This work evaluates particle size-composition distributions simulated by the Community Multiscale Air Quality (CMAQ) model using Micro-Orifice Uniform Deposit Impactor (MOUDI) measurements at 18 sites across North America. Size-resolved measurements of particulate SO42âˆ’, NO3âˆ’, NH4+, Na+, Clâˆ’, Mg2+, Ca2+ and K+ are compared to CMAQ model output for discrete sampling periods between 2002 and 2005. The observation sites were predominantly in remote areas (e.g. National Parks) in the United States and Canada, and measurements were typically made for a period of roughly one month. For SO42âˆ’ and NH4+, model performance was consistent across the US and Canadian sites, with the model slightly overestimating the peak particle diameter and underestimating the peak particle concentration compared to the observations. Na+ and Mg2+ size distributions were generally well represented at coastal sites, indicating reasonable simulation of emissions from sea spray. CMAQ is able to simulate the displacement of Clâˆ’ in aged sea spray aerosol, though the extent of Clâˆ’ depletion relative to Na+ is often underpredicted. The model performance for NO3âˆ’ exhibited much more site-to-site variability than that of SO42âˆ’ and NH4+, with the model ranging from an underestimation to overestimation of both the peak diameter and peak particle concentration across the sites. Computing PM2.5 from the modeled size distribution parameters rather than by summing the masses in the Aitken and accumulation modes resulted in differences in daily averages of up to 1 Î¼g mâˆ’3 (10%), while the difference in seasonal and annual model performance compared to observations from the IMPROVE, CSN and AQS networks was very small. Two updates to the CMAQ aerosol model â€“ changes to the assumed size and mode width of emitted particles and the implementation of gravitational settling â€“ resulted in small improvements in modeled size distributions.