THE EVALUATION OF SOME PHOTOCHEMICAL SMOG REACTION-MECHANISMS .3. DILUTION AND EMISSIONS EFFECTS

The effects of dilution and emissions on the performance of four widely-used photochemical smog reaction mechanisms are examined, using newly available experimental data from the CSIRO outdoor smog chambers. It is found that the Caltech, CB-XR and CB-IV mechanisms overpredicted the smog produced (SP) maximum associated with the dilution by 14-29% when the dilution rate was 22% h-1 and by 53-73% when the rate was 55% h-1. The CB-III mechanism underpredicted the maximum by 17% at the lower dilution rate (when the cumulative light flux was moderate), but agreed with the observations at the higher dilution rate (when the cumulative light flux was high). Five emissions experiments were conducted at moderate to very high HC/NO(x) ratios (8.3-51.0). When HC were injected during the light-limited regime the rate of formation of SP increased markedly; when NO was injected in this regime there was less effect (a slight decrease in slope). When HC were injected during the NO(x)-limited regime there was a gradual decline in SP concentration, but when NO was injected during the NO(x)-limited regime there was a dramatic increase in SP, leading to a second light-limited regime and a second NO(x)-limited regime. When both HC and NO(x) were injected during the light-limited regime, marked increases in slope occurred after each injection. The Caltech model gave good to excellent agreement with the data. In one experiment it overestimated the final SP by 8%. The CB-III model performed next best, sometimes in excellent agreement with the data, but at other times underpredicting the final SP by 14-18%. The CB-XR and CB-IV models underpredicted the final SP for most of the experiments by up to 38%.