OH production from the photolysis of isoprene-derived peroxy radicals: cross-sections, quantum yields and atmospheric implications

• Published in: Physical chemistry chemical physics : PCCP 2017, Vol.19 (3), p.2332-2345
• Main Authors: Hansen, Robert F, Lewis, Tom R, Graham, Lee, Whalley, Lisa K, Seakins, Paul W, Heard, Dwayne E, Blitz, Mark A
• Format: Article
• Language: English
• Subjects: Absorption; Atmospheric models; Atmospherics; Mathematical models; Nitrogen oxides; Peroxy radicals; Photolysis; Wavelengths
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c6cp06718b

In environments with high concentrations of biogenic volatile organic compounds and low concentrations of nitrogen oxides (NO = NO + NO ), significant discrepancies have been found between measured and modeled concentrations of hydroxyl radical (OH). The photolysis of peroxy radicals from isoprene (HO-Iso-O ) in the near ultraviolet represents a potential source of OH in these environments, yet has not been considered in atmospheric models. This paper presents measurements of the absorption cross-sections for OH formation (σ ) from the photolysis of HO-Iso-O at wavelengths from 310-362.5 nm, via direct observation by laser-induced fluorescence of the additional OH produced following laser photolysis of HO-Iso-O . Values of σ for HO-Iso-O ranged from (6.0 ± 1.6) × 10 cm molecule at 310 nm to (0.50 ± 0.15) × 10 cm molecule at 362.5 nm. OH photodissociation yields from HO-Iso-O photolysis, ϕ , were determined via comparison of the measured values of σ to the total absorption cross-sections for HO-Iso-O (σ ), which were obtained using a newly-constructed spectrometer. ϕ was determined to be 0.13 ± 0.04 at wavelengths from 310-362.5 nm. To determine the impact of HO-Iso-O photolysis on atmospheric OH concentrations, a modeling case-study for a high-isoprene, low-NO environment (namely, the 2008 Oxidant and Particle Photochemical Processes above a South-East Asian Tropical Rainforest (OP-3) field campaign, conducted in Borneo) was undertaken using the detailed Master Chemical Mechanism. The model calculated that the inclusion of HO-Iso-O photolysis in the model had increased the OH concentration by only 1% on average from 10:00-16:00 local time. Thus, HO-Iso-O photolysis alone is insufficient to resolve the discrepancy seen between measured OH concentrations and those predicted by atmospheric chemistry models in such environments.