Predicting the climate impact of aviation for en-route emissions: the algorithmic climate change function submodel ACCF 1.0 of EMAC 2.53
Authors: Yin Feijia , Grewe Volker , Castino Federica , Rao Pratik , Matthes Sigrun , Dahlmann Katrin , Dietmüller Simone , Frömming Christine , Yamashita Hiroshi , Peter Patrick , Klingaman Emma , Shine Keith P. , Lührs Benjamin , Linke Florian 
Published: June 2023

Abstract. Using climate-optimized flight trajectories is one
essential measure to reduce aviation's climate impact. Detailed knowledge of
temporal and spatial climate sensitivity for aviation emissions in the
atmosphere is required to realize such a climate mitigation measure. The
algorithmic Climate Change Functions (aCCFs) represent the basis for such
purposes. This paper presents the first version of the Algorithmic Climate
Change Function submodel (ACCF 1.0) within the European Centre HAMburg
general circulation model (ECHAM) and Modular Earth Submodel System (MESSy)
Atmospheric Chemistry (EMAC) model framework. In the ACCF 1.0, we implement
a set of aCCFs (version 1.0) to estimate the average temperature response
over 20 years (ATR20) resulting from aviation CO2 emissions and
non-CO2 impacts, such as NOx emissions (via ozone production and
methane destruction), water vapour emissions, and contrail cirrus. While the
aCCF concept has been introduced in previous research, here, we publish a
consistent set of aCCF formulas in terms of fuel scenario, metric, and
efficacy for the first time. In particular, this paper elaborates on
contrail aCCF development, which has not been published before. ACCF 1.0
uses the simulated atmospheric conditions at the emission location as input
to calculate the ATR20 per unit of fuel burned, per NOx emitted, or per flown
kilometre. In this research, we perform quality checks of the ACCF 1.0 outputs in two
aspects. Firstly, we compare climatological values calculated by ACCF 1.0 to
previous studies. The comparison confirms that in the Northern Hemisphere
between 150–300 hPa altitude (flight corridor), the vertical and latitudinal
structure of NOx-induced ozone and H2O effects are well
represented by the ACCF model output. The NOx-induced methane effects
increase towards lower altitudes and higher latitudes, which behaves
differently from the existing literature. For contrail cirrus, the
climatological pattern of the ACCF model output corresponds with the
literature, except that contrail-cirrus aCCF generates values at low
altitudes near polar regions, which is caused by the conditions set up for
contrail formation. Secondly, we evaluate the reduction of NOx-induced
ozone effects through trajectory optimization, employing the tagging
chemistry approach (contribution approach to tag species according to their
emission categories and to inherit these tags to other species during the
subsequent chemical reactions). The simulation results show that
climate-optimized trajectories reduce the radiative forcing contribution
from aviation NOx-induced ozone compared to cost-optimized
trajectories. Finally, we couple the ACCF 1.0 to the air traffic simulation
submodel AirTraf version 2.0 and demonstrate the variability of the flight
trajectories when the efficacy of individual effects is considered. Based on
the 1 d simulation results of a subset of European flights, the total
ATR20 of the climate-optimized flights is significantly lower (roughly
50 % less) than that of the cost-optimized flights, with the most
considerable contribution from contrail cirrus. The CO2 contribution
observed in this study is low compared with the non-CO2 effects, which
requires further diagnosis.