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. Eight years of upper-tropospheric (UT) ice crystal measurementswith the backscatter cloud probe (BCP), installed on commercial aircraftoperated as part of the In-Service Aircraft for a Global Observing System(IAGOS), have been analyzed to assess the frequency and characteristics ofextreme ice crystal events (EIEs), defined in this study as encounters withclouds that have number concentrations exceeding 5000 L−1. A total of3196 events, in clouds of horizontal extent ≥ 2.5 km, were identified during the period from December 2011 to March 2020 inthe latitude band between 30∘ S and 30∘ N. Regions ofanthropogenic sources of carbon monoxide, with particles that can altercloud microphysics, were attributed to these EIEs in UT clouds using theSOFT-IO model. The evaluation of low- and upper-level kinematic variablesfrom the European Centre for Medium-Range Weather Forecasts (ERA5)reanalysis, combined with spatial distributions of aerosol optical depth andregions of biomass burning, highlights the physical mechanisms by which theparticles are lofted to flight levels in regions of deep convection. Themaps of lightning frequency, derived from the World Wide Lightning LocationNetwork (WWLLN), provide additional evidence of the role of deep convectionin transporting aerosol particles, cloud hydrometeors and carbon monoxide toaircraft cruising altitudes. The evaluation of aerosol particle massconcentrations and composition from the Modern-Era Retrospective analysisfor Research and Applications, Version 2 (MERRA-2) contributes additionalevidence for a link between regions of EIEs and surface emissions of dust,black carbon (BC), organic carbon (OC) and sulfate particles. Given thecomposition of the source aerosols and the role of deep convection in theirtransport to the UT, the sampled ice clouds likely originate from thehomogeneous or heterogeneous freezing of droplets formed on these particles,as has been reported in previous studies. The results from this study,which have been obtained from a large sample of measurements, haveramifications related to satellite measurement validation, weatherforecasting and climate change. In addition, over 2000 of the randomlysampled clouds had derived ice water contents larger than 1 g m−3, aconcentration that is considered potentially hazardous to commercialaircraft operations.

High concentrations of ice crystals in upper-tropospheric tropical clouds: is there a link to biomass and fossil fuel combustion?