Air quality trends of the Kathmandu Valley: A satellite, observation and modeling perspective

Kathmandu is the capital city of Nepal and a 'hotspot' of urban air pollution in South Asia. Its bowl-shaped structure (altitude 1300 m above sea level (masl); floor area 340 km2) surrounded by tall mountains provides a unique case study for analyzing pollution trapped by topography and local meteorology. In the absence of long-term in-situ observations, for the first time the columnar aerosol loading trend of the Kathmandu Valley was analyzed using satellite-derived aerosol optical depths (AOD). AOD from MODIS (Moderate Resolution Imaging Spectroradiometer) onboard Aqua and Terra (3 × 3 km, Level 2) was used during the dry season (November-June) in 2000-2015. Trend analysis of Kathmandu AOD (KAOD) (the Kathmandu Valley AOD (KValAOD) + background AOD (BAOD)) suggested an increase of ∼35% during the study period. To derive the KValAOD trend, BAOD was subtracted from KAOD. The KValAOD trend indicated an increase of ∼50-60% during the study period, based on MODIS Aqua and Terra data. Thereafter, the background contribution only at the valley layer (1300-1400 masl) was determined using Cloud-Aerosol LiDAR and Infrared Pathfinder Satellite Observation (CALIPSO) profiles, in-situ observation and modeling techniques. The CALIPSO-based analysis indicated that background pollution contributed an additional 20-25% to local pollution. This finding was further supported by short-term in-situ measurements from Dhulikhel (the site where Kathmandu background measurements were taken) and Ratnapark (a Kathmandu city center site). Case studies were conducted using chemical transport models (WRF-STEM and WRF-Chem) to quantify the contribution of background air pollution to the Kathmandu valley pollution. These model results contradicted the satellite and in-situ observation by highly underestimating the Kathmandu Valley pollution levels. Comparison of visibility in Kathmandu with AOD suggests a profound role of BAOD on decreasing long-distance visibility in particular months.