Investigation of Potential Interferences in the Detection of OH Radicals by Laser-Induced Fluorescence under Dark Conditions

Abstract:

Recent findings from atmospheric OH measurements by laser-induced fluorescence (LIF) instruments have shown that the detected OH signals may contain a significant interference in the presence of biogenic VOCs and ozone. The fractional contribution of the interfering signal to the total measured signal in field campaigns was observed to be largest during nighttime, but was also present during daytime. The exact nature of this interference is not fully clear, but it has been proposed to be related to the short-lived products of the ozonolysis of alkenes (Criegee intermediates). Here, we will show investigations of potential interferences in the OH detection for the Juelich LIF instruments for nighttime conditions in laboratory studies. The inlet of the instrument was overflown by excess synthetic air containing one or more reactants. In order to distinguish between OH produced by reactions upstream of the inlet and artificial signals produced inside the instrument, a chemical titration scheme for OH was applied. Experiments included the investigation of potential interferences related to the nitrate radical and related to the ozonolysis of alkenes, monoterpenes, and isoprene. A small interference signal was found, if nitrate radicals are sampled by the Juelich LIF instrument. Internally produced OH was observed, if exceptionally large, non-atmospheric concentrations of ozone and alkenes were sampled. However, this signal is most likely not related to the presence of Criegee intermediates.