Arctic tropospheric ozone: assessment of current knowledge and model performance

Abstract. As the third most important greenhouse gas (GHG) after CO₂ and methane, tropospheric ozone (O₃) is also an air pollutant causing damage to human health and ecosystems. This study brings together recent research on observations and modeling of tropospheric O₃ in the Arctic, a rapidly warming and sensitive environment. At different locations in the Arctic, the observed surface O₃ seasonal cycles are quite different. Coastal Arctic locations, for example, have a minimum in the springtime due to O₃ depletion events resulting from surface bromine chemistry. In contrast, other Arctic locations have a maximum in the spring. The 12 state-of-the-art models used in this study lack the surface halogen chemistry needed to simulate coastal Arctic surface O₃ depletion in the springtime, however, the multi-model median (MMM) has accurate seasonal cycles at non-coastal Arctic locations. There is a large amount of variability among models, which has been reported previously, and we show that there continues to be no convergence among models, nor improved accuracy in simulating tropospheric O₃ and its precursor species. The MMM underestimates Arctic surface O₃ by 5 % to 15 % depending on the location. The vertical distribution of tropospheric O₃ is studied from recent ozonesonde measurements and the models. The models are highly variable, simulating free-tropospheric O₃ within a range of +/- 50 % depending on the model and the altitude. The MMM performs best, within +/- 8 % at most locations and seasons. However, nearly all models overestimate O₃ near the tropopause (~300 hPa or ~8 km), likely due to ongoing issues with underestimating the altitude of the tropopause and excessive downward transport of stratospheric O₃ at high latitudes. For example, the MMM is biased high by about 20 % at Eureka. Observed and simulated O₃ precursors (CO, NO_x and reservoir PAN) are evaluated throughout the troposphere. Models underestimate wintertime CO everywhere, likely due to a combination of underestimating CO emissions and possibly overestimating OH. Throughout the vertical profile (compared to aircraft measurements), the MMM underestimates both CO and NO_x but overestimates PAN. Perhaps as a result of competing deficiencies, the MMM O₃ matches the observed O₃ reasonably well. Our findings suggest that despite model updates over the last decade , model results are as highly variable as ever, and have not increased in accuracy for representing Arctic tropospheric.