Abstract
Abstract. 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 α-pinene, β-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 × 1013 and 10 × 1013 cm−3) and BVOC concentrations (between approximately 0.1 × 1012 and 40 × 1012 cm−3) was observed that was not laser generated and was independent of the ozonolysis reaction time. For the ozonolysis of α- and β-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.
Highlights
The hydroxyl radical (OH) plays an important role in the chemistry of the atmosphere
The open symbols are the measured OH concentration produced from the ozonolysis reaction without addition of C3F6, and the filled symbols represent the OH signal after the signal measured with C3F6 addition is removed using the 0.6 mm nozzle, shown on the same plot to illustrate the magnitude of the interference
Similar to that observed by Novelli et al (2017), estimates of the ambient concentration of stabilized Criegee intermediates (SCIs) on the order of approximately 4–5 × 104 molecules cm−3 for similar environments (Percival et al, 2013; Novelli et al, 2017) suggest that the observed interference in these measurements may not be solely due to ambient SCIs unless there are other significant sources of Criegee radicals that are not accounted for in these models. These results suggest that, SCIs may be contributing to the observed interference, there may exist an unknown interference in these measurements that correlates with the concentration of ozone and biogenic volatile organic compounds (BVOCs), perhaps due to oxidation products of BVOCs not tested in the experiments reported here (Fuchs et al, 2016; Novelli et al, 2017)
Summary
The hydroxyl radical (OH) plays an important role in the chemistry of the atmosphere. Because of its high reactivity, measurements of OH can provide a rigorous test of our understanding of the fast radical chemistry in the atmosphere. Several field campaigns have identified significant discrepancies between measured and modeled OH concentrations, especially in low-NOx forested environments (Rohrer et al, 2014). Ren et al (2008) found that OH concentrations were well predicted by models to within their combined estimated uncertainty when mixing ratios of isoprene were less than approximately 500 pptv, but measurements acquired in areas with higher mixing ratios of isoprene showed observed OH concentrations that were 3–5 times larger than model predictions. Measurements in a northern Michigan forest found daytime OH concentrations approximately 3 times larger and nighttime concentrations 3–10 times larger than model predictions (Tan et al, 2001; Faloona et al, 2001).
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