Multidecadal increases in global tropospheric ozone derived from ozonesonde and surface site observations: can models reproduce ozone trends?
Authors: Christiansen Amy , Mickley Loretta J. , Liu Junhua , Oman Luke D. , Hu Lu 
Published: November 2022

Abstract. Despite decades of effort, the drivers of global
long-term trends in tropospheric ozone are not well understood, impacting
estimates of ozone radiative forcing and the global ozone budget. We analyze
tropospheric ozone trends since 1980 using ozonesondes and remote surface
measurements around the globe and investigate the ability of two atmospheric
chemical transport models, GEOS-Chem and MERRA2-GMI, to reproduce these
trends. Global tropospheric ozone trends measured at 25 ozonesonde sites
from 1990–2017 (nine sites since 1980s) show increasing trends averaging 1.8 ± 1.3 ppb per decade across sites in the free troposphere
(800–400 hPa). Relative trends in sondes are more pronounced closer to the
surface (3.5 % per decade above 700 hPa, 4.3 % per decade below
700 hPa on average), suggesting the importance of surface emissions
(anthropogenic, soil NOx, impacts on biogenic volatile organic compounds (VOCs) from land use
changes, etc.) in observed changes. While most surface sites (148 of 238) in
the United States and Europe exhibit decreases in high daytime ozone values
due to regulatory efforts, 73 % of global sites outside these regions (24
of 33 sites) show increases from 1990–2014 that average 1.4 ± 0.9 ppb per decade. In all regions, increasing ozone trends both at the surface
and aloft are at least partially attributable to increases in 5th
percentile ozone, which average 1.8 ± 1.3 ppb per decade and
reflect the global increase of baseline ozone in rural areas. Observed ozone
percentile distributions at the surface have shifted notably across the
globe: all regions show increases in low tails (i.e., below 25th
percentile), North America and Europe show decreases in high tails (above
75th percentile), and the Southern Hemisphere and Japan show increases
across the entire distribution. Three model simulations comprising different
emissions inventories, chemical schemes, and resolutions, sampled at the
same locations and times of observations, are not able to replicate
long-term ozone trends either at the surface or free troposphere, often
underestimating trends. We find that ∼75 % of the average
ozone trend from 800–400 hPa across the 25 ozonesonde sites is captured by
MERRA2-GMI, and <20 % is captured by GEOS-Chem. MERRA2-GMI
performs better than GEOS-Chem in the northern midlatitude free
troposphere, reproducing nearly half of increasing trends since 1990 and
capturing stratosphere–troposphere exchange (STE) determined via a
stratospheric ozone tracer. While all models tend to capture the direction
of shifts in the ozone distribution and typically capture changes in high
and low tails, they tend to underestimate the magnitude of the shift in
medians. However, each model shows an 8 %–12 % (or 23–32 Tg) increase in
total tropospheric ozone burden from 1980 to 2017. Sensitivity simulations
using GEOS-Chem and the stratospheric ozone tracer in MERRA2-GMI suggest
that in the northern midlatitudes and high latitudes, dynamics such as STE are most
important for reproducing ozone trends in models in the middle and upper
troposphere, while emissions are more important closer to the surface. Our
model evaluation for the last 4 decades reveals that the recent version of
the GEOS-Chem model underpredicts free tropospheric ozone across this long
time period, particularly in winter and spring over midlatitudes to high latitudes.
Such widespread model underestimation of tropospheric ozone highlights the
need for better understanding of the processes that transport ozone and
promote its production.