94
B. Koo et al.
16.1 Introduction
Atmospheric particulate matter (PM) or aerosol has been recognized to have adverse
effects on human health [1] and visibility [6]. Atmospheric PM is a complex mixture
of inorganic ions, carbonaceous material, crustal elements, trace metals and water.
The inorganic components, which are mainly comprised of sulfate (SO 4
2− ), nitrate
(NO 3
− ), ammonium (NH 4
+ ), sodium (Na
+ ) and chloride (Cl
− ), are important contributors to PM 2.5 (PM with aerodynamic diameter less than or equal to 2.5 µm)
mass globally [11].
Atmospheric PM models typically assume thermodynamic equilibrium to determine partitioning of volatile inorganic components such as NO 3
− and NH 4
+ between
the gas and aerosol phases. A variety of thermodynamic equilibrium models have
been developed ([8] and references therein). Among them, ISORROPIA [4, 10] is
widely used by regional and global chemical transport models because of its numerical efficiency. ISORROPIA reduces computational costs by dividing the relative
humidity (RH) and composition space into subdomains that minimize the number
of equations to be solved. However, ISORROPIA computes activity coefficients
via an iterative procedure that adds to computational cost. Solving thermodynamic
equilibrium for inorganic ions remains one of the most computationally demanding
processes in large scale air quality modeling.
EQSAM4clim [8], originally developed for climate simulations, is based on a
single solute coefficient approach [7] that efficiently parameterizes single solution
hygroscopic growth accounting for aerosol water uptake from the deliquescence RH
up to supersaturation. EQSAM4clim extends the single solute coefficient approach
to treat water uptake for multi-component mixtures. The advantage of this approach
is that the gas-aerosol partitioning and the mixed solution water uptake can be solved
analytically eliminating the need for iterations, which brings potentially significant
speed-up. EQSAM4clim has been implemented and evaluated in a global chemistry
climate model [9].
The Comprehensive Air quality Model with extensions (CAMx; www.camx.com)
has provided alternative schemes for several model processes, but ISORROPIA has
been the only approach for determining thermodynamic equilibrium partitioning of
inorganic PM components. In this study, we implemented EQSAM4clim in CAMx as
an alternative to ISORROPIA, and evaluated model predictions of inorganic PM 2.5
components by EQSAM4clim and ISORROPIA over a continental US modeling
domain.
16.2 Modeling Platform
The latest version of CAMx (version 6.40) was applied to simulate two monthlong episodes (January and July 2011) from a US Environmental Protection Agency
(EPA) 2011 modeling platform [3]. The modeling grid covers the continental US with
B. Koo et al.
16.1 Introduction
Atmospheric particulate matter (PM) or aerosol has been recognized to have adverse
effects on human health [1] and visibility [6]. Atmospheric PM is a complex mixture
of inorganic ions, carbonaceous material, crustal elements, trace metals and water.
The inorganic components, which are mainly comprised of sulfate (SO 4
2− ), nitrate
(NO 3
− ), ammonium (NH 4
+ ), sodium (Na
+ ) and chloride (Cl
− ), are important contributors to PM 2.5 (PM with aerodynamic diameter less than or equal to 2.5 µm)
mass globally [11].
Atmospheric PM models typically assume thermodynamic equilibrium to determine partitioning of volatile inorganic components such as NO 3
− and NH 4
+ between
the gas and aerosol phases. A variety of thermodynamic equilibrium models have
been developed ([8] and references therein). Among them, ISORROPIA [4, 10] is
widely used by regional and global chemical transport models because of its numerical efficiency. ISORROPIA reduces computational costs by dividing the relative
humidity (RH) and composition space into subdomains that minimize the number
of equations to be solved. However, ISORROPIA computes activity coefficients
via an iterative procedure that adds to computational cost. Solving thermodynamic
equilibrium for inorganic ions remains one of the most computationally demanding
processes in large scale air quality modeling.
EQSAM4clim [8], originally developed for climate simulations, is based on a
single solute coefficient approach [7] that efficiently parameterizes single solution
hygroscopic growth accounting for aerosol water uptake from the deliquescence RH
up to supersaturation. EQSAM4clim extends the single solute coefficient approach
to treat water uptake for multi-component mixtures. The advantage of this approach
is that the gas-aerosol partitioning and the mixed solution water uptake can be solved
analytically eliminating the need for iterations, which brings potentially significant
speed-up. EQSAM4clim has been implemented and evaluated in a global chemistry
climate model [9].
The Comprehensive Air quality Model with extensions (CAMx; www.camx.com)
has provided alternative schemes for several model processes, but ISORROPIA has
been the only approach for determining thermodynamic equilibrium partitioning of
inorganic PM components. In this study, we implemented EQSAM4clim in CAMx as
an alternative to ISORROPIA, and evaluated model predictions of inorganic PM 2.5
components by EQSAM4clim and ISORROPIA over a continental US modeling
domain.
16.2 Modeling Platform
The latest version of CAMx (version 6.40) was applied to simulate two monthlong episodes (January and July 2011) from a US Environmental Protection Agency
(EPA) 2011 modeling platform [3]. The modeling grid covers the continental US with