Abstract
Abstract. The oxidative capacity of past atmospheres is highly uncertain. We present here a new climate–biosphere–chemistry modeling framework to determine oxidant levels in the present and past troposphere. We use the GEOS-Chem chemical transport model driven by meteorological fields from the NASA Goddard Institute of Space Studies (GISS) ModelE, with land cover and fire emissions from dynamic global vegetation models. We present time-slice simulations for the present day, late preindustrial era (AD 1770), and the Last Glacial Maximum (LGM, 19–23 ka), and we test the sensitivity of model results to uncertainty in lightning and fire emissions. We find that most preindustrial and paleo climate simulations yield reduced oxidant levels relative to the present day. Contrary to prior studies, tropospheric mean OH in our ensemble shows little change at the LGM relative to the preindustrial era (0.5 ± 12 %), despite large reductions in methane concentrations. We find a simple linear relationship between tropospheric mean ozone photolysis rates, water vapor, and total emissions of NOx and reactive carbon that explains 72 % of the variability in global mean OH in 11 different simulations across the last glacial–interglacial time interval and the industrial era. Key parameters controlling the tropospheric oxidative capacity over glacial–interglacial periods include overhead stratospheric ozone, tropospheric water vapor, and lightning NOx emissions. Variability in global mean OH since the LGM is insensitive to fire emissions. Our simulations are broadly consistent with ice-core records of Δ17O in sulfate and nitrate at the LGM, and CO, HCHO, and H2O2 in the preindustrial era. Our results imply that the glacial–interglacial changes in atmospheric methane observed in ice cores are predominantly driven by changes in its sources as opposed to its sink with OH.
Highlights
The oxidative capacity of the troposphere – primarily characterized by the burden of the four most abundant and reactive oxidants (OH, ozone, H2O2, and NO3) – determines the lifetime before removal of many trace gases of importance to climate and human health, including air pollutants and the greenhouse gas methane (Fiore et al, 2012)
This study introduces the ICE age Chemistry And Proxies (ICECAP) project, which adopts as its central component a stepwise, offline-coupled framework for simulating the chemical composition of the atmosphere at and since the Last Glacial Maximum (LGM)
The stratosphere is a net source of NOx into the troposphere and a net sink of tropospheric CO. Stratospheric concentrations of these species are calculated from monthly climatological 3-D production rates and loss frequencies archived from the Global Modeling Initiative (GMI) chemical transport model (CTM) driven by MERRA assimilated meteorology for 2004–2010 (Allen et al, 2010; Duncan et al, 2007; Considine et al, 2008) and re-gridded to the ModelE grid
Summary
The oxidative capacity of the troposphere – primarily characterized by the burden of the four most abundant and reactive oxidants (OH, ozone, H2O2, and NO3) – determines the lifetime before removal of many trace gases of importance to climate and human health, including air pollutants and the greenhouse gas methane (Fiore et al, 2012). Tropospheric oxidant levels respond in well known but complex ways to meteorological conditions, changes in emissions of key chemical species, and changes in surface and stratospheric boundary conditions These parameters include temperature, clouds, water vapor concentrations, overhead ozone columns, aerosols, and emissions of reactive oxides of nitrogen (NOx), carbon monoxide, and volatile organic compounds (VOCs) such as methane (e.g., Thompson and Stewart, 1991; Spivakovsky et al, 2000; Lelieveld et al, 2002; Holmes et al, 2013). This sensitivity suggests that multi-millennial global environmental change, including changes in climate and land cover, should affect tropospheric oxidative capacity.
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