The contribution of transport emissions to ozone mixing ratios in 2015 and 2050 in the Shared Socioeconomic Pathways (SSPs)

The transport sector (i.e. aviation, land-based transport and shipping) is an important source of emissions of ozone precursors such as nitrogen oxides (NO_x), volatile organic compounds (VOCs) and carbon monoxide (CO). The formation of ozone from these precursors is a highly non-linear process which depends strongly on atmospheric background conditions (i.e., emissions from other sectors and meteorological conditions) and on the amount of transport emissions. As the different transport sectors emit at different geographical locations under different background conditions, their impact on tropospheric ozone cannot be estimated simply by comparing the emission shares. Instead, model-based spatially resolving approaches are needed that include a detailed ozone source attribution. To do this, we quantified for the first time the contribution of each transport sector to tropospheric ozone in a consistent way using the EMAC global chemistry-climate model equipped with an ozone source apportionment technique (called TAGGING). We performed simulations for present-day (2015) and for 2050 under the Shared Socioeconomic Pathways (SSP) SSP1-1.9, SSP2-4.5 and SSP3-7.0 and analysed the contributions of land-based transport from different geographical regions, shipping, and aviation to ozone. Based on these calculated contributions, we quantify the ozone radiative forcing (RF) attributable to emissions from the transport sector.

For 2015, we estimate an ozone RF attributable to emissions from land-based transport, shipping and aviation of 121 mWm^-2, 60 mWm^-2, and 31 mWm^-2,respectively. Compared to 2015, only SSP1-1.9 shows a strong decrease of ozone RF attributable to the entire transport sector in 2050. For the SSP2-4.5 scenario, we find similar RFs of the entire transport sector as for 2015, while the RFs in SSP3-7.0 increase compared to 2015.