Theory 100/Energy & Environment /Atmospheric chemistry
Air Quality Is a Dilution Ratio
Air quality is managed by emissions. The weather decides how much room they have.
Urban air pollution is set by the ratio of emission flux to mixing height rather than by emissions alone, so a city with high emissions and deep mixing can be cleaner than one with lower emissions and a shallow boundary layer.
01The evidence
Pollution episodes occur on days with similar emissions and shallow mixing, and concentrations vary several-fold with boundary layer depth.
02The tension
Emission inventories are the regulatory tool, which predicts concentration from emissions and treats meteorology as a correction.
03The mechanism
A pollutant released into a mixed layer is diluted by the volume of that layer. The concentration is the flux divided by the height of the layer and the wind that ventilates it.
That makes the ratio the determinant, and predicts episodes should be predictable from mixing height rather than from emission changes.
04What this is not
Inventories predict from source strength. This predicts from dilution, which explains episodes without emission changes.
05Predictions
- P1Concentration should track the flux-to-mixing ratio.
- P2High-emission deep-mixing cities should be cleaner.
- P3Mixing height should predict episodes better than emissions.
06Refutation
Concentration tracking emissions at controlled mixing would restore inventory management.
07What it changes
It would change air quality forecasting and advisories.