TL;DR—While not suitable for regulatory purposes, low-cost air quality sensors have a wide range of applications for non-regulatory supplemental and informational monitoring (NSIM). Using sensor technology in urban environments presents an opportunity to collect cost-effective, high-resolution air quality data that provides deeper insight into pollution trends in cities than might be possible with FRM/FEM equipment. When best practices for planning, deploying, and maintaining these devices are followed, low-cost monitoring networks can be effectively used to fill in the air quality monitoring gaps that exist in urban environments and paint a more complete picture of air quality—providing the data points that are desperately needed to protect human health in increasingly populated and polluted areas.

Note: To hear from a panel of air quality experts from cities like London, Houston, and Denver on best practices for deploying low-cost sensors in urban environments, register for our webinar on Friday, June 11th, 2021, at 9 AM Pacific: Best Practices for the Use of Low-Cost Air Quality Sensors in the Urban Environment

Air pollution in cities

A majority of the world’s population lives in urban areas, and these numbers are increasing. By 2050, it is estimated that 2.5 billion more people will live in urban areas than in 2018. With high population density and a variety of pollution sources—such as high motor vehicle congestion, heat and power generation facilities, and municipal waste incineration, among others—cities desperately need better air pollution monitoring. 

A study of air pollution trends across the United States found that urban counties experience a greater number of days with elevated particulate matter (PM2.5) and ozone (O3) levels compared to non-urban counties. These differences are stark: for ozone, metropolitan counties were found to average 47.54 days with ambient concentrations greater than the U.S. Environmental Protection Agency’s (EPA’s) National Ambient Air Quality Standards (NAAQS), compared to 3.81 days in other counties. For PM2.5, the study showed 11.21 days for urban counties versus 0.95 days for non-urban counties. Baseline levels of PM2.5 were also found to be higher in urban counties.

Air pollution in urban environments is a function of not only the number and density of residents, but also of the associated facilities typically located in cities. Landfills, wastewater treatment, and intensive farming account for the release of volatile organic compounds (VOCs), adding to air pollution. Due to the variety of pollutant sources, air quality may not be uniform across an urban area. Elevated pollutant levels known as hotspots can occur, and certain neighborhoods—such as an area located near a pollution source like a powerplant—can experience heightened air pollution levels. Air pollution may not be dispersed evenly across a geographic region—for example, pollution from vehicle emissions may be seen at greater concentrations, and stick around for longer, in a highly developed area with a busy road.

Traditional air quality monitoring equipment in the urban landscape

Regulatory air quality monitors, such as Federal Reference Method (FRM) and Federal Equivalent Method (FEM) equipment, represent a traditional approach to air quality monitoring. While these types of monitors meet stringent performance parameters, they can be expensive and difficult to site. Few government monitors tend to be present in any given city—for example, the entire city of San Francisco has only one regulatory monitor. Because of the variable dispersion of air pollution across a city, these types of monitors often fall short of reflecting the dynamic nature of air quality. They can miss the disparities in air pollution experienced by certain neighborhoods or groups of residents, failing to provide a full picture of the pollution in a given area.

The Air Quality Monitoring Map above from the Bay Area Air Quality Management District shows the single regulatory air quality station in San Francisco. While there are over 30 ambient air quality monitors in this nine-county region, these federal monitors can often miss local trends and pollution disparities in specific neighborhoods. (Image source: Bay Area Air Quality Management District)

Low-cost sensors’ lower price tag and ease of deployment make them a more flexible and adaptable solution to air quality monitoring in cities. Low-cost sensors allow for greater spatial and temporal resolution in monitoring, and they can help “fill in the gaps” for air pollution trends that government monitors may miss. Dozens of low-cost sensors can be deployed for the same cost as a single FRM or FEM, and they also tend to have greater flexibility in where they are sited. Low-cost sensors do not require extensive infrastructure like data shelters, unlike government monitors. Furthermore, as some low-cost sensors rely on solar power and cellular connectivity, they can be entirely self-sufficient and require no pre-existing infrastructure to function.

With more real-time, immediate data, air quality managers and citizens alike can utilize the findings from low-cost sensors to take prompt action when air pollution is high. Chicago, Illinois has explored this capability with a network of sensors to measure both air quality metrics and other parameters, such as light and ambient noise, around the city. Integrating different types of technology helps cities obtain a more complete understanding of the factors that influence air pollution given the unique conditions in their jurisdiction. 

London has also embraced low-cost sensors as part of its air quality monitoring program. Through the Breathe London project, more than 100 low-cost sensors have been deployed at hospitals, schools, and other key locations to complement London’s existing government monitors with their availability of real-time data. The city has achieved greater data coverage and resolution as well as greater community involvement, driven by the access to this air quality data. To learn more about London’s adoption of low-cost sensors and how they will be integrated with the regulatory London Air Quality Network, check out our press release here.

The Breathe London team was able to deploy more than half a dozen sensors at schools across London in a single day. Using solar-powered, cellular-connected sensors can drastically simplify deployment and minimize operational costs.

Planning and deploying an air quality monitoring network

While low-cost sensor networks can be easier and more cost-effective to implement than a traditional monitoring network, their successful implementation still requires a good deal of planning. We provide an overview of the steps that should be taken to ensure a successful deployment below here—for more detail, our full Guide to Leveraging Low-cost Sensors for Air Quality Monitoring 2.0 is available for download here

This infographic provides an overview of the steps that should be taken when planning a low-cost sensor air quality monitoring network.

Planning the network

Because low-cost sensors differ significantly from regulatory equipment—in terms of both operational requirements and data outcomes—it is important to consider the infrastructure already in place. This will help determine which technology and network design best complement the systems available in a given city or environment.

In the initial stages of planning an air quality monitoring network, begin by looking at the existing infrastructure and asking, “What questions need to be answered that cannot be addressed with the existing infrastructure?” This will help think through air monitoring goals and determine the equipment needed to address these goals.

The level of technical knowledge required for the installation and operation of the sensors should also be noted, as well as how vandalism and theft of the monitors will be prevented. Low-cost monitors differ in their level of resilience to weather and other environmental conditions, so the needs and conditions at play in the specific urban environment should be considered before the technology is selected.

Certain types of low-cost sensors leverage solar energy for self-powered air quality monitoring. Ensuring that a solar-powered sensor is exposed to adequate sunlight, is adjusted depending on the season, and has a debris-free solar panel are important steps to take for effective solar-powered monitoring.

Lastly, consider the budget available to implement a low-cost sensors network. Funding an air quality sensor network may require some adjustments to planed budgets—or looking to external sources for funding—but the low upfront and operational costs associated with low-cost sensors make them a great fit for initiating a new air monitoring program or supplementing an existing one. 

Validating the technology 

Choosing the specific air quality sensor technology to use is a crucial step in planning a network that will be the right fit for a given urban environment. Air quality managers should determine what pollutants they wish to monitor and the precision of measurements that are needed to achieve their monitoring goals to come up with a shortlist of potential equipment to use.  The USEPA Performance Testing Protocols, Metrics, and Target Values for Air Sensors detail how to carry out laboratory and field testing of air sensor equipment to determine the quality of data that low-cost sensors produce. 

They should then consider whether they wish to simply purchase the equipment or work with an air quality monitoring provider that can serve as a partner throughout the lifecycle of the network. 

For example, air quality managers or community groups wishing to establish a monitoring network should also pay attention to who is responsible for data calibration. This is a vital part of gaining helpful insights from air sensor data but requires a good deal of technical expertise. For more information on calibration, how it helps with data accuracy, and how low-cost sensor performance is typically evaluated, check out our Guide to Accurate Particulate Matter Measurements: How to Meet USEPA, EU, and Other Performance Standards for Air Sensors.

Data security and accessibility are also factors, as well as which party owns the data under the air quality monitoring provider’s standard terms of service. A full checklist of the factors to consider when choosing an air quality monitoring partner is available here

Working in partnership

A successful monitoring network typically brings together a cohort of technology, community, and deployment partners to take a collaborative approach to air quality management. Beyond choosing appropriate monitoring technology, leveraging community groups’ knowledge about air pollution sensing needs helps ensure that the network suits the specific area’s demands. 

Working with trusted community members when making decisions about the network and involving local residents in the air quality monitoring program also helps effectively communicate the data findings that have been interpreted by air quality scientists. When it comes time to set up the network, local deployment partners can lend insight into the ideal locations to place devices and can help physically deploy them as well.

Siting and deploying air quality monitors

The decision of where to site low-cost air quality sensors in an urban environment is a combination of technical and practical considerations. This comes back to ensuring the network is designed with the goals of the air quality monitoring program in mind. For example, to look at how air pollution is affecting children, a city may choose to site their network at schools.

Whether the objective is to measure a certain pollutant source, a certain site, or general air pollution in the community will affect siting decisions, as will the level of spatial and temporal resolution required. Setting performance targets beforehand can help clarify these goals. 

Wherever the low-cost sensors are sited, it is important to reach out ahead of time to secure site permissions to ensure access to the site for installation and maintenance. One advantage of low-cost sensors is that they can be sited almost anywhere, making it easier to find an appropriate site and obtain a permit.

A Clarity air sensor is installed on a rooftop in San Francisco, CA, by Daniela Cortes of Brightline Defense. Brightline worked with local business owners to obtain permission to deploy sensors on their rooftops, which can offer a secure location in good proximity to areas where air pollution should be measured.  

When surveying the site, it is important to determine where and how the devices will be mounted. Depending on what is available in the given environment, a pole, fence, railing, or wall mount can be used. 

As part of the Breathe London project, additional low-cost air quality monitors were recently deployed in schools, nurseries, and around the community in the Stratford region of London. (Image source: Breathe London)

Deploy the sensors according to the manufacturer’s guidelines—in Clarity’s case, we recommend that sensors be placed: 

  • At least 6 feet away from obvious pollution sources like an exhaust vent, generator, or grill
  • Where the air inlet/outlet is unobstructed and experiences adequate airflow
  • At a height of 10 to 16 feet from the street level if measuring the general population exposure, or higher if measuring regional air quality
  • In a secure location
  • With access to sunlight if solar-powered
A Node-S sensor is installed on a streetlight in San Francisco, CA, by Eddie Ahn of Brightline Defense. Streetlights are often a good siting option as sensors can be deployed at an appropriate height and in proximity to pollutant sources. 

We always recommend taking photos of the deployed devices as this can be helpful when managing the network down the line. For more information about sensor deployment, read our deployment guide.

Planning a colocation

Colocating a low-cost sensor with a reference monitor in the same area where monitoring will take place is an essential step to enable the effective calibration of data from the network. The outcome will be a dataset of time-matched data from both the reference monitor and the low-cost sensors, which should be used to create a calibration model that is optimized for local environmental conditions. 

Low-cost sensor maintenance

Because different types of sensors vary in the level of maintenance required, this is an important factor to consider when planning a monitoring network. Sensors that require Internet or an electrical connection may require more maintenance and site visits, and those which have to be physically calibrated or adjusted tend to incur more operational costs. Our self-sufficient devices and Remote Calibration model help minimize the operational costs associated with a low-cost sensor network—learn more about our maintenance requirements here and our Remote Calibration approach here.

How to use and communicate the data 

While gathering data is the first step in learning more about air pollution trends in a given environment, the data alone will not create an impetus for change. The effective communication of findings and what they mean for the community is vital to improving air quality. 

Network managers should determine the extent to which community residents will have access to the data—such as whether the data will be publicly available on a website or app—and how the results of this data will be presented. Would it make sense to publish an annual or quarterly report including data from the network? Who would be the intended audience for the report? 

When communicated effectively, air quality data can impact a city’s actions to control air pollution. For example, a manager may decide to add more air sensors in areas with high readings to further investigate the local air pollution trends, or push for policies that would better control the pollution sources in that area. Air quality monitoring data may also act as supporting evidence for local, state, or national policy changes or other pollution abatement initiatives.

The future of urban air quality monitoring

Urban environments have a clear need for high-resolution air quality monitoring due to their high levels of air pollution, significant variability of pollution at the neighborhood level, and denser and larger populations. By supplementing existing infrastructure with low-cost sensors, air quality managers and community organizations can leverage the lower cost, greater flexibility, and higher resolution made possible by this technology to paint a more complete picture of air quality in a given urban environment.

Understanding the needs of the city or urban environment is a primary and essential step to success to ensure that low-cost monitor data is leveraged to best suit the city’s needs. This allows for sensors to be sited, deployed, and maintained according to these goals. Effective air quality monitoring programs also recognize the importance of data calibration, because accurate data is essential for effective decision-making.

Low-cost sensors are an excellent way to fill in the air quality monitoring gaps that often exist in urban environments. To hear from a panel of air quality experts from cities like London, Houston, and Denver on best practices for deploying low-cost sensors in urban environments, register for our webinar on Friday, June 11th, 2021 at 9 AM Pacific: Best Practices for the Use of Low-Cost Air Quality Sensors in the Urban Environment