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

Abstract

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.

Airborne in situ vertical profiling of HDO / H<sub>2</sub><sup>16</sup>O in the subtropical troposphere during the MUSICA remote sensing validation campaign

“…On 21 July 2013, 25 July 2013, and 30 July 2013 the marine boundary layer (MBL) was well separated from the FT by a temperature-inversion layer at an altitude of around 1100 m. This inversion is a consequence of large-scale subsidence in the downward branch of the Hadley circulation (Cuevas et al, 2013). The subsiding air is adiabatically warmed as it reaches lower altitudes, thus yielding a welldeveloped temperature-inversion layer.…”

“…This Saharan air layer was positioned above a layer of MBL air, and it presumably originated from the African boundary layer (Rodríguez et al, 2004;Alonso-Pérez et al, 2011;Cuevas et al, 2013). We refer to these conditions as Sahara.…”

Empirical validation and proof of added value of MUSICA's tropospheric δD remote sensing products

“…It is responsible for a strong subsidence inversion layer which hinders mixing of planetary boundary layer (PBL) air into the FT. Under such situation the FT air mass is transported from higher latitudes and/or higher altitudes (Galewsky et al, 2005;Barnes and Hartmann, 2010;Cuevas et al, 2013). In addition, the summertime FT of the northern subtropical Atlantic is frequently affected by air masses that are advected from the African continent (e.g.…”

“…The PM 10 aerosol data show the aerosol mass concentration considering all particles with aerodynamic diameter smaller than 10 µm, and at Izaña, PM 10 is, by far, dominated by Saharan dust particles (Rodríguez et al, 2011). For the calculation of the backward trajectories we use HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory -some examples and explanations of the HYSPLIT calculations are given in Cuevas et al, 2013).…”

Atmospheric CO2, CH4, and CO with CRDS technique at the Izaña Global GAW station: instrumental tests, developments and first measurement results

“…on Tenerife (Canary Islands, Spain), usually above a strong subtropical temperature inversion. Since it is located at the summit of a mountain, during night-time 15 there are North-Atlantic-free-troposphere background conditions, whereas during daytime there is a slight perturbation of these conditions by the arrival of an upslope thermal wind close to the terrain surface (e.g., Cuevas et al, 2013;GomezPelaez et al, 2013;Rodríguez et al, 2009). Detailed information about the numerous measurement programmes operative at IZO are provided by Cuevas et al (2015).…”