Abstract
Abstract. Isoprene and its oxidation products are major players in the oxidative chemistry of the troposphere. Current understanding of the factors controlling biogenic isoprene emissions and of the fate of isoprene oxidation products in the atmosphere has been evolving rapidly. We use a climate–biosphere–chemistry modeling framework to evaluate the sensitivity of estimates of the tropospheric oxidative capacity to uncertainties in isoprene emissions and photochemistry. Our work focuses on two climate transitions: from the Last Glacial Maximum (LGM, 19 000–23 000 years BP) to the preindustrial (1770s) and from the preindustrial to the present day (1990s). We find that different oxidants have different sensitivities to the uncertainties tested in this study. Ozone is relatively insensitive, whereas OH is the most sensitive: changes in the global mean OH levels for the LGM-to-preindustrial transition range between −29 and +7 % and those for the preindustrial-to-present-day transition range between −8 and +17 % across our simulations. We find little variability in the implied relative LGM–preindustrial difference in methane emissions with respect to the uncertainties tested in this study. Conversely, estimates of the preindustrial-to-present-day and LGM-to-preindustrial changes in the global burden of secondary organic aerosol (SOA) are highly sensitive. We show that the linear relationship between tropospheric mean OH and tropospheric mean ozone photolysis rates, water vapor, and total emissions of NOx and reactive carbon – first reported in Murray et al. (2014) – does not hold across all periods with the new isoprene photochemistry mechanism. This study demonstrates how inadequacies in our current understanding of isoprene emissions and photochemistry impede our ability to constrain the oxidative capacities of the present and past atmospheres, its controlling factors, and the radiative forcing of some short-lived species such as SOA over time.
Highlights
A key player in the coupling between climate change and atmospheric chemical composition is the oxidative capacity of the troposphere, primarily characterized by the burden of the four most abundant and reactive oxidants: OH, ozone, H2O2, and NO3
Results from the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) demonstrate that uncertainties remain in our understanding of the long-term trends in OH and methane lifetime and that these uncertainties primarily stem from a lack of adequate constraints on natural precursor emissions and on the chemical mechanisms in the current generation of chemistry–climate models (Naik et al, 2013)
Using a detailed climate–biosphere–chemistry framework, we evaluate the sensitivity of modeled tropospheric oxidant levels to recent advances in our understanding of biogenic isoprene emissions and of the fate of isoprene oxidation products in the atmosphere
Summary
A key player in the coupling between climate change and atmospheric chemical composition is the oxidative capacity of the troposphere, primarily characterized by the burden of the four most abundant and reactive oxidants: OH, ozone, H2O2, and NO3. Recent studies have suggested the need to revise our understanding of the environmental factors controlling biogenic isoprene emissions and of its atmospheric photo-oxidation mechanism (e.g., Paulot et al, 2009a, b; Possell and Hewitt, 2011) These advances call into question the validity of existing model estimates of the oxidative capacity of past atmospheres. Murray et al (2014) found that (1) the oxidative capacities of the preindustrial and LGM atmospheres were both lower than that of the present day; (2) tropospheric mean OH levels appear to be well buffered in the LGM-to-preindustrial transition – a result at odds with most prior studies; (3) past changes in atmospheric methane concentrations were predominantly source driven; and (4) the key parameters controlling the oxidative capacity over LGM–present-day timescales are tropospheric mean ozone photolysis rates, water vapor abundance, and total emissions of NOx and reactive carbon. In a systematic manner, the effects of all of the above developments on the chemical composition of the troposphere over the last glacial–interglacial time interval and the industrial era
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