Tropospheric ozone is a short-lived trace gas that either originates in the stratosphere or is produced in situ by precursor gases and sunlight (e.g., Monks et al., 2009). An important GHG with an estimated RF of 0.40 ± 0.20 W/m2 (Chapter 8), tropospheric ozone also impacts human health and vegetation at the surface. Its average atmospheric lifetime of a few weeks produces a global distribution highly variable by season, altitude and location. These characteristics and the paucity of long-term measurements make the assessment of long-term global ozone trends challenging. However, new studies since AR4 provide greater understanding of surface and free tropospheric ozone trends from the 1950s through 2010. An extensive compilation of measured ozone trends is presented in the Supplementary Material, Figure 2.SM.1 and Table 2.SM.2.
The earliest (1876–1910) quantitative ozone observations are limited to Montsouris near Paris where ozone averaged 11 ppb (Volz and Kley, 1988). Semiquantitative ozone measurements from more than 40 locations around the world in the late 1800s and early 1900s range from 5 to 32 ppb with large uncertainty (Pavelin et al., 1999). The low 19th century ozone values cannot be reproduced by most models (Section 184.108.40.206), and this discrepancy is an important factor contributing to uncertainty in RF calculations (Section 220.127.116.11). Limited quantitative measurements from the 1870s to 1950s indicate that surface ozone in Europe increased by more than a factor of 2 compared to observations made at the end of the 20th century (Marenco et al., 1994; Parrish et al., 2012).
Satellite-based tropospheric column ozone retrievals across the tropics and mid-latitudes reveal a greater burden in the NH than in the SH (Ziemke et al., 2011). Tropospheric column ozone trend analyses are few. An analysis by Ziemke et al. (2005) found no trend over the tropical Pacific Ocean but significant positive trends (5 to 9% per decade) in the mid-latitude Pacific of both hemispheres during 1979–2003. Significant positive trends (2 to 9% per decade) were found across broad regions of the tropical South Atlantic, India, southern China, southeast Asia, Indonesia and the tropical regions downwind of China (Beig and Singh, 2007).
Long-term ozone trends at the surface and in the free troposphere (of importance for calculating RF, Chapter 8) can be assessed only from in situ measurements at a limited number of sites, leaving large areas such as the tropics and SH sparsely sampled (Table 2.SM.2, Figure 2.7). Nineteen predominantly rural surface sites or regions around the globe have long-term records that stretch back to the 1970s, and in two cases the 1950s (Lelieveld et al., 2004; Parrish et al., 2012; Oltmans et al., 2013). Thirteen of these sites are in the NH, and 11 sites have statistically significant positive trends of 1 to 5 ppb per decade, corresponding to >100% ozone increases since the 1950s and 9 to 55% ozone increases since the 1970s. In the SH, three of six sites have significant trends of approximately 2 ppb per decade and three have insignificant trends. Free tropospheric monitoring since the 1970s is more limited. Significant positive trends since 1971 have been observed using ozone sondes above Western Europe, Japan and coastal Antarctica (rates of increase range from 1 to 3 ppb per decade), but not at all levels (Oltmans et al., 2013). In addition, aircraft have measured significant upper tropospheric trends in one or more seasons above the north-eastern USA, the North Atlantic Ocean, Europe, the Middle East, northern India, southern China and Japan (Schnadt Poberaj et al., 2009). Insignificant free tropospheric trends were found above the Mid-Atlantic USA (1971–2010) (Oltmans et al., 2013) and in the upper troposphere above the western USA (1975–2001) (Schnadt Poberaj et al., 2009). No site or region showed a significant negative trend.
In recent decades ozone precursor emissions have decreased in Europe and North America and increased in Asia (Granier et al., 2011), impacting ozone production on regional and hemispheric scales (Skeie et al., 2011). Accordingly, 1990–2010 surface ozone trends vary regionally. In Europe ozone generally increased through much of the 1990s but since 2000 ozone has either levelled off or decreased at rural and mountaintop sites, as well as for baseline ozone coming ashore at Mace Head, Ireland (Tarasova et al., 2009; Logan et al., 2012; Parrish et al., 2012; Oltmans et al., 2013). In North America surface ozone has increased in eastern and Arctic Canada, but is unchanged in central and western Canada (Oltmans et al., 2013). Surface ozone has increased in baseline air masses coming ashore along the west coast of the USA (Parrish et al., 2012) and at half of the rural sites in the western USA during spring (Cooper et al., 2012). In the eastern USA surface ozone has decreased strongly in summer, is largely unchanged in spring and has increased in winter (Lefohn et al., 2010; Cooper et al., 2012). East Asian surface ozone is generally increasing (Table 2.SM.2) and at downwind sites ozone is increasing at Mauna Loa, Hawaii but decreasing at Minami Tori Shima in the subtropical western North Pacific (Oltmans et al., 2013). In the SH ozone has increased at the eight available sites, although trends are insignificant at four sites (Helmig et al., 2007; Oltmans et al., 2013).
Owing to methodological changes, free tropospheric ozone observations are most reliable since the mid-1990s. Ozone has decreased above Europe since 1998 (Logan et al., 2012) and is largely unchanged above Japan (Oltmans et al., 2013). Otherwise the remaining regions with measurements (North America, North Pacific Ocean, SH) show a range of positive trends (both significant and insignificant) depending on altitude, with no site having a negative trend at any altitude (Table 2.SM.2).
In summary, there is medium confidence from limited measurements in the late 19th through mid-20th century that European surface ozone more than doubled by the end of the 20th century. There is medium confidence from more widespread measurements beginning in the 1970s that surface ozone has increased at most (non-urban) sites in the NH (1 to 5 ppb per decade), while there is low confidence for ozone increases (2 ppb per decade) in the SH. Since 1990 surface ozone has likely increased in East Asia, while surface ozone in the eastern USA and Western Europe has levelled off or is decreasing. Ozone monitoring in the free troposphere since the 1970s is very limited and indicates a weaker rate of increase than at the surface. Satellite instruments can now quantify the present-day tropospheric ozone burden on a near-global basis; significant tropospheric ozone column increases were observed over extended tropical regions of southern Asia, as well as mid-latitude regions of the South and North Pacific Ocean since 1979.