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Isoprene and its oxidation products are major players in the oxidativechemistry of the troposphere. Current understanding of the factorscontrolling biogenic isoprene emissions and of the fate of isoprene oxidationproducts in the atmosphere has been evolving rapidly. We use aclimate–biosphere–chemistry modeling framework to evaluate the sensitivity ofestimates of the tropospheric oxidative capacity to uncertainties in isopreneemissions and photochemistry. Our work focuses on two climate transitions:from the Last Glacial Maximum (LGM, 19 000–23 000 years BP) to thepreindustrial (1770s) and from the preindustrial to the present day (1990s).We find that different oxidants have different sensitivities to theuncertainties tested in this study. Ozone is relatively insensitive, whereasOH is the most sensitive: changes in the global mean OH levels for theLGM-to-preindustrial transition range between −29 and +7 % and thosefor the preindustrial-to-present-day transition range between −8 and+17 % across our simulations. We find little variability in theimplied relative LGM–preindustrial difference in methane emissions withrespect to the uncertainties tested in this study. Conversely, estimates ofthe preindustrial-to-present-day and LGM-to-preindustrial changes in theglobal burden of secondary organic aerosol (SOA) are highly sensitive. Weshow that the linear relationship between tropospheric mean OH andtropospheric mean ozone photolysis rates, water vapor, and total emissions ofNO<sub><i>x</i></sub> and reactive carbon – first reported in Murray et al. (2014) – doesnot hold across all periods with the new isoprene photochemistry mechanism.This study demonstrates how inadequacies in our current understanding ofisoprene emissions and photochemistry impede our ability to constrain theoxidative capacities of the present and past atmospheres, its controllingfactors, and the radiative forcing of some short-lived species such as SOAover time

atmospheric chemistry and physics

Uncertainties in isoprene photochemistry and emissions: implications for the oxidative capacity of past and present atmospheres and for climate forcing agents