title: OH production from the photolysis of isoprene-derived peroxy radicals: cross-sections, quantum yields and atmospheric implications.
abstract: In environments with high concentrations of biogenic volatile organic compounds and low concentrations of nitrogen oxides (NOx= 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 (σRO2,OH) from the photolysis of HO-Iso-O2at 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 σRO2,OHfor HO-Iso-O2ranged from (6.0 ± 1.6) × 10-20cm2molecule-1at 310 nm to (0.50 ± 0.15) × 10-20cm2molecule-1at 362.5 nm. OH photodissociation yields from HO-Iso-O2photolysis, ϕOH,RO2, were determined via comparison of the measured values of σRO2,OHto the total absorption cross-sections for HO-Iso-O2(σRO2), which were obtained using a newly-constructed spectrometer. ϕOH,RO2was determined to be 0.13 ± 0.04 at wavelengths from 310-362.5 nm. To determine the impact of HO-Iso-O2photolysis on atmospheric OH concentrations, a modeling case-study for a high-isoprene, low-NOxenvironment (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-O2photolysis in the model had increased the OH concentration by only 1% on average from 10:00-16:00 local time. Thus, HO-Iso-O2photolysis alone is insufficient to resolve the discrepancy seen between measured OH concentrations and those predicted by atmospheric chemistry models in such environments.
authors.affiliation: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK. m.blitz@leeds.ac.uk.
authors.forename: Robert F
authors.identifier: http://orcid.org/0000-0003-4455-7161
authors.initials: RF
authors.lastname: Hansen
authors.affiliation: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK. m.blitz@leeds.ac.uk.
authors.forename: Tom R
authors.initials: TR
authors.lastname: Lewis
authors.affiliation: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK. m.blitz@leeds.ac.uk.
authors.forename: Lee
authors.initials: L
authors.lastname: Graham
authors.affiliation: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK. m.blitz@leeds.ac.uk and National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, UK.
authors.forename: Lisa K
authors.initials: LK
authors.lastname: Whalley
authors.affiliation: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK. m.blitz@leeds.ac.uk and National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, UK.
authors.forename: Paul W
authors.initials: PW
authors.lastname: Seakins
authors.affiliation: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK. m.blitz@leeds.ac.uk and National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, UK.
authors.forename: Dwayne E
authors.identifier: http://orcid.org/0000-0002-0357-6238
authors.initials: DE
authors.lastname: Heard
authors.affiliation: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK. m.blitz@leeds.ac.uk and National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, UK.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Blitz