Experimental evidence for efficient hydroxyl radical regeneration in isoprene oxidation

Abstract

Most pollutants in the Earth's atmosphere are removed by oxidation with highly reactive hydroxyl radicals. Field measurements have revealed much higher concentrations of hydroxyl radicals than expected in regions with high loads of the biogenic volatile organic compound isoprene 1-8 . Different isoprene degradation mechanisms have been proposed to explain the high levels of hydroxyl radicals observed 5,9-11 . Whether one or more of these mechanisms actually operates in the natural environment, and the potential impact on climate and air quality, has remained uncertain 12-14 . Here, we present a complete set of measurements of hydroxyl and peroxy radicals collected during isoprene-oxidation experiments carried out in an atmospheric simulation chamber, under controlled atmospheric conditions. We detected significantly higher concentrations of hydroxyl radicals than expected based on model calculations, providing direct evidence for a strong hydroxyl radical enhancement due to the additional recycling of radicals in the presence of isoprene. Specifically, our findings are consistent with the unimolecular reactions of isoprenederived peroxy radicals postulated by quantum chemical calculations 9-11 . Our experiments suggest that more than half of the hydroxyl radicals consumed in isoprene-rich regions, such as forests, are recycled by these unimolecular reactions with isoprene. Although such recycling is not sufficient to explain the high concentrations of hydroxyl radicals observed in the field, we conclude that it contributes significantly to the oxidizing capacity of the atmosphere in isoprene-rich regions.Biogenic emissions by terrestrial vegetation are responsible for 90% of the volatile organic compounds (VOCs) in the atmosphere 15,16 . Isoprene (C 5 H 8 ) being the most abundant biogenic VOC has the highest emission rates in tropical forests owing to the light-and temperature-dependent emission process. The high isoprene emission rates coincide with high production rates of the main atmospheric oxidant, the hydroxyl radical (OH), which is mainly formed by photodissociation of ozone (O 3 ) and subsequent reaction of electronically excited oxygen atoms with water vapour. Isoprene reacts readily with OH (ref. 5). This process constitutes the main sink of atmospheric isoprene, and in turn influences the OH concentration, which determines the atmospheric lifetime of greenhouse gases (for example, methane) and pollutants (for example, CO).The atmospheric oxidation efficiency of OH is greatly enhanced by nitrogen monoxide (NO)-dependent radical chain reactions, which regenerate OH. The reaction of OH with isoprene, for example, forms hydroxy-peroxy isoprene radicals (RO 2 ), which can be converted to hydroperoxy radicals (HO 2 ) through their reaction with NO followed by the reaction of HO 2 with NO reforming OH ROOH IEPOX+OH +OH 17% 25% +HO 2 Isoprene+OH+ O 2 1 ,5 -H s h if t 29% <1% MVK, MACR +HCHO+OH y(OH): 0.29 /<0.01 1 ,6 -H s h if t +NO RO 2 41% 62% 13% 13% LIM-J LIM-CS MVK, MACR +HCHO+HO 2 y(OH): 0.29/0.43 HPAL...

Understanding in situ ozone production in the summertime through radical observations and modelling studies during the Clean air for London project (ClearfLo)

“…Observations of OH and HO2 in the urban atmosphere have primarily been 5 made using fluorescence assay by gas expansion (FAGE), and comparisons with predicted radical concentrations using chemistry box models constrained with co-located radical precursor measurements have revealed varying levels of success in replicating observations. Radical concentrations have been reported to be under-predicted by models (Ren et al, 2003;Martinez et al, 2003;Emmerson et al, 2005a;Chen et al, 2010;Lu et al, 2012;Lu et al, 2013), over-predicted (George et al, 1999;Konrad et al, 2003;Dusanter et al, 2009) and, at times, models and measurements have been reported to be in 10 reasonable agreement, to within 40%, (Shirley et al, 2006;Emmerson et al, 2007;Kanaya et al, 2007;Sheehy et al, 2010;Elshorbany et al, 2012;Ren et al, 2013;Griffith et al, 2016). Often the level of agreement observed was found to be dependent on time of day (Brune et al, 2016); with poorest agreement between modelled and measured OH concentrations generally observed during the night.…”

“…The extent of this interference will be dependent upon the level of interference suffered by the specific FAGE instrument utilised and the concentration of those RO2 species that interfere (principally aromatic, alkene and >C3 alkane-derived RO2 species) that were present in a particular environment. Similarly, two FAGE groups have reported interferences in their OH measurements made using wavelength modulation in the presence of ambient levels of ozone and alkenes (Mao et al, 2012;Novelli et al, 2014), 25 whilst, in contrast, good agreement between OH measurements made using FAGE and differential optical absorption spectroscopy (DOAS) during chamber measurements suggests minimal interferences in the presence of ozone and alkenes for a third FAGE instrument (Fuchs et al, 2013). This lack of interference was further corroborated in recent laboratory tests (Fuchs et al, 2016) although an artefact signal under dark conditions (deriving from NO3 in the presence of H2O) was identified.…”

“…Owing to the prevalence of carbonyl and dicarbonyl species in the urban atmosphere, a number of studies have highlighted the role that the photolysis of these species play as key radical precursors (and, hence, ozone precursors) 20 in the summertime: During the SHARP-2009 project that took place in Houston, Texas, the photolysis of formaldehyde accounted for 14% of radical production, with the photolysis of other OVOCs contributing a further 15% (Ren et al, 2013). During the CAREBeijing2006 HCHO was estimated to contribute ~30% to the overall radical production (Lu et al, 2013).…”

“…2016) and radical measurements from the MEGAPOLI project which took place at a suburban site close to Paris employed the chemical ionisation mass spectrometry (CIMS) technique to make observations of OH and the sum of HO2 and RO2 species rather than FAGE (Michoud et al, 2012). HO2 * (= [HO2] +∑iαi [RO2i], and αi is the mean fractional contribution of the RO2 species that interfere (RO2i)) model-measurement comparisons are now often reported (rather than HO2) to take into account contributions from RO2 species (Lu et al, 2013;Griffith et al, 2016) and very recently Tan et al (2017) presented interference-5 free HO2 observations alongside RO2 observations which were made using the FAGE technique coupled to a flow reactor (Fuchs et al, 2008) at a rural site in Wangdu, China. In contrast to some of the earlier HO2 model-measurement comparisons which diverged at NO concentrations > 1ppbv with models increasingly under-predicting the levels of HO2 observed, the predicted levels of HO2 were in good agreement with HO2 observations made in Wangdu over the whole range (0.1 -4 ppbv) of NO encountered (Tan et al, 2017).…”

The impact of the chemical production of methyl nitrate from the NO + CH&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt; reaction on the global distributions of alkyl nitrates, nitrogen oxides and tropospheric ozone: a global modelling study

“…Therefore the discrepancy could be linked to distribution and/or fluxes introduced for these species in the emission inventory. Also the isoprene oxidation scheme included could be considered outdated, considering the latest experimental studies presented in Fuchs et al (2013). For the UT the comparisons again suggest an unidentified source term.…”