Re-Evaluating Springtime as Southern Arizona's Dust Season

The existing literature largely identifies spring as Arizona's predominant dust season, when synoptic-scale dust events are most frequent and "Fine Soil" measurements from the IMPROVE (Interagency Monitoring of Protected Visual Environments) network reach a yearly maximum. Because IMPROVE observations are primarily derived from remote monitoring sites, however, they may not reflect exposure-relevant dust seasonality in Arizona's urban areas, where anthropogenic activities can strongly influence coarse particulate matter (PM) concentrations. Consistent with this possibility, the notion of "spring as dust season" is less clear in other surface data sets that serve as dust proxies, including PM coarse and ${\text{PM}}_{10}$ . Here we analyze (a) regulatory monitoring of hourly ${\text{PM}}_{10}$ and PM coarse concentrations, (b) IMPROVE measurements of 24-hr Fine Soil and PM coarse concentrations, and (c) sub-hourly PM coarse concentrations from a low-cost urban sensor network (SUNSET, Sonoran Unified Network of Sensors for Environmental Tracking). Our analysis reveals a different picture of dust seasonality. Summer has the highest concentrations of regulatory ${\text{PM}}_{10}$ and PM coarse, followed by fall and then spring, whereas IMPROVE-measured Fine Soil peaks in spring. Summer also has the most ${\text{PM}}_{10}$ NAAQS exceedances. Seasonal patterns differ between urban and rural environments, and concentrations are generally higher in more anthropogenically influenced areas. Diurnal cycles, weekday-weekend differences, a COVID-era anomaly in 2020, and urban spatial gradients further indicate strong anthropogenic influences. Together, these results suggest that while springtime dust is climatologically important at remote sites, it does not represent exposure-relevant dust seasonality in Arizona's urban areas.