Abstract
Abstract. This study evaluates the distribution and variation of carbon monoxide (CO) in the upper troposphere and lower stratosphere (UTLS) during 2004–2012 as simulated by two chemical transport models, using the latest version of Aura Microwave Limb Sounder (MLS) observations. The simulated spatial distributions, temporal variations and vertical transport of CO in the UTLS region are compared with those observed by MLS. We also investigate the impact of surface emissions and deep convection on CO concentrations in the UTLS over different regions, using both model simulations and MLS observations. Global Modeling Initiative (GMI) and GEOS-Chem simulations of UTLS CO both show similar spatial distributions to observations. The global mean CO values simulated by both models agree with MLS observations at 215 and 147 hPa, but are significantly underestimated by more than 40 % at 100 hPa. In addition, the models underestimate the peak CO values by up to 70 % at 100 hPa, 60 % at 147 hPa and 40 % at 215 hPa, with GEOS-Chem generally simulating more CO at 100 hPa and less CO at 215 hPa than GMI. The seasonal distributions of CO simulated by both models are in better agreement with MLS in the Southern Hemisphere (SH) than in the Northern Hemisphere (NH), with disagreements between model and observations over enhanced CO regions such as southern Africa. The simulated vertical transport of CO shows better agreement with MLS in the tropics and the SH subtropics than the NH subtropics. We also examine regional variations in the relationships among surface CO emission, convection and UTLS CO concentrations. The two models exhibit emission–convection–CO relationships similar to those observed by MLS over the tropics and some regions with enhanced UTLS CO.
Highlights
Carbon monoxide (CO) plays important roles in atmospheric chemistry and radiation balance
We evaluate the spatial distribution and temporal variation of CO in the upper troposphere and lower stratosphere (UTLS) during 2004–2012 simulated by two chemical transport models (GMI and Goddard Earth Observing System (GEOS)-Chem) using the latest version (V4.2) of Aura Microwave Limb Sounder (MLS) data
The relationships between emission, convection, and CO mixing ratio in the UTLS are investigated over different regions using MLS observations and model simulations
Summary
Carbon monoxide (CO) plays important roles in atmospheric chemistry and radiation balance. With a typical lifetime of 1–2 months in the troposphere, CO has been often used as a tracer for studying the transport of polluted air masses that originate in regions of biomass burning or fossil fuel combustion (e.g., Allen et al, 1996; Edwards et al, 2006; Huang et al, 2012) Previous studies using both satellite observations and model simulations have shown that CO has strong seasonal and interannual variations in the upper troposphere and lower stratosphere (UTLS) (e.g., Schoeberl et al, 2006; Liu et al, 2007, 2010, 2013; Huang et al, 2012, 2014). Strong interannual variation of CO in the UT has been found to be mainly associated with intense drought-induced fires in Indonesia and South America during El Niño periods (Liu et al, 2013; Livesey et al, 2013; Huang et al, 2014) Both surface emissions and convective transport could influence the seasonal peaks of CO in the UTLS, the relative importance of each factor varies between regions.
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