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CHAPTER SIX

ATTAINMENT DEMONSTRATION

Chapter Six describes the modeling that was performed to demonstrate that attainment of

the 24-hour PM-10 standard will be achieved in 2012. The modeling was conducted to

show attainment under high wind conditions since violations of the standard under stagnant

conditions have not occurred since 2006 (see Chapter 3 for monitoring data information).

A modeling attainment demonstration was performed for a 36 square mile area in the Salt

River area, which includes the West 43rd Avenue monitor that has recorded the most

exceedances of the PM-10 standard under high wind conditions during 2005-2010. A

regional attainment demonstration for the entirety of the nonattainment area is also

provided in this chapter. Both the modeling demonstration in the Salt River area and the

entire nonattainment area are based upon a distance-weighted rollback approach.

Windblown PM-10 emission estimates during high wind hours are calculated from back

trajectories while low wind hour emission estimates are derived from land use distribution

within each modeling domain. The benefits of the control measures described in Chapter

Five provide the reductions necessary to model attainment. 

In addition to the modeling attainment demonstrations, this chapter addresses other

requirements of a nonattainment area plan, including reasonable further progress,

contingency measures, and the onroad motor vehicle emissions budget for conformity. The

chapter concludes with a discussion of the control measures in the Five Percent Plan that

will reduce PM-10 emissions sufficiently to achieve attainment as expeditiously as

practicable and the need for an extension of the attainment date deadline to December 31,

2012. 

 

SALT RIVER AREA MODELING

This section describes the modeling and results for the attainment demonstration in the 

Salt River Area. A detailed discussion of the technical methods and assumptions used to

perform the modeling for the Salt River Area is provided in the TSD (Chapter V of Appendix

B, Exhibit 1).

Design Day Selection

PM-10 monitors in the Maricopa County nonattainment area recorded 17 exceedances of

the 24-hour average PM-10 national ambient air quality standard in 2007 (the last year

before the control measures in this plan were implemented). Exceedances were recorded

at six monitoring sites in the nonattainment area: Coyote Lakes, Central Phoenix, Durango,

Higley, South Phoenix and West 43rd Avenue. The Buckeye monitor also exceeded the

standard on two days in 2007, but this monitor is outside the nonattainment area and

therefore was not modeled in this plan.

6 - 1
Of the 17 exceedances recorded within the nonattainment area in 2007, the West 43rd

Avenue monitor recorded the highest number of exceedances with six. The Higley monitor

was next highest with five. The remaining exceeding monitors in the nonattainment area

recorded two or less exceedances in 2007. All but one of the exceedances in 2007

occurred under high wind conditions. For the purposes of this attainment demonstration,

high wind conditions are defined as days when five-minute or hourly average wind speeds

are greater than 12 miles per hour. This value is identical to the threshold for windblown

dust production as identified in the 2008 PM-10 Periodic Emissions Inventory (Appendix

A, Exhibit 1). Table 6-1 provides a tabulation by monitor of the exceedance days, 24-hour

average concentrations and the general meteorological conditions associated with the

exceedances.

The following primary criteria were applied in selecting the design days for PM-10

modeling:

1. Exceedance days with five-minute or hourly average wind speeds > 12 miles

per hour; and 

2. Exceedance days that are unlikely to be considered as a high wind

exceptional event.

The West 43rd Avenue monitor recorded six exceedances when hourly average winds were

greater than 12 miles per hour in 2007. Four of the six exceedance days were associated

with high winds from a frontal storm system, while the remaining two days were associated

with high winds from monsoon season thunderstorm outflows. Days on which

thunderstorm outflow activity occurs have been excluded as candidate days for modeling,

as these days are almost always flagged as uncontrollable exceptional events. 

Two of the four frontal system exceedances days recorded at the West 43rd Avenue monitor

were days in which only the West 43rd Avenue monitor exceeded the standard (March 27

and May 4). These days suggest that the local sources around the West 43rd Avenue

monitor may have been important contributors to the exceedance value. Of these two

dates, May 4, 2007 was chosen as the design day as wind speeds on March 27th were

significantly higher making it a candidate for an exceptional event demonstration. 

For a demonstration of regional attainment across the nonattainment area, the other

exceedance days in 2007 were evaluated using the same criteria employed to select May

4, 2007 for the Salt River area modeling. All high wind days associated with thunderstorm

outflows were removed for consideration due to their designation as uncontrollable

exceptional events. Of the remaining days with high wind days, two days (April 12 and

June 6) recorded exceedances at multiple monitors, indicating that the high winds had a

more regional effect on PM-10 concentrations. Of the these two dates, June 6, 2007 was

chosen as the design day as April 12, 2007 presented very strong state-wide high winds

indicating this date was also a candidate for an exceptional event demonstration. A more 

6 - 2
Table 6-1

Maricopa County and PM-10 Nonattainment Area 24-Hour Average 

PM-10 Standard Exceedance Dates, Measured Concentrations

 and Associated Meteorological Conditions in 2007

Monitor Date PM-10 Conc.

(ȝg/m3

)

Meteorological Conditions

Buckeye July 19 195.0 Low Winds

November 15 166.2 Irregular Winds

Coyote Lakes July 19 273.4 Thunderstorm Outflow 

October 21 312.9 Frontal System High Winds

Central Phoenix July 19 267.8 Thunderstorm Outflow

Durango Complex November 15 155.7 Irregular Winds

Higley . / 0 1 2 3 4 3 5 6 7 8 9 0 : ; < = > ? : 1 @ A B C D E B 0 F ?

July 19 199.5 Thunderstorm Outflow

August 16 195.6 Thunderstorm Outflow

August 23 230.4 Thunderstorm Outflow

October 24 174.8 Frontal System High Winds

South Phoenix April 12 171.1 Frontal System High Winds

August 13 159.5 Thunderstorm Outflow

West 43rd

Avenue

March 27 227.9 Frontal System High Winds

April 12 202.7 Frontal System High Winds

G H I J K L M N O P Q R S T H U V I W T X Y Z [ \ ] ^ [ S _ W

` a S X b c c d N M P Q R S T H U V I W T X Y Z [ \ ] ^ [ S _ W

July 19 178.0 Thunderstorm Outflow

August 16 215.1 Thunderstorm Outflow

Note: The selected design days are highlighted in bold italics. The Buckeye monitor is located outside the PM10 nonattainment area and is not part of the attainment demonstration. The Coyote Lakes monitor was a

special purpose monitor that no longer operates and has since been replaced by the Zuni Hills monitor in late

2009. As such, the Coyote Lakes monitor is not a good candidate for producing a design day. 

6 - 3
detailed discussion on design day selection is presented in the TSD (Chapter V of

Appendix B, Exhibit 1). 

Plots of hourly wind speed and PM-10 concentration for the high wind design days of May

4, 2007 and June 6, 2007 at the West 43rd Avenue and Higley monitors are shown in Figure

6-1 through 6-3.

Modeling Domain

On a high wind design day, the areal extent impacting monitors is largely a function of wind

speed. As such it is necessary to divide the selected design days into high and low wind

hours. High wind hours are represented by sources located along back trajectories and low

wind hours are represented by sources within a modeling domain surrounding the selected

monitoring site. The basis for distinguishing between high and low wind hours is the 12

mph windblown dust aerodynamic entrainment threshold established in the 2008 PM-10

Periodic Emissions Inventory (Appendix A, Exhibit 1). 

Five-minute average wind speed and direction as measured at the West 43rd Avenue

monitor is employed to construct the back trajectories for each high wind hour. Past

modeling of back trajectories indicate that the lands significantly contributing to elevated

PM-10 concentrations recorded at the monitor lie generally within a mile of either side of

the path of the upwind back trajectories (see Section entitled “Establishing the Boundaries

of a High Wind Domain” in Chapter V of Appendix B, Exhibit 1). 

The modeling domain used during the low wind hours for the West 43rd Avenue monitor is

similar to the area initially defined in the ADEQ TSD for the 2005 PM-10 attainment plan

and is bounded by Van Buren Street to the north, Baseline Road to the south, 59th Avenue

to the west and 7th Street to the east. Due to the diversity and number of PM-10 sources

in the Salt River area, it is considered to be a worst-case representation of sources

throughout the nonattainment area. 

Figure 6-4 shows the low wind domain and the high wind back trajectories on May 4, 2007

as originating from the West 43rd Avenue monitor. The highlighted area surrounding the

back trajectories represent the land use domain used for calculating windblown PM-10

emissions during the high wind hours.

Inventory Development

For the high wind hours, hourly windblown PM-10 emissions are calculated based upon the

land uses that fall within the hourly back trajectory. Emissions from the land uses within

each hourly back trajectory are calculated per the methodology described in the 2008 PM10 Periodic Emissions Inventory. Initially, the land uses within each back trajectory are

evaluated for their potential to emit windblown dust. Some land uses are assumed to have

no windblown dust emissions (office/retail), some are partially emissive

(commercial/industrial) and others are assumed to be 100% emissive (vacant lands). 

6 - 4
Figure 6-1

Summary of the Monitoring Conditions at West 43rd Avenue

on the High Wind Design Day

(May 4, 2007)

6 - 5
Figure 6-2

Summary of the Monitoring Conditions at West 43rd Avenue

on the High Wind Design Day

(June 6, 2007)

6 - 6
Figure 6-3

Summary of the Monitoring Conditions at Higley

on the High Wind Design Day

(June 6, 2007)

6 - 7
Figure 6-4

Salt River Area Low Wind Domain and 

May 4, 2007 Hourly High Wind Back Trajectories

and Associated Domains as Originating from West 43rd Avenue Monitor

6 - 8
Land uses that are conservatively assumed to have 100% of their acreage available for

windblown dust production include: vacant and open lands, sand and gravel/mining

lands, lands under development, landfills and unpaved automotive test tracks. Land

uses where only portions of the acreage is available for windblown dust production

include: agricultural lands, industrial lands, commercial lands and large public lands.

Windblown PM-10 emissions from these land uses are calculated by use of emission

factors based upon wind speed and the disturbance level of each land use. The

disturbance level of each land use is derived from rule effectiveness studies of the

fugitive dust rules that regulate the various land uses. The effectiveness of the rule is

assumed to be an approximate surrogate for the average level of disturbance of each

regulated land use. The details of the calculation of the windblown PM-10 emissions

are provided in the TSD (Chapter V of Appendix B, Exhibit 1).

Since there have been no stagnation violations of the 24-hour PM-10 standard since

2006, detailed low wind design-day specific emission inventories are not needed.

However, emissions reductions achieved as a result of controls in this plan during the

low wind hours of the high wind design days are quantifiable and a necessary part of

the attainment demonstration. In order to account for these emissions reductions,

annual low wind emission inventories for the PM-10 nonattainment area are allocated to

the Salt River area domain based upon land use distribution. This allows for

representation of the low wind emissions, and associated control measure benefits, in

the Salt River area domain without having to create a design day inventory. The details

on how low wind emissions are allocated to the Salt River area domain based upon land

use is included in the TSD (Chapter V of Appendix B, Exhibit 1). 

Distance-Weighted Rollback Modeling

The fundamental assumption underlying any rollback method is that pollutant

concentrations are directly proportional to total emissions over the area of interest. A

weighted rollback approach applies a distance reduction factor to the emissions of each

source in the modeling domain to help assess the impact of emissions as distance from

the monitor increases. The reduction factor is calculated based upon the distance

between each source and the impacting monitor.

An investigation of results from a limited-term (2010-mid 2011) saturation monitoring

study conducted in the Salt River area and beyond under high wind conditions

suggested the need to account for distance between emission sources and the

impacted monitor. Further testing of different weighting factors through the analysis of

dispersion model AERMOD confirmed that a 1/d weighting factor proved to be the best

form of weight to use in adjusting PM-10 emissions developed through the back

trajectory domains (See section entitled “Source Weighting” in Chapter V of Appendix B,

Exhibit 1 for more information). Thus, all high wind design day emissions inventories

were weighted to account for a 1/d distance from the modeled monitor.

6 - 9
Distance weighting was not applied to the low wind domain, as the low wind domain

primarily serves to identify the predominant land uses near the monitor and thus the

predominant low wind emissions associated with the land uses. The overall reduction in

low wind emissions gained through the implementation of control measures was applied

as the reduction factor to the low wind hourly concentrations without the need to weight

these reductions by distance.

A detailed explanation of the saturation monitoring study and the AERMOD evaluation

can be found in the TSD (Chapter V of Appendix B, Exhibit 1). 

Background Concentrations

The temporary saturation monitoring study also provided insights into nonanthropogenic levels of PM-10 within the nonattainment area. These background values

were established by comparing monitored readings under high and low wind conditions

at the temporary, rural Arlington School monitor and the existing Buckeye monitor

located just outside the western edge of the nonattainment area. The Arlington School

monitor was located approximately 11 miles west of the Buckeye monitor, surrounded

by mostly natural desert land. 

Under high wind conditions (> 12 mph), when the wind is coming primarily from the

west, the Arlington monitor consistently recorded lower PM-10 concentrations than the

Buckeye monitor, which is primarily impacted by PM-10 emissions from agricultural

activities. The median value under high wind conditions at the Arlington monitor was

found to be 21.9 ȝg/m3

 and is assumed to be the non-anthropogenic background PM-10

concentration for the PM-10 nonattainment area under high wind conditions. A similar

comparison under low wind conditions yielded a median value of 14.9 ȝg/m3 and is

assumed to be the non-anthropogenic background PM-10 concentration for the PM-10

nonattainment area under low wind conditions. A detailed explanation of the methods

and data used to develop the background values can be found in the TSD (Chapter V of

Appendix B, Exhibit 1).

Control Measure Analysis

To assess the benefits of the control measures under high wind conditions, separate

hourly windblown PM-10 inventories were developed along back trajectory domains

using base case soil disturbance rates in 2007 and then compared against inventories

developed using the predicted soil disturbance rates in 2012. The high wind inventories

are then weighted by distance from the monitor to more accurately represent the

lessening impact of emission sources that are farther away from the modeled monitor. 

The reductions seen during the high wind hours is strictly due to the benefits of control

measures in this plan, as land use is held constant between 2007 and 2012. Table 6-2

provides a summary of the hourly high wind inventories for 2007 and 2012.

6 - 10