Transport Pathways Versus Dust Sources: A Negative-Control Test of Trajectory-Statistical Attribution in Riyadh, Saudi Arabia

Backward-trajectory statistics locate arid-region dust sources on the assumption that trajectories separate emission from transport, an assumption seldom tested where the local contribution is known. We build one from ten years (2016–2025) of routine METAR observations at Riyadh, Saudi Arabia, where present-weather codes distinguish locally lifted dust (BLDU/BLSA) from dust in suspension (DU/SA). Suspension-dominated days outnumbered locally lifted days 345 to 179 (1.9:1), and ERA5-driven models reproduced the locally lifted class substantially better (held-out AUC 0.914 versus 0.765), indicating distinct meteorological signatures. Dust activity then fell 85.9% below the 2016–2022 baseline in 2023–2024, recovering only to 56.0% below it in 2025. The model did not anticipate this: observed dust days in 2023–2024 were 13–27% of the calibrated expectation in every class while local surfaces in 2024 stayed favorable for emission; upwind dust availability and transport remain hypotheses. We then computed PSCF and CWT fields from 14,612 HYSPLIT trajectories, running the locally lifted class, for which a distant source is not required, as an imperfect negative control, tested against permutation nulls. Under unmatched and season-matched backgrounds, the control was more enriched than the suspension-dominated class over the eight a priori candidate regions (p ≤ 0.007), and wind matching removed most regional structure from both. Both fields are shaped largely by the shared Shamal transport climatology and the wind selection of dust days, limiting their ability to identify emitting terrain. At wind-selected, single-regime receptors, PSCF and CWT source fields should be interpreted alongside such a control.