Atmos. Chem. Phys. Discuss., 11, C11059–C11064, 2011 www.atmos-chem-phys-discuss.net/11/C11059/2011/ © Author(s) 2011. This work is distributed under the Creative Commons Attribute 3.0 License. Atmospheric Chemistry and Physics Discussions Interactive comment on “Aerosols in the CALIOPE air quality modelling system: validation and analysis of PM levels, optical depths and chemical composition over Europe” by S. Basart et al. S. Basart et al. sara.basart@bsc.es Received and published: 31 October 2011 On behalf of all the co-authors I would like to thank the referee #1 for his/her construc- tive comments on the manuscript. Overall, the referee recognizes the interest of the manuscript in the particulate matter (PM) evaluation of the CALIOPE modelling sys- tem over Europe. Modelling data for 2004 has been compared with experimental data (observations) of PM10 and PM2.5 concentrations and chemical composition and with aerosol optical depth data (AOD). The manuscript has been revised after the referee’s comments in order to introduce the suggestions for improving the quality of the paper. A revision of the manuscript has already been sent to the Editorial Office. Please find C11059 hereafter an item-by-item response following to all the statements of the referee. Referee #1: The number of stations with data of chemical speciation available is high for sulphate (53 sites across Europe) and nitrate (27 sites). For other compounds, the number of stations providing experimental data is low. For the case of OC and EC, only data from two stations across Europe, have been used. I do not really think that any conclusion about the suitability of the model can been reached with comparison with only two stations across Europe. Something similar occurs with ammonium in Spain. No data of this compound from Spain have been used and this difficult the interpretation of the origin of the discrepancies between the model and experimental data of sulphate and nitrate (see details below). I know that there are several research groups that have been producing chemical speciation data of ammonium in many sites across Spain during the last years, including 2004. CALIOPE is a worthy system, and this is a very interesting article. In my opinion, ammonium should also be validated in Spain. Also try to find some OC and EC data from more sites in Europe. To include these data will sure improve the validation and identification of some questions of the system. It will also help to understating how key features of nitrate changes across Europe. Authors: As the reviewer indicates, the experimental data for some chemical com- pounds is low, particularly for carbonaceous matter. Unfortunately, a detailed inves- tigation of the organic aerosol cannot be done for the present study since there are only very few measurements available that cover a longer time, therefore OC and EC could only be evaluated at Birkenes (NO01) and Melpitz (DE44) in 2004 on an annual basis. Here, the modelling system underestimates the measurements by a factor of 4 (DE44: measured mean (OC+EC) 3.21 μg m-3, modelled 0.66 μg m-3; NO01: mea- sured mean (OC+EC) 0.97 μg m-3, modelled 0.23 μg m-3). This cannot be said more precisely because measurements were missing at the other sites. As indicate in P. 20590, l 14, Matthias (2008) also found that modelled OC+EC concentration were un- derestimated by a factor of 3 at Birkenes using CMAQ over Europe. This factor is lower C11060