There are several layers of the atmosphere with varying structure and temperature. The troposphere is the zone where ecosystems exist, and temperature decreases with altitude until stability is reached around 30-32,000 feet. The stratosphere, characterized by warmer upper layers, lacks convection and has fixed layers. The troposphere allows for easy mixing of air due to temperature variations. Vertical changes in an urban space follow a diurnal cycle, with mixing occurring during the day and limited mixing at night due to inversions. Lack of vertical mixing can lead to trapped pollutants and poor air quality. Understanding these systems is crucial in understanding our impact on the atmospheric layers.

The very first thing to consider when trying to understand the environmental chemistry of the atmosphere is that most chemistry takes place in the lower 10-12 km. 

The atmosphere has a structure that can be defined by vertical temperature variations. As you can see here, we pass upwards through the troposphere - the zone in which we, in fact all ecosystems, exist and see the atmospheric collapse rate in the decreasing temperatures. It is not until we get to approximately 30-32,000 feet do we get some sort of temperature stability. This is about the same height as commercial airplanes fly. 

From there, the temperature increases mainly due to an exothermic reaction between UV radiation from the sun being absorbed by oxygen and making ozone. 

It is important to note here that air in the upper stratosphere is warmer than air lower in the stratosphere. Their density variations mean that we see no convection, simply as the warm cannot mix with the colder air beneath it. This lends itself to quite fixed layers in this part of the atmosphere - hence the name stratosphere. 

This is not the case in the Troposphere where mixing can happen easily as warmer air sits beneath colder air. 

Above these layers, we find the Mesosphere and then the thermosphere. In terms of environmental chemistry these are of less importance for us, simply because 99% of atmospheric molecules are found in the lower layers - overwhelmingly in the Troposphere. 

If we look more locally at the vertical changes we see some very important patterns. The diurnal cycle of mixing - that is, the mixing over a typical 24-hour cycle - in an urban space is also defined by its temperature profile.

During the day, when solar insolation heats the ground, the temperature decreases vertically to a considerable height. This allows mixing in a large part of the atmospheric profile. In this case, pollutants are dissipated. 

At night, we see the opposite; the ground becomes cool as there is no solar heating, leaving colder air below the warmer air that has risen from the previous day. This creates an inversion: colder air beneath warmer air, and so mixing is very limited and contained in the space below the inversion. This is also true of early morning, although the inversion may be slightly higher - perhaps the same height as the buildings in an urban space. 

The implications of this lack of vertical mixing, particularly in winter or during a time of high pressure, are very significant. Daily pollution emissions, both gaseous and particulate, are trapped in the space close to the ground in which we live. Industrial or high traffic areas can generate incredibly low air quality in these times, and it is worth noting that high pressure systems can last for weeks.

These controls on vertical mixing also have an important temporal impact. We know from research that it can take 1-3 days to mix within the lower 3 km of the atmosphere; 1 week to mix through the troposphere; 1 month to reach the tropopause; and 5-10 years to reach the stratosphere. 

This means that some chemical species are entirely uncoupled from some parts of the atmospheric system. Pollutants such as short-lived radicals simply cannot impact the stratosphere. That said, compounds with longer lifespans, like freon that can exist for 20-30 years, can bring about significant long term change.

In all cases of understanding our impact on the atmosphere, we must appreciate the systems in which our chemical processes take place.