A photochemical modeling study of ozone and formaldehyde generation and budget in the Po basin

Abstract

In this work, a photochemical dispersion model, CAMx (Comprehensive Air quality Model with eXtensions) was used to simulate a high ozone episode observed in the Po basin during the 2003 FORMAT (Formaldehyde as a Tracer of Oxidation in the Troposphere) campaign. The study focuses on formaldehyde and ozone, and a budget analysis was set up for interpreting the importance of different processes, namely emission, chemistry, transport and deposition, for three different areas (urban, downwind, suburban) around the Milan metropolitan region. In addition, a sensitivity study was carried out based on 11 different VOC emission scenarios. The results of the budget study show that the strongest O 3 production rate (4 ppbv/hour) occurs in the downwind area of the city of Milan, and that accumulated O 3 is transported back to Milan city during nighttime. More than 80% of the HCHO concentration over the Milan metropolitan region is secondary, i.e., photochemically produced from other VOCs. The sensitivity study shows that the emissions of isoprene are not, on average, a controlling factor for the peak concentrations of O 3 and HCHO over the model domain because of very few oak trees in this region. Although the paraffinic (PAR) emissions are fairly large, a 20% reduction of PAR yields only 1.7% of HCHO reduction and 2.7% reduction of the O 3 peak. The largest reduction of O 3 levels can be obtained by reduced xylene (XYL) emissions. A 20% reduction of the total anthropogenic VOC emissions leads to 15.5% (20.3 ppbv) reduction of O 3 peak levels over the Milan metropolitan region.