Differences between CMAQ fine mode particle and PM2.5 concentrations and their impact on model performance evaluation in the lower Fraser valley

This paper calculates PM2.5 mass concentrations (PM2.5) from CMAQ output quantities, and focuses on analysing the uncertainty caused by using mass concentrations of particles in the two CMAQ fine modes (PMi+j) as approximations of PM2.5 in model evaluations. Conceptually, CMAQ fine mode and PM2.5 particles are different in terms of both concentrations and compositions. Quantitatively, the modelling results of the Pacific 2001 scenario at the Lower Fraser Valley shows that PMi+j and PM2.5 can be substantially or even extremely different, depending on relative humidity (RH) values. Under low RHs, PMi+j and PM2.5 generally correlate well and their quantitative differences are mostly moderate, although their maximum differences can still be substantial. Under high RHs, the correlation between PMi+j and PM2.5 deteriorates considerably and the quantitative differences increase dramatically. This is true whether the analysis is conducted on an all-component or a dry-component-only basis. Therefore, PMi+j could be used as an approximation of PM2.5 only on an average basis when RHs are low, but not under more general conditions. When compared with measured PM2.5 concentrations, the modelled concentrations of PM2.5 dry components (PM2.5,dry) performed much better than the modelled concentrations of fine mode dry components (PMi+j,dry) for the modelling domain and period, since the overall positive bias of the modelled PMi+j,dry was partially compensated by the lower PM2.5,dry values in comparison with PMi+j,dry. In addition, by using PM2.5,dry, the model demonstrated a better skill in simulating 24-h moving averages (MA) of measured concentrations in comparison with simulating hourly concentrations. This is different from the case of using PMi+j,dry, where the model could not clearly show a better skill in simulating 24-h MAs. Therefore, it is highly desirable to calculate PM2.5 values from CMAQ output and use them instead of PMi+j in model evaluations, especially under situations when RHs can be high. The method outlined in this paper can also be readily used for the calculations of PM concentrations at any cut-off diameter of interest, in addition to PM2.5 discussed here.