Results of a trajectory box model simulating the size distribution evolution of stratospheric particles (H2SO4/H2O and H2SO4/HNO3/H2O solutions). A case study during SESAME

Abstract

A revised interpretation of ER-2 polar stratospheric cloud (PSC) observations during AASE I (1989) and MOE (1987) Tabazadeh et al., 1995) suggests a picture of the PSC formations which evidences the importance of the air mass thermal histories. A series of ER-2 measurements are consistent with the thennodynamical properties of the liquid H2SO4/HNO3/H20 solution particles, but the data collected in other flights are in agreement with the possible formation of amorphous solid solutions of HNO3 and H2O through a peculiar coolingiheating cycle below the solid sulfuric acid tetrahydrate (SAT) melting temperatures and above the water ice saturation temperatures (frost point). During this cycle the liquid H2SO4 stratospheric aerosols may undergo a phase transition to SAT particles, required for the growing of solid nitric acid hydrates. On the other hand Koop et al., (1995) report laboratory experiments which show that H2SO4/HNO3/H20 liquid particles never freeze under stratospheric conditions for temperatures higher than the frost point. despite the change in composition due to the HNO3 uptake when cooling. In addition, when solid particles are heated, they start to become liquid at the SAT melting point. Then the analysis of any PSC data should start from air mass trajectory studies, and some thermodynamical criteria could be used to infer the physical state (liquid or solid) of the sampled particles. We use a trajectory box model to study the microphysical properties of stratospheric clouds observed during SESAME by the MOANA (Measurements and modelling of Ozone and Aerosols in the Northern Atmosphere) lidar at Sodankyla (SF). Our models treats the gas to particle conversion of H2SO4, HNO3, H2O and the microphysics of Aitken particles (ATK), H2SO4/H20 (WS) and H2SO4/HNO3/H20 (WSN) solution droplets; H2SO4 -nH2O (SA) and HNO3 -nH20 (NA) solid hydrates particles are also taken into account. To analyze the MOANA observations, within a prescribed air mass thermal history, we adopt a simple criteria which states that the particles should be liquid just after having performed temperatures above the SAT melting point. while they are solid if the water ice saturation temperatures are reached. The model simulations along the air mass trajectories reaching the lidar site during the observations are in agreement with the aerosol size distribution optically retrieved by the MOANA multiwavelength lidar (Masci et al., .1995).