An overview of airborne measurement in Nepal – Part 1: Vertical profile of aerosol size, number, spectral absorption, and meteorology

The paper provides an overview of an airborne measurement campaign with a microlight aircraft, over the Pokhara Valley region, Nepal, a metropolitan region in the central Himalayan foothills. This is the first aerial measurements in the central Himalayan foothill region, one of the polluted but relatively poorly sampled regions of the world. Conducted in two phases (in May 2016 and December 2016–January 2017), the goal of the overall campaign was to quantify the vertical distribution of aerosols over a polluted mountain valley in the Himalayan foothills, as well as to investigate the extent of regional transport of emissions into the Himalayas. This paper summarizes results from first phase where test flights were conducted in May 2016 (pre-monsoon), with the objective of demonstrating the potential of airborne measurements in the region using a portable instrument package (size with housing case: 0.45 m × 0.25 m × 0.25 m, 15 kgs) onboard an ultralight aircraft (IKARUS-C42). The limited dataset collected during the test flight also provides useful insights into the impact of regional emissions and meteorology on aerosol vertical profiles. A total of five sampling test flights were conducted (each lasting for 1–1.5 h) in the Pokhara Valley to characterize vertical profiles of aerosol properties such as aerosol number and size distribution (0.3–2 µm), total particle concentration (> 14 nm), aerosol absorption (370–950 nm), black carbon (BC), and meteorological variables. The vertical profiles of aerosol species showed decreasing concentrations with altitude (815 to 4500 m a.s.l.); steep concentration gradient below 2000 m (a.s.l.) in the morning and a more mixed profiles (up to ca. 4000 m a.s.l.) in the afternoon. The strong gradient in the morning hours was mainly contributed by the primary emissions from the valley floor, including occasional open agriculture burning. The near-surface (< 1000 m a.s.l.) BC concentrations observed in the Pokhara Valley were much lower than pre-monsoon BC concentrations in the Kathmandu Valley, and similar in range to Indo-Gangetic Plain (IGP) sites such as Kanpur in India. The sampling test flight also detected an elevated polluted aerosol layer (around 3000 m a.s.l.) over the Pokhara Valley, which could be associated with the regional transport. The total aerosol and black carbon concentration in the polluted layer was comparable with the near-surface values (< 1000 m a.s.l.). The elevated polluted layer was also characterized by high aerosol extinction co-efficient (at 550 nm) and was identified as smoke and a polluted dust layer. Long-term observations of aerosol optical depth (AOD) in the Pokhara Valley (2010–16) showed strong seasonality, with a pre-monsoon maximum which is also indicative of westerly advection transporting a mixture of dust and other aerosols from IGP into Himalayan foothills and mountain valleys. The observed shift in the westerlies (at 20–30° N) entering Nepal during the test flight period is an important factor for the presence of elevated polluted layers in the Pokhara Valley. The intrusions (in the form of a trough) of the cold and humid air mass from the mid-latitude (~ 40–50° N) a shift in the direction of synoptic airmass entering Himalayas. This synoptic-scale interaction is likely to drive the transport into the mountain valleys and higher Himalayas.