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Technical Evaluation of Sensor Technology (TEST) Program

PurpleAir PA-II Sensor

2019 – 1st Quarter

San Joaquin Valley Air Pollution Control District PurpleAir PA-II Sensor

Technical Evaluation of Sensor Technology (TEST) Program 2019 – 1st Quarter

2

Introduction and Sensor Profile

This analysis report is focused on assessing the performance of the PurpleAir PA-II sensor as a

part of the District’s Technical Evaluation of Sensor Technology (TEST) Program. The PurpleAir

PA-II sensor uses an optical laser-based particle counting methodology to estimate the mass of

varying diameters of particulate matter, including PM1, PM2.5, and PM10. The PA-II sensor

also measures temperature, pressure, and relative humidity.

Background and Approach of Evaluation Test

In November of 2017, NASA began an air quality study to compare the performance of

PurpleAir sensors to regulatory PM2.5 analyzers. The study is focused on the conditions in the

San Joaquin Valley and is based at the ARB air monitoring sites of Fresno-Garland, Visalia-

Church, Modesto-14th St., and Bakersfield-California. The data sets analyzed for this report

compare PM2.5 data collected from PurpleAir sensors and Federal Equivalent Method (FEM)

monitors that are collocated at the ARB air monitoring sites listed above. The scatter plots and

time series graphs below show how the datasets compare for both hourly values and the 24-

hour average.

Overview of Analysis Findings from Current Period

This assessment compares the Purple Air PA-II sensor performance against the MetOne BAM-

1020 continuous particulate monitor. The analysis for this report covers the time period of

January 2019 through March 2019 (2019 – 1st quarter). During this this period, hourly data was

removed from the calculation of bias and average concentrations when either the Purple Air

sensor or regulatory monitor did not have a valid sample. For the 24 hour average line graphs,

all available data is shown for each collocated analyzer and sensor.

The 1st quarter of 2019 was dominated by an active weather pattern where good dispersion

conditions brought lower particulate concentrations. Brief periods of high pressure resided

over the Valley causing stagnation and an increase in PM2.5 concentrations. The strongest

ridge of high pressure over the Valley during these three months was from January 22 until

January 30. PM2.5 concentrations for the Purple Air sensors and regulatory monitors were

highest during this late January period of higher stability. As the plots below show, Purple Air

data was biased higher than the District’s regulatory data during this period for all sites.



3

Site Specific Analysis of PurpleAir PA-II Sensor Performance

Fresno-Garland

For the 24-hour average, PurpleAir data had a 9.0 µg/m3 high bias during the January 1st, 2019

through March 31st, 2019 period. For the hourly average, PurpleAir data had a high bias of

8.5 µg/m3 over the same period.

y = 1.8311x - 0.3831

R² = 0.8724

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80

Pu

rple

Air

MetOne BAM FEM

Fresno 24-hour Average Comparison

y = 1.5605x + 2.6453

R² = 0.8327

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140

Pu

rple

Air

MetOne BAM FEM

Fresno Hourly Average Comparison

0

10

20

30

40

50

60

70

80

PM

2.5

(µ

g/m

3)

Fresno 24-hour PM2.5 Average FEM vs PurpleAir Q1 2019

Fresno FEM Fresno Purple Air



4

Visalia-Church

For the 24-hour average, PurpleAir data had an 8.1 µg/m3 high bias during the January 1st, 2019

through March 31st, 2019 period. For the hourly average, PurpleAir data had an 8.2 µg/m3 high

bias over the same period.

y = 1.7873x - 0.8746

R² = 0.9068

0

10

20

30

40

50

60

70

80

0 20 40 60 80

Pu

rple

Air

MetOne BAM FEM

Visalia 24-hour Average Comparison

y = 1.6572x + 0.8836

R² = 0.888

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140

Pu

rple

Air

MetOne BAM FEM

Visalia Hourly Average Comparison

0

10

20

30

40

50

60

70

80

PM

2.5

(µ

g/m

3)

Visalia 24-hour PM2.5 Average FEM vs PurpleAir Q1 2019

Visalia FEM Visalia Purple Air



5

Bakersfield-California

For the 24-hour average, PurpleAir data had a 5.2 µg/m3 high bias during the January 1st, 2019

through March 31st, 2019 period. For the hourly average, PurpleAir data also had a 5.2 µg/m3

high bias over the same period.

y = 1.4722x - 0.1737

R² = 0.8929

0

10

20

30

40

50

60

70

80

0 20 40 60 80

Pu

rple

Air

MetOne BAM FEM

Bakersfield 24-hour Average Comparison

y = 1.3383x + 1.5102

R² = 0.8412

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Pu

rple

Air

MetOne BAM FEM

Bakersfield Hourly Average Comparison

0

10

20

30

40

50

60

70

80

PM

2.5

(µ

g/m

3)

Bakersfield 24-hour average FEM vs PurpleAir Q1 2019

Bakersfield FEM Bakersfield Purple Air



6

Modesto-14th St.

For the 24-hour average, PurpleAir data had an 8.6 µg/m3 high bias during the January 1st, 2019

through March 31st, 2019 period. For the hourly average, PurpleAir data had a 9.2 µg/m3 high

bias over the same period.

y = 2.0617x - 1.6213

R² = 0.8436

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80

Pu

rple

Air

MetOne BAM FEM

Modesto 24-hour Average Comparison

y = 1.7959x + 1.7012

R² = 0.8838

0

10

20

30

40

50

60

70

80

90

100

110

0 10 20 30 40 50 60 70 80 90 100 110

Pu

rple

Air

MetOne BAM FEM

Modesto Hourly Average Comparison

0

10

20

30

40

50

60

70

80

PM

2.5

(µ

g/m

3)

Modesto 24-hour average FEM vs PurpleAir Q1 2019

Modesto FEM Modesto Purple Air



7

Statistical Summary

The following table provides a statistical summary of the data collected during the analysis

period of this report.

Statistic Fresno-Garland Visalia-Church Bakersfield-Cal Modesto

FEM Avg 24-hr 11.2 11.8 11.4 8.6

Sensor Avg 24-hr 20.2 19.9 16.6 17.2

FEM Max 1-hr 120.9 83.0 89.0 72.0

Sensor Max 1-hr 117.0 108.3 91.5 105.7

FEM Max 24-hr 47.0 45.0 53.0 34.0

Sensor Max 24-hr 72.6 78.0 73.5 70.3

1-hr R2 0.8327 0.888 0.8412 0.8838

1-hr Slope 1.5605 1.6527 1.3383 1.7595

1-hr Intercept 2.6453 0.8836 1.5102 1.7012

24-hr R2 0.8724 0.9068 0.8929 0.8436

24-hr Slope 1.8311 1.7873 1.4722 2.0617

24-hr Intercept -0.3831 -0.8746 -0.1737 -1.6213

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