Large NAT particle formation by mother clouds: Analysis of SOLVE/THESEO‐2000 observations

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Abstract
During the SOLVE/THESEO‐2000 Arctic stratospheric campaign in the winter 1999/2000 widespread occurrences of very large HNO3‐containing particles, probably composed of nitric acid trihydrate (NAT), were observed in situ by instruments on board the ER‐2 stratospheric research aircraft. These large NAT particles were found with low number densities (n ≈ 10−4 cm−3) in vast regions, in air generally supersaturated with respect to NAT. Within the same campaign other instruments have performed airborne and ground‐based measurements of polar stratospheric clouds (PSCs), often showing the existence of type 1a and type 1a‐enh clouds. Such PSCs often occur on the mesoscale with particle number densities n ≳ 10−2cm−3 and are also most likely composed of NAT. We use forward trajectories for the path of NAT particles, which are advected by winds based on ECMWF analyses and sediment due to gravity, to show that high number density NAT PSCs (mother clouds) could give rise to low number density NAT particle populations several days downstream.