The atmospheric boundary layer (ABL) is usually defined as the portion of the atmosphere that is directly influenced by the land surface. It ranges in height from around 50 to 4000m but is usually in the range of 1 to 2 km in depth. Due to a combination of turbulence and static stability (in simple terms: the way in which air naturally arranges itself when subjected to different temperatures and pressures) the air tends to arrange itself in layers (stratification). In the case of the boundary layer there is a region of very stable air which ‘caps’ the air below it that is interacting with the earth’s surface.

You can observe the boundary layer using equipment that can detect aerosols. The aerosols will be restricted by the cap at the top of the boundary layer, so the instrument will see high concentrations of the aerosols inside the boundary layer then a ‘pause’ followed by a well mixed zone in the free atmosphere above.

Speaking generally, the boundary layer is unstable during the day, when the warm surface is heating the air and creating the turbulence that is mixing the air well within the boundary layer itself. It is stable at night, as the ground is cooler than the air and mixing is reduced. At night the boundary layer is much shallower than during the day. The height of the boundary layer will begin to increase as the ground heats up and decrease as the ground cools down. All of this is driven quite strongly by two things: the weather conditions (windy and cloudy = a neutral boundary layer) and the land surface. At the time of writing, only one of these things we can control.

In a previous blog article, I wrote a little about how different land surfaces change the fluxes of heat as a result of the differences in how they absorb or reflect radiation and how they behave around water (affecting the amount of heat remaining after water is evaporated). One of the most important things about this difference in the heat balance is the effect that it has on the boundary layer, which has been observed to behave differently in the city to in the countryside. This difference is largely related to how the boundary layer develops. For example: if the city is warmer than the country, the unstable boundary layer is likely to persist later into the day. The city not only affects the heat balance but also the wind, a fact that anybody who has had to walk down a road between tall buildings on a windy day can attest to. Particularly if they were carrying an umbrella at the time. This increased turbulence as a result of the extra ‘surface roughness’ (I’ll come back to this another time) in the city, might also affect the dynamics of the forming or deforming boundary layer.

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One thing that all this certainly does affect is the distribution and concentration of pollutants in the city. A stable boundary layer (like overnight) is shallow and doesn’t mix very well with the atmosphere above. The result is increased pollutants close to the surface even without taking into account the actual emission of the pollutants because:

a) they cannot mix easily with the atmosphere above

and

b) the amount of space in the atmosphere that they can mix in has become limited.

The result is high pollutant concentrations in the air. In some cities under certain conditions ‘urban plumes’ of pollutants develop, spreading downstream of the city. In very hot cities there is the added complication of high turbulence inside the city creating enhanced updrafts that can rise above the rural boundary layer. When combined with weak downdrafts in the surrounding countryside, the net effect could be continual recirculation of pollutants into the city and the associated negative impact on air quality.

We can make adjustments to the heat balance of the city using effective city planning and energy management techniques. This is a field of science that is new and the importance of many of the individual contributors to the overall picture are not yet quantified (for example: the net cooling effect of trees, the net heat loss from buildings at the city scale).

In my next blog article I will discuss this topic in a little more detail, focusing more on the effect that the city has on the wind and the resulting influence on pollution.

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