The contribution of transport emissions on ozone and methane lifetime in 2015 and 2050 in the Shared Socioeconomic Pathways (SSPs)

Abstract

The transport sector, i.e. land transport, shipping and aviation, is an important source of emissions of NOx and further contributes to emissions of CO and VOCs. These components act as precursors for the formation of tropospheric ozone and impact the lifetime of methane by affecting the oxidation capacity of the atmosphere. Both of these effects in turn have an impact on climate. However, only a few self-consistent quantifications of the contribution of all transport modes on ozone and the effects on methane lifetime exist. Often, only specific transport modes are considered or different methods/different emission data sets are applied. Therefore, we analyze the contribution of emissions from all transport modes for 2015 and for three different projections for 2050 in the Shared Socioeconomic Pathways (SSPs) emissions within the DLR project TraK. For the first time, we apply a source attribution method (tagging) to attribute the transport emissions to ozone and OH, and thus to methane lifetime. For the present study, the chemistry-climate model EMAC has been used in in the T42L90MA resolution (approx. 2.8 by 2.8° in latitude and longitude with 90 vertical hybrid pressure levels up to 0.01 hPa). Four simulations for the period 07/2012-12/2017 have been performed with different inputs for emissions of short-lived components – one simulation with emissions for 2015 and three simulations with emissions for 2050, using the SSP2-4.5, SSP1-1.9 and SSP3-7.0 pathway, respectively. Emissions of long-lived species (e.g. CH4) were kept constant in all simulations. In addition, dynamics and chemistry have been decoupled, allowing for the same dynamics in all simulations. Stratospheric adjusted ozone radiative forcing (RF) of the ozone attributed to specific emission sectors have been calculated during runtime with additional calls of the radiation scheme. Our results show that land transport emissions are the transport mode with the largest contribution to ozone (ground-level ozone and ozone RF) as well as the largest effect on the methane lifetime in 2015 and for all emission pathways in 2050. While the shipping sector is the second largest contributor to ozone RF and also has the second largest effect on methane lifetime in 2015, the aviation sector gets more important in all pathways for 2050, overtaking the shipping sector. In the SSP2-4.5 and SSP3-7.0 emission pathways, transport emissions contribute significantly to ozone and also have the largest effect on methane lifetime. Only the SSP1-1.9 emission pathway transport emissions are reduced strongly, reducing climate effects due to ozone changes of the methane lifetime. This highlights the urgent need for more stringent emission reduction policies in the transport sector.