Denver Metro/North Front Range 2008 Ozone Standard Moderate Area State

Implementation Plan: Air Quality Technical Support Document (AQTSD)

Final

Prepared for:

Regional Air Quality Council 1445 Market Street, Suite 260

Denver, Colorado 80202

Prepared by:

Ramboll Environ 773 San Marin Drive, Suite 2115

Novato, California, 94998 www.ramboll-environ.com

415-899-0700

Alpine Geophysics, LLC 7341 Poppy Way

Arvada, Colorado 80007 (303) 421-2211

October 2016

06-35902E1

October 2016 i

CONTENTS

EXECUTIVE SUMMARY ......................................................................................................... 1

1.0 INTRODUCTION ............................................................................................................. 4

1.1 BACKGROUND ................................................................................................................. 4

1.2 OVERVIEW OF APPROACH ............................................................................................... 5

2.0 DEVELOPMENT OF CAMX 2011 BASE CASE INPUTS ......................................................... 8

2.1 Meteorological Inputs ..................................................................................................... 8

2.2 EMISSION INPUTS ............................................................................................................ 9

2.2.1 2011 Base Case Emission Inputs ........................................................................... 9

2.2.2 2017 Base Case Emission Inputs ......................................................................... 10

2.2.3 Summary of 2011 and 2017 Emission Modeling Results .................................... 12

2.3 OTHER PHOTOCHEMICAL MODEL INPUTS .................................................................... 14

2.4 MODEL PERFORMANCE EVALUATION ........................................................................... 16

3.0 2017 ATTAINMENT DEMONSTRATION MODELING ........................................................ 18

3.1 EPA RECOMMENDED OZONE PROJECTION PROCEDURES ............................................ 18

3.2 2017 OZONE ATTAINMENT DEMONSTRATION MODELING .......................................... 19

3.2.1 Attainment Demonstration at the Monitoring Sites .......................................... 19

3.2.2 Unmonitored Area Analysis ................................................................................ 20

3.2.3 Modeled Attainment Demonstration Conclusions ............................................. 22

3.3 MODELED WEIGHT OF EVIDENCE ANALYSIS TO SUPPORT THE ATTAINMENT DEMONSTRATION .......................................................................................................... 22

3.3.1 Ozone Projection Sensitivity Removing Flagged Exceptional Events Observations ....................................................................................................... 22

3.3.2 Ozone Projection Sensitivity to Model Performance ......................................... 23

3.3.3 Conclusions of Ozone Projection Sensitivity Analysis ......................................... 24

4.0 REFERENCES ................................................................................................................ 25

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TABLES

Table ES-1. Observed 2012-2014 ozone Design Values (DV) at the four monitoring sites in the DM/NFR NAA that failed to achieve the 75 ppb 2008 ozone NAAQS. .................................................................................................................... 1

Table ES-2. Base year observed ozone Design Values (DVB) and projected future year 2017 ozone Design Values (DVF) using MATS with the 7x7 and 3x3 projection procedures. ............................................................................................ 1

Table ES-3a. Base year observed ozone DVB (ppb) and projected 2017 ozone DVF (ppb) sensitivity analysis results for four key monitoring sites in the DM/NFR NAA using the 7x7 projection approach. ................................................. 2

Table ES-3b. Base year observed ozone DVB (ppb) and projected 2017 ozone DVF (ppb) sensitivity analysis results for four key monitoring sites in the DM/NFR NAA using the 3x3 projection approach. ................................................. 2

Table 2-1. Physics options used in the WAQS WRF Version 3.5.1 simulation of the 2011 calendar year and 36/12/4 km modeling domains. ....................................... 9

Table 2-2. Summary of 2011 and 2017 planning emissions (tons per day) in the DM/NFR NAA (Source: Moderate Area Ozone SIP for the DM and North Front Range Nonattainment Area, RAQC, 2016). ....................................... 13

Table 2-3. Percent reduction in 2017 emissions from 2011 levels. ........................................... 14

Table 2-4. CAMx model configuration used for the Denver ozone SIP modeling. .................... 15

Table 2-5. Summary of ozone model performance statistics across monitoring sites in the DM/NFR NAA for the final CAMx 2011c 4 km base case simulation. ............................................................................................................. 16

Table 3-1. Base year observed 8-hour ozone Design Values (DVB) and projected 8-hour ozone 2017 future year Design Values (DVFs) using the 3x3 and 7x7 projection approach. ...................................................................................... 19

Table 3-2. Base year (DVB) and 2017 future year (DVF) ozone Design Values (ppb) at key ozone monitors with and without including flagged exceptional event days in the 2009-2013 DVB. ........................................................................ 23

Table 3-4a. Future year ozone DVFs using different model performance evaluation (MPE) threshold criteria at key ozone monitors using standard DVBs (without excluding exceptional event days) and the 7x7 DVF projection approach.. .............................................................................................................. 24

Table 3-4b. Future year ozone DVFs using different model performance evaluation (MPE) threshold criteria at key ozone monitors using standard DVBs (without excluding exceptional event days) and the 3x3 DVF projection approach.. .............................................................................................................. 24

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FIGURES

Figure 1-1. Observed 2012-2014 ozone Design Value concentrations (ppb) in the Denver Metro/NFR NAA. ......................................................................................... 5

Figure 1-2. DM/NFR ozone SIP 36/12/4 km CAMx modeling domains. ...................................... 7

Figure 1-3. Locations of ozone and NOX monitoring sites operating in 2011 from the AQS and CASTNet networks within the DM/NFR NAA and vicinity. ....................... 7

Figure 3-1. Unmonitored area analysis using MATS for Denver 2017 ozone attainment demonstration showing interpolated base year DVBs (top left), 2017 projected DVFs (top right) and their differences (DVF-DVB; bottom).................................................................................................................. 21

October 2016 1

EXECUTIVE SUMMARY

The Denver Metropolitan and North Front Range (DM/NFR) ozone Nonattainment Area (NAA) failed to attain the March 2008 75 ppb ozone National Ambient Air Quality Standard (NAAQS) by 2015 due to 2012-2014 ozone Design Values (DVs) above the 75 ppb ozone NAAQS at four key monitoring sites (Table ES-1).

Table ES-1. Observed 2012-2014 ozone Design Values (DV) at the four monitoring sites in the DM/NFR NAA that failed to achieve the 75 ppb 2008 ozone NAAQS.

Site ID Site Name 2012-2014 DV (ppb)

RFNO Rocky Flats North 82

CHAT Chatfield 81

NREL National Renewable Energy Lab. 80

FTCW Fort Collins West 78

The DM/NFR NAA was “bumped up” to a Moderate NAA that required the region to demonstrate attainment of the ozone NAAQS by 2017. The Comprehensive Air Quality Model with extensions (CAMx) photochemical grid model (PGM) was used in the DM/NFR NAA ozone attainment demonstration modeling. CAMx was first applied for a 2011 base case on a Colorado domain with a 4 km grid resolution using meteorological conditions for the ozone season (May – August 2011). A Model Performance Evaluation (MPE) was conducted that compared predicted and observed ozone concentrations. Emissions were then projected to 2017 and CAMx was applied for 2017 emission conditions using the same 2011 May – August meteorological conditions.

The Modeled Attainment Test Software (MATS) was used using the 7x7 and 3x3 projection approaches to project base year ozone Design Values (DVB) to estimate future year 2017 ozone Design Values (DVF). The 7x7 and 3x3 projection approaches refer to the array of 4 km grid cells around the monitoring where modeling results are selected to make the 2017 ozone projections. The 2017 projected DVFs using both the MATS 7x7 and 3x3 projection approaches are shown in Table ES-2. The maximum 2017 DVF using the 7x7 projection approach is 75.8 ppb that is below 76.0 ppb indicating that ozone levels in the DM/NFR NAA would be at or below the 75 ppb ozone NAAQS in 2017. However, using the 3x3 projection approach, the 2017 DVF at the CHAT and RFNO monitoring sites are 76.2 ppb that are 0.3% above the ozone NAAQS.

Table ES-2. Base year observed ozone Design Values (DVB) and projected future year 2017 ozone Design Values (DVF) using MATS with the 7x7 and 3x3 projection procedures.

Design Value Projection Approach RFNO CHAT+ NREL FTCW

Current DVB (ppb) 80.3 80.7 78.7 78.0

2017 DVF (ppb) 7x7 75.8 75.7 74.3 70.9

2017 DVF (ppb) 3x3 76.2 76.2 75.4 71.5

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Additional sensitivity analysis of the 2017 ozone projection procedures was conducted as shown in Table ES-3. When observed data that were flagged as being exceptional event days were removed from the DVB calculation, the maximum projected 2017 ozone DVF was below the NAAQS using both the 7x7 (74.3 ppb) and 3x3 (74.7 ppb) projection approaches. Additional 2017 ozone DVF projection sensitivity analysis was performed that required the modeling days used in the 2017 DVF projections to meet model performance evaluation (MPE) criteria that the modeled and observed MDA8 ozone at a monitoring site agree within 20%, 15% and 10% of each other. Using the 20% and 15% MPE requirement all 2017 DVFs achieve the ozone NAAQS using the 7x7 projection approach. However, when the 10% MPE requirement is invoked using the 7x7 projection approach, both RFNO and CHAT estimate a 76.2 ppb 2017 DVF that is 0.4% above the NAAQS. And when the 20%, 15% and 10% MPE requirement is used with the 3x3 projection approach, the maximum 2017 DVFs are 0.3% to 0.9% above the NAAQS. Note that by invoking more stringent MPE requirement, lower modeled ozone days are used in the 2017 projections that have less local ozone to control so the model is less responsive to emission controls. Also note if the flagged exceptional event days were removed in the DVB calculation, all projected 2017 DVFs would be below the NAAQS using both the 7x7 and 3x3 projection approaches and all MPE projection sensitivity tests.

Table ES-3a. Base year observed ozone DVB (ppb) and projected 2017 ozone DVF (ppb) sensitivity analysis results for four key monitoring sites in the DM/NFR NAA using the 7x7 projection approach.

Design Value NREL CHAT NREL FTCW

Remove Flagged Exceptional Events from DVB on Potential Exceptional Event Days

Current DVB 78.7 78.7 77.7 76.3

2017 DVF 74.3 73.9 73.4 69.4

Model Performance Evaluation (MPE) Requirement

Current DVB 80.3 80.7 78.7 78.0

2017 DVF 20% MPE 75.7 75.7 74.3 72.1

2017 DVF 15% MPE 75.6 75.8 74.7 72.2

2017 DVF 10% MPE 75.7 76.8 74.9 72.4

Table ES-3b. Base year observed ozone DVB (ppb) and projected 2017 ozone DVF (ppb) sensitivity analysis results for four key monitoring sites in the DM/NFR NAA using the 3x3 projection approach.

Design Value NREL CHAT NREL FTCW

Remove Flagged Exceptional Events from DVB on Potential Exceptional Event Days

Current DVB 78.7 78.7 77.7 76.3

2017 DVF 74.4 74.7 74.5 70.0

Model Performance Evaluation (MPE) Requirement

Current DVB 80.3 80.7 78.7 78.0

2017 DVF 20% MPE 76.1 76.1 75.6 72.5

2017 DVF 15% MPE 76.1 76.3 75.8 72.6

2017 DVF 10% MPE 76.6 76.3 75.9 73.0

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In conclusion, the MATS 7x7 2017 projection approach used with the CAMx 2011 and 2017 4 km base case modeling results projects that the DM/NFR NAA would be at or below the 2008 75 ppb ozone NAAQS in 2017, but estimated 2017 DVF values that were 0.4% (0.3 ppb) above the NAAQS using the MATS 3x3 projection approach. Additional sensitivity analysis on the projection procedures mostly supports the modeled attainment demonstration, but under some of the sensitivity tests the maximum ozone DVF was projected to be above the NAAQS in 2017.

Ozone observations in the DM/NFR NAA are influenced by exceptional events, including wildfires and stratospheric ozone intrusions. When such potential exceptional events are removed when calculating base year ozone DVBs, all projected future year ozone DVFs are below the ozone NAAQS using both the 7x7 and 3x3 projection approaches. Observed ozone in the DM/NFR NAA exhibits a high degree of year-to-year variability with some years more conducive to ozone formation than others. Thus, whether ozone levels in the DM/NFR NAA are below the ozone NAAQS in 2017 will depend in part on the weather conditions.

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1.0 INTRODUCTION

1.1 BACKGROUND

This report is the Air Quality Technical Support Document (AQTSD) for the Denver Metropolitan Area and North Front Range (DM/NFR) 2017 8-hour ozone State Implementation Plan (SIP) attainment demonstration modeling. The DM/NFR 2017 ozone SIP AQTSD is a concise summary of the modeling analysis used to demonstrate that the DM/NFR ozone nonattainment area (NAA) will attain the March 2008 0.075 ppm (75 ppb) ozone National Ambient Air Quality Standard (NAAQS) by 2017. Details on the DM/NFR 2017 ozone SIP attainment demonstration modeling are provided in companion reports (Ramboll Environ and Alpine Geophysics, 2016a,b). Addressing attainment of the new October 2015 0.070 ppm (70 ppb) ozone NAAQS will occur in the future after EPA designates ozone nonattainment areas by October 2017.

In July 2012, the DM/NFR was classified as a Marginal ozone NAA under the 2008 75 ppb ozone NAAQS with an attainment year of 2015. Based on observed ozone concentrations during 2012-2014, four monitoring sites failed to attain the ozone NAAQS so the region was bumped up to a Moderate ozone NAA with an attainment date of 2017. Figure 1-1 displays the 2012-2014 observed ozone Design Values in the DM/NFR NAA. The four key monitoring sites that failed to attain the ozone NAAQS based on 2014-2016 observations are:

Rocky Flats North (RFNO);

Chatfield (CHAT);

National Renewable Energy Laboratory (NREL); and

Fort Collins West (FTCW).

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Figure 1-1. Observed 2012-2014 ozone Design Value concentrations (ppb) in the Denver Metro/NFR NAA.

1.2 OVERVIEW OF APPROACH

The procedures for conducting the Denver Metro/NFR NAA 2017 ozone attainment demonstration modeling are contained in an August 2015 Modeling Protocol (Ramboll Environ and Alpine Geophysics, 2015). The Denver Metro/NFR ozone attainment demonstration modeling was carried out by a contracting team consisting of Ramboll Environ US Corporation and Alpine Geophysics, LLC under contract to the Denver Regional Air Quality Council (RAQC). Working closely with the RAQC on the Denver 2017 ozone SIP development were the Colorado Department of Public Health and Environment (CDPHE) Air Pollution Control Division (APCD), Denver Regional Council of Governments (DRCOG), Colorado Department of Transportation (CDOT), the Northern Front Range Metropolitan Planning Organization (NFRMPO), U.S. Environmental Protection Agency Region 8 (EPA R8) and other local agencies.

The DM/NFR 2017 ozone attainment demonstration modeling highly leveraged a photochemical grid model (PGM) modeling database developed by the Western Air Quality Study (WAQS1) that was available through the Intermountain West Data Warehouse (IWDW2).

1 The WAQS is also called the Three State Air Quality Study (3SAQS)

2 http://views.cira.colostate.edu/tsdw/

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The 2017 attainment demonstration modeling used the following models:

Weather Research Forecast (WRF) meteorological model (Skamarock et al., 2004; 2005; 2006).

Sparse Matrix Operator Kernel Emissions (SMOKE) modeling system (Coats 1995; UNC, 2015).

Model for Emissions of Gases and Aerosol from Nature (MEGAN) (Guenther et al, 2014; Sakulyanontvittaya et al., 2012);

Comprehensive Air-Quality Model with extensions (CAMx) PGM model (Ramboll Environ, 2015); and

Supporting pre- and post-processing programs.

The procedures for conducting the 2017 attainment demonstration modeling followed EPA’s PGM modeling guidance (EPA, 2007; 2014) and were as follows:

Development of a Modeling Protocol (Ramboll Environ and Alpine Geophysics, 2015):

o Selection of May – August 2011 ozone season modeling period;

Definition of 36 CONUS, 12 km WESTUS; and 4 km Colorado modeling domains (Figure 1-2); and

o Selection of WRF/CAMx/SMOKE/MEGAN modeling system.

Processing of the WAQS WRF 36/12/4 km meteorological model output to generate CAMx meteorological inputs for the 36/12/4 km domains and May-August 2011 modeling period.

Processing of the 2011 National Emissions Inventory (NEI) and CDPHE/APCD 2011 emissions to generate CAMx 2011 36/12/4 km base case emission inputs using SMOKE, MEGAN and other emission tools.

Perform CAMx 2011 base case modeling:

o Perform diagnostic sensitivity tests and model improvements;

o Conduct final CAMx 2011c1 base case simulation;

o Perform Model Performance Evaluation (MPE); and

o Prepare CAMx 2011 base case and MPE technical report (Ramboll Environ and Alpine Geophysics, 2016a).

Processing of 2017 emissions for CAMx 36/12/4 km modeling domains.

Conduct CAMx 2017 base case simulation:

o Project base year ozone Design Values to the 2017 future year following EPA’s modeling guidance (EPA, 2007; 2014);

o Demonstrate that the DM/NFR NAA would attain the 2008 ozone NAAQS by 2017 (see Figure 1-3 for monitoring site locations);

o Perform additional Weight of Evidence (WOE) analysis to support the 2017 ozone attainment demonstration; and

o Prepare CAMx 2017 attainment demonstration modeling technical report (Ramboll Environ and Alpine Geophysics, 2016b).

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Figure 1-2. DM/NFR ozone SIP 36/12/4 km CAMx modeling domains.

Figure 1-3. Locations of ozone and NOX monitoring sites operating in 2011 from the AQS and CASTNet networks within the DM/NFR NAA and vicinity.

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2.0 DEVELOPMENT OF CAMX 2011 BASE CASE INPUTS

This section summarizes the procedures used to develop the meteorological, emissions, and air quality inputs to the CAMx model for the 8-hour ozone modeling. The DM/NFR 2011 36/12/4 km CAMx modeling database is based on a database developed by the Western Air Quality Study (WAQS). Thus, more details on the DM/NFR 2011 CAMx database development are provided in the WAQS documentation that includes:

Modeling Protocol for 2011 Emissions & Air Quality Modeling (UNC and ENVIRON, 20143).

Weather Research Forecast 2011 Meteorological Model Application/Evaluation (UNC and ENVIRON, 20154).

Emissions Modeling Report – Simulation Years 2008 and 2011. (Adelman and Baek, 20155).

CAMx Photochemical Grid Model Performance Evaluation Simulation Year 2011 reports (Adelman, Shanker, Yang and Morris, 20146; 20167).

2.1 Meteorological Inputs

Meteorological inputs for the 2017 DM/NFR ozone SIP modeling were based on the WAQS application of the Weather Research and Forecasting (WRF8) model (UNC and ENVIRON, 2015). The Advanced Research WRF (ARW) core of WRF modeling system was used by WAQS. Table 2-1 summarizes the physics options used in the WAQS 2011 WRF simulation.

The WAQS 2011 WRF Application/Evaluation report (UNC and ENVIRON, 2015) contains details on the WRF model performance evaluation (MPE) within the WAQS Three-State 4 km modeling domain as well separately within the states of Colorado, Utah and Wyoming. The Denver 2011 base case modeling and MPE reports present highlights of the WAQS WRF evaluation for the state of Colorado (Ramboll Environ and Alpine Geophysics, 2016a).

The WAQS WRF Application/Evaluation report (UNC and ENVIRON, 2015) concluded that the WAQS 2011 WRF simulation reproduced the observed meteorological conditions in the Three-State (CO, UT and WY) 4 km modeling domain as good or better than past meteorological model applications so was suitable for conducting photochemical grid modeling.

The Denver 2017 ozone SIP modeling study re-processed the WAQS 36/12/4 km WRF output using the latest (v4.0 released May 2013) WRFCAMx9 processor to generate meteorological inputs for the CAMx 36/12/4 km domains. Thus, the Denver CAMx 2011 meteorological inputs

3 http://vibe.cira.colostate.edu/wiki/Attachments/Modeling/3SAQS_2011_WRF_MPE_v05Mar2015.pdf

4 http://vibe.cira.colostate.edu/wiki/Attachments/Modeling/3SAQS_2011_WRF_MPE_v05Mar2015.pdf

5 http://vibe.cira.colostate.edu/wiki/Attachments/Emissions/3SAQS_Emissions_Modeling_Report_v18Feb2015.pdf

6 http://views.cira.colostate.edu/tsdw/Documents/

7 http://vibe.cira.colostate.edu/wiki/Attachments/Modeling/WAQS_Base11b_MPE_Draft_21Jan2016.doc

8 http://wrf-model.org/index.php

9 Available at www.camx.com

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are slightly different than those used by the WAQS, although CAMx produced essentially identical model performance over the 4km modeling domain.

Table 2-1. Physics options used in the WAQS WRF Version 3.5.1 simulation of the 2011 calendar year and 36/12/4 km modeling domains.

WRF Treatment Option Selected Notes

Microphysics Thompson A scheme with ice, snow, and graupel processes suitable for high-resolution simulations.

Longwave Radiation RRTMG Rapid Radiative Transfer Model for GCMs includes random cloud overlap and improved efficiency over RRTM.

Shortwave Radiation RRTMG Same as above, but for shortwave radiation.

Land Surface Model (LSM) NOAH Two-layer scheme with vegetation and sub-grid tiling.

Planetary Boundary Layer (PBL) scheme

YSU Yonsie University (Korea) Asymmetric Convective Model with non-local upward mixing and local downward mixing.

Cumulus parameterization Kain-Fritsch in the 36 km and 12 km domains. None in the 4 km domain.

4 km can explicitly simulate cumulus convection so parameterization not needed.

Analysis nudging Nudging applied to winds, temperature and moisture in the 36 km and 12 km domains

Temperature and moisture nudged above PBL only

Observation Nudging Nudging applied to surface wind and temperature only in the 4 km domain

moisture observation nudging produces excessive rainfall

Initialization Dataset 12 km North American Model (NAM)

2.2 EMISSION INPUTS

CAMx-ready emission inputs were developed for the 2011 base and 2017 future years.

2.2.1 2011 Base Case Emission Inputs

The 2011 base case emissions inventories developed for the 2017 DM/NFR 8-hour ozone modeling study were based on several sources:

For Colorado, the CDPHE/APCD provided 2011 emissions for all anthropogenic emission source categories except on-road mobile sources and NOX/SO2 emissions for sources with Continuous Emissions Monitors (CEM) were based on observed hourly CEM data.

For on-road mobile source emissions within the DM/NFR NAA, local traffic demand model (TDM) output from the Denver Regional Council of Governments (DRCOG) and North Front Range Metropolitan Planning Organization (NFRMPO) provided link-level vehicle activity data that were used with emissions factors from the MOVES2014 mobile

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source emissions model using Colorado-specific vehicle fleet distribution data and hourly gridded WRF 2011 meteorology.

For on-road mobile source emissions in the remainder of Colorado outside of the NAA, SMOKE-MOVES was used with Colorado-specific MOVES2014 emission factor lookup table and county-level vehicle activity data from the CDPHE/APCD and hourly WRF meteorological data.

On-road mobile source emissions inputs outside of Colorado were based on the SMOKE-MOVES model using the EPA national MOVES2014 emission factors and hourly gridded WRF meteorological data.

2011 emissions from oil and gas sources within Colorado and the Rocky Mountain states were based on the 2011 emission estimates developed by the WAQS using the WRAP phase III methodology with the 2011 NEI oil and gas emissions used for the remainder of the domain.

For other anthropogenic emissions sources, Colorado emissions were provided by CDPHE/APCD and outside of Colorado the 2011 NEI emissions with enhancements developed by the WAQS (Adelman and Baek, 2015) were used.

Biogenic emissions were day-specific hourly gridded based on 2011 WRF model-derived temperatures using the latest MEGAN10 biogenic emissions model Version 2.10 (Guenther et al., 2014).

2011 emissions from open-land burning, including wildfires, prescribed burns and agricultural burning were based on the Joint Fire Sciences Program PMDETAIL11 project (Moore et al., 2015).

Lightning NOX emissions were generated using the 2011 WRF information on locations of convective activity.

The 2011 emissions were processed using SMOKE to generate hourly gridded speciated inputs for the 36/12/4 km modeling domains, May – August 2011 modeling period and the CAMx PGM. Details on the development of the CAMx 2011 base case emission inputs can be found in Ramboll Environ and Alpine Geophysics (2016a).

2.2.2 2017 Base Case Emission Inputs

The procedures for developing the 2017 emission inputs were the same as used for the 2011 emissions only using 2017 anthropogenic emissions from CDPHE/APCD for Colorado and EPA’s 2017 emission projections for anthropogenic emissions outside of Colorado. Natural emissions and emissions from Mexico and Canada were held constant at 2011 emission levels.

Within Colorado, most of the 2017 emissions were provided by the CDPHE/APCD. The exceptions to this were on-road mobile sources and natural sources. Special considerations were also taken with the hourly temporal variations in the emissions from Electrical Generating

10

http://lar.wsu.edu/megan/ 11

https://pmdetail.wraptools.org/

October 2016 11

Units (EGUS) using the 2017 annual emissions provided by CDPHE/APCD and 2011 hourly variations from the observed 2011 CEM data.

The same procedures as used for generating 2011 on-road mobile source emissions within the DM/NFR NAA, remainder Colorado outside of the NAA and outside of Colorado were used for 2017 only using 2017 fleet distribution and other information in the MOVES2014 emission factor simulation and 2017 vehicle activity data for the SMOKE-MOVES modeling. In particular, within the DM/NFR NAA detailed link-level vehicle activity data was used from the DRCOG and NFRMPO 2017 TDM modeling.

Both the 2011 and 2017 oil and gas point source emissions inventories were developed in the same manner. Actual 2011 reported Air Pollutant Emission Notice (APEN) data was used for the 2011 emissions inventory and actual 2014 reported APEN data was grown by the projected increase in oil production from 2014 to 2017 to estimate the 2017 emissions inventory. For oil and gas area source emissions, the 2011 inventory was derived from the 3–State Study/ENVIRON 2011 update which was based on the 2006 Independent Petroleum Association of Mountain States (IPAMS) Survey grown to 2011 based upon production data from the Colorado Oil and Gas Conservation Commission (COGCC). The 2017 area source emissions projections were provided by five of the top six producers, which took into account estimated future production, well count, and technology advancements as a result of state and federal rules. This data was summed by each category and scaled-up for the rest of the producers using either 2017 production estimates or well count estimates depending on the category.

Condensate tanks were developed through a separate process. The 2011 condensate tank inventory for VOC emissions was initially developed in 2014 as one of the requirements for a Marginal ozone nonattainment area. The emissions inventory was based on production data from the COGCC using a VOC emission factor of 13.7 pounds per barrel (lbs/bbl), which was the approved emission factor for condensate tanks in the nonattainment area. This emission factor was derived by the APCD in 2002 based on a study by Lesair Environmental12 on flashed gas from condensate oil samples. For 2017, the CDPHE/APCD and RAQC met with petroleum industry representatives to discuss the development of new, more representative emission factors based on the advancements in technology in recent years. Based upon these discussions, these producers agreed to provide site specific information related to drilling, fracing, production, etc. for both 2014 and 2017 to aid in the development of a much more detailed inventory than had ever historically been crafted. From the top six producers, 2014 actual data was provided, which helped in developing emission factors based on well orientation (i.e. horizontal and vertical) and stage of separation (i.e. tankless, 1 stage, 2 stage, and 3 stage). Five of the six companies also provided estimated 2017 production data by well, which was applied to established emission factors to determine future year emissions. Estimates for rest of the producers resembled those for the top six companies, with future production being primarily from 1 and 2 stages of separation at horizontal wells. More

12

E&P Storage Tank VOC Emission Study.” Prepared for Colorado Oil & Gas Association. Prepared by Lesair Environmental, Inc.

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information is provided in the 2011 and 2017 Oil and Gas Emissions Inventory Development Technical Support Document (TSD; CDPHE and RAQC, 2016).

The 2017 anthropogenic emissions were processed using the SMOKE modeling system to generate the CAMx-ready 2017 hourly gridded speciated emission inputs.

2.2.3 Summary of 2011 and 2017 Emission Modeling Results

Table 2-2 summarizes the planning emissions inventories within the DM/NFR NAA for the 2011 and 2017 emission scenarios with Table 2-3 showing their percent differences. Several source categories (i.e., on-road mobile, EGUs, biogenic, lightning NOX, windblown dust and fires) use day-specific hourly varying emissions in the photochemical modelling, so the day-specific modelling inventories differ slightly from the planning inventories given in Table 2-2.

Between 2011 and 2017 total VOC emissions in the NAA are projected to go down by approximately 170 tons per day (TPD) (-25% total emissions and -33% anthropogenic emissions only). Most (74%) of these VOC emission reductions come from the O&G sector (-126 TPD). On-road mobile VOC reductions is next most important sector (-39 TPD or 23% of the reduction) followed by non-road mobile (-14 TPD or 8% of the reduction).

Total NOX emissions in the NAA are projected to go down 86 TPD (-26%) between 2011 and 2017 despite the fact that O&G NOX emissions are expected to increase by 24 TPD. Large reductions in on-road mobile (-69 TPD), non-road mobile (-21 TPD) and point source (-20 TPD) NOX emissions more than compensate for the increase in O&G NOX to result in a net -26% reduction in total NOX emissions across the NAA.

CO emissions are reduced 298 TPD (-18%) in the NAA between 2011 and 2017, which is primarily (90%) due to reductions in CO emissions from on-road mobile sources.

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Table 2-2. Summary of 2011 and 2017 planning emissions (tons per day) in the DM/NFR NAA (Source: Moderate Area Ozone SIP for the DM and North Front Range Nonattainment Area, RAQC, 2016).

2017 2011

Description

VOC NOX CO VOC NOX CO

Oil and Gas Sources

Point Sources Subtotal

16.3 20.6 19.7 14.8 18.1 17.0

Condensate Tanks Subtotal

78.7 0.6 2.3 216.0 1.1 2.3

Area Sources Subtotal

59.0 44.6 31.4 48.9 22.2 12.9

TOTAL

154.0 65.8 53.4 279.7 41.4 32.2

Point Sources (EGU and Non-Oil and Gas)

Electric Generating Units (EGU)

0.4 19.2 2.9 0.7 39.7 3.6

Point (Non-Oil and Gas)

28.0 20.9 14.4 25.9 21.0 14.1

TOTAL

28.4 40.1 17.3 26.5 60.7 17.7

Area Sources (Non-Oil and Gas)

TOTAL

67.5 - 1.6 60.6 - 1.4

Non-Road Mobile Sources

TOTAL

44.3 54.9 759.7 58.2 75.9 800.2

On-Road Mobile Sources

Light-Duty Vehicles

52.4 50.3 538.6 90.0 102.5 812.2

Medium/Heavy-Duty Vehicles

2.6 23.0 16.2 3.7 39.6 20.6

TOTAL

55.0 73.3 554.7 93.7 142.0 832.8

Total Anthropogenic Emissions

349.2 234.0 1,386.6 518.8 320.0 1,684.4

Total Biogenic Sources

170.5 6.1 21.6 170.5 6.1 21.6

Total Nonattainment Area Emissions

519.7 240.1 1,408.2 689.3 326.1 1,706.0

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Table 2-3. Percent reduction in 2017 emissions from 2011 levels.

2017-2011 (%)

Description

VOC NOX CO

Oil and Gas Sources

Point Sources Subtotal 10.4% 14.2% 15.6%

Condensate Tanks Subtotal -63.6% -47.7% -2.1%

Area Sources Subtotal 20.5% 100.4% 143.8%

TOTAL -45.0% 58.9% 65.5%

Point Sources (EGU and Non-Oil and Gas)

Electric Generating Units (EGU) -37.1% -51.8% -20.8%

Point (Non-Oil and Gas) 8.3% -0.4% 2.0%

TOTAL 7.1% -34.0% -2.7%

Area Sources (Non-Oil and Gas)

TOTAL 11.4%

11.0%

Non-Road Mobile Sources

TOTAL -23.8% -27.7% -5.1%

On-Road Mobile Sources

Light-Duty Vehicles -41.8% -50.9% -33.7%

Medium/Heavy-Duty Vehicles -31.2% -41.8% -21.5%

TOTAL -41.4% -48.4% -33.4%

Total Anthropogenic Emissions -32.7% -26.9% -17.7%

Total Biogenic Sources 0.0% 0.0% 0.0%

Total Nonattainment Area Emissions -24.6% -26.4% -17.5%

2.3 OTHER PHOTOCHEMICAL MODEL INPUTS

Table 2-4 summarizes the CAMx configuration used in the DM/NFR 2017 ozone SIP modeling. The latest version of CAMx at the time the modeling was initiated (Version 6.2, released March 2015) was used in the Denver ozone modeling. The model was configured to predict both ozone and PM species. CAMx was set up to operate using just the 4 km Colorado domain as a stand-alone application. Running on just the 4 km domain alone allows for more efficient model simulations and faster turn-around for sensitivity and emission control modeling within Colorado. CAMx 4 km boundary conditions were extracted from CAMx three dimensional outputs of the 12 km WESTUS domain results from the CAMx 36/12 km domain simulations. CAMx was also set up to run on the 36/12/4 km domains using linked two-way grid nesting, but the ozone attainment demonstration modeling was run on the 4 km Colorado domain using on-way grid nesting with the 36/12 km domain simulations.

September 2017 15

Table 2-4. CAMx model configuration used for the Denver ozone SIP modeling.

Science Options Configuration Details

Model Codes CAMx V6.2 – March 2015 Release Latest version of CAMx available at time of modeling (CAMx v6.3 released in April 2016)

Horizontal Grid Mesh 36/12/4 km

36 km grid 148 x 112 cells 36/12/4 two-way grid nesting

12 km grid 224 x 227 cells 36/12 km domains match the WAQS

4 km grid 173 x 128 cells Also set up 4 km domain as a one-way nest

Vertical Grid Mesh 25 vertical layers, defined by WRF Layer 1 thickness ~24 m. Model top at ~19 km AGL

Grid Interaction 36/12/4 km two-way nesting 4 km also set up as a one-way nest

36/12/4 km to examine regional/international transport 4 km alone to examine local source contributions

Initial Conditions 9 day spin-up on 4 km grid IC off of 36/12 km run; first high ozone day May 10, 2011

Boundary Conditions 2011 MOZART GCM for 36 km CONUS domain

Cap MOZART dust concentrations as not day-specific

Emissions

Baseline Emissions Processing

SMOKE, SMOKE-MOVES2014, MEGAN 2011 NEI plus CDPHE/APCD for Colorado

Sub-grid-scale Plumes Plume-in-Grid for major NOX sources

Chemistry

Gas Phase Chemistry CB6r2 Latest chemical reactions and kinetic rates (Yarwood et al., 2010)

Meteorological Processor WRFCAMx v4.4 Publicly released in April 2016

Horizontal Diffusion Spatially varying K-theory with Kh grid size dependence

Vertical Diffusion CMAQ-like Kz Minimum Kz 0.1 to 1.0 m2/s

Diffusivity Lower Limit Kz-min = 0.1 to 1.0 m2/s Depends on percent urban land use fraction

Deposition Schemes

Dry Deposition Zhang dry deposition scheme (Zhang et. al, 2001; 2003)

Wet Deposition CAMx -specific formulation rain/snow/graupel

Numerics

Gas Phase Chemistry Solver

Euler Backward Iterative(EBI) EBI fast and accurate solver

Vertical Advection Scheme

Implicit scheme w/ vertical velocity update

Horizontal Advection Scheme

Piecewise Parabolic Method (PPM) scheme

Colella and Woodward (1984)

Integration Time Step Wind speed dependent ~0.5-1 min (4-km), 1-5 min (12-km), 5-15 min (36-km)

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2.4 MODEL PERFORMANCE EVALUATION

The model performance evaluation (MPE) for the CAMx DM/NFR 2011c base case simulation focused on ozone model performance within the DM/NFR NAA and vicinity. The Denver CAMx 2011c base case model configuration, inputs and model performance are similar to the WAQS CAMx 2011b base case. The WAQS conducted a comprehensive model evaluation for ozone and particulate matter, their precursor and product species, visibility and deposition that is contained in the WAQS MPE reports (Adelman, Shanker, Yang and Morris, 2014; 2016) with additional MPE products available on the Intermountain West Data Warehouse (IWDW13).

As recommended by EPA (1991; 2007; 2014; 2015; Simon, Baker and Philips, 2012), the Denver model performance evaluation used a combination of quantitative and qualitative evaluation approaches using model performance statistics measures as well as graphical displays of model performance that includes time series, scatter, soccer and spatial plots of model performance. Table 2-5 displays the hourly and maximum daily average 8-hour ozone (MDA8) ozone model performance statistics across the DM/NFR NAA during the May – August 2011 modeling period and compares them with ozone model performance goals (EPA, 1991). The performance statistics were calculated using no cut-off and a 60 ppb observed ozone cut-off concentration, as recommended by EPA (2007; 2014; 2015; Simon, Baker and Phillips, 2012) in order to focus the ozone evaluation for high observed ozone conditions. Using no observed ozone cut-off concentration, the model tends to overestimate the observed hourly (+12%) and MDA8 (+6%) ozone concentrations. Whereas, when the 60 ppb observed ozone cut-off concentration is used, the hourly (-7%) and MDA8 (-3%) ozone tends to be underestimated; in both cases the model achieves the ≤±15% ozone bias performance goal. The model’s error performance statistics (9% - 25%) also achieve the ≤35% ozone error performance goal.

Table 2-5. Summary of ozone model performance statistics across monitoring sites in the DM/NFR NAA for the final CAMx 2011c 4 km base case simulation.

Cut-Off Bias Error

EPA Goal -- ≤±15% ≤35%

Hourly O3 None 11.8% 25.4%

Hourly O3 60 ppb -7.2% 12.5%

MDA8 O3 None 6.3% 13.3%

MDA8 O3 60 ppb -2.5% 8.8%

The Denver ozone SIP 2011 base case and MPE report (Ramboll Environ and Alpine Geophysics, 2016a) provides details on the CAMx 2011c final base case simulation ozone model performance. In particular, it examined the spatial distribution of ozone model performance using spatial bas plots and hourly modeled ozone plots with superimposed observations, scatter and time series plots of predicted and observed hourly and MDA8 ozone concentrations. Particular emphasis was placed on the MDA8 ozone model performance for the ten highest modeled ozone days that are used to make future year ozone Design Value projections as recommended in EPA’s latest modeling guidance (EPA, 2014). The top 10 13

http://views.cira.colostate.edu/TSDW/

October 2016 17

modeled days at the four key monitoring sites (RFNO, CHAT, NREL and FTCW) with 2011 MDA8 ozone that are within 20%, 15% and 10% of the observed MDA8 ozone were also identified and used in the attainment demonstration weight of evidence (WOE) analyses. Additional model performance results, including ozonesonde, VOC measurement, particulate matter, visibility and deposition that follows EPA’s MPE check list (EPA, 2015), are available in the WAQS 20011 CAMx/CMAQ MPE reports (Adelman, Shaker, Yang and Morris,2014; 2016).

The conclusion of the MPE was that it was appropriate to use the DM/NFR 2011 modeling platform for air quality planning.

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3.0 2017 ATTAINMENT DEMONSTRATION MODELING

The results from the CAMx 2011 and 2017 4 km base case were used to project 8-hour ozone Design Values from 2011 to 2017 for comparison with the ozone NAAQS.

3.1 EPA RECOMMENDED OZONE PROJECTION PROCEDURES

The recommended procedures for making future year ozone Design Value projections are outlined in EPA’s latest draft modeling guidance (EPA, 2014), which are very similar to the procedures used in the Denver 2008 ozone SIP attainment demonstration modeling (Morris et al., 2008; 2009) that followed EPA’s previous modeling guidance (EPA, 2007). These procedures use the modeling results in a relative fashion to scale the base year 8-hour ozone Design Value (DVB) to project the future year ozone Design Value (DVF) using model-derived scaling factors called Relative Response Factors (RRFs):

DVF = DVB x RRF

DVB: EPA guidance recommends the base year ozone Design Value (DVB) be defined as the three year average of ozone Design Values (DVs) centered on the base year, which for the 2011 modeling year is as follows:

DVB = (DV2009-2011 + DV2010-2012 + DV2011-2013) / 3

RRF: The RRFs are defined as the ratio of the average of the future year (FY) to base year (BY) MDA8 ozone concentrations near the monitoring site for the 10 highest MDA8 ozone modeling days:

RRF = Σ Model-O3FY / ΣModel-O3BY

Near the Monitor: The latest draft EPA guidance (EPA, 2014) defines “near the monitoring site” as the maximum BY modeled MDA8 ozone (Model-O3BY) within a 3x3 array of grid cells centered on the monitoring site. In the previous modeling guidance (EPA, 2007), the array of grid cells near the monitoring site was grid cell dependent with a 7x7 array used for a 4 km grid resolution.

Modeled Ozone Concentration: The latest draft EPA guidance recommends using the FY modeling results (Model-O3FY) from the same grid cell in the array (e.g., 3x3 or 7x7) cells around the monitoring site as used for Model-O3BY. This is slightly different that EPA’s 2007 guidance that recommends using the maximum modeled MDA8 ozone within the array of grid cells for both the Model-O3BY and Model-O3FY.

EPA has developed the Modeled Attainment Demonstration Software (MATS; Abt, 2014) that implemented EPA’s 2014 modeling guidance approach for making FY ozone Design Value projections. The 2014 guidance version of MATS with the 7x7 projection procedure was used to make the FY ozone projections in the Denver 2017 ozone SIP.

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3.2 2017 OZONE ATTAINMENT DEMONSTRATION MODELING

2017 ozone DVF projections were made using the MATS tool (EPA, 2014) with the maximum MDA8 ozone modeling results within a 7x7 4 km array of grid cells around the monitoring site and the top 10 modeled base year (BY) ozone days. The 7x7 array of grid cells provides better model performance with the observed MDA8 ozone on the top 10 modeled ozone days than when a 3x3 array of grid cells is used as recommended in EPA’s December 2014 draft modeling guidance (EPA, 2014). A 7x7 array is also the recommended approach in the current EPA modeling guidance (EPA, 2007) as well as what was used in the Denver 2008 ozone SIP future year (FY) ozone DVF projections (Morris et al., 2008; 2009).

3.2.1 Attainment Demonstration at the Monitoring Sites

Table 3-1 displays the observed 2009-2013 BY ozone Design Values (DVB) and the projected FY 2017 ozone Design Values (DVF) using MATS with the 3x3 and 7x7 projection approaches. Using the 7x7 projection approach the maximum projected 2017 8-hour ozone DVF is 75.8 ppb at the Rocky Flats North (RFNO) monitoring site, which is below 76.0 so attains the March 2008 ozone NAAQS. However, when the 3x3 projection approach is used the projected 2017 8-hour ozone DVFs at RFNO and CHAT are 76.2 ppb that exceeds the 2008 ozone NAAQS. All other projected 2017 ozone DVFs are below the 2008 ozone NAAQS suggesting that ozone levels in 2017 will likely be below the ozone NAAQS.

Table 3-1. Base year observed 8-hour ozone Design Values (DVB) and projected 8-hour ozone 2017 future year Design Values (DVFs) using the 3x3 and 7x7 projection approach.

AIRS ID Station County DVB

(ppb) DVF 3x3

(ppb) DVF 7x7

(ppb)

DM/NFR NAA

80350004 CHAT Douglas 80.7 76.2 75.7

80590006 RFNO Jefferson 80.3 76.2 75.8

80590011 NREL Jefferson 78.7 75.4 74.3

80690011 FTCW Larimer 78.0 71.5 70.9

80013001 WELB Adams 76.0 72.2 73.8

80050002 HIGH Arapahoe 76.7 72.9 72.3

80050006 AURE Arapahoe 73.5 68.8 68.8

80130011 SOBC Boulder 74.7 70.7 70.5

80310014 CARR Denver 71.0 68.8 68.1

80310025 DASH Denver 65.0 63.0 61.8

80590002 ARVA Jefferson 74.0 71.9 70.2

80590005 WELC Jefferson 75.7 72.2 71.3

80590013 ASNP Jefferson 74.5 69.9 69.8

80690007 RMNP Larimer 75.7 71.6 71.0

80690012 RIST Larimer 71.0 65.6 65.0

80691004 FTCO Larimer 68.7 63.3 62.4

81230009 WELD Weld 74.7 70.3 68.9

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3.2.2 Unmonitored Area Analysis

EPA’s 8-hour ozone projection procedure also includes an unmonitored area analysis procedure (EPA, 2007; 2014) that has been codified in MATS. The unmonitored area analysis uses the future-year 8-hour ozone Design Value projection procedure applied to each grid cell in the modeling domain. In this procedure, the monitor-based DVBs are interpolated to each grid cell in the modeling domain. This interpolation scheme uses the modeled concentration gradients so that the gridded DVBs may have some locations that are higher or lower than any of the observed DVBs at the monitoring sites. RRFs are then obtained for each grid cell in the modeling domain using essentially the same approach as used for the monitored ozone projections, only RRFs are based on the model estimates within each grid cell rather than near a grid cell (e.g., 3x3 or 7x7) as done for the projections at the monitor.

The top left panel in Figure 3-1 displays the interpolated 2009-2013 8-hour ozone DVBs using the MATS unmonitored area analysis. Interpolated base year ozone DVBs in excess of 76 ppb are estimated to the south, west and northwest of Denver stretching to Fort Collins and then west of Fort Collins. The projected DVFs for the 2017 future case (Figure 3-1, top right) have greatly removed all areas with the DVFs in excess of 76 ppb. The peak value is 75.9 ppb near the Jefferson/Boulder County border (i.e., near RFNO monitoring site).

The differences in the 2011 and 2017 unmonitored area analysis Design Values (DVF – DVB) are shown in the bottom panel of Figure 3-1. The largest ozone decreases are in Eastern Larimer and Western Weld Counties and West and South of Denver, with relatively small ozone changes in the core DM area. The relatively larger ozone reductions in western Weld and eastern Larimer Counties are likely due in part to the large reductions in oil and gas VOC emissions in this region. The reductions in ozone in the Denver Metro area are most likely mainly due to the reductions in mobile source NOX and VOC emissions due to use of cleaner vehicles, although reductions in emissions from non-road engines and point sources also likely contributed to the ozone reductions. However, in the core of the Denver Metro area, the NOX emission reductions have competing effects on ozone concentrations with ozone reductions due to less precursors participating in photochemistry and ozone increases due to less ozone titration from the fresh NO emissions and a reduction in the inhibition effect NOX has on ozone formation under higher NOX concentration conditions, as can occur in an urban core area.

October 2016 21

Figure 3-1. Unmonitored area analysis using MATS for Denver 2017 ozone attainment demonstration showing interpolated base year DVBs (top left), 2017 projected DVFs (top right) and their differences (DVF-DVB; bottom).

October 2016 22

3.2.3 Modeled Attainment Demonstration Conclusions

Using the 7x7 projection approach the projected 2017 DVFs at all monitoring sites within and near the DM/NFR NAA are below 76.0 ppb so demonstrate that that NAA will attain the 2008 75 ppb ozone NAAQS by 2017. However, using the 3x3 projection approach the modelled ozone concentration estimates are less responsive to the emissions reductions between 2011 and 2017 so that the 2017 projected DVF at RFNO and CHAT are 76.2 ppb, which is 0.4% above the 2008 ozone NAAQS. It is anticipated that the 2017 modeled ozone concentrations could be lower in either projection approach as discussed below in the supplemental WOE analysis such that ozone concentrations in 2017 will likely be below the 2008 ozone NAAQS. The unmonitored area analysis shows that the 2017 projected DVFs at all grid cells within the Colorado 4 km domain would also be below the ozone NAAQS..

3.3 MODELED WEIGHT OF EVIDENCE ANALYSIS TO SUPPORT THE ATTAINMENT DEMONSTRATION

Although the 2017 ozone DVF projections using the Denver 2011 and 2017 base case 4 km modeling results suggests that ozone concentrations in the DM/NFR ozone NAA will be at or below the 75 ppb ozone standard by 2017, additional Weight of Evidence (WOE) analyses were conducted to help assess the uncertainties in the future year ozone projection approach as described below.

3.3.1 Ozone Projection Sensitivity Removing Flagged Exceptional Events Observations

During the 2009 through 2013 period used to determine the base year ozone DVB, several observed MDA8 ozone concentrations were flagged by CDPHE monitoring staff as potentially being influenced by exceptional events, which make the observation data of questionable appropriateness for use in air quality planning. In 2010 and 2011, days were flagged at some monitoring sites due to being impacted by stratospheric ozone intrusion events. And in 2012 and 2013, days were flagged at some monitoring sites due to being impacted by wildfire smoke events. Observed ozone concentrations for a total of 13 days during the 5-year DVB period were flagged by the CDPHE as being potentially influenced by exceptional events. The CDPHE did not prepare a formal exceptional event document for submission to EPA to approve the removal of these days from the regulatory record since they it would not affect the attainment classification of the area. However, their removal would reduce the DVBs and resultant 2017 DVF projections so the sensitivity of the 2017 DVF projections to removing ozone observations from these flagged days at the four key monitoring sites was examined. Table 3-2 presents the sensitivity of the DVBs and DVFs to the removal of the flagged data. Removal of the flagged data reduces the design values by approximately 1 to 2 ppb. In particular, the maximum projected 2017 DVF at any monitor was reduced from 75.8 ppb including to 74.3 ppb excluding the flagged exceptional event days using the 7x7 projection approach and from 76.2 ppb to 74.7 ppb using the 3x3 projection approach.

October 2016 23

Table 3-2. Base year (DVB) and 2017 future year (DVF) ozone Design Values (ppb) at key ozone monitors with and without including flagged exceptional event days in the 2009-2013 DVB.

Monitor DVB Including

All Days DVB Excluding Flagged Days

DVF Including All Days

DVF Excluding Flagged Days

7x7 Projection Approach

CHAT 80.7 78.7 75.7 73.9

RFNO 80.3 78.7 75.8 74.3

NREL 78.7 77.7 74.3 73.4

FTCW 78.0 76.3 70.9 69.4

3x3 Projection Approach

CHAT 80.7 78.7 76.2 74.4

RFNO 80.3 78.7 76.2 74.7

NREL 78.7 77.7 75.4 74.5

FTCW 78.0 76.3 71.5 70.0

3.3.2 Ozone Projection Sensitivity to Model Performance

As was shown in the 2011 base case modeling and Model Performance Evaluation (MPE) report (Ramboll Environ and Alpine Geophysics, 2016a), the ozone performance of the CAMx model varied day-to-day and across monitoring sites. Ozone model performance was also better for the latter half of the ozone season (July and August) than the first half (May and early June). Overall, the ozone performance of the 2011 base case was deemed sufficiently good to use the model for assessing future ozone planning, but the model performed better at certain monitors on some days than others.

To assess the impact of model performance on the model’s estimation of future ozone DVFs, a MPE WOE analysis was conducted where the modeling results were only used on days when the predicted and observed MDA8 ozone at a monitoring site achieved a certain level of model performance. That is, instead of selecting the top 10 modeled ozone days near the monitoring site from the 2011 base case regardless how the model performed, we selected the top 10 highest ozone modeled days considering only those days in which the predicted and observed MDA8 ozone concentrations at the monitoring site agreed with each other to a certain threshold of model performance. The ozone projection MPE analysis examined three thresholds of model performance that required the predicted and observed MDA8 ozone at a monitoring site for a day to be within 10%, 15% and 20% of each other. The results of the 2017 ozone DVF projections using the MPE threshold criteria at the key ozone monitors is presented in Table 3-4. The model shows modeled attainment (i.e., DVF values less than 76.0) at all monitors for all MPE thresholds except for the Chatfield monitor (CHAT) using the 10% MPE threshold (76.8 ppb) using the 7x7 projection approach (Table 3-4a). Using the 3x3 projection approach, for all four MPE thresholds (none, 20%, 15% and 10%), the projected 2017 DVFs at CHAT and RFNO range from 76.1 to 76.6 ppb, which is 0.2% to 0.9% above the 2008 ozone NAAQS.

October 2016 24

Table 3-4a. Future year ozone DVFs using different model performance evaluation (MPE) threshold criteria at key ozone monitors using standard DVBs (without excluding exceptional event days) and the 7x7 DVF projection approach..

AIRS ID Station

10% MPE Threshold DVF (ppb)

15% MPE Threshold DVF (ppb)

20% MPE Threshold DVF (ppb)

No MPE Threshold DVF (ppb)

80350004 CHAT 76.8 75.8 75.7 75.7

80590006 RFNO 75.7 75.6 75.7 75.8

80590011 NREL 74.9 74.7 74.3 74.3

80690011 FTCW 72.4 72.2 72.1 70.9

Table 3-4b. Future year ozone DVFs using different model performance evaluation (MPE) threshold criteria at key ozone monitors using standard DVBs (without excluding exceptional event days) and the 3x3 DVF projection approach..

AIRS ID Station

10% MPE Threshold DVF (ppb)

15% MPE Threshold DVF (ppb)

20% MPE Threshold DVF (ppb)

No MPE Threshold DVF (ppb)

80350004 CHAT 76.6 76.1 76.1 76.2

80590006 RFNO 76.3 76.3 76.1 76.2

80590011 NREL 75.9 75.8 75.6 75.4

80690011 FTCW 73.0 72.6 72.5 71.5

3.3.3 Conclusions of Ozone Projection Sensitivity Analysis

Use of MATS with the 7x7 projection resulted in a maximum projected 2017 ozone DVF of 75.8 ppb that is below 76.0 ppb so achieves the 2008 ozone NAAQS, whereas use of the 3x3 projection approach both the CHAT and RFNO monitoring sites had projected 2017 DVFs (76.2 ppb) that were 0.4% above the ozone NAAQS. Additional WOE analysis using the 7x7 project approach that removed flagged days observations from potential exceptional event days (74.3 ppb) or invoking a 20% (75.7 ppb) or 15% (75.8 ppb) MPE criteria when selecting the top 10 modeled ozone days for making projections also corroborate that ozone levels in the DM/NFR NAA will be at or below the ozone NAAQS by 2017. However, using the MATS with the 3x3 projection approach with the 20% (76.1 ppb), 15% (76.3 ppb) or 10% (76.6 ppb) MPE threshold or using the 7x7 projection approach invoking the 10% MPE threshold (76.8 ppb) in selecting modeling days for making projections resulted in projected 2017 DVFs that were from 0.2% to 0.9% above the NAAQS. Although if the flagged exceptional event days were removed from the DVB calculation, all 2017 DVFs would be below the NAAQS. These results illustrate the uncertainties in making future year DVF projections. The DM/NFR NAA exhibits a high degree of year-to-year variability in observed ozone concentrations that is highly dependent on meteorological conditions. Although the modeling suggests that the emission reduction between 2011 and 2017 should be sufficient for the region to achieve the 2008 ozone NAAQS by 2017, adverse meteorological conditions that are highly conducive to ozone formation in 2017 could potentially result in higher values than predicted in future years.

October 2016 25

4.0 REFERENCES

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Adelman, Z. and B. Baek. 2015. Three-State Air Quality Modeling Study Emissions Modeling Report – Simulation Years 2008 and 2011. University of North Carolina at Chapel Hill. February 18. (http://vibe.cira.colostate.edu/wiki/Attachments/Emissions/3SAQS_Emissions_Modeling_Report_v18Feb2015.pdf).

Adelman, Z. U. Shanker, D. Yang and R. Morris. 2016. Three-State Air Quality Modeling Study CAMx Photochemical Grid Model Draft Model Performance Evaluation Simulation Year 2011. University of North Carolina at Chapel Hill and ENVIRON International Corporation, Novato, CA. January (http://vibe.cira.colostate.edu/wiki/Attachments/Modeling/WAQS_Base11b_MPE_Draft_21Jan2016.doc).

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Morris R., T. Sakulyanontvittaya, E. Tai, D. McNally and C. Loomis. 2008. 2010 Ozone Attainment Demonstration Modeling for the Denver 8-Hour Ozone State Implementation Plan Control Strategy. ENVIRON International Corporation, Novato, California. Prepared for Denver Regional Air Quality Council (RAQC), Denver, Colorado. September 22. (http://www.ozoneaware.org/documents/modeling/Denver_2010ControlStrat_Draft_Sep22_2008.pdf)

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RAQC. 2016. Moderate Area Ozone SIP for the Denver Metro and North Front Range Area. Regional Air Quality Council (RAQC) and Colorado Department of Health and Environment (CDPHE) Air Pollution Control Division (APCD). June 30.

Sakulyanontvittaya, T., G. Yarwood and A. Guenther. 2012. Improved Biogenic Emission Inventories across the West. ENVIRON International Corporation, Novato, CA. March 19. (http://www.wrapair2.org/pdf/WGA_BiogEmisInv_FinalReport_March20_2012.pdf).

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