The influence of mid-latitude cyclones on European background surface ozone.

K. Emma Knowland,Kevin I. Hodges,Ruth M. Doherty,Lesley E. Ott

doi:10.5194/acp-17-12421-2017

Abstract

The relationship between springtime mid-latitude cyclones and background ozone (O3) is explored using a combination of observational and reanalysis data sets. First, the relationship between surface O3 observations at two rural monitoring sites on the west coast of Europe - Mace Head, Ireland and Monte Velho, Portugal - and cyclone track frequency in the surrounding regions is examined. Second, detailed case study examination of four individual mid-latitude cyclones and the influence of the associated frontal passage on surface O3 is performed. Cyclone tracks have a greater influence on the O3 measurements at the more northern coastal European station, Mace Head, located within the main North Atlantic (NA) storm track. In particular, when cyclones track north of 53° N, there is a significant relationship with high levels of surface O3 (> 75th percentile). The further away a cyclone is from the NA storm track, the more likely it will be associated with both high and low (< 25th percentile) levels of O3 at the observation site during the cyclone's life cycle. The results of the four case studies demonstrate a) the importance of the passage of a cyclone's cold front in relation to surface O3 measurements, b) the ability of mid-latitude cyclones to bring down high levels of O3 from the stratosphere and c) that accompanying surface high pressure systems and their associated transport pathways play an important role in the temporal variability of surface O3. The main source of high O3 to these two sites in springtime is from the stratosphere, either from direct injection into the cyclone or associated with aged airstreams from decaying downstream cyclones that can become entrained and descend toward the surface within new cyclones over the NA region.

Highlights

Tropospheric ozone (O3), which is harmful to human health and vegetation (Scherrer et al, 2006; Krzyzanowski and Cohen, 2008), has both natural and man-made sources
We have shown that passing cyclones have a discernible influence on surface O3 concentrations; especially when cyclones track north of 53◦ N, there is a significant probability that the surface O3 at Mace Head will be high (> 75th pc)
Cyclones are more likely to be associated with both low O3 (< 25th pc) and high O3 if the cyclones track to the south of the observation site or if the cyclones are to the west of the observation site

Summary

Introduction

Tropospheric ozone (O3), which is harmful to human health and vegetation (Scherrer et al, 2006; Krzyzanowski and Cohen, 2008), has both natural and man-made sources. Rural monitoring ground stations, which are not influenced by local emission sources, are used to monitor the “clean” hemispheric baseline, or background, levels of pollutants such as O3 (e.g., Parrish et al, 2012; Torseth et al, 2012; Wilson et al, 2012). The measurements from these remote stations in Europe previously have been used to study the concentrations of O3 in polluted air arriving from different source regions separate from the background O3 levels (e.g., Li et al, 2002; Wilson et al, 2012). A large number of observations (about 70 %) taken at the Mace Head station on the west coast of Ireland have been found to be associated with maritime air from over the North Atlantic (NA) and the remainder are associated with continental European air (Jennings et al, 1991; Simmonds et al, 1997).

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