Atmospheric Environment 34 (2000) 5183}5189 Background ozone variations at Mt. Cimone Station P. Bonasoni*, A. Stohl, P. Cristofanelli, F. Calzolari, T. Colombo, F. Evangelisti CNR } ISAO, via Gobetti 101, 40129, Bologna, Italy Technical University of Munich, Chair of Bioclimatology and Air Pollution Research, Am Hochanger 13, D-85354 Freising-Weihenstephan, Germany ITAV}AM, Via delle Ville 100, 41029, Sestola (Mo), Italy Received 18 January 2000; received in revised form 20 April 2000; accepted 10 May 2000 Abstract In order to evaluate the background ozone concentration at Mt. Cimone (2165 m a.s.l.), the contribution of air masses characterised by di!erent provenances is analysed in this paper. The analysis method is based on identi"cation of background air masses which travelled above the 780 hPa pressure level for at least 48 h before arriving at Mt. Cimone. Not being recently mixed with boundary layer air, these air masses are characterised by a chemical age greater than 2 days. This analysis has shown that under background conditions the yearly principal maximum of ozone is recorded in spring and a secondary maximum is recorded in summer. In contrast, if we consider non-background conditions, the principal maximum is found in late summer and a secondary one in spring. In addition, the analysis indicates the presence of a smooth latitudinal gradient of background ozone concentrations in air masses arriving at Mt. Cimone, with higher concentrations coming from the north and lower ones from the south.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Background ozone; Free troposphere; Trajectory analysis; Atmospheric transport; Background station 1. Introduction In the last decades, the ozone concentration in the clean, background troposphere has doubled compared to the preindustrial years. Today its mixing ratio varies between 20 ppbv in winter and 60 ppbv in summer (Staehelin et al., 1994; Seinfeld and Pandis, 1998). This ozone increase took place throughout the 1970s and in the free troposphere of Northern mid-latitudes an in- crease of about 10% per decade is registered (London and Liu, 1992). An increase in anthropogenic activities leading to higher emissions of NO  and hydrocarbons can be considered as the principal reason for this rise in tropospheric ozone concentrations. Ozone and photo- oxidants can be transported from the atmospheric * Corresponding author. E-mail address: p.bonasoni@isao.bo.cnr.it (P. Bonasoni). boundary layer to the free troposphere and dispersed over large regions by long-range transport phenomena (Memmesheimer et al., 1997). In order to determine the background concentration of ozone at Mt. Cimone (MTC), we analysed the ozone concentration in an `un- perturbeda free troposphere. In this paper `background conditionsa are de"ned by a chemical age of air masses greater than 2}3 days (TOR-2, 1999). This prevents the surface ozone concentration measured at MTC from being directly a!ected by the transformation and trans- port of ozone and its precursors from polluted regions. This allowed us to determine the concentration of the background ozone present at MTC (2165 m a.s.l.) and in the Northern Mediterranean free troposphere. In fact, measurements at high mountain sites permit the invest- igation and assessment of the ozone concentration and other trace gases in the lower free troposphere. For this purpose, continuous measurements have been under- taken at four mountain peak stations during the EC project `Vertical Ozone Transports in the Alpsa 1352-2310/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 2 6 8 - 5