ISO 9359:1989

Overview

ISO 9359:1989 specifies a stratified sampling method for assessment of ambient air quality. The standard describes how to estimate percentiles (fractiles/quantiles) and, in certain cases, means of the frequency distribution of ambient air quality measurements by dividing the population of measurement conditions into strata (sub-populations). The method reduces the number of measurements needed or improves precision by allocating samples across strata that reflect different meteorological, temporal or spatial conditions. Results can be weighted to provide assessments independent of the short-term meteorological conditions during the measurement campaign and extrapolated to longer time periods.

Key topics and technical requirements

• Stratification principles: choose strata so that stratum means differ and within-stratum variances are smaller than the overall variance. Typical k = 2–4 strata is often sufficient.
• Stratification criteria: meteorology (wind speed/direction, stability, mixing height, air mass), temporal factors (season, time-of-day), topography, emission patterns and associations with other pollutants.
• Sample allocation: formulas in the standard (and Annex A) show optimal allocation of total samples n to strata ni to minimize variance. Allocation depends on stratum probabilities (Wi) and variances (si) or fractions fi.
• Weighting and extrapolation: use weighting factors Wi corresponding to the target assessment period; use predicted weights for prospective assessments.
• Independence and randomness: measurements must be independent (avoid autocorrelation); random selection in time and/or space is required; measurement interval should be less than stratum duration to avoid bias.
• Practical limits and checks: estimation of means is restricted unless distributional assumptions or sufficient independent measurements are available. Some equations require minimum ni (e.g., certain confidence-limit formulas assume ni > 15; confidence for weighted mean more robust if ni > 5).
• Supporting activities: pilot surveys and air quality models are recommended where a priori information is insufficient.

Applications

• Short-term ambient air quality surveys that must represent longer-term conditions
• Estimating percentile concentrations (e.g., 90th percentile) for regulatory assessment or health-impact studies
• Focused studies near highways, industrial basins, or point sources where stratification by distance, wind direction, or time-of-day improves efficiency
• Designing monitoring campaigns and pilot studies to optimize resource allocation
• Prospective assessments (e.g., projected traffic increases) using predicted weighting factors

Who should use this standard

• Environmental consultants and field technicians designing sampling programs
• Air quality monitoring agencies and regulators conducting compliance and exposure assessments
• Researchers and modelers using stratified designs to validate or calibrate air quality models

Related standards

• ISO 2602:1980 (confidence intervals for means)
• ISO 2854:1976 (statistical techniques for means/variances)
• ISO 3534:1977 (statistics vocabulary)
• ISO 7168:1985 (presentation of ambient air quality data)

Keywords: ISO 9359, stratified sampling, ambient air quality, percentiles, stratification, sampling design, air quality assessment.