The accuracy of the solution obtained with EBI and MEBI is

controlled by the size of the chemistry time step and

convergence tolerances for each species for the Euler

iterations. In addition, the MEBI solvers include

convergence tolerances for the Newton-Raphson routine. In

CMAQ, the time step has a default value of 2.5 minutes, and

convergence tolerances are set for each species individually

on the basis of the work done by Huang and Chang [90] and

subsequent testing and comparison with results obtained

using the SMVGEAR solver. These defaults were selected

to provide relatively accurate solutions without greatly

sacrificing computational efficiency.



The original U.S. EPA PM NAAQS, issued in 1971 as total

suspended particulates (TSP), were revised in 1987 to

protect against human health effects associated with

exposure to ambient PM having particle diameters less than

10 μm (PM10). In 1997, a re-evaluation of the issue using

newly available information provided key scientific bases

for promulgation of additional standards for fine particles

(i.e., those with diameters less than 2.5 μm, known as

PM2.5), while retaining current standards for PM10. The vast

majority of fine particulate mass across the Eastern U.S. and

Canada is secondary in origin and sulfate is a dominant

constituent involved with both the aerosol and aqueous


processes. In the following we describe CMAQ algorithms

implemented for these processes.

8.1 Aerosol Processes in the CMAQ CTM

In the CCTM, aerosol particles are divided into two groups:

fine particles and coarse particles. The fine particles

resulting from combustion and secondary production

processes are considered to have the same chemical

composition and consist of sulfates, nitrates, organic carbon,

and elemental carbon. The coarse particles consist of windblown

dust (crustal materials), marine particles, and some

anthropogenic contributions. Fine and course particles do

not interact in this version of the aerosol module. We

assume that the fine aerosol particles are in equilibrium with

the ambient gas and vapor phase species at the ambient

relative humidity and form an aqueous solution. Detailed

descriptions of the aerosol algorithms and test results are

available in Binkowski and Roselle [91] and, therefore, only

a few highlights of the aerosol processes are described here.

The present implementation of the aerosol module in the

CCTM is based on a modal aerosol modeling approach.

Polydisperse fine particles are characterized by a bimodal

lognormal distribution and coarse particles are described

with a unimodal lognormal distribution. In the modal

approach, particle distributions are characterized by the

lognormal moments. The k-th moment of the distribution is

defined as



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