AbstractThis work analyzes and classifies stratospheric airmass transport events (ST) detected at the Nepal Climate Observatory–Pyramid (NCO-P; 27°57′N, 86°48′E, 5079 m MSL) Global Atmospheric Watch–World Meteorological Organization station from March 2006 to February 2008. For this purpose, in situ ozone (O3), meteorological parameters (atmospheric pressure and relative humidity), and black carbon (BC) are analyzed. The paper describes the synoptic-scale meteorological scenarios that are able to favor the development of ST over the southern Himalaya, by analyzing the meteorological fields provided by the ECMWF model (geopotential height, wind speed, and potential vorticity), satellite Ozone Monitoring Instrument data (total column ozone), and three-dimensional back trajectories calculated with the Lagrangian Analysis Tool (LAGRANTO) model. The study, which represents the first "continuous" classification of ST in the southern Himalaya, permitted classification of 94% of ST days within four synoptic-scale scenarios: stratospheric potential vorticity structures (PVS), subtropical jet stream (SJS), quasi-stationary ridges (QSR), and monsoon depressions (MD). SJS and PVS were the most frequent scenarios (48% and 30% of occurrences, respectively), QSR occurred for 12% of the ST days, and MD were detected only during the monsoon season (3%). SJS and PVS scenarios presented a peak frequency during the nonmonsoon seasons, when the jet stream and westerly disturbances influence atmospheric circulation over the southern Himalaya. During the identified ST, significant variations of O3 (+24%) and BC (−56%) were recorded relative to the averaged 2-yr mean values. On average, PVS and SJS were the most effective synoptic-scale scenarios in modifying the O3 and BC levels at NCO-P from postmonsoon to premonsoon seasons, and ST is one of the leading processes in defining the "background" BC variability at NCO-P
