Editing IPCC:AR6/WGI/Chapter-2 (section)

==== 2.2.5.3 Tropospheric Ozone ====

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The AR5 assessed ''medium confidence'' in large-scale increases of tropospheric ozone at rural surface sites across the NH (1970–2010), and in a doubling of European surface ozone during the 20th century, with the increases of surface ozone in the SH being of ''low confidence'' . Surface ozone ''likely'' increased in East Asia, but levelled off or decreased in the eastern USA and western Europe. Free tropospheric trends (1971–2010) from ozonesondes and aircraft showed positive trends in most, but not all, assessed regions, and for most seasons and altitudes. This section focuses on large scale ozone changes; chemical and physical processes and regional changes in tropospheric ozone are assessed in Section 6.3.2.1 and Section 7.3.2.5 assesses radiative forcing.

Prior to 1850 ozone observations do not exist, but a recent analysis using clumped-isotope composition of molecular oxygen ( <sup>18</sup> O <sup>18</sup> O in O <sub>2</sub> ) trapped in polar firn and ice, combined with atmospheric chemistry model simulations, constrains the global tropospheric ozone increase to less than 40% between 1850 and 2005, with most of this increase occurring between 1950 and 1980 ( [[#Yeung--2019|Yeung et al., 2019]] ). Recently, the Tropospheric Ozone Assessment Report identified and evaluated 60 records of surface ozone observations collected at rural locations worldwide between 1896 and 1975, which were based on a range of measurement techniques with potentially large uncertainties ( [[#Tarasick--2019|Tarasick et al., 2019]] ). They found that from the mid-20th century (1930s to the early 1970s) to 1990–2014, rural surface ozone increased by 30–70% across the northern extra-tropics. This is smaller than the 100% 20th-century increase reported in AR5, which relied on far fewer measurement sites, all in Europe. In the northern tropics limited low-elevation historical data (1954–1975) provide no clear indication of surface ozone increases ( [[#Tarasick--2019|Tarasick et al., 2019]] ). However, similar to the northern mid-latitude increases, lower-free tropospheric ozone at Mauna Loa, Hawaii increased by approximately 50% from the late 1950s to present ( [[#Cooper--2020|Cooper et al., 2020]] ). Historical observations are too limited to draw conclusions on surface ozone trends in the SH tropics and mid-latitudes since the mid-20th century, with tropospheric ozone exhibiting little change across Antarctica ( [[#Tarasick--2019|Tarasick et al., 2019]] ; [[#Cooper--2020|Cooper et al., 2020]] ). Based on reliable UV absorption measurements at remote locations (surface and lower troposphere), ozone trends since the mid-1990s varied spatially at northern mid-latitudes, but increased in the northern tropics (2–17%; 1–6 ppbv per decade; ( [[#Cooper--2020|Cooper et al., 2020]] ; [[#Gaudel--2020|Gaudel et al., 2020]] ). Across the SH these more recent observations are too limited to determine zonal trends (e.g., tropics, mid-latitudes, high latitudes).

The earliest observations of free tropospheric ozone (1934–1955) are available from northern mid-latitudes where limited data indicate a tropospheric column ozone increase of 48 ± 30% up to 1990–2012 ( [[#Tarasick--2019|Tarasick et al., 2019]] ). Starting in the 1960s, records from ozonesondes show no significant changes in the free troposphere over the Arctic and mid-latitude regions of Canada, but trends are mainly positive elsewhere in the northern mid-latitudes ( [[#Oltmans--2013|Oltmans et al., 2013]] ; [[#Cooper--2020|Cooper et al., 2020]] ). Tropospheric column and free tropospheric trends since the mid-1990s based on commercial aircraft, ozonesonde observations and satellite retrievals (Figure 2.8b,c), are overwhelmingly positive across the northern mid-latitudes (2–7%; 1–4 ppbv per decade) and tropics (2–14%; 1–5 ppbv per decade), with the largest increases (8–14%; 3–6 ppbv per decade) in the northern tropics in the vicinity of southern Asia and Indonesia. Observations in the SH are limited, but indicate average tropospheric column ozone increases of 2–12% (1–5 ppbv) per decade in the tropics (Figure 2.8c), and weak tropospheric column ozone increases (&lt;5%, &lt;1 ppbv per decade) at mid-latitudes ( [[#Cooper--2020|Cooper et al., 2020]] ). Above Antarctica, mid-tropospheric ozone has increased since the late 20th century ( [[#Oltmans--2013|Oltmans et al., 2013]] ). The total ozone ERF from 1750 to 2019 best estimate is assessed as 0.47 W m <sup>–2</sup> (Section 7.3.2.5) and this is dominated by increases in the troposphere. The underlying modelled global tropospheric ozone column increase ( [[#Skeie--2020|Skeie et al., 2020]] ) from 1850 to 2010 of 40–60%, is somewhat higher than the isotope based upper-limit of [[#Yeung--2019|Yeung et al. (2019)]] . At mid-latitudes (30°–60°N) model increases of 30–40% since the mid-20th century are broadly consistent with observations.

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[[File:29dfc27e28be57444bcf16f98ff974d1 IPCC_AR6_WGI_Figure_2_8.png]]

'''Figure 2.8''' '''|''' '''Surface and tropospheric ozone trends. (a)''' Decadal ozone trends by latitude at 28 remote surface sites and in the lower free troposphere (650 hPa, about 3.5 km) as measured by IAGOS aircraft above 11 regions. All trends are estimated for the time series up to the most recently available year, but begin in 1995 or 1994. Colours indicate significance (p-value) as denoted in the in-line key. See Figure 6.5 for a depiction of these trends globally. '''(b)''' Trends of ozone since 1994 as measured by IAGOS aircraft in 11 regions in the mid-troposphere (700–300 hPa; about 3–9 km) and upper troposphere (about 10–12 km), as measured by IAGOS aircraft and ozonesondes. '''(c)''' Trends of average tropospheric column ozone mixing ratios from the TOST composite ozonesonde product and three composite satellite products based on TOMS, OMI/MLS (Sat1), GOME, SCIAMACHY, OMI, GOME-2A, GOME-2B (Sat2), and GOME, SCIAMACHY, GOME-II (Sat3). Vertical bars indicate the latitude range of each product, while horizontal lines indicate the ''very likely'' uncertainty range. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

In summary, ''limited'' available isotopic ''evidence'' constrains the global tropospheric ozone increase to less than 40% between 1850 and 2005 ( ''low confidence'' ). Based on sparse historical surface/low altitude data tropospheric ozone has increased since the mid-20th century by 30–70% across the NH ( ''medium confidence'' ). Surface/low altitude ozone trends since the mid-1990s are variable at northern mid-latitudes, but positive in the tropics [2 to 17% per decade] ( ''high confidence'' ). Since the mid-1990s, free tropospheric ozone has increased by 2–7% per decade in most regions of the northern mid-latitudes, and 2–12% per decade in the sampled regions of the northern and southern tropics ( ''high confidence'' ). Limited coverage by surface observations precludes identification of zonal trends in the SH, while observations of tropospheric column ozone indicate increases of less than 5% per decade at southern mid-latitudes ( ''medium confidence'' ).

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