Atmospheric composition, even of trace gases, has a huge influence on weather and climate. Carbon dioxide is the most abundant trace gas – its mixing ratio is 400 ppm and growing, but other trace gases are also emitted into the atmosphere. The atmosphere cleans itself of these gases by atmospheric chemistry, which oxidizes the gas emissions and produces new chemicals that contain oxygen and so are stickier and more water soluble.
These new chemicals can be removed from the atmosphere either by hitting surfaces and sticking or by being taken up in clouds or rain drops and precipitated to the ground. The main oxidant is hydroxyl (OH), which is made with ozone, UV sunlight, and water vapor and starts the removal sequence by reacting with gas emissions. In these reaction sequences ozone pollution is produced if the pollutant nitric oxide (NO) is also present.
This pollutant ozone is nearby and is harmful to human health and agriculture. Stratospheric ozone, on the other hand, shields Earth from harmful UV, and is made a completely different way – by the breaking apart of O2 to produce O, which reacts readily with O2 to form O3. Some of this stratospheric ozone is then transported to Earth, but at levels much lower than pollutant levels. Methane oxidation is an example of the VOC reactions that produce ozone and particles.
An important concept is the atmospheric lifetime of gases and particles. This can be determined by solving a simple linear differential equation. The methane lifetime was shown to be about 10 years.
Particles have many natural and anthropogenic sources; some are emitted directly from the sources (primary particles) and some are produced by atmospheric chemistry (secondary particles). Particles affect human health, visibility, scattering and absorption of light, and are essential for cloud formation, as will be seen in the next lesson on cloud physics.
Reminder - Complete all of the Lesson 4 tasks!
You have reached the end of Lesson 4! Double-check that you have completed all of the activities before you begin Lesson 5.