Air quality, Atmospheric stability and measure of air quality

Air quality, AIr pollution, measuring techniques

1. Atmospheric Stability  Concepts are applied to motions of air parcels in the atmosphere The topic of stability in atmospheric science is important because the formation of clouds is closely related to stability or instability in the atmosphere.

2. Adiabatic Lapse Rate Adiabatic processes  do not exchange heat and they are reversible. For an air parcel, this means that no thermal energy is entering or leaving the air parcel from the outside. However, internal processes are allowed However, temperature changes in the air parcel can still occur, but it is not due to mixing, it is due to changes in the internal energy of the air parcel. To summarize, rising air parcels expand and cool adiabatically without exchanging heat with the environment.

3. Dry Adiabatic Lapse Rate If an air parcel is dry, meaning unsaturated, stability is relatively straightforward. An atmosphere where the environmental lapse rate is the same as the dry adiabatic lapse rate, meaning that the temperature in the environment also drops by 9.8 K·km -1 , will be considered neutrally stable. After some initial vertical displacement, the temperature of the air parcel will always be the same as the environment so no further change in position is expected. Air rises, expands, and cools at the dry adiabatic lapse rate, approximated as a 10°C decrease per km

4. Moist Adiabatic Lapse Rate When water vapor condenses, it goes from a higher energy state to a lower energy state. Energy is never created nor destroyed, especially in phase changes, the energy gets released in the form of latent heat.  As latent heat is added from the process of condensation, it offsets some of the adiabatic cooling from expansion. Because of this, the air parcel will no longer cool at the dry adiabatic lapse rate, but will cool as a slower rate, known as the  moist adiabatic lapse rate . 

5. The atmosphere is said to be  absolutely stable  if the environmental lapse rate is less than the moist adiabatic lapse rate. This means that a rising air parcel will always cool at a faster rate than the environment, even after it reaches saturation. If an air parcel is cooler at all levels, then it will not be able to rise, even after it becomes saturated

6. The atmosphere is said to be  absolutely unstable  if the environmental lapse rate is greater than the dry adiabatic lapse rate. This means that a rising air parcel will always cool at a slower rate than the environment, even when it is unsaturated. This means that it will be warmer (and less dense) than the environment, and allowed to rise. The atmosphere is said to be  conditionally unstable  if the environmental lapse rate is between the moist and dry adiabatic lapse rates. This means that the buoyancy (the ability of an air parcel to rise) of an air parcel depends on whether or not it is saturated.