Detection of deep stratospheric intrusions by cosmogenic 35S

The extent to which stratospheric intrusions on synoptic scales influence the tropospheric ozone (O₃) levels remains poorly understood, because quantitative detection of stratospheric air has been challenging. Cosmogenic ³⁵S mainly produced in the stratosphere has the potential to identify stratospheric air masses at ground level, but this approach has not yet been unambiguously shown. Here, we report unusually high ³⁵S concentrations (7,390 atoms m^-3; ∼16 times greater than annual average) in fine sulfate aerosols (aerodynamic diameter less than 0.95 µm) collected at a coastal site in southern California on May 3, 2014, when ground-level O₃ mixing ratios at air quality monitoring stations across southern California (43 of 85) exceeded the recently revised US National Ambient Air Quality Standard (daily maximum 8-h average: 70 parts per billion by volume). The stratospheric origin of the significantly enhanced ³⁵S level is supported by in situ measurements of air pollutants and meteorological variables, satellite observations, meteorological analysis, and box model calculations. The deep stratospheric intrusion event was driven by the coupling between midlatitude cyclones and Santa Ana winds, and it was responsible for the regional O₃ pollution episode. These results provide direct field-based evidence that ³⁵S is an additional sensitive and unambiguous tracer in detecting stratospheric air in the boundary layer and offer the potential for resolving the stratospheric influences on the tropospheric O₃ level.