# Modeling Protocol for the OTC CALGRID

Screening-Level Modeling Platform for the Evaluation of Ozone, Fine Particulate Matter, and

Regional Haze

December 29, 2009

New Hampshire Department of Environmental Services

Air Resources Division 29 Hazen Drive Concord, NH 033021

1.0 Introduction
The Ozone Transport Commission (OTC) has sponsored the development of a photochemical modeling system to assist states in t he preparation of their State Implementation Plans (SIPs) for attaining the eight -hour National Ambient Air Quality Standard (NAAQS) for ozone. The Community Multisca le Air Quality Model (CMAQ) is the basis of the OTC SIP-quality modeling platfo rm that is being used in states’ eighthour ozone attainment SIPs. CMAQ was also an impor tant analysis tool in states' SIPs for regional haze. It was used to assess source co ntributions to regional haze and to evaluate the effectiveness of control strategies fo r the mitigation of future visibility impairment.
Running the CMAQ modeling platform is very resource intensive and usually requires the use of a Unix-based operating system. Therefor e, the CMAQ modeling that is being done in support of the eight-hour ozone and regiona l haze SIPs is limited to a handful of modeling centers that have the necessary expertise and computer resources. Similarly, because of the substantial effort, time, and storag e space required to run the SIP-quality CMAQ platform, it is impractical to assess a large number of run scenarios.
Consequently, OTC sponsored the development of an a dditional grid-based photochemical modeling platform that states could u se as a screening tool for running additional emissions control strategies or performi ng sensitivity runs. The requirements for this additional modeling platform were that it be flexible, portable, and easily accessible to agencies that wish to use it. In thi s manner, a larger number of run scenarios could be modeled, allowing states to scre en a wider variety of potential control strategies.
The California Grid Model (CALGRID) was selected as the basis of this screening platform. CALGRID is a grid-based photochemical mo deling platform that is designed to be run in a Windows environment. CALGRID has re cently undergone a series of improvements and enhancements, with the most recent version being designated CALGRID Version 2.45. CALGRID Version 2.45 is bein g used as the screening platform for the OTC eight-hour ozone attainment SI Ps. CALGRID is also being used in screening-level analyses for fine particulate matte r (particulate matter with an equivalent aerodynamic diameter of 2.5 microns or less, also k nown as PM2.5) and regional haze. This document outlines the manner in which CALGRID will be used in these efforts.

2.0 Platform Description and Model Inputs
To make the CALGRID modeling platform the best poss ible tool to supplement the SIPquality CMAQ platform, it was decided that CALGRID would be run on the same modeling domain using a common set of model inputs. All boundary conditions, meteorology, and emissions inputs were the same as those used for the SIP-quality CMAQ platform. Processing programs were written to convert these inputs into a format useable by the CALGRID model. The modeling domain and model inputs are described in the sections below.2 2.1 Modeling Domain
The OTC CMAQ/CALGRID modeling domain was set up on a Lambert Conic Conformal projection and covers the eastern United States and parts of Canada. The southwest corner of the domain was set at 264 km, - 888 km and the northeast corner was set to 2328 km, 1176 km. A 12-km grid cell resolut ion was used with 172 grid cells in the east-west direction and 172 grid cells in the n orth-south direction. For the vertical grid definition, 22 layers were used in the CMAQ si mulations. For CALGRID, the processing programs that were used to reformat the boundary conditions, meteorology, and emissions mapped the 22 layers used for CMAQ to a total of nine vertical layers for the CALGRID modeling. The OTC CMAQ/CALGRID modelin g domain is shown in Figure 1.
Figure 1: Map of the OTC CMAQ/CALGRID Modeling Doma in



## 0 20 40 60 80 100 120 140 160 0 20 40 60 80 100 120 140 1603 2.2 Boundary Conditions
For the OTC CMAQ modeling, boundary conditions were derived from GEOS-Chem (Goddard Earth Observing System) global atmospheric simulations by running CMAQ on the 36-km continental grid. For the CALGRID pla tform, initial conditions, side boundary conditions, and top layer concentrations w ere derived from the CMAQ boundary conditions files by means of a processing program. This program mapped the 22 vertical layers used in the CMAQ simulations to the nine layers used with CALGRID. It produced the necessary side boundary file and to p layer concentration file for each modeled episode day. It also produced an initial c onditions file for each episode day processed; these files could be retained so that th e user could begin a CALGRID simulation on any desired episode day.

2.3 Meteorology
The meteorological modeling that was done in suppor t of the OTC CMAQ SIP-quality modeling platform was performed by the University o f Maryland (UMD) in conjunction with staff at the New York State Department of Envi ronmental Conservation (NYSDEC). UMD used the Penn State/National Center for Atmosph eric Research 5 th Generation Mesoscale Model (MM5) Version 3.6 to generate year 2002 meteorological inputs. The details of this work are described in NYSDEC’s Tech nical Support Document TSD-1a, Meteorological Modeling Using Penn State/NCAR 5 th Generation Mesoscale Model (MM5), February 1, 2006. For the CALGRID modeling platform, a processing p rogram was used to interpolate the MM5 outputs to the CALG RID modeling grid and map them to CALGRID’s vertical layer structure. QA/QC plots were generated during the processing to ensure the accuracy of the interpolat ed data. Figures 2 through 4 show example QA/QC plots from the re-formatting of the m eteorological data.4

Figure 2: QA/QC Plot of Humidity Data

August 6, 2002 Hour 12 Humidity Layer 1 CALGRID 2.45 Modeling Domain

Humidity (ppm)

NHDES 7/18/06

400 600 800 1000 1200 1400 1600 1800 2000 2200 LCC Easting (km) -800 -600 -400 -200 0 200 400 600 800 1000LCC Northing (km)

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000

Figure 3: QA/QC Plot of Temperature Data

August 6, 2002 Hour 18 Temperature Layer 1 CALGRID 2.45 Modeling Domain

Temp (K)

NHDES 7/23/06

400 600 800 1000 1200 1400 1600 1800 2000 2200 LCC Easting (km) -800 -600 -400 -200 0 200 400 600 800 1000LCC Northing (km)

0 280 282 284 286 290 292 294 296 298 300 302 304 306 308 3105

Figure 4: QA/QC Plot of Rainfall Data

August 12, 2002 Hour 17

Rain CALGRID 2.45 Modeling Domain

Rain (inches)

NHDES 7/17/06

400 600 800 1000 1200 1400 1600 1800 2000 2200 LCC Easting (km) -800 -600 -400 -200 0 200 400 600 800 1000LCC Northing (km)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

2.4 Emissions
The Regional Planning Organizations (RPOs) and thei r contractors were responsible for preparing modeling emissions inventories for use in the OTC CMAQ SIP-quality modeling. The RPOs prepared a set of accurate, up- to-date, and quality assured emissions inventories for the 2002 Base Case and th ree future years, 2009, 2012, and 2018. For the future years, emissions inventories were prepared for two emissions control scenarios: one reflecting those emissions c ontrols that are in place or likely to be in place by 2009, and another that reflects additio nal control strategies that may be needed to demonstrate attainment with the eight-hou r ozone standard. The first scenario is referred to as “On The Books/On The Way” (OTB/OT W), and the second is referred to as “Beyond On The Way” (BOTW). A third scenario, s ometimes referred to as "Best & Final", was developed to assess additional potentia lly reasonable control programs aimed at improving visibility. All of these inventories were broken down into the following general emissions source categories: biogenic, poin t, area, non-road mobile, and on-road mobile.
Using these inventories, NYSDEC and the other model ing centers, the Virginia Department of Environmental Quality (VADEQ) and Nor theast States for Coordinated Air Use Management (NESCAUM), generated the necessa ry emissions inputs for CMAQ using the Sparse Matrix Operator Kernel Emissions ( SMOKE) model. For the CALGRID modeling effort, the pre-merged SMOKE emiss ions files were obtained from the modeling centers and re-formatted for input int o EMSPROC, the emissions pre-6

processor for the CALGRID modeling system. The pre -merged SMOKE files that were obtained from the modeling centers were broken down into the biogenic, point, area, nonroad, and on-road categories mentioned earlier. Th ese files by component were then converted for use with EMSPROC, thus giving users o f the CALGRID modeling system the flexibility to analyze a wide variety of emissi ons control strategies. QA/QC plots were generated during the re-formatting of the emis sions data to ensure that an accurate modeling inventory was generated for the CALGRID pl atform. Example emissions QA/QC plots are shown in Figures 5 through 8.

Figure 5: QA/QC Plot of Surface Source CO Emissions7

Figure 6: QA/QC Plot of Surface Source VOC Emission s

Figure 7: QA/QC Plot of Point Source NOx Emissions8

Figure 8: QA/QC Plot of Point Source SO2 Emissions

3.0 CALGRID Model Outputs
A variety of post-processing programs have been dev eloped to assist users of the CALGRID modeling platform in visualizing and tabula ting model results. These postprocessors are designed to take the hourly concentr ation output generated by the CALGRID model and produce statistical results for d esired averaging periods and geographic areas. CALGRID can also generate output suitable for plotting with standard graphical packages such as the Center for Ocean-Lan d-Atmosphere Studies’ Grid Analysis and Display System (GrADS) or Golden Softw are’s SURFER.
As with the CMAQ SIP-quality modeling platform, it was decided that the CALGRID screening results should be used in a relative mann er. Ratios of model-predicted base case and future scenario concentrations would be ap plied to current monitored ozone design values to generate estimated future design v alues (DVFs) for the future-year scenario of interest. It is these DVFs that are us ed in the determination of attainment.

4.0 Episode Selection
For the CMAQ SIP-quality ozone modeling analysis, t he five-month ozone season of 2002 was selected for modeling because it met EPA’s episode selection criteria of containing episode days that are meteorologically r epresentative of typical high ozone exceedance days and that are severe enough that any control strategy that is predicted to reach attainment on those days would reach attainme nt on other less severe days.9

Similarly, the CALGRID screening platform was run f or the summer period from May 15 to September 15, 2002 for the 2002 Base Case and 20 09 OTB/OTW and BOTW scenarios. However, running the full summer episod e, even with the CALGRID screening platform, is very time-consuming and reso urce intensive. In order to increase production and maximize the number of scenarios tha t can be evaluated with the CALGRID screening platform, it was decided that sho rter modeling episodes would be selected that adequately reproduced the results obt ained by running the full summer episode. An extensive analysis was undertaken to r un a number of shorter summer episodes and compare the resulting predicted DVFs w ith those obtained for the full summer episode. The 2002 Base Case and 2009 OTB/OT W scenarios were used in this evaluation. It was found that the combined DVFs ca lculated from the July 6 to July 23, 2002 and July 30 to August 16, 2002 modeling episod es best represented those from the full summer episode (note that July 6 th and July 30 th are considered to be model warm-up days).

5.0 CALGRID Model Performance Evaluation
A qualitative and quantitative assessment was made of the CALGRID screening platform’s ability to reproduce observed ozone, PM2 .5, and visibility. To assess its performance, the CALGRID screening platform was run for the 2002 Base Case for the July 30 to August 16, 2002 modeling episode. Predi cted maximum eight-hour ozone concentrations at monitor locations were compared a gainst observations for 2002. Comparisons were also made of predicted vs. observe d PM2.5 and regional haze. All of these comparisons are described below and shown in the subsequent figures.
Figure 9 shows the relative bias between maximum ei ght-hour ozone observations and predicted CALGRID eight-hour ozone concentrations f or the 2002 Base Case for the episode from July 30 through August 12. Figures 10 through 13 show plots of predicted 2002 Base Case ozone concentrations against 2002 ob servations for selected episode days. Although the CALGRID model has a tendency to over-predict, especially in the urban core areas of northern Virginia, Maryland, so utheastern Pennsylvania, New York City and Boston, it does a reasonable job of reprod ucing the pattern of the 2002 maximum ozone observations.
Figures 14 through 19 present predicted maximum 24- hour PM2.5 concentrations (top of each figure) against observations (bottom of each f igure) for selected episode days. Included for comparison are corresponding maximum P M2.5 concentrations predicted with the Regional Modeling System for Aerosols and Dispersion (REMSAD). Observations include measurements taken at monitors in the IMPROVE network (Interagency Monitoring of Protected Visual Environ ments). These figures show that CALGRID reasonably reproduces the pattern of maximu m daily PM2.5 observations and provides a similar pattern of predictions as REMSAD , which is state-of-the-science model for predicting particulate matter and other p ollutants.10

Figure 20 compares predicted haze index (bottom of figure) with observations at IMPROVE sites (top of figure) for a relatively clea r episode day (May 14th). Figure 21 compares predicted haze index with observations for a relatively hazy day (August 12th).
Lastly, Figures 22 through 28 show time-series plot s of predicted vs. observed haze index for selected days and Class I areas.
Figure 9: Relative Bias Between Maximum 8-hour Ozon e Concentrations Predicted with CALGRID and Observations 

# Relative Bias (CALGRID minus Observed)

-20 to -15 -15 to -10 -10 to -5 -5 to -2 -2 to 2 2 to 5 5 to 10 10 to 15 15 to 20 20 to 50 

### Based on Model Data for July 30 - August 12, 2002 2002 Base Case Emissions versus Observed Data

Green Circles are where CALGRID Underpredicts
Red Triangles are where CALGRID Overpredicts

NHDES 11/21/0611

Figure 10: Comparison of Maximum 8-hour Ozone Conce ntrations for August 11, 2002

Maximum 8-Hour Ozone Concentrations CALGRID - 2002 Base Case OTC Modeling Domain - August 11, 2002

Ozone Concentrations (PPB)

NHDES 11/21/06 0 to 40 40 to 50 50 to 60 60 to 70 70 to 80 80 to 90 90 to 100 100 to 150

Figure 11: Comparison of Maximum 8-hour Ozone Conce ntrations for August 12, 2002

Maximum 8-Hour Ozone Concentrations CALGRID - 2002 Base Case OTC Modeling Domain - August 12, 2002

Ozone Concentrations (PPB)

NHDES 11/21/06 0 to 40 40 to 50 50 to 60 60 to 70 70 to 80 80 to 90 90 to 100 100 to 15012

Figure 12: Comparison of Maximum 8-hour Ozone Conce ntrations for August 13, 2002

Maximum 8-Hour Ozone Concentrations CALGRID - 2002 Base Case OTC Modeling Domain - August 13, 2002

Ozone Concentrations (PPB)

NHDES 11/27/06 0 to 40 40 to 50 50 to 60 60 to 70 70 to 80 80 to 90 90 to 100 100 to 150

Figure 13: Comparison of Maximum 8-hour Ozone Conce ntrations for August 14, 2002

Maximum 8-Hour Ozone Concentrations CALGRID - 2002 Base Case OTC Modeling Domain - August 14, 2002

Ozone Concentrations (PPB)

NHDES 11/27/06 0 to 40 40 to 50 50 to 60 60 to 70 70 to 80 80 to 90 90 to 100 100 to 15013

Figure 14: Comparison of Maximum 24-hour PM2.5 Conc entrations for July 31, 2002 

# July 31,

2002

REMSAD
1 5 10 15 20 25 30 35 40 50 60

PM2.5 Concentrations (UG/M3)

0 - 4

5 - 9

10 - 14

15 - 19

20 - 24

25 - 29

30 - 34

35 - 39

40 - 49

50 - 59

>= 60 

### CALGRID Observations

IMPROVE14

Figure 15: Comparison of Maximum 24-hour PM2.5 Conc entrations for August 3, 2002 

# August 3,

2002

REMSAD
1 5 10 15 20 25 30 35 40 50 60

PM2.5 Concentrations (UG/M3)

0 - 4

5 - 9

10 - 14

15 - 19

20 - 24

25 - 29

30 - 34

35 - 39

40 - 49

50 - 59

>= 60 

### CALGRID Observations

IMPROVE15

Figure 16: Comparison of Maximum 24-hour PM2.5 Conc entrations for August 6, 2002 

# August 6,

2002

REMSAD
1 5 10 15 20 25 30 35 40 50 60

PM2.5 Concentrations (UG/M3)

0 - 4

5 - 9

10 - 14

15 - 19

20 - 24

25 - 29

30 - 34

35 - 39

40 - 49

50 - 59

>= 60 

### CALGRID Observations

IMPROVE16

Figure 17: Comparison of Maximum 24-hour PM2.5 Conc entrations for August 9, 2002 

# August 9,

2002

REMSAD
1 5 10 15 20 25 30 35 40 50 60

PM2.5 Concentrations (UG/M3)

0 - 4

5 - 9

10 - 14

15 - 19

20 - 24

25 - 29

30 - 34

35 - 39

40 - 49

50 - 59

>= 60 

### CALGRID Observations

IMPROVE17

Figure 18: Comparison of Maximum 24-hour PM2.5 Conc entrations for August 12, 2002 

# August 12,

2002

REMSAD
1 5 10 15 20 25 30 35 40 50 60

PM2.5 Concentrations (UG/M3)

0 - 4

5 - 9

10 - 14

15 - 19

20 - 24

25 - 29

30 - 34

35 - 39

40 - 49

50 - 59

>= 60 

### CALGRID Observations

IMPROVE18

Figure 19: Comparison of Maximum 24-hour PM2.5 Conc entrations for August 15, 2002 

# August 15,

2002

REMSAD
1 5 10 15 20 25 30 35 40 50 60

PM2.5 Concentrations (UG/M3)

0 - 4

5 - 9

10 - 14

15 - 19

20 - 24

25 - 29

30 - 34

35 - 39

40 - 49

50 - 59

>= 60 

### CALGRID Observations

IMPROVE19

Figure 20: Comparison of Observed and Predicted Haz e Index for May 14, 2002

Observed Haze Index at IMPROVE Monitors for May 14, 2002 (in Deciviews)

NHDES 10/5/07 CALGRID 172x172 Modeling Domain Haze Index

(in Deciviews)
0 to 7 7 to 14 14 to 19 19 to 30 30 to 37 37 to 39 39 to 50 Visual Range

(in Kilometers)
400 to 200 200 to 100 100 to 60 60 to 20 20 to 10 10 to 8 8 to 4 Great Gulf: Obs = 3 Pred = 0

Brigantine: Obs = 19 Pred = 18

Shenandoah: Obs = 16 Pred = 18 Lye Brook: Obs = 3 Pred = 7

CALGRID Predicted Haze Index at Ozone Monitors for May 14, 2002 (in Deciviews)

NHDES 10/5/07 CALGRID 172x172 Modeling Domain Haze Index

(in Deciviews)
0 to 7 7 to 14 14 to 19 19 to 30 30 to 37 37 to 39 39 to 50 Visual Range

(in Kilometers)
400 to 200 200 to 100 100 to 60 60 to 20 20 to 10 10 to 8 8 to 420

Figure 21: Comparison of Observed and Predicted Haz e Index for August 12, 2002

Observed Haze Index at IMPROVE Monitors for August 12, 2002 (in Deciviews)

NHDES 10/5/07 CALGRID 172x172 Modeling Domain Haze Index

(in Deciviews)
0 to 7 7 to 14 14 to 19 19 to 30 30 to 37 37 to 39 39 to 50 Visual Range

(in Kilometers)
400 to 200 200 to 100 100 to 60 60 to 20 20 to 10 10 to 8 8 to 4 Acadia NP: Obs = 28 Pred = 27

Great Gulf: Obs = 33 Pred = 28

Brigantine: Obs = 25 Pred = 27

Shenandoah: Obs = 33 Pred = 25 Lye Brook: Obs = 33 Pred = 25

CALGRID Predicted Haze Index at Ozone Monitors for August 12, 2002 (in Deciviews)

NHDES 10/5/07 CALGRID 172x172 Modeling Domain Haze Index

(in Deciviews)
0 to 7 7 to 14 14 to 19 19 to 30 30 to 37 37 to 39 39 to 50 Visual Range

(in Kilometers)
400 to 200 200 to 100 100 to 60 60 to 20 20 to 10 10 to 8 8 to 421

Figure 22: Comparison of Predicted and Observed Haz e Index at Acadia National Park, ME

Predicted vs. Observed Haze Index at Acadia NP, ME for Selected Dates

0 5 10 15 20 25 30 35

1 /1 4/2 0 02

2 /1 0/2 0 02 3/15/

2 00 2 4/1 4 / 2 00 2

5 /1 4 / 2 0 02 6/16/200

2 7/13/200

2

8 / 15/2 0 02

9 / 14/2002

1

0 /17 /2 0 0 2

1 1/ 1 6/2 0 02

1 2 /13 /2 0 02Deciviews Predicted

Observed '

Figure 23: Comparison of Predicted and Observed Haz e Index at Great Gulf Wilderness Area, NH

Predicted vs. Observed Haze Index at Great Gulf Wil derness Area, NH for Selected Dates

-5 0 5 10 15 20 25 30 35

1/14/2002 2/1

0 /2002 3/15/2

0 0 2 4/14/200

2 5/14/2002

6

/16/2 0 0 2 7/13/2 0 0 2

8 / 1 5/2 0 0 2

9 /14/2 0 0 2 10/17/ 2 002

1 1 / 16 / 2002

1 2 / 13/2002Deciviews Predicted

Observed22

Figure 24: Comparison of Predicted and Observed Haz e Index at Brigantine National Wildlife Refuge, NJ

Predicted vs. Observed Haze Index for Brigantine NW R, NJ for Selected Dates

0 5 10 15 20 25 30 35

1/14/ 2 002 2/10/

2 002 3/15/2002 4/14/

2 002
5/ 1 4/ 20 02 6/16/

2 002
7/ 1 3/ 20 02 8/15/

2 002 9/14/

2 002 10/17/2002

1

1 /16 / 20 0 2 12/13/20

0 2Deciviews Predicted

Observed

Figure 25: Comparison of Predicted and Observed Haz e Index at Addison Pinnacle, NY

Predicted vs. Observed Haze Index at Addison Pinnac le, NY for Selected Dates

0 5 10 15 20 25 30

1/1 4 /2002
2/

1 0 / 2002
3/

1 5 / 2002 4/1

4 /2002
5/

1 4 / 2002
6/

1 6 / 2002 7/1

3 /2002 8/1

5 /2002
9/

1 4 / 2002 10/

1 7/2002 11/

1 6/2002 12/1

3/ 200 2Deciviews Predicted

Observed23

Figure 26: Comparison of Predicted and Observed Haz e Index at Arendtsville, PA

Predicted vs. Observed Haze Index at Arendtsville, PA for Selected Dates

0 5 10 15 20 25 30 35

1/1 4/2 0 0 2 2/10 / 2002 3/1

5/ 2002 4/14/2002

5

/14/2 0 0 2

6 /16/2 0 0 2

7 / 1 3/2 0 0 2

8 /15/2 0 0 2 9/1 4/2 0 0 2 10 /17 /2 0 02 11/ 16 /2 0 02

1 2 /13 / 200 2Deciviews Predicted

Observed

Figure 27: Comparison of Predicted and Observed Haz e Index at Lye Brook, VT

Predicted vs. Observed Haze Index at Lye Brook, VT for Selected Dates

0 5 10 15 20 25 30 35

1/1 4 / 2 00 2
2/ 10/2 00 2 3/15/2002 4/1

4/2 00 2
5/ 14/2 00 2 6/16/2002 7/1

3/2 00 2

8 /15/2002 9/14/2002
10

/ 17/2002 11/

16 /20 0 2 12/13/2002Deciviews Predicted

Observed24

Figure 28: Comparison of Predicted and Observed Haz e Index at Shenandoah, VA

Predicted vs. Observed Haze Index at Shenandoah, VA for Selected Dates

0 5 10 15 20 25 30 35

1 / 1 4 / 2 00 2
2/ 1 0 / 2 0 02

3 / 1 5 / 2 00 2
4/ 1 4 / 2 0 02 5/14/2002 6/16/2

0 02 7/13/2002 8/15/2

0 02 9/14/2002 10/17/20

0 2

11 / 16/20 0 2 12/13/20

0 2Deciviews Predicted

Observed

6.0 CALGRID Model Runs
The OTC CALGRID modeling platform will be run for t he 2002 Base Case, 2009 OTB/OTB, and 2009 BOTW. These simulations will be run for the full summer (May 15 to September 15, 2002) episode as well as for the s horter summer modeling episodes described earlier. A variety of control scenarios and sensitivity runs will be performed for the shorter episodes as requested by the OTC Mo deling Committee or individual state agencies. The results of these runs may be used to evaluate, at a screening level, any additional emissions reductions that may be require d, thus relieving the technical burden on the CMAQ modeling centers. CALGRID screening re sults may also be used in individual states’ weight-of-evidence (WOE) analyse s. If states request it, the CALGRID modeling platform will be delivered so that they ma y run additional modeling scenarios at their discretion.