Secondary Particle Production in Urban Areas

Abstract

Suspended particles in air and their corresponding mass can originate from three different types of sources. One is primary, expressing the release into the atmosphere as a particle straight away. This includes mineral dust, sea salt, soot, heavy metals, clay and biological material (pollen, bacteria, etc.). Those are usually located at larger diameters above half a micron. The second type originates from atmospheric trace gases (precursors), which react in the gas phase to form products of different volatility and reactivity. Some of them will either form new particles in number or produce new aerosol mass by partitioning between the gas and the aerosol phase. The third source type refers to the cloud phase and is essentially a mixture of both other types. Gases are absorbed in the cloud water, subsequently processed chemically and either stick primary aerosols included in the cloud water as well or form new aggregates. When the cloud starts evaporating as nine of ten clouds do, re-entering the atmosphere either as gases or as particulate matter. The primary particles dissolved in the cloud phase and interacting with the processed chemicals consist mainly of dissolved salts and water-soluble compounds, organic as well as inorganic.