Long-term changes in lower tropospheric baseline ozone concentrations: Comparing chemistry-climate models and observations at northern mid-latitudes

Speaker: David Parrish, NOAA ESRL CSD

When: Wednesday, October 30, 2013, 3:30 p.m. Mountain Time
Location: Room 2A305, DSRC (NOAA Building), 325 Broadway, Boulder
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Chemistry-climate models (CCMs) synthesize our knowledge of atmospheric chemistry, and provide the means to calculate the composition of relevant species through the depth of the atmosphere over the entire globe, not only for present time, but they also can reproduce the past and predict the future. These calculations thereby provide the basis for determining the radiative forcing of tropospheric O3 and its contribution to climate change, as well as the background concentrations upon which O3 from pollutant sources is added in regions of degraded air quality. A long-standing, significant concern regarding such model results is that, compared to existing observations, the models overestimate preindustrial O3 concentrations, and thus may underestimate ozone's radiative forcing. However, questions have been raised regarding the reliability of the early O3 measurements, so that it can be plausibly argued that the model-derived O3 concentrations are more representative of the pre-industrial atmosphere than are the observations.

This talk will present a quantitative comparison between results from three CCMs and the most reliable, longest-term O3 measurements beginning in the 1950s and earlier. The model results do capture many features of the observations, but there are substantial quantitative disagreements. The overestimate of preindustrial O3 concentrations by the models is confirmed, current trends of ozone concentrations are poorly reproduced in many cases, and the rates at which those trends are slowing and in many cases reversing are greatly underestimated. These disagreements are profound enough that we are left with significant concerns that can be summarized in three questions. First, since our models cannot accurately produce past O3 changes, how much confidence can we place in their prediction of future changes? Second, what confidence can we put in the estimates of present-day radiative forcing of tropospheric O3 that are provided by CCMs? Finally, what is missing from our understanding of the tropospheric O3 budget, at least in the context of how our knowledge is incorporated into the present generation of CCMs?