Transport pathways of ozone to marine and free-troposphere sites in Tenerife, Canary Islands
Introduction
Tropospheric ozone studies in the North Atlantic have become a question of interest because of the importance of the ozone long-range transport processes. North Atlantic tropospheric-ozone-columns (vertically integrated concentrations) and surface ozone present a latitudinal gradient with a maximum at mid-latitudes (Weller et al., 1996; Fishman and Brackett, 1997). Satellite observations, model simulations and experimental data, have shown that ozone pollution from Eastern North America enter the North Atlantic and is eastward transported following the mid-latitudes circulation (Dickerson et al., 1995; Parrish et al., 1998; Derwent et al., 2002; Li et al., 2002a). Although this ozone exportation has been documented at surface level (Parrish et al., 1998), it was argued that convective activity lifts ozone pollution to the mid-upper troposphere, where advections are more efficient due to higher wind speed (Derwent et al., 2002; Stohl et al., 2002). Subsequent subsidence over the ocean would produce high ozone events at the surface over the remote North Atlantic (Dickerson et al., 1995).
Recent modelling has shown that European ozone pollution may contribute to the North Atlantic ozone budget by a southwestward transport pathway toward subtropical latitudes (Li et al., 2002a). In contrast with the behaviour over North America, ozone pollution exportation from Europe has a strong tendency to occur at low altitude because of the minor convective activity in Europe (Stohl et al., 2002; Li et al., 2002a).
At several subtropical North Atlantic marine sites, high surface ozone events are associated with rapid subsidence of air masses from the middle troposphere (Savoie et al., 1992; Oltmans and Levy, 1994; Rodrı́guez and Guerra, 2001). There is a continuing debate about the “natural” vs. “anthropogenic” origin of this ozone. These downward transport events are typically associated with upper tropospheric cyclones and tropopause folding. Thus, these high ozone episodes are frequently interpreted as ozone transport from the upper troposphere/lower stratosphere. Owing to these events are related to cyclonic activity, this downward transport is especially important from mid-latitudes to subtropical regions (Davies and Schuepbach, 1994; James et al., 2003).
The eastward transatlantic transport of ozone pollution may influence the regional air quality in Western Europe and NW Africa (Li et al., 2002a). At North Atlantic islands affected by a predominant wind direction, this long-range transport plays a key role on the air quality. This is due to the fact that the predominant wind pattern induces a downwind dispersion of the pollutants emitted in the island and an upwind supply of oceanic air bringing ozone (Rodrı́guez and Guerra, 2001).
In this study we analysed two time series of surface ozone recorded in Tenerife. One measurement site was Aguere (580 m.a.s.l.), located into the trade wind layer (marine boundary layer) and affected by the N–NE airflow carrying air masses from higher latitudes. The other site was Izaña (2367 m.a.s.l.), located in the free troposphere above the trade wind inversion layer. Although prior studies highlighted the significance of the long-range transport, different ozone features and transport scenarios have been described at these sites (Cuevas, 1995; Prospero et al., 1995; Graustein and Turekian, 1996; Kentarchos et al., 2000; Rodrı́guez and Guerra, 2001). Thus, we performed a comparative study of ozone at these sites. The seasons of high and low ozone events, the transport pathways of ozone in several concentration ranges, the role of the downward transport processes, and the meteorological scenarios associated with high ozone events are some points included in this study following the same methodology at both sites.
Section snippets
Measurements sites
Owing to the quasi-permanent subsidence conditions and the trade wind regime, a strong and stable temperature inversion layer (IL) is present below 2000 m.a.s.l. over the Canaries (Fig. 1). This IL separates the cool and moist marine boundary layer (MBL) from the dry and relatively warmer free troposphere (FT), resulting in a strongly stratified low troposphere. In the MBL, the trade wind (NE) is the predominant airflow, whereas in the low FT winds present a higher variability, being the NW the
Seasonal cycles
Fig. 3 shows the monthly means of ozone at Aguere and Izaña. At Aguere, ozone maximises in spring as observed at other sites of the North Atlantic MBL (30°N–65°N; Oltmans and Levy, 1994; Parrish et al., 1998). In the Western North Atlantic ozone is maximum in March–April (e.g. Sable, Reykjavik and Bermuda; Fig. 1A), whereas in the Eastern North Atlantic ozone maximises in April–May (e.g. Mace-Head and Aguere-Tenerife; Fig. 1A). The magnitude of the maximum ozone monthly mean is higher at
Discussion, summary and conclusions
The results obtained in this study, have shown that ozone exhibits significantly differenced seasonal cycles at Izaña (FT) and Aguere (MBL) in Tenerife. For example, the high O3 events (>90 Pth) are mostly recorded from March to June at Aguere, and from April to August at Izaña. These events account for the 30–40% of the days at the two sites in spring and for 14% of the days in summer at Izaña.
The results obtained after a detailed analysis of the seasonal O3 transport pathways for the
Acknowledgements
O3 measurements were performed into the framework of several research projects supported by the Government of the Canaries and by the Global Atmospheric Watch Program financed by INM. We are also indebted to the staff responsible for the maintenance operations. Synoptic charts were provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, USA (http://www.cdc.noaa.gov/). We are also indebted to John Merrill for providing the air trajectories into the framework of AEROCE project.
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