Evaluation of isoprene emissions and chemistry for photochemical ozone modelling in the lower Fraser Valley, British Columbia

Abstract

We have developed a photochemical modelling system, MC2-CALGRID, and implemented it for the Lower Fraser Valley of British Columbia. The modelling system has been used to simulate an ozone episode of July 17-21, 1985, to evaluate the model performance and to provide a basis for future year scenarios. Although the model performance was judged adequate, there was a systematic underestimation of peak ozone which led us to examine in more detail the biogenic emissions and photochemistry of isoprene. The spatial distribution of the emissions inventory for isoprene in the photochemical model was compared with the spatial distribution of ambient concentrations of isoprene over several summers. With the exception of one site, which could have been influenced by vehicle emissions, the correlation appeared adequate, given the uncertainties associated with this type of comparison. Although this suggests that the relative emissions in the regions of the monitoring locations are reasonable, conclusions cannot be drawn concerning the absolute emissions. Three dimensional photochemical modelling of isoprene concentrations would eliminate some of the uncertainties in the current assessment. Additional measurements of isoprene in more rural and remote areas would provide even greater confidence in the isoprene emissions inventory. We have added explicit reactions for isoprene and its oxidation products, methacrolein and methyl vinyl ketone, to the CALGRID chemical mechanism. Using this mechanism in a trajectory model, we find good agreement with the daytime ambient concentration ratio of the isoprene oxidation products, [methacrolein]/[methyl vinyl ketone], measured during the Pacific 93 field study in the Lower Fraser Valley. The representation in the model of the daytime chemistry for isoprene oxidation is considered satisfactory. Further detailed simulations are required to assess the nighttime chemistry and the contribution of possible anthropogenic sources of methacrolein and methyl vinyl ketone.