Terra Ay Ay Project empowers St. Croix residents with real-time air quality data

DOCUMENT DISCLAIMER: This information is not intended to be an endorsement, but rather the Terra Ay Ay Project collaborators sharing what we’ve learned during our experience of building a community-based air quality network on St. Croix. There are many different brands of air quality monitors available. We encourage you to conduct your own research before acquiring an air monitor of your own. Terra Ay Ay is a citizen-science project collecting data on St. Croix’s air, water, and soil quality. The project is a collaboration between The St. Croix Foundation, CHANT (Crucian Heritage and Nature Tourism), SEA (St. Croix Environmental Association), Virgin Islands Good Food, the University of the Virgin Islands – as well as St. Croix scientists, teachers, students, and volunteers. This project has been funded wholly or in part by the United States Environmental Protection Agency under assistance agreement 96241922 to St. Croix Foundation. The contents of this document do not necessarily reflect the views and policies of the Environmental Protection Agency, nor does the EPA endorse trade names or recommend the use of commercial products mentioned in this document.

Knowing what is in the air you breathe and how it can affect your health,

helps empower you to make healthier choices, whenever possible.

Collecting data on air quality can help community initiatives
when they advocate for safer air quality practices and
increased transparency from potential polluting sources.

The information shared here is for any community member interested in or concerned about air quality in and around St. Croix and how it may affect human and environmental health.

The data collected helps inform current and future projects that aim to advocate for more data, reporting, and transparency to protect the health and wellness of St. Croix’s community.

We hope teachers, students, and citizen scientists find this network and its data engaging and informative and that it inspires future generations of air quality stewards and enquiring minds.

💡To learn more about the Terra Ay Ay Project, visit our website: https://terra-ayay-project.org/

Building the Air Quality Network on St. Croix:

The Terra Ay Ay project will install 15 PurpleAir PA-ll FLEX Monitors at participating St. Croix residents’ homes, to build a community-based air quality network that provides hyper-local air quality data, in real-time, to anyone with access to the internet.

>> This real-time data map of St. Croix can be seen here ( Link to PA map ) <<

💡 How to use the PurpleAir Map

The Terra Ay Ay collaborators specifically chose the PurpleAir brand of air monitors because they are considered “low-cost sensors” and are used globally by many community air quality projects. These projects have laid extensive groundwork that can be used to help support the growth and sustainability of St. Croix’s community-based air quality network.

“Known for their highly precise data, PurpleAir air quality monitors have been gaining widespread acceptance in the air quality research and academic community. Reputable organizations like ExactAQ, AirNow, and NASA, also use them for air quality maps and programs.

More than that, PurpleAir air quality monitors are user-friendly. That means anyone, even those without an academic or technical background, can install and use the air quality monitors themselves. As a result, PurpleAir has a vast air quality network across the globe.”

⇨ Case Study: How the RISE Communities Program is Fighting for Environmental Justice

PurpleAir hosts an extensive web forum (https://community.purpleair.com/categories) and Facebook group (https://www.facebook.com/share/g/15iA6G8YZG/) where interested folks can engage, ask questions, and look for answers and feedback.

PurpleAir also has years of helpful articles published on their blog to help guide individuals and community groups along the path of air quality advocacy and citizen science. (https://www2.purpleair.com/blogs/blog-home)

“PurpleAir is built on the ideals of sharing data and empowering individuals to institute real change.” – PurpleAir

The Map/The Data:

The air quality data collected by the network of air monitors can be found on the PurpleAir Map which:

displays readings from thousands of sensors globally.
is accessible to anyone who has access to the internet.
supports a graph feature that can be used to easily view/download data from sensors. Which can be useful when looking to understand the recent history of a monitor and its air quality readings.

💡 Tips on to use the PurpleAir map, here.

💡 How to use PurpleAir graph, here.

💡 Tips on downloading map graph data, here.

The PurpleAir map automatically presents the US EPA PM2.5 data set and correlates the results to indicate where the air quality data falls on the EPA AQI (Air Quality Index).

You can switch the map to display other data sets, see how here.

“The U.S. Air Quality Index (AQI) is EPA’s tool for communicating about outdoor air quality and health. The AQI includes six color-coded categories, each corresponding to a range of index values. The higher the AQI value, the greater the level of air pollution and the greater the health concern. For example, an AQI value of 50 or below represents good air quality, while an AQI value over 300 represents hazardous air quality.”

–AirNow.gov

💡 Read more about the Air Quality Index here.

💡 Read more about air quality effects on health here.

💡Download the EPA’s guide on the AQI and your health here.

💡Download the EPA’s guide on Particle Pollution and your health here.

💡 Air Quality Guide for Schools here

🙋🏾Do you have questions about the PurpleAir Map and the data that you see?

You can discuss it with a global community of air quality advocates and citizen scientists at the

The PurpleAir Forum and

The PurpleAir Facebook Page

Clicking on multiple monitor icons lets you compare their data, side by side, on the graph – this is especially helpful when you are trying to determine a pollution source.

Evident in the graph above, which charts data from many sensors, you can see that sensors often register short-lasting, but high AQI data spikes.

With the feedback from our network of air sensor hosts, we have deduced that these short spikes are most commonly caused by hyper-local* pollution sources. (*Hyper-local, in this context, means the air quality spike is only registering on one sensor in the area, while other local sensors are showing more common air quality levels.)

Some common examples of these hyper-local pollution sources have been reported to be:

grass cutting
broom sweeping
nearby yard waste fires
vehicle emissions: sometimes caused by a car sitting while running in the driveway, sometimes caused by commuter traffic.
BBQ Grills
incense or smoking near a sensor

To help visualize some of these spikes and their causes, we captured a few examples of hyper-local air quality ‘events’ since installing the first air monitors on St. Croix in 2024.

The above images of multiple air quality spikes above 75 are from a construction crew cutting/drilling concrete, 20-50 feet from a sensor.

⚠️ Don’t be alarmed by the high AQI reading – this was a citizen science experiment, conducted to see what might cause a high spike in the air quality data ⚠️

The above images of an air quality spike to 323 are from a citronella incense stick lit and left to burn, 10 feet from a sensor.

One example of a broader air quality ‘event’ would be Saharan Dust Plumes. Since these plumes mostly affect the whole Caribbean region you will usually see the AQI readings spike across sensors on multiple islands.

The Caribbean region experienced a Saharan Dust plume in Dec 2024 (https://viconsortium.com/vi-community_center/virgin-islands-saharan-dust-event-to-impact-usvi-and-puerto-rico–public-health-advisory-issued )

The screenshot on the left, from Dec 23, 2024, shows elevated AQI readings on sensors island-wide, which coincided with the beginnings of a Saharan dust plume in December 2024.

The screenshot on the right, from Dec 10, 2024, shows AQI readings that are more common for St. Croix when there is not an air quality ‘event.’

“The VI Department of Health (VIDOH) advises the public to be cautious about spending time outdoors when the Virgin Islands are experiencing clouds of Saharan Dust.” – VIDOH:
Article: DOH Advises Residents to Be Cautious When Saharan Dust is Present

The Sensors:

The PA-ll FLEX Outdoor monitors have three individual sensors measuring data in real-time.

Two Plantower PMS6003 laser particle counters measure Particulate Matter (PM.)

On the PurpleAir map, these will show up on graph data as sensors ‘A’ or ‘B’

Particulate Matter – Particulate Matter or PM is a complex mixture of small solid particles and liquid droplets in the air. Some particulate matter, like soot, smoke, dust, or dirt, is large enough to see. Fine particulate matter is so small that it can only be seen through an electron microscope. (source: EPA Particulate matter basics)

The Plantower PMS6003 laser particle counters have laser beams that detect particles by their reflectivity.

These sensors count suspended particles in sizes of 0.3, 0.5, 1.0, 2.5, and 10 micrometers

These particle counts are processed by the sensor using a complex algorithm to calculate the:

PM1.0 = particulate matter 1.0 microns (micrometers) in diameter and smaller

PM2.5 = particulate matter 2.5 microns (micrometers) in diameter and smaller

PM10 = particulate matter 10 microns (micrometers) in diameter and smaller)

💡Read more about particulate matter here: https://www.epa.gov/pm-pollution/particulate-matter-pm-basic

The third sensor, a Bosch BME680/688 sensor, measures:

Pressure
Temperature
Humidity (%)
Volatile Organic Compound (VOCs) experimental*

Volatile Organic Compounds (VOC): Many are human-made chemicals, industrial solvents, often components of petroleum fuels, hydraulic fluids, paint thinners, and dry cleaning agents. VOCs are common ground-water contaminants. Volatile organic compounds (VOCs) are emitted as gasses from certain solids or liquids. VOCs include a variety of chemicals, some of which may have short- and long-term adverse health effects. (source: EPA What are Volatile Organic Compounds)

*At this time, VOC readings are in an experimental phase. Further details about the BME680 are available in its data sheet. It should be noted that the VOC measurements from PurpleAir devices are purely experimental. We have not seen any research or evaluations indicating that they are accurate.

The sensors inside of the PA-ll FLEX Outdoor monitors have an average lifespan of around 2 years; before the readings become unreliable. The sensors inside of the monitors can be replaced without having to purchase a whole new sensor.

Moving Forward:

This air quality sensor network is a community resource to help us not only better understand air quality but also to help advocate for cleaner air practices. When we collect our own data and make it available to our community we are equipping ourselves with more tools to help advocate for cleaner air and healthier communities.

Many community projects exist globally, that are advocating for their air quality rights, collecting their own data, and engaging in citizen science. These projects have helped to lay groundwork that our community can study and replicate, to serve our specific needs.

💡To explore other community projects that have utilized PurpleAir sensors follow this link: https://community.purpleair.com/t/purpleair-in-the-community/809

“Citizen Science is a collaboration between scientists and those of us who are just curious or concerned and motivated to make a difference.

Four common features of citizen science practice:

(a) anyone can participate,

(b) participants use the same data collection methods so data can be combined and be high quality,

(d) a wide community of scientists and volunteers work together and share data to which the public, as well as scientists, have access.”

–The GLOBE program

We sincerely hope that these sensors and the data they collect are of value to community members and that even when The Terra Ay Ay Project is gone (September 2025) folks are impassioned to stay active in maintaining a local St. Croix air quality network.

There are many air quality sensors available and the sensor capabilities evolve as rapidly as air quality science evolves. The collection of data helps inform the growing scientific evolution of clear-air science.

We recommend everyone does their own research before purchasing a sensor of your own. We also encourage the use of community forums to discuss air quality, and to share resources with those in your community who you know are interested and/or concerned about air quality.

Websites, Articles & Additional Air Quality Resources:

AirNow Fire & Smoke Map (includes data from public PurpleAir sensors)

NASA Earthdata: Air Quality

American Lung Association: Clean Air Page

CDC: Air Pollution Page

PurpleAir Blog: 5 Environmental Activists Advocating for Better Air Quality to Follow

AQMD: Community in Action – A Comprehensive Guidebook on Air Quality Sensors

EPA’s Air Sensor Toolbox (Information on sensors, including sensor performance, guides to siting & installation, and best practices. The Air Sensor Toolbox provides information for technology developers, air quality managers, citizen scientists and the public.

EPA Webinar Series: Community Air Monitoring Fundamentals

>> USVI and PR Air Quality Resources:

DPNR’s Air Pollution Control Program

VIDOH Air Quality Page

El Programa de Asma de Puerto Rico se encarga de crear y mantener alianzas para controlar la morbil

Port Hamilton Refining and Transportation

EPA: Benzene Fenceline Monitoring Data Collection and Reporting Dashboard

> Intro to the Benzene Fenceline Monitoring Data Collection and Reporting Dashboard (video)

>> Resources for Air Quality and Human Health:

AirNow: Older Adults and Air Quality

PurpleAir Blog: Air Quality Illnesses: How Poor Air Quality Affects Your Respiratory System

EPA Fact Sheet: Age Healthier Breathe Easier Information for Older Adults and their Caregivers