České vysoké učení technické v Praze Fakulta dopravní 1. listopadu 2018 Praha, Česká republika Aircraft Contrail Research Sébastien Láni, Tereza Topkováii, Iveta Kameníkováiii Abstrakt: Kondenzační stopa je umělý oblak mající podobu pruhu, který se tvoří za letadly. Její životnost může dosahovat několika hodin, přičemž se může dále rozšiřovat a přejít v indukovanou cirrovitou oblačnost. Kondenzační stopy způsobují pozitivní radiační působení a přispívají tak k oteplování atmosféry. Tento článek popisuje vznik kondenzačních stop, jejich dopad na podnebí a výzkum zaměřený na kondenzační stopy, který probíhá na Ústavu letecké dopravy, Fakultě dopravní, Českém vysokém učení technickém v Praze. Výzkum je založený na pozorování kondenzačních stop a využití zpráv ADS- B, zpráv módu S a výsledků aerologického měření. Cílem je zjistit, jak často se kondenzační stopy tvoří, jaká je jich životnost a další vlastnosti v závislosti na meteorologických podmínkách. Klíčová slova: letadlo, kondenzační stopa, emise, podnebí, radiační působení, ADS-B, Mód S, BDS registr, aerologické měření Abstract: Contrails are line-shaped clouds formed behind aircraft which may persist for hours while growing to resemble cirrus clouds. Contrails cause positive radiative forcing which tends to warm the atmosphere. This paper describes the contrail formation, climate effect and contrail research carried out by the Department of Air Transport, Faculty of Transportation Sciences, Czech Technical University in Prague. The research is based on contrail observation, ADS-B messages, Mode S messages and aerological measurement. The aim of the research is to identify the frequency of contrail occurrence, their lifetime and other properties and compare them to meteorological conditions. Keywords: aircraft, contrail, emission, climate, radiative forcing, ADS-B, Mode S, BDS register, aerological measurement 1. Introduction Condensation trail or contrail is one of the most visible anthropogenic effects on the atmosphere. Contrail is a white strip composed of ice crystals which forms behind an aircraft flying in cold air due to water vapor emissions. It has become a common sight since the 1960s due to civil jet aircraft traffic increase. A contrail forms due to high content of water vapor in the engine exhaust plume. When the air is supersaturated, contrails might cause contrail cirrus. Contrails together with contrail induced cloudiness increase the global cloud coverage and have negative impact on the radiation balance of the Earth-atmosphere system. Therefore, it is important to research contrail formation, properties and techniques to avoid contrails. This paper describes the thermodynamic principle of contrail formation, contrail negative impact on i Ing. Sébastien Lán, Czech Technical University in Prague, Faculty of Transportation Sciences, Department of Air Transport, Horská 3, 128 03 Praha 2, Czech Republic, e-mail: [email protected] ii Ing. Tereza Topková, Czech Technical University in Prague, Faculty of Transportation Sciences, Department of Air Transport, Horská 3, 128 03 Praha 2, Czech Republic, e-mail: [email protected] iii Mgr. Iveta Kameníková, Czech Technical University in Prague, Faculty of Transportation Sciences, Department of Air Transport, Horská 3, 128 03 Praha 2, Czech Republic, e-mail: [email protected] 1 České vysoké učení technické v Praze Fakulta dopravní 1. listopadu 2018 Praha, Česká republika the atmosphere and ongoing scientific research carried out by the Department of Air Transport at the Czech Technical University in Prague [1, 2]. 2. Contrail formation 2.1. Principle of contrails formation Contrail is an artificial cloud which looks like cirrus or cirrocumulus and forms behind jet engines in the upper troposphere and lower stratosphere. Contrail is initially 5 m to 10 m wide and forms at a distance of 50 m to 100 m behind an aircraft. Its lifetime is usually less than forty minutes. It forms at altitudes from 7 km to 12 km where the ambient temperature is about -40 ℃ to -50 ℃ [3]. Contrail formation is caused by increase in relative humidity in the engine plume as a result of mixing hot and moist exhaust gases coming out of the engine with cold ambient air. Fuel used by present jet aircraft is kerosene, primarily composed of hydrocarbons. There are two main products of kerosene combustion – carbon dioxide and water vapor. The second mentioned product has a major impact on contrail formation. The kerosene water vapor emission index is about 1,23. It means that combustion of one kilogram of kerosene gives 1,23 kg of water vapor. The consequence of large value of emission index is high content of water vapor in aircraft engine exhaust [4, 5]. If the ambient air is cold enough the moisture in the engine plume may reach saturation point, i.e. state when the air water content cannot be higher. The moisture may even reach supersaturation point. Supersaturated air contains more water molecules then possible. These molecules tend to go from gaseous state to liquid state and solid state, respectively. When the air is supersaturated condensation (gas to liquid state transition) eventually deposition (gas to solid state transition) begins. Water droplets and ice crystals formation is necessary to achieve equilibrium state. Due to condensation and deposition the ambient air goes from the supersaturated state back to the saturated state. During the condensation process water droplets are being developed. The water vapor condenses mainly on ambient and exhaust aerosols called condensation nuclei. Especially soot particles are very suitable to act as condensation nuclei. Due to very low ambient temperature the water droplets instantly freeze, form icy crystals and grow via deposition. The ice crystals grow as long as the humidity with respect to ice is above the saturation point [1, 4]. Contrail shape and lifetime is dependent on several meteorological factors which include relative humidity, turbulence, atmosphere vertical movements. Contrail eventually dissipates via sublimation (solid to gas state transition) if relative humidity with respect to ice is below the saturation point or by precipitation into non-saturated layers below the flight level [1]. 2.2. Schmidt-Appleman criterion The basic rule defining whether a contrail will occur or not is the Schmidt-Appleman criterion. According to this criterion contrail forms if the humidity in the engine plume reaches liquid saturation. Saturation with respect to ice is not sufficient for contrail formation. When the air is supersaturated with respect to liquid water, the water vapor starts to condense [6, 7]. 2 České vysoké učení technické v Praze Fakulta dopravní 1. listopadu 2018 Praha, Česká republika The Schmidt-Appleman criterion is met when the ambient temperature is below the threshold temperature. The threshold temperature sometimes called critical temperature is the highest temperature which allows contrail formation for a given ambient water vapor partial pressure, exhaust gases temperature and exhaust water vapor partial pressure [7]. Jet aircraft with overall propulsion efficiency of 0,3 cause contrails as follows. If the ambient temperature corresponds to the standard atmosphere and the relative humidity with respect to liquid water is 100 % (the air is saturated), aircraft cause contrails within the altitude range from 8,2 km to 19 km. If the relative humidity with respect to liquid water is 0 % (the air is completely dry), aircraft cause contrails within the altitude range from 10,2 km to 14 km [4]. The ambient air temperature is very low in places of high altitude. The lower the temperature is, the lower the water vapor partial pressure sufficient for contrail formation is. Hence, contrails form more easily at high altitudes where the ambient temperature is low. 3. Climate impact of contrails Aircraft emit gases (mainly greenhouse gases carbon dioxide and water vapor) and particles in the upper troposphere and lower stratosphere. These substances cause changes in greenhouse gases concentrations, may trigger contrails formation, increase cirrus coverage and change other clouds properties, and hence they may contribute to climate change [8]. 3.1. Contrail coverage Contrails formed in a dry air dissipate very quickly. The global short-lived contrail coverage is very low and has a negligible impact on the climate. If there is a high content of water vapor and low ambient temperature, contrails might persist for hours. During their lifetime they might spread and become contrail cirrus. Contrail induced cirrus can form more easily than natural cirrus because the formation of cirrus requires higher humidity than for contrail persistence and spreading [5]. Unlike the line shaped contrails, it is very difficult to determine the coverage of contrail induced cloudiness. Contrail cirrus looks like a natural cirrus, so it is hard to distinguish a contrail cirrus from natural cloudiness. The largest coverage of persistent contrails and contrail induced cloudiness has been found in regions with high density of air traffic, namely over central Europe, over the east coast of the United States of America and over the east coast of southeast Asia [9, 10]. 3.2. Radiative forcing Contrails and contrail cirrus affect the cloudiness of Earth’s atmosphere and therefore might affect the atmospheric temperature and climate. They reflect some solar (short-wave) radiation that would otherwise warm the Earth-atmosphere system and absorb some infrared (long-wave) radiation that cools the system. The overall