4. City-Wide Air Quality Aggregation Methodology
The Challenge of City-Level Averages
A single air quality measurement or simple average for an entire city is often not a good indicator of air pollution exposure. Air pollution levels vary significantly across different neighbourhoods within the same city due to traffic patterns, industrial activities, human behaviour (cooking, burning, transport), geographic features, meteorological conditions, and land use characteristics.
Recommended Methodological Approaches
Approach 1: Spatial Heterogeneity Analysis (Preferred)
Maintain spatial granularity in your analysis rather than collapsing to a single city average. Analyse air quality patterns by district, division, or neighbourhood. Identify pollution hotspots and clean areas. Relate air quality levels to land use, traffic density, and population exposure.
Approach 2: Background Site Averaging
If a city-level indicator is necessary: identify monitors in background sites with minimal local emission sources, calculate their mean for the period, and also report the highest reading from any monitor to represent worst-case exposure.
Approach 3: Population-Weighted Average
For exposure assessment studies, weight monitor readings by the population residing within each monitor's representative area. This provides a better estimate of average population exposure than simple arithmetic means.
- Simple arithmetic mean of all monitors — can misrepresent exposure by over-representing densely monitored areas.
- Ignoring spatial variation — treating a city as having uniform air quality.
- Using near-source monitors (roads, industries) to represent the whole city.