Machine learning prediction of transboundary PM2.5 episodes in Colombo and Kandy, Sri Lanka

Abstract

This study quantifies the transboundary contribution of fine particulate matter (PM2.5) in Colombo and Kandy, Sri Lanka (2021–2023), originating from other countries in the Indian subcontinent, and evaluates a machine learning (ML) approach to predict these episodes. Transboundary pollution days were identified using ground-based PM2.5 and meteorological data, particularly when winds from 315◦–45◦ (NW–N–NE) transported pollutants from the continent. Temporal patterns, meteorological influences, and pollutant persistence were examined. Observed data revealed frequent pollution episodes associated with northeasterly winds, indicating longrange transport from external sources. Random Forest regression models performed strongly, achieving R2 = 0.91 (RMSE = 2.60 μg/m3) for kandy and R2 = 0.90 (RMSE = 3.99μg/m3 for Colombo. Classification models to detect high-pollution days (≥50 μg/m3) achieved 100% accuracy and ROC AUC = 1.00 at both cities. These results demonstrate the high predictability of pollution episodes when combining local meteorological factors with persistence features. The findings underscore the importance of transboundary air pollution control and highlight the potential of machine learning–based forecasting to strengthen early warning and public health response in Sri Lanka. The strongest predictors were 7-day and 30-day PM2.5 averages and wind components, indicating that both historical pollution and regional airflows strongly influence concentrations. However, the models struggled to detect extreme events due to the limited number of high-pollution days (22 in Colombo and only one in Kandy). The findings demonstrate that transboundary pollution substantially impacts PM2.5 in Colombo during the Northeast Monsoon and inter-monsoon periods. This approach highlights the potential of simple ML systems to forecast transboundary episodes, support targeted warnings, promote regional cooperation, and integrate cross-border pollution signals into air-quality alert systems