The impact of Los Angeles basin pollution and stratospheric intrusions on the San Gabriel Mountains as seen by surface measurements, lidar, and numerical models

In this work, the impact of Los Angeles basin pollution transport and stratospheric intrusions on the surface ozonelevels observed in the San Gabriel Mountains is investigated based on a combination of surface and lidar measurements as wellas WRF-Chem (Weather Research and Forecasting with Chemistry) and WACCM (Whole Atmosphere Community ClimateModel) model runs. The number of days with observed surface ozone levels exceeding the National Ambient Air QualityStandards exhibit a clear seasonal pattern, with a maximum during summer, when models suggest a minimum influence of5stratospheric intrusions and the largest impact from Los Angeles basin pollution transport. Additionally, measured and modeledsurface ozone and PM10 were analyzed as a function of season, time of the day and wind direction. Measurements and modelsare in good qualitative agreement, with maximum surface ozone observed for south-west and west winds. For the prevailingsummer wind direction, slightly south of the ozone maximum and corresponding to south south-west winds, lower ozonelevels were observed. Back-trajectories suggest that this is associated with transport from the central Los Angeles basin, where10titration limits the amount of surface ozone. A quantitative comparison of the lidar profiles with WRF-Chem and WACCMmodels revealed good agreement near the surface, with models showing an increasing positive bias as function of altitude,reaching 75% at 15kmabove sea level. Finally, three selected case studies covering the different mechanisms affecting thenear-surface ozone concentration over the San Gabriel mountains, namely stratospheric intrusions and pollution transport, areanalyzed based on surface and ozone lidar measurements, as well as co-located ceilometer measurements and models.