Spatiotemporal Assessment of Black Carbon in the Urban Environment of Dun Valley, Himalayan Foothills, India

Rapid urbanisation and rising vehicular emissions have become one of the major causes of declining air quality in the Himalayan foothill regions/cities. Black Carbon (BC), a short-lived climate pollutant primarily emitted due to the incomplete combustion of fossil fuels, biomass fuels, and open burning, is not only associated with atmospheric warming but also with adverse human health. In the present study, the spatial distribution of BC was assessed in Dehradun, the capital city of Uttarakhand state, India. Dehradun resides in the valley sandwiched between the Shiwalik range to the south and the Lesser Himalayan range to the north.  In the present study, the integration of a comprehensive mobile and fixed-site monitoring approach has been applied to the assessment of BC in the region during 2022–2023. The microAeth MA350 was used for mobile monitoring, while the Aethalometer AE33 was used for fixed-site monitoring. Further, personal exposure to BC was evaluated to assess the spatial-temporal variation of human exposure to it along four different routes.  The selected routes represented different microenvironments, including urban regions with heavy traffic, industrial regions, and green corridors.  A total of ~100 hours of mobile monitoring was conducted. The mean BC concentration of 3.78 ± 3.01 µg/m³ at the fixed site was much lower than the mean BC concentration along the mobile-monitored routes, which varied from 18.09 ± 15.51 µg/m³ to 27.22 ± 17.90 µg/m³.  For the determination of individual inhalation doses, three different respiratory rates (RRs) were used, indicating diverse commuting intensities in the region, viz. passive travel (0.47 m³/hr), walking (0.63 m³/hr), and cycling/motorcycling (0.70 m³/hr). It was realised that at the lowest RR, the total inhalation doses were estimated within the range of 10.08 µg to 14.06 µg, while at the highest RR, the inhalation doses ranged from 15.01 µg to 20.94 µg. However, during moderate activity levels, inhalation doses remained within the range of 13.51-18.84 µg. Whereas higher RR results in increased air intake, the inhalation dose of pollutants, such as BC, also increases with greater physical exertion. This pattern is evident for all routes, reinforcing the idea that physical exertion plays a crucial role in determining personal exposure levels. Individuals engaged in more physically demanding commuting modes, such as cycling or motorcycling, inhale more air per hour, which increases their intake of airborne pollutants. The inhalation dose shows a consistent increase with respiratory rate across all chosen routes, thereby indicating the impact of physical activity on pollutant intake. The preliminary results of this unique regional study emphasise the need to consider mobility patterns and the choice of path when determining pollutant exposure. This study also highlights the uncertainty and challenges associated with the estimation of BC concentrations within our environment. The present study is an initiative that may help in understanding and managing the urban air quality, thereby suggesting the importance of sustainable transportation strategies and public awareness initiatives aimed at reducing the health risks within the valley.