Evolution of Brown Carbon Aerosols during Atmospheric Long-Range Transport in the South Asian Outflow and Himalayan Cryosphere

The Himalayas sustain the water sources for ∼2 billion people in South and East Asia. The Himalayan cryosphere is particularly sensitive to climate change as a result of long-range transport of anthropogenic emissions from the Indo-Gangetic Plain. Atmospheric warming as a result of light-absorbing aerosols, i.e., black carbon (BC) and brown carbon (BrC), is responsible for accelerated loss of glaciers in the region. The BrC lifetime in the atmosphere over the Himalayan cryosphere has never been studied before, and observational determination of the lifetime of BrC is imperative in improving model performance. In this work, we collected bulk water-soluble BrC (WS-BrC) data from multiple campaigns conducted over the past 15 years in South Asia and the Himalayas, with an aim to provide constraints for two distinct receptor regions: South Asia outflow and Himalayan atmosphere. We observed decay in WS-BrC absorptivity for both receptor regions as air masses undergo long-range transport from the source region. However, the decay rate of the WS-BrC imaginary refractive index (k WS‑BrC‑365) in the Himalayan atmosphere (0.09 ± 0.02 day–1) was ∼2 times slower than the South Asian outflow (0.17 ± 0.04 day–1), implying their longer atmospheric half-life in the Himalayan atmosphere than the South Asian outflow (∼8 and 4 days, respectively). The comparison in the evolution of k WS‑BrC‑365 and absorption Ångström exponent revealed the less pronounced role of photochemical bleaching in the decay of BrC in the Himalayan atmosphere than in the South Asian outflow. Slower decay is consistent with highly viscous organic aerosol as a result of a low temperature and relative humidity in the cryospheric atmosphere.