EVALUATION OF ORIGINAL AND IMPROVED VERSIONS OF CALPUFF USING THE 1995 SWWYTAF DATA BASE. Technical Report. Prepared by

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1 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 Prepared for American Petroleum Institute 1220 L Street NW Washington, DC Document CP October 2009

2 TABLE OF CONTENTS Executive Summary 1. Introduction Additional Improvements to CALPUFF Modeling System...3 Improvements to CALPUFF Ammonia Treatment...3 Improvement of CALPUFF POSTUTIL Processor CALPUFF Application and Evaluation...6 CALMET Simulations...6 CALPUFF Simulations...7 CALPUFF simulations with different chemistry options...7 CALPUFF sensitivity studies Summary and Conclusions References...29 Evaluation of CALPUFF using the 1995 SWWYTAF Data Base i

3 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 NH 3 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 NH 3 data for 2007 were provided by Shell and these data were processed to calculate seasonally averaged background NH 3 concentrations for CALPUFF. CALPUFF was also modified to optionally read monthly files of vertically varying background NH 3 concentrations. This improvement addresses the limitation of CALPUFF that it cannot account for the variation with altitude of concentrations of NH 3, 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). Evaluation of CALPUFF using the 1995 SWWYTAF Data Base ES1

4 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 NH 3 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 NH 3 concentration of 1 ppb for arid land. 2. Original CALPUFF RIVAD chemistry (MCHEM=3) using background values of NH 3 concentrations based on measurements in the Pinedale, Wyoming area. 3. Improved CALPUFF RIVAD chemistry (MCHEM=5) using background values of NH 3 concentrations based on measurements in the Pinedale, Wyoming area. 4. Improved CALPUFF RIVAD chemistry (MCHEM=5) using background values of surface NH 3 concentrations based on measurements in the Pinedale, Wyoming area with vertically varying NH 3 concentrations based on CMAQ outputs for 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- Evaluation of CALPUFF using the 1995 SWWYTAF Data Base ES2

5 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 NH 3 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 NH 3 concentrations (obtained from 2001 CMAQ outputs) may not be representative for this CALPUFF application. Although using a vertical NH 3 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 NH 3 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/m 3 to 0.3 µg/m 3 ) PM nitrate than the revised RIVAD chemistry. There was a northsouth 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/m 3, 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 NH 3 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. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base ES3

6 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 ES4

7 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 Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 1

8 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 preprocessor, 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. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 2

9 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 NH 3 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 NH 3 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 NH 3 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 NH 3 in CALPUFF is the lack of a vertical concentration profile to account for the variations in NH 3 concentrations with altitude. Ammonia is primarily emitted by surface sources, and NH 3 concentrations decrease sharply with height. Because CALPUFF uses spatially and vertically homogeneous NH 3 concentrations, it may overestimate particulate nitrate formation for emissions from elevated sources. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 3

10 To address the first limitation, we used background NH 3 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 NH 3 concentrations for the Pinedale, Wyoming area at the Boulder Monitoring Station operated by Shell Exploration and Production Company. Twice-weekly NH 3 data for 2007 were provided to AER by Shell for the CALPUFF simulations. These data were processed to calculate seasonally averaged background NH 3 concentrations for CALPUFF. To address the second limitation, we modified CALPUFF to read monthly files of vertically varying background NH 3 concentrations. The file contains the NH 3 concentration at each model layer. For the application described here, we obtained typical seasonal NH 3 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 NH 3 concentrations measured at the Boulder Monitoring Station by Shell Exploration and Production Company. Improvement of CALPUFF POSTUTIL processor In CALPUFF, the total nitrate (TNO 3 ) is partitioned into the gas (HNO 3 ) and particulate (NO 3 ) phases based on the available ammonia and the temperature and humidity dependent equilibrium relationships between HNO 3 and NO 3. 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 NH 3 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. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 4

11 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 (HNO 3 ) and particulate (NO 3 ) phases to account for ammonia limitation. In POSTUTIL, ammonia availability is computed based on receptor concentrations of total sulfate and TNO 3, not on a puff-bypuff 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 postprocessor to use the ISORROPIA model instead of the original CALPUFF thermodynamic gas/particle partitioning algorithm. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 5

12 CALPUFF APPLICATION AND EVALUATION In this section, we describe the application of different versions of CALPUFF to the SWWYTAF modeling domain for 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. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 6

13 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 NH 3 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 Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 7

14 Table 1. Matrix of CALPUFF simulations for testing chemistry options Case Chemistry Option Background NH 3 POSTUTIL for NH 3 limitation 1a MCHEM = 1 1 ppb No 1b MCHEM = 1 1 ppb Yes 2a MCHEM = 3 Seasonal values based on Pinedale measurements 2b MCHEM = 3 Seasonal values based on Pinedale measurements 3a MCHEM = 5 Seasonal values based on Pinedale measurements 3b MCHEM = 5 Seasonal values based on Pinedale measurements 4a MCHEM = 5 Seasonal surface values based on Pinedale measurements; vertical profile based on CMAQ outputs 4b MCHEM = 5 Seasonal surface values based on Pinedale measurements; vertical profile based on CMAQ outputs No Yes No Yes No Yes Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 8

15 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 NH 3 concentrations for the Pinedale, Wyoming area at the Boulder Monitoring Station operated by Shell Exploration and Production Company. The twice-weekly NH 3 data for 2007 were processed to calculate seasonally averaged background NH 3 concentrations for CALPUFF. The seasonal NH 3 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 NH 3 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 NH 3 measurements discussed above. Table 2 shows the seasonal vertical profiles of NH 3 concentrations used in the CALPUFF simulations for Case 4. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 9

16 Table 2. Seasonal vertical profiles for background NH 3 concentrations (ppb) CALPUFF Model Layer Height (m) Winter NH 3 Spring NH 3 Summer NH 3 Fall NH Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 10

17 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 underpredicts 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 NH 3 profile for the improved RIVAD chemistry simulation (Case 4a) results in significant underpredictions 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. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 11

18 Table 3a. Statistics for PM nitrate at the Pinedale CASTNET site without postprocessing with POSTUTIL to repartition total nitrate Case 1a Case 2a Case 3a Case 4a Mean Observed Value (µg/m 3 ) Mean Modeled Value (µg/m 3 ) Ratio of Means Gross Bias (µg/m 3 ) Normalized Bias Root Mean Square Error Coefficient of Determination (r 2 ) < Observed Standard Deviation Modeled Standard Deviation 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 Mean Observed Value (µg/m 3 ) Mean Modeled Value (µg/m 3 ) Ratio of Means Gross Bias (µg/m 3 ) Normalized Bias Root Mean Square Error Coefficient of Determination (r 2 ) < Observed Standard Deviation Modeled Standard Deviation Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 12

19 Table 4a. Statistics for PM nitrate at the Bridger IMPROVE site without postprocessing with POSTUTIL to repartition total nitrate Case 1a Case 2a Case 3a Case 4a Mean Observed Value (µg/m 3 ) Mean Modeled Value (µg/m 3 ) Ratio of Means Gross Bias (µg/m 3 ) Normalized Bias Root Mean Square Error Coefficient of Determination (r 2 ) <0.001 <0.001 Observed Standard Deviation Modeled Standard Deviation 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 Mean Observed Value (µg/m 3 ) Mean Modeled Value (µg/m 3 ) Ratio of Means Gross Bias (µg/m 3 ) Normalized Bias Root Mean Square Error Coefficient of Determination (r 2 ) < Observed Standard Deviation Modeled Standard Deviation Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 13

20 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 NH 3 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 NH 3 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 NH 3 concentrations (obtained from 2001 CMAQ outputs) may not be representative for this CALPUFF application. Although using a vertical NH 3 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 NH 3 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 NH 3 profiles (case 4a) again show a lower frequency distribution compared to the observed distribution. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 14

21 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. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 15

22 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. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 16

23 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/m 3 to 0.3 µg/m 3 ) 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/m 3, 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. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 17

24 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. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 18

25 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. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 19

26 Figure 5. Differences between CALPUFF predictions of PM nitrate (µg/m 3 ) using the improved and original RIVAD chemistry. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 20

27 CALPUFF sensitivity studies The sensitivity studies described in this section were conducted to examine the effect of background NH 3 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 NH 3 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/m 3 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/m 3. In the southern portion of the modeling region, the MESOPUFF II chemistry predictions are about 0.2 µg/m 3 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/m 3 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/m 3 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/m 3. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 21

28 Table 5. Matrix of CALPUFF sensitivity studies to test effect of background NH 3 concentrations Case Chemistry Option Background NH 3 5 MCHEM = 1 1 ppb 6 MCHEM = ppb 7 MCHEM = 1 10 ppb 8 MCHEM = 5 1 ppb 9 MCHEM = ppb 10 MCHEM = 5 10 ppb Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 22

29 Figure 6. Differences between improved RIVAD chemistry and MESOPUFF II chemistry predictions of PM nitrate (µg/m 3 ) with a background ammonia concentration of 1 ppb. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 23

30 Figure 7. Differences between improved RIVAD chemistry and MESOPUFF II chemistry predictions of PM nitrate (µg/m 3 ) with a background ammonia concentration of 0.5 ppb. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 24

31 Figure 8. Differences between improved RIVAD chemistry and MESOPUFF II chemistry predictions of PM nitrate (µg/m 3 ) with a background ammonia concentration of 10 ppb. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 25

32 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/m 3 ) of the differences over the entire domain is comparable to the simulations with 1 ppb and 0.5 ppb background ammonia. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 26

33 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 NH 3 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. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 27

34 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 NH 3 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. Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 28

35 REFERENCES Escoffier-Czaja, C. and J. Scire. The effects of ammonia limitation on nitrate aerosol formation and visibility impacts in Class I areas, 12 th Joint AMS Conference on the Applications of Air Pollution Meteorology with the Air and Waste Management Association, Norfolk VA, May Karamchandani, P., S.-Y. Chen, N. Kumar and M. Gupta. A comparative evaluation of two reactive puff models using power plant plumes measurements, AWMA Guideline on Air Quality Models Conference, Denver CO, April Karamchandani, P., S.-Y. Chen and C. Seigneur. CALPUFF Chemistry Upgrade, AER Final Report CP prepared for API, Washington, DC, February Morris, R.E., S. Lau and B. Koo. Evaluation of the CALPUFF chemistry algorithms, Paper No presented at the 98th Annual Meeting of the Air & Waste Management Association, Minneapolis, MN, Morris, R.E. et al. Further evaluation of the chemistry algorithms used in the CALPUFF modeling system, AWMA Guideline on Air Quality Models Conference, Denver CO, April Nehrkorn, T. and J. Henderson. Evaluation of the 1995 SWWYTAF MM5 Model Results, AER Report prepared for API, Washington, DC, August Nenes, A., C. Pilinis and S.N. Pandis. Continued development and testing of a new thermodynamic aerosol module for urban and regional air quality models, Atmos. Environ., 33, (1999). Santos, L. and R. Paine. Case study of reactive plume using CALPUFF and SCICHEM models, AWMA Guideline on Air Quality Models Conference, Denver CO, April Stelson, A.W. and J.H. Seinfeld. Relative humidity and temperature dependence of the ammonium nitrate dissociation constant, Atmos. Environ., 16, (1982). Evaluation of CALPUFF using the 1995 SWWYTAF Data Base 29