What happens to traffic pollution particles as they spread from a roadside into the surrounding city?

Details

This abstract describes a study of how aerosol processes influence particle number (PN) concentrations in three major European cities over 1 h, at neighborhood and city scales. The researchers used selected measurements of particle size distributions from previous campaigns in Helsinki, Oslo and Rotterdam.

They evaluated aerosol transformations with the aerosol dynamics model MAFOR, combined with a simplified treatment of roadside and urban atmospheric dispersion. They compared predicted particle number size distributions with measurements and performed sensitivity analyses of various model inputs. They also developed a simplified parameterization of aerosol processes based on the more complex process calculations, intended for easy implementation in both Gaussian and Eulerian urban dispersion models.

The processes considered were particle coagulation—particles joining together—the condensation and evaporation of two organic vapors, and dry deposition. Chemical transformations of gas-phase compounds were not included. Calculations started with roadside concentrations and particle size distributions. With this starting point, nucleation of gas-phase vapors from exhaust was treated as post-tailpipe emission, avoiding the need to include nucleation in the process analysis.

Dry deposition and coagulation were identified as the most important aerosol dynamic processes controlling particle evolution and removal. Uncertainty in measured deposition velocities produced an error ranging from −76 to +64 % in the contribution of dry deposition to PN losses. Nanoparticle removal through coagulation increased considerably when the calculations accounted for the fractal nature of soot aggregates and the combined effects of van der Waals and viscous interactions.

The researchers examined how condensation and evaporation of vehicle-emitted organic vapors affected particle numbers and particle size distributions. Under inefficient dispersion conditions, the model predicted that condensational growth contributed to changes in PN from roadside to neighborhood scale.

The simplified aerosol-process parameterization predicted the change in particle number concentrations between roadside and urban background within 10 % of the prediction from the fully size-resolved MAFOR model.