Evaluation of the Community Multiscale Air Quality model (CMAQ) version 4.4 in urban areas in the Pacific Northwest has shown an under-prediction of aerosol number by a factor of 10 to 100, especially for particles smaller than 100 nm in diameter. While many factors could be responsible for this error, the nucleation of new particles and the emission size distributions in the released version of CMAQ attract attention for their scientific deficiencies.

Previously presented results revealed that the addition of ammonia to the nucleation process dramatically increased the number of predicted particles. Ternary H2SO4-H2O-NH3 nucleation produced too many particles by 2-4 orders of magnitude. However, most of these particles are below the detection limit of the measurement and additionally do not fit into the three-mode structure of CMAQ.

One solution that retains the goal of an efficient air quality model is to parameterize the nucleation mode process within the existing Aitken, accumulation, and coarse mode paradigm. Using nucleation mode dynamics, fresh particles are either grown by condensation to the Aitken mode or are lost by coagulation to the accumulation mode. Now number concentrations are often within an order of magnitude of observed, although there are cases of no improvement over the released version of CMAQ. The new algorithm incorporates ternary nucleation, the processes that occur between nucleation and a particle's appearance in observations and in the Aitken mode, and conservation of sulfur.

The emission size distributions in CMAQ do not represent aerosol size distributions at mesoscale grid resolutions. Recent measurements of fresh aerosol particles representative of emission into a 4-12 km grid are used to redefine the emitted size distributions. Typically much more of the mass of emitted aerosol now goes into the Aitken mode. To retain the same emitted mass, the number of emitted particles increases dramatically. Results from these updates are compared to observations in the Pacific Northwest.