Why nitric oxide monitoring matters alongside NO2. Learn about the NOx cycle, industrial NO sources, and how combined NO/NO2 data improves emissions analysis.
When environmental consultants assess nitrogen oxides, the focus typically falls on nitrogen dioxide — the regulated pollutant with established ambient air quality limits. But between 90% and 95% of the NOx leaving a combustion source is emitted as nitric oxide (NO), not NO2. Without dedicated nitric oxide monitoring, you are observing only the end product of atmospheric conversion, not the full emission picture.
Measuring NO alongside NO2 transforms NOx data from a compliance checkbox into a diagnostic tool. The ratio between the two gases reveals how far pollution has travelled, how recently it was emitted, and which source is responsible. For environmental consultants working near industrial operations, traffic corridors, or construction sites, that distinction matters.
What Is Nitric Oxide and How Does It Differ from NO2?
Nitric oxide (NO) is a colourless, odourless gas produced whenever fuel burns at high temperatures. It forms when the heat of combustion — typically above 1,300 degrees Celsius — forces atmospheric nitrogen (N2) and oxygen (O2) to react. Every diesel engine, gas turbine, boiler, and furnace generates NO as a primary combustion byproduct.
Nitrogen dioxide (NO2), by contrast, is a reddish-brown gas with a sharp, acrid odour. Although a small fraction of NO2 is emitted directly from combustion, the majority forms in the atmosphere when NO reacts with available oxidants — principally ozone (O3). This is the critical distinction: NO is the primary emission, while NO2 is largely a secondary pollutant created after release.
Understanding NO vs NO2 matters because regulatory frameworks target NO2 for ambient air quality (40 micrograms per cubic metre annual mean in the UK), yet the pollutant actually leaving the source is predominantly NO. Effective NOx measurement requires tracking both species.
The NOx Cycle — How NO Becomes NO2 in the Atmosphere
The conversion of NO to NO2 is not a one-way process. It follows a continuous photochemical cycle that depends on sunlight, ozone, and volatile organic compounds (VOCs).
When NO is emitted into the atmosphere, it reacts rapidly with ozone. One molecule of NO combines with one molecule of O3 to produce NO2 and O2. Depending on the ozone concentration and ambient conditions, this conversion can occur within minutes to a few hours.
In sunlight, the process reverses. Ultraviolet radiation breaks NO2 apart, releasing NO and a free oxygen atom. That oxygen atom combines with O2 to regenerate ozone. The ozone then oxidises more NO back to NO2, and the cycle continues. This equilibrium — known as the photostationary state — means that in clean air, NO and NO2 constantly interchange without any net production of ozone.
The balance breaks down when volatile organic compounds are present. VOCs react with hydroxyl radicals to produce peroxy radicals (HO2 and RO2), which provide an additional pathway for converting NO to NO2 without consuming ozone. The result is a net accumulation of ground-level ozone — the photochemical smog that affects urban and industrial areas during warm, still weather.
For NOx ambient air monitoring, this chemistry has a practical consequence: the NO:NO2 ratio you observe at any point depends on how far the air has travelled from the emission source and what it encountered along the way.
Industrial and Environmental Sources of Nitric Oxide
The UK emitted 538,000 tonnes of NOx in 2024, representing an 80% decrease since 1990 — a substantial reduction driven by catalytic converters, stricter industrial standards, and the shift from coal to gas and renewables.
The current sectoral breakdown tells a clear story about where NO monitoring matters most:
- ·Road transport accounts for 30% of UK NOx emissions, although this share is falling as fleet electrification accelerates.
- ·Energy industries contribute 20%, primarily from gas-fired power generation.
- ·Industrial combustion makes up 15% — and since 2010, industrial combustion has overtaken road vehicles as the single largest NOx source category in the UK.
- ·Non-road transport — shipping, aviation, and rail — adds 13%.
At the point of emission, virtually all of this NOx is NO. A diesel exhaust plume is typically 90-95% NO by volume. A gas turbine stack is similar. This is why NO monitoring is indispensable for source characterisation: if you measure only NO2, you are detecting the pollutant only after atmospheric processing has occurred.
When Is Dedicated Nitric Oxide Monitoring Required?
Several scenarios demand separate NO measurement rather than relying on NO2 alone:
Source characterisation near industrial operations. When an environmental permit requires understanding which process generates NOx, the NO:NO2 ratio at the monitoring point identifies fresh versus aged emissions. A monitoring station showing high NO relative to NO2 indicates close proximity to an active source.
Environmental permit compliance. The Environmental Permitting Regulations 2010 and the Medium Combustion Plant Directive require regulated combustion installations to monitor and report NOx. Stack emissions testing (continuous emissions monitoring systems, or CEMS) measures total NOx, which requires quantifying both NO and NO2 separately.
Traffic and roadside assessment. Fresh vehicle exhaust is predominantly NO. Transport studies that rely solely on NO2 data underestimate total NOx at kerbside and overestimate the contribution of background pollution.
Tunnel and enclosed-space monitoring. In confined environments where atmospheric mixing is limited, NO accumulates before it has the opportunity to convert to NO2. A nitric oxide sensor is essential in these settings to capture the actual exposure.
Vegetation and ecosystem protection. The UK applies a total NOx limit of 30 micrograms per cubic metre (annual mean, expressed as NO2 equivalent) for the protection of vegetation. Meeting this standard requires measuring both components.
Interpreting NO and NO2 Data Together
The real value of combined NO monitoring and NO2 monitoring lies in what the ratio between them reveals.
High NO, low NO2 indicates fresh emissions close to the source. You are seeing combustion exhaust before atmospheric processing has converted the NO. This pattern appears at roadside monitoring points during rush hour, or downwind of an industrial stack during active operations.
Low NO, high NO2 signals aged pollution. The air mass has had time — and sufficient ozone — to convert most NO to NO2. This is the typical pattern at urban background monitoring stations, away from direct source influence.
Sharp NO spikes with a delayed NO2 rise point to discrete emission events. A piling rig starting, a furnace cycle beginning, or a convoy of heavy vehicles passing — each creates a signature that separate NO and NO2 data can resolve. This temporal pattern allows consultants to link air quality exceedances to specific operations, supporting both compliance evidence and mitigation planning.
Total NOx is reported as the sum of NO and NO2 (expressed as NO2 equivalent). For regulatory submissions, this combined figure is what matters. But the breakdown between the two species adds a layer of intelligence that total NOx alone cannot provide.
How Sensorbee Monitors Nitric Oxide
The Sensorbee NO Sensor Module (SB4242) uses electrochemical technology to measure nitric oxide directly in ambient air. The electrochemical cell oxidises NO at a working electrode, generating an electrical current proportional to the gas concentration — providing continuous, real-time readings without the size, cost, or power requirements of chemiluminescence analysers.
Paired with the SB4202 NO2 Sensor Module, the SB4242 delivers a complete NOx measurement capability from a single Sensorbee Pro2 station. Both modules connect to the Pro2 base unit (SB8202/SB8203), which handles data logging, solar power management, and cellular transmission via NB-IoT or LTE-M.
The practical advantage is straightforward: one solar-powered station at an industrial boundary, a roadside location, or a construction perimeter provides continuous NO and NO2 data without mains power, without a separate NOx analyser, and without manual data collection. Combined with the Pro2's dust, noise, and vibration modules, it becomes a multi-parameter monitoring point that captures the full environmental picture from a single device.
Common Questions About Nitric Oxide Monitoring
Does the UK have specific air quality limits for NO? There is no standalone UK ambient air quality limit for nitric oxide. Nitrogen dioxide (NO2) carries the health-based limits — 40 micrograms per cubic metre annual mean and 200 micrograms per cubic metre hourly mean. However, total NOx (NO + NO2 expressed as NO2 equivalent) has a vegetation protection critical level of 30 micrograms per cubic metre annual mean.
What is the difference between NO and NOx? NOx is the collective term for nitrogen oxides in the atmosphere, primarily comprising NO (nitric oxide) and NO2 (nitrogen dioxide). NO is one component of the NOx family. When regulators or reports refer to NOx, they mean the sum of both gases.
Can one sensor measure both NO and NO2? No. The two gases require different electrochemical cells with distinct electrode chemistries and sensitivities. Sensorbee uses separate dedicated modules — the SB4242 for NO and the SB4202 for NO2 — to ensure accurate measurement of each species independently. This approach avoids the cross-sensitivity issues that affect combined sensors.
