Analysis of particles and carbon dioxide concentrations and fluxes in an urban area: Correlation with traffic rate and local micrometeorology

Number particle concentrations and fluxes were measured, synchronously with CO2 concentrations and fluxes, in an urban area. Measurements were taken with an eddy-correlation station located near the busiest road of the town of Lecce (Italy). Upward fluxes dominate completely over deposition and the area behaved as a source of aerosol and CO2 with an average particle flux FN = 71,100 #/cm2 s (median 64,000 #/cm2 s) and an average CO2 flux FC = 0.76 mg/m2 s (median 0.46 mg/m2 s). Pronounced diurnal and weekly cycles of FN and FC were observed, well correlated with measured traffic rate, TR, indicating that traffic is the main source of CO2 and particles in the area. Biogenic cycle on CO2 fluxes and concentrations was also distinguishable, decreasing the correlation between FN and FC. The relationships between particle and CO2 fluxes with TR, friction velocity and atmospheric stability were analysed. Measured FN/TR increased when friction velocity increased and, correspondingly, number concentration decreased. Particle fluxes showed a dependence on the atmospheric stability. These dependencies were used to derive an empirical parameterisation of aerosol concentration and fluxes, based on TR and micrometeorological parameters, that could be used to estimate traffic emissions, in real operating conditions, for applications in dispersion and climate modelling. Measured FC/TR showed a limited correlation with friction velocity and stability, because of the influence of the biogenic cycle, thereby micrometeorological parameters were not used in the parameterisation of FC.