Improved global modelling of HOx recycling in isoprene oxidation: evaluation against the GABRIEL and INTEX-A aircraft campaign measurements
Abstract. Stimulated by recent important developments regarding the oxidation chemistry of isoprene, this study evaluates and quantifies the impacts of different mechanism updates on the boundary layer concentrations of OH and HO2 radicals using the IMAGESv2 global chemistry transport model. The model results for HOx, isoprene, NO, and ozone are evaluated against air-based observations from the GABRIEL campaign, conducted over the Guyanas in October 2005, and from the INTEX-A campaign over the Eastern US in summer 2004. The version 2 of the Mainz Isoprene Mechanism (MIM2, Taraborrelli et al., 2009) used as reference mechanism in our simulations, has been modified to test (i) the artificial OH recycling proposed by Lelieveld et al. (2008), (ii) the epoxide formation mechanism proposed by Paulot et al. (2009b), and finally (iii) the HOx regeneration of the Leuven Isoprene Mechanism (LIM0) proposed by Peeters and Müller (2010). The simulations show that the LIM0 scheme holds by far the largest potential impact on HOx concentrations over densely vegetated areas in the Tropics as well as at mid-latitudes. Strong increases, by up to a factor of 4 in the modelled OH concentrations, and by a factor of 2.5–3 in the HO2 abundances are estimated through the LIM0 mechanism compared to the traditional isoprene degradation schemes. Comparatively much smaller OH increases (<25%) are associated with the implementation of the mechanism of Paulot et al. (2009b); moreover, the global production of epoxides is strongly suppressed (by a factor of 4) when the LIM0 scheme is combined with this mechanism. Hydroperoxy-aldehydes (HPALDs) are found to be major first-generation products in the oxidation of isoprene by OH, with a combined globally averaged yield of 50–60%. The use of the LIM0 chemistry in the global model allows for reconciling the model with the observed concentrations at a satisfactory level, compared to the other tested mechanisms, as the observed averaged mixing ratios of both OH and HO2 in the boundary layer can be reproduced to within 30%. In spite of the remaining uncertainties in the theoretically-predicted rates of critical radical reactions leading to the formation of HPALDs, and even more in the subsequent degradation of these new compounds, the current findings make a strong case for the newly proposed chemical scheme. Experimental confirmation and quantification is urgently needed for the formation of HPALDs and for their fast OH-generating photolysis.