Formation of secondary aerosols over Europe: comparison of two gas-phase chemical mechanisms

The impact of two recent gas-phase chemical kinetic mechanisms (CB05 and RACM2) on the formation of secondary inorganic and organic aerosols is compared for simulations of PM2.5 over Europe between 15 July and 15 August 2001. The host chemistry transport model is Polair3D of the Polyphemus air-quality platform. Particulate matter is modeled with a sectional aerosol model (SIREAM), which is coupled to the thermodynamic model ISORROPIA for inorganic species and to a module (MAEC) that treats both hydrophobic and hydrophilic species for secondary organic aerosol (SOA). Modifications are made to the gas-phase chemical mechanisms to handle the formation of SOA. In order to isolate the effect of the original chemical mechanisms on PM formation, the addition of reactions and chemical species needed for SOA formation was harmonized to the extent possible between the two gas-phase chemical mechanisms. Model performance is satisfactory with both mechanisms for speciated PM2.5. The monthly-mean difference of the concentration of PM2.5 is less than 1 μg m−3 (6%) over the entire domain. Secondary chemical components of PM2.5 include sulfate, nitrate, ammonium and organic aerosols, and the chemical composition of PM2.5 is not significantly different between the two mechanisms. Monthly-mean concentrations of inorganic aerosol are higher with RACM2 than with CB05 (+16% for sulfate, +11% for nitrate, and +10% for ammonium), whereas the concentrations of organic aerosols are slightly higher with CB05 than with RACM2 (+22% for anthropogenic SOA and +1% for biogenic SOA). Differences in the inorganic and organic aerosols result primarily from differences in oxidant concentrations (OH, O3 and NO3). Nitrate formation tends to be HNO3-limited over land and differences in the concentrations of nitrate are due to differences in concentration of HNO3. Differences in aerosols formed from aromatic SVOC are due to different aromatic oxidation between CB05 and RACM2. The aromatic oxidation in CB05 leads to more cresol formation, which then leads to more SOA. Differences in the aromatic aerosols would be significantly reduced with the recent CB05-TU mechanism for toluene oxidation. Differences in the biogenic aerosols are due to different oxidant concentrations (monoterpenes) and different particulate organic mass concentrations affecting the gas-particle partitioning of SOA (isoprene). These results show that the formulation of a gas-phase chemical kinetic mechanism for ozone can have significant direct (e.g., cresol formation) and indirect (e.g., oxidant levels) effects on PM formation. Furthermore, the incorporation of SOA into an existing gas-phase chemical kinetic mechanism requires the addition of reactions and product species, which should be conducted carefully to preserve the original mechanism design and reflect current knowledge of SOA formation processes (e.g., NOx dependence of some SOA yields). The development of chemical kinetic mechanisms, which offer sufficient detail for both oxidant and SOA formation is recommended.