## 4 Discussion  
This study can be viewed as the next step of the effort to develop and test 2D-VBS schemes for the description of primary and secondary OA. In previous work [20], different types of parameterizations with different assumptions were optimized for the polluted conditions of the Po Valley in Italy. The Po Valley is dominated by anthropogenic emissions (transportation, industry, agriculture). In the present work, the robustness of the same optimized parameterizations has been tested when applied to a dramatically different environment usually dominated by biogenic emissions. By necessity, the modeling tools and necessarily the approach used are quite similar in the two steps. However, an important difference is that there is no effort here to optimize again the various parameterizations as done in the original work. The same parameterizations are used and tested.  
The performance of the seven parameterizations was similar, leading to the conclusion that no parameterization was substantially superior. The simple schemes (1-bin and 2-bin) have the advantage that they can also be implemented in the 1D-VBS, with much lower computational cost. The other schemes cannot be easily simplified for the 1D-VBS without introduction of errors. Regarding the aging mechanisms, available findings suggest that there is significant later-generation production of SOA from anthropogenic precursors, while the corresponding later-generation production of SOA from biogenic precursors may be a lot smaller. These results, despite their uncertainty, suggest that one may need to treat the chemical aging of anthropogenic and biogenic compounds independently following previous modeling efforts. Furthermore, fragmentation of organic compounds during chemical aging is an important reaction pathway that should be included in some way in modeling efforts. The value of the use of the ensemble of the seven schemes in the present work is that they demonstrate which conclusions are robust and which are more uncertain (e.g., the contribution of biogenic SOA) and require additional testing with suitable field measurements.  
The major difference of the schemes taking into account bSOA aging (cases 2, 5 and 7) from the rest is that they assume that the chemical aging reactions of the semivolatile biogenic SOA components lead to significant additional SOA formation. The other four schemes assume that in these reactions the fragmentation and functionalization of the biogenic SOA components balance each other and there is no change in total biogenic SOA mass. The other difference is fragmentation of the anthropogenic SOA components in these schemes. They assume fragmentation, which in turn results in reduction of the predicted anthropogenic SOA concentrations.  
In the present study for the boreal forest, all seven models were able to reproduce the observed diurnal profile of the OA similarly to the study of the Po Valley. As expected, the biogenic SOA contribution (41-63% of the OA) for the boreal forest was significantly higher than in the Po Valley (17-45% of the OA) for all parameterizations. However, in both cases this was the most uncertain contribution of all components across the parameterizations. Additional studies to constrain this important OA component are needed.