Overview
ISO 9835:1993 specifies a standardized method to determine the black smoke index of ambient air by measuring the staining (light absorption) caused by particulate matter collected on filter paper. The method uses air sampling, reflectance measurement of the stained filter, and calculation of an absorption coefficient, which can be converted to traditional black smoke units via Annex A. The procedure is intended for measurements in the black smoke index range of about 6 to 375 and is based on reflectometry rather than direct gravimetric mass.
Key topics and technical requirements
- Sampling: Air is drawn through a filter membrane (collection efficiency ~0.1–5 µm) using a sampler with a nominal flow of 1.4 L/min (2 m3/day) for a 24-hour sample. Alternative arrangements with flow controllers or dry gas meters are allowed. Typical pump capability up to 2.0 L/min.
- Sampling location and hardware: Inlet funnel (30–50 mm), mounting height 2.5–5 m above ground, inlet ≥1 m from walls. Connection tubing PVC, internal diameter ~8 mm, maximum 6 m length. Filter holder must be conductive, chemically inert, aperture ~5 cm2, and leakage ≤2% of flow.
- Reflectance measurement: Use a reflectometer (0–100% linear reflectance or logarithmic extinction). Calibrate per manufacturer instructions and at least monthly; verify on clean filters frequently (e.g., after every 10 stains). Measured reflectance should lie between about 35% and 95%.
- Calculation: The absorption coefficient a (m−1) is calculated from measured reflectance R, reference reflectance R0, stain area A, and sampled volume V using the standard equation:
a = (A / (2 V)) × ln(R0 / R)
Report a to one decimal place. Annex A provides conversion tables/curves to convert a into traditional black smoke index units.
- Quality and precision: Reflectance readings to ±1 percentage unit; instruments designed per ISO should measure a with precision better than 5% above certain absorption levels. Test report must document sampling details, filter and reflectometer types, sampled volume, reflectance, and any unusual observations.
Applications and users
ISO 9835 is practical for:
- Environmental monitoring and air quality networks measuring soot-like particulate staining
- Regulatory agencies and compliance testing where historical “black smoke” indices are used
- Research labs comparing historical black smoke datasets with modern optical measurements
- Industrial emission monitoring where a quick staining-based index is needed rather than gravimetric mass
Keywords: ISO 9835, black smoke index, ambient air, reflectometer, absorption coefficient, filter paper, black smoke measurement, air quality monitoring.
Related standards
- OECD and EEC (EEC Black Smoke Reference Method) - ISO 9835 references these historical methods and provides Annex A conversion guidance to align with traditional black smoke units.
