At elevated levels they limit visibility and a human health hazard. There is a growing body of epidemiological data that suggests that increasing levels of aerosols may cause a significant increase in human mortality. For many years it was thought that atmospheric aerosols did not interact in any appreciable way with the cycles of trace gases. We now know that particles in the air affect climate and interact chemically in heretofore unrecognized ways with atmospheric gases. Volcanic aerosols in the stratosphere, for example, participate in the catalytic destruction of ozone by chlorine compounds, not directly, but through the intermediate step of NOx chemistry.
Aerosols reflect solar radiation back to space, and, in so doing, cool the Earth.
Aerosols are also the nuclei around which clouds droplets form - no aerosols, no clouds. Clouds are one of the most important elements of our climate system, so the effect of increasing global aerosol levels on the Earth's cloudiness is a key problem in climate studies. Historically urban air pollution and its effects were studied more or less separately from that of the chemistry of the Earth's atmosphere as a whole. It is now recognized, in quantitative scientific terms, that the Earth's atmosphere is a continuum. The urban atmosphere, the remote troposphere, the marine boundary layer, and the stratosphere are but points along the continuum from the smallest turbulent eddies and the fastest time scales of free radical chemistry to global circulations and the decadal time scales of the longest-lived trace gases.