BIOGENIC EMISSIONS IN EUROPE .2. IMPLICATIONS FOR OZONE CONTROL STRATEGIES

As shown in a companion paper, uncertainties in the emissions of isoprene in Europe are significant, quite possibly of the order of 500%, and this uncertainty must be taken into account when assessing the effects of emission control measures in reducing ozone. The EMEP MSC-W model has been used to estimate the effects of 50% reductions in man-made NOx (nitrogen oxides = NO + NO2) and volatile organic compounds (VOCs) on rural ozone concentrations in different parts of Europe, under four different assumptions concerning biogenic emissions: (1) medium isoprene emissions, (2) high isoprene emissions, (3) zero isoprene emissions, and (4) zero soil NOx emissions. The model results suggest that uncertainties in isoprene emissions are not very important for the evaluation of long-term (6 monthly) ozone scenarios in Europe, especially with regard to the relative merit of NOx versus VOC control. To explain these results, we have analyzed the sources and sinks of important radical species under different isoprene emission levels. It is shown that isoprene emissions are not sufficient to affect production or loss rates of key-radical species significantly. The main effect of isoprene is to depress OH levels in isoprene-rich areas, thereby reducing somewhat the share of ozone production possible from anthropogenic VOC. However, in such-areas it is NOx emissions which are limiting as far as ozone production is concerned, so the policy advice from the model is quite robust against the isoprene uncertainties. The results obtained with respect to the effects of control measures on maximum ozone proved more sensitive to the assumed isoprene level. It is also shown that changes in isoprene emission level can alter the dates upon which ozone maxima occur, so predictions of the effects of man-made control cannot rely upon a consideration of the chemistry occurring at the time of the base case maxima but rather require modeling over the summer period.