CCTM uses a modal approach to simulate PM2.5 (particulate matter of diameter equal to or less than 2.5 microns), coarse particulate matter (particulate matter of diameter greater than 2.5 microns and equal to or less than 10 microns), and PM10 (particulate matter of size equal to or less than 10 microns). PM2.5 is further divided into the Aitken and accumulation modes. Coarse particulate matter is represented currently by fugitive dust and a generic anthropogenic species. PM10 is the sum of the PM2.5 and coarse particulate matter.
Particulate matter in the atmosphere can either be primary or secondary. Primary particulate matter is emitted directly into the atmosphere from natural or anthropogenic emissions. Secondary particulate matter is formed in the atmosphere either from precursors, as a result of chemical reactions, or from condensation or deposition onto primary particles that are already present in the atmosphere. CCTM can predict fine particulate speciated concentrations of sulfate, nitrate, ammonium, organics, and aerosol water, and it includes methods for simulating nucleation, dry deposition, and cloud processes. The 3rd generation CMAQ aerosol module (aero3) takes chemical species concentrations and reactivity rates from the chemistry solvers and primary particulate concentrations from the emissions processor to compute fine and coarse particulate concentrations.
Invoking the AERO3 module will influence the sulfate, ammonia, and nitric acid concentrations predicted by the gas phase chemistry module. The sulfate is partitioned between the vapor (sulfuric acid) and particle phases. The greater part of the sulfate is put into the aerosols with a very small residual amount remaining in the vapor phase. Nitric acid and ammonia are equilibrated with the aerosol species.
The aerosol module in CMAQ version 4.5 is called AERO4. The difference between AERO4 and AERO3 is that AERO4 includes consideration of sea salt aerosols. Emissions of sea salt from the open ocean are calculated as a function of wind speed and relative humidity, following the parameterizations of Gong (2003) and Zhang et al. (27), respectively. These emissions are speciated into sodium, chloride, and sulfate, and are distributed by size to the accumulation and coarse modes by fitting the emission parameterization to a bimodal distribution. Note that sea-salt emissions are calculated within the CMAQ model and do not require special pre-processing in SMOKE. Equilibrium between the accumulation mode (which now contains sodium and chloride) and the gas phase (which now contains hydrochloric acid) is calculated within the ISORROPIA thermodynamic module. As in previous CMAQ releases, mass transfer is not simulated between the coarse mode and the gas phase. These sea-salt processes can be activated by selecting the AERO4 module and deactivated by using AERO3. For easier comparison of CMAQ output with measurement, AERO4 now provides three new variables (PM25AT, PM25AC, and PM25CO) that are the fractional amounts of the Aitken, accumulation, and coarse modes, respectively, that are composed of particles less than 2.5 m in aerodynamic diameter (Jiang et al., 2006)
Unlike gases, the deposition velocity for particles must be calculated from the aerosol size distribution, as well as meteorological and land-use information. The aero_depv2 module calculates the size distribution from the mass and number concentration for each of the three modes and calculates the dry deposition velocity. The dry-deposition algorithm has been modified to include an impaction term in the coarse and accumulation modes. Finally, two diagnostic tools, for tracking the sulfate budget and sources of elemental and primary organic carbon have been added.
CCTM’s AERO module is also useful for evaluating visibility. CCTM integrates Mie scattering (a generalized particulate light-scattering mechanism that follows from the laws of electro-magnetism applied to particulate matter) over the entire range of particle sizes to obtain a single visibility value for each model grid cell at each time step. A more detailed description of the particulate matter simulation techniques used in CCTM can be found in Binkowski and Shankar (1995), and Byun and Schere (2006).