Influence of vertical mixing and nighttime transport on surface ozone variability in the morning in Paris and the surrounding region

International audienceVertical profiles of ozone (O3, aerosols, and winds in Paris, France have been measured via remote sensing (Lidar) during a 36 h intensive observation period (IOP) on 9–10 September 2014. The measurements show an ozone poor layer (ozone concentrations as low as 40 μg.m-3is present above the nocturnal boundary layer (NBL) during the IOP (~400m AGL, 10 Sept. 2014 06:00 LT). This ozone poor layer is reincorporated in the morning into the developing daytime boundary layer, resulting in lower measured surface ozone concentrations in Paris. Specifically, the surface ozone concentration at 12:30 LT is 33 μg.m-3 smaller on 10 Sept. 2014 than on 9 Sept. 2014 in Paris. This ozone poor layer is also co-located with a nocturnal low-level jet (NLLJ), which transports air away from the Paris region, resulting in lower ozone concentrations downwind of Paris the following day. We track transport and dispersion of this airmass using the regional Weather Research and Forecasting (WRF) model combined with the FLEXible PARTicle dispersion model (FLEXPART-WRF). This dataset demonstrates that clean air masses above the NBL have the ability to impact “next-day” ozone concentrations in cities and the surrounding areas. Our analysis shows that vertical mixing and incorporation of ozone located above the boundary layer in the morning is a significant factor that can control daytime surface ozone concentrations in Paris and the surrounding area