Regional transport of the urban workweek: Methylchloroform cycles in the Nevada‐Arizona Desert

The halocarbons CFCl3, CH3CCl3, CCl4, and C2Cl4 have been monitored at four locations in southern California, southern Nevada, and northwestern Arizona. Summertime concentrations of CH3CCl3 and C2Cl4 at the Nevada and Arizona sites exhibit strong weekly cycles that lag similar cycles observed in the Los Angeles Basin by 1–2 days. The observed patterns imply a nearly complete weekend shutdown of emissions, and remarkably consistent long-range (300–400 km) transport through complex terrain. The average amplitude of the summertime CH3CCl3 cycle is about 1200 ppt in air leaving the Los Angeles Basin, 80 ppt at the mountain-top Nevada observatory, and 40 ppt at the level-terrain Arizona observatory.