2008 PM2.5 Plan 

This appendix has been provided by the California Air Resources Board (ARB). 

For more information on modeling, please see Chapter 3 and Appendix F. 

Appendix G 

 

Regional Air Quality Modeling 
2008 PM2.5 Plan 

This page intentionally blank.
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-1 

Appendix G: Regional Air Quality Modeling

Regional Air Quality Modeling and PM2.5 Attainment Demonstration 

Introduction 

The San Joaquin Valley (SJV) is currently designated as nonattainment for 

PM2.5. Addressing this issue properly requires rigorous long-term chemical transport 

modeling to support the development of a plan to attain the National Ambient Air Quality 

Standard (NAAQS) for PM2.5. These modeling exercises serve to determine future year 

attainment status for the SJV given projected emissions scenarios and also the most 

effective emissions reduction pathways to control PM concentrations for different 

seasons and regions of the domain. 

A complex interplay between meteorology and chemistry shapes the aerosol size 

and composition distribution. Atmospheric particulate matter is made up of both directly 

emitted particles, like road dust or soot, as well as secondary pollutants formed via 

chemical reactions with gas-phase compounds. Both primary particles and secondary 

particles (e.g., ammonium nitrate (NH4NO3) and ammonium sulfate ((NH4)2SO4)) make 

up significant portions of the total PM2.5 levels in the SJV. Strong seasonal variations 

and many different sources, pathways, and components make PM2.5 in the region 

impossible to characterize simply. Effectively understanding the PM2.5 problem in the 

SJV therefore requires comprehensive modeling with well-characterized emissions, 

meteorology, and chemistry in addition to a thorough knowledge of the observed 

concentrations of bulk PM2.5 and its individual PM components. Model predictions 

combined with observed particulate concentrations provide the foundation for the EPArecommended attainment demonstration for PM2.5. 

While model predictions represent a significant contribution to the attainment 

demonstration, it is recommended that models be used in a “relative” sense in 

conjunction with observations. Given that models may perform differently for each PM 

component, it may be generally assumed that models will be more successful at 

predicting concentration changes than absolute concentrations. To dampen the effects 

of varying degrees of performance amongst modeled species and the potential model 

bias in predicting absolute species concentrations, the EPA recommends that the 

models provide “relative response factors” (RRF) to quantify the effects of emissions 

changes between base and future years. These relative response factors in conjunction 

with speciated PM2.5 measurements form the basis of the Speciated Modeled 

Attainment Test (SMAT). 

In the following sections, the regional air quality modeling performed for use in 

the Speciated Modeled Attainment Test is outlined (EPA, 2007). Following a 

description of the modeling methodology and inputs, the steps of the EPArecommended Speciated Modeled Attainment Test are described in detail. Finally the 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-2 

modeling results are presented alongside observational data in order to determine the 

future year attainment status for the San Joaquin Valley. 

Regional Air Quality Methodology 

As stipulated in the EPA Modeling Guidance, a grid-based photochemical model 

is necessary to perform the modeled attainment test for PM2.5 (EPA, 2007). Such 

models offer the best available representation of important atmospheric processes and 

are an essential tool in analyzing the impacts of proposed emissions controls on 

pollutant concentrations. The EPA recommends guidelines for choosing a model for 

use in the attainment test. For example, the model source code should be free or low 

cost, modeling elements should have undergone rigorous scientific peer-review, and it 

should have been shown to perform well in the past for similar applications. 

The Community Multiscale Air Quality Modeling System (CMAQ) has been 

selected for use in the PM2.5 modeled attainment demonstration for the San Joaquin 

Valley Air Pollution Control District. CMAQ is a state-of-the-science “one-atmosphere” 

system that treats major atmospheric and land processes (e.g., advection, diffusion, gas 

phase chemistry, gas-particle mass transfer, nucleation, coagulation, wet and dry 

deposition, aqueous phase chemistry, etc.) and a range of species (e.g., anthropogenic 

and biogenic, primary and secondary, gaseous and particulate) in a comprehensive 

framework (EPA, 1999; CMAS, 2007). 

CMAQ has been extensively peer-reviewed, is well-documented, and is regularly 

updated to reflect the latest changes in scientific understanding. CMAQ has been 

applied successfully in a range of environments and on many spatial and temporal 

scales. Given that CMAQ has also been applied successfully to episodic modeling in 

Central California, the CMAQ modeling system version 4.6 with California-specific 

updates, as described in Liang and Kaduwela (2005), was selected for use in support of 

the PM2.5 modeled attainment demonstration. 

Chemical Mechanism 

There are a number of gas-phase chemical mechanisms readily available for 

application in CMAQ (e.g., CB-IV, CB-V, SAPRC-99). The user has the additional 

option of whether to couple the chosen gas phase mechanism with aerosol and/or 

aqueous phase chemical processes. In order to simulate the complex mixture of PM2.5

species in the San Joaquin Valley, SAPRC99 coupled with CMAQ aerosol code version 

4 and aqueous phase chemistry has been chosen for this application. SAPRC-99, a 

complete update of SAPRC-90, is a detailed mechanism describing the gas-phase 

reactions of volatile organic compounds (VOCs) and oxides of nitrogen (NOx) (Carter, 

2000). AE4-AQ, the fourth-generation CMAQ aerosol code with aqueous phase 

chemistry, when coupled with a gas phase mechanism, represents such phenomena as 

gas–aerosol/aqueous phase mass transfer, chemical transformation of particulate 

species and their gas phase precursors, and the evolution of the aerosol size 

distribution. 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-3 

Model Inputs and Setup: 

Domain Structure:

Two modeling domains were used for this work. The first modeling domain 

(“CCAQS”) covers the Central Valley and its surroundings with 63x63 lateral 12x12 km2

grid cells for each vertical layer. The CCAQS domain extends from the Pacific Ocean in 

the west to the Mojave Desert and western Nevada in the east and runs from the 

northern Sacramento Valley to the Tehachapi Mountains in the south. The second 

domain (“SJV”) is nested within the CCAQS domain and covers the San Joaquin Valley 

with 80x89 lateral 4x4 km2 grid cells for each vertical layer (Figure 1). The vertical 

structure for both domains is composed of 15 layers of varying thickness up to the top of 

the meteorological domain (100 mb). The finest resolution belongs to those layers 

closest to the surface and is determined largely by the vertical structure of the 

meteorological inputs. The surface layer is approximately 30 meters thick. 

Initial and Boundary Conditions:

Boundary conditions for the CCAQS domain were taken from the global chemical 

transport Model for Ozone And Related chemical Tracers (MOZART). Model boundary 

conditions for major species were extracted for the Central California modeling domain 

from MOZART results representative of the year 2000. In addition to VOCs and 

inorganic gases, boundary conditions were extracted for ammonium, nitrate, sulfate, 

and organic and elemental carbon. Initial conditions were estimated as an average of 

the extracted boundary conditions for each species. The CCAQS domain provided the 

initial and boundary conditions for the SJV domain. All species reported in the threedimensional output fields for the simulation over the CCAQS domain were included in 

the initial and boundary conditions for the SJV domain. While boundary conditions for 

the CCAQS domain were held constant for each month, boundary conditions for the 

SJV domain varied for each hour. The impact of initial conditions was minimized for 

each domain by simulating 8-day spin up periods prior to the simulation of each month 

of the year. 

Emissions:

A spatially, temporally, and chemically resolved emissions inventory of combined 

area, mobile, and point sources was generated using the California Emissions 

Forecasting System (CEFS) version 1.06 with offline adjustments. The inventory 

includes emissions estimates for gaseous and particulate species of anthropogenic and 

biogenic origin. Gridded hourly emissions were developed for the CMAQ modeling 

domain for the years 2000, 2005, and 2014 (baseline). Quality assurance checks of 

domain emissions totals and spatial distribution were performed at various steps in 

emissions processing in order to ensure that the CMAQ emissions input files were 

sound. 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-4 

In order to better estimate future air quality, a second 2014 emissions sensitivity 

scenario was generated incorporating expected reductions from future state and local 

controls beyond the baseline. These were applied uniformly over the domain on a 

percentage basis. The percentage difference between baseline and “controlled” 

emissions was taken from the information in Table 1. 

Meteorological Inputs:

The meteorological input fields to CMAQ were generated with the MeteorologyChemistry Interface Processor (MCIP) version 3.0. MCIP serves as a link between 

meteorological models like MM5 or WRF with CMAQ and generates model-ready 

meteorological inputs like the wind and temperature fields necessary to drive the 

transport and chemistry calculations in CMAQ. Inputs to MCIP were generated using 

the PSU/NCAR Mesoscale Model (MM5) (version 3.6) (Grell et al, 1995). Planetary 

boundary layer and radiation characteristics were calculated in MCIP, and the Models-3 

dry deposition routine (Pleim - with chlorine and mercury species) was chosen to 

represent dry deposition. 

MM5 Simulation

MM5 is a limited area, terrain-following sigma coordinate model on Lambert 

Conformal projection that was developed by Penn State and NCAR as a community 

mesoscale model. The model is based on non-hydrostatic, fully compressible motions 

that allow users to study the atmospheric motions at small scales by explicitly treating 

the effects of convective motions on atmospheric circulations. The MM5 model has 

been improved over more than two decades by contributions from a broad scientific 

community. 

MM5 was set up for a 14 month simulation (December 1999 – January 2001) 

with three nested grids using (70 x 70), (133 x 133), and (94 x 85) grid points in (x, y) or 

(south-north, west-east) direction with 36, 12, and 4 km horizontal resolution in each 

grid, respectively (Figure 2). The vertical structure of the domain was defined with 30 

layers extending to 100 mb at the top of the domain. The first two coarse grids defined 

the atmospheric initial and boundary conditions for the area at large scale, while the 

innermost grid with 4 km horizontal resolution resolved the fine details of atmospheric 

motions within the SJV modeling domain. MM5 has several options to calculate the 

components of internal and external forces acting on a volume of air, such as those for 

radiation, convection, cloud microphysics, soil fluxes, and boundary layer physics. 

While many sensitivity studies were conducted using various model options to find the 

best agreement with observations, an effort was also made to use the same model 

options from one simulation to the next. The Grell (1995) cumulus parameterization 

scheme for coarse grids was used along with the Blackadar boundary layer scheme for 

calculation of fluxes (Blackadar, 1979, Grell, 1995). The Dudhia simple ice scheme was 

used for the treatment of excess moisture (Dudhia, 1989) and the Dudhia cloud 

radiation scheme (Dudhia, 1993) was used for radiational heating and cooling of the 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-5 

atmosphere. The Blackadar multi-layer, force-restore method soil model (Blackadar, 

1976) was used for soil physics in all grids. 

Initial and boundary conditions were prepared using the analyses of observations 

prepared by the National Center for Environmental Prediction (NCEP) archived at 

NCAR. The 14-month period (December 1999 through January 2001) was first 

simulated using initial and boundary conditions (IC/BCs) with the analysis nudging 

option on the two coarse grids. Then, IC/BCs were prepared from the 12 km grid output 

for the initialization of the 4 km grid. The three-dimensional (3-D) wind and temperature 

values simulated by the model were compared against surface observations obtained 

from the four SJV stations (Fresno, Bakersfield, Arvin, and Parlier) to study the temporal 

and 3-D spatial structure of atmospheric motions as well as to evaluate the model 

performance within SJV. Figure 2 shows the MM5 domains employed to generate 

meteorological fields for the CMAQ simulation. 

Model Years: 

CMAQ was run for the year 2000 to provide the basis for the model performance 

evaluation. It was during 2000 that the California Regional PM10/PM2.5 Air Quality Study 

(CRPAQS) took place. The study resulted in a wealth of data with which to evaluate 

model performance. As it is necessary to execute simulations for a model reference 

year and a future year to perform the recommended modeled attainment demonstration, 

2005 and 2014 were also simulated. Simulations for all years were driven by the 

meteorological inputs for 2000, while emissions varied from year to year. 

Model Performance Evaluation: 

To assure that the modeling system (emissions, meteorology, and air quality) is a 

satisfactory representation of the period modeled, the estimated emissions, 

meteorology, and air quality of the base-case simulation need to be compared with 

observations. Satisfactory performance of the model in simulating observed conditions 

and responses is a prerequisite for use of the modeling system to evaluate control 

strategies. 

CRPAQS was an extensive and intensive measurement campaign designed to 

characterize the important chemical and physical processes involved in the formation 

and evolution of particulate matter in Central California (Chow et al. 2006). The 

CRPAQS measurement campaign extended from December 1999 through January 

2001 and provided a wealth of data from diverse areas for model evaluation. 

There are hourly and daily concentration data for a range of gaseous and 

particulate species at numerous sites available for comparison with CMAQ modeled 

concentrations. These data will be used to assess model performance temporally and 

spatially, with a focus on monthly average performance, as recommended for long-term 

model simulations. Appropriate goals for model performance will be based on the EPA 

Modeling Guidance and recommendations in the scientific literature on appropriate 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-6 

measures of model performance for long term PM simulations (EPA, 2007; Boylan and 

Russell, 2006). A model performance analysis is forthcoming which will be provided at 

a later date as an appendix to this work. 

Speciated Modeled Attainment Test 

Regional air quality modeling only represents a portion of the attainment test. In 

order to perform the EPA-recommended Speciated Modeled Attainment Test, or SMAT, 

the relative response between the modeled reference and future years must be 

considered in conjunction with observations. This approach minimizes the uncertainties 

in predicting future year attainment that result from potential model bias in predicting 

absolute species concentrations. In the following sections, the Speciated Modeled 

Attainment Test, as described in the EPA “Guidance on the Use of Models and Other 

Analyses for Demonstrating Attainment of Air Quality Goals for Ozone, PM2.5, and 

Regional Haze”, is outlined in general terms. Here the recommended procedure for 

combining model results (relative response factors (RRFs)) with speciated and bulk 

observations (STN and FRM measurements) in order to determine future year 

attainment status is explained. This is followed by a description of the application of the 

SMAT procedure to the San Joaquin Valley to determine future year PM2.5 attainment 

status for the region. 

General Procedure of SMAT 

Step 1. Calculate the observed quarterly mean bulk PM2.5 concentration and 

composition for each measurement site. The quarterly mean species concentrations 

can be calculated by multiplying the observed percentage contribution of each species 

against the quarterly mean bulk PM2.5 design value. This design value is calculated 

from the bulk FRM concentrations averaged over a number of years (generally three but 

can be a weighted average spanning 5 years), one of which should be the modeled 

reference year. The procedure to speciate this bulk design value is described in the 

“SANDWICH” section below. 

Step 2. Calculate the RRFs for each quarter and measurement site. Relative 

response factors are calculated using model results. For species i, site j, and quarter k, 

the RRF is given by the following equation: 

 RRFijk = ([Ci, quarter k of future year]/[Ci, quarter k of the base year])j

Ci represents the quarterly modeled concentration (often averaged over a 

number of model cells near the location of the measurement site) for the reference year 

and the future year attainment target. The number of model cells recommended for use 

in the average depends on the size of the grid cell and is justified by the long PM2.5

sampling times, the representative spatial scale of the monitors, and the desire to offset 

any potential errors stemming from the geometry of the superimposed grid system. 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-7 

Step 3. Multiply the quarterly, site-specific model-based RRFs from step 2 and 

speciated observations from step 1 to estimate future quarterly species concentrations. 

Step 4. Sum the future quarterly species concentration estimates from step 3 to 

estimate a future quarterly PM2.5 estimate at each monitoring site and then average 

these for a projected future year annual PM2.5 concentration for each monitoring site. 

Step 5. Compare the future year annual average PM2.5 concentrations from step 

4 to the annual PM2.5 standard of 15.0 μg/m3

. If all sites have projected PM2.5

concentrations below the standard, the attainment test is passed. 

These are the basic steps of SMAT. The following sections examine the 

attainment test in more detail, starting with the calculation of the base year design 

values against which the model-based RRFs are multiplied. 

Speciated Base Year Design Values 

Federal Reference Method 

Federal Reference Method (FRM) PM2.5 mass measurements provide the basis 

for nonattainment designations. For this reason it is recommended that the FRM data 

also be used to project future air quality and progress towards attainment of the healthbased National Ambient Air Quality Standard (NAAQS) for PM2.5. However, given the 

complex physicochemical nature of PM2.5, it is necessary to consider individual species 

as well. While the FRM measurements give the mass of the bulk sample, a method for 

apportioning this bulk mass to individual PM2.5 components is a first step towards 

determining the best targets for emissions controls in order to reach NAAQS levels in a 

timely manner. 

The FRM measurement protocol finds its roots in the past epidemiological 

studies of health effects associated with PM2.5 exposure. It is upon these studies that 

the NAAQS is based. The protocol is sufficiently detailed so that results might be easily 

reproducible and involves the measurement of filter mass before and after sampling 

after equilibrating at narrowly defined conditions. Filters are equilibrated for more than 

24 hours at a standard relative humidity between 30 and 40% and temperature between 

20 and 23 oC. Due to the sampler construction and a lengthy filter equilibration period, 

however, FRM measurements are subjected to a number of known positive and 

negative artifacts. FRM measurements do not necessarily capture the PM2.5

concentrations in the atmosphere and can differ substantially from what is measured by 

speciation monitors (e.g., STN). Nitrate and semi-volatile organics can be lost from the 

filter during the equilibration process, and particle bound water associated with 

hygroscopic species like sulfate provides a positive artifact. These differences present 

an area for careful consideration when one attempts to utilize speciated measurements 

to apportion the bulk FRM mass to individual species. 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-8 

Given that (1) attainment status is currently dependent upon FRM measurements 

and (2) concentrations of individual PM2.5 species need to be considered in order to 

understand the nature of and efficient ways to ameliorate the PM2.5 problem in a given 

region, a method has been developed to speciate bulk FRM PM2.5 mass with known 

FRM limitations in mind. This method is referred to as the measured Sulfate, Adjusted 

Nitrate, Derived Water, Inferred Carbonaceous material balance approach or 

“SANDWICH”. SANDWICH is based on speciated measurements from other (often colocated) samplers, such as those from the Speciated Trends Network (STN), and the 

known sampling environment of the FRM. The approach serves to provide mass 

closure, reconciliation between speciated and bulk mass concentration measurements, 

and the basis for a connection between observations, modeled PM2.5 concentrations, 

and the air quality standard. 

SANDWICH 

Given that the FRM is the basis for attainment status, the reconstructed PM2.5

mass should be based on the composition of the mass measured by the FRM. 

SANDWICH was developed in order to attribute portions of the bulk FRM mass to 

different components using STN measurements while also considering the limitations of 

the FRM monitor. Nitrates are often lost from the FRM filter along with semivolatile 

organics, and water bound to the inorganics also comprises a portion of the FRM mass. 

The main steps in estimating the PM2.5 composition are as follows: 

(1) calculate the nitrate retained on the FRM filter using hourly relative humidity and 

temperature alongside STN nitrate measurements, 

(2) calculate quarterly averages for retained nitrate, sulfate, elemental carbon, 

ammonium (or sulfate degree of neutralization, [NH4

+

]/[SO4

2-], if not using ammonium 

measurements directly), 

(3) calculate particle bound water using the concentrations of ammonium, sulfate, and 

nitrate, using an equilibrium model like the Aerosol Inorganic Model or a polynomial 

equation derived from model output, and 

(4) calculate organic carbon mass (OCMmb) by difference, subtracting all inorganic 

species (including blank mass) from the PM2.5 mass. 

Total PM2.5 mass is given by 

 PM2.5 FRM = [SO4

2-] + [NO3-

FRM] + [NH4

+

FRM] + [EC] + [Other ] + [OCMmb] + 

 [H2O] + [blank mass = 0.5 μg/m3

] 

where all concentrations have units of μg/m3

 and 

[SO4

2-] = measured sulfate 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-9 

[NO3

-

FRM] = nitrate retained on the FRM filter 

[NH4

+

FRM] = ammonium associated with the nitrate and sulfate on the FRM filter 

[EC] = measured EC 

Other = other inorganic mass; e.g., crustal material or sea salt 

[OCMmb] = organic carbon mass calculated by difference by adding all inorganic species 

and subtracting from the FRM PM2.5 mass. Organic carbon measurements may also be 

used if it seems that the OC by mass balance is clearly under or overestimated. 

[H2O] = water bound to the hygroscopic species; here, calculated using a polynomial 

equation dependent on the concentrations of sulfate, nitrate, and ammonium or using 

an inorganic aerosol equilibrium model. 

Blank mass represents the mass passively collected on the filter and is assumed to be 

held constant at a value of 0.5 μg/m3

. 

SANDWICH may be applied directly in areas where FRM and speciation 

monitors are collocated. When that is not the case, the following calculations and 

adjustments may still be applied using spatially interpolated speciation data or some 

other means to estimate the speciation. The speciation concentrations referred to in the 

following paragraphs can be represented, therefore, by measurements from a 

speciation monitor collocated with the FRM or by interpolated values for the species 

measured by speciation monitors in the region. 

The first step in estimating the speciation of the bulk sample is to assume that, 

due to the stability and nonvolatility of sulfate, sulfate measured by the speciation 

samplers is similar to the sulfate captured in the FRM sampler. The same is assumed 

for [EC]. As mentioned above, a passively collected mass of 0.5 μg/m3

 is assumed. 

Estimation of other species’ contributions to the FRM mass is described in the following 

sections. 

Adjusted Nitrate 

The FRM does not retain all of the semi-volatile PM2.5 mass, and at warmer 

temperatures, loss of particulate nitrate from filters has been widely observed (Chow et 

al., 2005). In order to estimate how much nitrate is retained on the FRM filter, simple 

thermodynamic equilibrium relations may be used. Necessary inputs include 24-hour 

average nitrate measurements and hourly temperature and relative humidity data. 

Frank (2006) suggests the following methodology for estimating retained nitrate: 

(1) For each hour (i) of the day, calculate the dissociation constant, Ki, from ambient 

temperature and relative humidity. 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-10 

For RH < 61%: ln(K) = 118.87 – (24084/T) – 6.025 ln(T), where T is temperature in 

kelvins and K is in nanobars. 

For RH >= 61%, K is replaced by: K’ = [P1 - P2 (1-a) + P3 (1-a)2

] x (1-a)1.75 x K, where a 

is “fractional” relative humidity and 

 ln(P1) = -135.94 + 8763/T + 19.12ln(T) 

 ln(P2) = -122.65 + 9969/T + 16.22ln(T) 

 ln(P3) = -182.61 + 13875/T + 24.46ln(T) 

(2) From that, calculate the nitrate retained on the filter: 

Retained Nitrate = STN nitrate – [745.7/TR x (κ-γ) x 1/24 x Σ (i=1 to 24) (Ki)

0.5] 

where TR is the daily average temperature for the sampled air volume (K), Ki is the 

dissociation constant for NH4NO3 at ambient temperature for hour i, and (κ-γ) relates to 

the temperature rise of the filter and vapor depletion from the inlet surface and is 

assumed to have a value equal to 1 (Hering and Cass, 1999). For further details, 

please refer to Frank (2006). 

Ammonium 

All nitrate on the FRM filter is assumed to be neutralized by ammonium. 

Ammonium associated with nitrate on the FRM filter, therefore, is given by 

 [NH4]NO3,FRM = 0.29 x [NO3]FRM

where [NO3]FRM is the nitrate remaining on the FRM filter and 0.29 is the mass ratio of 

NH4 to NO3 if fully neutralized. 

The form of (NH4)2SO4, however, can vary depending on location and season. 

Sulfate may or may not be fully neutralized. In order to determine the amount of 

ammonium associated with the sulfate aerosol, the ammonium difference between the 

STN and FRM filter needs to be determined. Some studies point to a loss of STN NH4

during sampling conditions where nitrate volatilization is favored. While the volatilized 

NO3 will be recaptured on the basic nylon filter and reported as nitrate, the associated 

ammonium, not sharing the same affinity for the basic filter, may pass through. In some 

cases, an adjustment assuming a loss of STN NH4 corresponds more closely with the 

NH4 on the FRM filter; in other cases, the unadjusted measurement agrees better with 

FRM NH4. For the calculations for the SJV, an area rich in NH3, NH4 on the filter is 

assumed to be the measured STN NH4 minus any losses associated with fully 

neutralized nitrate (as NH4NO3) that may have volatilized off the FRM filter. 

[NH4]FRM = [NH4]STN - 0.29 x ([NO3] STN -[NO3]FRM) 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-11 

Here, ([NO3] STN -[NO3]FRM) represents the amount of nitrate mass lost due to 

volatilization. 

Particle Bound Water 

At the FRM filter equilibration conditions, hygroscopic aerosol will retain its 

particle bound water (PBW) and be included in the observed FRM PM2.5 mass. PBW 

can be calculated using an equilibrium model like the Aerosol Inorganics Model (AIM). 

AIM requires the concentrations of ammonium, nitrate, sulfate, and estimated H+

 as 

inputs. In addition to inorganic concentrations, the equilibration conditions are also 

necessary model inputs. In this case, a temperature of 294.15 K and 35% RH is 

recommended. For simplification, a polynomial regression equation may be constructed 

by fitting the calculated water concentration from an equilibrium model and the 

concentrations of nitrate, ammonium, and sulfate. 

Other Species 

Other components that may be represented on the FRM filter include elemental 

carbon, crustal material, sea salt, and passively collected mass. Depending on location, 

for example, certain species may be neglected (like sea salt for inland areas). 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-12 

Carbonaceous Mass 

While carbonaceous aerosol may make up a large portion of airborne aerosol, 

speciated measurements of carbonaceous PM are considered highly uncertain. This is 

due to the large number of carbon compounds in the atmosphere and the measurement 

uncertainties associated with samplers of different configurations. In the SANDWICH 

approach, organic carbonaceous mass is calculated by difference. The sum of all nonorganic carbon components may be subtracted from the FRM PM2.5 mass as an 

estimate of organic carbon. 

After having calculated the species concentrations as outlined above, one can 

calculate the percentage contribution of each species to the measured FRM mass 

(minus the blank concentration of 0.5 μg/m3

) for each quarter of the years represented 

by the speciated data. One can then apply those percentages to the base year FRM 

design value, with the assumption that the years for which the speciation was calculated 

are representative of all years over which the design value is averaged. For the annual 

PM2.5 attainment demonstration, the speciated reference year FRM design value for 

each quarter is multiplied by the RRF for each species at each site, averaged together, 

and summed to get a future year PM2.5 concentration. Note that blank mass is kept 

constant at 0.5 μg/m3 between the base and future years, and future year particle bound 

water needs to be calculated for the future year values of nitrate, ammonium, and 

sulfate. A numerical example of the SMAT for the annual PM2.5 standard is given in the 

EPA modeling guidance on pp. 53-56 (U.S. EPA, 2007). 

24-hour Attainment Test 

The 24-hour attainment test is similar to the attainment test for the annual PM2.5

standard. In the case of the 24-hour test, however, the goal is to have the future 24-

hour PM2.5 design value at or below 65 μg/m3

 for all sites. The test has the following 

steps: 

Step 1. Identify the observed 98th percentile 24-hour PM2.5 average 

concentrations for each year (e.g., of the three year period that makes up the reference 

year design value) and the next highest concentrations for the other quarters. 

Step 2. Using the SANDWICH methodology outlined above, calculate the 

species percentages of the bulk FRM samples for each site, quarter, and year. These 

fractions should be calculated using days at the high end of the distribution (i.e., days 

around or above the standard). 

Step 3. Calculate RRFs for each species at each site for each quarter from 

modeled days at the high end of the PM2.5 distribution (i.e., modeled days having PM2.5

concentrations around or above 65 μg/m3

). As in the case of the annual test, for 

species i, site j, and quarter k, the RRF is given by the following equation: 

 RRFijk = ([Ci, quarter k of future year]/[Ci, quarter k of the base year])j
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-13 

Step 4. Multiply the quarterly, site-specific model-based RRFs from step 3 and 

speciated observations from step 2 to estimate future quarterly species concentrations. 

Step 5. Sum the future quarterly species concentration estimates from step 4 to 

estimate a quarterly “potential” 98th percentile PM2.5 estimate at each monitoring site. 

Step 6. Average the highest quarterly values for each year and then compare 

the future year design values at each site to the 24-hour PM2.5 standard of 65 μg/m3

. If 

all sites are at or below the standard, the 24-hour PM2.5 attainment test is passed. 

A numerical example of an application of the 24-hour PM2.5 attainment test is 

given on pp. 60-63 of the EPA Model Guidance (U.S. EPA, 2007). 

SJV Attainment Demonstration for the Annual PM2.5 Standard 

The SMAT procedure as outlined above was applied for FRM monitors operating 

in the San Joaquin Valley with minimal deviations from the recommended procedure. 

The 2006 design value was used as a basis from which to project forward the future 

year design value. Speciation data for four STN (speciation) sites was used to speciate 

the FRM mass for all FRM sites. For those sites not collocated with STN monitors, 

“surrogate” speciation sites were determined based on analysis of CRPAQS data to 

determine which sites had similar speciation profiles. The composition was assumed to 

be the same at all three Bakersfield sites (BAC, BGS, and BEP). Similarly, the percent 

composition at the two Fresno sites (FSF and FSH) was assumed to be the same. In 

addition, Stockton (SOH), Clovis (CLO), Corcoran (COP), and Modesto (MRM), were 

assumed to have the same speciation as one of the four speciation sites based on 

CRPAQS data analysis. The results of that analysis are given in Table 2. For a list of 

all FRM sites and their associated speciation site, see Table 3. 

The steps followed in order to determine future year design values for the FRM 

sites of the SJV were similar to those outlined above in the generalized description of 

the SMAT/SANDWICH procedure. Specific points to note are that in the speciation 

calculations, only days with all the necessary hourly temperature and relative humidity 

data in addition to values for all major components (ammonium ion, nitrate ion, sulfate 

ion, and elemental carbon) plus FRM mass were included in the species average 

calculations. Temperature and relative humidity data were obtained from the ARB’s Air 

Quality and Meteorological Information System (AQMIS, 

www.arb.ca.gov/aqd/aqinfo.htm) database for each monitoring station (collocated with 

the STN sites or nearby if the STN site was not available) for each available hour of the 

years 2004-2006. 

Quarterly average species concentrations were calculated at each STN site 

using the SANDWICH procedure described above. In addition, a blank mass average 

of 0.5 μg/m3

 was assumed for each quarter and held constant into the future. Modeled 

concentrations for the reference year (2005) and future year (2014) for each component 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-14 

were extracted for the FRM sites as a nine-cell average. The relative response factors 

(as defined above) were calculated for each component for each quarter. These 

calculations were performed using all modeled days, as we assumed that the selected 

FRM measurements provided a stable quarterly average value. Finally, particle bound 

water was calculated using a polynomial algorithm provided by Dr. Bong-Mann Kim of 

the South Coast Air Quality Management District. Organic mass was calculated by 

difference between the average total FRM mass and the sum of all other species and 

blank.

These quarterly species percentages were then multiplied against the reference 

year design value for 2006 (the average FRM PM2.5 concentrations for 2004, 2005, and 

2006). The quarterly observed species concentrations were then multiplied by the 

RRFs and summed and averaged to get a future year PM2.5 design value at each FRM 

site. See Table 3 for the predicted baseline and controlled 2014 PM2.5 design values. 

For the “controlled” 2014 emissions sensitivity scenario described above, future annual 

PM2.5 concentrations at all FRM sites are below the annual PM2.5 NAAQS of 15.0 μg/m3

, 

and, therefore, the San Joaquin Valley has passed the speciated modeled attainment 

test for the annual PM2.5 NAAQS. 

SJV Attainment Demonstration for the 24-Hour PM2.5 Standard 

A similar procedure to the attainment demonstration for the annual PM2.5

standard was followed for the 24-hour PM2.5 standard attainment demonstration. The 

exception was that only the top 25% of the measured and modeled days for each 

quarter were used instead of all available days. The top 25% of the days are expected 

to be more representative of the 24-hour design value than would all available days for 

a given quarter. 

Table 4 shows the projected future year 24-hour PM2.5 design values for the top 

five 2006 design value sites for the controlled emissions sensitivity case. As shown, all 

sites in the SJV attained the 24-hour standard in 2006, and further emissions controls 

do not cause any monitors to become non-attainment. 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-15 

Unmonitored peaks 

EPA’s modeling guidance (EPA, 2007) describes the “Unmonitored Area 

Analysis” as an analysis used to ensure that a proposed control strategy will be effective 

in reducing PM2.5 at locations without air quality monitors so that attainment is shown 

throughout a nonattainment area. The purpose of the analysis is to use a combination 

of model output and ambient data to identify areas that might exceed the NAAQS if 

monitors were located there. 

This analysis has four major steps. They are: 

1. Interpolation of the base year design values to create a set of spatial fields 

2. Adjustment of the spatial design value fields using gridded model output gradients 

(base year values) 

3. Application of the gridded model RRFs to the gradient adjusted spatial design value 

fields 

4. Determination if any unmonitored areas are predicted to exceed the NAAQS in the 

future 

We will present a complete analysis of unmonitored areas in a future version of 

this document. A screening analysis that is designed to assess the possibility of 

unmonitored violations of the annual PM2.5 NAAQS is presented here. 

First, an annual-averaged modeled PM2.5 field is generated for the entire 

modeling domain as shown in figure 3. This field is then scrutinized to see if there will 

be gradients in the field that would give rise to higher values away from monitors if this 

field were to be used to adjust the interpolated annual-averaged design value field. As 

seen from figure 3, there are no areas with steep gradients that would result in higher 

design values than those measured at monitors. That is, for the central and southern 

part of the Valley where design values are high, the highest modeled concentrations 

occur at monitors so that there can not be higher values away from monitors. 

As mentioned before, this is a simple screening analysis and a complete analysis 

of the unmonitored areas will be presented in the future. 

References 

Blackadar, A. K. (1976) Modeling the Nocturnal Boundary Layer. Preprints of Third 

Symposium on Atmospheric Turbulence and Air Quality, Raleigh, NC. 19-22, 

October 1976, Amer. Meteor. Soc., Boston, 46-49. 

Blackadar, A. K. (1979) Advances in Environmental Science and Engineering, 1. No. 1. 

Pfaflin and Ziegler, Eds. Gordon and Breach Publishers, 50-85. 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-16 

Boylan, J.W. and A.G. Russell (2006) PM and light extinction model performance 

metrics, goals, and criteria for three-dimensional air quality models. Atmospheric 

Environment, 40:4946-4959. doi:10.1016/j.atmosenv.2005.09.087 

Carter, W. (2000) Documentation of the SAPRC-99 chemical mechanism for VOC 

reactivity assessment. Final Report to California Air Resources Board. Contract 

92-329 and Contract 95-308, 

http://pah.cert.ucr.edu/ftp/pub/carter/pubs/s99txt.pdf 

Chow, J.C., Chen, L.W.A., Watson, J.G., Lowenthal, D.G., Magliano, K.A., Turkiewicz, 

K., and D.E. Lehrman (2006) PM2.5 chemical composition and spatiotemporal 

variability during the California Regional PM10/PM2.5 Air Quality Study 

(CRPAQS). J. Geophys. Res., 111, D10S04, doi:10.1029/2005JD006457. 

Chow, J.C., Watson, J.G., Lowenthal, D.H., Magliano, K. (2005) Loss of PM2.5 Nitrate 

from Filter Samples in Central California. J. Air & Waste Manage. Assoc. 

55:1158-1168. 

Community Modeling and Analysis System (2007) CMAQ v4.6 Operational Guidance 

 Document. http://www.cmaq-model.org/op_guidance_4.6/manual.pdf

Dudhia, J. (1989) Numerical Study of Convection Observed During the Winter Monsoon 

Experiment Using a Two-Dimensional Model. J. Atmos. Sci., 46, 3077-3107. 

Dudhia, J. (1993) Radiation Studies with a High-Resolution Mesoscale Model. 

Proceedings of the Third Atmospheric Radiation Measurement (ARM) Science 

Team Meeting. March 1-4, 1993, Norman, OK. pp. 363-366. 

Frank, Neil H. (2006) Retained Nitrate, Hydrated Sulfates, and Carbonaceous Mass in 

Federal Reference Method Fine Particulate Matter for Six Eastern U.S. Cities. J. 

Air & Waste Manage. Assoc. 56:500-511. 

Grell, G. A., J. Dudhia and D. R. Stauffer. (1995) A Description of the Fifth-Generation 

Penn State/NCAR Mesoscale Model (MM5). NCAR Technical Note, NCAR/TN398+STR. 

Hering, S.V. and Cass, G.R. (1999) The Magnitude of Bias in the Measurement of 

PM2.5 Arising from Volatilization of Particulate Nitrate from Teflon Filters. J. Air 

& Waste Manage. Assoc. 49:725-733. 

Liang, J. and A. Kaduwela, 2005: Microdevelopment of CMAQ for California Regional 

Particulate-Matter Air Quality Study. Proceedings of the 4th Annual CMAS 

Models-3 Users’ Conference; September 26-28, 2005, Chapel-Hill, NC. 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-17 

U.S. Environmental Protection Agency (1999) Science Algorithms of the EPA Models-3 

 Community Multiscale Air Quality (CMAQ) Modeling System. EPA/600/R-

 99/030. http://www.epa.gov/asmdnerl/CMAQ/CMAQscienceDoc.html

U.S. Environmental Protection Agency. (2007) Guidance on the Use of Models and 

Other Analyses for Demonstrating Attainment of Air Quality Goals for Ozone, 

PM2.5, and Regional Haze. EPA-454/B-07-002 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-18 

Figure 1. The CCAQS modeling domain with the SJV modeling domain inset. 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-19 

Figure 2. The location of the three nested grids adopted for the numerical modeling of 

SJV using MM5. 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-20 

Figure 3. The annual-averaged modeled PM2.5 concentrations in the domain.
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-21 

Table 1. Percentage reduction between 2014 baseline and controlled emissions 

NOx PM2.5 SOx ROG

Defined State Measures (Strategy 

4/26/07) 20 5 0 6

District Rules (Dft Plan 12/11/07) 4 8 4 0

Total Reduction 24 13 4 6

Table 2. Percent Composition Ratio Based on 2000 Average CRPAQS Data 

Site AmmNitrate AmmSulfate EC OC Geological Elements

CLO % 30.78 9.09 10.49 43.22 4.14 2.27

FSF % 26.79 10.15 7.07 49.31 4.16 2.52

CLO to FSF Ratio 1.15 0.90 1.48 0.88 1.00 0.90

COP % 36.44 9.86 8.37 37.87 5.06 2.39

VCS % 35.38 9.53 9.04 39.53 4.37 2.14

COP to VCS Ratio 1.03 1.03 0.93 0.96 1.16 1.12

MRM % 29.33 9.08 10.92 45.50 2.82 2.35

M14 % 26.39 9.84 11.43 46.35 2.71 3.27

MRM to M14 Ratio 0.90 1.08 1.05 1.02 0.96 1.39

SOH % 24.80 11.76 13.13 44.07 2.93 3.31

M14 % 26.39 9.84 11.43 46.35 2.71 3.27

SOH to M14 Ratio 0.94 1.19 1.15 0.95 1.08 1.01

Table 3. Reference and future year annual design values for SJV FRM sites 

Site Code Speciation 2006 

DV 

2014 

Baseline 

DV 

2014 

"Controlled" 

DV 

Bakersfield - 5558 California BAC BAC 18.51 15.86 14.28 

Bakersfield - 410 E Planz 

Road BEP BAC 18.86 16.26 14.70 

Bakersfield - Golden State BGS BAC 18.64 15.98 14.39 

Clovis - N Villa Avenue CLO FSF 16.39 14.10 12.72 

Corcoran - Patterson 

Avenue COP VCS 17.24 14.75 13.27 

Fresno - 1st Street FSF FSF 16.68 14.43 13.01 

Fresno - Hamilton and 

Winery FSH FSF 17.16 14.93 13.47 

Merced - 2334 M Street MRM M14 14.69 12.85 11.76 

Modesto - 14th Street M14 M14 14.10 12.52 11.44 

Stockton - Hazelton Street SOH M14 12.93 11.77 10.87 

Visalia - N. Church Street VCS VCS 18.20 16.05 14.47 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix G: Regional Air Quality Modeling 

2008 PM2.5 Plan

G-22 

Table 4. Reference and future year 24-hour design values for SJV FRM sites 

Site Code Speciation 2006 

DV 

2014 

"Controlled" 

DV 

Bakersfield - 5558 California BAC BAC 62.4 46.2 

Bakersfield - 410 E Planz 

Road BEP BAC 65.2 45.9 

Bakersfield - Golden State BGS BAC 64.4 45.3 

Fresno - 1st Street FSF FSF 58.0 41.2 

Fresno - Hamilton and 

Winery FSH FSF 58.5 41.7