Evaluation of the condensed carbon bond (CB-IV) mechanism against smog chamber data at low VOC and NOx concentrations

The condensed Carbon Bond mechanism (CB-IV) has been evaluated against smog chamber data obtained under conditions representative of urban air. A synthetic urban VOC mixture was studied at NOx concentrations ranging from 25 to 167 ppb and VOC/NOx ratios ranging from 2.7 to 10. The results indicate that the mechanism underpredicts the rates of O3 formation and NO oxidation by peroxy radicals by 15–30%. The maximum ozone concentration for VOC/NOx ratios of 8–10 was underpredicted by only about 10%. At lower VOC/NOx ratios the maximum amount of ozone formed is underpredicted by about 30%; however, an ozone concentration maximum is not always obtained. PAN is severely underpredicted (60–90%). Measured formaldehyde concentrations are in good agreement with the calculated concentrations. The individual hydrocarbons ethene and propene were also studied under similar conditions and the performance of the CB-IV mechanism for these compounds was tested.