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Chartres, nestled in the heart of the Centre-Val de Loire region of France, occupies a strategic position on the gently rolling plains of the Beauce, a vast agricultural landscape. The city is built on a plateau overlooking the Eure and Loir rivers, which converge nearby, influencing local microclimates and drainage patterns. At an elevation of approximately 130-150 meters, Chartres avoids the more significant temperature inversions common in valleys, but isn’t entirely immune. Historically, the city’s location facilitated trade and transport, but modern traffic contributes to localised pollution. The surrounding Beauce is intensely farmed, with extensive cereal crops; agricultural practices, including fertiliser use, contribute to ammonia emissions, a precursor to particulate matter formation. While Chartres isn’t directly within a major industrial belt, proximity to the Paris Basin means long-range transport of pollutants is a factor. The urban-rural gradient is relatively sharp, transitioning quickly from the city’s dense core to open farmland. This gradient impacts dispersion patterns, with pollutants tending to concentrate within the city itself, particularly during calm weather conditions. The cathedral, a dominant feature, influences wind flow around the city centre, creating localised eddies that can trap pollutants. Understanding these geographical factors is crucial for modelling and mitigating air quality issues in Chartres.

Chartres experiences a temperate oceanic climate, resulting in distinct seasonal air quality patterns. Winter (December-February) often sees poorer air quality due to increased residential heating, particularly wood burning, leading to elevated particulate matter (PM2.5 and PM10) concentrations. Calm, stable atmospheric conditions and frequent temperature inversions trap pollutants near the ground. Spring (March-May) brings improvements as temperatures rise and heating demand decreases, but agricultural activities begin, releasing ammonia. Summer (June-August) generally offers the best air quality, with prevailing winds dispersing pollutants and rainfall acting as a natural scrubber. However, heatwaves can exacerbate ozone formation. Autumn (September-November) sees a return to poorer conditions as heating starts again, combined with agricultural harvesting and potential for stagnant air. Fog, common in autumn and winter, can trap pollutants, reducing visibility and worsening air quality. Sensitive individuals – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during winter months and periods of fog. Monitoring pollen levels is also important in spring and summer. Avoiding peak traffic hours can further reduce exposure. The months of June, July and August generally present the most favourable conditions for outdoor pursuits.