0-D-Modelling of Carbonaceous Aerosols over Greater Paris Focusing on the Organic Particle Formation

To improve our understanding of aerosol formation and ageing in urban atmospheres, we have tested the ORISAM 0-D aerosol module (ORganic and Inorganic Spectral Aerosol Model). This module accounts for both types of primary carbonaceous particles (black carbon BC and primary organic carbon OC_p) and also simulates the formation of secondary inorganic and organic particles (sulfates, nitrates, ammonium, water and secondary organic carbon particles OC_sec) by attachment of gas precursors to pre-existing carbonaceous particles. Simulations were performed for surface aerosols over Greater Paris area during the ESQUIF summer 1998 and winter 2000 experiments. Results show that OC_sec formation is highly dependent on temperature and insolation with more intense secondary formation in summer than in winter. Moreover in Summer, when atmospheric conditions shift from warm and humid to hot and dry, the model indicates a decreasing formation of secondary organic aerosols OC_sec as shown by an increase of the OC_p/(OC_p+OC_sec) ratio from 42 to 56%. These results satisfactorily compare with the few experimental available data for BC/(OC_p+OC_sec) ratios increasing from 24 to 37% against modelled values in the range 21–32%. ORISAM module sensitivity to initial size distributions, background concentrations and emissions of gases and primary carbonaceous particles was documented too. One main result is that the formation of secondary organic particles with ORISAM is very sensitive to the concentrations of gaseous precursors. At the present stage of ORISAM development, OC_sec build up appears to be however less sensitive to particulate background concentrations.