Concentrations and size distributions of fine aerosol particles measured at roof level in urban zone

Abstract

During the experimental Field Experiments to Constrain Models of Atmospheric Pollution and Transport of Emissions (ESCOMPTE) campaign in June–July 2001, concentrations and size distributions of fine particles (14–722 nm) were measured at roof level in downtown Marseille (France). Part of the campaign was dedicated to the study of aerosol behavior in relation to strong photochemical events (which were identified as “IOP” days) and their regional modeling. The analysis of the concentration variations and the evolution of average diurnal size distribution showed that an “IOP day” is not characterized by a specific concentration or its variation, nor by a specific evolution of the average size distribution. The morning traffic rush is detected at roof level by a net increase in particle concentration over the whole size range measured, indicating a production of ultrafine particles by the traffic but also the raising to roof level of particles of the accumulation mode. The increase is observed about 1 hour after the traffic peak at street level, which is characterized by strong increases in NOx and CO concentrations. The corresponding flux of particles at roof level has been estimated around 3 × 104 cm−2 s−1. A specific signature characterized by a strong and rapid burst of concentration (factor 2 to 4 in 15 min) of particles between 25 and 50 nm, independent of the traffic source, has been detected on six occasions during the campaign. These events occur systematically around noon, in cases of strong radiation, low relative humidity, and common wind direction. Despite the high‐diameter value of these particles, it is suggested that they could result from a specific “secondary aerosol process” event involving ozone, biogenic, and/or anthropogenic gas precursors like iodine and VOCs.