What controls tropospheric ozone?

TLDR

A global three‑dimensional chemistry‑transport model was run for 1979–1993 using ECMWF reanalyses and time‑varying emissions to quantify tropospheric O₃ photochemistry and compare source categories. STE contributes significantly to O₃ in photochemically quiescent regions, yet its impact is smaller than its column abundance implies; extratropical Northern Hemisphere O₃ is dominated by industrial/fossil‑fuel emissions, while natural sources prevail in the tropics and Southern Hemisphere, and future trends show that low‑latitude emissions, especially in southern and eastern Asia, will drive continued surface O₃ growth despite regulatory reductions.

Abstract

We have applied a global three‐dimensional chemistry‐transport model to quantify the photochemistry of tropospheric O 3 and compare the main source categories. We simulated a 15 year period (1979–1993) on the basis of the European Centre for Medium‐Range Weather Forecasts meteorological reanalyses and a time‐varying emission data set. We calculate that stratosphere‐troposphere exchange (STE) strongly contributes to O 3 in regions where the photochemistry is quiescent. Since such regions play a minor role in radiative and chemical processes, we argue that STE‐derived O 3 is much less important than is suggested by its column abundance. By distinguishing between photochemical pathways in the model we calculate that tropospheric O 3 in the extratropical Northern Hemisphere is strongly affected by industrial and fossil fuel‐related emissions. In the tropics and Southern Hemisphere, natural emissions still play a major role. Our model results indicate a less important role for man‐made biomass burning emissions than previous analyses. Further, the results show that tropospheric O 3 trends are strongly influenced by transports of pollution and by meteorological variability. Scenario calculations for the year 2025 suggest that man‐made emissions at low northern latitudes, in particular in southern and eastern Asia, will become a very strong tropospheric O 3 source in the next decades. This will influence O 3 levels on a hemispheric scale so that despite pollution regulations in Europe and North America, surface O 3 will continue to grow.