A study of the influence of tropospheric subsidence on spring and summer surface ozone concentrations at the JRC-Ispra station in northern Italy

Abstract. The influence of tropospheric ozone to the surface ozone concentrations is investigated at the monitoring station of JRC-Ispra, based on 10 years of measurements (2006–2015) of surface ozone data. In-situ hourly measurements of ozone and other air-pollutants, meteorological parameters and weekly averaged 7 Be (as indicator of upper-tropospheric/stratospheric influence) and 210 Pb measurements (as indicator of boundary layer influence), have been used for the analysis. It is observed that frequently 7 Be and ozone weekly peaks coincide, which might be explained by the impact of deep atmospheric subsidence on surface ozone, particularly during late spring and early summer. Based on this observation, a detailed analysis of selected 7 Be and ozone episodes occurring during that period of the year has been performed in order to further elucidate the mechanisms of tropospheric influence to the surface pollutant concentrations. For the analysis composite NOAA/ESRL reanalysis synoptic meteorological charts in the troposphere have been used as well as IASI satellite ozone measurements and NOAA-HYSPLIT back trajectories. The JRC-station hourly measurements during subsidence episodes show very low values of local pollution parameters (e.g. NO x , 222 Rn, nephelometer data, PM 10 ), close to zero. On the contrary, during these periods ozone levels reach usually values around 45–60 ppb during the afternoon hours but also show significantly higher values than the average during the night and morning hours, which is a sign of direct tropospheric influence to the surface ozone concentrations.