Overview
ISO 7996:1985 specifies a chemiluminescence method for determining the mass concentration of nitrogen oxides (NOx) in ambient air. The procedure covers measurement of nitrogen monoxide (NO) and nitrogen dioxide (NO2) at standard conditions (25 °C, 101.3 kPa) and is applicable up to approximately 12.5 mg/m3 for NO and 19 mg/m3 for NO2. The method is widely used in air quality monitoring to provide sensitive, real‑time NOx data.
Key topics and technical requirements
- Principle: NO reacts with ozone (O3) in a reaction chamber producing excited NO2 that emits near‑infrared light; the emitted radiation is converted to an electrical signal by a photomultiplier tube.
- Analyzer types: dual (simultaneous NO and NOx streams) and cyclic (alternating bypass through a converter).
- Main components:
- Sample line and intake - short, inert tubing (PTFE or FEP recommended) to minimize reactions and sorption.
- Particle filter - inert materials (PTFE or stainless steel) to prevent particulate sorption of NO2.
- Converter - heated furnace (stainless steel, molybdenum, tungsten or spectroscopically pure carbon) to reduce NO2 to NO; must convert ≥95% of NO2 at temperatures not exceeding 400 °C.
- Ozone generator - provides excess O3; supply air must be dry/filtered.
- Reaction chamber - optimized residence time; often operated at reduced pressure to improve sensitivity.
- Optical filter - blocks wavelengths below 600 nm to avoid hydrocarbon interferences; emitted NO2 radiation centered ~1200 nm.
- Photomultiplier tube - usually thermostatically controlled to reduce noise.
- Ozone removal (activated charcoal) downstream to protect ambient air.
- Calibration & verification:
- Calibrate using permeation systems (ISO 6349) or certified gas mixtures; generate several concentrations across the range and plot response curves.
- Confirm converter efficiency by comparing total NOx response for varying NO/NO2 mixes; maintain efficiency close to unity.
- Conversion factors between mg/m3 and ppm for NO and NO2 are provided for standard conditions.
Applications and users
- Practical uses: continuous ambient NOx monitoring, regulatory air‑quality surveillance, urban pollution studies, source characterization, and compliance testing.
- Typical users: environmental monitoring agencies, air quality laboratories, instrument manufacturers and calibration labs, research institutions studying atmospheric chemistry.
Related standards
- ISO 6142, ISO 6144 - gas mixture preparation (weighing and static volumetric methods)
- ISO 6349 - permeation method for calibration gas generation
- ISO 6711 - checking calibration gas mixtures
Keywords: ISO 7996:1985, chemiluminescence method, nitrogen oxides, NOx measurement, ambient air quality, NO2 converter, ozone generator, calibration.