Assessment of atmospheric processes driving ozone variations in the subtropical North Atlantic free troposphere

Abstract

An analysis of the 22-yr ozone (O₃) series (1988–2009) at the subtropical high mountain Izaña~station (IZO; 2373 m a.s.l.), representative of free troposphere (FT) conditions, is presented. Diurnal and seasonal O₃ variations as well as the O₃ trend (0.19 ± 0.05 % yr^−1 or 0.09 ppbv yr^−1), are assessed. A climatology of O₃ transport pathways using backward trajectories shows that higher O₃ values are associated with air masses travelling above 4 km altitude from North America and North Atlantic Ocean, while low O₃ is transported from the Saharan continental boundary layer (CBL). O₃ data have been compared with PM₁₀, ²¹⁰Pb, ⁷Be, potential vorticity (PV) and carbon monoxide (CO). A clear negative logarithmic relationship was observed between PM₁₀ and surface O₃ for all seasons. A similar relationship was found between O₃ and ²¹⁰Pb. The highest daily O₃ values (90th percentile) are observed in spring and in the first half of summer time. A positive correlation between O₃ and PV, and between O₃ and ⁷Be is found throughout the year, indicating that relatively high surface O₃ values at IZO originate from the middle and upper troposphere. We find a good correlation between O₃ and CO in winter, supporting the hypothesis of long-range transport of photochemically generated O₃ from North America. Aged air masses, in combination with sporadic inputs from the upper troposphere, are observed in spring, summer and autumn. In summer time high O₃ values seem to be the result of stratosphere-to-troposphere (STT) exchange processes in regions neighbouring the Canary Islands. Since 1995–1996, the North Atlantic Oscillation has changed from a predominantly high positive phase to alternating between negative, neutral or positive phases. This change results in an increased flow of the westerlies in the mid-latitude and subtropical North Atlantic, thus favouring the transport of O₃ and its precursors from North America, and a higher frequency of storms over North Atlantic, with a likely higher incidence of STT processes in mid-latitudes. These processes lead to an increase of tropospheric O₃ in the subtropical North Atlantic region after 1996 that has been reflected in surface O₃ records at IZO.