Environmental Engineering Reference
In-Depth Information
2.28 Modeling Chemically Reactive Air Toxics
with CAMx
Chris Emery and Greg Yarwood
ENVIRON International Corporation, Novato, CA, USA
Abstract
Hazardous air pollutants (HAPs or “toxics”) are of concern because they
cause acute and/or chronic health impacts and may have sufficiently long
atmospheric lifetimes that allow them to be transported and transformed over great
distances. Traditional Gaussian plume and puff dispersion models are applicable
for simulating primary pollutants (e.g., diesel particulate matter) over short scales.
Such dispersion models are not suitable, however, for simulating chemically reactive
pollutants (e.g., formaldehyde, 1,3-butadiene, acrolein) and secondary pollutants
(e.g., acrolein, formaldehyde) over larger scales where variable chemical regimes
and transport/removal processes are important aspects of the long-term distri-
bution and ultimate fate of these compounds. On the other hand, photochemical
grid models can address complex chemical transformation pathways for toxics,
and simulate their three-dimensional dispersion and removal over wide areas
and for long periods. ENVIRON has developed a highly flexible Reactive
Tracer Chemical Mechanism Compiler (RTCMC) for the Comprehensive Air
quality Model with extensions (CAMx; ENVIRON, 2008). CAMx is a regional
photochemical/particulate grid model used for regulatory applications in the U.S.
and is applied widely throughout the world to address complex air quality issues.
The RTCMC module allows a user-defined toxic chemistry mechanism to run
in parallel with, and to draw oxidant information from, a standard gas-phase
photochemical simulation (i.e., Carbon Bond or SAPRC). In this paper we describe
the RTCMC for modeling air toxics on regional and annual scales.
Keywords
Hazardous air pollutants, toxics, air quality, grid modeling, dispersion,
photochemistry, CAMx
1. RTCMC Overview
The RTCMC allows users to define, in an external text-based (ASCII) format, a
set of chemical species and reactions (a mechanism) to be treated as “reactive
tracers” within a CAMx photochemical simulation. The core model simulates
photochemistry using standard oxidant mechanisms (e.g., CB05 or SAPRC99) and
the reactive tracers simulate emission, dispersion, chemical decay/production and
