Modelling the global tropospheric ozone budget: exploring the variability in current models O. Wild Frontier Research Center for Global Change, JAMSTEC, Yokohama, Japan now at: Centre for Atmospheric Science, University of Cambridge, UK
Abstract. What are the largest uncertainties in modelling ozone in the troposphere,
and how do they affect the calculated ozone budget?
Published chemistry-transport model studies of tropospheric ozone differ
significantly in their conclusions regarding the importance of the key
processes controlling the ozone budget: influx from the stratosphere,
chemical processing and surface deposition. This study surveys ozone
budgets from previous studies and demonstrates that about two thirds of
the increase in ozone production seen between early assessments
and more recent model intercomparisons can be accounted for by
increased precursor emissions. Model studies using recent estimates of
emissions compare better with ozonesonde measurements than studies using
older data, and the tropospheric burden of ozone is closer to that
derived here from measurement climatologies, 335±10 Tg. However,
differences between individual model studies remain large and cannot be
explained by surface precursor emissions alone; cross-tropopause transport,
wet and dry deposition, humidity, and lightning also make large contributions.
The importance of these processes
is examined here using a chemistry-transport model to investigate the
sensitivity of the calculated ozone budget to different assumptions about
emissions, physical processes, meteorology and model resolution. The budget
is particularly sensitive to the magnitude and location of lightning
NOx emissions, which remain poorly constrained; the 3–8 TgN/yr
range in recent model studies may account for a 10% difference in
tropospheric ozone burden and a 1.4 year difference in CH4
lifetime. Differences in humidity and dry deposition account for some
of the variability in ozone abundance and loss seen in previous studies,
with smaller contributions from wet deposition and stratospheric influx.
At coarse model resolutions stratospheric influx is systematically
overestimated and dry deposition is underestimated; these differences are
5–8% at the 300–600 km grid-scales investigated here, similar in magnitude
to the changes induced by interannual variability in meteorology. However,
a large proportion of the variability between models remains unexplained,
suggesting that differences in chemical mechanisms and dynamical schemes
have a large impact on the calculated ozone budget, and these should be
the target of future model intercomparisons.
Citation: Wild, O.: Modelling the global tropospheric ozone budget: exploring the variability in current models, Atmos. Chem. Phys., 7, 2643-2660, doi:10.5194/acp-7-2643-2007, 2007.