Using Fohn Conditions to Characterize Urban and Regional Sources of Particles

Characterizations of urban and regional sources of particulate matter (PM) were performed in the Milan area (North of Italy) during Fohn and stagnant (non-Fohn) conditions. The measurements were performed at two different places: in an urban area North of Milan (Bresso) and in a regional area at the EMEP-GAW station in Ispra (about 65 km NW from Milan) during the winter periods of the years 2002-2007. Particle size distributions and chemical bulk analysis of aerosols is combined with single particle mass spectrometry to obtain information about the chemical content of the particles and their mixing state. Fohn conditions are characterized by extremely clean background air from which background aerosol is scavenged, and consequently local sources determine the aerosol properties.
It was observed that during Fohn events the accumulation mode in the size range 50 nm < d < 300 nm practically disappears and that the size fraction below 50 nm dominates the total number distribution. The significant change in the number size distribution and the large decrease in PM10 mass during Fohn events are accompanied by a significant change in the chemical composition of the particles. Results from bulk chemical analysis showed high amounts of carbonaceous compounds and very low concentrations of ammonium nitrate (as indicator for secondary chemistry) during Fohn episodes, opposite during stagnant conditions secondary components are dominating the aerosol composition. Single particle measurements confirm the high contribution of carbonaceous compounds in locally emitted particles.
It was concluded that particles that originated in the urban area come mainly from combustion processes, especially direct traffic emissions, domestic heating and industrial activities, whereas the regionally emitted particles are different with much less traffic contribution.
We estimate that under prevailing (non-Fohn) winter conditions, about 50-65 % of the aerosol mass load in the city of Milan are caused by local emissions, and about 35-50 % come from regional background. This finding suggests that in order to improve air quality in a big city like Milan, it is important to combine local traffic restriction interventions with other long-term regional scale air-quality-measures.

Authors:

VAN DINGENEN Rita, GRUENING Carsten, PUTAUD Jean-Philippe, CAVALLI Fabrizia, CAVALLI Paolo, ERDMANN Nicole, DELL'ACQUA Alessandro, DOS SANTOS MARQUES Fernando, HJORTH Jens, RAES Frank, JENSEN Niels, MIRA-SALAMA D.

Publisher:

ELSEVIER SCIENCE INC

Format medium:

Printed