Analysis of 1970-1995 Tropospheric Ozone Trends

This figure compares observed changes in ozone profiles (black dashed lines) from 1970 to 1995 with GEOS-CHEM model calculated changes in ozone profiles (red lines) at three ozonesonde stations at Northern Hemisphere midlatitudes. Horizontal blue lines indicate mean tropopause height. Modeled changes stem from estimated changes in anthropogenic emissions of NOx, CO and VOCs, reported increases in methane levels, and an assumed 30% decrease in the cross-tropopause flux of ozone from 1970 to 1995. In the lower troposphere, rising anthropogenic emissions are responsible for a 10-20% increase in modeled ozone in the lower troposphere. Increases in NOx emissions (particularly from Asia) are responsible for most of the anthropogenic increase in modeled ozone. Rising methane levels are responsible for about one quarter of the increase. Agreement with observed changes is better over Europe and Asia than over Canada, and better in spring and summer than in fall and winter. In the upper troposphere, declining cross-tropopause ozone fluxes largely offset the increase in modeled ozone due to rising anthropogenic emissions. Over Canada, this gives rise to modeled decreases in ozone. Observed declines over Canada are even more pronounced, suggesting the effect of a declining flux may have been even stronger than the model indicates. The combined influence of these two factors in the model appears to explain qualitatively the changes in observed northern midlatitude ozone profiles. More information regarding this research is included in the full report of Fusco and Logan [in press, JGR, 2002].