Thumbnail Image

Variability of Anthropogenic Gases: Nitrogen Oxides, Sulfur Dioxide, Ozone and Ammonia in Kathmandu Valley, Nepal

Background: Kathmandu Valley is one of the largest and most polluted metropolitan regions in the Himalayan foothills. Rapidly expanding urban sprawl and growing fleet of vehicles, and industrial facilities such as brick factories across the valley have led to conditions where ambient concentrations of key gaseous air pollutants are expected to exceed Nepal’s National Ambient Air Quality Standards and World Health Organization guidelines. Objectives: The aim of this study is to quantify the distribution of gaseous pollutants across the valley characterized by different emission sources, and also assess influence of meteorology in the region on the temporal variability and spatial distribution of these gases, including differences at sites upwind and/or downwind of three major cities in the Kathmandu Valley. Methods: In order to understand the spatial variation of the trace gases in the Kathmandu Valley, passive samples of SO2, NOx, NO2, NH3, and O3 were simultaneously collected from fifteen locations between March and May 2013. A follow-up study during two separate campaigns in 2014 sampled these gases, except ammonia, one site at a time from thirteen urban, suburban and rural stationary sites. Results: In 2013, urban sites were observed to have higher weekly averaged NO2 and SO2 (22.4 ± 8.1 µg/m3 and 14.5 ± 11.1 µg/m3, respectively) than sub-urban sites (9.2 ± 3.9 µg/m3 and 7.6 ± 2.8 µg/m3, respectively). Regions located within 3 km of brick factories exhibited higher SO2 concentrations (22.3 ± 14.7 µg/m3) than sites at least than 3 km away (5.8 ± 1.1 µg/m3). Increased NH3 levels were observed at sites downstream from polluted rivers (25.8 ± 5.5 µg/m3) compared to upstream sites (19.9 ± 3.6 µg/m3). Increased O3 was observed in rural locations (108.5 ± 31.4 µg/m3) compared to urban sites (87.1 ± 9.2 µg/m3) (ANOVA, p Conclusions: Parallel to previous studies that found O3 levels that exceeded guideline, these results suggest that ground-level O3, as its levels frequently exceeded guidelines throughout the sampling periods, is an important concern throughout the valley. NH3 near polluted rivers, NO2 near high traffic activity and SO2 around brick factories are also important pollutants that need more intensive monitoring, primarily due to their importance in particulate matter formation chemistry.