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

摘要

This paper calculates PM_(2.5) mass concentrations (PM_(2.5)) from CMAQ output quantities, and focuses on analysing the uncertainty caused by using mass concentrations of particles in the two CMAQ fine modes (PM_(i+j)) as approximations of PM_(2.5) in model evaluations. Conceptually, CMAQ fine mode and PM_(2.5) 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 PM_(i+j) and PM_(2.5) can be substantially or even extremely different, depending on relative humidity (RH) values. Under low RHs, PM_(i+j) and PM_(2.5) generally correlate well and their quantitative differences are mostly moderate, although their maximum differences can still be substantial. Under high RHs, the correlation between PM_(i+j) and PM_(2.5) 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, PM_(i+j) could be used as an approximation of PM_(2.5) only on an average basis when RHs are low, but not under more general conditions. When compared with measured PM_(2.5) concentrations, the modelled concentrations of PM_(2.5) dry components (PM_(2.5,dry)) performed much better than the modelled concentrations of fine mode dry components (PM_(i+j,dry)) for the modelling domain and period, since the overall positive bias of the modelled PM_(i+j,dry) was partially compensated by the lower PM_(2.5,dry) values in comparison with PM_(i+j,dry). In addition, by using PM_(2.5,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 PM_(i+j,dry) where the model could not clearly show a better skill in simulating 24-h MAs. Therefore, it is highly desirable to calculate PM_(2.5) values from CMAQ output and use them instead of PM_(i+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 PM_(2.5) discussed here.