Valley inversion and a winter smog alert

At night, the snow surface and valley sides lose heat by long-wave radiation, so the air in contact with them cools. Cold air is denser and drains downslope into the valley floor, where it pools. That is why the shoulder station at 220 m is warmer (+3 °C) than the floor station at 40 m (-4 °C): the lapse rate is inverted. The inversion acts as a lid. With light anticyclonic winds there is little mechanical mixing, so the stable layer traps emissions from traffic, domestic heating and other local sources. PM2.5 is also produced by secondary chemical reactions in this shallow boundary layer. Snow increases albedo and reduces daytime heating, which helps the inversion persist. Because the anticyclone is forecast to last 36 hours, pollutants are not dispersed; the airport visibility of 600 m is consistent with a shallow, moist, polluted layer. The key reasoning is not simply that cold air sinks, but that radiative cooling, drainage, weak winds and a persistent synoptic pattern together create a small mixing volume.

Recommendation: issue a public health alert, move outdoor school sports indoors, and advise vulnerable groups to reduce outdoor exertion, but do not close schools or the airport on this reading alone. Justification: 78 µg/m³ is well above the WHO 24-hour PM2.5 guideline of 15 µg/m³ and more than six times the overnight reading; visibility at the airport has fallen to 600 m; the inversion is forecast to persist for 36 hours; and the valley floor is where most people live. The precautionary principle is appropriate because the cost of a false alarm is lower than the health cost of missing a real episode. Uncertainties: first, the town-centre monitor was calibrated 11 months ago and may be biased; second, the only temperature comparison comes from a farm station that is not co-located and does not measure PM2.5, so the vertical profile is inferred rather than measured. A third uncertainty is spatial representativeness, because one monitor may not capture the whole town. Issue the alert but verify with a calibrated handheld monitor and keep the advice under review.

Co-locate at least three reference-grade PM2.5 monitors on the valley floor, mid-slope and shoulder, with temperature and wind sensors at the same sites. Calibrate every six months and keep a log of local emissions. Add a sonic anemometer to measure mixing and a visibility sensor or ceilometer to track inversion height, then compare these with the airport record. A low-cost sensor network can give spatial coverage, but it must be corrected against a reference monitor. Collect data over at least one full winter season so that the council can distinguish a one-night episode from a recurring problem. Publish trigger thresholds in advance, for example a PM2.5 concentration and a forecast persistence time, so that decisions are consistent and defensible.