Photochemical ozone creation potentials
Goal, Scope and Background
Photochemical ozone creation potentials (POCPs) typically used in life cycle impact assessment (LCIA) to address the impact category ‘photo-oxidant formation’ only provide factors for particular volatile organic compounds and do not take into account background concentrations and meteorological conditions. However, the formation of ozone from volatile organic compounds (VOCs), carbon monoxide (CO) and nitrogen oxides (NOx) is highly dependent on the background pollutant concentrations and meteorological conditions. Some LCIA manuals therefore recommend working with potentials for high background concentrations of NOx (Derwent 1998), and potentials for low background concentrations of NOx (Andersson-Sköld 1992).
This study has introduced an improved set of POCPs independently of meteorological and emission conditions specific to a given period or location. Whereas current POCP values may be relevant to estimate the photo-oxidant formation over a certain (temporally and spatially well-defined) domain, this study has further introduced more relevant values with respect to potential impacts of ozone on human health and environment.
For the computation of POCP values on the scale of Western Europe, independently of meteorological and emission conditions specific to a given period or location, a Eulerian chemistry-transport numerical model (CHIMERE-continental) has been implemented over three summer seasons. POCPs have been evaluated for ten VOC species (including the whole VOC group), CO and NOx. The coherence of this new set of POCP values with previous studies has been checked. The spatial representa-tivity of POCP values over the simulation domain in Europe has also been addressed. The robustness of these POCP values to changes in the implemented chemical mechanism used in our model has been checked.
Results and Discussion
The POCPs computed in this study were generally lower than the POCPs calculated in previous studies. In the previous studies, but not here, the POCPs have been calculated with particular meteorological conditions (during anti-cyclonic, fair weather conditions) or emission levels (high polluted backgrounds) known to be optimal with respect to ozone formation. Despite the quantitative variations in the POCP values, we have found a good agreement in the relative ranking of the pollutant species between this study and previous studies. It was also shown that POCP values display significant spatial variability over Western Europe (the largest spatial differences were obtained for NOx where the sign of the POCP value even changes from region to region).
Finally, the temporally and spatially averaged values obtained here for the POCP index update previous values and represent an attempt to generate the most appropriate and accurate scale for European conditions independently of meteorological and emission conditions specific to a given period or location.
Recommendations and Outlook
These new PCOPs should be useful to LCIA-practitioners in further life cycle impact assessment. However, for the NOx species, we do not recommend the use of the POCP value for LCIA.
KeywordsCharacterization factors life cycle impact assessment (LCIA) photochemical atmospheric model photochemical ozone creation potentials (POCP) photo-oxidant formation
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