The Hidden Challenges of Measuring NO₂

If you’ve ever looked at purchasing an air quality monitor, you might have noticed something odd: while air quality monitors with PM2.5 sensors are everywhere and relatively affordable (starting at under $50), monitors that measure nitrogen dioxide (NO₂) are surprisingly rare and expensive, often costing over $1,000. Well, there’s an interesting technical story behind this price gap that reveals just how challenging it is to accurately measure certain air pollutants.

When we were developing the Open Air Max, we wanted to include an NO₂ sensor alongside our PM, CO₂, and other sensors. This led us into months of intensive research and testing, evaluating dozens of sensors. This journey really highlighted why the air quality monitoring industry has struggled with NO₂ sensors for years, and ultimately why so many monitors avoid NO₂ sensors altogether.

Today, we want to share some of these findings - some of which we were already familiar with, and some which were new to us - to provide a bit more background as to why it’s so hard to find (accurate) NO₂ sensors in affordable, or relatively affordable, air quality monitors.

Why NO₂ Matters for Your Health

Before we discuss NO₂ sensor technologies and their limitations, it’s worth first highlighting why we even wanted to measure this metric with our Open Air Max.

Nitrogen dioxide is one of the key pollutants that affects urban air quality, and it’s also one of the six pollutants that makes up the U.S. EPA AQI. It comes primarily from vehicle exhaust and industrial processes, and exposure to elevated levels can irritate airways, aggravate respiratory diseases like asthma, and contribute to the formation of other harmful pollutants.

Beyond its immediate effects on the lungs, long-term exposure to NO₂ has been linked to reduced lung function growth in children, increased risk of developing chronic respiratory conditions, and greater susceptibility to respiratory infections. People living in areas with consistently high levels of NO₂ often experience worsened overall cardiovascular health as well, since the gas contributes to systemic inflammation and oxidative stress. These health impacts mean that NO₂ is not just a short-term irritant but a long-term threat to public well-being, particularly in dense urban environments where sources are concentrated.

The Three Ways to Measure NO₂

Unlike particles that can be physically counted and sized, NO₂ is a reactive gas that interacts with other compounds in the atmosphere and even with surfaces inside the measuring device itself. This reactivity makes sensors prone to cross-sensitivity with gases like ozone, complicating readings and often requiring advanced correction algorithms or reference calibration. As a result, while affordable particle sensors are now widely available and reasonably reliable, obtaining trustworthy NO₂ measurements still demands more sophisticated equipment and careful data interpretation.

When it comes to detecting NO₂ in the air, we have three main sensor technologies, each with significant trade-offs:

1. Metal-Oxide Semiconductor (MOS) Sensors

Cheap but Unreliable

These sensors are very affordable ($5-20) and work by detecting changes in electrical resistance when NO₂ touches a heated sensing surface made of semiconductive materials. The problem? They have a high sensitivity to temperature and humidity changes, exhibit significant sensor drift and cross sensitivity to other pollutants, and they struggle to detect the low NO₂ concentrations typically found in ambient air.

MOS sensors still have some usefulness - for example, they may be suitable for indicative measurements and trend analysis, but they shouldn’t be relied on for absolute concentration measurements.

2. Optical Sensors

Laboratory-Grade but Impractical

On the opposite end of the spectrum, optical sensors use sophisticated light absorption techniques like NDIR or ultraviolet [UV] spectroscopy to measure NO₂ with exceptional accuracy and minimal cross sensitivity. This is the technology that most government and reference-grade monitoring stations use. The catch? Devices with these sensors often cost over $5,000, require significant power and maintenance, and are not very portable. With these limitations, they’re not exactly something well suited to a compact and affordable air quality monitor.

3. Electrochemical Sensors

The Middle Ground

This leaves us with electrochemical (EC) sensor modules (often costing $50-200), which seem like the perfect compromise. They work through chemical reactions that generate an electrical current proportional to NO₂ concentration (more details on exactly how these sensors work can be found here). In theory, they offer good accuracy at a reasonable price. However, in practice, even electrochemical sensors present a range of issues that need to be overcome for the best results.

The Four Major Headaches of Electrochemical NO₂ Sensors

When we were evaluating NO₂ sensors for the Open Air Max, electrochemical sensors seemed like the obvious choice - they represented the only viable middle ground between cheap-but-inaccurate MOS sensors and accurate-but-unaffordable optical sensors. However, our testing revealed why even this choice came with significant challenges.

1. The Ozone Problem

Electrochemical NO₂ sensors have a major weakness: they are known to be cross-sensitive to other gases, such as ozone (O₃), which is frequently present in ambient air. Both are oxidizing gases that produce similar signals in the sensor. This means that in urban environments where both pollutants are present, your NO₂ sensor might actually be measuring a mix of both gases without you knowing it.

There are a few ways to work around this, but even they come with caveats. You can add ozone filters as a coating, but they degrade over time - particularly in areas with high ozone levels - and can lead to decreasing accuracy over time. Another option is to add a dedicated ozone sensor (another significant cost) to correct for the cross sensitivity of the NO₂ sensor by using mathematical corrections (NO₂ (corrected) = NO₂ (raw) - a*O₃).

To overcome this issue, we added a second electrochemical (ozone) sensor to the Open Air Max.

2. Meteorological Conditions Can Cause issues

Unlike many other sensors that remain relatively stable across different weather conditions, NO₂ sensors are highly sensitive to changes in temperature and humidity. A sensor that reads accurately on a cool, dry morning might give completely different readings on a hot, humid afternoon – even if the actual NO₂ levels haven’t changed. Correcting for this requires additional temperature and humidity sensors, plus mathematical models that need to be calibrated for each specific environment.

Also accounting for ozone (as mentioned above), you can see that the correction formula quickly becomes more complex: NO₂ (corrected) = NO₂ (raw) - a*O₃ - b*Temp - c*RH

3. Every Sensor Is Unique

While PM sensors (among other sensors) from the same batch are usually quite consistent with each other, every single electrochemical NO₂ sensor behaves differently. Each one has its own baseline reading, sensitivity, and aging pattern. This means every sensor needs individual calibration against expensive reference equipment – you can’t just calibrate one and apply those settings to all your sensors

To ensure accuracy and account for each sensor’s unique characteristics, every NO₂ sensor must be calibrated individually against a reference instrument. Ideally, this calibration should be performed under the specific environmental conditions (e.g., NO₂ range, temperature, humidity, and O₃ levels) in which the sensor will be deployed. This “personalized” calibration ensures that the sensor’s readings are accurate, despite its intrinsic variability, and accounts for environmental factors that may influence sensor performance over time.

4. They Don’t Age Well

Perhaps the most frustrating aspect is that electrochemical NO₂ sensors degrade quite rapidly. While a PM sensor can often reliably operate for 4-5 years, electrochemical NO₂ sensors typically last only 250-600 days before needing replacement. The electrolyte dries out, the electrodes contaminate, and the sensitivity drifts. This means frequent recalibration and annual sensor replacements, driving up both costs and maintenance burden.

Real-World Testing

To illustrate just some of these challenges, our science team recently ran some tests in collaboration with the University of Cambridge. When four identical electrochemical NO₂ sensors from EC Sense were tested straight out of the box against laboratory-grade reference equipment (Chemoluminescence, Thermo Fisher Scientific Model 42i), the results were shocking:

• A relatively low signal-to-noise ratio - even with a time resolution of 5 minutes. This means the signal is significantly affected by unwanted interference or fluctuations (noise), reducing the accuracy and precision of measurements. This issue is particularly pronounced at lower concentrations, such as those typically found in ambient air.
• A shift in the baseline and capping of lower readings at ca. 10 ppb (i.e. the sensor is not able to produce readings below 10 ppb (which might, however, be recovered through post-processing).
• Poor correlation between the sensors’ NO₂ readings and the reference NO₂ measurements (R² values ranging from -0.1 to 0.23).
• Stronger correlation with the reference Ox measurements (R² values between 0.44 and 0.56), suggesting significant cross-sensitivity to O₃.

For comparison, PM2.5 sensors tested in similar conditions showed near-perfect agreement with each other (R² of 0.97-0.98) and strong correlation with reference instruments. While PM sensors might overestimate absolute values, this is easily corrected with simple calibration.

The Costs Add Up Quickly

When you factor in all these requirements, one of the hidden challenges of monitoring NO₂ quickly becomes apparent - it’s a lot more expensive than just purchasing a sensor:

• NO₂ sensor: $50-100
• O₃ sensor (for cross-sensitivity correction): $50-100
• Integration board: $100+
• Regular calibration against reference equipment
• Annual sensor replacement
• Ongoing maintenance and data processing

This explains why commercial NO₂ monitors start at $1,000 and can easily reach $5,000-7,000 for multi-pollutant systems. The ongoing operational costs can make NO₂ monitoring 5-10 times more expensive than PM monitoring over the system’s lifetime. Of course, this doesn’t even touch on the maintenance requirements!

What This Means for Air Quality Monitoring

The complexity and cost of NO₂ monitoring creates a challenging situation for community air quality initiatives and citizen science projects. While PM2.5 monitoring has become democratized with affordable, reliable sensors, NO₂ monitoring remains largely restricted to well-funded research projects and government monitoring stations.

This gap matters because NO₂ and PM2.5 often have different sources and patterns. A community might have low PM2.5 but high NO₂ from nearby traffic, or vice versa. Without affordable NO₂ monitoring, we’re only seeing part of the air quality picture. While electrochemical NO₂ sensors can provide sufficient accuracy, even for scientific and policy applications, achieving this level of accuracy requires careful calibration and compensation.

While progress with these sensors is being made, for now, if you’re considering monitoring NO₂, it’s important to understand these limitations. Measuring other pollutants (such as PM2.5) provides valuable insights at an affordable price. But for comprehensive air quality assessment including NO₂, be prepared for significantly higher costs and complexity – or advocate for more public monitoring stations in your area.

Unfortunately, not all environmental measurements are created equal and sometimes, the difference between a $100 and $1,000 monitor isn’t just about features, but instead is about the fundamental physics and chemistry that make some measurements inherently more challenging than others.