Impact of ozone layer depletion II:: changes in photodissociation rates and tropospheric composition

Seasonal trends in the ozone profile determined from the Nimbus-7 TOMS total ozone record and SAGE trends are used to estimate the trends in tropospheric photodissociation rates. The resultant photodissociation trends for tropospheric ozone, CH3CHO, HNO3, HNO4 and CH2O are presented. A three-dimensional chemical transport model is used to investigate the impact of changing photodissociation rates on tropospheric composition. First, the ‘linear trend’ method is applied. The trends in OH, tropospheric ozone, carbon monoxide, methane and NOx due to anthropogenic stratospheric ozone depletion are estimated from the trends in photodissociation rates assuming a mean stratospheric ozone seasonal cycle. Total ozone trends are reduced by their 2σ error estimates to exclude the influence that measurement errors and interannual natural variations in stratospheric ozone may have on trends in tropospheric composition. The resulting lower limits of trends in tropospheric composition due to anthropogenic ozone depletion over the time period 1978–1993 are presented at a 95% confidence level. Second, the TOMS total ozone record is used to calculate via parameterization the temporal dependence of tropospheric photodissociation rates within the chemical transport model and to identify the resulting trends in tropospheric composition. The trends in OH, O3, CO, NOx and CH4 determined from the TOMS total ozone record agree with the trends from the ‘linear trend’ method within the 2σ error limits.