evaluation of original and improved versions of calpuff using...

EVALUATION OF ORIGINAL AND IMPROVED VERSIONS OF CALPUFF USING THE 1995 SWWYTAF DATA BASE

Technical Report

Prepared by

Prakash Karamchandani, Shu-Yun Chen and Rochelle Balmori

Atmospheric and Environmental Research, Inc.

388 Market Street, Suite 750

San Francisco, CA 94111

Prepared for

American Petroleum Institute

1220 L Street NW

Washington, DC 20005

Document CP281-09-01

October 2009

Evaluation of CALPUFF using the 1995 SWWYTAF Data Base i

TABLE OF CONTENTS Executive Summary 1. Introduction..............................................................................................................1 2. Additional Improvements to CALPUFF Modeling System ....................................3 Improvements to CALPUFF Ammonia Treatment .................................................3 Improvement of CALPUFF POSTUTIL Processor.................................................4 3. CALPUFF Application and Evaluation...................................................................6 CALMET Simulations.............................................................................................6 CALPUFF Simulations............................................................................................7 CALPUFF simulations with different chemistry options ...............................7 CALPUFF sensitivity studies .......................................................................21 4. Summary and Conclusions ....................................................................................27 5. References..............................................................................................................29

Evaluation of CALPUFF using the 1995 SWWYTAF Data Base ES1

EXECUTIVE SUMMARY

This report describes the application and evaluation of an improved chemistry version of CALPUFF. The improvements to CALPUFF were made in a previous API-sponsored study and are documented in Karamchandani et al. (2008). These improvements include: a) the correction of errors in the treatment of background ozone concentrations in the RIVAD chemistry option of CALPUFF; b) the replacement of the original CALPUFF PM modules by newer algorithms that are used in contemporary 3-D air quality models such as CMAQ, CMAQ-MADRID, CAMx and REMSAD; and c) the incorporation of an aqueous-phase chemistry module based on the treatment in CMAQ. Sensitivity testing of the improved chemistry version of CALPUFF showed that the new inorganic thermodynamic aerosol module (based on the ISORROPIA aerosol treatment implemented in CMAQ and CAMx) resulted in lower formation of particulate nitrate than the original CALPUFF module for typical atmospheric conditions (Karamchandani et al., 2008). The improvements to CALPUFF were evaluated using the 1995 Southwest Wyoming Technical Air Forum (SWWYTAF) database. The SWWYTAF modeling study was conducted to quantify the contributions of emission sources, particularly in the relatively highly industrialized southwest quadrant of Wyoming, to visibility and acid deposition at the Bridger and Fitzpatrick Class I Wilderness Areas. As part of the CALPUFF model evaluation study described here, an evaluation of the meteorological fields used to drive the model was also conducted. This meteorological model evaluation, described in a separate report (Nehrkorn and Henderson, 2008), assessed both the original meteorological fields (MM5 outputs) and the meteorological fields produced by the CALPUFF meteorological pre-processor, CALMET, using the MM5 outputs. For the SWWYTAF application described in this report, additional minor improvements were made to the modeling system. These improvements were related to the treatment of ammonia in the model and the post-processor used to recalculate PM nitrate at receptor locations. Background NH3 concentrations, based on observations for the Pinedale, Wyoming area at the Boulder Monitoring Station operated by Shell Exploration and Production Company, were used for some of the CALPUFF simulations. Twice-weekly NH3 data for 2007 were provided by Shell and these data were processed to calculate seasonally averaged background NH3 concentrations for CALPUFF. CALPUFF was also modified to optionally read monthly files of vertically varying background NH3 concentrations. This improvement addresses the limitation of CALPUFF that it cannot account for the variation with altitude of concentrations of NH3, a species that is primarily emitted by surface sources. The final improvement was to the POSTUTIL ammonia limiting post-processor, in which the existing inorganic aerosol equilibrium algorithm was replaced by the ISORROPIA algorithm. The ISORROPIA module is a key component of the improved chemistry version of CALPUFF developed in the previous API study (Karamchandani et al., 2008).


The SWWYTAF database used in the CALPUFF simulations described in this report was provided by the Wyoming Department of Environmental Quality. It included MM5 output for 1995, CALMET and CALPUFF codes and control files, emissions for the Southwest Wyoming Regional modeling domain, and selected outputs from the CALPUFF simulations. Since the database did not include the CALMET outputs required for running CALPUFF, we used the latest available regulatory version of CALMET (Version 5.8, dated June 23, 2007) to generate the necessary meteorological fields for CALPUFF. A companion report (Nehrkorn and Henderson, 2008) describes the evaluation of the CALMET fields with observations. The CALPUFF simulations were also conducted with the latest available regulatory version of the model (Version 5.8, dated June 23, 2007), including the API chemistry improvements described in Karamchandani et al. (2008). A benchmark CALPUFF simulation was first conducted using the SWWYTAF inputs, run scripts, and model configuration to ensure that we could reasonably reproduce the SWWYTAF results obtained from simulations with the 1999 versions of CALMET and CALPUFF. The benchmarking was successful and a series of CALPUFF simulations were conducted using different chemistry options (original options as well as the improved API chemistry option) for the model evaluation. A number of sensitivity studies were also conducted to investigate the effect of background NH3 concentrations on model predictions of PM nitrate. The different CALPUFF chemistry options evaluated in this study include:

1. MESOPUFF II chemistry (MCHEM=1) using the FLAG recommended background NH3 concentration of 1 ppb for arid land.

2. Original CALPUFF RIVAD chemistry (MCHEM=3) using background values of NH3 concentrations based on measurements in the Pinedale, Wyoming area.

3. Improved CALPUFF RIVAD chemistry (MCHEM=5) using background values of NH3 concentrations based on measurements in the Pinedale, Wyoming area.

4. Improved CALPUFF RIVAD chemistry (MCHEM=5) using background values of surface NH3 concentrations based on measurements in the Pinedale, Wyoming area with vertically varying NH3 concentrations based on CMAQ outputs for 2001.

The model evaluation was conducted with both the raw outputs of the CALPUFF simulations as well as with the post-processed outputs using POSTUTIL. Model results for PM sulfate and nitrate were compared at the Bridger Wilderness Area IMPROVE site and the Pinedale CASTNET site. With the regulatory CALPUFF chemistry options (MESOPUFF II and original RIVAD), CALPUFF significantly over-predicted PM nitrate concentrations at both monitoring locations (by factors of 3 to 4). There was some improvement in the bias when the POSTUTIL processor was used to repartition total nitrate, but the over-predictions were still large (approximately factors of 2 to 3). When the API-improved RIVAD chemistry option was used, significantly lower biases in model predictions of PM nitrate were noted. At the Pinedale CASTNET site, the model under-predicted PM nitrate by about 5%, and at the Bridger IMPROVE site, it over-


predicts PM nitrate by about 25%. Using the POSTUTIL processor to repartition total nitrate for the improved RIVAD chemistry simulation had a small impact on the overall results. Introducing the constraint of a vertically decreasing background NH3 profile for the improved RIVAD chemistry simulation resulted in significant under-predictions of PM nitrate (by factors of 4 to 5). A possible reason for this underprediction is that the prescribed vertical profile for NH3 concentrations (obtained from 2001 CMAQ outputs) may not be representative for this CALPUFF application. Although using a vertical NH3 profile did not work well for this application, it is a user-selectable option and we believe it is important to retain this capability in the model for future applications where appropriate NH3 vertical profiles may be available. To identify the bias at the upper end of the frequency distribution (the values used in making regulatory decisions), the observed and predicted frequency distributions were compared using Quantile-quantile (Q-Q) plots. At both the Bridger and Pinedale monitoring locations, the predicted frequency distributions for PM nitrate with both the regulatory CALPUFF chemistry options (MESOPUFF II and original RIVAD) were significantly higher than the observed frequency distribution. In contrast, the predicted frequency distribution for PM nitrate with the improved RIVAD chemistry option showed a much better correspondence with the observed frequency distribution. For all the model configurations evaluated in this study, the results for PM sulfate were very similar, which was expected since the API improvements to the CALPUFF chemistry were expected to have the most impact on PM nitrate predictions. The Q-Q plots for sulfate concentrations showed that for all the chemistry options, the predicted frequency distribution of sulfate concentrations was within a factor of two of the observed distribution and concentrations at the higher end of the distribution tended to be underestimated. Spatial patterns of the differences between the annually averaged PM nitrate predictions from the CALPUFF simulations using the revised and original RIVAD chemistry options showed that that the original RIVAD formulation produced significantly more (from 0.2 µg/m3 to 0.3 µg/m3) PM nitrate than the revised RIVAD chemistry. There was a north-south gradient in the differences between the model predictions with the two different chemistry options, with the larger differences in the southern portion of the domain and the smaller differences in the northern portion of the domain. Considering that the observed annual average PM nitrate concentrations in the region are of the order of 0.1 µg/m3, i.e., a factor of 2 to 3 lower than the excess PM nitrate produced with the original CALPUFF chemistry, it is clear that the original chemistry options in CALPUFF significantly over-predict PM nitrate. Sensitivity studies were conducted to compare the effect of background NH3 concentrations on model predictions of PM nitrate formation with the FLAG configuration of CALPUFF (MESOPUFF II chemistry) and with the API revisions to the RIVAD chemistry. The results from these studies showed that, regardless of the background ammonia concentration, the PM nitrate predictions from the improved RIVAD chemistry were always lower than the MESOPUFF II chemistry predictions.


The results from this study indicate that PM nitrate concentrations in the SWWYTAF modeling domain are likely to be over-predicted by factors of 2 to 3 using the original CALPUFF chemistry options. The API improvements to the RIVAD chemistry, which include algorithms that are currently in use in today’s comprehensive air quality models, result in significantly lower biases in PM nitrate predictions. Additional studies are required for other regions of the United States to determine if the improvements to the chemistry consistently lead to better agreement between model predictions and observations of PM nitrate.

Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 1

INTRODUCTION

CALPUFF is the preferred model for assessing long range transport of pollutants

and their impacts on Federal Class I areas under the Prevention of Significant

Deterioration (PSD) program and on a case-by-case basis for certain near-field

applications involving complex meteorological conditions. CALPUFF is also the

preferred option in Best Available Retrofit Technology (BART) determinations for

assessing the visibility impacts of one or a small group of sources. In addition, CALPUFF

is being extensively used to prepare Environmental Impact Statements (EIS) under the

National Environmental Policy Act (NEPA) and the Federal Land Policy and

Management Act (FLPMA) to project cumulative air quality impacts from proposed new

Exploration & Production (E&P) development in adjacent Class I areas. Because the

decisions made on the basis of CALPUFF simulations can have a significant effect on the

costs and viability of new E&P development, it is necessary to ensure that the model is

based on sound science and accurate inputs.

Several studies (Karamchandani et al., 2006; Santos and Paine, 2006; Morris et

al., 2005; 2006) have shown that CALPUFF tends to overestimate the formation of

secondary aerosols because its treatment of atmospheric chemistry is highly simplified

and inadequate for particulate matter (PM) formation. These concerns about the

shortcomings of CALPUFF led to the development of an improved version of CALPUFF

under API funding, described in Karamchandani et al. (2008). Results from simulations

conducted with the revised version of CALPUFF showed that the new inorganic

thermodynamic aerosol module (based on the aerosol treatment implemented in grid

models such as CMAQ and CAMx) resulted in lower formation of particulate nitrate than

the original CALPUFF module for typical atmospheric conditions (Karamchandani et al.,

2008).

This report describes the application and evaluation of the improved version of

CALPUFF using the 1995 Southwest Wyoming Technical Air Forum (SWWYTAF)

database. The SWWYTAF modeling study was conducted to quantify the contributions

of emission sources, particularly in the relatively highly industrialized southwest

quadrant of Wyoming, to visibility and acid deposition at the Bridger and Fitzpatrick


Class I Wilderness Areas. As part of the CALPUFF model evaluation study described

here, an evaluation of the meteorological fields used to drive the model was also

conducted. This meteorological model evaluation, described in a separate report

(Nehrkorn and Henderson, 2008), assessed both the original meteorological fields (MM5

outputs) and the meteorological fields produced by the CALPUFF meteorological pre-

processor, CALMET, using the MM5 outputs.

In the following sections, we first describe additional minor improvements to the

CALPUFF modeling system since the improvements implemented by Karamchandani et

al. (2008). Next, we describe the evaluation of both the original and improved versions of

CALPUFF using the SWWYTAF database. We also present results from a number of

sensitivity studies based on varying background ammonia concentrations.


ADDITIONAL IMPROVEMENTS TO CALPUFF MODELING SYSTEM

In addition to the science improvements that were incorporated into CALPUFF in

the previous study conducted for API (Karamchandani et al., 2008), some minor

improvements were made in this evaluation study. These improvements were related to

the treatment of ammonia in the model and the post-processor used to recalculate PM

nitrate at receptor locations. The improvements are described briefly below.

Improvements to CALPUFF Ammonia Treatment

The partitioning of total nitrate into the gas (nitric acid) and particle (particulate

nitrate) phases is sensitive to variables such as temperature, humidity, and ambient sulfate

and ammonia levels. The formation of particulate nitrate is limited by the amount of

ammonia available. In CALPUFF, ammonia concentrations are specified as constant

background values (although monthly variations in ammonia concentrations are allowed).

There are several shortcomings in this treatment of ammonia in CALPUFF. One

limitation is in the specification of background NH3 values, particularly the

recommended values for regulatory applications. For example, the recommended

IWAQM (Interagency Workgroup on Air Quality Modeling) and FLAG (Federal Land

Managers’ AQRV Workgroup) values for CALPUFF NH3 background concentrations are

10 ppb for grassland, 1 ppb for arid land, and 0.5 ppb for forests. In current Western EIS

applications of CALPUFF, the IWAQM-recommended background NH3 concentration of

1 ppb is being used, which is higher than that used in the SWWYTAF modeling.

A second limitation of the current treatment of NH3 in CALPUFF is the lack of a

vertical concentration profile to account for the variations in NH3 concentrations with

altitude. Ammonia is primarily emitted by surface sources, and NH3 concentrations

decrease sharply with height. Because CALPUFF uses spatially and vertically

homogeneous NH3 concentrations, it may overestimate particulate nitrate formation for

emissions from elevated sources.


To address the first limitation, we used background NH3 concentrations that were

representative of the region for some of the CALPUFF simulations described in the

following section. These concentrations were based on measurements of NH3

concentrations for the Pinedale, Wyoming area at the Boulder Monitoring Station

operated by Shell Exploration and Production Company. Twice-weekly NH3 data for

2007 were provided to AER by Shell for the CALPUFF simulations. These data were

processed to calculate seasonally averaged background NH3 concentrations for

CALPUFF.

To address the second limitation, we modified CALPUFF to read monthly files of

vertically varying background NH3 concentrations. The file contains the NH3

concentration at each model layer. For the application described here, we obtained typical

seasonal NH3 vertical profiles for the modeling region from a previous 3-D modeling

study with the U.S. EPA Community Multiscale Air Quality (CMAQ) model. These

profiles were then adjusted to the CALPUFF model layers and the surface NH3

concentrations measured at the Boulder Monitoring Station by Shell Exploration and

Production Company.

Improvement of CALPUFF POSTUTIL processor

In CALPUFF, the total nitrate (TNO3) is partitioned into the gas (HNO3) and

particulate (NO3) phases based on the available ammonia and the temperature and

humidity dependent equilibrium relationships between HNO3 and NO3. Since CALPUFF

treats a continuous plume as a series of discrete puffs, the total concentration of any

modeled species at a receptor location is the sum of the contributions of all puffs

impacting that receptor. Thus, the PM nitrate concentration at a receptor location is the

sum of the PM nitrate contributions from all puffs influencing that receptor. Since the full

background NH3 concentration is available to all puffs for forming PM nitrate rather than

being shared among neighboring puffs, nitrate concentrations at a receptor will generally

be overestimated unless there is an abundance of ammonia. This is a limitation of the

model framework that can be correctly treated only in a grid model.


To account for this limitation, the CALPUFF developers created a post-processor

referred to as POSTUTIL (Escoffier-Czaja and Scire, 2002). The POSTUTIL processor

in CALPUFF repartitions the total predicted nitrate into the gas (HNO3) and particulate

(NO3) phases to account for ammonia limitation. In POSTUTIL, ammonia availability is

computed based on receptor concentrations of total sulfate and TNO3, not on a puff-by-

puff basis. POSTUTIL computes the total sulfate concentrations from all sources, and

estimates available ammonia for particulate nitrate formation after the preferential

scavenging of ammonia by sulfate. This allows non-linearity associated with ammonia

limiting effects to be included in the predicted PM nitrate concentrations at receptors of

interest.

The gas/particle partitioning scheme used in POSTUTIL is the same as that used

in CALPUFF for the MCHEM=1 (MESOPUFF II chemistry) and MCHEM=3 (original

CALPUFF RIVAD chemistry) options for chemistry. As discussed in Karamchandani et

al. (2008), this scheme, based on Stelson and Seinfeld (1982), tends to overestimate the

formation of PM nitrate as compared to more recent schemes, such as the ISORROPIA

model of Nenes et al. (1999). The ISORROPIA scheme is implemented in several

contemporary 3-D air quality models such as CMAQ, CMAQ-MADRID, CAMx and

REMSAD, and was implemented into CALPUFF in the recently completed API study to

improve the CALPUFF chemistry (Karamchandani et al., 2008). This scheme is used

when the revised chemistry options (MCHEM=5 or MCHEM=6) are selected by the

CALPUFF user.

In this study, we updated POSTUTIL by including an option for the post-

processor to use the ISORROPIA model instead of the original CALPUFF

thermodynamic gas/particle partitioning algorithm.


CALPUFF APPLICATION AND EVALUATION

In this section, we describe the application of different versions of CALPUFF to

the SWWYTAF modeling domain for 1995. The SWWYTAF database was obtained

from Ms. Darla Potter at the Wyoming Department of Environmental Quality as a set of

seven CDs. The database included MM5 output for 1995, CALMET and CALPUFF

codes and control files, emissions for the Southwest Wyoming Regional modeling

domain, and selected outputs from the CALPUFF simulations. The database did not

include the CALMET outputs required for running CALPUFF.

CALMET Simulations

As part of the current study, we first applied CALMET for the SWWYTAF

modeling domain to generate three-dimensional meteorological fields for the CALPUFF

simulations. We used the latest available regulatory version of CALMET (Version 5.8,

dated June 23, 2007) for this purpose. CALMET modifies the raw MM5 model output by

(a) applying vertical interpolation/extrapolation from model to near-surface levels, (b)

modifying the wind field to account for channeling effects using a higher resolution

terrain database (4 km vs. the 40 km MM5 resolution), and (c) adding analysis

increments to account for available observations. The resulting meteorological fields

from CALMET were evaluated against observations as described in our companion

report (Nehrkorn and Henderson, 2008). However, as pointed out by Nehrkorn and

Henderson (2008), the comparison of the CALMET fields against the verifying

observations does not provide a realistic assessment of the fidelity of these fields, since

the verifying observations have already been used in the analysis step of CALMET.

Thus, arbitrarily good agreement with these observations can be achieved. To provide a

more realistic assessment of the quality of CALMET fields, we repeated the CALMET

runs, but did not include any observations in the analysis step. This second set of

CALMET outputs was generated only for evaluation purposes and was not used in the

CALPUFF simulations described in this report.


CALPUFF Simulations

The CALPUFF simulations were also conducted with the latest available

regulatory version of the model (Version 5.8, dated June 23, 2007), including the API

chemistry improvements described in Karamchandani et al. (2008). As a first step, we

conducted a benchmark CALPUFF simulation by repeating the model application

described in the SWWYTAF modeling study report prepared by Earth Tech, Inc. (Earth

Tech, 2001) for the Wyoming Department of Environmental Quality. All the inputs and

model configuration options were exactly the same as those used in the SWWYTAF

modeling study. The only differences between our simulations from that conducted in the

SWWYTAF study were that the 2008 official releases of CALMET and CALPUFF were

used in our study, while the previous study was based on the 1999 versions of the models.

We compared observed and predicted 1995 averaged concentrations at the Bridger

Wilderness Area IMPROVE site and the Pinedale CASTNET site. The model

performance results were almost identical to those described in the SWWYTAF report

(Earth Tech, 2001) suggesting that the updates to the official releases of the CALMET

and CALPUFF models had negligible effects on the model results for the SWWYTAF

domain and also ensuring that the benchmarking was successful.

After completing the benchmarking simulation, we conducted a series of

CALPUFF simulations with the different chemistry options (including the latest API

chemistry improvements) as well as a number of sensitivity studies to investigate the

effect of background NH3 concentrations on model predictions of PM nitrate. These

simulations are described in the following sections.

CALPUFF simulations with different chemistry options

Table 1 shows the matrix of annual CALPUFF simulations conducted to

investigate the effects of different chemistry options and model configurations on model

predictions of PM nitrate. For each CALPUFF simulation, two sets of results were

generated. The first set of results is based on the raw CALPUFF outputs while the second


Table 1. Matrix of CALPUFF simulations for testing chemistry options

Case Chemistry Option Background NH3 POSTUTIL for NH3 limitation

1a MCHEM = 1 1 ppb No

1b MCHEM = 1 1 ppb Yes

2a MCHEM = 3 Seasonal values based on Pinedale measurements

No

2b MCHEM = 3 Seasonal values based on Pinedale measurements

Yes

3a MCHEM = 5 Seasonal values based on Pinedale measurements

No

3b MCHEM = 5 Seasonal values based on Pinedale measurements

Yes

4a MCHEM = 5 Seasonal surface values based on Pinedale measurements; vertical profile based on CMAQ outputs

No

4b MCHEM = 5 Seasonal surface values based on Pinedale measurements; vertical profile based on CMAQ outputs

Yes


set of results is based on the POSTUTIL outputs (i.e, post-processing of raw CALPUFF

outputs to repartition total nitrate into the gas and particle phases based on the available

background ammonia concentration).

Case 1 corresponds to the MESOPUFF II chemistry (MCHEM=1) using the

FLAG recommended background ammonia concentration of 1 ppb for arid land. Case 2

corresponds to the original CALPUFF RIVAD chemistry (MCHEM=3) while Case 3

corresponds to the new CALPUFF RIVAD chemistry with the API improvements

(MCHEM=5). The CALPUFF simulations for Case 2 and Case 3 were conducted using

background values of ammonia concentrations based on measurements of NH3

concentrations for the Pinedale, Wyoming area at the Boulder Monitoring Station

operated by Shell Exploration and Production Company. The twice-weekly NH3 data for

2007 were processed to calculate seasonally averaged background NH3 concentrations for

CALPUFF. The seasonal NH3 values were 0.43 ppb for winter, 0.7 ppb for spring, 1.1

ppb for summer and 0.59 ppb for fall.

Case 4 is the same as Case 3, with the additional constraint of vertically varying

background ammonia concentrations. The vertical profile is based on outputs of a 2001

U.S. simulation with the U.S. EPA Community Multi-scale Air Quality (CMAQ) model.

We created seasonal vertical NH3 profiles for the region corresponding to the

SWWYTAF modeling domain from the CMAQ outputs and applied these profiles to the

CALPUFF model layers with surface values corresponding to the Pinedale NH3

measurements discussed above. Table 2 shows the seasonal vertical profiles of NH3

concentrations used in the CALPUFF simulations for Case 4.


Table 2. Seasonal vertical profiles for background NH3 concentrations (ppb)

CALPUFF Model Layer

Height (m) Winter NH3 Spring NH3 Summer NH3 Fall NH3

1 20 0.43 0.70 1.1 0.59

2 40 0.42 0.69 1.1 0.59

3 100 0.41 0.69 1.1 0.57

4 140 0.40 0.69 1.1 0.56

5 320 0.35 0.67 1.0 0.52

6 580 0.31 0.61 0.96 0.47

7 1020 0.26 0.52 0.86 0.39

8 1480 0.22 0.41 0.70 0.30

9 2220 0.19 0.27 0.52 0.19

10 2980 0.17 0.21 0.41 0.14


Table 3a shows the model performance evaluation statistics for PM nitrate at the

Pinedale CASTNET site for the 4 chemistry options (Cases 1a, 2a, 3a, and 4a) without

repartitioning of PM nitrate with the POSTUTIL processor. Table 3b shows the statistics

with the post-processed CALPUFF outputs (Cases 1b, 2b, 3b and 4b) using the original

POSTTUTIL for the original CALPUFF chemistry options (Cases 1b and 2b) and the

improved POSTUTIL (described previously) for the improved RIVAD chemistry options

(Cases 3b and 4b). The corresponding statistics for the Bridger IMPROVE site are shown

in Tables 4a and 4b, respectively.

As can be seen from Tables 3 and 4, both CALPUFF simulations with the original

MESOPUFF II and RIVAD chemistries (Cases 1a and 2a) significantly over-predict PM

nitrate concentrations at both monitoring locations (by factors of 3 to 4). There is some

improvement in the bias when the POSTUTIL processor is used to repartition total nitrate

(Cases 1b and 2b), but the over-predictions are still large (approximately factors of 2 to

3).

When the improved RIVAD chemistry is used (Case 3a), we see significantly

lower biases in the model predictions. At the Pinedale CASTNET site, the model under-

predicts PM nitrate by about 5%, and at the Bridger IMPROVE site, it over-predicts PM

nitrate by about 25%. Using the POSTUTIL processor to repartition total nitrate for the

improved RIVAD chemistry simulation (Case 3b) has a small impact on the overall

results. At the CASTNET site, the PM nitrate is over-predicted about 4%, and at the

IMPROVE site, the PM nitrate is over-predicted by about 28%.

Introducing the constraint of a vertically decreasing background NH3 profile for

the improved RIVAD chemistry simulation (Case 4a) results in significant under-

predictions of PM nitrate (by factors of 4 to 5). However, when the POSTUTIL processor

is used to repartition total nitrate for this simulation (Case 4b), the predicted PM nitrate

concentrations are over-predicted by about 40 to 70%.

From these results, we see a definite improvement in model predictions of PM

nitrate with the API revisions to the RIVAD chemistry. For all the CALPUFF

simulations, however, the correlation coefficients are very small.


Table 3a. Statistics for PM nitrate at the Pinedale CASTNET site without post-processing with POSTUTIL to repartition total nitrate

Case 1a Case 2a Case 3a Case 4a

Mean Observed Value (µg/m3) 0.139

Mean Modeled Value (µg/m3) 0.401 0.41 0.131 0.03

Ratio of Means 2.88 2.95 0.95 0.21

Gross Bias (µg/m3) 0.263 0.271 -0.007 -0.109

Normalized Bias 3.664 3.384 0.526 -0.71

Root Mean Square Error 0.333 0.322 0.128 0.133

Coefficient of Determination (r2) <0.001 0.041 0.002 0.054

Observed Standard Deviation 0.078

Modeled Standard Deviation 0.194 0.175 0.108 0.035

Table 3b. Statistics for PM nitrate at the Pinedale CASTNET site after post-processing with POSTUTIL to repartition total nitrate

Case 1b Case 2b Case 3b Case 4b



Ratio of Means 2.64 2.23 1.04 1.4

Gross Bias (µg/m3) 0.227 0.17 0.006 0.055

Normalized Bias 3.301 2.387 0.701 1.327


Coefficient of Determination (r2) <0.001 0.01 0.004 0.008




Table 4a. Statistics for PM nitrate at the Bridger IMPROVE site without post-processing with POSTUTIL to repartition total nitrate

Case 1a Case 2a Case 3a Case 4a



Ratio of Means 3.72 3.78 1.24 0.27

Gross Bias (µg/m3) 0.269 0.275 0.023 -0.072

Normalized Bias 8.007 7.147 1.829 -0.401


Coefficient of Determination (r2) 0.035 0.044 <0.001 <0.001



Table 4b. Statistics for PM nitrate at the Bridger IMPROVE site after post-processing with POSTUTIL to repartition total nitrate

Case 1b Case 2b Case 3b Case 4b



Ratio of Means 3.35 2.86 1.28 1.67

Gross Bias (µg/m3) 0.232 0.183 0.028 0.067

Normalized Bias 7.225 5.612 1.89 2.545


Coefficient of Determination (r2) 0.03 0.014 <0.001 0.001




The above performance statistics show that the original CALPUFF chemistry

options overestimates the observed PM nitrate concentrations by factors of 3 to 4 and that

this bias is reduced with the API revisions to the RIVAD chemistry. However, it is also

important to identify the bias at the upper end of the frequency distribution, since these

values are used in making regulatory decisions. The observed and predicted frequency

distributions can be compared using Quantile-quantile (Q-Q) plots.

Figure 1 shows the Q-Q plots for PM nitrate concentrations at the Pinedale

CASTNET site. The results are shown for the 4 chemistry cases (1a, 2a, 3a, and 4a – see

Table 1). As seen in Figure 1, the predicted frequency distributions with both the original

CALPUFF chemistry options (cases 1a and 2a) are significantly higher than the observed

frequency distribution. In contrast, the predicted frequency distribution with case 3a

(improved RIVAD chemistry using observed NH3 concentrations in the Pinedale area)

shows a much better correspondence with the observed frequency distribution. The

predicted frequency distribution with case 4a (same as case 3a except that background

NH3 concentrations are allowed to vary with height) is consistently lower than the

observed distribution. A possible reason for this underprediction is that the prescribed

vertical profile for NH3 concentrations (obtained from 2001 CMAQ outputs) may not be

representative for this CALPUFF application. Although using a vertical NH3 profile did

not work well for this application, it is a user-selectable option and we believe it is

important to retain this capability in the model for future applications where appropriate

NH3 vertical profiles may be available.

Figure 2 shows the Q-Q plots for PM nitrate concentrations at the Bridger

IMPROVE site. As in the case of the Pinedale CASTNET site, the original CALPUFF

chemistry options significantly overpredict the observed frequency distribution. The

improved chemistry option (case 3a) also overpredicts the frequency distribution but is in

much closer agreement and well within the factor of 2 limits. The results with the

improved chemistry and vertically varying NH3 profiles (case 4a) again show a lower

frequency distribution compared to the observed distribution.


Figure 1. Q-Q plot showing the comparison of observed and estimated PM nitrate

concentration distributions at the Pinedale CASTNET site. The plot also shows the 1:1 line and the factor of two limits.


Figure 2. Q-Q plot showing the comparison of observed and estimated PM nitrate

concentration distributions at the Bridger IMPROVE site. The plot also shows the 1:1 line and the factor of two limits.


The API improvements to the CALPUFF chemistry are expected to have a small

impact on CALPUFF predictions of sulfate concentrations and this is shown in Figures 3

and 4. Figure 3 is the Q-Q plot for sulfate concentrations at the Pinedale CASTNET site

and Figure 4 is the Q-Q plot for sulfate concentrations at the Bridger IMPROVE site. For

all the chemistry options, the predicted frequency distribution of sulfate concentrations is

within a factor of two of the observed distribution and concentrations at the higher end of

the distribution tend to be underestimated.

Figure 5 shows the spatial pattern of differences between the annually averaged

PM nitrate predictions from the CALPUFF simulations using the revised and original

RIVAD chemistry options. The receptor locations shown in the figure are the same as the

gridded and discrete receptors used for the SWWYTAF CALPUFF modeling study

(Earth Tech, 2001). We see from the figure that the original RIVAD formulation

produces significantly more (from 0.2 µg/m3 to 0.3 µg/m3) PM nitrate than the revised

RIVAD chemistry. There is a north-south gradient in the differences between the model

predictions with the two different chemistry options, with the larger differences in the

southern portion of the domain and the smaller differences in the northern portion of the

domain.

Considering that the observed annual average PM nitrate concentrations in the

region are of the order of 0.1 µg/m3, i.e., a factor of 2 to 3 lower than the excess PM

nitrate produced in the original CALPUFF chemistry, it is clear that the original

chemistry options in CALPUFF significantly over-predict PM nitrate.


Figure 3. Q-Q plot showing the comparison of observed and estimated PM sulfate

concentration distributions at the Pinedale CASTNET site. The plot also shows the 1:1 line and the factor of two limits.


Figure 4. Q-Q plot showing the comparison of observed and estimated PM sulfate

concentration distributions at the Bridger IMPROVE site. The plot also shows the 1:1 line and the factor of two limits.


Figure 5. Differences between CALPUFF predictions of PM nitrate (µg/m3) using the improved and original RIVAD chemistry.


CALPUFF sensitivity studies

The sensitivity studies described in this section were conducted to examine the

effect of background NH3 concentrations on model predictions of PM nitrate formation.

The studies were conducted with the FLAG configuration of CALPUFF (MESOPUFF II

chemistry option, MCHEM=1) and the API revisions to the RIVAD chemistry

(MCHEM=5). The sensitivity studies conducted are summarized in Table 5. Because the

objective of these studies was to investigate the response of CALPUFF to the background

ammonia concentrations, we did not use the POSTUTIL processor to repartition total

nitrate for any of the cases shown in Table 5.

The spatial patterns of differences between the annually averaged PM nitrate

predictions from the CALPUFF simulations using the RIVAD chemistry with the API

improvements and the MESOPUFF II chemistry are shown in Figures 6 through 8 for the

various combinations of background NH3 concentrations. For a background ammonia

concentration of 1 ppb (the default FLAG value for arid land), Figure 6 shows that the

improved RIVAD chemistry predicts annual PM nitrate concentrations that are about

0.14 to 0.22 µg/m3 lower than the MESOPUFF II chemistry predictions. Over most of the

domain, the excess PM nitrate predicted with the MESOPUFF II chemistry is about 0.14

to 0.18 µg/m3. In the southern portion of the modeling region, the MESOPUFF II

chemistry predictions are about 0.2 µg/m3 higher than the improved RIVAD chemistry

predictions.

When we use a background ammonia concentration of 0.5 ppb (default FLAG

value for forests), the predicted PM nitrate concentrations from the CALPUFF

simulations with the improved RIVAD chemistry are about 0.12 to 0.2 µg/m3 lower than

the CALPUFF predictions with MESOPUFF II chemistry, as shown in Figure 7. In the

northern portion of the domain, the MESOPUFF II chemistry predictions are about 0.12

to 0,14 µg/m3 higher than the improved RIVAD chemistry predictions, while in the

southern portion of the domain, the excess PM nitrate predicted by the MESOPUFF II

chemistry is closer to 0.2 µg/m3.


Table 5. Matrix of CALPUFF sensitivity studies to test effect of background NH3 concentrations

Case Chemistry Option Background NH3

5 MCHEM = 1 1 ppb

6 MCHEM = 1 0.5 ppb

7 MCHEM = 1 10 ppb

8 MCHEM = 5 1 ppb

9 MCHEM = 5 0.5 ppb

10 MCHEM = 5 10 ppb


Figure 6. Differences between improved RIVAD chemistry and MESOPUFF II chemistry predictions of PM nitrate (µg/m3) with a background ammonia concentration of 1 ppb.


Figure 7. Differences between improved RIVAD chemistry and MESOPUFF II chemistry predictions of PM nitrate (µg/m3) with a background ammonia concentration of 0.5 ppb.


Figure 8. Differences between improved RIVAD chemistry and MESOPUFF II chemistry predictions of PM nitrate (µg/m3) with a background ammonia concentration of 10 ppb.


Finally, with a background ammonia concentration of 10 ppb, which is the default

FLAG value for grassland, we see from Figure 8 that there are more spatial gradients (as

compared to the results with the lower background ammonia values) in the differences

between the CALPUFF predictions of PM nitrate with the improved RIVAD and

MESOPUFF II chemistry options, but the range (0.09 to 0.22 µg/m3) of the differences

over the entire domain is comparable to the simulations with 1 ppb and 0.5 ppb

background ammonia.


SUMMARY AND CONCLUSIONS

The effects of improvements to the CALPUFF chemistry (Karamchandani et al.

2008) on predictions of PM nitrate concentrations were investigated in this study using

the SWWYTAF modeling database. Annual CALPUFF simulations were conducted with

the original and improved chemistry options and model performance for PM nitrate was

evaluated. We used the latest official release of CALPUFF for the study described here.

The meteorological inputs for the model were generated from MM5 outputs provided in

the SWWYTAF database using the latest official release of the CALPUFF

meteorological preprocessor, CALMET. The creation and evaluation of the meteorology

inputs are described in a companion report (Nehrkorn and Henderson, 2008). We also

used available measurements of NH3 concentrations for the Pinedale, Wyoming area at

the Boulder Monitoring Station operated by Shell Exploration and Production Company

to provide more realistic estimates of background ammonia concentrations. In addition to

the chemistry improvements to CALPUFF described in Karamchandani et al. (2008),

minor modifications to the CALPUFF code were made in this study to allow vertically

varying background ammonia concentrations. The CALPUFF postprocessor, POSTUTIL,

was also modified to incorporate the option of using the ISORROPIA algorithm for

partitioning of total nitrate into the gas and particle phases for consistency with the

partitioning algorithm used for the improved RIVAD chemistry in CALPUFF.

CALPUFF predictions of PM nitrate with the improved RIVAD chemistry were a

factor of 2 to 3 lower than the PM nitrate predictions with the original RIVAD chemistry

and the MESOPUFF II chemistry. These results are consistent with our findings from the

CALPUFF improvement study, described in Karamchandani et al. (2008). The improved

RIVAD chemistry predictions were also in better agreement with observed PM nitrate

values at CASTNET and IMPROVE sites than the original chemistry predictions, with

significantly lower biases and errors. The predicted frequency distributions for PM nitrate

with the MESOPUFF II chemistry and original RIVAD chemistry were also significantly

larger than the observed frequency distribution. When the improved RIVAD chemistry

option was selected, the predicted PM nitrate frequency distribution was in good

agreement with the observed distribution.


We also conducted sensitivity studies with different background ammonia

concentrations corresponding to FLAG recommended values for different land use

categories. These studies were conducted using the improved RIVAD chemistry and the

MESOPUFF II chemistry. The results from these sensitivity studies showed that,

regardless of the background ammonia concentration, the PM nitrate predictions from the

improved RIVAD chemistry were always lower than the MESOPUFF II chemistry

predictions.

The results from this study indicate that PM nitrate concentrations in the

SWWYTAF modeling domain are likely to be over-predicted by factors of 2 to 3 using

the original CALPUFF chemistry options and default FLAG recommended values of

background NH3 concentrations. The API improvements to the RIVAD chemistry, which

include algorithms that are currently in use in today’s comprehensive air quality models,

result in significantly lower biases in PM nitrate predictions. Additional studies are

required for other regions of the United States to determine if the improvements to the

chemistry consistently lead to better agreement between model predictions and

observations of PM nitrate.


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