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

Cuevas, E.; González, Y.; Rodríguez, S.; Guerra, J. C.; Gómez-Peláez, A. J.; Alonso-Pérez, S.; Bustos, J.; Milford, C.

An analysis of the 22-yr ozone (O 3) 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 O 3 variations as well as the O 3 trend (0.19 ± 0.05 % yr −1 or 0.09 ppbv yr −1), are assessed. A climatology of O 3 transport pathways using backward trajectories shows that higher O 3 values are associated with air masses travelling above 4 km altitude from North America and North Atlantic Ocean, while low O 3 is transported from the Saharan continental boundary layer (CBL). O 3 data have been compared with PM 10, 210Pb, 7Be, potential vorticity (PV) and carbon monoxide (CO). A clear negative logarithmic relationship was observed between PM 10 and surface O 3 for all seasons. A similar relationship was found between O 3 and 210Pb. The highest daily O 3 values (90th percentile) are observed in spring and in the first half of summer time. A positive correlation between O 3 and PV, and between O 3 and 7Be is found throughout the year, indicating that relatively high surface O 3 values at IZO originate from the middle and upper troposphere. We find a good correlation between O 3 and CO in winter, supporting the hypothesis of long-range transport of photochemically generated O 3 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 O 3 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 O 3 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 O 3 in the subtropical North Atlantic region after 1996 that has been reflected in surface O 3 records at IZO.

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Cuevas, E. / González, Y. / Rodríguez, S. / et al: Assessment of atmospheric processes driving ozone variations in the subtropical North Atlantic free troposphere. 2013. Copernicus Publications.

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