Transport and Distribution of Short-lived Climate Pollutants in the Himalaya

Major combustion sources in the Indo-Gangetic Plain (IGP) of southern Asia form persistent haze layers, also known as atmospheric brown clouds (ABC), from December through early June. ABC is a mixture of high concentrations of reactive trace gases and light-absorbing and light-scattering particles like black carbon (BC) and ozone (O3) precursors. Models and satellite imagery suggest that strong wind systems within deep Himalayan valleys are major pathways by which pollutants from the IGP are transported to the higher Himalaya. However, there is a lack of observational evidence to elucidate Himalayan valleys as major pollutant pathways. To evaluate the pathway of pollutants from IGP to higher Himalaya, I measured BC, O3, and associated meteorological conditions within the Kali-Gandaki Valley (KGV), Nepal, from January 2013 to July 2015. High concentrations of BC and O3 were observed during pre-monsoon (March-June) and post-monsoon (September- February) seasons. Frequent episodes of BC concentrations two to three fold higher than average persisted from several days to a week during non-monsoon months. Observation of increases in BC concentration and fluxes in the valley were found during pre- and post-monsoon seasons and in conjunction with widespread agricultural burning and wildfires over the IGP.

In addition to surface sources, tropospheric O3 can also be transported from the stratosphere that contributes significantly to background concentrations. Tropospheric O3 modulates the atmospheric lifetime of many atmospheric gases and like BC influences climate via direct and indirect radiative forcing. I collected ozonesonde data from Pokhara, Nepal in winter (December 2015 – January 2016) focusing on the distribution and sources of tropospheric O3 in the Himalayan region. Stratospheric influence was inferred when tropospheric air masses had potential vorticity (PV) above 1.5 PVU, relative humidity (RH) under 20% and potential temperature (theta) above 320 K. I found that the ozonesondes that stayed south of the Himalaya measured higher O3 concentrations between 3 and 10 km and ozonesondes launched during high PV period were 10-20 ppbv higher between 9 and 13 km. I also found that the topography and surface temperature induced a consistent thermal structure over the Tibetan plateau. When the jet was positioned south of the thermal gradient, high O3 concentrations were observed above 8 km. Higher O3 concentrations were detected when the jet stream had high wind speeds (above 60 ms-1) and during periods of high PV given the location of the SJS was south of the thermal gradient. Layers of diffused air masses were frequently detected with PV values showing both stratospheric and tropospheric origin. This dissertation supports the hypothesis that trans-Himalayan valleys are important conduits for transport of pollutants from the IGP to TP and provides insight, for the first time, on the vertical concentrations O3 adjacent to Himalaya using MERRA-2 as a tool to understand the role of SJS and the influence of stratospheric air.