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 NOx-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 HOx 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 NOx levels. In the pre-industrial scenario, the increases in CH3OOH and total ROOH abundances are even larger, reflecting the more efficient operation of HOx recycling mechanisms at lower NOx levels. RCO3H species contribute 40–50% of the global burden of ROOH; inclusion of HOx recycling mechanisms leads to an increase in these RCO3H species but there is no discernible change in the remaining ROOH (ROOH–RCO3H) burden.