                                       
                       Technical Support Document (TSD)
                                     for 
                              EPA's Rulemaking
                                   For the 
                       Arizona State Implementation Plan
                              Bullhead City Area
                 2[nd] Period Limited Maintenance Plan (LMP) 









                                 December 2021


This document presents calculations and data required to support the review of the Bullhead City PM10 Limited Maintenance Plan (LMP). Specifically, we discuss the LMP eligibility criteria related to average design value concentration and motor vehicle growth analysis.
1. Average Design Value Concentration
The memo "Limited Maintenance Plan Option for Moderate PM10 Nonattainment Areas" outlines the requirements for an area to qualify for the LMP option: 
      "To qualify for the limited maintenance plan option, an area should meet the following applicability criteria. The area should be attaining the NAAQS and the average PM10 design value for the area, based upon the most recent 5 years of air quality data at all monitors in the area, should be at or below 40 [ug]/m[3] for the annual and 98 [ug]/m[3] for the 24-hr PM10 NAAQS with no violations at any monitor in the nonattainment area. If an area cannot meet this test it may still be able to qualify for the LMP option if the average design values of the site are less than their respective site-specific CDV.
      We believe it is appropriate to offer this second method of qualifying for the LMP because, based on the air quality data we have studied, we believe there are some monitoring sites with average design values above 40 [ug]/m[3] or 98 [ug]/m[3], depending on the NAAQS in question, that have experienced little variability in the data over the years. When the CDV calculation was performed for these sites we discovered that their average design values are less than their CDVs, indicating that the areas have a very low probability (1 in 10) of exceeding the NAAQS in the future. We believe it is appropriate to provide these areas the opportunity to qualify for the LMP in this circumstance since the 40 [ug]/m[3] or 98 [ug]/m[3] criteria are based on a national analysis and don't take into account each local situation."
To calculate the average design value concentration, the LMP Option memo references a separate EPA guidance document, the PM10 SIP Development Guideline, which outlines multiple methods to perform this calculation, including a "table look-up" approach based on monitored PM10 concentrations. This approach selects the annual design value concentration from among the highest measured concentrations based on the total number of 24-hour PM10 concentrations available over three years. For example, if a PM10 -  monitor samples every day and provides 1,095 24-hour concentration measurements over the three years, then the annual design value concentration calculated by this approach is the fourth-highest concentration measured during the three-year period. A PM10 monitor that samples once every six days, in contrast, would have only approximately 183 24-hour concentration measurements available over three years, and the annual design value concentration would be the highest concentration measured during the three-year period. To calculate the average design value concentration based on five years of air quality data, three adjacent annual design value concentrations are calculated, and these annual design value concentrations are averaged together. 
The methodology to calculate a "site-specific CDV" is found in Attachment A to the LMP Option memo. The equation set forth is:
                             CDV = NAAQS/(1+tc*CV)
Where: 
      CDV = Critical Design Value in ug/m[3];
      NAAQS = 150 ug/m[3] for the 24-hour PM10 NAAQS;
      tc = critical t-value corresponding to a 10 percent probability of exceeding the NAAQS; and
      CV = coefficient of variation of the annual design values.
      
Eligibility for the LMP option is then determined by comparing the average design value concentration to the CDV calculated for the same period. The CDV can be calculated using five years of data or across a longer period of time. 
There is one PM10 monitor located in the Bullhead City PM10 -  maintenance area. This monitor is operated by the Arizona Department of Environmental Quality (ADEQ) and has been operating at its current location since November of 1997. Originally, ADEQ operated a Federal Reference Monitor (FRM) for PM10 at the site, that operated on a sampling schedule of once every six days. On July 1, 2012, this monitor was replaced by a continuous Federal Equivalent Method (FEM) instrument and provides data on a daily sampling schedule. The FEM monitor has continued to operate at the site since 2012. Because the sampling frequency of the monitor changed in 2012, the rank of the value used to calculate annual design values via the lookup method described above has changed over time. 
In evaluating the Bullhead City PM10 -  LMP for eligibility for the LMP option, we evaluated data from 2006-2010, which were the years analyzed in the Second 10-Year LMP. We also evaluated the most recently available data from the site. In addition, we  provided evaluations of the data for the time period of 2013-2020 (using all data years with daily monitoring) but did not rely on these data years for qualification for the LMP.
Table 1 below provides the total valid days, table look-up rank, and resulting design concentration for design value years 2004 (data years 2002-2004) through 2020 (data years 2018-2020).

Table 1: Design Value Years with Design Concentration Determination for the Bullhead City area
                              Design Value Years
                               Total Valid Days
                                  Lookup Rank
                        Design Concentration (ug/m[3])
                                   2002-2004
                                      167
                                       1
                                      55
                                   2003-2005
                                      175
                                       1
                                      48
                                   2004-2006
                                      178
                                       1
                                      72
                                   2005-2007
                                      176
                                       1
                                      72
                                   2006-2008
                                      179
                                       1
                                      72
                                   2007-2009
                                      177
                                       1
                                      98
                                   2008-2010
                                      179
                                       1
                                      98
                                   2009-2011
                                      181
                                       1
                                      132
                                   2010-2012
                                      333
                                       1
                                      185
                                   2011-2013
                                      628
                                       2
                                      185
                                   2012-2014
                                      932
                                       3
                                      108
                                   2013-2015
                                     1085
                                       4
                                      84
                                   2014-2016
                                     1081
                                       4
                                      90
                                   2015-2017
                                     1066
                                       4
                                      91
                                   2016-2018
                                     1064
                                       4
                                      110
                                   2017-2019
                                     1076
                                       4
                                      192
                                   2018-2020
                                     1081
                                       4
                                      102


As stated above, we evaluated the LMP qualification primarily based on the years provided in the original submission. As shown in Table 2 below, the five years evaluated in the original submission were 2006 - 2010. Based on these years, we calculated the average design value concentration to be 89 μg/m[3]. The average design value concentration was below 98 μg/m[3] and therefore qualifies for this criterion of the LMP. For the motor vehicle calculation described later in this document, the site-specific CDV was calculated to be 114 μg/m[3] as shown in Table 3 below.

Table 2: 2006  -  2010 Average Design Value Concentration Calculation (Most recent five years upon plan submission)
                              Design Value Years
                        Design Concentration (ug/m[3])
                 Average Design Value Concentration (ug/m[3])
                                   2006-2008
                                      72
                                      89
                                   2007-2009
                                      98
                                       
                                   2008-2010
                                      98
                                       


Table 3: 2006  -  2010 CDV Calculation
                                     Mean
                                     89.3
                                       n
                                       3
                                      df
                                       2
                                      tc
                                     1.886
                              Standard Deviation
                                    15.011
                                      CV
                                    0.16803
                                      CDV
                                      114

We also evaluated the most recent five years (2016-2020). As shown in Table 4, the average design value concentration for Bullhead City was 101 μg/m[3], above the 98 μg/m[3] threshold. Because the average design value concentration was not below 98 μg/m[3], the site-specific CDV was calculated for this site. The values and results for the CDV calculation were given below in Table 5. As indicated in Table 5, we calculated the CDV for 2016- 2020 to be 128 μg/m[3]. Because the average design value concentration for that period, 101 μg/m[3], is below the CDV, 128 μg/m[3], the monitor also passes the second test for LMP qualification in the five most recent years.

Table 4: 2016  -  2020 Average Design Value Concentration Calculation (Most recent five years)
                              Design Value Years
                        Design Concentration (ug/m[3])
                 Average Design Value Concentration (ug/m[3])
                                   2016-2018
                                      110
                                      101
                                   2017-2019
                                      92
                                       
                                   2018-2020
                                      102
                                       

Table 5: 2016  -  2020 CDV Calculation
                                     Mean
                                     101.3
                                       n
                                       3
                                      df
                                       2
                                      tc
                                     1.886
                              Standard Deviation
                                     9.018
                                      CV
                                   0.088998
                                      CDV
                                      128

Tables 6 and 7 provide the average design value concentration and site-specific CDVs for the 2013  -  2020 time period. As stated above, this time period was reviewed because it encompassed using all data years with continuous monitoring. 

As indicated in Tables 6 and 7, we calculate the 2013  -  2020 average design value concentration to be 95 μg/m[3] and the 2013  -  2020 site-specific CDV to be 131 μg/m[3]. Because the average design value concentration was below the CDV as well as the criteria of 98 μg/m[3] provided in the LMP Option memo, the area also passes the LMP qualification using all data with a daily sampling schedule from 2013  -  2020.

Table 6: 2013  -  2020 Average Design Value Concentration Calculation (All complete data years with daily sampling schedule)
                              Design Value Years
                        Design Concentration (ug/m[3])
                 Average Design Value Concentration (ug/m[3])
                                   2013-2015
                                      84
                                      95
                                   2014-2016
                                      90
                                       
                                   2015-2017
                                      91
                                       
                                   2016-2018
                                      110
                                       
                                   2017-2019
                                      92
                                       
                                   2018-2020
                                      102
                                       

Table 7: 2013  -  2020 CDV Calculation
                                     Mean
                                     94.83
                                       n
                                       6
                                      df
                                       5
                                      tc
                                     1.476
                              Standard Deviation
                                    9.43222
                                      CV
                                    0.09946
                                      CDV
                                      131

In its annual reports following submission of the Second 10-Year LMP, ADEQ identified eight monitored high PM10 values occurring between 2011 and 2018 that could have affected Bullhead City's CDV calculations. However, these events did not result in violations and therefore did not trigger the contingency measures in the Second 10-Year Plan. ADEQ indicated that these values were single-day episodes and attributed them to high wind events. EPA has not evaluated these possible events, so the high values are included in our average design value concentration calculations. 
Table 8: High PM10 Values 2011-2018
Date 
Measured 24-hr Conc. 
                           Cause Identified by ADEQ
August 19, 2011 
132 μg/m3 
Thunderstorm-driven, wind-blown dust event 
August 15, 2012 
185 μg/m3 
Thunderstorm-driven, windblown dust event
April 15, 2013 
208 μg/m3 
Regional Wind Event
May 10, 2014 
108 μg/m3 
Regional Wind Event 
January 31, 2016 
119 μg/m3 
Regional Wind Event
March 30, 2017 
125 μg/m3 
Regional Wind Event
February 11, 2018 
118 μg/m3 
Regional Wind Event
July 9, 2018 
102 μg/m3 
Regional Thunderstorm Wind Event

In summary, the Bullhead City area met the average design value concentration criterion for LMP eligibility for the years presented in the Second 10-Year LMP (2006-2010), for the most recent years (2016-2020), and for all intervening years with continuous monitoring (2013-2020).
 
2. Motor Vehicle Growth Analysis
 ADEQ's motor vehicle growth analysis demonstration for the Second 10-Year LMP years of 2006-2010 yielded 99.6 ug/m[3], which is less than the 101 ug/m[3] site-specific CDV threshold that ADEQ calculated. However, ADEQ calculated the motor vehicle design value concentration based only on the on-road mobile portion of the 2008 attainment year inventory, yielding a value of 5 ug/m[3]. According to table 3.6 of the Second 10-Year LMP, total motor vehicle emissions made up 8.4% of the emissions inventory for 2008. 
Using ADEQ's calculated design value concentration of 98 ug/m[3] with the 8.4% figure yields a motor vehicle design value concentration of 8.23 ug/m[3] and a motor vehicle regional analysis value of 100.62 ug/m[3]. Using EPA's calculated design value concentration for 2006-2010 of 89 ug/m3 yields a motor vehicle design value of 7.5 ug/m[3] and a motor vehicle regional analysis value of 91.4 ug/m[3]. Both 100.62 ug/m[3] and 91.4 ug/m[3] are lower than ADEQ's calculated site-specific CDV threshold of 101 ug/m[3] and below EPA's calculated site-specific CDV threshold of 114 ug/m[3] for the years 2006-2010.
In 2013, ADEQ provided EPA with updated exhaust, brake wear and tire wear emissions for the Second 10-Year LMP based on the development of EPA's Motor Vehicle Emission Simulator (MOVES) model. ADEQ found that using MOVES added 17.9 tons per year in total on-road PM10 emissions to the total emissions in Table 3.6 of its submittal. This would have yielded a motor vehicle design value concentration of 8.47 ug/m3 using ADEQ's maximum design value calculation of 98 ug/m[3]. Adding 0.24 ug/m3 to ADEQ's motor vehicle regional analysis value would not have raised it beyond ADEQ's calculated site-specific CDV. The updated motor vehicle emissions would also not have significantly impacted EPA's calculated motor vehicle regional analysis value, which was more than 20 ug/m3 below EPA's site-specific CDV calculation.
ADEQ's motor vehicle regional analysis calculation in the Second 10-Year LMP was a forward-looking projection of vehicle growth over the next 10 years. Given that Bullhead City's population grew less than 5% between the 2010 and 2020 Census, we find that the Second 10-Year LMP's motor vehicle growth analysis demonstration is sufficient and consistent with the area's continued attainment of the PM10 NAAQS.
Because the purpose of the LMP threshold is to indicate the probability that an area will exceed the NAAQS in the future, and because Bullhead City has stayed in attainment, the Second 10-Year LMP fulfilled the purpose intended by EPA's guidance.

