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The combination of low anthropogenic emissions and large biogenic  sources that characterizes the Southern Hemisphere (SH) leads to significant  differences in atmospheric composition relative to the better studied Northern  Hemisphere. This unique balance of sources poses significant challenges  for global models. Carbon monoxide (CO) in particular is difficult to simulate  in the SH due to the increased importance of secondary chemical production  associated with the much more limited primary emissions.  Here, we use aircraft observations from the 1991–2000 Cape Grim  Overflight Program (CGOP) and the 2009–2011 HIAPER (High-performanceInstrumented Airborne Platform for Environmental Research)Pole-to-Pole Observations (HIPPO), together with model output from the SH  Model Intercomparison Project, to elucidate the drivers of  CO vertical structure in the remote SH. Observed CO vertical profiles from Cape Grim are  remarkably consistent with those observed over  the southern mid-latitudes Pacific 10–20 years later, despite major  differences in time periods, flight locations, and sampling  strategies between the two data sets. These similarities suggest  the processes driving observed vertical gradients are  coherent across much of the remote SH and have not changed  significantly over the past 2 decades. Model ability to simulate CO profiles  reflects the interplay between biogenic emission sources, the chemical  mechanisms that drive CO production from these sources,  and the transport that redistributes this CO throughout the SH. The four chemistry-climate  and chemical transport models included in the intercomparison show large variability in their abilities  to reproduce the observed CO profiles. In particular, two of the four models significantly  underestimate vertical gradients in austral summer and autumn, which we find  are driven by long-range transport of CO produced from oxidation of biogenic compounds.  Comparisons between the models show that more complex chemical mechanisms  do not necessarily provide more accurate simulation of CO vertical gradients  due to the convolved impacts of emissions, chemistry, and transport.  Our results imply a large sensitivity of the  remote SH troposphere to biogenic emissions and  chemistry, both of which remain key uncertainties in global modeling.  We suggest that the CO vertical gradient  can be used as a metric for future model evaluation as it provides  a sensitive test of the processes that define  the chemical state of the background atmosphere

atmospheric chemistry and physics

Seasonal changes in the tropospheric carbon monoxide profile over the remote Southern Hemisphere evaluated using multi-model simulations and aircraft observations