The global budgets of organic hydroperoxides for present and preindustrial scenarios

The global 3-D chemistry-transport model, STOCHEM-CRI (Utembe et al., 2010), has been used to simulate the global distribution of organic hydroperoxides (ROOH) for both present day and pre-industrial scenarios. Globally, the formation of ROOH is solely from the reaction between RO₂ and HO₂, being more significant under NO_x-limited conditions; here the self and cross reactions of RO₂ and HO₂ radicals dominate over their reaction with NO. The predominant global loss processes for ROOH are reaction with OH (95%) and by photolysis (4.4%) with a minor loss (<1%) by deposition, in the present day scenario. The associated global burden of ROOH in our model study is found to be 3.8 Tg. The surface distribution of ROOH shows a peak near the equator corresponding with higher photochemical activity and large (biogenic) VOC emissions. The simulated abundances of ROOH are comparable with those recorded in field campaigns, but generally show a tendency towards underestimation, particularly in the boundary layer. ROOH displayed seasonal cycles with higher concentrations during the summer months and lower concentrations during the winter months. The effects of including proposed HO_x recycling schemes, including isomerisation of isoprene-derived peroxy radicals on the global budget of ROOH have also been investigated for the present and the pre-industrial environment. The present day simulations showed significant increases in CH₃OOH and ROOH (up to 80% and 30%, respectively) over tropical forested regions, due to a general increase in HO₂ and RO₂ levels in isoprene-rich regions at low NO_x levels. In the pre-industrial scenario, the increases in CH₃OOH and total ROOH abundances are even larger, reflecting the more efficient operation of HO_x recycling mechanisms at lower NO_x levels. RCO₃H species contribute 40–50% of the global burden of ROOH; inclusion of HO_x recycling mechanisms leads to an increase in these RCO₃H species but there is no discernible change in the remaining ROOH (ROOH–RCO₃H) burden.