Development of a Toxicity Wind Rose for Particulate Matter in Riyadh: A New Approach to Identifying Pollution Hotspots in the City

This study aimed to develop a “toxicity wind rose” for Riyadh City to investigate the di-rectional distribution of toxic PM₁₀ (particulate matter with aerodynamic diameter ≤ 10 µm) in the ambient atmosphere. The data collection approach combined an online moni-toring system with an offline sampling method. The monitoring instruments operated continuously, recording concentrations of key air pollutants including PM₁₀, PM₂.₅, NO₂, and SO₂, as well as meteorological parameters such as wind direction, temperature, and relative humidity. Concurrently, PM₁₀ samples were collected using Teflon and quartz fil-ters for subsequent chemical and toxicological analysis. The sampling campaign spanned from December 2019 to August 2020, covering both warm and cool seasons. The chemical and dithiothreitol (DTT) activity data were integrated with hourly wind direction meas-urements to construct pollutant and toxicity wind rose diagrams. During the warm sea-son, PM₁₀ concentrations exceeded 350 µg/m³ from the northwest and east, while the cool season showed elevated concentrations from the northeast. In contrast, the highest levels of oxidative potential (DTT activity) were associated with air masses originating from the north and west during the warm season and from the north during the cool season. These results demonstrate that high PM₁₀ mass concentrations do not necessarily coincide with high toxicity, highlighting the importance of considering both quantity and oxidative ca-pacity in air quality assessments.