Latitudinal variation of the effect of aviation NOx emissions on atmospheric ozone and methane and related climate metrics
Highlights
► Global model study on climate response to regional/latitudinal aircraft NOx changes. ► RF impacts from O3 and CH4 are larger for changes at lower than at higher latitudes. ► GWPs for all latitudes are positive whereas the GTP can exhibit a change in sign.
Introduction
Emissions from global aviation alter atmospheric composition. These emissions include oxides of nitrogen (NOx = NO + NO2) which result in the increased formation of ozone (O3) and in a reduced lifetime and concentration of methane (CH4) (see Lee et al., 2010; Myhre et al., 2011; Holmes et al., 2011, and references therein) and result in a radiative forcing (RF) that can drive climate change. The RF due to NOx emissions depends sensitively on where the emissions occur unlike emissions of carbon dioxide (CO2) which is long lived and hence globally well-mixed.
Air traffic has a heterogeneous global distribution with the majority concentrated in northern hemisphere mid-latitudes, particularly over North America, Europe, the North Atlantic and Japan (Eyers et al., 2005). A substantial increase in future air traffic is predicted in countries with fast growing economies such as China and India (ACI, 2011).
We investigate the link between the location of aircraft NOx emissions and the consequent RF, by examining the effect of emissions from four distinct regions: Two of these experience high air traffic today; in the other two air traffic is expected to grow significantly. We also examine systematically the effect of latitude of aircraft emissions at cruise altitude. We apply sustained regional perturbations to aircraft NOx emissions from a contemporary emissions inventory, using a global chemistry transport model in combination with an off-line radiative transfer model. We then calculate climate emission metrics (the Global Warming Potential and Global Temperature Change Potential) to illustrate the dependence on the type of climate metric.
A particular difficulty is that the net RF of NOx emissions is a residual of effects of opposing signs – a short-lived and positive ozone RF directly resulting from the NOx emissions is accompanied by a longer-lived negative forcing due to a consequent decrease in methane concentrations and an associated decrease in ozone. Further impacts from NOx emissions are the formation of nitrate particles (Kärcher, 1996) and indirectly a more effective conversion of SO2 to sulphuric acid due to an increase in OH and the subsequent formation of sulphate aerosol (Pitari et al., 2002). These further NOx impacts have been much less studied and are not considered here.
The net RF from aircraft NOx emissions for the year 2005 is reported by Lee et al. (2010) as the second largest (13.8 mW m−2) after that from CO2 (28 mW m−2). The more uncertain RF for aircraft-induced cloudiness could however be larger than these (estimated 33 mW m−2). Hoor et al. (2009), Myhre et al. (2011) and Holmes et al. (2011) report large inter-model differences in the RF of aircraft NOx. The relative importance of aircraft NOx is influenced by the background NOx concentrations, originating from surface or lightning emissions (Berntsen and Isaksen, 1999). The abundance of the oxides of hydrogen (HOx), volatile organic compounds (VOCs), as well as the available solar irradiance play further important roles (Jaeglé et al., 1998). Earlier modelling studies (e.g. Berntsen et al., 2005; Derwent et al., 2008) found that the global-mean RF and surface temperature change resulting from surface NOx emissions depend on where emissions occur. Köhler et al. (2008) have shown that changing the flight routing pattern at cruise altitudes significantly changes the impact on ozone and methane, and the associated RF. Grewe and Stenke (2008) found a significant difference in the RF from aircraft NOx emissions in four latitude bands and six height levels for a 2050 background scenario. Stevenson and Derwent (2009) calculated the effect of location of cruise-level aviation NOx emissions on the 100-year time-integrated RF (the absolute global warming potential) in response to pulse emissions of NOx in about 100 different locations. They show a strong dependence on the background NOx levels, and emphasise the compensation between the short-lived ozone effect and the longer-lived methane effect.
We quantify here the effect of the location of aircraft NOx emissions on RF by increasing NOx emissions by small amounts in different regions and at different latitudes. This will illustrate how the effect of aviation NOx emissions on ozone, methane, and the associated RF depend on the location of emissions, which is of particular importance as future growth in aviation emissions is likely to be globally heterogeneous. The results for metrics have potential applications in policy contexts (see e.g. Fuglestvedt et al., 2010) and when considering the climate impact of changes in aircraft design (e.g. Schwartz Dallara et al., 2011).
Sections 2 Model description, 3 Experiments describe the model and experiment setup. Section 4 focuses on composition changes and the associated RF. Section 5 presents the climate metrics. We conclude with a discussion of our findings.
Section snippets
Model description
The p-TOMCAT chemistry transport model was used in a similar configuration as in Hoor et al. (2009) and Myhre et al. (2011), an updated version compared with that used in Köhler et al. (2008); the most important updates concern the model's chemical reaction rates and surface emissions. As in Hoor et al. (2009) the model grid has a horizontal resolution of 5.6 × 5.6°, but with increased vertical resolution of 35 vertical levels between the surface and 10 hPa as in Köhler et al. (2008) resulting
Experiments
We have carried out sensitivity studies where in one region at a time small perturbations in NOx are applied. Emission scaling has been used previously to estimate the contribution from individual sectors to changes in composition and RF (e.g., Hoor et al., 2009; Myhre et al., 2011). Critical evaluation of this method (e.g., Wang et al., 2009; Grewe et al., 2010) has raised concerns about non-linearities in the atmospheric response which can introduce uncertainties when large scaling factors
Results
In p-TOMCAT the unaltered AERO2k NOx emissions (0.68 Tg (N) yr−1) cause an increase in the global annual mean O3 column by 0.57 DU which is approximately 30% less than the change reported in Köhler et al. (2008). This reduction is caused by the inclusion of updated chemistry and surface emissions in the model. The all-sky global-mean stratospheric temperature adjusted RF from short-lived ozone was 19.6 mW m−2 (compared to 22.7 mW m−2 for the instantaneous forcing). Köhler et al. (2008) found a
Climate emission metrics
Radiative forcing provides an indication of the equilibrium climate effect of sustained NOx emissions. Because of the differing lifetimes of the various components, RF does not give a useful view of the future impact of current emissions, which may be required for considering possible mitigation options and for comparison with other aviation climate impacts. We present values for two climate metrics – the Global Warming Potential (GWP) and the Global Temperature Change Potential (GTP),
Conclusions
We have carried out chemical transport model experiments with regional perturbations to aircraft NOx emissions. First emission perturbations were applied throughout the vertical extent of the atmosphere in four distinct geographical regions. Then NOx was increased at cruise altitude in discrete latitude bands.
Our results show strong regional sensitivity of ozone and methane to changes in NOx, particularly at cruise altitude. In general, low latitude emission increases result in stronger impacts
Acknowledgements
We are grateful to the AERO2k and QUANTIFY community for providing access to emissions data. GR acknowledges funding through the EU FP6 QUANTIFY Project. MOK acknowledges funding from NERC through the AIM Project. KS acknowledges support from the EU FP7 ECLIPSE project. We are grateful to O. Dessens for engaging discussions and to three reviewers for many helpful comments.
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