Climatology of ozone, PAN, CO, and NMHC in the free troposphere over the southern north Atlantic

Free tropospheric measurements of ozone, peroxyacetylnitrate and precursors (CO, NMHC) that were made within the framework of the EUROTRAC sub-project TOR (Tropospheric Ozone Research) between 1990 and 1995 at the GAW station Izana, Tenerife (28 degrees 18' N, 16 degrees 30' W) are discussed. The average annual cycles reveal the importance of transport from northern mid-latitudes and the role of photo-chemistry. According to air-mass trajectories, which were supplied to us from AEROCE (Atmosphere/Ocean Chemistry Experiment), transport from northern mid-latitudes is associated with high precursor concentrations in winter, whereas ozone concentrations in winter are not much influenced by transport patterns, suggesting a rather uniform distribution over the northern part of the Northern Hemisphere around mean value of 43 +/- 5 ppb. In summer, high ozone concentrations of up to 90 ppb are often encountered during transport from north, while the levels of precursors are much lower than in winter, because of photochemical destruction. Trajectories from southerly latitudes and the Sahara usually have the lowest ozone concentrations associated with them.