Some models simulate a secondary maximum at around 100 - 200 hPa, with concentrations much lower than the maximum near the surface; this is primarily due to condensation of
semi-volatile SOA at low temperatures but also due to OA accumulation above clouds, where dry and wet deposition is absent.
In order to capture the magnitude, seasonality, and the OA physical and chemical properties, several global models now treat
SOA as
semi-volatile, based on an continuously increasing amount of experimental data on
SOA formation in the atmosphere.