Variability in Contrail Ice Nucleation and Its Dependence on Soot Number Emissions

• Published in: Journal of geophysical research. Atmospheres 2019-03, Vol.124 (6), p.3384-3400
• Main Authors: Bier, A., Burkhardt, U.
• Format: Article
• Language: English
• Subjects: Aircraft; Aircraft control; alternative fuels; Atmospheric conditions; atmospheric state; Atmospheric temperature; Climate; Climate models; Climate variability; Contrail formation; Contrails; Cruises; Crystals; Dependence; Emissions; Geophysics; Global climate; Global climate models; Ice; Ice crystal formation; Ice crystals; Ice formation; Ice nucleation; Nucleation; Optical properties; Soot; soot emissions; Soot particles; Spatial variability; Spatial variations; Tropical environments
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2018JD029155

Contrail ice nucleation is mainly controlled by aircraft emissions and the atmospheric state. The nucleation rate can have a strong impact on microphysical processes, optical properties, lifetime, and, therefore, on the climate impact of contrail cirrus. We study contrail ice crystal formation offline for specified atmospheric conditions and its spatial variability in a global climate model. Assuming the standard atmosphere, above around 270 hPa (10 km) contrail ice nucleation is mainly controlled by aircraft soot number emissions and below additionally by atmospheric temperature. Parameterizing contrail ice nucleation in a global climate model, we find that in the northern extratropics contrails form frequently far away from their formation threshold. For current soot number emissions and in case of contrail formation, 90% of emitted soot particles form on average ice crystals around the cruise level and more than 70% between cruise altitudes and 300 hPa. The number of nucleated ice crystals in the extratropics decreases nearly at the same rate as soot number emissions. In contrast, in the tropics around cruise altitudes approximately 60% of contrails develop close to their formation threshold so that on average only about 50% of emitted soot particles can form ice crystals. Below, contrail formation occurs rarely and ice nucleation is reduced more strongly. Of the main air traffic areas, contrail ice nucleation is significantly limited by the atmospheric state over eastern Asia and over the southeastern United States. This limitation is enhanced during the summer months. Key Points Assuming the standard atmosphere, ice nucleation is controlled by soot number emissions above 270 hPa and below additionally by temperature For current aircraft emissions, around 90% of emitted soot particles form on average ice crystals at extratropical cruise altitudes At tropical cruise altitudes 60% of contrails form near the formation threshold and the ice crystal number formed is reduced by around 50%