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

Kathmandu Valley is one of the largest and most polluted metropolitan regions in the Himalayan foothills. Rapidly expanding urban sprawl and a 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 (NAAQS) and World Health Organization (WHO) guidelines. In order to understand the spatial variation of the trace gases in the Kathmandu Valley, passive samples of SO_2, NO_x, NO_2, NH_3, and O_3 were collected simultaneously 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. In 2013, urban sites were observed to have higher weekly averaged NO_2 and SO_2 (22.4 ± 8.1 μg m^(-3) and 14.5 ± 11.1 μg m^(-3), respectively) than sub-urban sites (9.2 ± 3.9 μg m^(-3) and 7.6 ± 2.8 μg m^(-3), respectively). Regions located within 3 km of brick factories had higher SO_2 concentrations (22.3 ± 14.7 μg m^(-3)) than distant sites (5.8 ± 1.1 μg m^(-3)). Higher O_3 (108.5 ± 31.4 μg m^(-3)) was observed in rural locations compared to urban sites (87.1 ± 9.2 μg m^(-3)), emphasizing the importance of meteorological factors and precursor species for ozone production and titration. Parallel to previous studies, these results suggest that ground-level O_3, as its levels frequently exceeded guidelines throughout the sampling periods, is an important concern throughout the valley. NH_3 near polluted rivers and SO_2 around brick factories are also important pollutants that need more intensive monitoring, primarily due to their importance in particulate matter formation chemistry.