Atmospheric Chemistry and Physics | 2021
Air traffic and contrail changes over Europe during COVID-19: a model study
Abstract
Abstract. The strong reduction of air traffic during the COVID-19\npandemic provides a unique test case for the relationship between air traffic\ndensity, contrails, and their radiative forcing of climate change. Here, air\ntraffic and contrail cirrus changes are quantified for a European domain for\nMarch to August 2020 and compared to the same period in 2019. Traffic data\nshow a 72\u2009% reduction in flight distance compared with 2019. This paper\ninvestigates the induced contrail changes in a model study. The contrail\nmodel results depend on various methodological details as discussed in\nparameter studies. In the reference case, the reduced traffic caused a\nreduction in contrail length. The reduction is slightly stronger than\nexpected from the traffic change because the weather conditions in 2020 were\nless favorable for contrail formation than in 2019. Contrail coverage over\nEurope with an optical depth larger than 0.1 decreased from 4.6\u2009% in 2019\nto 1.4\u2009% in 2020; the total cirrus cover amount changed by 28\u2009% to 25\u2009%. The\nreduced contrail coverage caused 70\u2009% less longwave and 73\u2009% less\nshortwave radiative forcing but, because of various nonlinearities, only 54\u2009% less net forcing in this case. The methods include recently developed\nmodels for performance parameters and soot emissions. The overall propulsion\nefficiency of the aircraft is about 20\u2009% smaller than estimated in\nearlier studies, resulting in 3\u2009% fewer contrails. Considerable\nsensitivity to soot emissions is found, highlighting fuel and engine\nimportance. The contrail model includes a new approximate method to account\nfor water vapor exchange between contrails and background air and for\nradiative forcing changes due to contrail–contrail overlap. The water vapor\nexchange reduces available ice supersaturation in the atmosphere, which is\ncritical for contrail formation. Contrail–contrail overlap changes the\ncomputed radiative forcing considerably. Comparisons to satellite\nobservations are described and discussed in a parallel publication.