Long-Term Spatiotemporal Variability of Fine Particulate Matter (PM2.5) Across Idaho, USA, Using Statistical and Geospatial Tools

Abstract Fine particulate matter (PM 2.5 ) is a leading environmental risk factor for human health, driven in the interior Pacific Northwest by summertime wildfire smoke and wintertime residential wood combustion trapped by valley inversions. This study characterized the seasonal, spatial and temporal structure of PM 2.5 across Idaho, USA, over 2005 to 2024, and inferred its dominant source regimes using statistical and geospatial tools. The complete daily PM 2.5 record of the US Environmental Protection Agency Air Quality System was aggregated to 4,260 site-months for 32 sites, and Mann-Kendall and Theil-Sen trend analysis, Pearson correlation, principal component analysis (PCA) and inverse distance weighting (IDW) interpolation were applied. The regional mean was 7.53 µg m − 3 , with a site range of 3.16 to 12.08 µg m − 3 . The climatology was bimodal, with an August to September wildfire maximum and a November to February wood-smoke and inversion maximum separated by a clean spring background, and July to September carried 36.6% of the annual mass. The cold-season and background series declined significantly, whereas the fire-season series rose at 2.54 µg m − 3 per decade, which held the annual mean flat. PCA separated the network into two source regimes explaining 57.5% of the between-site variance, and a temporal mode was dominated by July to September episodes, identifying wildfire smoke as the dominant driver. It was found that 32.7% of site-years exceeded the 2024 annual standard of 9.0 µg m − 3 , and the two regimes are diverging under a warming climate, so that season and source specific management is required.