Modeling organic aerosol concentrations and properties duringwinter 2014 in the northwestern Mediterranean region

Organic aerosols are measured at a remote site (Ersa) on Corsica Cape in the northwestern Mediterranean basin during the Chemistry-Aerosol Mediterranean Experiment (CharMEx) winter campaign of 2014, when high organic concentrations from anthropogenic origin are observed. This work aims at representing the observed organic aerosol concentrations and properties (oxidation state) using the air-quality model Polyphemus with a surrogate approach for secondary organic aerosol (SOA) formation. Because intermediate/semi-volatile organic compounds (I/S-VOC) are the main precursors of SOA at Ersa during the winter 2014, different parameterizations to represent the emission and ageing of I/S-VOC were implemented in the chemistry-transport model of the air-quality platform Polyphemus (different volatility distribution emissions, single-step oxidation vs multi-step oxidation within a Volatility Basis Set framework, inclusion of non-traditional volatile organic compounds NTVOC). Simulations using the different parameterizations are compared to each other and to the measurements (concentration and oxidation state). The high observed organic concentrations are well reproduced whatever the parameterizations. They are slightly under-estimated with most parameterizations, but they are slightly over-estimated when the ageing of NTVOC is taken into account. The volatility distribution at emissions influences more strongly the concentrations than the choice of the parameterization that may be used for ageing (single-step oxidation vs multi-step oxidation), stressing the importance of an accurate characterization of emissions. Assuming the volatility distribution of sectors other than residential heating to be the same as residential heating may lead to a strong under-estimation of organic concentrations. The observed organic oxidation and oxygenation states are strongly under-estimated in all simulations, even when a recently developed parameterization for modeling the ageing of I/S-VOC from residential heating is used. This suggests that uncertainties in the ageing of I/S-VOC emissions remain to be elucidated, with a potential role of organic nitrate from anthropogenic precursors and highly oxygenated organic molecules.