Comparing the characteristics of ambient fine particle pollution episodes across South Asian cities

South Asia has been experiencing recurring severe air pollution episodes in recent years. While many previous studies investigated such episodes focusing on individual cities and specific events, limited information exists on episode characteristics across multiple cities in this region. This study presents a comparative analysis of the characteristics of ambient PM_2.5 pollution episodes in 12 South Asian cities across five countries during 2019−2023. Daily mean PM_2.5 mass concentrations were decomposed into trend, seasonal, and residual components, and episodes were identified through anomalies in residuals. Furthermore, pollution episodes were characterized using magnitude, frequency, duration, and a relative severity index. The cities exhibited annual mean PM_2.5 mass concentrations ranging from 20.6 ± 2.5 μg m⁻³ (Colombo) to 116.6 ± 9.3 μg m⁻³ (Lahore), with six out of 12 cities having annual mean PM_2.5 mass concentrations > 50 μg m⁻³. Additionally, significant increasing trends (p < 0.05) in PM_2.5 levels were observed for Dhaka, Chennai, Hyderabad, Kolkata, Islamabad, and Lahore (Sen's slope: 1.00−4.33 μg m⁻³ y⁻¹), whereas decreasing trends (p < 0.05) were observed for Mumbai (−0.74 μg m⁻³ y⁻¹) and New Delhi (−2.00 μg m⁻³ y⁻¹). Mean PM_2.5 episode magnitudes varied in a wide range from 49.9 ± 6.1 μg m⁻³ (Colombo) to 367.1 ± 17.9 μg m⁻³ (Lahore) across the cities. Likewise, the mean episode frequency ranged from 1.6 y⁻¹ (Kathmandu) to 5.2 y⁻¹ (Dhaka), whereas duration ranged from 1.2 (Mumbai) to 2.6 (Kathmandu) days per episode. Based on the relative index of episode severity, Lahore, Dhaka, and New Delhi exhibited high episode severity, as well as high baseline PM_2.5 levels. In contrast, Karachi, Islamabad, Hyderabad, and Kathmandu showed moderate episode severity and moderate baseline PM_2.5 levels, whereas Colombo and Mumbai showed low episode severity with low to moderate baseline PM_2.5 levels. Moreover, annual PM_2.5 episode severity ranks among the cities changed dramatically during 2019−2023. The relative severity of baseline and episodic pollution levels presented in this study may help policymakers prioritize the control strategies targeting pollution episodes, long-term trends, or both, as well as protecting human health through mitigation, preparedness, and forecasting. The findings will also provide insights for formulating regional policies aimed at transboundary cooperation and collaboration to deal with air pollution challenges across South Asia.