Street-canyon air quality and a single funded measure
An air-quality monitor on a 25 m-wide street canyon records annual mean NO2 of 46 µg/m³ (EU limit 40) and PM10 of 38 µg/m³ (limit 40). Traffic is 12,000 vehicles/day, 18% diesel cars, and buses stop every 200 m. The street has narrow pavements, a primary school at one end and a clinic at the other. The council can fund one of three measures this year: (a) pedestrianise the street and reroute traffic 600 m away; (b) a low-emission zone banning pre-Euro 5 diesel; (c) a 10-minute bus frequency plus a green wall. Modelling suggests (a) cuts street NO2 by 25% but raises it 8% on the bypass; (b) cuts it by 15%; (c) cuts it by 6%.
1. Interpret the monitoring data. Are the limit values exceeded, and does compliance mean the air is safe?
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NO2: 46 µg/m³ is above the 40 µg/m³ annual limit value, an exceedance of 15%. This is a legal exceedance and a health concern, especially at the school and clinic.
PM10: 38 µg/m³ is below the 40 µg/m³ limit value, but only by 2 µg/m³. It is close enough that year-to-year weather or traffic changes could push it over. More importantly, the WHO guideline is far lower than 40 µg/m³, so 'compliance' does not mean safe.
Street-canyon effect: the 25 m width traps pollutants; buses stopping every 200 m add idling emissions at breathing height. The annual mean also hides peaks at drop-off and pick-up times, when children are present.
The correct interpretation is that NO2 is non-compliant and PM10 is marginal, and that the limit value is a legal floor, not a health-based target. This justifies action even before PM10 exceeds the limit.
2. Evaluate the three measures against health benefit, equity and feasibility.
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- (a) Pedestrianisation: largest street-level NO2 cut (25%), so 46 → about 34.5 µg/m³. Strong health benefit at the school and clinic. But it raises bypass NO2 by 8%; if the bypass baseline is, say, 30 µg/m³, it becomes 32.4, still compliant, but traffic and noise are displaced, possibly to a less affluent area. Feasibility is medium: rerouting, deliveries and bus routes need redesign.
- (b) LEZ banning pre-Euro 5 diesel: cuts NO2 by 15%, so 46 → about 39.1 µg/m³, just compliant. Feasible quickly, but it burdens owners of older diesels, who may be lower-income, and enforcement is needed.
- (c) Bus frequency plus green wall: only 6%, so 46 → about 43.2 µg/m³, still non-compliant. The green wall may help locally but is not a substitute for reducing emissions. It is cheap and popular but insufficient alone.
Equity matters: (a) displaces pollution, (b) displaces cost, (c) displaces responsibility onto vegetation. None is cost-free.
3. Recommend one measure for this year and state the monitoring and mitigation needed.
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Recommend (a) pedestrianisation, but only with conditions, because it delivers the largest health benefit where vulnerable people are and because (c) fails to reach compliance. The displacement to the bypass must be measured, not assumed.
Conditions:
- before/after continuous NO2 and PM10 monitors on the street, at the school gate, at the clinic and on the bypass;
- traffic counts and queue-length surveys on the bypass;
- a bypass baseline check: if it is already near 40 µg/m³, the scheme should be modified or paired with traffic calming;
- delivery and bus routing plan, and accessible design for the clinic;
- a commitment to review after 12 months and add an LEZ if NO2 remains above 40.
If the bypass baseline is high or the reroute is not feasible within the year, the fallback is (b), with targeted support for low-income owners of older diesels. The judgement should be explicit about the trade-off rather than claiming a win-win.