A breath of fresh air? Assessing the significant decline in PM2.5 and O3 pollution in Kathmandu valley during the coronavirus disease 2019 lockdown

Background: The COVID-19 pandemic led to unprecedented restrictions on human mobility and economic activities, providing a natural experiment to examine short-term changes in urban air quality. Kathmandu Valley, a densely populated basin-shaped city is particularly vulnerable to air pollution. Aims and Objectives: This study assessed changes and temporal trends in particulate matter (PM 2 . 5 ) and ground-level ozone (O 3 ) concentrations in the Valley before and during the COVID-19 lockdown. Materials and Methods: Daily air quality data from seven ground-based monitoring stations were obtained for the period January 2018 to mid-June 2020. Pollutant trends were analyzed using non-parametric Mann–Kendall tests and Theil–Sen slope estimators, with comparisons made between lockdown (24 March–14 June 2020) and corresponding pre-lockdown periods in previous years. Results: PM 2 . 5 and O 3 exhibited pronounced seasonal variability, with PM 2 . 5 peaking during winter and O 3 during spring and summer. During the lockdown period, mean PM 2 . 5 and O 3 concentrations declined by 31.66% and 8.17%, respectively, compared with the average of the same period in 2018–2019. Trend analysis showed statistically significant declining monthly trends for PM 2 . 5 (–9.55 µg/m³; 95% CI: –15.87 to –5.26) and O 3 (–1.68 µg/m³; 95% CI: –3.22 to –0.01). Conclusion: The observed reductions in PM 2 . 5 and O 3 during the lockdown period likely reflect decreased anthropogenic emissions, although seasonal and meteorological influences cannot be excluded. These findings highlight the potential for sustained, non-lockdown policy interventions, such as cleaner transport and energy strategies, to improve urban air quality in Kathmandu Valley.