Attribution of ozone radiative forcing trend to individual NO_x sources

Abstract. Decadal means of ozone fields from transient E39/C climate-chemistry simulations (1960–2019) are analysed and temporally developing ozone radiative forcings (RF) are investigated which result from individual ozone precursor sources like road traffic, industry, air traffic, etc. We study how effective NO_x emissions from different sources produce ozone. This ozone production efficiency is mainly dependent on the altitude of NO_x emission and on the amount of background NO_x. For example, our study shows that the ozone production efficiency of lightning and air traffic have a five and two time higher ozone production efficiency than ground based sources. The radiative efficiency of ozone (i.e. the radiative forcing per molecule) is mainly dependent on the surface temperature, but also, to a lesser degree, on the altitude of added ozone. Lightning, for example, causes the highest specific RF due to the fact that lightning primarily enhances ozone in low latitudes in the mid-troposphere. Superimposed on these effects, is a saturation effect which causes a decreasing RF efficiency with increasing background ozone. A consequence of this saturation effect is an underestimation of total RF by about 10% if the component RFs of individual ozone sources are calculated separately and added up afterwards. The results show that the time development of emissions (1960–2019) control the RF changes for most sources. RF changes are slightly reduced due to a changing atmospheric composition (10–25%) for all but the aircraft sources, and due to RF saturation (2–5%).