Global tropospheric ozone trends, attributions, and radiative impacts in 1995–2017: an integrated analysis using aircraft (IAGOS) observations, ozonesonde, and multi-decadal chemical model simulations
Authors: Wang Haolin , Lu Xiao , Jacob Daniel J. , Cooper Owen R. , Chang Kai-Lan , Li Ke , Gao Meng , Liu Yiming , Sheng Bosi , Wu Kai , Wu Tongwen , Zhang Jie , Sauvage Bastien , Nédélec Philippe , Blot Romain , Fan Shaojia 
Published: October 2022

Abstract. Quantification and attribution of long-term tropospheric ozone trends are critical for understanding the impact of human activity and
climate change on atmospheric chemistry but are also challenged by the
limited coverage of long-term ozone observations in the free troposphere
where ozone has higher production efficiency and radiative potential
compared to that at the surface. In this study, we examine observed
tropospheric ozone trends, their attributions, and radiative impacts from
1995–2017 using aircraft observations from the In-service Aircraft for a
Global Observing System database (IAGOS), ozonesondes, and a multi-decadal
GEOS-Chem chemical model simulation. IAGOS observations above 11 regions in
the Northern Hemisphere and 19 of 27 global ozonesonde sites have measured
increases in tropospheric ozone (950–250 hPa) by 2.7 ± 1.7 and 1.9 ± 1.7 ppbv per decade on average, respectively, with particularly
large increases in the lower troposphere (950–800 hPa) above East Asia,
the Persian Gulf, India, northern South America, the Gulf of Guinea, and
Malaysia/Indonesia by 2.8 to 10.6 ppbv per decade. The GEOS-Chem
simulation driven by reanalysis meteorological fields and the most
up-to-date year-specific anthropogenic emission inventory reproduces the
overall pattern of observed tropospheric ozone trends, including the large
ozone increases over the tropics of 2.1–2.9 ppbv per decade and above
East Asia of 0.5–1.8 ppbv per decade and the weak tropospheric ozone trends above North America, Europe, and high latitudes in both
hemispheres, but trends are underestimated compared to observations.
GEOS-Chem estimates an increasing trend of 0.4 Tg yr−1 of the
tropospheric ozone burden in 1995–2017. We suggest that uncertainties in
the anthropogenic emission inventory in the early years of the simulation
(e.g., 1995–1999) over developing regions may contribute to GEOS-Chem's
underestimation of tropospheric ozone trends. GEOS-Chem sensitivity
simulations show that changes in global anthropogenic emission patterns,
including the equatorward redistribution of surface emissions and the rapid
increases in aircraft emissions, are the dominant factors contributing to
tropospheric ozone trends by 0.5 Tg yr−1. In particular, we highlight
the disproportionately large, but previously underappreciated, contribution
of aircraft emissions to tropospheric ozone trends by 0.3 Tg yr−1,
mainly due to aircraft emitting NOx in the mid-troposphere and upper troposphere
where ozone production efficiency is high. Decreases in lower-stratospheric
ozone and the stratosphere–troposphere flux in 1995–2017 contribute to an
ozone decrease at mid-latitudes and high latitudes. We estimate the change in
tropospheric ozone radiative impacts from 1995–1999 to 2013–2017 is
+18.5 mW m−2, with 43.5 mW m−2 contributed by anthropogenic
emission changes (20.5 mW m−2 alone by aircraft emissions),
highlighting that the equatorward redistribution of emissions to areas with
strong convection and the increase in aircraft emissions are effective for
increasing tropospheric ozone's greenhouse effect.