Study of Processes Influencing Short-Lived Climate Forcers/Pollutants (Black Carbon and Ozone) Variability in the Himalayas

South Asia and the Himalayas are considered among the worldwide “hot spots” for the climate impacts of air pollution. Among the atmospheric pollutants are the short-lived climate forcers/pollutants (SLCF/P), i.e., those atmospheric substances characterized by short lifetimes, with harmful effects on climate, ecosystems and human health. The high levels of the SLCF/P derive from a variety of factors, both of anthropogenic and natural origin. In this thesis, several processes affecting the variability of two key SLCF/P (i.e., black carbon, BC, and tropospheric ozone, O3) in the southern Himalayas are investigated and discussed. The characterization of one of the “hot spots” for air pollution in the Himalayan foothills, i.e., the Kathmandu Valley, is given, by analyzing BC and O3 variability at the Paknajol urban measurement site, over February 2013–January 2014. Since the persistent poor air quality conditions of the valley could affect a broader area, up to the high Himalayan environment, the specific role of air-mass transport from the planetary boundary layer (PBL) over the Himalayan foothills to the southern Himalayas is investigated, by comparing BC and O3 concentrations at Paknajol and at the high altitude WMO/GAW global station Nepal Climate Observatory-Pyramid (5079 m a.s.l.). Results show that 50% and 65% of the BC and O3 variability at NCO-P can be explained by PBL variations occurring over Kathmandu, in days (9% of the period) in which air-mass transport between the two measurement sites is observed. Lastly, the influence of a natural process at NCO-P, i.e., stratospheric intrusions, is assessed. The application of the Stratosphere-to-Troposphere Exchange Flux tool (STEFLUX), a novel methodology based on the trajectories from the ERA-Interim reanalysis dataset, indicates that 13% of the 2006–2013 period at NCO-P was affected by this natural phenomenon, resulting in a significant change in O3 and BC concentrations at the measurement site.