Nitric Oxide (NO) Gas Sensor

Nitric oxide is the primary nitrogen oxide emitted directly from combustion processes — vehicle engines, construction generators, industrial boilers, and power plants all produce NO as a combustion byproduct. In the atmosphere, NO rapidly oxidises to nitrogen dioxide (NO2), making it a critical precursor pollutant. Measuring NO alongside NO2 enables calculation of total NOx (NO + NO2), the standard metric used in emissions monitoring and air quality assessment. The Sensorbee NO Sensor is an electrochemical module with a 3 ppb limit of detection, 1 ppb resolution, and a fast sub-45-second response time, designed to work alongside the NO2 sensor for comprehensive nitrogen oxide monitoring.

How It Works

The NO sensor uses electrochemical detection — nitric oxide molecules diffuse through a gas-permeable membrane and react at a working electrode, generating an electrical current proportional to the NO concentration. The electrochemical cell design is optimised for nitric oxide selectivity, minimising interference from other gases commonly present in ambient air.

At 55 × 40 × 30 mm and 25 grams, the module matches the form factor of all Sensorbee electrochemical gas sensors. It connects to the Air Pro 2 or Air Lite base station as a plug-in module, where the platform manages signal processing, temperature compensation, and data transmission. Factory calibration to CEN/TS 17660-1:2022 standards means the sensor is ready for deployment immediately upon installation.

The sub-45-second response time makes the NO sensor one of the faster-responding modules in the range — important for capturing the short-duration concentration peaks associated with passing traffic and intermittent combustion events.

Why NO Monitoring Matters

While nitrogen dioxide (NO2) receives more attention as a regulated pollutant, nitric oxide measurement is essential for understanding nitrogen oxide emissions and atmospheric chemistry:

• Total NOx calculation — air quality regulations and emission standards typically reference total NOx (NO + NO2). Measuring both species provides the complete nitrogen oxide picture that regulators and environmental assessments require
• Source proximity indicator — NO concentrations are highest close to emission sources (traffic, exhaust stacks) because NO is the primary emission product. High NO/NO2 ratios indicate fresh emissions, while low ratios suggest aged air mass where atmospheric oxidation has converted NO to NO2
• Ozone chemistry — NO reacts rapidly with ozone (O3), destroying ozone near emission sources. Understanding this NO-O3 interaction is critical for interpreting ozone monitoring data and predicting downwind ozone formation
• Emission characterisation — different combustion sources produce different NO/NO2 emission ratios. Monitoring both species helps identify contributing sources in mixed-source environments

Measurement Performance

The NO sensor delivers ±7 ppb typical accuracy with 4 ppb repeatability across a 0–5,000 ppb measurement range. Near busy roads and combustion sources, NO concentrations frequently reach several hundred to over 1,000 ppb during peak traffic or operational periods, making the 5,000 ppb range appropriate for most near-source monitoring applications.

The 3 ppb limit of detection (CEN/TS 17660-1:2022 certified) provides the sensitivity to measure background NO levels in suburban and rural locations — typically 1–10 ppb — where total NOx monitoring supports air quality assessment and atmospheric research.

The 20 ppm guarantee range provides headroom for extreme near-source exposure without sensor damage.

NOx Monitoring: NO + NO2 Together

The primary application for the NO sensor is paired deployment with the Sensorbee NO2 sensor (SB4202) on the same Air Pro 2 or Air Lite base station. Together, these two modules provide:

• Simultaneous NO and NO2 measurement — independent electrochemical sensors measure each species with their own calibrated accuracy
• Calculated total NOx — the Sensorbee Cloud computes and displays NO + NO2 = NOx values from the paired measurements
• NO2/NOx ratio analysis — the proportion of primary NO2 in total NOx provides information about emission source types and atmospheric processing
• Complete nitrogen oxide profile — visualise NO, NO2, and total NOx on the same time axis for comprehensive air quality reporting

This paired approach avoids the limitations of molybdenum converter-based NOx analysers, which convert NO2 to NO for measurement but also convert other nitrogen species, potentially overestimating true NOx.

Integration With Sensorbee Platforms

The NO sensor plugs directly into the Air Pro 2 or Air Lite base station. No configuration is required — the platform detects the sensor module and begins data acquisition automatically.

For a complete traffic and combustion emissions monitoring station, deploy the NO sensor alongside:

• NO2 sensor (SB4202) — for total NOx capability
• CO sensor (SB4262) — carbon monoxide from incomplete combustion
• Particulate matter module — PM from diesel and other combustion sources
• O3 sensor (SB4272) — to track the NO-O3 interaction and photochemical ozone formation

All sensor data flows through the base station's cellular connection to the Sensorbee Cloud, where multi-parameter analysis reveals the relationships between different pollutants and their sources.

Deployment and Maintenance

Insert the NO sensor into an available port on the Air Pro 2 or Air Lite. The base station recognises the module and measurement begins after the brief warm-up period. No field calibration, wiring, or software setup is required.

The sensor operates continuously for over one year. Replacement is a plug-and-play process: remove the depleted module and insert a new factory-calibrated unit in under two minutes. For traffic monitoring networks operating multiple stations, plan annual sensor replacement to maintain consistent data quality across all measurement points.

The -30 to +50°C operating range and 15–85% RH humidity tolerance ensure reliable operation across seasons and climatic conditions.

Applications

• Traffic emissions monitoring — measure NO at roadside and near-road locations, paired with NO2 for total NOx assessment of vehicle emission impacts
• Construction site monitoring — track NO from diesel generators, excavators, and other combustion plant at site boundaries alongside other combustion pollutants
• Industrial emission assessment — NO measurement at facility perimeters to characterise combustion emissions from boilers, furnaces, and process equipment
• Port and shipping monitoring — detect vessel engine emissions using the NO/NO2 pair to identify combustion-related pollution events at port facilities
• Air quality research — high-resolution NO data for atmospheric chemistry studies, source apportionment, and photochemical modelling of ozone formation
• Environmental impact assessments — NO and NOx baseline monitoring to support planning applications and environmental permit conditions for combustion sources

Frequently Asked Questions

Do I need both the NO and NO2 sensors?

For total NOx monitoring, yes — NOx is defined as NO + NO2, so both sensors are required. If your monitoring programme only requires NO2 measurement (the most commonly regulated species), the NO2 sensor alone is sufficient. However, adding the NO sensor provides significantly richer data about emission sources and atmospheric chemistry.

Why is the NO sensor faster than the NO2 sensor?

The NO sensor responds in under 45 seconds, compared to under 80 seconds for NO2. This reflects differences in the electrochemical cell design and the diffusion characteristics of the two gas molecules through the sensor membrane. The faster response is beneficial for capturing transient NO peaks from individual vehicles or intermittent combustion events.

How does the Sensorbee Cloud calculate NOx?

The platform sums the simultaneous NO and NO2 measurements from co-deployed sensors: NOx = NO + NO2. Both concentrations are in ppb, and the sum is reported as total NOx in ppb. For reporting in µg/m³, the platform applies appropriate conversion factors based on ambient temperature and pressure.

What is the NO/NO2 ratio and why does it matter?

Fresh combustion emissions typically have a high NO/NO2 ratio (e.g., 90% NO, 10% NO2 for older diesel engines). As the emission plume ages and NO oxidises to NO2, the ratio decreases. Monitoring this ratio over time and distance from sources provides information about emission freshness and atmospheric processing.

Can this sensor detect NO from specific vehicles?

The sensor measures ambient NO concentration, not emissions from individual vehicles. However, at near-road locations, concentration peaks associated with individual heavy vehicles or vehicle platoons can be observed in the real-time data, particularly at low wind speeds.