Determination of nitrogen oxides (NOx) in ambient air is a core task of air quality monitoring, environmental compliance verification, and human health risk assessment. This technical article explains the principal measurement techniques—including chemiluminescence, Griess-Saltzman spectrophotometry, electrochemical analysis, and passive sampling—together with applicable international and national standards, sampling requirements, quality control procedures, and typical applications. It is intended as a practical reference for environmental engineers, laboratory analysts, and facility operators who require accurate and defensible NOx data.
Technical Overview
Nitrogen oxides (NOx) is a collective term for gaseous compounds of nitrogen and oxygen, of which nitric oxide (NO) and nitrogen dioxide (NO2) are the most significant in atmospheric monitoring. NO is a colorless gas formed mainly during high-temperature combustion through reaction of atmospheric nitrogen and oxygen; after release, it undergoes rapid photochemical oxidation to NO2, a reddish-brown gas with a characteristic pungent odor. Because the two species interconvert continuously in the atmosphere, regulatory monitoring and most analytical methods report results for NO, NO2, and total NOx, with total NOx conventionally expressed as NO2 mass equivalent.
Principal anthropogenic sources include motor vehicle exhaust, fossil-fuel-fired power plants, industrial boilers and furnaces, nitric acid manufacturing, metal pickling and surface treatment, welding, and explosives use in mining and construction. NOx plays a central role in atmospheric chemistry: it is a key precursor of ground-level ozone and photochemical smog, contributes to acid deposition as nitric acid, and drives the formation of secondary nitrate particulate matter (PM2.5). Direct inhalation of NO2 is associated with airway inflammation and aggravation of asthma, which is why NO2 is one of the basic pollutants regulated under national and international ambient air quality frameworks such as GB 3095-2012, the EU Ambient Air Quality Directives, and the United States NAAQS program administered by the EPA.
Testing Samples
Ambient (Outdoor) Air
Samples collected at routine air quality monitoring stations, roadside and traffic-influenced microenvironments, industrial plant boundaries, and sensitive receptor locations such as schools, hospitals, and residential areas. Results are compared against hourly, 24-hour, and annual average limit values.
Indoor Air
Air in residences, offices, schools, commercial kitchens, and underground parking garages, where gas cooking appliances, unvented heaters, and infiltration of vehicle exhaust can generate locally elevated NO2 concentrations.
Workplace Air
Air in occupational settings such as nitric acid production plants, electroplating and acid pickling workshops, welding shops, blasting and mining operations, and other confined spaces where combustion or high-temperature processes occur, evaluated against occupational exposure limits.
Exhaust Gas from Stationary Sources
Flue gas sampled from boiler stacks, kilns, furnaces, gas turbines, and waste incinerators, including high-temperature and high-moisture streams. These measurements verify compliance with emission standards and confirm the performance of SCR or SNCR denitrification systems and low-NOx burners.
Testing Items
- Nitric oxide (NO) concentration, reported in μg/m³ or mg/m³ at specified reference conditions
- Nitrogen dioxide (NO2) concentration
- Total nitrogen oxides (NOx), reported as NO2 equivalent
- Conversion and oxidation efficiency of sample pretreatment devices, including NO-to-NO2 oxidizing columns and NO2-to-NO converters
- Ancillary stack parameters for stationary sources, including O2 and CO2 content, moisture, gas temperature, velocity, and volumetric flow, required for converting measured concentrations into standardized emission rates
Testing Methods
Method selection depends on the monitoring objective (real-time continuous data versus time-weighted averages), the expected concentration range, and the applicable regulatory standard. The most widely used techniques are described below.
Chemiluminescence Method
Chemiluminescence is the reference technique for continuous NOx monitoring and is specified in HJ 479-2009, ISO 7996, and EN 14211. Sample gas is drawn into a reaction chamber where NO reacts with excess ozone generated internally by the analyzer; the reaction produces excited NO2 molecules that emit light (photons) as they return to the ground state. The emission intensity, measured by a photomultiplier tube, is directly proportional to the NO concentration. To quantify NOx, a parallel sample stream passes through a converter, typically heated molybdenum at approximately 315–325 °C or a photolytic converter, which reduces NO2 to NO; NO2 is then obtained by difference between the NOx channel and the NO channel. Typical detection limits are in the low μg/m³ or sub-ppb range, with a linear response extending to ppm levels and response times of a few seconds. The method requires calibration with certified NO and NO2 standard gases traceable to a national metrology institute and periodic verification of converter efficiency, which should typically remain above 95–99 percent.
Griess-Saltzman Spectrophotometry
This classical manual method is specified in GB/T 15435-1995 and ISO 6768. NO2 is absorbed from a known volume of air into Saltzman reagent, an acidic sulfanilic acid solution, and couples with N-(1-naphthyl)ethylenediamine dihydrochloride to form a stable azo dye whose absorbance is measured spectrophotometrically at approximately 540–545 nm. For total NOx, the sample gas first passes through an oxidizing column, such as chromium trioxide or acidic potassium permanganate, which converts NO to NO2 before absorption. The method is well suited to short-term grab samples and laboratory-based analysis, with detection limits on the order of a few μg/m³. Absorbed samples should be analyzed promptly within the holding time specified by the standard, and field blanks and parallel samples are required for quality assurance.
Electrochemical Sensor (Fixed-Potential Electrolysis) Method
Portable instruments based on electrochemical cells, applied under HJ 57-2017 for stationary sources and widely used for workplace and indoor surveys. Sample gas diffuses through a membrane to a working electrode held at a fixed potential, where NO or NO2 undergoes electrochemical oxidation or reduction; the resulting current is proportional to concentration. These analyzers are lightweight, fast, and inexpensive, but sensors exhibit gradual drift and cross-sensitivity to other gases, so regular zero, span, and interference checks are essential, and results are generally supported by periodic comparison against a chemiluminescence reference analyzer.
Passive (Diffusive) Sampling
Diffusive tube samplers, standardized in EN 16339, rely on molecular diffusion of NO2 onto a collection substrate, typically a filter impregnated with triethanolamine, over an exposure period of one to four weeks. The trapped nitrite is subsequently extracted and quantified in the laboratory by spectrophotometry or ion chromatography. Passive sampling requires no power and no field instrumentation, making it ideal for dense spatial mapping of long-term average concentrations across urban areas, fenceline studies, and screening surveys, although it cannot resolve short-term peaks.
Long-Path and Remote Optical Methods
Open-path differential optical absorption spectroscopy (DOAS) measures path-averaged NO2 by exploiting its structured ultraviolet and visible absorption features along a light beam of several hundred meters, and is useful for fenceline monitoring of industrial sites, tunnels, airports, and city-scale studies. Cavity-attenuated phase-shift (CAPS) spectroscopy is accepted in several jurisdictions as an equivalent automated method, providing direct, converter-free NO2 measurement with very high sensitivity and reduced interference from other reactive nitrogen species.
Quality Assurance and Quality Control
- Calibration using standard gases traceable to a national metrology institute, with multipoint calibration linearity typically requiring a correlation coefficient of at least 0.999
- Routine zero and span checks on continuous analyzers, with documented drift correction and control charts
- Periodic verification of NO2-to-NO converter efficiency and replacement when efficiency falls below the specified threshold
- Use of inert sampling train materials such as PTFE tubing, with heated lines for moisture-laden stack gases, and particulate filters upstream of the analyzer
- Field blanks, parallel duplicate samples, breakthrough checks for absorbing solutions, and timely analysis within prescribed holding times for manual methods
- Participation in inter-laboratory proficiency testing and independent audits when data are submitted for regulatory purposes from accredited third-party laboratories
Applications
- Ambient air quality assessment and compliance monitoring against GB 3095-2012, under which the NO2 limits are 40 μg/m³ as an annual average, 80 μg/m³ as a 24-hour average, and 200 μg/m³ as a 1-hour average, with corresponding NOx limits of 50, 100, and 250 μg/m³
- Environmental impact assessment and post-construction acceptance monitoring for industrial, transportation, and energy projects
- Verification of denitrification system performance, including SCR, SNCR, and low-NOx burner retrofits, and compliance demonstration for stack emissions
- Calibration, auditing, and relative accuracy testing of continuous emission monitoring systems (CEMS) installed on regulated sources
- Input data generation for air dispersion modeling, source apportionment, and photochemical ozone and smog episode studies
- Indoor air quality investigations and occupational exposure surveys in workshops, parking structures, and confined work spaces
- Urban monitoring network design, low-cost sensor network validation, and academic or policy-driven research campaigns
FAQ
What is the difference between NO, NO2, and NOx?
NO is the primary product of high-temperature combustion and is relatively less toxic, while NO2 is the secondary, more toxic oxidized form that dominates health-based regulation. NOx is the sum of both species, expressed as NO2 equivalent, and is the parameter typically used for combustion source evaluation and atmospheric chemistry studies.
Which measurement method should be selected?
Chemiluminescence is preferred for continuous regulatory-grade monitoring of ambient air and stack gas. Griess-Saltzman spectrophotometry remains suitable for manual short-term sampling and laboratories without continuous analyzers. Electrochemical portable analyzers are appropriate for screening and spot checks, and passive samplers are the best choice when weekly or monthly average concentrations and wide spatial coverage are needed.
What interferences can affect the results?
In chemiluminescence systems with heated metal converters, other reactive nitrogen species such as nitric acid, PAN, and ammonia may be partially converted to NO and cause a positive bias on the NOx channel; photolytic converters reduce this bias. Water vapor and CO2 can attenuate signals in some optical and electrochemical systems, and acidic gases or particulates can contaminate sampling lines, which is why inert materials, filters, and regular maintenance are required.
How should sampling and sample transport be handled for manual methods?
Absorbing solutions must be protected from light and temperature extremes, and the impinger train must be leak-checked before sampling. After collection, samples should be analyzed within the holding time specified in the applicable method, typically within the same working day, with field blanks carried through the entire procedure.
How are stationary source results reported?
Measured concentrations are converted to dry basis and standard reference conditions and corrected to a specified reference oxygen content, as required by the applicable emission standard. This allows emissions from different combustion sources to be compared on a consistent basis regardless of excess air dilution.
What detection performance can be expected?
Modern chemiluminescence analyzers achieve detection limits of roughly 1–2 μg/m³ or lower for NO and NO2, with linear ranges covering ambient to source-level concentrations. Spectrophotometric manual methods typically reach detection limits of a few μg/m³, while passive samplers resolve weekly averages in the low μg/m³ range under standard exposure durations.
