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.
Highlights
Hydrology and O(Fzioshnme a(nOa3n)disCrauntziemn,p1o9rEt7a8an)t.rIttrthaacfeSfecgytassstthienemotxhieditzrionpgoscpahpearceity of the troposphere (Levy, 1S97c1;ieLnogcaen,s1985; Thompson, 1992), it is a phytotoxicant that can affect public health (Tiao et al, 1975), and is an important greenhouse gas (Ramanathan et al, 1985; Mitchel, 1989)
Oltmans et al (1996) reported evidence of high O3 concentration (∼ 100 ppb) layers in the middle and upper troposphere over the North Atlantic which are invariably associated with transport characteristics that strongly support a stratospheric source for these layers
Other authors (e.g. Moody et al, 1996; Creilson et al, 2003; Hegarty et al, 2009) suggest that a substantial amount of photochemically generated tropospheric O3 is transported from continental source regions over the North Atlantic
Summary
Hydrology and O(Fzioshnme a(nOa3n)disCrauntziemn,p1o9rEt7a8an)t.rIttrthaacfeSfecgytassstthienemotxhieditzrionpgoscpahpearceity of the troposphere (Levy, 1S97c1;ieLnogcaen,s1985; Thompson, 1992), it is a phytotoxicant that can affect public health (Tiao et al, 1975), and is an important greenhouse gas (Ramanathan et al, 1985; Mitchel, 1989). Spatial and temporal evolution of atmospheric processes driving O3 concentrations affect stations located in different regions of the world in different ways, as it has been recently reported by Oltmans et al (2013). In this regard Jonson et al (2006) address the question if the observed O3 trends can be understood. We propose, firstly, a detailed analysis of a long-term (22 yr) surface O3 time series (1988–2009) at the Izana high mountain station (hereafter IZO), which is representative of background conditions of the lower troposphere in the subtropical North Atlantic region, where there is very little information available.
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