The mixing layer (or boundary layer) is the zone of the planetary boundary layer where turbulence keeps pollutants, heat, and moisture well mixed. Its height determines surface pollutant concentrations.
Daily variation
At night, the layer shrinks to 100–300 m beneath a temperature inversion, concentrating pollutants (rush-hour smog). By midday, convection expands it to 1–3 km, diluting pollution. In persistent anticyclonic conditions, subsidence compresses the mixing layer, creating severe pollution episodes in cities like Madrid.