Aerosol size distribution modeling with the Community Multiscale Air Quality modeling system in the Pacific Northwest: 3. Size distribution of particles emitted into a mesoscale model

In order to improve the Community Multiscale Air Quality modeling system (CMAQ) performance for ultrafine particle concentrations in the Pacific Northwest, CMAQ v4.4 was modified so that particles are input to the model with an appropriate size distribution. CMAQ's default emission size distributions are based on outdated measurements which underrepresent ultrafine particles. At the same time, the size distribution must represent the results of all processes between the point of emission and the smallest resolvable spatial scale of the model. As a computationally efficient and simple solution, size distributions were compiled from published modern observations for traffic-dominated urban areas, power plants, and marine sources at the typical mesoscale air quality model spatial resolution of 4-15 km. CMAQ was modified so that all chemical species are input according to the emission size distribution of traffic-dominated urban areas because this source emits the majority of each aerosol species in the Pacific Northwest. For a summer 2001 case study based around field measurements, CMAQ v4.4 underpredicts the observable aerosol number concentrations by 1-2 orders of magnitude, while CMAQ with new emission size distributions underpredicts by similar to 1 order of magnitude. The modeled size distributions have improved properties in terms of more distinct Aitken and accumulation modes and a more prominent Aitken mode. Errors remain, especially in the accumulation mode.