Long-term trends and exposure implications of ambient air pollution in Riyadh, Saudi Arabia: A nine-year multi-site assessment.

Long-term ambient air pollution remains an important environmental exposure concern in rapidly growing arid cities, where desert dust and anthropogenic emissions jointly shape population exposure. This study examined trends from 2015 to 2023 in six regulated pollutants-PM2.5, PM10, NO2, SO2, O3, and CO-across thirteen fixed-site monitoring stations in Riyadh, Saudi Arabia. Monthly and annual concentrations were evaluated against the WHO Air Quality Guidelines (2021), spatial variability across monitoring districts was assessed, Air Quality Index (AQI) classifications were calculated using U.S. EPA methods, and simple linear extrapolations to 2030 were generated as exploratory trend-based extensions. Citywide annual mean concentrations of PM2.5, PM10, and NO2 exceeded WHO guideline values in all monitored years, indicating persistent long-term exposure to pollutants with established cardiopulmonary relevance. Annual means of O3, SO2, and CO generally remained within the corresponding WHO short-term reference values used for contextual comparison, although annual averages may not capture episodic peak exposures. Particulate matter was the primary driver of AQI levels, which remained predominantly within the High category for much of the study period. Monitoring locations in more intensively developed districts showed relatively elevated PM2.5, PM10, and NO2 concentrations, indicating spatial heterogeneity in long-term ambient exposure. Temporal analyses showed declines in PM2.5 (-40.7%), PM10 (-32.5%), and CO (-22.1%), alongside modest increases in NO2 (+7.8%) and O3 (+9.9%). Exploratory extrapolations suggested continued reductions in PM2.5 and CO, although uncertainty remained substantial, particularly for PM10 and O3. Overall, this study provides new insights into long-term ambient air pollution patterns across Riyadh and highlights the importance of continued monitoring and exposure assessment to support efforts aimed at reducing population exposure.