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

• Main Authors: E. Cuevas, Y. González, S. Rodríguez, J. C. Guerra, A. J. Gómez-Peláez, S. Alonso-Pérez, J. Bustos, C. Milford
• Format: Article
• Language: English
• Published: Copernicus Publications 2013-02-01
• Series: Atmospheric Chemistry and Physics
• Online Access: http://www.atmos-chem-phys.net/13/1973/2013/acp-13-1973-2013.pdf

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