Inversely modeling homogeneous H<sub>2</sub>SO<sub>4</sub>-H<sub>2</sub>O nucleation rate in exhaust-related conditions

Abstract. Homogeneous sulfuric acid-water nucleation rate in conditions related to vehicle exhaust was measured and modeled. The measurements were performed by evaporating pure sulfuric acid and water liquids and by diluting and cooling the sample vapor with a sampling system mimicking the dilution process occurring in a real-world driving situation. The nucleation rate inside the measurement system was modeled inversely using CFD (computational fluid dynamics) and the aerosol dynamics code, CFD-TUTMAM (Tampere University of Technology Modal Aerosol Model for CFD). The nucleation exponents for the concentrations of sulfuric acid and water and for the saturation vapor pressure of sulfuric acid were found to be 1.9 ± 0.1, 0.50 ± 0.05, and 0.75 ± 0.05, respectively. With these exponents, nucleation rate can be expressed with a function of the concentrations of sulfuric acid and water and of temperature. Results imply that the nucleation process of volatile nanoparticles in real vehicle exhaust cannot be fully explained by sulfuric acid; instead, it is likely that other compounds, e.g., hydrocarbons, are involved as well. In general, the obtained nucleation rate function can be used to examine the nucleation mechanisms occurring in exhaust from different combustion sources (internal combustion engines, power plant boilers, etc.) or in the atmosphere and, furthermore, to improve air quality models.