Measurements of a potential interference with laser-induced fluorescence measurements of ambient OH from the ozonolysis of biogenic alkenes

摘要

Reactions of the hydroxyl radical (OH) play a central role in the chemistry of the atmosphere, and measurements of its concentration can provide a rigorous test of our understanding of atmospheric oxidation. Several recent studies have shown large discrepancies between measured and modeled OH concentrations in forested areas impacted by emissions of biogenic volatile organic compounds (BVOCs), where modeled concentrations were significantly lower than measurements. A potential reason for some of these discrepancies involves interferences associated with the measurement of OH using the laser-induced fluorescence–fluorescence assay by gas expansion (LIF-FAGE) technique in these environments. In this study, a turbulent flow reactor operating at atmospheric pressure was coupled to a LIF-FAGE cell and the OH signal produced from the ozonolysis of iα/i-pinene, iβ/i-pinene, ocimene, isoprene, and 2-methyl-3-buten-2-ol (MBO) was measured. To distinguish between OH produced from the ozonolysis reactions and any OH artifact produced inside the LIF-FAGE cell, an external chemical scrubbing technique was used, allowing for the direct measurement of any interference. An interference under high ozone (between 2??×??10sup13/sup and 10??×??10sup13/sup?cmsup?3/sup) and BVOC concentrations (between approximately 0.1??×??10sup12/sup and 40??×??10sup12/sup?cmsup?3/sup) was observed that was not laser generated and was independent of the ozonolysis reaction time. For the ozonolysis of iα/i- and iβ/i-pinene, the observed interference accounted for approximately 40?% of the total OH signal, while for the ozonolysis of ocimene the observed interference accounted for approximately 70?% of the total OH signal. Addition of acetic acid to the reactor eliminated the interference, suggesting that the source of the interference in these experiments involved the decomposition of stabilized Criegee intermediates (SCIs) inside the FAGE detection cell. Extrapolation of these measurements to ambient concentrations suggests that these interferences should be below the detection limit of the instrument.