ROOOH: a missing piece of puzzle in the understanding of OH in remote environments?

Mohamad Al Ajamia), Sebastien Batuta), Valerio Ferraccib,c), Scott Archer-Nichollsb), Alexander T. Archibaldb,d) , Coralie Schoemaeckera) and Christa Fittschena)

a) Université Lille, CNRS, UMR 8522, PhysicoChimie des Processus de Combustion et de l’Atmosphère - PC2A, Lille, 59000, France

b) University of Cambridge, Centre for Atmospheric Science, Department of Chemistry, Lensfield Road, Cambridge, CB2 1EW, UK

c)Cranfield University, Centre for Environmental and Agricultural Informatics, College Road, Cranfield MK43 0AL, UK

d) National Centre for Atmospheric Science, Cambridge, UK

Field campaigns have been carried out 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 up to a factor of 10 higher than predicted by models and were interpreted as a major lack in our understanding of the chemistry of biogenic hydrocarbons.

In the following years, and until today, many experimental and theoretical studies have been carried out to improve the knowledge of the oxidation mechanism of biogenic VOCs under low NO conditions in order to bring into agreement model and measurement. Some new reaction paths have been found able to recycle OH radicals under low NO, especially for isoprene, but the strong disagreement between measurements and models still persists until today.

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 background 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 will present 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 University 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.