MEMORANDUM

TO:     Docket No. EPA‐HQ‐OAR‐2012‐0918

    Air Quality Designations for the 2012 PM2.5 Standards

FROM: Liz Naess

    Air Quality Assessment Division, OAQPS

SUBJECT:   Data Used for the Calculation of the Imperial County Design Values

This memorandum addresses EPA’s decision to use data that were pulled from the Air Quality

System (AQS) database in the calculation of design values for Imperial County, CA. The

California Air Resources Board (CARB) invalidated three exceedances of the 24‐hour PM2.5

standard from a monitor in Imperial County (Site ID: 060250005):  

October 15, 2011 (40.9 μg/m3

)

March 21, 2012 (56 μg/m3)  

May 25, 2012 (119.3 μg/m3)  

CARB removed these data based on their following concerns: non‐uniform deposits on filters,

the presence of particles larger than 2.5micrometers on the filters, high wind conditions, and

non‐federal equivalent method (FEM) continuous monitors not matching the federal reference

method (FRM) data. CARB has determined that the combination of these issues indicate a

possible FRM instrument issue. Due to the uncertainty associated with the combination of

these issues, CARB felt that the samples should be invalidated. Please see Attachment 1 for a

more complete discussion of the identified issues.

The technical justifications for invalidating these data are not supported by the analysis in the

Department of Public Health report and are not appropriate reasons for invalidating PM2.5 data

from AQS. Uncertainty remains regarding only one technical issue: the lack of agreement

between the non‐regulatory continuous instrument and the FRM. Given only that uncertainty,

the samples should remain in the dataset. Please see Attachment 2 for a more complete

discussion of EPA’s response to CARBs concerns.

Below is a table showing the implication to the PM2.5 design values before and after removal of

the data.

24‐Hour DV (μg/m3) 2011 2012 2013 Annual  DV (μg/m3) 2011 2012 2013

Before Invalidation 38 43 42 Before Invalidation 13.9 14.1 14.3

After Invalidation 35 32 30 After Invalidation 13.8 13.6 13.7

Change in DV  ‐3  ‐11  ‐12 Change in DV  ‐0.2  ‐0.5  ‐0.6
On August 7, 2014 EPA Region 9 formally asked CARB to resubmit these data to AQS. At the

time of this memo the data are not currently in AQS. EPA used these data in the calculation of

the 2011‐2013 design values per 40 CFR 50, Appendix N, Section 3a.  

Attachments:

1. Analysis of Particulate Matter collected by PM2.5 FRM Monitors in Calexico, Imperial

County, Department of Public Health, 2013.

2. EPA Region 9 Summary of Data Issues in Imperial County, CA, August 2014.

3. Letter from Meredith Kurpius, U.S. EPA Region IX, to Michael Benjamin, California Air

Resources Board dated August 7, 2014.
Lab. ID E1301013 04/11/13 

Analyses of Particulate Matter collected by PM2.5 FRM Monitors in 

Calexico, Imperial County 

Jeff Wagner, Ph.D. and Sutapa Ghosal, Ph.D. 

California Department of Public Health 

Environmental Health Laboratory Branch 

850 Marina Bay Parkway 

Richmond, CA 94804 

SUMMARY OF RESULTS 

Six Teflon PM2.5 filters collected in Calexico (four from 2012 and one each from 

2010 and 2011) were qualitatively analyzed using optical and scanning electron 

microscopy at magnifications of 40x – 16,000x. Two filters which resulted in 

unusually high PM2.5 values in 2012 exhibited heavy, non-uniform deposits. These 

two samples also contained a greater proportion of large particles (>2.5 m) than 

two 2012 PM2.5 filters which had resulted in more typical, lower values. The large 

PM was mostly in the 5-15 m size range, with elemental signatures and 

morphologies consistent with resuspended crustal material. The large particles 

were primarily aluminum silicates with substantial amounts of calcium, sodium, 

and chlorine. Calexico monitoring data shows unusually high evening wind speeds 

on both high-PM2.5 sample dates from 2012, suggesting the samplers may have 

been challenged by high levels of large windblown dust. A small but non-zero 

fraction of this dust could be expected to reach the filters based on the PM2.5 size 

selective sampler design. Fine, near-spherical iron-zinc-manganese particles 

characteristic of a high-temperature process were also found in both 2012 highPM2.5 filters. 

Wind speeds on the high-PM2.5 2010 and 2011 sampling dates were relatively low, 

so the moderate amounts of large crustal PM found on these filters may have 

resulted from dust resuspension from nearby activities, rather than transport due to 

high wind speeds. In addition, the 2010 filter contained a considerable layer of 

fine black soot consistent with a combustion source, and the 2011 filter exhibited 

non-uniform deposits and a gouge in the filter material which likely occurred postsampling. 

Preliminary Raman microspectroscopy (RMS) analysis of a representative large 

particle in the 2010 sample demonstrated its capability to characterize chemical 

compounds (rather than elemental finger print). Future work with windblown dust 

modeling, passive sampling, and RMS could improve understanding of the 

presence of large particles on these filters. 

Attachment 1
Lab. ID E1301013 04/11/13 

INTRODUCTION 

The Environmental Health Laboratory Branch (EHLB) Microscopy/Special Studies Unit of the 

California Department of Public Health received six Teflon (PTFE) PM2.5 filters from the 

California Air Resources Board (CARB) Monitoring and Laboratory Division on January 28, 

2012. All six filters were collected using a Federal Reference Method (FRM), filter-based 

monitor at the Imperial County Calexico – Ethel station. Two of the filters represented unusually 

high-PM2.5 values (mass concentrations of airborne particulate matter with aerodynamic 

diameters less than 2.5 m) recorded during March and May of 2012. Two other filters were 

submitted which represented more typical, lower PM2.5 measurements, each of them sampled 

immediately after the two respective high-PM2.5 days. Two additional high-PM2.5 filters from 

2010 and 2011 were also submitted to CARB by Imperial County APCD for supplemental 

analyses by EHLB (see Table 1 and attached LIMS report). 

The two high-PM2.5 FRM sampler measurements in 2012 were unusual since agreement with colocated PM2.5 beta attenuation monitors (BAMs) was poor. The respective heights of the FRM 

and BAM inlets during the filter collection dates were approximately 3 and 5 meters. 

The Calexico-Ethel test site is located approximately 0.5 miles North of the border crossing to 

the large, urbanized area of Mexicali, Mexico, and approximately 30 miles Southeast of the 

Salton Sea, with desert, agricultural, military, geothermal power generation, and industrial lands 

in between. Vehicle parking lot areas immediately to the south and west of the site have the 

potential to accumulate regional dusts, and other potential dust sources include parking areas, 

recreational fields, and vacant lots within 500 feet of the site in all directions. 

CARB requested EHLB analyze the particulate matter collected on these filters to help determine 

the cause of the 4 unusually high FRM PM2.5 measurements, which were not recorded by the 

PM2.5 BAMs. EHL analyzed particles from the filters using optical and electron microscopy to 

determine the morphology and composition of unexpectedly large particles >2.5 um that 

penetrated the size selective sampler designed to measure PM2.5. 

Table 1. Analyzed filters from Calexico-Ethel FRM sampler with CARB-reported results 

Filter ID Sampling Date PM2.5 ( g/m3

)

2309112 3/31/12 56.37

2309412 4/3/12 11.16

2314612 5/25/12 119.37

2314812 5/27/12 14.29

2317910 6/28/10 41.04

2328811 10/15/11 40.95

METHODS 

Each 37 mm diameter Teflon filter was shipped in its standard, barcoded, plastic Petri-slide 

container. A nominal mismatch between the barcode on one filter (2314612) and the 

Attachment 1
Lab. ID E1301013 04/11/13 

accompanying sampling data sheets was confirmed via emails with CARB to be the result of a 

documented sampling date change. Filters were first documented photographically, then 

inspected using a low-power, Leica SD6 stereozoom reflecting light microscope. Nondestructive surveys were also conducted at ambient pressure on untreated, uncut whole filters 

using differential interference contrast (DIC) reflected light microscopy. This technique 

employed a Nikon E600 light microscope with 100x and 200x magnifications. 

Filter subsamples were prepared for scanning electron microscopy (SEM) / energy-dispersive 

spectroscopy (EDS) by first cutting each filter in half to preserve one half for any future analyses 

with minimal disturbance to the PM deposit.. Each remaining half was then placed flat upon on 

an adhesive carbon tab mounted on a standard SEM aluminum stub, from which an 

approximately hexagonal, 12 mm-diameter subsample was cut with a fresh sterile scalpel blade. 

SEM/EDS analysis was conducted with an FEI XL30 Environmental SEM and Thermo Fisher 

Scientific Noran System 7 EDS system. Improved image resolution and particle stability were 

achieved relative to typical high-vacuum SEM by injecting an atmosphere of 0.5-1 mBar of 

water vapor into the Environmental SEM. Manual analyses were conducted at 600x with a backscattered electron (BSE) detector to identify major constituents and any distinctive trace species 

(e.g., heavy metals), and also with a gaseous secondary electron (GSE) detector to detect 

carbonaceous and sub-micron particles. EDS spectra from Teflon filter backgrounds exhibit 

primarily fluorine with a very small carbon peak and interfere minimally with particle analysis 

results. Spectral Imaging (SI) EDS maps were obtained to illustrate the spatial distribution of the 

major chemical elements. 

RESULTS 

Visual inspection of the filters upon receipt revealed reddish-tan PM deposits on the 4 filters 

from 2012 and gray-black deposits on the filters from 2010 and 2011 (Figure 1). The low-PM2.5

filter from 5/27/12 possessed the lightest color, while the nearly black 2010 filter appeared the 

darkest. The high-PM2.5 filters from 3/31/12, 5/25/12, and 2011 exhibited obvious non-uniform 

deposits. 

Under the low-magnification stereozoom microscope, all six filters exhibited relatively rare, very 

large, black particles (Figure 2) and a regular array of 150 m-diameter, circular deposits of 

smaller particles corresponding to the grid pattern in the sampler cassette backing screens. Nonuniform deposits were confirmed on the filters from 3/31/12, 5/25/12, and 2011, the latter of 

which was especially non-uniform and also exhibited a 5 mm-long gouge in the filter surface 

(Figure 2f). 

Reflected-light DIC microscopy of the unprepared, whole filters revealed numerous opaque 

particles in the 5-15 m range, with a few as large as 30 m (Figure 3). These 2-dimensional, 

physical sizes will be somewhat different than their aerodynamic diameters. This reflected-light 

technique has limited ability to image optically transparent particles such as quartz sand. The use 

of transmitted illumination to “backlight” the filter revealed many additional particles which 

were either just below the surface, or directly on the surface but previously transparent under 

reflected light. This technique shows promise for future development, especially in cases where 

destructive sample preparations are prohibited. 

Attachment 1
Lab. ID E1301013 04/11/13 

SEM/EDS confirmed the presence of large conchoidal particles and flakes >2.5 m, many >10 

m, on all filters (Figure 4, Figure 6). These large particles were rare on the 4/3/12 and 5/27/12 

filters, but dominated the collected PM in the 5/25/12 filter, and to a lesser extent, the 3/21/12, 

2010, and 2011filters. Figure 5 shows SI EDS maps, obtained from randomly-chosen locations 

on each filter, for 4 elements: a) silicon (the major constituent of aluminum silicate clays and 

quartz sand), b) calcium (gypsum, lime -based soils), c) sodium (sodium-chlorine salts), and d) 

fluorine (from the Teflon filter). The SI maps and EDS data show that the large particles are 

primarily composed of aluminum and silicon, often enriched with calcium and sodium-chlorine 

salts. The aluminum silicates also exhibited oxygen, magnesium, sulfur, potassium, and 

occasionally, prominent iron, titanium or barium. This collection of elements and morphologies 

is consistent with wind-blown or otherwise resuspended crustal materials (soil, sand, and salt). 

The lower-PM2.5 filters from 2012 possessed fewer large particles, but otherwise contained 

particles of a similar, crustal composition, as did the majority of the analyzed mass <2.5 m on 

all filters. 

The 2010 filter was unique in that it also exhibited a nearly continuous layer of lacey soot 

agglomerates over its entire surface (Figure 7), consistent with its uniformly black appearance in 

visible light. These particles are indicative of a combustion source such as burning diesel or 

biomass. The primary soot nano-particles (approximately 100 nm in diameter) only became 

visible when using the GSE detector at magnifications >1000x. EDS data from the very thin 

primary particles were generally not different than the background, but in areas where the 

agglomerates were particularly compact and dense, their EDS data exhibited measurable carbon 

peaks. Soot was observed to a lesser extent in the other filters (Figure 8) in the form of isolated 

agglomerates 5-20 m in diameter, most easily detected in the less heavily-loaded samples. 

Overall, the PM2.5 mass contributed by soot was likely negligible for all but the 2010 filter. 

The gouge in the 2011 filter was not observed to contain any unusual or non-crustal elements in 

SEM/EDS. The gouge was in the form of a shallow, roughly-carved-out trough with a raised 

mound of filter material at the end. Very few particles were contained within it, implying that the 

filter damage occurred after the PM sampling period. If the damage occurred before the postweighing, it could indicate inadvertent contact which may have contributed to the filter’s high 

PM2.5 value.

Several fine (0.2-2 m), nearly-spherical particles enriched with iron, zinc, and manganese were 

observed in the high-PM2.5 filters from 2012 (3/31 and 5/25) (Figure 9); one was also observed in 

the 2011 filter. This morphology and chemistry implies emissions from a high-temperature 

process such as a metalworking facility or incinerator. These particles did not likely contribute 

substantial PM2.5 mass to the filters, but they suggest that these filters were exposed to a common 

air mass containing this source. 

Other metal-containing particles were observed but were not concentrated in specific filters. The 

most common were iron oxides, or less commonly, particles containing iron in combination with 

copper, sulfur, or chromium. Cerium-phosphorus (a weathering product of naturally-occurring 

apatite) and lead-chlorine (paint or gasoline, or possibly a blank filter contaminant) were found 

in one particle each. 

Attachment 1
Lab. ID E1301013 04/11/13 

DISCUSSION 

The presence of substantial large, windblown PM in Calexico air on 3/31/12 and 5/25/12 is 

plausible. Publicly accessible monitoring data (http://www.arb.ca.gov/aqmis2/aqmis2.php and 

http://www.airnowtech.org) from Calexico-Ethel station show unusually high winds and 

changing weather on both 3/31/12 and 5/25/12. During the period of 3/24/12 – 5/30/12, hourly 

wind averages only exceeded 20 MPH on three occasions: 3/31/12, 5/25/12, and a third period in 

which no FRM PM2.5 filters were collected (4/13/12-4/14/12). The morning winds for both 

3/31/12 and 5/25/12 were low and out of the East (<5 MPH, 80-120 degrees), switching to high 

winds from the West-Northwest (>10 MPH, 280-320 degrees) at midday. Corresponding to this 

shift in winds, the co-located BAM PM2.5 levels on both days rose dramatically in the evening, 

reaching approximately that of the FRM 24-hr integrated value during the last hour in each case. 

In contrast, winds during the following, low-PM2.5 sampling periods (4/3/12 and 5/27/12, 

respectively) were low (all hourly averages <7 MPH). Winds on the 2010 and 2011 dates were 

also fairly low (all hourly averages <10 MPH and 5MPH, respectively,) so the moderate amounts 

of large crustal PM found on these filters may have resulted from dust resuspension from local 

activities. 

The large particles observed on the 3/31/12 and 5/25/12 filters would require substantial energy 

to lift off the ground and settle relatively quickly. As such, they would be expected to decrease 

rapidly in concentration as a function of height. The 2 m difference in height reported for the 

FRM and BAM samplers is thus a plausible contributor to the disagreement in the two sampler 

types’ results on those dates. 

FRM PM2.5 sampler maintenance, inlet cleaning or calibration issues could all potentially 

enhance collection of windblown dust. Even when operated properly, the S-shaped particle size 

cut-point curves of the PM10 and PM2.5 inlets are designed to admit a small but non-zero fraction 

of particles larger than their nominal cut-points. Depending on the size and quantity of large 

particles aloft and their respective inlet penetrations, the increase to PM2.5 could be substantial. 

PRELIMINARY INVESTIGATIONS AND POTENTIAL FOR FUTURE WORK

Raman microspectroscopy could improve understanding of the presence of large particles on 

these filters. As a preliminary proof of concept, the 2010 sample was further analyzed by Raman 

micro-spectroscopy (RMS) to probe molecular speciation of the particulate matter. The RMS 

analyses were performed directly on the filter sample using a Renishaw inVia dispersive Raman 

microscope with 785 nm laser. Figure 10 show RMS analyses of a selected large particle along 

with the relevant spectral matches to specific compounds which were identified using an 

automated spectral search algorithm against a spectral library of known compounds (GRAMS 

Spectral ID). For instance, the particle is shown to contain iron sulfide and iron oxide minerals 

along with other components. Figure 11 shows RMS analyses of the filter substrate in close 

vicinity of the particle and which is correctly identified as Teflon based on spectral matching. 

Both the substrate and the particle also contain elemental carbon which is indicative of 

combustion sources. The carbon observed on the substrate likely corresponds to the fine soot 

layer that covered the filter surface (Figure 7). The particular advantage of RMS analysis is that 

it provides molecular compositional information not available from electron microscopy and 

Attachment 1
Lab. ID E1301013 04/11/13 

therefore is an attractive technique for particle characterization as well as being complementary 

to SEM/EDS. 

Future work using passive aerosol sampling would be essential to provide a more complete 

analysis of windblown dust events. Automated image analyses of particles collected on the UNC 

passive aerosol sampler would enable quantitative comparisons of particle size distributions. 

This was not possible with the 24-hr Teflon samples which were analyzed manually due to the 

frequently overlapping particles and obtrusive background structures typical of this sample type. 

Use of 24-hr passive UNC samplers, which are optimized for microscopy and relatively 

inexpensive, could be made available for rapid deployment when anomalous meteorological 

conditions are predicted similar to 3/31/12 and 5/25/12.

More detailed theoretical models using the combined FRM inlet penetration curves and a range 

of windblown dust size distributions could be developed in the future to determine more realistic, 

potential increases to PM2.5 on windy days. 

Attachment 1
Lab. ID E1301013 04/11/13 

FIGURES 

(a) (b) 

(c) (d) 

(e) (f) 

Figure 1. Filters as received: (a) 2309112 (b) 2309412 (c) 2314612 (d) 2314812 (e) 2317910 (f) 

2328811.

(3/31/12) 56 g/m3(4/3/12) 11 g/m3

(5/25/12) 120 g/m3(5/27/12) 14 g/m3

(6/28/10) 41 g/m3(10/15/11) 41 g/m3

Attachment 1
Lab. ID E1301013 04/11/13 

(a) (b) 

(c) (d) 

(e) (f) 

Figure 2. 40x stereozoom images: (a) 2309112 (b) 2309412 (c) 2314612 (d) 2314812 (e) 

2317910 (f) 2328811. 

(3/31/12) 56 g/m3(4/3/12) 11 g/m3

(5/25/12) 120 g/m3(5/27/12) 14 g/m3

(6/28/10) 41 g/m3(10/15/11) 41 g/m3

Attachment 1
Lab. ID E1301013 04/11/13 

(a) (b) 

(c) (d) 

(e) (f) 

Figure 3. 100x DIC microscopy images: (a) 2309112 (b) 2309412 (c) 2314612 (d) 2314812 (e) 

2317910 (f) 2328811. 

(3/31/12) 56 g/m3(4/3/12) 11 g/m3

(5/25/12) 120 g/m3(5/27/12) 14 g/m3

(6/28/10) 41 g/m3(10/15/11) 41 g/m3

Attachment 1
Lab. ID E1301013 04/11/13 

(a) (b) 

(c) (d) 

(e) (f) 

Figure 4. 500x SEM BSE images: (a) 2309112 (b) 2309412 (c) 2314612 (d) 2314812 (e) 

2317910 (f) 2328811. 

(5/25/12) 120 g/m3(5/27/12) 14 g/m3

(6/28/10) 41 g/m3(10/15/11) 41 g/m3

(3/31/12) 56 g/m3(4/3/12) 11 g/m3

Attachment 1
Lab. ID E1301013 04/11/13 

(a) (b) 

(c) (d) 

(e) (f) 

Figure 5. 4,000x SEM/EDS SI map images. (a) 2309112 (b) 2309412 (c) 2314612 (d) 2314812

(e) 2317910 (f) 2328811. The presence of silicon is denoted by blue, calcium is yellow, sodium 

is green, and fluorine (indicative of the Teflon filter background) is red. 

(5/25/12) 120 g/m3(5/27/12) 14 g/m3

(6/28/10) 41 g/m3(10/15/11) 41 g/m3

(3/31/12) 56 g/m3(4/3/12) 11 g/m3

Attachment 1
Lab. ID E1301013 04/11/13 

(a) 

(b) 

Figure 6. Large (20-30 m), flat, mineral particles on filter 2328811: (a) 200x DIC microscopy 

image showing characteristic semi-transparency with interference fringes. (b) 2,000x BSE image 

from SEM/EDS of flake in center showing typical crustal elements Al, Si, K, and Ca. 

Attachment 1
Lab. ID E1301013 04/11/13 

Figure 7. 2,000x SEM GSE image from filter 2317910 showing widespread, dark, lacey soot 

agglomerates and bright crustal PM. Inset: 16,000x close-up of 100 nm soot primary particles. 

Attachment 1
Lab. ID E1301013 04/11/13 

(a) 

(b) 

Figure 8. Soot agglomerates on filter 2309412. a) 200x DIC microscopy image showing 2 

agglomerates (arrows). b) 10,000x SEM GSE image (different location than (a)). 

Attachment 1
Lab. ID E1301013 04/11/13 

(a) 

(b) 

(c) 

Figure 9. 16,000x BSE SEM/EDS data showing fine, often-spherical particles (bright regions, 

0.2-2 m) enriched with iron, zinc, and manganese from a) 2309112 and b-c) 2314612. 

Attachment 1
Lab. ID E1301013 04/11/13 

(a) 

(b) 

Figure 10. RMS analyses of a representative large particle from 2317910 showing spectral 

matches with a variety of mineral components and elemental carbon for spectral regions 

corresponding to (a) 100-2000 cm-1, inset : optical image showing the particle location; (b) 100-

800 cm-1.

Attachment 1
Lab. ID E1301013 04/11/13 

Figure 11. RMS analyses of the filter substrate (shown in the optical image) with spectral 

matches for elemental carbon and Teflon. 

Attachment 1
Attachment 2

SUMMARY OF IMPERIAL PM2.5 MONITORING ISSUES (Michael Flagg, R9)       8‐07‐2014

A. CARB Data Invalidation:

Based on the results of a scanning electron microscopy (SEM) analysis performed by the Department of Public Health

(DPH)1 and other factors, CARB has invalidated three exceedances (10/15/11, 3/31/2012, & 5/25/2012) of the 24‐hour

PM2.5 standard from the AQS database.  

Timeline of EPA Review and Coordination:

April 23, 2013 – Received DPH report from CARB

May 6, 2013 – Initial feedback from ORD/OAQPS

May 7, 2013 – 1st meeting with EPA, CARB, & ICAPCD

May 22, 2013 – Received detailed comments from ORD and agreement from OAQPS

June 5, 2013 – Received further detailed comments from ORD

June 5, 2013 – 2nd meeting with EPA, CARB, & ICAPCD

August 7, 2013  – In person meeting with EPA, CARB, & ICAPCD at Region 9 offices

August 6, 2014  – Meeting with CARB, EPA

August 7, 2014 – Formal notification and request for data submittal to AQS2

  

Outcome or EPA Review and Coordination:

After multiple meetings discussing the technical aspects of the DPH report, there remains a fundamental disagreement

between EPA & CARB/ICAPCD on the invalidation these data. The table below outlines CARB/ICAPCD’s justifications and

EPA’s technical responses based on consultation with OAQPS and ORD.

Technical Issue

CARB & ICAPCD

Justification for Invalidation EPA Response  

Non‐uniform deposits  

on filters

Non‐uniform deposits indicate a sampler

malfunction, therefore data should be

invalidated.

Non‐uniform deposits do not indicate a

sampler malfunction and is not an acceptable

justification for invalidation. There is no

requirement for uniform deposition of

collected particles on a filter. Variation of the

filter media is expected, as there are no

requirements for homogeneity or thickness

uniformity during filter production. Variation

in material homogeneity or thickness will

naturally result in variations of pressure drop

across the filter, resulting in non‐uniform

aerosol deposition. During ORD development

and evaluation of FRM methods, uniform

deposition normally occurred, but there were

occasional apparent differences in aerosol

deposition that were observed. These non‐

uniform deposits did not affect the inter‐

sampler precision of the mass concentration

during gravimetric analysis.   

                                                            

1 Analysis of Particulate Matter collected by PM2.5 FRM Monitors in Calexico, Imperial County, DPH, 2013 2 see letter from Meredith Kurpius, U.S. EPA Region IX, to Michael Benjamin, California Air Resources Board dated August 7, 2014
Attachment 2

SUMMARY OF IMPERIAL PM2.5 MONITORING ISSUES (Michael Flagg, R9)       8‐07‐2014

Technical Issue

CARB & ICAPCD  

Justification for Invalidation EPA Response  

Presence of particles larger

than 2.5micrometers on the

filters

Particles larger than 2.5 micrometers on

Filter indicate a sampler malfunction and  

thus the measurements are not

representative.

The cut‐point of the PM2.5 inlet is not

perfectly sharp (by design) and allows a

percentage of particles larger than the

sampler’s cut‐point onto the filter sample.

The SEM analysis provided can only

characterize the types of particles collected

on the FRM filters. This type of analysis

cannot be used to evaluate the aerodynamic

size distribution, or the size selective

performance of the FRM sampler. It is

expected that as wind speed increases the

size distribution (and total concentration)

may shift toward larger particle sizes. As a

result, the number of observed particles

larger than 2.5 micrometers can naturally be

expected to increase, but this does not

indicate that the fraction of large particles

(i.e. penetration efficiency) has actually

increased. Therefore, the qualitative evidence

of the presence of these large particles does

not indicate sampler malfunction.

High wind conditions*  

*High winds only occurred on  

3/31 and 5/25/12. Wind

speeds on 10/15/11 were

below 5 mph.

High wind conditions are not representative

of typical PM2.5 exceedances.

Invalidating data due to the source or cause

of the exceedance is inappropriate. In arid

desert environments, PM2.5 exceedances

caused by windblown dust should be

considered valid and representative

measurements. Exclusion of samples due to

high wind can only be done through the

exceptional events rule process.  

Non‐FEM continuous  

monitors do not match  

the FRM data

The difference in these measurements

indicates a FRM sampler malfunction.

The non‐regulatory designation of the

monitors, the distances from the FRM (2 m

vertically and 10 m horizontally), and the

wind direction during high PM2.5 are plausible

explanations of the differences in

measurements.