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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 differsignificantly in their conclusions regarding the importance of the keyprocesses controlling the ozone budget: influx from the stratosphere,chemical processing and surface deposition. This study surveys ozonebudgets from previous studies and demonstrates that about two thirds ofthe increase in ozone production seen between early assessmentsand more recent model intercomparisons can be accounted for byincreased precursor emissions. Model studies using recent estimates ofemissions compare better with ozonesonde measurements than studies usingolder data, and the tropospheric burden of ozone is closer to thatderived here from measurement climatologies, 335±10 Tg. However,differences between individual model studies remain large and cannot beexplained by surface precursor emissions alone; cross-tropopause transport,wet and dry deposition, humidity, and lightning also make large contributions.The importance of these processesis examined here using a chemistry-transport model to investigate thesensitivity of the calculated ozone budget to different assumptions aboutemissions, physical processes, meteorology and model resolution. The budgetis particularly sensitive to the magnitude and location of lightningNO<sub>x</sub> emissions, which remain poorly constrained; the 3–8 TgN/yrrange in recent model studies may account for a 10% difference intropospheric ozone burden and a 1.4 year difference in CH<sub>4</sub>lifetime. Differences in humidity and dry deposition account for someof 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 systematicallyoverestimated and dry deposition is underestimated; these differences are5–8% at the 300–600 km grid-scales investigated here, similar in magnitudeto 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 schemeshave a large impact on the calculated ozone budget, and these should bethe target of future model intercomparisons

Modelling the global tropospheric ozone budget: exploring the variability in current models