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

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 LGMpreindustrial 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 NO x 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.

Southeast Atmosphere Studies: learning from model-observation syntheses

“…Background ozone, however, may increase due to increasing methane ( West et al, 2012 ). A major challenge in quantifying the future trends in surface air quality is our lack of knowledge in temperature-dependent isoprene emissions and photochemistry ( Achakulwisut et al, 2015 ).…”

Aerosol-Climate Interactions During the Last Glacial Maximum

“…In terms of other biogenic emissions from land vegetation, global simulations with an Earth system model estimated isoprene emissions between~250 and 850 TgC a −1 in the LGM depending on the assumptions on CO 2 sensitivity and temperature boundary conditions, and a decrease by 42-44% in total SOA burden compared to their pre-industrial control for their central case [136,137].…”

“…In synthesis, the overall knowledge on LGM carbonaceous aerosol is very limited. By combining information from the source of fires with information of past vegetation [138] and linking with past and modern data from sinks like snow and ice [134], we might have the tools to evaluate and constrain models simulating past fire activity, hence potentially the emissions of carbonaceous aerosols from fire [136,139,140] and vegetation [137]. Additional information is therefore needed, to provide a constraint on the sinks of carbonaceous aerosols, from polar (and potentially some alpine) ice cores spanning the LGM.…”

Understanding the glacial methane cycle

“…Changes in emissions of nitrogen oxides (NO x ) from soils and lightning, and changes in photolysis rates do not alter this conclusion 16 . However, these past studies may have overestimated the reduction in emissions of BVOCs during the LGM, because they did not include the leaf-level stimulation of isoprene emissions caused by the low (185 ppmv) LGM atmospheric CO 2 , which potentially counters the effects from a cooler climate 17 18 .…”