Increasing interannual and altitudinal ozone mixing ratios in the Catalan Pyrenees Maria Dı ´az-de-Quijano a, * , Josep Pen ˜ uelas a ,A ` ngela Ribas b a Ecophysiology and Global Change Unit CREAF-CEAB-CSIC, CREAF (Center for Ecological Research and Forestry Applications), Edifici C, Universitat Auto `noma de Barcelona, 08193 Bellaterra, Catalonia, Spain b Forest Technology Centre of Catalonia, Crta. St. Llorenç de Morunys, km 2. 25280 Solsona, Lleida, Spain article info Article history: Received 30 May 2009 Received in revised form 4 August 2009 Accepted 19 August 2009 Keywords: AOT40 Bel-W3 Biomonitoring Mediterranean region Nitrogen oxides Phytotoxicity abstract Interannual, seasonal, daily and altitudinal patterns of tropospheric ozone mixing ratios, as well as ozone phytotoxicity and the relationship with NO x precursors and meteorological variables were monitored in the Central Catalan Pyrenees (Meranges valley and Forest of Guils) over a period of 5 years (2004–2008). Biweekly measurements using Radiello passive samplers were taken along two altitudinal transects comprised of thirteen stations ranging from 1040 to 2300 m a.s.l.. Visual symptoms of ozone damage in Bel-W3 tobacco cultivars were evaluated biweekly for the first three years (2004–2006). High ozone mixing ratios, always above forest and vegetation protection AOT40 thresholds, were monitored every year. In the last 14 years, the AOT40 (Apr–Sept.) has increased significantly by 1047 mgm 3 h per year. Annual means of ozone mixing ratios ranged between 38 and 67 ppb v (38 and 74 ppb v during the warm period) at the highest site (2300 m) and increased at a rate of 5.1 ppb v year 1 . The ozone mixing ratios were also on average 35–38% greater during the warm period and had a characteristic daily pattern with minimum values in the early morning, a rise during the morning and a decline overnight, that was less marked the higher the altitude. Whereas ozone mixing ratios increased significantly with altitude from 35 ppb v at 1040 m–56 ppb v at 2300 m (on average for 2004–2007 period), NO 2 mixing ratios decreased with altitude from 5.5 ppb v at 1040 m–1 ppb v at 2300 m. The analysis of meteorological variables and NO x values suggests that the ozone mainly originated from urban areas and was transported to high- mountain sites, remaining aloft in absence of NO. Ozone damage rates increased with altitude in response to increasing O 3 mixing ratios and a possible increase in O 3 uptake due to more favorable microclimatic conditions found at higher altitude, which confirms Bel-W3 as a suitable biomonitor for ozone concentrations during summer time. Compared to the valley-bottom site the annual means of ozone mixing ratios are 37% larger in the higher sites. Thus the AOT40 for the forest and vegetation protection threshold is greatly exceeded at higher sites. This could have substantial effects on plant life at high altitudes in the Pyrenees. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction During the pre-industrial period ozone concentrations were approximately 10–15 ppb v (Finlayson-Pitts and Pitts, 1997). However, these concentrations rapidly increased when NO x emis- sions became more intense due to the switch to fossil fuels that took place in the industrial revolution (Finlayson-Pitts and Pitts, 1997). Nowadays, the annual average background ozone concen- trations over the mid-latitudes of the Northern Hemisphere ranges approximately between 20 and 45 ppb v , with variability depending on location, atmospheric elevation and distance to other pollutants’ emission sources (Finlayson-Pitts and Pitts, 2000). Overall, in mountainous areas ozone mixing ratios seem to rise with increasing altitude (Aneja et al., 1994a,b; Skelly et al., 1997; Cooper and Peterson, 2000; Ribas and Pen ˜uelas, 2006; Sanz et al., 2007). Higher solar radiation (Volz and Kley, 1988), local air recir- culation linked to orographic systems (eg. valley-mountain systems or mixing boundary layers) (Lefhon, 1992; Sanz and Milla ´n, 2000), ozone intrusions from the stratosphere (Viezee et al., 1983) and less chemical ozone loss under low NO concentrations (Naja et al., 2003) are all factors which contribute to high ozone mixing ratios at high altitude. Moreover, stomatal conductance is found to increase with rising altitude (Wieser et al., 2000), since trees rarely have the need for restricting water loss at high altitudes (Tranquillini, 1979). Therefore, vegetation at higher altitudes can be more easily affected by ozone toxicity than at lower sites (Ribas and Pen ˜uelas, 2006). * Corresponding author. Tel.: þ34 93 581 48 50; fax: þ34 93 581 41 51. E-mail addresses: m.diaz@creaf.uab.cat (M. Dı ´az-de-Quijano), josep.penuelas@ uab.cat (J. Pen ˜ uelas), angela.ribas@ctfc.cat (A ` . Ribas). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.08.035 Atmospheric Environment 43 (2009) 6049–6057