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

作者：Cuevas E^*; Gonzalez Y; Rodriguez S; Guerra J C; Gomez Pelaez A J; Alonso Perez S; Bustos J; Milford C

来源：Atmospheric Chemistry and Physics, 2013, 13(4): 1973-1998.

DOI：10.5194/acp-13-1973-2013

An analysis of the 22-yr ozone (O-3) series (1988-2009) at the subtropical high mountain Izana 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 PM10, Pb-210, Be-7, potential vorticity (PV) and carbon monoxide (CO). A clear negative logarithmic relationship was observed between PM10 and surface O-3 for all seasons. A similar relationship was found between O-3 and Pb-210. 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 Be-7 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.

• 出版日期2013