the puzzle in our understanding of OH in clean atmospheres.
Highlights
OH radicals are the most important oxidant in the atmosphere, and the detailed understanding of their formation and reactivity is key for the understanding of the overall chemistry
It should be noted that the intensity of interferences or even the presence at all can depend on the design of the FAGE instrument, and the results presented here are only valid for the FAGE instrument of the University of Lille
In this work we have shown that the product of the reaction of RO2 radicals with OH radicals leads to an OH interference signal in the UL-FAGE instrument
Summary
OH radicals are the most important oxidant in the atmosphere, and the detailed understanding of their formation and reactivity is key for the understanding of the overall chemistry. The concentration of OH radicals has been measured for several decades (Holland et al, 2003; Creasey et al, 1997; Brune et al, 1995), and comparison of OH concentration profiles with model outputs is taken as a good indicator of the degree of our understanding of the chemistry. Good agreement is often obtained between measurements and models for polluted environments (where levels of nitrogen oxides (NOx = NO + NO2) are in excess of 500 pmol mol−1, or ppt); how-. Fittschen et al.: ROOOH: a missing piece of the puzzle for OH measurements in low-NO environments?
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