Formation, Nature, and Mobility of Ions of Atmospheric Importance

Abstract

An understanding of the evolution of atmospheric small ions requires the knowledge of 1) the type of radiation, 2) the atmospheric trace gas composition as a function of altitude and the physical and chemical molecular parameters of these trace gas molecules such as ionization potential, electron affinity, proton affinity, polarizability, chemical reactivity, etc., 3) ion-molecule reaction rates for eventually every collisional interaction of ions with the neutral gaseous environment and 4) the theory of ionic mobility. Partial information on 1) –4) is available today and presented in this paper. Based upon laboratory and field measurement, we can now derive a preliminary picture of the nature of atmospheric ions as a function of altitude. The atmospheric ions are thought of being in a state of constant change, reflecting in their chemical nature the collisional interaction with atmospheric trace gas molecules. Molecular rearrangement of attached molecules can lead to the formation of new chemical species on the “surface” of small ions, which allows the ion to grow into larger ion clusters. The mobility spectrum of atmospheric small ions is thought to be pseudo-continuous in the lower part of the atmosphere due to the presence of numerous trace gases (on the level of less than 10–9 of all parts) interacting with the ions during their lifetime. However, direct mass spectrometric in situ measurements of tropospheric and stratospheric ions are still missing.