In Situ Observations of Light-Absorbing Carbonaceous Aerosols at Himalaya: Analysis of the South Asian Sources and Trans-Himalayan Valleys Transport Pathways

The mountain-valleys along the south margin of the Himalaya-Tibetan Plateau (HTP), referred to as trans-Himalayan valleys, are transport pathways of atmospheric pollutants from the South Asia lifting to the HTP based on atmospheric models and satellite detections. However, few field studies can be conducted to confirm the aerosol transport along these valleys due to the harsh environment and limited power supply. Individual particle collection instruments offer the unique advantage of collecting samples and tracing particle sources in remote areas due to their portability and efficiency. In this study, transmission electron microscopy was used to study individual particles collected in two trans-Himalayan valleys. We found that 49-70% of thousands of particles were light-absorbing carbonaceous aerosols including soot-bearing and tarball-bearing particles in December of 2017. The dominant sizes of soot-bearing and tarball-bearing particles were 151-370 and 198-255 nm, respectively. The WRF-Chem model and wind field analyses show that the intense regional air pollution with high mass concentrations of light-absorbing carbonaceous aerosol occurred in the South Asia and southwest wind normally prevailed along the trans-Himalayan valleys in the afternoon of each day during wintertime. Backward trajectory and CALIPSO data further confirm that these light-absorbing carbonaceous aerosols from the South Asia were transported to the southern HTP. These carbonaceous aerosols exhibited significantly internally mixing with secondary sulfates. Here individual particle analysis coupled with the WRF-Chem model, the meteorological observation data, and satellite data confirm the trans-Himalayan valleys as the direct channels for abundant carbonaceous aerosols lifting from the South Asia to the HTP.