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

In environments with high concentrations of biogenic volatile organic compounds and low concentrations of nitrogen oxides (NO_x = NO + NO₂), significant discrepancies have been found between measured and modeled concentrations of hydroxyl radical (OH). The photolysis of peroxy radicals from isoprene (HO-Iso-O₂) 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_RO) from the photolysis of HO-Iso-O₂ 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-O₂. Values of σ_RO for HO-Iso-O₂ ranged from (6.0 ± 1.6) × 10^-20 cm² molecule^-1 at 310 nm to (0.50 ± 0.15) × 10^-20 cm² molecule^-1 at 362.5 nm. OH photodissociation yields from HO-Iso-O₂ photolysis, ϕ_OH,RO, were determined via comparison of the measured values of σ_RO to the total absorption cross-sections for HO-Iso-O₂ (σ_RO), which were obtained using a newly-constructed spectrometer. ϕ_OH,RO was determined to be 0.13 ± 0.04 at wavelengths from 310-362.5 nm. To determine the impact of HO-Iso-O₂ photolysis on atmospheric OH concentrations, a modeling case-study for a high-isoprene, low-NO_x 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-O₂ photolysis in the model had increased the OH concentration by only 1% on average from 10:00-16:00 local time. Thus, HO-Iso-O₂ photolysis alone is insufficient to resolve the discrepancy seen between measured OH concentrations and those predicted by atmospheric chemistry models in such environments.