Three-dimensional modelling of photochemical air pollution in Switzerland and comparison with a multi-parcel Lagrangian model

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The photochemical smog situation in Switzerland on 29 July, 1993 was simulated using the 3–D grid model UAM (Urban Airshed Model) and the results were compared with those of a multi–parcel Lagrangian model. Sensitivity analyses concerning initial conditions, emissions, deposition, biogenic volatile organic compounds (VOC), and emission reductions were carried out. Results were compared with field measurements. Calculations showed that although ozone (O3) formation in Switzerland is mainly NOx (NO + NO2) limited, some areas seem also to be sensitive to VOC emissions. According to the calculations, reducing anthropogenic VOC emissions by 20–80% would lead to a decrease of 5–25% in O3 levels in a few regions, whereas the calculated decrease in O3 concentrations by NOx emission reductions was 6–38% and is expected to be effective over a larger area. Impacts of emission reductions on other species, such as peroxyacetyl nitrate (PAN), hydrogen peroxide (H2O2), nitric acid (HNO3) and formaldehyde (HCHO) were investigated. A correlation was found between the sensitivity of O3 to emission reductions, and NOz (NOy–NOx, where NOy = NOx + PAN + HNO3 + other nitrates) at the time of peak O3. VOC controls seem to be effective only at NOz concentrations higher than about 2 ppb, whereas NOx controls are effective also below this range. Exclusion of biogenic VOC decreased the calculated O3 concentrations by only 5% in certain regions. PAN and H2O2 concentrations, however, decreased significantly. Differences and similarities between the results of this study and those of a multi–parcel Lagrangian model are discussed, and criteria are outlined to decide on the choice for specific problems.