On the wintertime low bias of Northern Hemisphere carbon monoxide found in global model simulations

Abstract

Abstract. Despite the developments in the global modelling of chemistry and of the parameterization of the physical processes, carbon monoxide (CO) concentrations remain underestimated during Northern Hemisphere (NH) winter by most state-of-the-art chemistry transport models. The consequential model bias can in principle originate from either an underestimation of CO sources or an overestimation of its sinks. We address both the role of surface sources and sinks with a series of MOZART (Model for Ozone And Related Tracers) model sensitivity studies for the year 2008 and compare our results to observational data from ground-based stations, satellite observations, and vertical profiles from measurements on passenger aircraft. In our base case simulation using MACCity (Monitoring Atmospheric Composition and Climate project) anthropogenic emissions, the near-surface CO mixing ratios are underestimated in the Northern Hemisphere by more than 20 ppb from December to April, with the largest bias of up to 75 ppb over Europe in January. An increase in global biomass burning or biogenic emissions of CO or volatile organic compounds (VOCs) is not able to reduce the annual course of the model bias and yields concentrations over the Southern Hemisphere which are too high. Raising global annual anthropogenic emissions with a simple scaling factor results in overestimations of surface mixing ratios in most regions all year round. Instead, our results indicate that anthropogenic CO and, possibly, VOC emissions in the MACCity inventory are too low for the industrialized countries only during winter and spring. Reasonable agreement with observations can only be achieved if the CO emissions are adjusted seasonally with regionally varying scaling factors. A part of the model bias could also be eliminated by exchanging the original resistance-type dry deposition scheme with a parameterization for CO uptake by oxidation from soil bacteria and microbes, which reduces the boreal winter dry deposition fluxes. The best match to surface observations, satellite retrievals, and aircraft observations was achieved when the modified dry deposition scheme was combined with increased wintertime road traffic emissions over Europe and North America (factors up to 4.5 and 2, respectively). One reason for the apparent underestimation of emissions may be an exaggerated downward trend in the Representative Concentration Pathway (RCP) 8.5 scenario in these regions between 2000 and 2010, as this scenario was used to extrapolate the MACCity emissions from their base year 2000. This factor is potentially amplified by a lack of knowledge about the seasonality of emissions. A methane lifetime of 9.7 yr for our basic model and 9.8 yr for the optimized simulation agrees well with current estimates of global OH, but we cannot fully exclude a potential effect from errors in the geographical and seasonal distribution of OH concentrations on the modelled CO.

Tropospheric ozone and its precursors from the urban to the global scale from air quality to short-lived climate forcer

“…22a (Fortems-Cheiney et al, 2011) and Fig. 22b (Tohjima et al, 2014), more particularly at midlatitudes of the Northern Hemisphere in winter (Kopacz et al, 2010;Stein et al, 2014). However, an analysis of satellite and aircraft observations has also shown that emissions inventories in North America might provide overestimated values (Miller et al, 2008).…”

Regional data assimilation of multi-spectral MOPITT observations of CO over North America

“…The estimated winter emissions of Kopacz et al (2010) are about 50 % larger than the summer emissions. Kopacz et al (2010) and Stein et al (2014) attributed the higher wintertime emissions in the Northern Hemisphere to vehicular emissions, which are not account for in the a priori emission inventory.…”

Uncertainties of fluxes and <sup>13</sup>C ∕ <sup>12</sup>C ratios of atmospheric reactive-gas emissions

“…B00 and E04; see also Stein et al, 2014, and references therein). Stein et al (2014) show that a more detailed representation of the strength and seasonality of CO dry deposition fluxes and traffic emissions in Europe and North America leads to more adequately reproduced NH CO mixing ratios. It is noteworthy that their hypothesis that the missing traffic CO is due to emission inventories not accounting for cold-start engine conditions should be verifiable through 18 O / 16 O ratio of emitted CO: the latter (but unfortunately not 13 C / 12 C ratio) differ substantially between the BB and FF sources (see Kato et al, 1999a, also Sect.…”

“…2), as well as in other employed models/inventories (e.g. B00 and E04; see also Stein et al, 2014, and references therein). Stein et al (2014) show that a more detailed representation of the strength and seasonality of CO dry deposition fluxes and traffic emissions in Europe and North America leads to more adequately reproduced NH CO mixing ratios.…”