13Traditional aerosol mechanisms underestimate the observed organic aerosol concentration, especially due to 14 the lack of information on secondary organic aerosol (SOA) formation and processing. In contrast VBS slightly underestimates the contribution from fossil-fuel combustion (HOA), indicating that 31 POA emissions related to road transport are either underestimated or associated to higher volatility classes. 32The VBS scheme under-predictes the SOA too, but to a lesser extent than CAMx-SOAP. SOA 33 underestimation can be related to corresponding underestimation of either aging processes or precursor 34emissions. This indicates that improvements in the emission inventories for semi-and intermediate-volatility 35 organic compounds are needed for further progress in this area. Finally, the comparison between modeled 36 and observed SOA sources points out the urgency to include processing of OA in particle water phase into 37 SOA formation mechanisms, to reconcile model results and observations. 38
Highlights
39• CAMx performance for OA are worse than for other PM components 40• SOAP scheme shows a better performance than VBS, due to an error compensation 41• VBS allows a better repartition of primary and secondary OA than SOAP scheme 42• POA volatility distribution and SVOC and IVOC emissions need improvement 43• Aqueous phase mechanism is necessary to reconcile OA observations and modeling 44 45 CTMs implementing standard SOA chemistry often over-predict fresh POA and underpredict SOA, 74 especially in summer (Bergstrom et al., 2012). 75
KeywordsThe volatility basis set (VBS) approach Donahue et al. (2006Donahue et al. ( , 2011 allows to take into account POA 76 volatility and multiple generation SOA production. VBS can be used in two configurations. The 1-dimension 77 approach (1D-VBS) describes OA evolution based on OA volatility (Donahue et al., 2006). The 2-dimension 78 approach (2D-VBS) describes OA evolution in the 2-D space defined by effective saturation concentration 79 C* (μg m -3 ) and the oxidation degree (Donahue et al., 2012). We use a hybrid VBS approach 1.5D-VBS 80 (Koo et al., 2014), where OA evolution follows specific path in such space, reducing computational cost. The 81 implementation of VBS approach in CTMs introduced a valuable improvement both in model performance 82 as well as in the knowledge of the key processes influencing the modeled results. For example Zhang et al. 83 (2013) showed that in a simulation with non-volatile POA and a simplified SOA formation mechanism POA 84 are largely overestimated, while SOA are underestimated. The application of the VBS scheme indicated that 85 also the volatility distribution of the aerosols is extremely important (Lane et al., 2008; Fountoukis et al., 86 2011). Namely, the distribution of OA emissions into the low volatility bins appears to be important for the 87 predicted POA because it has great impact on the initial partitioning between the aerosol and the gas phase 88 (Tsimpidi et al., 2010). Bergstrom et al. (2012) showed...