This document summarizes the important considerations for evaluating the quality of data from low-cost air sensors for different applications. It discusses how the needed data quality depends on the intended use of the data. For example, identifying sources of pollution may require high precision, while personal exposure monitoring could use lower precision. The document then provides examples of how low-cost sensors have been used successfully for research, supplementing regulatory monitoring, and education with data of sufficient quality for the intended purpose.

1. 22 em august 2014 awma.org
em • feature
by Timothy S. Dye,
Briana J. Gordon, and
Paul T. Roberts
Timothy S. Dye, CCM,
is a senior vice president
and manages the
Meteorological and Air
Quality Operations and
Measurements Division;
Briana J. Gordon is a
meteorologist and project
manager; and Paul T.
Roberts, Ph.D., is chief
scientific officer and execu-
tive vice president; all with
Sonoma Technology Inc.
(STI), Petaluma, CA. E-mail:
tim@sonomatech.com.
Air quality monitoring is currently the domain of
trained professionals using expensive, highly accu-
rate instruments; however, the fast-paced evolution
of air sensor technology is putting new types of
sensors into the hands of non-experts. Individuals,
community organizations, and air quality agencies
have tremendous new opportunities to expand air
quality monitoring, but the idea of so many people
measuring air quality with many different devices
raises concerns about whether the resulting data
will be any good. There is a void of information
and examples about the quality and appropriate-
ness of the data generated by newer sensors.
The level of data quality needed varies with the
intended use. There is no single way to define
quality, because the intended uses are vast. For
example, a person with health issues could use a
portable sensor to compare air pollution levels at
Important considerations for evaluating the
level of data quality needed for different air
sensor applications.
Quality Data for the Right Application
AirSensors
22_EM0814-FT3-Dye.indd 22 7/22/14 11:47 AM
Copyright 2014 Air & Waste Management Association

2. august 2014 em 23awma.org
several locations, such as at home and at work, to
determine which location or time of day has better
air quality; this person might not need scrupulously
precise measurements. However, justifying regula-
tory actions requires data collected under rigorous
standards. Users and regulators need a clear, com-
mon understanding of the quality requirements for
a range of potential air sensor applications. This
article qualitatively discusses important consider-
ations for evaluating the data quality needed for
different applications and gives real-world exam-
ples of different applications for air sensors.
Defining ‘Quality’
Several factors play a part in the “quality” of data:
accuracy, precision, minimum detection limit
(MDL), potential bias, and degradation of the
instrument over time, to name a few. Other factors
also affect quality: reproducibility of the data, reli-
ability, frequency of maintenance, clearly defined
data formatting and processing, and ease of use.
The level of data quality needed for a successful
measurement study is determined by the purpose
of the measurements and the data’s intended use.
Data with a high level of precision and low amount
of bias may be needed to meet the objectives of
one study, while moderate levels of precision and
bias may suffice for another study. For example,
lower precision and accuracy may be acceptable if
large emissions from an industrial plant are being
measured; higher precision and accuracy may be
needed to measure small differences in ozone
concentrations throughout an urban area.
During the Air Sensors 2013 workshop, a diverse
group of people from academia, government
agencies, and the private sector discussed the
quality issue during a breakout session. It was
clear from the active discussions that many are
concerned about the quality and the potential use
or misuse of data from these sensors. Some of the
issues raised included:
1. standardizing determinations of the quality of
lower-cost air quality sensors;
2. choosing statistics to evaluate quality and facili-
tate comparison among different sensor devices
(e.g., bias, accuracy, precision);
3. developing guidelines for choosing the appro-
priate sensor for an application; and
4. the need for data quality guidelines for various
sensor applications. Note: the U.S. Environmen-
tal Protection Agency (EPA) recently released
guidelines for sensor developers, sensor users,
and state/local air quality agencies.1
Air Sensor Applications
An exciting number of applications exist for all
potential users of these new air sensors, whether
the users are researchers, educators, or the gen-
eral public. In this section, we provide examples of
how sensors are being deployed to produce data
of sufficient quality for particular applications.
Figure 2. Locations of
ozone sensor sites and
Air District FEM sites.
Notes: Di Giorgio is the new
site and Bear Mountain is the
old site.
Figure 1. A pollution rose showing NO concentrations as a function of wind direction
at sensor sites (SNAQ41 and SNAQ08) located north of runway 27 and also near a
perimeter road at Heathrow Airport.2
Notes: NO concentrations indicate the different sources: (1) from the southwest of the site due to
aircraft emissions, and (2) from the northeast from the perimeter road. Note that dominant winds
are from the west-southwest and east-northeast.
22_EM0814-FT3-Dye.indd 23 7/22/14 11:47 AM
Copyright 2014 Air & Waste Management Association

3. Research
Scientific studies aimed at discovering new infor-
mation about air pollution have often required
highly accurate and precise instruments. But
that’s now changing; professors are finding new
insights, as the sheer number of data points pro-
vide greater granularity of the atmosphere in the
same way a camera with higher pixel density pro-
vides sharper pictures.
For example, the Sensor Network for Air Quality
(SNAQ) project at Heathrow Airport deployed a
40-sensor network in and around London’s Heath-
row Airport for 12 months.2
The study goals were to
better understand optimal sensor network deploy-
ment approaches, perform source attribution, and
validate pollution models. Researchers developed
and deployed sensor nodes that monitored a suite
of air quality and meteorological information, includ-
ing carbon monoxide (CO), carbon dioxide (CO2),
ozone (O3), sulfur dioxide (SO2), nitric oxide (NO),
nitrogen dioxide (NO2), volatile organic compounds
(VOCs), size-speciated particles, wind speed, wind
direction, temperature, and relative humidity (RH).
Preliminary analyses indicate that pollutant sensors
captured the diurnal cycle in airport activity—busy
periods during the day and less activity during the
overnight hours. In addition, wind observations
allowed for identification and quantification of indi-
vidual sources using data from individual sensor
nodes (see Figure 1). More results are expected to
be published within the next two years.
Supplementing Existing Monitoring
When air quality agencies place sensors within an
existing state/local regulatory monitoring network,
they can fill in spatial gaps and address questions
about spatial representativeness. A recent study in
the Central Valley of California illustrated the value
of low-cost sensors and demonstrated that study
design and execution are as critical to the data’s
quality as the individual sensor performance.3
The San Joaquin Valley Unified Air Pollution Con-
trol District (the District) recently moved a monitor-
ing site two miles due to leasing issues. Operation
of both sites for a year and the resulting analyses
showed that both 1-hr and 8-hr average O3 con-
centrations measured at the new site were approx-
imately 10% lower than concentrations measured
at the old site. This difference raised concerns in
the community, and EPA indicated that the differ-
ence might hinder the agency’s ability to deter-
mine whether the region has attained federal O3
standards. To respond to EPA’s concerns, the Dis-
trict conducted a study saturating the area with 21
low-cost sensors to examine O3 gradients in and
around the area.
Figure 3. Ozone sensor
systems (top) during the
collocation period with an
FEM and (bottom) in the
field with its solar panel,
battery, tripod, and
supporting hardware.
24 em august 2014 awma.org
22_EM0814-FT3-Dye.indd 24 7/22/14 11:47 AM
Copyright 2014 Air & Waste Management Association

4. Using a network of low-cost ozone sensors (see Fig-
ure 2) was the only cost-effective method to obtain
sufficient coverage to detect and understand the
O3 gradients. To ensure data quality, a rigorous
study was designed, including collocation periods
with an FEM ozone instrument and all sensors at
the beginning and end of the study (see Figure 3).
In addition, data quality was continuously evaluated
and monitored during the study. Analysis methods
combined with the study design helped achieve
data quality with sufficient precision to address the
study objectives. The study found that:
1. The low-cost O3 sensors provided very good
accuracy and precision for assessing spatial gra-
dients. The accuracy of the 1-hr measurements
was within 3 parts per billion (ppb) and the pre-
cision was ±4% at the 95% confidence level.
2. Strong spatial gradients for peak 1-hr and 8-hr
O3 concentrations existed within and around
the study area. Peak 1-hr O3 concentrations
at each site on a given day can
vary by as much as 30 ppb.
3. O3 concentrations measured at
the new site were very repre-
sentative of the O3 concentra-
tions measured at the old site;
they were well-correlated and
of about the same magnitude.
Education
Sensors in educational settings can
be used for science, technology,
engineering, and math lessons.
An educational application does
not mean a less accurate sensor
should be used; learning requires
sensors with sufficient data qual-
ity and usability for students to
pose hypotheses, collect data, and
answer questions. For example,
HabitatMap and Sonoma Technol-
ogy Inc. were funded by the Knight News Foun-
dation to implement a pilot program, called Kids
Making Sense, which aims to teach youth how to
measure particle pollution using air quality sensors
and to “make sense” of the data they collect.4
Prior to their deployment in pilot projects, the par-
ticle sensors were evaluated against a reference
instrument to ensure sufficient responsiveness for
students. Pilot projects were conducted in Brook-
lyn and San Francisco; students first learned about
particle pollution, planned sampling routes in
their neighborhoods, and ventured out into the
neighborhood in teams to collect particle mea-
surements. The teams shared their data using the
crowd-sourced platform AirCasting.org (see Fig-
ure 4). The students reviewed the patterns in their
data, comparing the high particle concentrations
august 2014 em 25awma.org
(b)
(c)
Figure 4. (a; see page 22) Youth measure particles in a Brooklyn, NY, subway station; (b) real-time
readings report to a student’s cellphone; and (c) crowd-sourced measurements from five teams
measuring particles throughout this Brooklyn neighborhood.
22_EM0814-FT3-Dye.indd 25 7/22/14 11:47 AM
Copyright 2014 Air & Waste Management Association

5. Here’s just a sampling of the benefits you receive as an A&WMA Member:
Quality Information: from the Journal of the Air & Waste Management Association to EM
magazine to technical books and publications, you get the technical, practical, and professional
information you need.
Professional Development and Education: improve your professional skills and expertise
at more than 30 continuing education programs, specialty conferences, and a wide variety of
workshops held each year, in addition to the Annual Conference & Exhibition.
Networking and Contacts: take advantage of numerous opportunities to meet with your
peers and expand your circle of valuable business contacts.
Job Search and Employment: find a position in the environmental field or fill a vacancy in
your organization through our online job board.
Membership and Resource Directories: your quick reference for finding colleagues and a
range of products and services.
Discounts: members receive substantial discounts on publications, conferences, educational
seminars, insurance coverage, logo merchandise, and more.
Recognition: get recognized for your accomplishments through the A&WMA awards
programs, publishing an article in EM or the Journal, or serving on an A&WMA committee.
A&WMAMembership Benefits
For more information on these great member benefits,
contact A&WMA Member Services at 1-800-270-3444
or visit us online at www.awma.org.
MissingPages.indd 1MissingPages.indd 1 4/24/14 1:59 PM4/24/14 1:59 PM
Copyright 2014 Air & Waste Management Association

6. they detected in underground subway stations,
along windy and dusty streets, and near smokers.
In their feedback, students expressed continued
interest in the subject matter. More importantly,
the sensors detected changes in particle pollution
consistent with nearby activities, thereby helping
the youth use the real-time readings to identify
nearby sources of particles.
There are many other applications of sensors for
the public, organizations, and researchers:
1. Information for informal air quality awareness.
2. Environmental Justice/Advocacy to under-
stand exposure to and health effects of air pol-
lutants in EJ communities.
3. Personal Exposure Monitoring to quantify
an individual’s exposure to pollution.
4. Hotspot Identification and Characteriza-
tion to detect individual sources using fixed-
location and/or mobile-sensor systems.
Conclusion
Sensor technology is rapidly being developed
and deployed for applications, ranging from edu-
cation to supplementing regulatory monitoring.
There’s an urgent need for the entire community
to experiment and conduct pilot projects in real-
world conditions to understand the capabilities,
benefits, and pitfalls of the new technologies—and
ultimately, the adequacy of the data for address-
ing air quality issues. em
august 2014 em 27awma.org
References
1. Williams, R.; Kilaru, V.; Snyder, E.; Kaufman, A.; Dye, T.; Rutter, A.; Russell, A.; Hafner, H. Air Sensor Guidebook; EPA 600/R-14/159; U.S.
Environmental Protection Agency, June 2014; available online at http://epa.gov/research/airscience/docs/air-sensor-guidebook.pdf.
2. Bright, V.; Mead, M.I.; Popoola, O.A.; Baron, R.P.; Saffell, J., Stewart, G., Kaye, P., Jones, R. A low-cost high-density sensor network for air
quality at London Heathrow Airport. Presented at the AGU Fall meeting, San Francisco, CA, December 3-7, 2012; Poster A11H-0159.
3. MacDonald, C.P.; Roberts, P.T.; McCarthy, M.C.; DeWinter, J.L.; Dye, T.S.; Vaughn, D.L.; Henshaw, G.; Nester, S.; Minor, H.A.; Rutter, A.P.;
Smith, K.; Winegar, E. Ozone concentrations in and around the City of Arvin, California. Final report prepared for the San Joaquin Valley
Unified Air Pollution Control District, Fresno, CA, by Sonoma Technology, Inc., Petaluma, CA, STI-913040-5865-FR2, May 2014; available
online at http://www.valleyair.org/air_quality_plans/docs/2013attainment/ozonesaturationstudy.pdf.
4. Kids Making Sense. Web site describing projects funded by the Knight Foundation, Miami, FL. See http://knightfoundation.org/
grants/201347645/ (accessed April 24, 2014).
33rd International Conference on Thermal Treatment
Technologies & Hazardous Waste Combustors
October 13-15, 2014 • Baltimore, MD
Conference Location:
Embassy Suites Baltimore – Inner Harbor
Hotel Rate: $155
+1-410-727-2222
Register Early and Save!
Register before September 15, 2014 and save up to
$150. Visit the registration page on the conference
website for pricing or to sign up now!
Professional Development Course
IT3/HWC is pleased to also offer a full day Professional
Development Course on October 13, 2014:
Air Pollution Control Basics Course for
Incineration & Thermal Treatment Technologies
View the course syllabus on the conference website.
Conference Supported by ASME-MER and WTERT
IT3/HWC, a thermal treatment conference, provides a forum for the
discussion of state of the art technical information, regulations, and
public policy on thermal treatment technologies and their relationship
to air emissions, greenhouse gases, climate change, renewable energy or
alternative energy production, and sustainability.
What are the Benefits of Attending the IT3/HWC Conference?
• Interface with facility owners and operators
• Develop new business and network with industry professionals and
colleagues
• Obtain updates on state of the art technical developments
• Stay current on regulatory and public policy initiatives
• Participate in training/courses related to thermal treatment
• Attend plant tours of state-of-the art facilities
For more information please visit http://it3.awma.org.
22_EM0814-FT3-Dye.indd 27 7/22/14 11:47 AM
Copyright 2014 Air & Waste Management Association