3E: Response of Ground-Level Ozone to Stratospheric Ozone Recovery

Our recently improved numerical models were used to examine how ground-level ozone will change in response to future changes in stratospheric ozone.  Ground-level ozone is a major pollutant that causes ~1 million premature human deaths per year, as well as substantial damage to agriculture and natural ecosystems.  It is made in the atmosphere as a byproduct of the oxidation of volatile organic compounds catalyzed by nitrogen oxides and energized by solar UV radiation.  Much of this incoming solar UV radiation is modulated by stratospheric ozone that also protects humans against harmful UV rays.  Thus, changes in “good” stratospheric ozone can affect the amounts of “bad” ground-level ozone.

Stratospheric ozone is currently lower than its natural amount, by 5-10% at mid latitudes, due to anthropogenic ozone-depleting substances (e.g. chlorofluorocarbons) produced and emitted over the past half century.  Because of international agreements (the 1987 Montreal Protocol and its subsequent adjustments and amendments) emissions of these compounds have been greatly reduced worldwide, and the stratospheric ozone layer is expected to recover back to pre-1980 levels by the second half of this century.  This means that less UV radiation will reach the surface, which in turn could affect the amount of ground level ozone.  An earlier modeling study suggested that ground-level ozone will generally increase due to this stratospheric recovery, but the low resolution of that model (4x5 deg.) prevented evaluation of the effect on urban scales where pollution is usually most severe.  We used the WRF-Chem at high resolution (12x12 km) over the US, to assess the changes in ground-level ozone resulting from stratospheric ozone recovery.  UV radiation and its effects on chemical reactions were modeled with the updated TUV model within WRF-Chem. The results indicate that the lower UV radiation will lead to cleaner (less ozone) urban atmospheres due to slower production, but more polluted (more ozone) regional atmosphere due to slower ozone destruction.  The net effects are not large, about 1 part per billion (ppb) in either direction compared to (e.g.) current background levels of 40-50 ppb, but are not negligible considering the large magnitude of the ground-level ozone problem.  (Hodzic and Madronich, in preparation, 2017).