A mysterious air pollution event occurred in South China on April 1-2, 2024. The predominant winds in Hong Kong were steady southerly, typically associated with clean air. However, visibility declined, reaching a low of 4.4 km overnight between April 1 and 2. Meanwhile, the maximum PM2.5 level exceeded the 99th percentile of historical levels. Additionally, the increase in PM2.5 levels spread northward across Hong Kong and the adjacent Pearl River Delta region on April 1. The deterioration of air quality and visibility suggests the presence of an unusual source or transport mechanism that overrode the typical clean air conditions associated with southerly winds in the region.

Potential sources of air pollutants for such events include biomass burning and volcanic emissions from Southeast Asia. This event first caught the attention of the Hong Kong Environmental Protection Department and the Hong Kong Observatory. Subsequently, our research team at HKUST collaborated with colleagues from these two government units to analyze real-time chemical composition data, satellite data, and conduct air quality modeling.

Satellite measurements and chemical transport modeling reveal a compelling connection between this pollution event and the transboundary transport of a volcanic plume originating from the Taal volcano in the Philippines. Furthermore, ground measurements of aerosol chemical composition in Hong Kong recorded unusually high levels of sulfate and phosphorus, characterized by remarkably synchronized variations, further supporting the link to volcanic emissions.

Our comprehensive analyses of chemical and meteorological measurement data led to the development of a real-time chemical-wind index incorporating sulfur, phosphorus, and wind measurements as a tool to identify air pollution events associated with volcanic emissions. Using this index, we have successfully identified several additional air pollution events in 2024, conclusively attributing them to ongoing emissions from the Taal volcano.

This study exemplifies the immense value of integrating multi-scale real-time data capabilities. By promptly attributing sudden and unexpected air pollution events, such as volcanic eruptions and wildfires, to their sources, we can take proactive measures to protect at-risk populations.